Psychological stressors such as social stress and bullying are prevalent in today’s society. Disorders such as PTSD, depression and social anxiety disorder can be either caused or exacerbated by social stress and treatment options are not always effective in providing relief for these disorders. Our laboratory studies a form of social stress termed conditioned defeat, whereby a defeated Syrian hamster no longer displays species-typical territorial aggression but instead is submissive and defensive toward an intruder in its own cage. We hypothesized that the nucleus accumbens is a necessary component of the circuit mediating the acquisition and expression of conditioned defeat and that dopamine is necessary within the nucleus accumbens for inducing memory processes as well as expression of behavioral responses to stressful situations. We also hypothesized that defeat activates dopaminergic and/or nondopaminergic neurons in the ventral tegmental area (VTA) and that dopamine released by neurons projecting from the VTA to the nucleus accumbens and basolateral amygdala (BLA) increases neuronal activation of these structures during defeat. We found that dopamine, but not GABA, modulates memory of social defeat within the nucleus accumbens. However, GABA does affect the expression of behavioral responses to social defeat. Defeat also increased Fos activation of non-dopaminergic neurons, but it did not increase activation of dopaminergic neurons. Baclofen infusion into the VTA prior to defeat, which was hypothesized to specifically inhibit dopaminergic neurons, did not affect Fos activation within the nucleus accumbens and the basolateral amygdala. These experiments determined that dopamine does modulate memory of social defeat within the nucleus accumbens, but it is currently unclear what the source of this dopamine is. Future experiments are planned to determine this source of dopamine that could be a target of treatment for disorders that are caused or exacerbated by social stress.

M5 muscarinic receptors are associated with dopamine neurons of the ventral tegmental area (VTA) and substantia nigra, and provide an important excitatory input to the mesolimbic dopamine system. Here, I studied locomotion induced by systemic morphine (3, 10, 30 mg/kg, i.p.) in M5 knockout mice of the C57Bl/6 (B6) and CD1 x 129SvJ (129) background strains. M5 knockout mice of both strains showed reduced locomotion in response to 30 mg/kg morphine, while only B6 M5 knockout mice showed reduced locomotion in response to 10 mg/kg morphine. In B6 wild-type mice VTA pre-treatment with the non subtype-selective muscarinic receptor antagonist atropine (3 mg per side), but not the non subtype-selective nicotinic receptor antagonist mecamylamine (5 mg per side), reduced locomotion in response to 30 mg/kg (i.p.) morphine to a similar extent as systemic M5 knockout, suggesting that the reduced morphine-induced locomotion in M5 knockout mice was due to the loss of M5 receptors on VTA dopamine neurons. By contrast, in M5 knockout mice, either intra-VTA atropine or mecamylamine alone increased locomotion by almost 3 times relative to saline, and potentiated morphine-induced locomotion. Therefore, in M5 knockout mice, more clearly than in wild-type mice, blockade of either VTA muscarinic or nicotinic receptors activated locomotion. Infusions of morphine (50 ng) into the VTA increased nucleus accumbens dopamine efflux in urethane-anesthetized wild-type mice. Either M5 knockout or pre-treatment with VTA scopolamine (50 ug) in wild-type mice blocked accumbal dopamine efflux in response to VTA morphine. Therefore, M5 receptors are critical for excitation of dopamine neurons by intra-VTA morphine, suggesting that the reduced locomotion produced by systemic morphine in M5 knockout mice was, in part, due to loss of M5-mediated excitation of VTA dopamine neurons by opiates. The locomotion data also show that in the absence of M5 receptors, cholinergic afferents to mesolimbic dopamine neurons are inhibitory. This supports and extends the conclusions from many studies that non-M5 muscarinic receptors inhibit, and M5 receptors excite, dopamine neurons. Loss of M5-mediated excitation results in reduced acute effects of opiates.

