Repeated cocaine exposure inhibits the adrenocorticotropic hormone response to the serotonin releaser d-fenfluramine and the 5-HT1A agonist, 8-OH-DPAT

Effects of low-doses of methamphetamine on d-fenfluramine-induced head-twitch response (HTR) in mice during ageing and c-fos expression in the prefrontal cortex

BACKGROUND

The head-twitch response (HTR) in mice is considered a behavioral model for hallucinogens and serotonin 5-HT_2A receptor function, as well as Tourette syndrome in humans. It is mediated by 5-HT_2A receptor agonists such as ( ±)- 2,5-dimethoxy-4-iodoamphetamine (DOI) in the prefrontal cortex (PFC). The 5-HT_2A antagonist EMD 281014, can prevent both DOI-induced HTR during ageing and c-fos expression in different regions of PFC. Moreover, the nonselective monoamine releaser methamphetamine (MA) suppressed DOI-induced HTR through ageing via concomitant activation of inhibitory 5-HT_1A receptors, but enhanced DOI-evoked c-fos expression. d-Fenfluramine is a selective 5-HT releaser and induces HTR in mice, whereas MA does not. Currently, we investigated whether EMD 281014 or MA would alter: (1) d-fenfluramine-induced HTR frequency in 20-, 30- and 60-day old mice, (2) d-fenfluramine-evoked c-fos expression in PFC, and (3) whether blockade of inhibitory serotonergic 5-HT_1A- or adrenergic ɑ₂-receptors would prevent suppressive effect of MA on d-fenfluramine-induced HTR.

RESULTS

EMD 281014 (0.001-0.05 mg/kg) or MA (0.1-5 mg/kg) blocked d-fenfluramine-induced HTR dose-dependently during ageing. The 5-HT_1A antagonist WAY 100635 countered the inhibitory effect of MA on d-fenfluramine-induced HTR in 30-day old mice, whereas the adrenergic ɑ₂ antagonist RS 79948 reversed MA's inhibitory effect in both 20- and 30- day old mice. d-Fenfluramine significantly increased c-fos expressions in PFC regions. MA (1 mg/kg) pretreatment significantly increased d-fenfluramine-evoked c-fos expression in different regions of PFC. EMD 281014 (0.05 mg/kg) failed to prevent d-fenfluramine-induced c-fos expression, but significantly increased it in one PFC region (PrL at - 2.68 mm).

CONCLUSION

EMD 281014 suppressed d-fenfluramine-induced HTR but failed to prevent d-fenfluramine-evoked c-fos expression which suggest involvement of additional serotonergic receptors in the mediation of evoked c-fos. The suppressive effect of MA on d-fenfluramine-evoked HTR is due to well-recognized functional interactions between stimulatory 5-HT_2A- and the inhibitory 5-HT_1A- and ɑ₂-receptors. MA-evoked increases in c-fos expression in PFC regions are due to the activation of diverse monoaminergic receptors through increased synaptic concentrations of 5-HT, NE and/or DA, which may also account for the additive effect of MA on d-fenfluramine-evoked changes in c-fos expression. Our findings suggest potential drug receptor functional interaction during development when used in combination.

Serotonin at the nexus of impulsivity and cue reactivity in cocaine addiction

Cocaine abuse and addiction remain great challenges on the public health agendas in the U.S. and the world. Increasingly sophisticated perspectives on addiction to cocaine and other drugs of abuse have evolved with concerted research efforts over the last 30 years. Relapse remains a particularly powerful clinical problem as, even upon termination of drug use and initiation of abstinence, the recidivism rates can be very high. The cycling course of cocaine intake, abstinence and relapse is tied to a multitude of behavioral and cognitive processes including impulsivity (a predisposition toward rapid unplanned reactions to stimuli without regard to the negative consequences), and cocaine cue reactivity (responsivity to cocaine-associated stimuli) cited as two key phenotypes that contribute to relapse vulnerability even years into recovery. Preclinical studies suggest that serotonin (5-hydroxytryptamine; 5-HT) neurotransmission in key neural circuits may contribute to these interlocked phenotypes well as the altered neurobiological states evoked by cocaine that precipitate relapse events. As such, 5-HT is an important target in the quest to understand the neurobiology of relapse-predictive phenotypes, to successfully treat this complex disorder and improve diagnostic and prognostic capabilities. This review emphasizes the role of 5-HT and its receptor proteins in key addiction phenotypes and the implications of current findings to the future of therapeutics in addiction. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Stimulation of medial prefrontal cortex serotonin 2C (5-HT(2C)) receptors attenuates cocaine-seeking behavior

