Differential serotonergic inhibition of in vitro striatal [3H]acetylcholine release in prenatally cocaine-exposed male and female rats

Cocaine-induced neurodevelopmental deficits and underlying mechanisms

Exposure to drugs early in life has complex and long-lasting implications for brain structure and function. This review summarizes work to date on the immediate and long-term effects of prenatal exposure to cocaine. In utero cocaine exposure produces disruptions in brain monoamines, particularly dopamine, during sensitive periods of brain development, and leads to permanent changes in specific brain circuits, molecules, and behavior. Here, we integrate clinical studies and significance with mechanistic preclinical studies, to define our current knowledge base and identify gaps for future investigation. Birth Defects Research (Part C) 108:147-173, 2016. © 2016 Wiley Periodicals, Inc.

Dopamine signaling in C. elegans is mediated in part by HLH-17-dependent regulation of extracellular dopamine levels

In Caenorhabditis elegans, the dopamine transporter DAT-1 regulates synaptic dopamine (DA) signaling by controlling extracellular DA levels. In dat-1(ok157) animals, DA is not taken back up presynaptically but instead reaches extrasynpatic sites, where it activates the dopamine receptor DOP-3 on choligeneric motor neurons and causes animals to become paralyzed in water. This phenotype is called swimming-induced paralysis (SWIP) and is dependent on dat-1 and dop-3. Upstream regulators of dat-1 and dop-3 have yet to be described in C. elegans. In our previous studies, we defined a role for HLH-17 during dopamine response through its regulation of the dopamine receptors. Here we continue our characterization of the effects of HLH-17 on dopamine signaling. Our results suggest that HLH-17 acts downstream of dopamine synthesis to regulate the expression of dop-3 and dat-1. First, we show that hlh-17 animals display a SWIP phenotype that is consistent with its regulation of dop-3 and dat-1. Second, we show that this behavior is enhanced by treatment with the dopamine reuptake inhibitor, bupropion, in both hlh-17 and dat-1 animals, a result suggesting that SWIP behavior is regulated via a mechanism that is both dependent on and independent of DAT-1. Third, and finally, we show that although the SWIP phenotype of hlh-17 animals is unresponsive to the dopamine agonist, reserpine, and to the antidepressant, fluoxetine, hlh-17 animals are not defective in acetylcholine signaling. Taken together, our work suggests that HLH-17 is required to maintain normal levels of dopamine in the synaptic cleft through its regulation of dop-3 and dat-1.

Effects of psychotropic drugs on second messenger signaling and preference for nicotine in juvenile male mice

RATIONALE

A common treatment strategy for pediatric attention deficit/hyperactivity disorder (ADHD) and major depressive disorder (MDD) is combined methylphenidate (MPH) and fluoxetine (FLX). This has raised concerns because MPH + FLX treatment may have pharmacodynamic properties similar to cocaine, potentially increasing drug abuse liability.

OBJECTIVES

To examine the short- and long-term consequences of repeated vehicle, MPH, FLX, MPH + FLX, and cocaine treatment on gene expression in juvenile (postnatal days [PD] 20-34) and adult (PD 70-84) male mice. We further assessed whether juvenile drug treatment influenced subsequent sensitivity for nicotine in adulthood.

METHODS

Juvenile and adult C57BL/6J mice received vehicle, MPH, FLX, MPH + FLX, or cocaine twice-daily for 15 consecutive days. Mice were sacrificed 24 h or 2 months after the last drug injection to assess drug-induced effects on the extracellular signal-regulated protein kinase-1/2 (ERK) pathway within the ventral tegmental area. Subsequent sensitivity for nicotine (0.05, 0.07, and 0.09 mg/kg) was measured using the place-conditioning paradigm (CPP) 24 h and 2 months after juvenile drug exposure.

RESULTS

MPH + FLX, or cocaine exposure in juvenile mice increased mRNA expression of ERK2 and its downstream targets (CREB, cFos, and Zif268), and increased protein phosphorylation of ERK2 and CREB 2 months after drug exposure. Similar mRNA findings were observed in the adult-treated mice. Findings on gene expression 24 h following drug treatment were variable. Juvenile drug exposure increased preference for nicotine when tested in adulthood.

CONCLUSIONS

Early-life MPH + FLX, or cocaine exposure similarly disrupts the ERK pathway, a signaling cascade implicated in motivation and mood regulation, and increases sensitivity for nicotine in adulthood.

