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J Physiol. 2000 Dec 1; 529(Pt 2): 431–443.
PMCID: PMC2270201

Responses of aortic depressor nerve-evoked neurones in rat nucleus of the solitary tract to changes in blood pressure

Abstract

  1. Using electrophysiological techniques, the discharge of neurones in the nucleus of the solitary tract (NTS) receiving aortic depressor nerve (ADN) inputs was examined during blood pressure changes induced by I.V. phenylephrine or nitroprusside in anaesthetized, paralysed and artificially ventilated rats.
  2. Various changes in discharge rate were observed during phenylephrine-induced blood pressure elevations: an increase (n = 38), a decrease (n = 5), an increase followed by a decrease (n = 4) and no response (n = 11). In cells receiving a monosynaptic ADN input (MSNs), the peak discharge frequency response was correlated to the rate of increase in mean arterial pressure (P < 0.01) but was not correlated to the absolute increase in blood pressure. The peak discharge frequency response of cells receiving a polysynaptic ADN input (PSNs) was correlated to neither the absolute increase in blood pressure nor the rate of increase in mean arterial pressure.
  3. Diverse changes in discharge rate were observed during nitroprusside-induced reductions in blood pressure: an increase (n = 3), a decrease (n = 10), an increase followed by a decrease (n = 3) and no response (n = 6). Reductions in pressure of 64 ± 2 mmHg produced weak reductions in spontaneous discharge of 1.3 ± 0.9 Hz and only totally abolished spontaneous discharge in one neurone.
  4. These response patterns of NTS neurones during changes in arterial pressure suggest that baroreceptor inputs are integrated differently in MSNs compared to PSNs. The sensitivity of MSNs to the rate of change of pressure provides a mechanism for the rapid regulation of cardiovascular function. The lack of sensitivity to the mean level of a pressure increase in both MSNs and PSNs suggests that steady-state changes in pressure are encoded by the number of active neurones and not graded changes in the discharge of individual neurones. Both MSNs and PSNs receive tonic excitatory inputs from the arterial baroreceptors; however, these tonic inputs appear to be insufficient to totally account for their spontaneous discharge.

The action potential discharge of arterial baroreceptors is determined by the degree and rate of vessel distension. This discharge encodes both dynamic and static information (Brown, 1980) so that information about the rate of change in blood pressure and the mean level of blood pressure is relayed to the central nervous system. This information is initially integrated by neurones within the nucleus of the solitary tract (NTS). Therefore, insights into how NTS neurones respond to activation of arterial baroreceptors is critical to our understanding of how the neuro-humoral systems regulating cardio-vascular function are organized.

Baroreceptor afferent fibres are a heterogeneous population as regards their responses to a change in pressure and conduction velocity (Brown, 1980). The extent to which this heterogeneity is reflected in the responses of neurones that integrate such inputs is not known. Several studies have undertaken the daunting task of characterizing the initial integration of arterial baroreceptor inputs by neurones in the NTS. Previous studies have indicated that the output of an NTS neurone, measured as action potential discharge, is not a simple linear transformation of the mean pressure signal (Mifflin et al. 1988; Suzuki et al. 1993; Rogers et al. 1993, 1996; Hayward & Felder, 1995; Seagard et al. 1995). Rogers et al. (1996) reported that the discharge frequency response of NTS neurones to an increase in pressure was best correlated to the rate and direction of change in arterial pressure, rather than the absolute change in pressure. Seagard et al. (1995) found two NTS neuronal response patterns to a ramp increase in carotid sinus pressure; one group exhibited a sudden increase in discharge followed by adaptation and the other group exhibited a ramp increase in discharge that followed the ramp pressure stimulus fairly well. These studies support the concept that there are parallel, partially separate channels of signal processing within the NTS that encode either rate or mean pressure information.

Previous analyses of NTS neuronal responses to changes in arterial pressure have not attempted to discern second order from higher order neurones (Mifflin et al. 1988; Suzuki et al. 1993; Hayward & Felder, 1995; Seagard et al. 1995), or have concentrated on neurones receiving short latency, presumably myelinated, monosynaptic afferent inputs (Rogers et al. 1993, 1996). Furthermore, relatively little attention has been paid to neuronal responses to reductions in arterial pressure. Therefore, the goal of the present study was to characterize the integration of arterial baroreceptor inputs by putative second and higher order NTS neurones. Neuronal discharge frequency during pharmacologically induced increases and decreases in arterial pressure was measured to determine what particular component(s) of the pressure stimulus was best correlated to the neuronal response. Our results are consistent with the concept that rate and mean pressure information are encoded by separate populations of NTS neurones. Our results extend this concept by suggesting that rate information is encoded by cells receiving monosynaptic baroreceptor inputs whilst mean pressure information is encoded by the number of neurones discharging at a given pressure, rather than graded changes in the discharge of individual neurones.

