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J Neurosci. Author manuscript; available in PMC 2009 Jun 17.
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PMCID: PMC2688724
NIHMSID: NIHMS84452

Chronic Gαs signaling in the striatum increases anxiety-related behaviors independent of developmental effects

Abstract

Current research in the field of anxiety disorders is largely receptor-centric, leaving intracellular pathways largely unexplored. Gαs, the G-protein which stimulates adenylyl cyclase and L-type voltage gated calcium channels, may be one intracellular molecule regulating anxiety-related behaviors as increased efficacy of Gαs signaling has been noted in patient populations that suffer from anxiety. We report here anxiety-related behaviors in two lines of transgenic mice expressing a constitutively active isoform of Gαs (or Gαs*). The first line expressed Gαs* throughout postnatal forebrain neurons, while the second line of mice conditionally expressed Gαs* selectively in the striatum (Gαs*str mice). In the open field, both lines of mice showed a significant preference for the periphery suggesting that expression of Gαs* in the striatum alone was sufficient to produce an anxiogenic phenotype. In the light/dark box, Gαs*str mice exhibited longer latencies to enter the light and spent significantly less time in the lit compartment. Similarly, Gαs*str mice showed longer latencies to enter the open quadrants and spent less time in the open quadrants of the elevated zero maze. Interestingly, these anxiety-related phenotypes were largely unrelated to developmental effects as mice expressing the Gαs*str transgene during development, but not at testing, were normal on most measures. These observations show that chronic Gαs signaling in the striatum is sufficient to trigger anxiety-related behaviors largely independent of developmental effects and suggest the cAMP pathway or L-type voltage gated calcium channels may be viable targets for future pharmacological intervention in the treatment of anxiety disorders.

Keywords: GNAS, anxiety, panic disorder, striatum, cAMP, L-VGCC

Anxiety disorders are estimated to have a lifetime prevalence rate in the United States of 31% (Kessler et al., 2007). In addition to the psychological symptoms (e.g., irrational fear, worry, restlessness, and sleep disturbances) classically associated with anxiety disorders, patients show an increased risk for a variety of physical illnesses, including heart disease, obesity, and cancer (c.f., Balon, 2006). Thus, anxiety disorders pose a threat to both the mental and physical well being of a large percentage of the population, greatly affecting quality of life and producing an economic burden (e.g., Hoffman et al., 2008).

Despite the widespread prevalence of anxiety disorders, the biochemical substrates underlying these pathological behavioral responses remain largely unknown. One potential intracellular mechanism contributing to anxiety disorders may be increased signaling through Gαs, the G-protein that stimulates adenylyl cyclase (c.f., Neves, 2002) and L-type voltage-gated calcium channels (VGCC; Yatani and Brown, 1989; Lader et al., 1998). Gurguis and colleagues (1999) found increased efficacy of Gαs signaling in neutrophils of drug-free patients with Panic Disorder as well as in polymorphonuclear leukocytes of patients with Panic Disorder (Gurguis et al., 1997). In addition, increased levels and/or efficacy of Gαs have repeatedly been measured in peripheral and central measures from patients with bipolar disorder (Young et al., 1991, 1993, 1994; Avissar et al., 1997), a patient population with high rates of comorbid anxiety (c.f., Oswald et al., 2007). Together, these data raise the possibility that chronic Gαs signaling may contribute to anxiety-related phenotypes.

