Browsing by Subject "Serotonin"

Rationale

Objectives

Methods

Results

Conclusions

Activation of monoaminergic receptors is essential to the mechanism by which psychostimulants and antidepressants induce changes in behavior. Although these drugs rapidly increase monoaminergic transmission, they need to be administered for several weeks or months in order to produce long-lasting alterations in behavior. This observation suggests that it is likely that molecular mechanisms downstream of receptor activation contribute to the effects of psychostimulants and antidepressants on behavior.

Recently, we and others have demonstrated that the methyl-CpG-binding protein 2 (MeCP2) contributes to both neural and behavioral adaptations induced by repeated psychostimulant exposure (Deng et al, 2010, Im et al, 2010). Psychostimulants induce rapid and robust phosphorylation of MeCP2 at Ser421 (pMeCP2), a site that is thought to modulate MeCP2-dependent chromatin regulation (Cohen et al, 2011), and this phosphorylation event is selectively induced in the GABAergic interneurons of the nucleus accumbens (NAc). In order to understand the signaling pathways that contribute to the pattern of pMeCP2 we observe, I characterized the monoaminergic signaling pathways that regulate pMeCP2. I found that activation of dopamine (DA) and serotonin (5-HT) transmission is sufficient to induce pMeCP2. The novel finding that drugs that activate serotonergic signaling induce pMeCP2 suggests that pMeCP2 may be involved in serotonergic mediated behaviors.

To determine the requirement of pMeCP2 in serotonergic mediated behaviors, I utilized mice that bear a knockin (KI) mutation that converts serine to alanine at 421 (S421A) (Cohen et al, 2011). After characterizing the behavioral phenotype of these mice, I conducted tests to assess anxiety- and depression-like behavior. I found that the KI mice do not display heightened anxiety in several assays. However, the KI mice exhibit depression-like behavior in the forced swim and tail suspension but show no differences compared to wild-type (WT) littermates in the sucrose preference test, suggesting that pMeCP2 may be implicated in the behavioral response to stressful stimuli.

Because we are interested in examining the role of pMeCP2 in the behavioral adaptations to chronic monoaminergic signaling, I then put the KI mice and their WT littermates through chronic social defeat stress, a behavioral paradigm in which repeated exposure to aggressive mice causes social avoidance that is reversed by chronic but not acute antidepressant treatment. Although the WT mice show an increase in social interaction following chronic imipramine treatment, the KI mice fail to show a behavioral response to chronic treatment. These data suggest that pMeCP2 may be implicated in the antidepressant action of chronic imipramine. Finally, investigation of the brain regions in which pMeCP2 may be contributing to the behavioral response to chronic imipramine treatment revealed that chronic but not acute imipramine treatment induces pMeCP2 in the lateral habenula (LHb), a brain region involved in the behavioral response to stress and reward. Together, these data implicate a novel role for pMeCP2 in depression-like behavior and the behavioral response to chronic antidepressant treatment.

Negotiating the complex decisions that we encounter daily requires coordinated neu-

ronal activity. The enormous variety of decisions we make, the intrinsic complexity

of the situations we encounter, and the extraordinary flexibility of our behaviors

suggest the existence of intricate neural mechanisms for negotiating contexts and

making choices. Further evidence for this prediction comes from the behavioral al-

terations observed in illness and after injury. Both clinical and scientific evidence

suggest that decision signals are carried by electrical neuronal activity and influenced

by neuromodulatory chemicals. This dissertation addresses the function of two puta-

tive contributors to decision-making: neuronal activity in posterior cingulate cortex

and modulatory effects of serotonin. I found that posterior cingulate neurons respond

phasically to salient events (informative cues; intentional saccades; and reward deliv-

ery) across multiple contexts. In addition, these neurons signal heuristically guided

choices across contexts in a gambling task. These observations suggest that posterior

cingulate neurons contribute to the detection and integration of salient information

necessary to transform event detection to expressed decisions. I also found that

lowering levels of the neuromodulator serotonin increased the probability of making

risky decisions in both monkeys and mice, suggesting that this neurotransmitter con-

tributes to preference formation across species. These results suggest that posterior

cingulate cortex and serotonin each contribute to decision formation. In addition, the

unique serotonergic pro jections to posterior cingulate cortex, as well as the frequent

implication of altered serotonergic and posterior cingulate function in psychiatric dis-

orders, suggest that the confluence of cingulate and serotonergic activity may offer

key insights into normal and pathological mechanisms of decision making.

The loss of nigrostriatal dopaminergic neurons is the fundamental hallmark of Parkinson’s disease (PD). In early PD stages, this is ameliorated by dopamine (DA) supplementation; however, as the disease progresses, the complete loss of this key dopaminergic pathway forces the central nervous system to find alternative routes to regain motor control. It has previously been shown that serotonergic routes must take on the role of the failed dopaminergic system throughout the progression of the disease. Previously studied 5-HT1A anxiolytic and anti-depressive therapeutics have yet to be successfully repurposed for Parkinson’s disease patients. Herein is described the current efforts towards the employment non-dopaminergic agonists in the investigation of motor control in Parkinson’s disease. This research outlines the development of non-dopaminergic therapeutics inspired by the core structure of the clinically approved 5-HT1A agonist Befiradol. This motif has been infused with a trans-2-arylcyclopropylamine moiety which has been independently shown to reduce motor symptoms in Parkinson’s disease via a prior collaboration from the McCafferty lab. While it was originally hypothesized that these therapeutics would act as bifunctional agonists at the 5-HT1A and M4 GPCRs, affinity assays reveal dualistic agonism at the 5-HT1A and 1 receptors, offering a new class of potential bifunctional therapeutics.

