Kathryn A. Cunningham

Serotonergic mechanisms involved in the discriminative stimulus, reinforcing and subjective effects of cocaine

Abstract The purpose of the present manuscript is to review the current status of the role of serotonin (5-hydroxytryptamine; 5-HT) systems in the stimulus and reinforcing properties of cocaine in non-humans and the subjective effects of cocaine in humans. Review of the current literature suggests that general enhancement (via precursor administration) or depletion of brain 5-HT content (via neurotoxin administration or tryptophan depletion) impact the reinforcing effects of cocaine in non-humans and its subjective effects in humans. Selective 5-HT reuptake inhibitors (SSRIs) enhance the discriminability of cocaine and decrease cocaine self-administration in animals, although data to the contrary also exist. Studies in humans suggest that SSRIs attenuate the subjective effects of cocaine in humans. Although few drugs with selectivity for 5-HT2 receptors have been studied systematically, a 5-HT2 agonist and several antagonists show some efficacy in enhancing and reducing, respectively, the reinforcing effects of cocaine in non-humans. Limited data from humans suggest that a 5-HT2 antagonist may also decrease the subjective effects of cocaine; thus, 5-HT2 compounds deserve further attention. The majority of studies evaluating the 5-HT3 antagonists have reported negative results across all paradigms. In summary, while the functional significance of 5-HT receptors has not been fully elucidated, these data suggest that changes in serotonergic activity can modulate the effects of cocaine in both animals and humans under a variety of experimental conditions. One commonality among the studies with positive findings is that cocaine effects are only partially modified by 5-HT agents regardless of the direction of change.

Dopamine D1 and D2 mediation of the discriminative stimulus properties of d-amphetamine and cocaine

Evidence suggests that stimulants such asd-amphetamine and cocaine act presynaptically by increasing the amount of dopamine (DA) available to stimulate postsynaptic DA receptors. Since two subpopulations of DA receptors (D1 and D2) exist, we investigated the role of both of these receptor subtypes in mediating the internal “state” produced by these stimulants. Two groups of rats (N=8/group) were trained to discriminate intraperitoneal (IP) injections of eitherd-amphetamine (1 mg/kg) or cocaine (10 mg/kg) from saline in a two-lever, water-reinforced, drug discrimination task. After stable performance was established (i.e., more than 85% correct under each training condition), substitution and combination tests were conducted with selective D1 and D2 agonists and antagonists. The D2 agonist quinpirole (0.0313–0.125 mg/kg) mimicked both stimulant cues while the D1 agoinst SKF 38393 (5–20 mg/kg) substituted partially for cocaine but notd-amphetamine. Combination tests with DA antagonists indicated that both the D1 antagonist SCH 23390 (0.0063–0.25 mg/kg) and the D2 antagonist haloperidol (0.125–0.5 mg/kg) attenuated the effects of both stimulants; in addition, the substitution of cocaine (20 mg/kg) ford-amphetamine was blocked by both DA antagonists. The ability of both D1 and D2 antagonists to attenuate the stimulus effects ofd-amphetamine and cocaine raises the possibility that a synergistic (“enabling”) interaction between D1 and D2 receptors may modulate stimulant cues.

Monoamine reuptake inhibitors enhance the discriminative state induced by cocaine in the rat