METHODS

Animals

Successful experiments were performed on 47 male Sprague-Dawley rats (375–500 g; Charles River Laboratories, Willington, MA, USA or Harlan Sprague Dawley, Inc., Indianapolis, IN, USA). Rats were housed in a fully accredited (AAALAC and USDA) laboratory animal room with free access to food and water. All experimental rats were housed for at least 1 week before use. All experimental protocols were approved by the Institutional Animal Care and Use Committee. Some of the neurones were obtained in the course of other studies (Zhang & Mifflin, 1998a, 2000).

Surgical preparation

Rats were initially anaesthetized with pentobarbital sodium (60 mg kg−1, i.p.) and placed on a thermostatically controlled heating pad. Body temperature was maintained at 36–38°C throughout the experiment. After placement of a venous catheter (tail vein) and cannulation of the trachea, the animal was artificially ventilated with oxygenated room air. Maintenance doses of pentobarbital were given as an infusion of 10–20 mg kg−1 h−1 (i.v.). Gallamine triethiodide (20 mg kg−1 per half hour to hour as needed, i.v.) was also given for paralysis. A femoral artery was cannulated for arterial blood pressure monitoring. The depth of anaesthesia was assessed by monitoring the stability of arterial pressure and heart rate during a pinch of the hindpaw and was adjusted by appropriate changes in the anaesthetic infusion rate. ADNs were isolated bilaterally and marked with small pieces of black suture. After all surgical procedures had been performed, the rat was placed in a stereotaxic head frame and an occipital craniotomy was performed to expose the dorsal surface of the medulla in the region of the calamus scriptorius. An ADN ipsilateral to the central recording site was mounted, intact, on bipolar stimulating electrodes placed as far as possible from the vagus nerve (typically 4–10 mm). The electrode and the exposed ADN were isolated from other tissues with a mixture of Vaseline and mineral oil. The ADN was activated using square-wave, constant current stimuli of 1 ms duration, at 10 times the threshold intensity necessary to evoke a fall in blood pressure (typically 300–500 μA) delivered at 0.5 Hz. At these intensities, the total population of myelinated and unmyelinated afferent fibres in the ADN should be activated (Sapru et al. 1981; Numao et al. 1985). Blood pressure was manipulated pharmacologically by i.v. injections of phenylephrine (2–4 μg kg−1) or nitroprusside (40–60 μg kg−1).

Extracellular recordings and microionophoresis

All recordings were carried out with multi-barrel electrodes (ASI Instrument, Warren, MI, USA). The recording barrel was filled with a solution of 0.5 m sodium acetate containing 2% Chicago sky blue (impedance, 8–30 MΩ). Although other barrels of the multi-barrel electrode were not always used in these experiments, they were filled with either saline or excitatory amino acid (EAA) agonists. EAAs were administered by application of microionophoretic ejecting currents to the drug-containing barrels. The drug solutions for microionophoresis were l-glutamic acid (monosodium salt, 100 mm, Sigma Chemical Co.) or (RS)-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA, 10 mm, Tocris Neuramin, Bristol, UK). All EAAs were ejected as anions. Retaining currents of appropriate polarity were applied to the drug barrels to retard the passive diffusion of the drug from the electrode tip during non-ejection periods. One barrel of each five-barrel electrode was filled with a solution of 3 m NaCl, and was used for automatic current balancing if microionophoresis was performed. The five-barrel electrode was lowered into the tissue in 2.0–2.5 μm steps by a stepdriver controller (Burleigh Instrument Inc., Fishers, NY, USA). Recorded signals were DC amplified (World Precision Instruments, New Haven, CT, USA) and then passed to an AC filter/amplifier (Grass Instrument Co., Quincy, MA, USA). The signal was then sent to a digital oscilloscope (Nicolet Instrument Co., Madison, WI, USA), an audiomonitor (Grass Instrument Co.), a video tape recorder (A. R. Vetter, Co., Reberburg, PA, USA), and a window discriminator (World Precision Instruments). The window discriminator output was led to a CED1401 analog-to-digital converter (Cambridge Electronic Design, Cambridge, UK) interfaced with a PC. Spike2 data acquisition software (Cambridge Electronic Design) was used for on- and off-line analyses. Peristimulus-time histograms (PSTHs; duration, 0.5 s; bin width, 1 ms; 40 sweeps of single pulse stimulation at an interval of 1.5 s) and ratemeter histograms (duration, 180 s; bin width, 1 s) were constructed to analyse extracellular ADN-evoked and spontaneous discharge, respectively. Responses were measured as changes relative to baseline by subtracting the spontaneous discharge frequency from the response. Only data from ADN-evoked NTS neurones whose action potentials exhibited no or minimal changes in amplitude during drug-induced blood pressure changes were reported and used for the statistical analysis. Large changes in action potential amplitude during blood pressure changes were considered to be artifactual and neurones exhibiting such behaviour were not used in the present study. Neurones that responded to ADN stimulation were designated as monosynaptic neurones (MSNs) or polysynaptic neurones (PSNs) using previously reported criteria (Scheuer et al. 1996), primarily the ability to respond to each of two ADN stimuli separated by 5 ms. Data are presented as peak discharge frequency responses which were calculated as the difference between the absolute peak discharge frequency response following an increase in pressure and the spontaneous discharge frequency before such an increase in pressure. Arterial pressure was measured using a Cobe CDX transducer (Cobe Laboratories, Lakewood, CO, USA). Mean arterial pressure and heart rate were determined from the pulsatile arterial pressure signal using a Coulburn blood pressure processor. At the conclusion of the experiment the animal was killed by i.v. injection of a lethal dose of pentobarbital.