Given that disease-related symptoms are likely due to long term changes in neuronal function, we test here the behavioral effect of chronically increasing Gαs signaling. To do so, we evaluated two lines of transgenic mice that express a constitutively active isoform Gαs (Gαs*). In the first line, forebrain-specific expression of Gαs* was driven by the CaMKIIα promoter (Figure 1A; Wand et al., 2001; Kelly et al., 2007, 2008). In the second line, conditional expression of Gαs* was driven with the reversible tetracycline-regulated system (Figure 1B), which spatially restricted expression of the transgene to forebrain neurons and enabled expression of the transgene to be suppressed with administration of doxycycline (Figure 1C). By driving expression of our transgene with this reversible system, we were able to determine if expressing Gαs* during development was sufficient to alter anxiety-related behaviors in adults. By virtue of where in the genome the tetO construct randomly inserted, the new transgenic line showed Gαs* expression enriched in the striatum consistent with increased cAMP levels being found only in this region (referred to hereafter as Gαs*str mice; Figure 1). Thus, Gαs*str mice impart enhanced temporal and spatial resolution relative to our initial line. Focusing on anxiety-related behaviors, we tested Gαs* mice in the open field, light/dark box, and elevated zero maze. We show that, across all paradigms, Gαs*str mice exhibited a significant increase in anxiety-related behaviors relative to control littermates and that this phenotype was largely independent of developmental effects.

Figure 1
Gαs*str mice showed striatally-enriched transgene expression and cAMP elevation restricted to the striatum. A) The tetracycline-regulated transgenic system used herein employs the CaMKIIα promoter to restrict expression of the transcription ...

MATERIALS AND METHODS

Subjects

Transgenic mice were bred and group-housed in breeding colonies at the University of Pennsylvania in the hemizygous state on a C57BL/6J background. Transgenic mice express an isoform of the of the Gαs protein subunit that is constitutively active due to a point mutation (Q227L), as previously described (Wand et al., 2001). This mutation prevents the hydrolysis of bound GTP and, in so doing, results in increased basal adenylyl cyclase activity in the brain (Wand et al., 2001; Kelly et al., 2007). CaMKIIα-Gαs* mice (N12-13 onto C57Bl/6J) were bred as previously described (Kelly et al., 2007, 2008). The tetO-Gαs* transgene was microinjected into pronuclei from superovulated B6SJLF1/J mice by the Transgenic and Chimeric Mouse Facility at the University of Pennsylvania. The line reported here (line 1485, referred to as Gαs*str mice) was then backcrossed onto C57BL/6J for 5–6 generations in the hemizygous state before being mated to CaMKIIα-tTA(B) mice (Mayford et al., 1996;N20+ on C57BL/6J). Only mice positive for both the tetO and CaMKIIa driven constructs express the Gαs* transgene. The control group, then, includes wild-type mice (with neither construct), tetO-only mice and CaMKIIα-only mice. For experiments employing doxycycline, 200 mg/kg was administered via the chow to both Gαs*str and control littermates either ≥ 2.5 weeks prior to test (i.e., Gαs*str-dev mice) or from gestation through testing (i.e., for Gαs*str-dox mice). This dose was chosen based on our previous experience that 40 mg/kg dox was insufficient to reliably suppress transgene expression in our tetracycline regulated Gαswt mice(unpublished). In situ hybridization confirmed 2.5 weeks of dox was sufficient to suppress transgene expression (Figure 1D). Using transgene specific probes, animals are genotyped by Southern blot, but all experiments are conducted blind to genotype. For both lines, transgenic mice 2–5 months old were compared to sex-matched control littermates. All experiments were conducted in accordance with National Institutes of Health guidelines for animal care and use, and all experiments were approved by the Institutional Animal Care and Use Committee of the University of Pennsylvania.

Behavior

Locomotor activity was measured for 30 minutes in a 41 cm × 41 cm San Diego Instruments Open Field (San Diego, CA; Stein et al., 2006). The box was equipped with 16 × 16 motion detector beams (every 2.54 cm). Ambulations (breaks of 2 contiguous beams) and preference for the periphery (% ambulations in outer 2 beams on all sides) were calculated.

Light/Dark Box

Partiality to light vs dark was measured in a modified housing cage (Stein et al., 2006). Animals were introduced to the dark chamber and were allowed to explore the apparatus freely for 10 minutes. Latency to enter (operationally defined as the time when all four paws first crossed the threshold of the 6 cm × 6 cm opening) and the total time spent in the lit compartment were recorded.