M5 muscarinic receptors are associated with dopamine neurons of the ventral tegmental area (VTA) and substantia nigra, and provide an important excitatory input to the mesolimbic dopamine system. Here, I studied locomotion induced by systemic morphine (3, 10, 30 mg/kg, i.p.) in M5 knockout mice of the C57Bl/6 (B6) and CD1 x 129SvJ (129) background strains. M5 knockout mice of both strains showed reduced locomotion in response to 30 mg/kg morphine, while only B6 M5 knockout mice showed reduced locomotion in response to 10 mg/kg morphine. In B6 wild-type mice VTA pre-treatment with the non subtype-selective muscarinic receptor antagonist atropine (3 mg per side), but not the non subtype-selective nicotinic receptor antagonist mecamylamine (5 mg per side), reduced locomotion in response to 30 mg/kg (i.p.) morphine to a similar extent as systemic M5 knockout, suggesting that the reduced morphine-induced locomotion in M5 knockout mice was due to the loss of M5 receptors on VTA dopamine neurons. By contrast, in M5 knockout mice, either intra-VTA atropine or mecamylamine alone increased locomotion by almost 3 times relative to saline, and potentiated morphine-induced locomotion. Therefore, in M5 knockout mice, more clearly than in wild-type mice, blockade of either VTA muscarinic or nicotinic receptors activated locomotion. Infusions of morphine (50 ng) into the VTA increased nucleus accumbens dopamine efflux in urethane-anesthetized wild-type mice. Either M5 knockout or pre-treatment with VTA scopolamine (50 ug) in wild-type mice blocked accumbal dopamine efflux in response to VTA morphine. Therefore, M5 receptors are critical for excitation of dopamine neurons by intra-VTA morphine, suggesting that the reduced locomotion produced by systemic morphine in M5 knockout mice was, in part, due to loss of M5-mediated excitation of VTA dopamine neurons by opiates. The locomotion data also show that in the absence of M5 receptors, cholinergic afferents to mesolimbic dopamine neurons are inhibitory. This supports and extends the conclusions from many studies that non-M5 muscarinic receptors inhibit, and M5 receptors excite, dopamine neurons. Loss of M5-mediated excitation results in reduced acute effects of opiates.

The goal of this dissertation was to investigate the role of μ-opioid receptors in the mechanism of ethanol-stimulated dopamine release in the nucleus accumbens shell (NAcS) of rats. The underlying hypothesis is that blockade of the μ-opioid receptors leads to an attenuation of ethanol-stimulated mesolimbic dopamine release. We prepared ethanol-naïve male Long Evans rats (n = 95) for intravenous (i.v.) drug administration and in vivo microdialysis (in awake, freely moving animals), and analyzed our samples using HPLC and GC for dopamine and ethanol detection, respectively. In one set of experiments, we looked at the effects of naltrexone, a non-selective opioid antagonist, on ethanol-stimulated mesolimbic dopamine release. First of all, we checked to see if naltrexone affected basal dopamine levels in the NAcS. Thereafter, we looked for a dose of naltrexone (i.v.) that was effective in suppressing the release of dopamine in the NAcS evoked by morphine (1 mg/kg, i.v.). Subsequently, we checked to see if doses of naltrexone that inhibited morphine-evoked dopamine were also effective in attenuating dopamine release due to ethanol (1g/kg, 10% w/v, i.v.). To do this, we pretreated rats with naltrexone doses, followed 20 min later by morphine, ethanol or saline (all drugs were administered i.v.). In another set of experiments, we looked at the effect of β-funaltrexamine, a selective μ-opioid antagonist, on ethanol-stimulated dopamine release in the NAcS. Similarly to the previous set of experiments, we looked for a dose of β-funaltrexamine (s.c.) that was effective in suppressing the release of dopamine the NAcS evoked by morphine (1 mg/kg, i.v.), and checked to see if this dose of β-funaltrexamine was also effective in attenuating ethanol-stimulated dopamine release in the NAcS. For the β- funaltrexamine experiments, rats were pretreated with β-funaltrexamine (s.c.) 20- 25 h before i.v. infusions of saline, morphine and ethanol. Morphine increased dopamine release in the NAcS. Naltrexone and β- funaltrexamine significantly attenuated morphine-evoked dopamine release. Also, ethanol increased dopamine release in the NAcS. Naltrexone and β- funaltrexamine, at doses effective in attenuating morphine-evoked dopamine release, suppressed the prolongation, but not the initiation of dopamine release in the NAcS due to ethanol. Naltrexone and β-funaltrexamine did not affect the peak concentration and clearance of ethanol in the brain. The conclusion of this study is that the μ-opioid receptors are involved in a delayed component of ethanol-stimulated dopamine release in the NAcS in ethanol-naïve rats. This is the first study to show that the ethanol-stimulated dopamine response consists of a delayed μ-opioid mechanism. / text

Thesis advisor: Michael Numan / This paper will review research from our laboratory dealing with the neural basis of maternal behavior in rats. Specifically, my work investigates hypothalamic interaction with the mesolimbic dopamine system and the regulation of maternal responsiveness. Recent evidence has shown that increased dopamine activity in the nucleus accumbens, a major terminus of the mesolimbic dopamine pathway, results in a facilitation of maternal behavior in female rats who have been partially primed by hormones. However, the way in which dopamine and hormones act on these neural circuits is unclear. We hypothesize that one of these hormones, estradiol, acts on the MPOA and mesolimbic dopamine system through similar intracellular mechanisms as dopamine. My research goals are twofold: (1) to discern which G-protein coupled pathway dopamine utilizes to act in the nucleus accumbens and (2) to investigate whether estradiol is having rapid effects at the cell membrane and whether these effects are mediated by G-protein coupled receptors. / Thesis (BA) — Boston College, 2008. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Psychology. / Discipline: College Honors Program.