Data analysis

Data were analysed with MANOVA (ANOVA), correlation or ×2 tests (StatSoft, Tulsa, OK, USA). All values are expressed as means ±s.e.m. and significance was accepted at P < 0.05.

RESULTS

General characteristics of ADN-evoked NTS neurones

Data were obtained from 66 NTS neurones that responded to ADN stimulation. Thirty-five of them were characterized as MSNs (Fig. 1A) and 31 as PSNs (Fig. 1B). The mean onset latency of the ADN-evoked input in MSNs (9.0 ± 1.3 ms) was significantly shorter (P < 0.001) than that of PSNs (24.3 ± 1.5 ms). The onset latency variability of MSNs (5.6 ± 0.7 ms) was less (P < 0.001) than that of PSNs (12.0 ± 1.3 ms). Twenty-seven of 35 MSNs and 28 of 31 PSNs were spontaneously active. The mean discharge rate for the spontaneously active MSNs (3.9 ± 0.9 Hz) was lower (P < 0.05) than that of the spontaneously active PSNs (8.3 ± 1.9 Hz).

Figure 1
Characterization of NTS neurones receiving ADN inputs as monosynaptic (MSN, A) and polysynaptic (PSN, B)

Responses to phenylephrine-induced increases in blood pressure

MSNs

Thirty-one MSNs were examined during phenylephrine-induced increases in mean blood pressure of 16–61 mmHg (23 spontaneously active, 8 silent). Twenty-six of these responded to blood pressure increases with an increase in discharge (17 spontaneously active MSNs and 6 silent MSNs), an increase followed by a decrease (2 spontaneously active MSNs) or a decrease (1 spontaneously active MSN). Five MSNs had no obvious response to blood pressure elevations (3 spontaneously active MSNs and 2 silent MSNs). The ADN nerve was examined after finding ADN-evoked cells which were non-responsive to changes in blood pressure and was found to be capable of evoking a reflex decrease in blood pressure, which suggests that the lack of response was not due to nerve damage. During ionophoretic application of EAAs sufficient to increase discharge, MSNs that did not respond to an increase in blood pressure remained unresponsive to blood pressure elevations (n = 2).

In 7 of the 23 MSNs responding to an increase in pressure with solely an increase in discharge, the increased discharge was transient (Fig. 2A and and3)3) while in the remaining 16 MSNs the initial peak increase in discharge frequency was followed by adaptation to a lower frequency level that was sustained for the duration of the pressure increase (Fig. 2B and and4).4). The peak increases in discharge frequency occurred either before/during (5 MSNs, Fig. 2A) or after (19 MSNs, Fig. 2B) the peak increase in blood pressure. The delay between the peak response and peak pressure ranged from 1 to 9 s in 16 MSNs and was 13, 16 and 25 s in the remaining three MSNs. There was no correlation between the peak discharge frequency response of MSNs and the time interval between the peak discharge frequency response and the peak increase in pressure (r =−0.17, P = 0.42). There was no difference between the peak discharge rate increase of the spontaneously active MSNs (11.0 ± 2.8 Hz, n = 23) and the peak discharge rate increase of the silent MSNs (32.4 ± 20.0 Hz, n = 8; P = 0.09) during similar elevations in blood pressure (35 ± 2 and 37 ± 4 mmHg, P = 0.63).

Figure 2
MSN responses to phenylephrine-induced increases in blood presure
Figure 3
Responses of a single, short latency MSN to blood pressure increases with differing rates of rise
Figure 4
Responses of a long latency MSN to blood pressure increases with differing rates of rise

Six MSNs had long latency responses (20–29 ms) to ADN stimulation (Figs 1A and and2D).2D). These MSNs were obviously a different group from MSNs with short latency responses (2–10 ms) and there was no overlap between the onset latencies of the two groups (Fig. 2D). Estimates of afferent fibre conduction velocity suggest that these two groups of MSNs represent cells receiving monosynaptic A and C fibre afferent inputs. A larger percentage of the short latency MSN population (23/25, 92%) responded to increases in blood pressure than in the longer latency MSN population (3/6, 50%). Short latency MSNs had a more robust response to increases in blood pressure (Fig. 2A and D, and and3)3) than MSNs with longer latency ADN-evoked responses (Fig. 2B and D, and and4;4; 20.1 ± 6.6 vs. 1.6 ± 0.7 Hz increase in discharge rate, P < 0.01) during similar blood pressure increases (39.6 ± 4.7 and 34.6 ± 1.8 mmHg, P = 0.25).