Elevated Zero Maze

Preference for walled versus exposed quadrants was measured in an elevated circular track (Stein et al., 2006). Animals were introduced to a walled quadrant and permitted to explore the Elevated Zero Maze for 10 minutes. Latency to enter (operationally defined as time when all 4 paws first entered the open) and total time spent in the open quadrants were recorded.

Contextual fear conditioning was conducted as previously described (Kelly et al., 2008), by pairing a training context with a 1.5 mA footshock and measuring the level of freezing exhibited in the same context 24 hours later (3 minute session).

Prepulse inhibition (PPI) was conducted as previously described (Kelly et al., 2007; Kanes et al., 2007), employing prepulses 4–16 dB above a 68 dB background and a pulse of 120 dB and measuring the startle response with an accelerometer (SD Instruments; San Diego, CA).

Biochemistry

In situ hybridization and cAMP assays were conducted as previously described (Kelly et al., 2007, 2008). For biochemical experiments, animals were killed by cervical dislocation and brains were immediately harvested, hemisected (one half intact for in situ the other dissected for cAMP assays), and placed on dry ice. ‘Parahippocampal cortex’ was defined as cortical regions overlying the hippocampus. Levels of cAMP were measured from a single hemisphere using a radioimmune competition assay kit (Perkin-Elmer, Wellesley, MA). All animals were killed from the home cage.

Data Analyses

Data were analyzed for effect of genotype by t-test using Sigmastat (v2.03; Systat; Point Richmond, CA 94804). In biochemical experiments, balanced groups of animals were raised at different times and killed on different days. Therefore, to control for any confounds due to day of killing, data for each group were normalized and expressed as a percentage of the control (CT) mean, as previously described (Kelly et al., 2007, 2008). Normalized data were then combined and analyzed for effects of genotype. Statistical outliers >2 standard deviations from the mean were removed from analyses. Significance was determined at p < 0.05. All values reported are means ± SEM.

RESULTS

Gαs*str mice showed increased cAMP levels in the striatum

Autoradiographic in situ hybridization showed transgene expression was more spatially restricted in the Gαs*str mice (Figure 1C) versus our original CaMKIIα-Gαs* mice (Figure 1B). Biochemical analyses show that Gαs*str mice show elevated cAMP levels only in striatum (not cortex nor hippocampus; Figure 1E; n = 5–9 per genotype; t(12)= 3.10, p = 0.009), as previously reported in CaMKIIα-Gαs* mice (41% increase; Kelly et al., 2007). Developmental expression of Gαs*str is not sufficient to cause permanent alterations in striatal cAMP levels, because Gαs*str-dev mice show striatal cAMP levels equivalent to those of control littermates (Figure 1F; n = 7 per genotype; t(6) = 0.47, p = 0.657).

CaMKIIα-Gαs* and Gαs*str mice exhibited a significant preference for the periphery of an open field

To determine if expressing Gαs* in forebrain neurons would be sufficient to increase anxiety-related behaviors, we characterized our initial line of CaMKIIα-Gαs* mice in an open field. Relative to wildtype mice (n = 7–8, per genotype), CaMKIIα-Gαs* mice exhibited a higher percentage of ambulations in the periphery of the open field (t(13) = 2.28, p = 0.048; Figure 2A). In addition, CaMKIIα-Gαs* mice showed significantly more ambulations overall (t(12) = 3.50, p = 0.004; Figure 2B). These results suggest that Gαs* expression is sufficient to cause hyperlocomotion and an increase in anxiety-related behavior.

Figure 2
Gαs* mice exhibited an increase in anxiety-like behavior in the Open Field. A) Relative to control littermates (CT), CaMKIIα-Gαs* transgenic mice performed a greater percentage of ambulations in the periphery of an Open Field, ...

