Cannabinoid and Neuropeptide Modulation of Fear-Potentiated Startle
1Michael W. Leonard, Alisa R. Gutman, Jasmeer P. Chhatwal, Michael Davis
1Department of Psychiatry & Behavioral Sciences and Center for Behavioral Neuroscience, Emory University, Atlanta, GA 30322



Abstract

Neuropeptides and endocannabinoids have both been found to modulate fear and anxiety systems. Our laboratory is interested in endogenous systems that both enhance and inhibit fear to better understand the circuitry underlying diseases such as post-traumatic stress disorder and depression. Previous work from our laboratory has shown that rimonabant (RIM), a cannabinoid CB1 receptor antagonist, inhibits the extinction of fear-potentiated startle (FPS). In this study, we examined the effect of RIM on expression of FPS to evaluate the mechanism by which RIM inhibits extinction. We hypothesized that RIM would inhibit within-session extinction which may underlie the inhibition of between-session extinction. Alpha-melanocyte stimulating hormone (α-MSH) is a neuropeptide that is best known for its role in feeding, but has also been implicated in anxiety systems. With this in mind, we hypothesized that central administration of α-MSH would enhance the expression of FPS. Our results show that RIM does inhibit within-session extinction of FPS. A two way repeated measures ANOVA yielded a significant effect of test block (p=0.011, F(5,89) = 3.28). A post-hoc analysis revealed a significant difference between blocks 1 and 5 for the vehicle group (p< 0.05) and no significant difference for the RIM group, illustrating that the vehicle group has significant within-session extinction during the expression test and the RIM group did not. When examining data from the α-MSH study, we found that intracerebroventricular (i.c.v.) administration enhanced the expression of FPS; however the sample size was not large enough to show significance. These data are promising, as a trend is seen. In the future, we intend to replicate the α-MSH study with a larger sample size and to look at interactions between α-MSH or RIM with other neuropeptide systems.


Introduction

We are interested in endogenous systems that both enhance and inhibit fear circuitry; neuropeptides and endocannabinoids have both been found to modulate fear and anxiety systems. Endocannabinoids are widely investigated as a novel therapeutic target; CB1 (cannabinoid receptor) knockout mice have been found to display increased anxiety; rimonabant is a CB1 antagonist, which therefore may lead to a heightened state of anxiety. Previous work from our laboratory has shown that rimonabant inhibits the extinction of fear-potentiated startle α-MSH is a neuropeptide found in many of the same brain regions as Neuropeptide Y (NPY) α-MSH produces anorexia and anxiety after i.c.v. infusion in rats. In contrast, NPY i.c.v. infusion conversely leads to anxiolytic-like effects. We hypothesized that:

• i.p. infusion of rimonabant would inhibit within-session extinction of fear-potentiated startle

• i.c.v. infusion of α-MSH would enhance the expression of fear-potentiated startle


Methods and Materials








Results



Fear-Potentiated Startle Across Trials – 2 way repeated measures ANOVA: Significant effect for block (p=0.011); Post hoc Student-Neuman-Keuls test revealed a significant difference for block within the vehicle group (block 1 vs. 5 p<0.05)



Fear Retention – An independent samples t-test was used to examine the first block of a second expression test to examine fear retention (p<0.05)



Fear-Potentiated Startle Across Trials – These data show a trend towards enhanced FPS with α-MSH administration



Effects on Baseline Startle – There is no baseline startle difference between groups (vehicle, n = 5; α-MSH, n = 4)


Conclusions and Future Studies

Rimonabant: We found that the vehicle group had significant within-session extinction, (comparing block 1 to block 5), while the rimonabant group did not. This suggests that rimonabant administration led to a blockade of within-session extinction and we believe this may represent a mechanism for the inhibition of between -session extinction found previously

α-MSH: Our data show a trend towards enhanced FPS with central α-MSH administration with no effect on baseline startle

Future Research: The α-MSH study was a pilot experiment. Based on the trends we found with the testing, our laboratory is planning to continue research on α-MSH with a larger sample size, and also to examine interactions between α-MSH and other neuropeptides, primarily NPY.


Resources

This research was supported by MH047840 and NSF agreement # IBN-9876754 and by the Howard Hughes Medical Institute under Grant No.52003727


Techniques

Cannulae-placement surgery, fear-potentiated startle training (behavioral), flash freezing, cryostat slicing


Keywords

Fear-potentiated startle, anxiety, Rimonabant, α-MSH