Thesis advisor: Michael Numan / Maternal behavior is a primary social characteristic of mammals. By studying maternal behavior in rats, broader inferences can be made about the neural circuits that influence maternal behavior in other mammals, including humans. Maternal behavior of rats includes nest building, pup grooming, nursing, and pup retrieval. The projections from the medial preoptic area of the hypothalamus (MPOA) to the ventral tegmental area (VTA) of the mesolimbic dopamine system are known to regulate maternal behavior in post-partum rats. The aim of the present study was to examine how inhibition of the basolateral amygdala (BLA), an area that projects to the nucleus accumbens-ventral palldium (NA-VP) circuit of the mesolimbic dopamine system, bilaterally with muscimol (a GABA-A agonist) might interrupt the retrieval of pups by post-partum rats. Females injected with muscimol, but not those injected with saline, displayed significant deficits in retrieval behavior, suggesting that the BLA is a region important for the promotion of maternal behavior. The effects were also reversible, as all females displayed normal maternal behavior 24-hours post-injection. Follow-up studies should use asymmetric neuron-specific lesions of the BLA and the VP to show that the projections from the BLA to the VP are essential for maternal behavior. / Thesis (BA) — Boston College, 2010. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: College Honors Program. / Discipline: Psychology Honors Program. / Discipline: Psychology.

Melanocortins (MCs) play an important role in feeding, metabolism, and energy expenditure. While melanocortin receptor (MCR) mRNA has been found in the mesolimbic dopamine (DA) pathway, the ability of melanocortins to regulate feeding and other behaviors through actions on the mesolimbic DA system have not been examined. Short-hairpin RNAs (shRNAs) were created targeting MC3R and MC4R and were tested via in vitro studies for their ability to knockdown their target receptor. A total of three shRNAs were created targeting each receptor, and each shRNA caused successful knockdown. These shRNAs are tools that can be used for future in vivo studies to examine the various behavioral effects of melanocortins on the mesolimbic DA pathway.

Mesolimbic dopamine plays an important role in ethanol reinforcement, and studies have shown that accumbal dopamine increases during operant ethanol self-administration. However, no one has ever studied this dopaminergic response during the acquisition of ethanol self-administration. Furthermore, some studies have shown that the dopamine signal does not correlate with the pharmacological effects of ethanol, but with the time during which the animal consumes the majority of the ethanol solution and when the sensory stimuli of ethanol are strongest. However, there is currently no direct evidence showing that the sensory stimuli of ethanol is indeed what causes the brief increase in accumbal dopamine during ethanol self-administration. The studies in this dissertation attempted to elucidate these issues. We designed and tested a placebo spout, which was to be used to study the relationship between accumbal dopamine and the sensory stimuli of ethanol during self-administration. Unfortunately, the placebo designs were either not feasible for performing microdialysis or did not show promising behavioral data. We also developed and tested a self-administration protocol in which the concentrations of ethanol (10%) were kept constant throughout the study. The new protocol was successful in initiating and maintaining ethanol self-administration, and the animals doubled their intake from day 1 to day 2 of ethanol consumption. Using this protocol, we trained male Long Evans rats to self-administer ethanol and measured accumbal dopamine during the first two days of ethanol self-administration through microdialysis. The behavioral and neurochemical data matched. A single exposure to ethanol was sufficient for the animals to double their ethanol consumption by day 2 and to cause an increase in accumbal dopamine during the first 5 minutes of ethanol self-administration. The dopamine response was observed during the time when the sensory stimuli of ethanol were strongest, but before ethanol reached peak concentrations in the brain. Overall, these results suggest that the dopamine response to ethanol self-administration may not be solely pharmacological and that a single exposure to ethanol is sufficient to learn the association between ethanol and its cues. These findings give us greater insight into mesolimbic dopamine's role in the early stages of ethanol reinforcement. / text

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