To further isolate where in the brain and when (i.e., development vs adulthood) Gαs* acts to influence locomotor and anxiety-related behavior, we also tested Gαs*str and control littermates in the open field (n = 14–18 per genotype). Consistent with the observations above, Gαs*str mice showed a greater preference for the periphery of the open field (t(30) = 3.19, p = 0.003; Figure 2a). In contrast to the CaMKIIα-Gαs* line, however, Gαs*str mice displayed no difference in total ambulations relative to control littermates (Figure 2B), showing the effect of Gαs* signaling on the anxiety-related phenotype in the open field is dissociable from the effect on hyperactivity.

Gαs*str mice showed anxiety-related phenotypes in a light/dark box and an elevated zero maze

To determine if the Gαs*-induced anxiety-related phenotype observed above in the open field would extend to additional anxiety-related paradigms, we tested Gαs*str mice and control littermates in the light/dark box and elevated zero maze paradigms. In the light/dark box, Gαs*str mice exhibited a longer latency to enter the lit chamber relative to their control littermates (n = 7–11 per genotype; t(16) = 3.62, p = 0.002; Figure 3A). Additionally, Gαs*str mice spent significantly less time in the lit compartment relative to control littermates (t(16) = 2.27, p < 0.037; Figure 3B). This pattern of behavior is paralleled in the Elevated Zero Maze, where Gαs*str mice showed a longer latency to enter the open quadrants (n = 7–10, per genotype; t(15) = 3.01, p = 0.009; Figure 3C) and spent less total time in the open quadrants relative to control littermates (t(14) = 2.99, p = 0.01; Figure 3D). These behaviors confirm the hypothesis that chronically increasing Gαs* signaling is sufficient to increase anxiety-related phenotypes suggest the effect is localized to the striatum.

Figure 3
Gαs*str mice showed an increase in anxiety-related behaviors in the Light/Dark Box and Elevated Zero Maze. A) Relative to control littermates (CT), Gαs*str transgenic mice exhibited a longer latency to enter the lit chamber of the Light/Dark ...

Anxiety-related phenotypes observed in Gαs*str mice were largely unrelated to developmental effects

Next we determined if the anxiety-related phenotypes observed in adult Gαs*str mice were due to developmental effects. To do so, we tested adult Gαs*str mice that experienced transgene expression throughout development, but not at the time of adulthood testing due to doxycycline administration (referred to as Gαs*str-dev mice). Gαs*str-dev mice did not exhibit a significant preference for the periphery of an open field relative to control littermates (Figure 2A; n = 7 per genotype; t(12) = 1.52, p = 0.154). Similarly, Gαs*str-dev mice showed no significant difference in the latency to enter the lit compartment of the light/dark box (Figure 3A; n = 7, per genotype; t(12) = 0.77, p = 0.456). In contrast, however, Gαs*str-dev mice did spend significantly less time in the light in total (Figure 3B; t(12) = 2.58, p = 0.024). This effect observed in Gαs*str-dev mice was specifically due to developmental expression of the transgene (as opposed to a confounding effect of transgene insertion) because Gαs*str mice administered dox throughout life (i.e., that never expressed the transgene; Gαs*str-dox) showed no difference with respect to latency to enter the lit chamber (Figure 3A; n = 4 per genotype; t(6) = 0.75, p = 0.48) nor total time spent in the light (Figure 3B; t(6) = 0.30, p = 0.77). In the elevated zero maze, Gαs*str-dev mice showed no significant difference in the latency to enter the open quadrants (Figure 3C; n = 7 per genotype; t(12) = 1.46, p = 0.172) nor in the total time spent in the open quadrants (Figure 3D; t(12) = 1.96, p = 0.074).

DISCUSSION

Here, we showed that chronically increasing Gαs signaling in the forebrain triggers an increase in anxiety-related behaviors in the open field, light/dark box, and elevated zero maze. The fact that chronic Gαs signaling increases anxiety-related behaviors largely independent of developmental effects suggests that such anxiety-related pathophysiology is readily reversible in the adult. These observations are particularly relevant to patients with Panic Disorder, in whom increased efficacy of Gαs signaling has been noted (Gurguis et al., 1997, 1999).