Cocaine inhibits the reuptake of dopamine (DA), norepinephrine (NE), and serotonin (5-HT). To investigate the relative role of such reuptake processes in the discriminative stimulus properties of cocaine, male rats (N=16) were trained to discriminate cocaine (10 mg/kg) from saline in a two-lever, water-reinforced drug discrimination task and were administered neuroactive compounds during substitution or combination tests. The DA reuptake inhibitor GBR 12909 (2–16 mg/kg) completely mimicked cocaine. The reuptake inhibitors for NE (desipramine; 2–8 mg/kg) and 5-HT (fluoxetine; 0.625–5 mg/kg) did not substitute for the training drug. A low dose of either desipramine (3 mg/kg), fluoxetine (1.25 mg/kg), or GBR 12909 (2 mg/kg) coadministered with low doses of cocaine (0.625–2.5 mg/kg) enhanced the discriminative stimulus properties of this psychostimulant. The dose predicted to elicit 50% drug-lever responding is reduced (ED50) in the presence of desipramine (0.38 mg/kg), fluoxetine (0.79 mg/kg) or GBR 12909 (0.84 mg/kg) compared to the ED50 for cocaine (1.57 mg/kg) in the absence of any reuptake inhibitor. The finding that GBR 12909 mimics the cocaine cue corroborates the hypothesis that the stimulus properties of cocaine are mediated predominantly by DA systems. The potentiation of the stimulus effects of cocaine by monoamine reuptake inhibitors in rats suggests that these drugs could also amplify the subjective effects of cocaine in humans, a possibility that should be considered given the current use of antidepressants in the treatment of cocaine abusers.

Perimembrane Localization of the Estrogen Receptor α Protein in Neuronal Processes of Cultured Hippocampal Neurons

There is clear evidence of rapid, nongenomic responses to estrogen in a variety of neuronal model systems. To address the question of whether some of these rapid estrogen signals might be transduced by the classical estrogen receptor (ER) α or a closely related protein in nontransformed neurons, we undertook the present study using isolated fetal rat hippocampal neurons. Several antibodies developed to detect ERα were tested in this system and showed positive membrane staining in nonpermeabilized neurons. MC-20, an affinity purified anti-ERα, rabbit polyclonal IgG antibody which does not recognize ERβ was selected to carry out the majority of the experiments. When permeabilized, the hippocampal neurons exhibited low levels of nuclear staining for ERα, but abundant labeling for ERα throughout the entire cell including the neurites. In addition to traditional immunocytochemistry controls, incubation of neurons for 24 h in the presence of 10 µM antisense oligonucleotide directed against the translation start site of ERα reduced ERα immunoreactivity throughout the neurons providing further evidence that the immunostaining was specific for ERα. Confocal and conventional microscopy demonstrated that the antigen was predominately extranuclear and localization of ERα in the neurites suggests that the receptor is in close proximity to the plasma membrane. This localization is consistent with a role for ERα as a transducer of rapid, nongenomic estrogen responses in hippocampal neurons.

Distribution of serotonin 5-HT2C receptors in the ventral tegmental area.

Serotonin 2C receptors (5-HT2CR) appear to exert tonic inhibitory influence over dopamine (DA) neurotransmission in the ventral tegmental area (VTA), the origin of the mesolimbic DA system, thought to be important in psychiatric disorders including addiction and schizophrenia. Current literature suggests that the inhibitory influence of 5-HT2CR on DA neurotransmission occurs via indirect activation of GABA inhibitory neurons, rather than via a direct action of 5-HT2CR on DA neurons. The present experiments were performed to establish the distribution of 5-HT2CR protein on DA and GABA neurons in the VTA of male rats via double-label immunofluorescence techniques. The 5-HT2CR protein was found to be co-localized with the GABA synthetic enzyme glutamic acid decarboxylase (GAD), confirming the presence of the 5-HT2CR on GABA neurons within the VTA. The 5-HT2CR immunoreactivity was also present in cells that contained immunoreactivity for tyrosine hydroxylase (TH), the DA synthetic enzyme, validating the localization of 5-HT2CR to DA neurons in the VTA. While the degree of 5-HT2CR+GAD co-localization was similar across the rostro-caudal levels of VTA subnuclei, 5-HT2CR+TH co-localization was highest in the middle relative to rostral and caudal levels of the VTA, particularly in the paranigral, parabrachial, and interfascicular subnuclei. The present results suggest that the inhibitory influence of the 5-HT2CR over DA neurotransmission in the VTA is a multifaceted and complex interplay of 5-HT2CR control of the output of both GABA and DA neurons within this region.