Serotonin 2C receptor (5-HT(2C)R) agonists administered systemically attenuate both cocaine-primed and cue-elicited reinstatement of extinguished cocaine-seeking behavior. To further elucidate the function of these receptors in addiction-like processes, this study examined the effects of microinfusing the 5-HT(2C)R agonist MK212 (0, 10, 30, 100 ng/side/0.2 microl) into the medial prefrontal cortex (mPFC) on cocaine self-administration and reinstatement of extinguished cocaine-seeking behavior. Male Sprague-Dawley rats were trained to self-administer cocaine (0.75 mg/kg, i.v.) paired with light and tone cues. Once responding stabilized, rats received MK212 microinfusions before tests for maintenance of cocaine self-administration. Next, extinction training to reduce cocaine-seeking behavior, defined as responses performed without cocaine reinforcement available, occurred until low extinction baselines were achieved. Rats then received MK212 microinfusions before tests for reinstatement of extinguished cocaine-seeking behavior elicited by cocaine-priming injections (10 mg/kg, i.p.) or response-contingent presentations of the cocaine-associated cues; operant responses during cocaine-primed reinstatement tests produced no consequences. MK212 microinfusions into the prelimbic and infralimbic, but not anterior cingulate, regions of the mPFC dose-dependently attenuated both cocaine-primed and cue-elicited reinstatement of extinguished cocaine-seeking behavior, but did not reliably affect cocaine self-administration. A subsequent experiment showed that the effects of MK212 (100 ng/side/0.2 microl) on reinstatement of extinguished cocaine-seeking behavior were blocked by co-administration of the 5-HT(2C)R antagonist SB242084 (200 ng/side/0.2 microl). MK212 administered alone into the mPFC as a drug prime produced no discernable effects on cocaine-seeking behavior. These findings suggest that stimulation of 5-HT(2C)Rs in the mPFC attenuates the incentive motivational effects produced by sampling cocaine or exposure to drug-paired cues.

Behavioral evidence for the significance of serotoninergic (5-HT) receptors in cocaine addiction

Cocaine addiction has somatic, psychological, psychiatric, socio-economic and legal implications in the developed world. Presently, there is no medication approved for the treatment of cocaine addiction. In recent years, data from the literature (pre-clinical studies and clinical trials) have provided several lines of evidence that serotonin (5-HT) and 5-HT receptors play a modulatory role in the mechanisms of action of cocaine. Here we review the contribution of 5-HT receptor subtypes to cocaine sensitization, discrimination, conditioned place preference, self-administration, reinstatement of seeking behavior and withdrawal symptoms in laboratory animals. Additionally, the consequences of chronic cocaine exposure on particular 5-HT receptor-assigned functions in pre-clinical studies are presented.

Dual dopamine/serotonin releasers: potential treatment agents for stimulant addiction

"Agonist therapy" for cocaine and methamphetamine addiction involves administration of stimulant-like medications (e.g., monoamine releasers) to reduce withdrawal symptoms and prevent relapse. A significant problem with this strategy is that many candidate medications possess abuse liability because of activation of mesolimbic dopamine (DA) neurons in the brain. One way to reduce DA-mediated abuse liability of candidate drugs is to add in serotonin (5-HT) releasing properties, since substantial evidence shows that 5-HT neurons provide an inhibitory influence over mesolimbic DA neurons. This article addresses several key issues related to the development of dual DA/5-HT releasers for the treatment of substance use disorders. First, the authors briefly summarize the evidence supporting a dual deficit in DA and 5-HT function during withdrawal from chronic cocaine or alcohol abuse. Second, the authors discuss data demonstrating that 5HT release can dampen DA-mediated stimulant effects, and the "antistimulant" role of 5-HT-sub(2C) receptors is considered. Next, the mechanisms underlying potential adverse effects of 5-HT releasers are described. Finally, the authors discuss recently published data with PAL-287, a novel nonamphetamine DA/5-HT releasing agent that suppresses cocaine self-administration but lacks positive reinforcing properties. It is concluded that DA/5-HT releasers could be useful therapeutic adjuncts for the treatment of cocaine and alcohol addictions, as well as for obesity, attention-deficit disorder, and depression.