Prenatal Cocaine Disrupts Serotonin Signaling-Dependent Behaviors: Implications for Sex Differences, Early Stress and Prenatal SSRI Exposure

Prenatal cocaine (PC) exposure negatively impacts the developing nervous system, including numerous changes in serotonergic signaling. Cocaine, a competitive antagonist of the serotonin transporter, similar to selective serotonin reuptake inhibitors (SSRIs), also blocks dopamine and norepinephrine transporters, leaving the direct mechanism through which cocaine disrupts the developing serotonin system unclear. In order to understand the role of the serotonin transporter in cocaine's effect on the serotonergic system, we compare reports concerning PC and prenatal antidepressant exposure and conclude that PC exposure affects many facets of serotonergic signaling (serotonin levels, receptors, transporters) and that these effects differ significantly from what is observed following prenatal SSRI exposure. Alterations in serotonergic signaling are dependent on timing of exposure, test regimens, and sex. Following PC exposure, behavioral disturbances are observed in attention, emotional behavior and stress response, aggression, social behavior, communication, and like changes in serotonergic signaling, these effects depend on sex, age and developmental exposure. Vulnerability to the effects of PC exposure can be mediated by several factors, including allelic variance in serotonergic signaling genes, being male (although fewer studies have investigated female offspring), and experiencing the adverse early environments that are commonly coincident with maternal drug use. Early environmental stress results in disruptions in serotonergic signaling analogous to those observed with PC exposure and these may interact to produce greater behavioral effects observed in children of drug-abusing mothers. We conclude that based on past evidence, future studies should put a greater emphasis on including females and monitoring environmental factors when studying the impact of PC exposure.

Juvenile administration of concomitant methylphenidate and fluoxetine alters behavioral reactivity to reward- and mood-related stimuli and disrupts ventral tegmental area gene expression in adulthood

There is a rise in the concurrent use of methylphenidate (MPH) and fluoxetine (FLX) in pediatric populations. However, the long-term neurobiological consequences of combined MPH and FLX treatment (MPH + FLX) during juvenile periods are unknown. We administered saline (VEH), MPH, FLX, or MPH + FLX to juvenile Sprague Dawley male rats from postnatal day 20 to 34, and assessed their reactivity to reward- and mood-related stimuli 24 h or 2 months after drug exposure. We also assessed mRNA and protein levels within the ventral tegmental area (VTA) to determine the effect of MPH, FLX, or MPH + FLX on the extracellular signal-regulated protein kinase-1/2 (ERK) pathway--a signaling cascade implicated in motivation and mood regulation. MPH + FLX enhanced sensitivity to drug (i.e., cocaine) and sucrose rewards, as well as anxiety (i.e., elevated plus maze)- and stress (i.e., forced swimming)-eliciting situations when compared with VEH-treated rats. MPH + FLX exposure also increased mRNA of ERK2 and its downstream targets cAMP response element-binding protein (CREB), BDNF, c-Fos, early growth response protein-1 (Zif268), and mammalian target of rapamycin (mTOR), and also increased protein phosphorylation of ERK2, CREB, and mTOR 2 months after drug exposure when compared with VEH-treated rats. Using herpes simplex virus-mediated gene transfer to block ERK2 activity within the VTA, we rescued the MPH and FLX-induced behavioral deficits seen in the forced-swimming task 2 months after drug treatment. These results indicate that concurrent MPH + FLX exposure during preadolescence increases sensitivity to reward-related stimuli while simultaneously enhancing susceptibility to stressful situations, at least in part, due to long-lasting disruptions in ERK signaling within the VTA.

Short- and long-term functional consequences of fluoxetine exposure during adolescence in male rats

BACKGROUND

Fluoxetine (FLX), a selective serotonin reuptake inhibitor, is prescribed for the treatment of major depressive disorder in young populations. Here, we explore the short- and long-term consequences of adolescent exposure to FLX on behavioral reactivity to emotion-eliciting stimuli.

METHODS

Adolescent male rats received FLX (10 mg/kg) twice daily for 15 consecutive days (postnatal days 35-49). The influence of FLX on behavioral reactivity to rewarding and aversive stimuli was assessed 24 hours (short-term) or 3 weeks after FLX treatment (long-term). A separate group of adult rats was also treated with FLX (postnatal days 65-79) and responsiveness to forced swimming was assessed at identical time intervals as with the adolescents.