Data presented here suggest that the effect of Gαs* on anxiety-related behaviors is localized to the striatum. This conclusion is based on the fact that Gαs*str mice, which exhibited increased anxiety-related behaviors, showed cAMP changes in striatum but not in hippocampus nor cortex. Further, Gαs*str mice showed intact contextual fear conditioning and PPI (Figure 4), behaviors influenced by chronic Gαs signaling in the hippocampus and cortex (Kelly et al., 2007, 2008). Although studies here do not rule out completely the possibility that Gαs* is acting elsewhere in the brain, a striatally localized effect of Gαs* on anxiety would be consistent with an extensive literature implicating this brain region as key to the circuitry underlying anxiety-related behaviors and pathologies (e.g., panic disorder, Reiman et al., 1989; Yoo et al., 2005; social anxiety disorder, c.f., Mathew and Ho, 2006; bipolar disorder, Konarski et al., 2008; Killgore et al., 2008). Dysfunction within the dorsal striatum, in particular, may disrupt processing of safety signals that would otherwise decrease expression of fearful or anxiety-related behaviors (Rogan et al., 2005).

Figure 4
Gαsstr mice showed normal performance in contextual fear conditioning and prepulse inhibition of acoustic startle. A) Relative to control littermates, Gαs*str mice showed equivalent levels of freezing during training (prior to administration ...

It remains to be determined through which downstream target Gαs* elicits an anxiogenic profile. Chronically increasing cAMP signaling could increase anxiety, by virtue of decreasing GABA functionality (Moss et al., 1992; Shekhar et al., 2003; Kalueff and Nutt, 2007). Although increasing cAMP signaling acutely appears to reduce anxiety-related behaviors in rodents (Silvestre et al., 1999; Barrot et al., 2002; Masood et al., 2008), chronically increasing cAMP signaling has the opposite effect. Genetic deletion of the cAMP-degrading enzyme Phosphodiesterase 4B or overexpression of the striatally-enriched cAMP-producing enzyme Adenylyl Cyclase 5, increases anxiety-related behavior in mice (Zhang et al., 2007; Kim et al., 2008). In addition, genetic deletion of 5HT1A receptors, which couple negatively to adenylyl cylase via Gαi, increases anxiety-related behaviors in mice, and a reduction in 5HT1A receptors has been measured in patients with social anxiety disorder (c.f., Lesch et al., 2003). Increased activity of L-type VGCCs could also contribute to Gαs*-induced increases in anxiety as they play a role in the anxiogenic effects associated with amphetamine and nicotine administration (Biala and Budzynka, 2006; Biala and Kruk, 2007, 2008). Further, calcium channel blockers have been proposed as a novel class of anxiolytics, particularly for treatment of Panic Disorder (Balon and Ramesh, 1996). As such, we hypothesize that Gαs*-induced anxiety is likely due to the chronic nature of the cAMP upregulation and/or increased activation of L-type VGCCs. Our results identify Gαs as an intracellular signaling molecule regulating the anxiety response, suggesting Gαs or it downstream effectors may prove effective therapeutic targets in the treatment of anxiety disorders.

Acknowledgments

The authors would like to thank Dr. Raquel Gur for helpful discussions and Dr. Jean Richa for efforts in generating our transgenic mice. The authors would also like to acknowledge the following sources of funding: Tourettes Syndrome Association (MPK), NIMH T32 MH019112 (to MPK, R. Gur, P.I.) and Merck, Whitehall and Packard Foundations as well as NIMH R01 MH60244, NIA R01 AG18199, and P50 MH 6404501 (Project 3 to TA; R. Gur, Conte Center P.I.). MPK is currently employed by Wyeth Research.

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