Serotonin2C receptor localization in GABA neurons of the rat medial prefrontal cortex: implications for understanding the neurobiology of addiction.

Serotonin (5-HT) action via the 5-HT(2C) receptor (5-HT(2C)R) provides an important modulatory influence over neurons of the prefrontal cortex (PFC), which is critically involved in disorders of executive function including substance use disorders. In the present study, we investigated the distribution of the 5-HT(2C)R in the rat prelimbic prefrontal cortex (PrL), a subregion of the medial prefrontal cortex (mPFC), using a polyclonal antibody raised against the 5-HT(2C)R. The expression of 5-HT(2C)R immunoreactivity (IR) was highest in the deep layers (layers V/VI) of the mPFC. The 5-HT(2C)R-IR was typically most intense at the periphery of cell bodies and the initial segment of cell processes. Approximately 50% of the 5-HT(2C)R-IR detected was found in glutamate decarboxylase, isoform 67 (GAD 67)-positive neurons. Of the subtypes of GABA interneurons identified by expression of several calcium-binding proteins, a significantly higher percentage of neurons expressing IR for parvalbumin also expressed 5-HT(2C)R-IR than did the percentage of neurons expressing calbindin-IR or calretinin-IR that also expressed 5-HT(2C)R-IR. Since parvalbumin is located in basket and chandelier GABA interneurons which project to cell body and initial axon segments of pyramidal cells, respectively, these results raise the possibility that the 5-HT(2C)R in the mPFC acts via the parvalbumin-positive GABAergic interneurons to regulate the output of pyramidal cells in the rat mPFC.

Estrogen regulation of gene expression in the brain: a possible mechanism altering the response to psychostimulants in female rats

Acute behavioral responses to cocaine are more pronounced in female than in male rats. We have shown that 3 weeks of treatment with 17beta-estradiol (E(2)) implants significantly enhanced the hyperactivity induced by cocaine in ovariectomized (OVX) rats. The ligand-bound estrogen receptor (ER) functions as a transcription factor to regulate the expression of E-responsive genes. Thus, we hypothesized that estrogen (E) modulates the behavioral response to cocaine via regulation of expression of components of dopamine (DA) and serotonin (5-HT) systems in mesolimbic nuclei important in the response to cocaine as well as the hypothalamus, a brain area known to be E-responsive. Adult female Sprague-Dawley rats were OVX; half of them then received E(2) implant (OVX+E) (n=8/group, two groups). Twenty-seven days later, brain tissue was collected to study E(2) effects on mRNA expression for DA D(1)-like (D(1)) and D(2)-like (D(2S), D(2L), D(3)) receptors, DA transporter (DAT), 5-HT(1A), 5-HT(1B), 5-HT(2A), 5-HT(2C) receptors, and 5-HT transporter (SERT) as well as ERalpha and ERbeta in amygdala, hypothalamus, nucleus accumbens, midbrain, and ventral tegmental area (VTA). We found that E(2) implants in OVX rats increased mRNA levels for D(1) receptor in hypothalamus, D(2L) receptor in midbrain, and D(3) receptor in VTA, and decreased D(3) receptor mRNA levels in midbrain relative to OVX controls. E(2) also increased 5-HT(2C) receptor mRNA levels in midbrain and hypothalamus. In addition, E(2) decreased mRNA levels for ERalpha in amygdala and hypothalamus and ERbeta in amygdala. The present study demonstrates that E can regulate mRNA expression for specific DA and 5-HT receptors in a region-specific manner and suggests that such modifications may contribute to the behavioral response to cocaine.