RESULTS

Fluoxetine exposure during adolescence resulted in long-lasting decreases in behavioral reactivity to forced swimming stress and enhanced sensitivity to sucrose and to anxiety-eliciting situations in adulthood. The FLX-induced anxiety-like behavior was alleviated by re-exposure to FLX in adulthood. Fluoxetine treatment during adolescence also impaired sexual copulatory behaviors in adulthood. Fluoxetine-treated adult rats did not show changes in behavioral reactivity to forced swim stress as observed in those treated during adolescence and tested in adulthood.

CONCLUSIONS

Treating adolescent rats with FLX results in long-lived complex outputs regulated by the emotional valence of the stimulus, the environment in which it is experienced, and the brain circuitry likely being engaged by it. Our findings highlight the need for further research to improve our understanding of the alterations that psychotropic exposure may induce on the developing nervous system and the potential enduring effects resulting from such treatments.

Serotonin modulates glutamatergic transmission in the rat olfactory tubercle

The olfactory tubercle (OT) is found in the brains of mammals that are highly dependent on their sense of smell. Its human analogue is the poorly understood anterior perforated substance. Previous work on rat brain slices identified two types of field potential responses from the OT. The association fibre (AF) pathway was sensitive to muscarinic modulation, whereas the lateral olfactory tract (LOT) fibre pathway was not. Here, we establish that serotonin (5-hydroxytryptamine; 5-HT) also inhibits field potential excitatory postsynaptic potentials (EPSPs) in the AF, but not in the LOT fibre, pathway. Parallel experiments with adenosine (ADO) excluded ADO mediation of the 5-HT effect. Exogenous 5-HT at 30 microm caused a long-lasting approximately 40% reduction in the amplitude of AF postsynaptic responses, without affecting the time-course of EPSP decline, indicating a fairly restricted disposition of the 5-HT receptors responsible. The 5-HT(1)-preferring, 5-HT(5)-preferring and 5-HT(7)-preferring agonist 5-carboxamidotryptamine caused similar inhibition at approximately 100 nm. The 5-HT(1A)-preferring ligand 8-hydroxy-di-n-propylamino-tetralin at 10 microm, and the 5-HT uptake inhibitor citalopram at 3 microm, caused inhibition of AF-stimulated field potential responses in the 5-10% range. Order-of-potency information suggested a receptor of the 5-HT(1B) or 5-HT(1D) subtype. The 5-HT(1D) agonist L-694,247 (1 microm) suppressed the AF response by approximately 10% when used on its own. After washing out of L-694,427, inhibition by 30 microm 5-HT was reduced to negligible levels. Allowing for a partial agonist action of L-694,427 and complex interactions of 5-HT receptors within the OT, these results support the presence of active 5-HT(1D)-type receptors in the principal cell layer of the OT.

The role of acetylcholine in cocaine addiction

Central nervous system cholinergic neurons arise from several discrete sources, project to multiple brain regions, and exert specific effects on reward, learning, and memory. These processes are critical for the development and persistence of addictive disorders. Although other neurotransmitters, including dopamine, glutamate, and serotonin, have been the primary focus of drug research to date, a growing preclinical literature reveals a critical role of acetylcholine (ACh) in the experience and progression of drug use. This review will present and integrate the findings regarding the role of ACh in drug dependence, with a primary focus on cocaine and the muscarinic ACh system. Mesostriatal ACh appears to mediate reinforcement through its effect on reward, satiation, and aversion, and chronic cocaine administration produces neuroadaptive changes in the striatum. ACh is further involved in the acquisition of conditional associations that underlie cocaine self-administration and context-dependent sensitization, the acquisition of associations in conditioned learning, and drug procurement through its effects on arousal and attention. Long-term cocaine use may induce neuronal alterations in the brain that affect the ACh system and impair executive function, possibly contributing to the disruptions in decision making that characterize this population. These primarily preclinical studies suggest that ACh exerts a myriad of effects on the addictive process and that persistent changes to the ACh system following chronic drug use may exacerbate the risk of relapse during recovery. Ultimately, ACh modulation may be a potential target for pharmacological treatment interventions in cocaine-addicted subjects. However, the complicated neurocircuitry of the cholinergic system, the multiple ACh receptor subtypes, the confluence of excitatory and inhibitory ACh inputs, and the unique properties of the striatal cholinergic interneurons suggest that a precise target of cholinergic manipulation will be required to impact substance use in the clinical population.