Adolescence is the period of transition between childhood and adulthood, and is characterized across mammalian species by changes in behavior that include increases in risk taking, novelty/sensation seeking, and social behavior. Immaturity of the central serotonergic system during adolescence could contribute to risk taking behavior by resulting in lower avoidance of aversive stimuli in adolescents than adults. The purpose of this dissertation was to investigate whether immature serotonergic function could contribute to adolescent risk taking. We studied pre- and postsynaptic serotonergic function and circuit-level mechanisms relevant to risk taking behavior using behavioral and neurochemical approaches.

Serotonergic modulation of behavior was assessed in adult (67-74 day old) and adolescent (28-34 day old) male rats in the novelty induced hypophagia (NIH), elevated plus maze, (EPM), and light/dark (LD) tests for anxiety-like behavior. Serotonin depletion with the synthesis inhibitor p-chlorophenylalanine (PCPA) produced anxiolytic effects only in adult rats in the NIH test and in both age groups in the EPM. These data showed that some serotonin-mediated behavioral inhibition is present during adolescence. However, adolescent rats were less sensitive than adults to the anxiogenic effects of the serotonin releasing drugs fenfluramine and methylenedioxymethamphetamine (MDMA) and the serotonin uptake inhibitor fluoxetine in the LD test, suggesting that serotonin is not as effective at inhibiting behavior in adolescents as it is in adults.

Microdialysis conducted in medial prefrontal cortex (mPFC) showed that adolescent rats exhibited lower increases in extracellular serotonin after treatment with the releasing drug fenfluramine, but not the uptake inhibitor fluoxetine. Further investigation of presynaptic serotonin function in adults and adolescents revealed that adolescent rats have lower tissue serotonin content than adults in several forebrain regions, but similar rates of serotonin synthesis, density of serotonin transporter (SERT)-immunoreactive innervation, and SERT radioligand binding. These data suggest that adolescents may have a lower increase in extracellular serotonin than adults after a releasing drug, but not an uptake inhibitor, due to lower tissue serotonin stores. Lower serotonin stores may limit the ability of a releasing drug to increase extracellular serotonin, but are unlikely to affect response to an uptake inhibitor. These findings also indicate that extracellular serotonin does not completely account for lower serotonin-mediated behavioral inhibition in adolescents.

Since presynaptic serotonin function did not explain age differences in the anxiogenic effects of indirect serotonin agonists, we investigated postsynaptic serotonin signaling by testing the behavioral effects of serotonin receptor agonists in the LD test. Adolescent rats were less sensitive than adults to the anxiogenic effects of the 5-HT_1A agonist 8-hydroxy-2-(dipropylamino)tetralin (8-OH DPAT) in the LD test, but not to the 5-HT₂ agonist meta-chlorophenylpiperazine (mCPP). No age differences were observed in 3H-8-OH DPAT binding in prefrontal cortex, amygdala, or hippocampus between adolescents and adults, and infusion of 8-OH DPAT into mPFC (prelimbic cortex), ventral hippocampus, or basolateral amygdala was unable to replicate the systemic effects of 8-OH DPAT. These data suggest that lower adolescent sensitivity to the anxiogenic effects of 8-OH DPAT is not due to age differences in receptor expression, and show that 5-HT_1A stimulation in mPFC, ventral hippocampus, and basolateral amygdala alone is not sufficient to mimic the effects of systemic 8-OH DPAT.

We tested the circuit-level effects of fluoxetine and 8-OH DPAT, since stimulating 5-HT_1A receptors in single brain regions failed to reproduce age differences in systemic 8-OH DPAT administration. Both drugs activated regions of the amygdala more in adults than adolescents, and 8-OH DPAT also produced greater prefrontal cortical activation in adults. Fluoxetine produced greater expression of the immediate early gene c-Fos in regions of the extended amygdala in adult rats, and 8-OH DPAT produced greater activation of the lateral orbital cortex and central amygdala in adult rats. Lower activation of cortical and amygdala brain regions could underlie the lower behavioral effects of these drugs in adolescents, as these brain regions are important in mediating behavioral inhibition and anxiety-like behavior. These data are also consistent with human studies showing immature cortical and amygdala function during adolescence.

This dissertation shows that adolescents are less sensitive than adults to serotonin mediated behavioral inhibition, and that this may be due to immature activation of neural circuits modulated by the 5-HT_1A receptor between the prefrontal cortex and amygdala. This immature serotonin mediated behavioral inhibition could contribute to adolescent risk taking, drug abuse, and increased risk for suicidality during SSRI therapy for depression and mood disorders.