The spontaneous discharge and/or pressure-evoked discharge of 4 of 23 spontaneously active MSNs and 2 of 8 silent MSNs (Fig. 3) exhibited a rhythmic discharge pattern. The rhythmic discharge of these neurones was correlated to the inspiratory phase of the ventilatory rhythm. Turning the ventilator off either eliminated the rhythmic discharge or changed it to an irregular pattern of discharge (n = 2, data not shown). All of these neurones had short latency ADN-evoked inputs (2–5 ms) and were very sensitive to the phenylephrine-induced blood pressure increases (Fig. 3). All six of these MSNs exhibited an increase in discharge rate during blood pressure elevations.

The relationships between the responses of MSNs and the net blood pressure increases, the latency for ADN-evoked responses, the basal spontaneous discharge, the rate of the pressure increase and the resting level of blood pressure were analysed. For the two biphasically responding MSNs only the excitatory responses were used for statistical analysis. There was a significant correlation between the discharge frequency response of MSNs and the rate of the pressure change (Fig. 2E, r = 0.44, P = 0.01). The increase in discharge frequency in MSNs was not correlated to the net increase in blood pressure (r = 0.07, P = 0.70, Fig. 2C), the onset latency of the ADN-evoked response (r =−0.28, P = 0.12, Fig. 2D), the spontaneous activity (r = 0.06, P = 0.72) or the basal level of blood pressure (r = 0.08, P = 0.65). In 30 MSNs, there was no relationship between the spontaneous rate of discharge and the resting level of blood pressure (r =−0.06, P = 0.74).

The responses of seven MSNs were tested to variable increases in blood pressure produced by repeated phenylephrine injections. The net increase in blood pressure (i.e. the static component) and the rate of increase of the blood pressure (i.e. the dynamic component) were related to the dose of phenylephrine and the speed of the injections. Five MSNs, including those receiving short latency or long latency ADN inputs, showed a strong correlation between the peak discharge response and the rate of change of pressure during the blood pressure increase (Figs 3 and and4).4). Sensitivity to the rate of change of pressure was also evident during the recovery from nitroprusside-induced reductions in blood pressure (Fig. 4D). None of the MSNs tested showed a correlation between the peak discharge response and the static component of the increase in blood pressure.

PSNs

Twenty-seven PSNs were tested for their responses to blood pressure increases (16–62 mmHg; 25 spontaneously active with discharge rates of 0.7–45.7 Hz, 2 silent). Twenty-one responded to blood pressure increases with an increase in discharge (14 spontaneously active and 1 silent), an increase in discharge followed by a decrease (2 spontaneously active) and a decrease in discharge (4 spontaneously active). Six PSNs had no obvious discharge response to the blood pressure increases (5 spontaneously active and 1 silent).

The peak excitatory responses of PSNs (1.5–20.4 Hz increase in discharge rate) occurred either before/during (5 PSNs, Fig. 5A) or after (14 PSNs, Fig. 5B) the peak increase in blood pressure. The delay between the peak response and peak pressure ranged from 1 to 12 s in 13 PSNs and was 30 s in the remaining PSN. There was no correlation between the peak discharge frequency response of PSNs and the time interval between the peak discharge frequency response and the peak increase in pressure (r =−0.18, P = 0.48). Sustained increases in discharge followed the peak responses in seven PSNs. Two PSNs did not show an obvious peak discharge response, but only showed the lower frequency, sustained discharge response (Fig. 5C). Five PSNs responded with only a peak discharge response or a transient discharge response with no obvious later discharge response.

Figure 5
Responses of PSNs to phenylephrine-induced increases in blood pressure

The discharge responses of PSNs were analysed in the same manner as the MSN data. As in the analysis of MSNs, if a neurone was tested with several blood pressure challenges, only the first response was used for the analysis. The peak discharge response to an increase in blood pressure was correlated to the spontaneous activity of the cell prior to the increase in blood pressure (r = 0.54, P < 0.01). The peak discharge responses of PSNs were not correlated to the net blood pressure increase (r = 0.12, P = 0.54, Fig. 5D), the onset latency of the ADN-evoked response (r = 0.10, P = 0.64, Fig. 5E), the rate of the pressure increase (r =−0.06, P = 0.78, Fig. 5F), or the resting level of blood pressure (r =−0.20, P = 0.32). There was no relationship between the spontaneous rate of discharge and the resting level of blood pressure (r = 0.14, P = 0.49).

The discharge responses of PSNs to an increase in blood pressure (3.7 ± 1.1 Hz increase in discharge rate, n = 27) were relatively weaker (P < 0.05) than those of MSNs (16.5 ± 5.6 Hz increase in discharge rate, n = 31) during equivalent blood pressure increases (36 ± 2 mmHg for MSNs and 38 ± 2 mmHg for PSNs, P = 0.48; Fig. 7A).

Figure 7
Comparison of the blood pressure responses and the peak discharge frequency responses of MSNs and PSNs

The responses of eight PSNs were also observed during the increases in blood pressure produced by repeated phenylephrine injections. PSNs showed very inconsistent responses to repeated increases in blood pressure and the inconsistent responses were not related to differences in the rate of rise or amplitude of the pressure changes (Fig. 5B). In individual neurones, no correlation was found between the discharge responses and the static or the dynamic components of the increase in blood pressure (Fig. 5A-C).