Antidepressant treatment can normalize adult behavioral deficits induced by early-life exposure to methylphenidate

BACKGROUND

Methylphenidate (MPH) is prescribed for the treatment of attention-deficit/hyperactivity disorder. Exposure to MPH before adulthood causes behavioral deficits later in life, including anxiety- and depression-like behaviors and decreased responding to natural and drug rewards. We examined the ability of fluoxetine (FLX), a selective serotonin reuptake blocker, to normalize these MPH-induced behavioral deficits.

METHODS

Male rats received MPH (2.0 mg/kg) or saline (VEH) during preadolescence (postnatal day [PD] 20-35). When adults, rats were divided into groups receiving no treatment, acute or chronic FLX, and behavioral reactivity to several emotion-eliciting stimuli were assessed.

RESULTS

The MPH-treated rats were significantly less responsive to natural (i.e., sucrose) and drug (i.e., morphine) rewards and more sensitive to stress- and anxiety-eliciting situations. These MPH-induced deficits were reversed by exposure to FLX.

CONCLUSIONS

These results indicate that exposure to MPH during preadolescence leads to behavioral alterations that endure into adulthood and that these behavioral deficits can be normalized by antidepressant treatment. These results highlight the need for further research to better understand the effects of stimulants on the developing nervous system and the potential enduring effects resulting from early-life drug exposure.

Anterior hypothalamic vasopressin modulates the aggression-stimulating effects of adolescent cocaine exposure in Syrian hamsters

Repeated low-dose cocaine treatment (0.5 mg/kg/day) during adolescence induces offensive aggression in male Syrian hamsters (Mesocricetus auratus). This study examines the hypothesis that adolescent cocaine exposure predisposes hamsters to heightened levels of aggressive behavior by increasing the activity of the anterior hypothalamic-vasopressinergic neural system. In a first experiment, adolescent male hamsters were treated with low-dose cocaine and then scored for offensive aggression in the absence or presence of vasopressin receptor antagonists applied directly to the anterior hypothalamus. Adolescent cocaine-treated hamsters displayed highly escalated offensive aggression that could be reversed by blocking the activity of vasopressin receptors within the anterior hypothalamus. In a second set of experiments, adolescent hamsters were administered low-dose cocaine or vehicle, tested for offensive aggression, and then examined for differences in vasopressin innervation patterns and expression levels in the anterior hypothalamus, as well as the basal- and stimulated-release of vasopressin in this same brain region. Aggressive, adolescent cocaine-treated hamsters showed no differences in vasopressin afferent innervation and/or peptide levels in the anterior hypothalamus compared with non-aggressive, saline-treated littermates. Conversely, significant increases in stimulated, but not basal, vasopressin release were detected from the anterior hypothalamus of aggressive, cocaine-treated animals compared with non-aggressive, saline-treated controls. Together, these data suggest that adolescent cocaine exposure increases aggression by increasing stimulated release of vasopressin in the anterior hypothalamus, providing direct evidence for a causal role of anterior hypothalamic-vasopressin activity in adolescent cocaine-induced offensive aggression. A model for how alterations in anterior hypothalamic-vasopressin neural functioning may facilitate the development of the aggressive phenotype in adolescent-cocaine exposed animals is presented.

Cocaine in adolescent rats produces residual memory impairments that are reversible with time

Rats received injections (subcutaneous) of either 10 or 20 mg/kg cocaine on postnatal days 26-33, while lab chow-fed and pair-fed controls received saline. Spatial memory in a Morris water maze was assessed on four different occasions commencing 10 days postcocaine and ending approximately 12 months later. To determine whether there existed long-term changes in cholinergic processes, maze performance was evaluated following 1 mg/kg scopolamine challenge 4 months postcocaine. Subjects survived under standard laboratory housing conditions until they died. Results from the first assessment indicated a working memory deficit in the low-dose cocaine group and a long-term memory impairment in the high-dose cocaine group. These decrements neither were permanent nor were exacerbated by age-related processes in that cocaine-treated subjects performed at control levels on subsequent assessments. An exception to this was the results derived from the third assessment indicating that animals previously treated with 20 mg/kg cocaine were impaired when challenged with scopolamine. Examination of mortality rates revealed that cocaine-treated rats died significantly sooner than lab chow-fed control subjects. Taken together, these data indicate that cocaine during adolescence causes residual, but not permanent, deleterious effects on memory that may be mediated by alterations in cholinergic neurochemistry. More provocatively, the results showed that cocaine during adolescence shortened the lifespan of rats. This latter finding suggests that cocaine during adolescence may produce residual physiological effects that last well into adulthood.