Interfacial dopamine concentration was transduced at a platinum microelectrode with the constant potential amperometry technique, which involved applying a +500 mV potential with respect to an AgCl electrode. Simultaneous calibration across four channels of the array was performed (A) prior to and (B) after implantation with dopamine solution concentrations ranging from 100 nM to 20 µM. (C) In vivo amperometric recordings were transformed into a dopamine concentration with the post-implant calibration fits. Although sites were sensitive to dopamine after probe explantation, the sensitivity was decreased (paired t-test, p=0.013, n=4). (D) To verify that the electrically-evoked in vivo electrochemical signals corresponded to changes in extra-synaptic dopamine concentrations, rats were injected i.p. with a dopamine uptake inhibitor (nomifensine, 12 mg/kg). Response curves represent an average of five trials with standard errors indicated by the gray fills. (E) MFB stimulation did not elicit detectable dopamine transients when a microelectrode site was positioned in the corpus callosum (CC), which contrasted with the large peaks observed simultaneously in the striatum (Str). Shown are three stimulation trials for 1-s long consecutive pulse trains using frequencies of 10, 50, and 100 Hz (150 µA, 2 ms duration biphasic waveforms). Subtraction of the two recordings decreased common noise between the two platinum microelectrodes.

MFB stimulation elicited local field potential modulation in striatum. (A) The figure shows an example recording session sampled continuously during electrochemical recordings for dopamine on adjacent sites (indicated by numbers 1–4 next to the probe). Following MFB stimulation (indicated by the gray rectangle), slow-wave activity was suppressed considerably and coincided with the emergence of higher frequency activity. (B,C) Event-averaged spectrograms showed significant modulation in the 0–20 Hz and 40–90 Hz bands following stimulation (n=5 trials). Above each plot is the event-averaged dopamine response recorded on an adjacent site. Recordings reflect the presence of nomifensine (10 mg/kg).

Linear microelectrode arrays implanted in rat dorsal striatum recorded unit-spike and local field potential activity on iridium sites (177 µm²) with adjacent platinum sites (450 µm²) used for measuring electrochemical dopamine overflow signals through constant potential amperometry. Arrays were fabricated from a boron-doped, silicon substrate with metal contacts and phosphorus-doped polysilicon traces insulated with a multi-layer stack of silicon dioxide and silicon nitride.

Concurrent electrochemical recordings across microelectrode sites within the striatum were characterized by spatially heterogeneous responses following MFB stimulation. Response curves exhibited different peak dopamine levels on sites as close as 200 µm and their magnitudes depended on stimulation frequency. Each trace corresponds to an average of five trials with fill bars indicating standard errors and gray bars indicating stimulus duration. Recordings reflect the presence of nomifensine (10 mg/kg).

Responses to MFB stimulation evoked distinct changes in striatal spike activity as shown in representative examples of neurons recorded simultaneously along an implanted array. (A) This neuron responded with an excitatory response (top row) that paralleled the duration of elevated dopamine overflow. Other neurons (bottom row) exhibited an inhibitory response, which also paralleled dopamine overflow. This particular neuron exhibited a higher incidence of bursting than the neuron shown in the top row as indicated by the autocorrelograms. Recording modalities were separated by 200 µm. MFB stimulation consisted of a 100 pulse, 100 Hz train of 2 ms duration biphasic waveforms at 150 µA amplitude. (B, C) An example of neuronal firing rates and dopamine overflow depending on stimulation frequency for a striatal neuron excited during MFB stimulation. Both dopamine overflow and neuronal firing rates were smaller in magnitude and shorter in duration during randomized trials at (B) 10 Hz than at (C) 100 Hz. Recordings reflect the presence of nomifensine (10 mg/kg). Dashed lines represent 95% confidence intervals. Stimulation artifacts prevented including spikes during the 100 Hz stimulation train in the analysis.