Responses to nitroprusside-induced reductions in blood pressure

MSNs

The responses of 11 MSNs were observed during nitroprusside-induced reductions in blood pressure (42–79 mmHg). All were spontaneously active (discharge rate, 0.4–25.9 Hz). Eight MSNs responded to blood pressure reductions with an increase in discharge rate (n = 1), an increase followed by a decrease (n = 1) or a decrease in discharge rate of 1.5–7.5 Hz (n = 6, including short latency and long latency MSNs as illustrated in Figs 4 and and6A).6A). The mean decrease in discharge rate during the mean 62 ± 4 mmHg reduction in blood pressure was 1.3 ± 0.8 Hz (n = 8). Similar to the responses of MSNs to phenylephrine-induced blood pressure increases, the responses of MSNs to nitroprusside-induced blood pressure reductions were not correlated to the net blood pressure reduction (r = 0.44, P = 0.18, Fig. 6C), the onset latency of the ADN-evoked input (r = 0.06, P = 0.86), the spontaneous activity (r = 0.54, P = 0.08) or the basal level of blood pressure (r =−0.37, P = 0.26).

Figure 6
Responses to nitroprusside-induced reductions in blood pressure

PSNs

The responses of 11 PSNs were observed during blood pressure reductions of 58–81 mmHg. All were spontaneously active (discharge rates of 0.5–23.4 Hz). PSNs responded to nitroprusside-induced blood pressure decreases with an increase in discharge rate (n = 2, Fig. 6B), an increase followed by a decrease (n = 2) or a decrease in discharge rate of 1.1–8.5 Hz (n = 4). The decrease in discharge rate during a 64 ± 2 mmHg reduction in blood pressure averaged 1.3 ± 0.9 Hz. Compared during equivalent reductions in blood pressure (P = 0.52), the responses of PSNs (n = 11) and MSNs (n = 11) were similar (P = 0.97; Fig. 7B).

The responses of PSNs to nitroprusside-induced reductions in blood pressure were correlated to the net decrease in blood pressure (r =−0.80, P < 0.01, Fig. 6D), the spontaneous discharge rate (r =−0.90, P < 0.001) and the basal blood pressure (r = 0.64, P < 0.01), but not the onset latency of the ADN-evoked input (r = 0.28, P = 0.41).

DISCUSSION

Several recent studies have measured the responses of individual NTS neurones to changes in pressure to determine the ability of a given cell to encode mean pressure and/or the rate of change of pressure (Rogers et al. 1993, 1996; Hayward & Felder, 1995; Seagard et al. 1995). In the present study, we performed similar analyses; in addition we plotted individual neuronal responses to equivalent pressure stimuli to observe how the population of neurones studied encodes information about the mean level of pressure and the rate of change of pressure. The resting level of mean arterial pressure observed in the animals studied and the standardized intravenous doses and rates of injection used resulted in similar changes in arterial pressure between animals which facilitated this approach. This analysis provides new insights into the parameters of arterial pressure which are encoded by NTS neurones as a population and therefore what information is relayed to subsequent central sites.

The peak discharge frequency responses of individual MSNs and the population as a whole were correlated to the rate of rise of the increase in blood pressure but were not correlated to the net increase in blood pressure. PSNs did not respond in a graded fashion to either the static or the dynamic components of a change in blood pressure. How, then, is mean pressure information encoded within the NTS? As proposed for NTS neurones receiving cardiopulmonary afferent inputs (Hines et al. 1994) and short latency ADN inputs (Rogers et al. 1996), information regarding changes in the steady-state level of blood pressure may be signalled by a population-encoding mechanism. Graded increases in pressure lead to a graded increase in the discharge of baroreceptor afferent fibres and this leads to a graded increase in the number of NTS neurones discharging action potentials. Neuronal discharge increases once the pressure stimulus is suprathreshold; however, further increases in stimulus intensity do not result in further increases in discharge frequency. In this model, mean pressure information is encoded by the number of neurones discharging at a given pressure as opposed to graded changes in the discharge frequency of a given neurone. The lack of a relationship between the spontaneous discharge frequency of MSNs and PSNs and the resting level of blood pressure further suggests that once a neurone receives a suprathreshold baroreceptor input, the absolute discharge frequency of the neurone is not altered by further increases in mean arterial pressure. The failure of increased afferent input to increase the discharge of NTS neurones could be the result of frequency-dependent limitations on transmission (Seller & Illert, 1969; Miles, 1986; Mifflin & Felder, 1988; Liu et al. 2000). A recent study found that the composition of baroreflex networks fluctuates with the respiratory cycle and during baroreceptor stimulation (Arata et al. 2000). These dynamics are not obvious in analyses of the discharge rates of individual neurones suggesting that network processes may regulate the encoding of afferent signal intensity.

In the present study, a small number of MSNs and PSNs did not show any obvious response to an increase in blood pressure. This is puzzling since the ADN is considered to contain only baroreceptor afferent fibres (Sapru & Krieger, 1977; Sapru et al. 1981). The reasons why these ADN-evoked NTS neurones were non-responsive to an increase in pressure are not known. However, a plausible explanation could be related to recent reports of the existence of chemoreceptor afferent fibres in the ADN (Cheng et al. 1997; Brophy et al. 1999).