Serotonin and psychostimulant addiction: Focus on 5-HT1A-receptors

Serotonin(1A)-receptors (5-HT(1A)-Rs) are important components of the 5-HT system in the brain. As somatodendritic autoreceptors they control the activity of 5-HT neurons, and, as postsynaptic receptors, the activity in terminal areas. Cocaine (COC), amphetamine (AMPH), methamphetamine (METH) and 3,4-methylenedioxymethamphetamine ("Ecstasy", MDMA) are psychostimulant drugs that can lead to addiction-related behavior in humans and in animals. At the neurochemical level, these psychostimulant drugs interact with monoamine transporters and increase extracellular 5-HT, dopamine and noradrenalin activity in the brain. The increase in 5-HT, which, in addition to dopamine, is a core mechanism of action for drug addiction, hyperactivates 5-HT(1A)-Rs. Here, we first review the role of the various 5-HT(1A)-R populations in spontaneous behavior to provide a background to elucidate the contribution of the 5-HT(1A)-Rs to the organization of psychostimulant-induced addiction behavior. The progress achieved in this field shows the fundamental contribution of brain 5-HT(1A)-Rs to virtually all behaviors associated with psychostimulant addiction. Importantly, the contribution of pre- and postsynaptic 5-HT(1A)-Rs can be dissociated and frequently act in opposite directions. We conclude that 5-HT(1A)-autoreceptors mainly facilitate psychostimulant addiction-related behaviors by a limitation of the 5-HT response in terminal areas. Postsynaptic 5-HT(1A)-Rs, in contrast, predominantly inhibit the expression of various addiction-related behaviors directly. In addition, they may also influence the local 5-HT response by feedback mechanisms. The reviewed findings do not only show a crucial role of 5-HT(1A)-Rs in the control of brain 5-HT activity and spontaneous behavior, but also their complex role in the regulation of the psychostimulant-induced 5-HT response and subsequent addiction-related behaviors.

Dual dopamine-5-HT releasers: potential treatment agents for cocaine addiction

Biogenic amine transporters (BATs) are integral membrane proteins that translocate biogenic amine neurotransmitters [norepinephrine, dopamine (DA) and 5-hydroxytryptamine (5-HT)] across cell membranes. BATs are the principal sites of action for many psychotropic drugs, including abused stimulants such as cocaine and methamphetamine. Preclinical and human data demonstrate that withdrawal from long-term cocaine administration produces a dual deficit of synaptic DA and 5-HT in the brain, indicating the advantage of developing medications that normalize impairments in both neurotransmitter systems. In this article, we review data supporting the notion that stimulant effects normally produced by increased levels of extracellular DA can be antagonized by concurrent increases in levels of extracellular 5-HT. Accordingly, nonselective BAT substrates that can release both DA and 5-HT, such as the novel compound PAL287, have low abuse potential while maintaining the ability to suppress drug-seeking behavior. The collective findings indicate that such drugs will provide neurochemical normalization therapy for cocaine addiction and might also be useful for treating depression, obsessive-compulsive disorder, attention deficit disorder and obesity.

Estrogen treatment increases the levels of regulator of G protein signaling-Z1 in the hypothalamic paraventricular nucleus: possible role in desensitization of 5-hydroxytryptamine1A receptors