Prospects for serotonin 5-HT2R pharmacotherapy in psychostimulant abuse

The serotonin (5-HT) neurotransmitter system provides fundamental modulatory regulation of the limbic-corticostriatal circuitry known to be vital in the development of addiction as well as the aspects of addiction that hinder recovery and contribute to relapse. Thus, components of the 5-HT system may provide novel targets for the development of pharmacological treatments for psychostimulant dependence, which is associated with significant aberrations in dopamine (DA) neurotransmission. Two key modulators of DA signalling within the limbic-corticostriatal circuit are the 5-HT(2A) receptor (5-HT(2A)R) and the 5-HT(2C)R. These receptors are known to control the neurochemical and behavioural effects of psychostimulants, and in particular the in vivo effects of cocaine. Pre-clinical studies indicate that 5-HT(2A)R antagonists and/or 5-HT(2C)R agonists may effectively reduce craving and/or relapse, and likewise, enhance abstinence, while 5-HT(2C)R agonists may also effectively reduce cocaine intake in active cocaine users. At present, the progression of studies to probe the effectiveness of 5-HT(2A)R and 5-HT(2C)R ligands in the clinical setting is hindered by a lack of available, selective 5-HT(2A)R antagonists or 5-HT(2C)R agonists for use in human cocaine abusers. However, a number of selective 5-HT(2)R ligands currently under development, or in early clinical trials for psychiatric and/or neurological disorders, may soon be available for translational studies to explore their effectiveness in modulating drug use and dependence.

Electrophysiological effects of cocaine and procaine on dorsal raphe serotonin neurons

The effects of cocaine and procaine on the spontaneous activity of serotonin (5-hydroxytryptamine, 5-HT) neurons in the dorsal raphe nucleus of the chloral hydrate anesthetized male rat were characterized using extracellular single-unit and microiontophoretic recording techniques. Intravenous cocaine elicited a potent, dose-dependent inhibition of 5-HT cell firing with a cumulative dose to 50% inhibition of cell firing of 0.66 +/- 0.11 mg/kg (means +/- S.E.M.; N = 17). In contrast, systemic administration of the local anesthetic procaine did not significantly alter cell firing in cumulative doses up to 16 mg/kg (N = 10). Microiontophoretic application of cocaine, but not procaine, also depressed spontaneous cell firing of 5-HT-containing neurons and potentiated the inhibitory effects of 5-HT in the dorsal raphe nucleus. The depressant effects on 5-HT neurons following systemic or microiontophoretic application of cocaine may result from autoinhibition of 5-HT neurons, presumably as a consequence of cocaine-induced 5-HT reuptake inhibition.

Pharmacological studies of the acute effects of (+)-3,4-methylenedioxymethamphetamine on locomotor activity: Role of 5-HT1B/1D and 5-HT2 receptors

The role of serotonin 5-HT2 receptors (5-HT2R) in the hyperactivity induced by (+)-3,4-methylenedioxy-methamphetamine ((+)-MDMA; 3 mg/kg) was investigated. Hyperactivity induced by (+)-MDMA was robustly potentiated by the 5-HT2B/2CR antagonist SB 206553 (1.0, 2.0, and 4.0 mg/kg). Administration of the 5-HT1B/1DR antagonist GR 127935 (2.5 mg/kg) or the 5-HT2AR antagonist M100907 (1.0 mg/kg) partially suppressed the potentiated hyperactivity seen following SB 206553 plus (+)-MDMA; a blockade to activity levels seen with (+)-MDMA alone was observed following the combination of GR 127935 plus M100907. A modest potentiative interaction was seen when SB 206553 was combined with the DA releaser amphetamine (0.5 mg/kg) or amphetamine plus the 5-HT releaser fenfluramine (4.0 mg/kg). SB 206553 (1–4 mg/kg), GR 127935 (2.5 mg/kg) and M100907 (1 mg/kg) did not alter spontaneous activity upon administration singly or in combination. These data suggest that activation of 5-HT2CR exerts a strong inhibitory influence on the hyperactivity induced by (+)-MDMA, and that 5-HT2CR blockade unmasks hyperactivity mediated through several mechanisms.