Responses of MSNs to increases in blood pressure

The present study provides further evidence that the majority of MSNs receiving ADN inputs respond to baroreceptor stimulation induced by increases in blood pressure. In contrast to Rogers et al. (1993), we did not find any differences between the responses of spontaneously active MSNs and the responses of silent MSNs. We found no silent MSNs which discharged only one to four action potentials during the rising phase of the increase in blood pressure and again during the period when blood pressure returned to control levels, whereas Rogers et al. (1993) found this to be the prominent pattern of response in silent neurones.

While the majority of MSNs responded to an increase in blood pressure with a ‘pure’ increase in discharge, some MSNs responded with biphasic (excitatory-inhibitory) or even ‘pure’ inhibitory responses. We have previously reported that electrical stimulation of the ADN evokes biphasic excitatory-inhibitory responses in some MSNs and PSNs (Scheuer et al. 1996; Zhang & Mifflin, 1997, 1998a; see also Fig. 1B), and previous reports identified biphasic and inhibitory responses to baroreceptor stimulation (Lipski & Trzebski, 1975; Feldman & Moises, 1987; Mifflin et al. 1988; Suzuki et al. 1993). The present study confirms that these various patterns of response to baroreceptor stimulation are observed in the population of MSNs.

The response of 62% (16/26) of MSNs to an increase in blood pressure consisted of two components, a peak response which occurred during or soon after the peak increase in blood pressure followed by adaptation to a lower frequency discharge that was sustained for the duration of the pressure increase. Aortic baroreceptors have similar dynamic and static responses to a step increase in pressure (Thoren et al. 1977; Brown, 1980). Therefore, the decay of the peak, transient or dynamic response could be the result of baroreceptor adaptation. Alternatively, it could be the result of adaptive properties at the first synapse (activation of pre-synaptic inhibitory receptors, feedback inhibition, frequency-dependent filtering). The later, lower frequency, discharge that followed the peak response represents the response to a suprathreshold static level of blood pressure. Once pressure was sufficient to evoke discharge that persisted after the peak response, further increases in arterial pressure did not increase discharge frequency during the later response. Because the peak discharge frequency responses were not correlated to the net blood pressure change, the major determinant of the peak discharge frequency response is probably the sensitivity of MSNs to dynamic changes in blood pressure.

In 27% (7/26) of MSNs only the initial dynamic response was observed with no adaptation to a lower frequency level of steady-state discharge, similar to the extinction responses reported by Rogers et al. (1993, 1996). The lack of steady-state discharge in these cells could reflect the nature of the afferent input, intrinsic neuronal properties or inhibitory feedback network interactions.

The responses of six MSNs receiving a long latency, presumably C fibre, afferent input were also observed during increases in blood pressure. Three of the six responded weakly to the increase in blood pressure, compared to the robust and high frequency discharge observed in MSNs receiving short latency inputs (compare Figs 3 and and4).4). As in MSNs receiving short latency, myelinated, ADN inputs, long latency MSNs responded to an increase in pressure with a dynamic, peak frequency response and a later, lower frequency discharge that was not related to the absolute level of arterial pressure (Fig. 4C). The weaker responses of MSNs receiving C fibre afferents to increases in blood pressure could be related to the weak pressure sensitivity of unmyelinated aortic baroreceptor afferent fibres (Thoren et al. 1977).

Responses of PSNs to increases in blood pressure

Previous reports from this laboratory have identified several differences between the electrophysiological responses of MSNs and PSNs to ADN stimulation and application of excitatory and inhibitory amino acids (Scheuer et al. 1996; Zhang & Mifflin, 1997, 1998a,b). We observed differences in the responses of PSNs to increases in blood pressure compared to MSNs. During similar increases in blood pressure, the increase in discharge frequency of PSNs was less than the increase observed in MSNs (Fig. 7A). The discharge rate increase in most PSNs in response to an increase in blood pressure was less than 10 Hz, much lower than the responses of MSNs, which could discharge at frequencies of up to 100 Hz during an increase in blood pressure. This finding is consistent with the observation that ADN afferent inputs to most PSNs, but not to most MSNs, exhibit time-dependent inhibition (Scheuer et al. 1996). This time-dependent inhibition has been proposed to serve as a low-pass filter and could explain why the high frequency discharge evoked in MSNs by the baroreceptors is not transferred to PSNs. The filtering of rapid, transient events during transmission from MSNs to PSNs could lend stability to baroreceptor regulation of cardiovascular function.

The responses of PSNs to repeated increases in blood pressure were variable. In contrast to MSNs, the responses of some PSNs to a second or third blood pressure increase were markedly reduced compared to the responses to the first blood pressure increase. This ‘adaptive’ property of PSNs suggests that unknown factors modulate the response of these neurones to repeated increases in arterial pressure (e.g. neuronal properties, endocrine factors, feedback from other peripheral and/or central sites).