Desensitization of post-synaptic serotonin1A (5-HT1A) receptors may underlie the clinical improvement of neuropsychiatric disorders. In the hypothalamic paraventricular nucleus, Galphaz proteins mediate the 5-HT1A receptor-stimulated increases in hormone release. Regulator of G protein signaling-Z1 (RGSZ1) is a GTPase-activating protein selective for Galphaz proteins. RGSZ1 regulates the duration of interaction between Galphaz proteins and effector systems. The present investigation determined the levels of RGSZ1 in the hypothalamic paraventricular nucleus of rats subjected to four different treatment protocols that produce desensitization of 5-HT1A receptors. These protocols include: daily administration of beta estradiol 3-benzoate (estradiol) for 2 days; daily administration of fluoxetine for 3 and 14 days; daily administration of cocaine for 7 or 14 days; and acute administration of (+/-)-1-(2,5 dimethoxy-4-iodophenyl)-2-amino-propane HCl (DOI; a 5-HT2A/2C receptor agonist). Estradiol treatment was the only protocol that increased the levels of RGSZ1 protein in the hypothalamic paraventricular nucleus in a dose-dependent manner (46%-132% over control). Interestingly, previous experiments indicate that only estradiol produces a decreased Emax of 5-HT1A receptor-stimulation of hormone release, whereas fluoxetine, cocaine and DOI produce a shift to the right (increased ED50). Thus, the desensitization of 5-HT1A receptors by estradiol might be attributable to increased levels of RGSZ1 protein. These findings may provide insight into the adaptation of 5-HT1A receptor signaling during pharmacotherapies of mood disorders in women and the well-established gender differences in the vulnerability to depression.

Short-Term Cocaine Treatment Causes Neuroadaptive Changes in Gαq and Gα11 Proteins in Rats Undergoing Withdrawal

One of the characteristics of drug dependence is that a drug has to be administered repeatedly before withdrawal effects can be observed. We have previously shown that withdrawal after 14 days of cocaine treatment produces a supersensitivity of hypothalamic 5-hydroxytryptamine (serotonin) 2A (5-HT(2A)) receptors, which is accompanied by increases in the levels of Galpha(q) and Galpha(11) proteins. Unfortunately, the exact duration of cocaine treatment necessary to induce alterations in G protein levels during cocaine withdrawal is unknown. The present study investigated the minimum cocaine treatment period required to produce changes in protein levels of membrane- and cytosol-associated Galpha(q) and Galpha(11) proteins in the hypothalamic paraventricular nucleus, amygdala, and frontal cortex. Rats were injected with cocaine (15 mg/kg i.p., b.i.d.) for 0, 1, 3, 5, and 7 days and tested after 2 days of withdrawal. The levels of Galpha(q) and Galpha(11) proteins were increased in the paraventricular nucleus and the amygdala but not in the frontal cortex. Although 1 and 3 days of cocaine treatment were sufficient to maximally elevate the protein levels of Galpha(11) and Galpha(q) proteins in the amygdala, 5 days of treatment were required to maximally increase the levels of Galpha(11) and Galpha(q) proteins in the paraventricular nucleus. The data suggest that the amygdala shows a faster neuroadaptation to the effects of cocaine than the hypothalamic paraventricular nucleus. These findings provide insight into the relative importance of individual components of 5-HT(2A) receptor signal transduction system in regulating the overall sensitivity of this signaling in cocaine-treated rats.

Blunted fenfluramine-evoked prolactin secretion in hypertensive rats

Plasma prolactin (PRL) levels after acute administration of fenfluramine (FEN) have been used as a probe of brain serotonin activity. FEN-evoked increases in PRL levels inversely correlate with arterial blood pressure (ABP) in humans (Muldoon et al. Hypertension. 1998;32:972-975), thereby suggesting that brain serotonin activity may be reduced in hypertension. The present study sought to determine whether the relation between FEN-evoked PRL levels and ABP was present in two rat models of hypertension. Experiments were performed in awake male rats that were instrumented with femoral arterial and venous catheters 2 days before experiments. FEN (3.0 mg/kg IV) significantly increased plasma PRL levels in both spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY); however, FEN-evoked PRL levels were significantly lower in SHR compared with WKY, though baseline levels were similar between strains. Similar results were obtained in rats with chronic hypertension produced by figure-8 renal wrap plus contralateral nephrectomy. In contrast, the increase in PRL levels evoked by the serotonin receptor agonist m-CPP or the dopamine receptor antagonist eticlopride did not differ between SHR and WKY, indicating that PRL secretion is not generally blunted in chronic hypertensive rats. Furthermore, FEN-evoked PRL levels were not attenuated in rats made acutely hypertensive by an infusion of the alpha-adrenergic agonist phenylephrine. Thus, the present findings are consistent with the human data and suggest that chronic hypertension is associated with a presynaptic alteration in brain serotonin function.