Responses to decreases in blood pressure

In most MSNs and PSNs, unloading of the arterial baroreceptors resulted in a decrease in spontaneous discharge. This indicates that MSNs and PSNs receive a tonic excitatory input from the arterial baroreceptors. However, the degree of tonic excitation from the arterial baroreceptors appears to be less than one might expect. In the present study, reductions in discharge were moderate, with only one MSN being silenced during baroreceptor unloading. We also observed excitatory and biphasic excitatory-inhibitory responses during nitroprusside-induced decreases in blood pressure. The significance of excitatory responses to baroreceptor unloading is not readily apparent; however, it could represent disinhibition or a convergent excitatory input from arterial chemoreceptors that may be activated during reductions in carotid blood flow (Mifflin, 1992).

What determines the spontaneous discharge of MSNs and PSNs? We have found that the spontaneous discharge of NTS neurones receiving ADN inputs can be eliminated by ionophoretic application of EAA antagonists (Zhang & Mifflin, 1998a), therefore this activity appears to be the result of excitatory synaptic inputs. Because lowering of the blood pressure to 50 mmHg unloads most, but not necessarily all, baroreceptors (Brown, 1980), the fact that baroreceptor unloading did not totally abolish spontaneous discharge leads to the conclusion that a significant amount of the excitatory drive to these neurones originates from other sources. The site of these other sources could be inputs from other central nuclei, recurrent excitatory network connections within the NTS (Fortin & Champagnat, 1993) or convergent inputs from other peripheral afferent sites (arterial chemoreceptors, Mifflin, 1992; somatic afferents, Toney & Mifflin, 1994; vagus or laryngeal nerve afferents, Rogers et al. 1993; Hines et al. 1994; Mifflin, 1996).

It is important to keep in mind that the frequency-dependent inhibitory interactions first described by Seller & Illert (1969) and later by others (Miles, 1986; Mifflin & Felder, 1988) might also play a role in the relatively minor reductions in spontaneous discharge frequency we observed during reductions in blood pressure. Gabriel & Seller (1970) proposed that such afferent interactions not only limit the transmission of high frequency inputs through the NTS, but also minimize the effects of reductions in afferent input. Reductions in afferent input, as during baroreceptor unloading, reduce frequency- and time-dependent inhibitory interactions, allowing the remaining afferent inputs to be more fully expressed. This could attenuate the reduction in discharge frequency observed during baroreceptor unloading.

Summary

The majority of NTS neurones receiving an ADN input respond to increases in blood pressure with an excitatory response. There is an apparent segregation of A and C fibre inputs to different neurones at the level of the first synapse. Both MSNs and PSNs appear to receive weak tonic excitatory inputs from the arterial baroreceptors. As a population, MSNs encode information regarding the rate of change of pressure fairly well. In MSNs and PSNs information regarding the mean blood pressure level appears to be relayed to subsequent sites by a population-encoding mechanism. The ability of MSNs to faithfully transmit high frequency, dynamic information makes these neurones possible components of regulatory pathways with fast time constants (e.g. vagal regulation of heart rate, regulation of sympathetic nerve discharge), while the low-pass filtering of the afferent input observed in PSNs suggests these neurones might function in reflex pathways regulating comparatively slow events (e.g. hormone release) or function as modulators of more rapid events. An alternative interpretation is that MSNs provide a substrate for dynamic regulation of cardiovascular function, as suggested by Rogers et al. (1996), whereas PSNs are involved in the maintenance of tonic, steady-state levels of reflex function.

Acknowledgments

The authors gratefully acknowledge the expert technical assistance of Melissa Vitela and Myrna Herrera-Rosales. This work was supported by NIH grant HL-56637.

References

  • Arata A, Hernandez YM, Lindsey BG, Morris KF, Shannon R. Transient configurations of baroresponsive respiratory-related brainstem neuronal assemblies in the cat. Journal of Physiology. 2000;525:509–530. [PMC free article] [PubMed]
  • Brophy S, Ford TW, Carey M, Jones JFX. Activity of aortic chemoreceptors in the anaesthetized rat. Journal of Physiology. 1999;514:821–828. [PMC free article] [PubMed]
  • Brown AM. Receptors under pressure. Circulation Research. 1980;46:1–10. [PubMed]
  • Cheng Z, Powley TL, Schwaber JS, Doyle FJ., III A laser confocal microscopic study of vagal afferent innervation of rat aortic arch: Chemoreceptors as well as baroreceptors. Journal of the Autonomic Nervous System. 1997;67:1–4. [PubMed]
  • Feldman PD, Moises HC. Adrenergic responses of baroreceptive cells in the nucleus tractus solitarii of the rat: a microiontophoretic study. Brain Research. 1987;420:351–361. [PubMed]
  • Fortin G, Champagnat J. Spontaneous synaptic activities in rat nucleus tractus solitarius neurons in vitro: evidence for re-excitatory processing. Brain Research. 1993;630:125–135. [PubMed]
  • Gabriel M, Seller H. Interaction of baroreceptor afferents from carotid sinus and aorta at the nucleus tractus solitarius. Pflügers Archiv. 1970;318:7–20. [PubMed]
  • Hayward LF, Felder RB. Cardiac rhythmicity among NTS neurons and its relationship to sympathetic outflow in rabbits. American Journal of Physiology. 1995;269:H923–933. [PubMed]
  • Hines T, Toney GM, Mifflin SW. Responses of neurons in the nucleus tractus solitarius to stimulation of heart and lung receptors in the rat. Circulation Research. 1994;74:1188–1196. [PubMed]
  • Lipski J, Trzebski A. Bulbo-spinal neurons activated by baroreceptor afferents and their possible role in inhibition of preganglionic sympathetic neurons. Pflügers Archiv. 1975;356:181–192. [PubMed]
  • Liu Z, Chen C-Y, Bonham AC. Frequency limits on aortic baroreceptor input to nucleus tractus solitarii. American Journal of Physiology. 2000;278:H577–585. [PubMed]
  • Mifflin SW. Arterial chemoreceptor input to nucleus tractus solitarius. American Journal of Physiology. 1992;263:R368–375. [PubMed]
  • Mifflin SW. Convergent carotid nerve and superior laryngeal nerve afferent inputs to neurons in the NTS. American Journal of Physiology. 1996;271:R870–880. [PubMed]
  • Mifflin SW, Felder RB. An intracellular study of time-dependent cardiovascular afferent interactions in nucleus tractus solitarius. Journal of Neurophysiology. 1988;59:1798–1813. [PubMed]
  • Mifflin SW, Spyer KM, Withington-Wray DJ. Baroreceptor inputs to the nucleus tractus solitarius in the cat: Postsynaptic actions and the influence of respiration. Journal of Physiology. 1988;399:349–367. [PMC free article] [PubMed]
  • Miles R. Frequency dependence of synaptic transmission in nucleus of the solitary tract in vitro. Journal of Neurophysiology. 1986;55:1076–1090. [PubMed]
  • Numao Y, Siato M, Terui N, Kumada M. The aortic nerve-sympathetic nerve reflex in the rat. Journal of the Autonomic Nervous System. 1985;13:65–79. [PubMed]
  • Rogers RF, Paton JFR, Schwaber JS. NTS neuronal responses to arterial pressure and pressure changes in the rat. American Journal of Physiology. 1993;265:R1355–1368. [PubMed]
  • Rogers RF, Rose WC, Schwaber JS. Simultaneous encoding of carotid sinus pressure and dP/dt by NTS target neurons of myelinated baroreceptors. Journal of Neurophysiology. 1996;76:2644–2660. [PubMed]
  • Sapru HN, Gonzalez E, Krieger AJ. Aortic nerve stimulation in the rat: cardiovascular and respiratory responses. Brain Research Bulletin. 1981;6:393–398. [PubMed]
  • Sapru HN, Krieger AJ. Carotid and aortic chemoreceptor function in the rat. Journal of Applied Physiology. 1977;42:344–348. [PubMed]
  • Scheuer DA, Zhang J, Toney GM, Mifflin SW. Temporal processing of aortic nerve evoked activity in the nucleus of the solitary tract. Journal of Neurophysiology. 1996;76:3750–3757. [PubMed]
  • Seagard JL, Dean C, Hopp FA. Discharge patterns of baroreceptor-modulated neurons in the nucleus tractus solitarius. Neuroscience Letters. 1995;191:13–18. [PubMed]
  • Seller H, Illert M. The localization of the first synapse in the carotid sinus baroreceptor reflex pathway and its alteration of the afferent input. Pflügers Archiv. 1969;306:1–19. [PubMed]
  • Suzuki M, Kuramochi T, Suga T. GABA receptor subtypes involved in the neuronal mechanisms of baroreceptor reflex in the nucleus tractus solitarii of rabbits. Journal of the Autonomic Nervous System. 1993;43:27–36. [PubMed]
  • Thoren P, Saum WR, Brown AM. Characteristics of rat aortic baroreceptors with nonmedulated afferent nerve fibres. Circulation Research. 1977;40:231–237. [PubMed]
  • Toney GM, Mifflin SW. Time-dependent inhibition of hindlimb somatic afferent inputs to nucleus tractus solitarius. Journal of Neurophysiology. 1994;72:63–71. [PubMed]
  • Zhang J, Mifflin SW. Influences of excitatory amino acid receptor agonists on nucleus of the solitary tract neurons receiving aortic depressor nerve inputs. Journal of Pharmacology and Experimental Therapeutics. 1997;282:639–647. [PubMed]
  • Zhang J, Mifflin SW. Differential roles for NMDA and non-NMDA receptor subtypes in baroreceptor afferent integration in the nucleus of the solitary tract of the rat. Journal of Physiology. 1998a;511:733–745. [PMC free article] [PubMed]
  • Zhang J, Mifflin SW. Receptor subtype specific effects of GABA agonists on neurons receiving aortic depressor nerve inputs within the nucleus of the solitary tract. Journal of the Autonomic Nervous System. 1998b;73:170–181. [PubMed]
  • Zhang J, Mifflin SW. Subthreshold aortic nerve inputs to neurons in nucleus of the solitary tract. American Journal of Physiology. 2000;278:R1595–1604. [PubMed]

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