Evidence that 5-HT2A receptors in the hypothalamic paraventricular nucleus mediate neuroendocrine responses to (-)DOI

The present study determined whether the serotonin2A (5-HT2A) receptors in the hypothalamic paraventricular nucleus mediate the neuroendocrine responses to a peripheral injection of the 5-HT2A/2C receptor agonist (-)DOI [(-)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane]. The 5-HT2A receptor antagonist MDL100,907 ((+/-)-alpha(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidinemethanol), the 5-HT2C receptor antagonist SB-242084 (6-chloro-5-methyl-1-[[2-[(2-methyl-3-pyridyl)oxy]-5-pyridyl]carbamoyl]-indoline), or vehicle were microinjected bilaterally through a chronically implanted double-barreled cannula into the hypothalamic paraventricular nucleus 15 min before a peripheral injection of (-)DOI in conscious rats. (-)DOI significantly elevated plasma levels of oxytocin, prolactin, ACTH, corticosterone, and renin. Neither the 5-HT2A receptor antagonist nor the 5-HT2C receptor antagonist, injected alone, altered the basal levels of these hormones. MDL100,907 (0.748, 7.48, and 18.7 nmol) dose dependently inhibited the (-)DOI-induced increase in all of the hormones except corticosterone. In contrast, SB-242084 (10 nmol) did not inhibit (-)DOI-increased hormone levels. To confirm the presence of 5-HT2A receptors in the hypothalamic paraventricular nucleus, 5-HT2A receptors were mapped using immunohistochemistry. Densely labeled magnocellular neurons were observed throughout the anterior and posterior magnocellular subdivisions of the hypothalamic paraventricular nucleus. Moderately to densely labeled cells were also observed in parvicellular regions. Thus, it is likely that 5-HT2A receptors are present on neuroendocrine cells in the hypothalamic paraventricular nucleus. These data provide the first direct evidence that neuroendocrine responses to a peripheral injection of (-)DOI are predominantly mediated by activation of 5-HT2A receptors in the hypothalamic paraventricular nucleus.

Neuroendocrine effects of d-fenfluramine and bromocriptine following repeated smoked cocaine in humans

To study the consequences of repeated smoked cocaine use on central serotonergic and dopaminergic function, the effects of d-fenfluramine (d-FEN) and bromocriptine on plasma hormones were determined at three time-points following repeated cocaine self-administration under carefully controlled conditions. In a 20-day inpatient study, male cocaine abusers (d-FEN: n=10; bromocriptine: n=8) self-administered smoked cocaine (12-50 mg) for 3 days followed by 2 weeks of abstinence. The acute effects of d-FEN (0 or 30 mg po) or bromocriptine (0 or 1.25 mg po) on plasma neuroendocrine levels were determined 1-2, 7-8, and 13-14 days after the last cocaine dose. Blood was drawn before and then every 30-60 min for 4 h after capsule administration. The effects of d-FEN and bromocriptine were also determined in healthy, outpatient controls; d-FEN was removed from medical use in the US midway through the study due to complications associated with chronic administration, so all of the control participants were tested in Italy. Cocaine users had a blunted prolactin and cortisol response to d-FEN that lasted for at least 2 weeks of cocaine abstinence, but had a normal response to bromocriptine, which suppressed prolactin by 50% of baseline. The long-lasting and selective disruptions in serotonin pathways following chronic cocaine use may provide a neurochemical basis for changes in mood commonly reported during cocaine withdrawal.

Early postnatal cocaine exposure causes sequential, dose-dependent, enduring but reversible supersensitivity in 5-HT2A receptor-mediated function during development in male mice

This report investigated whether postnatal exposure to cocaine affects the index of 5-HT2A receptor function during development by utilizing the ability of the 5-HT2A/C agonist DOI to induce the head-twitch response (HTR) in mice. Thus, several groups of mice litters were treated with varying doses of cocaine (0, 0.5, 1, 5, and 10 mg/kg, IP) twice daily from postnatal days 5 to 14. Then, different groups of cocaine-exposed male mice pups along with their corresponding age-matched vehicle-exposed control groups were HTR tested once during development on the following postnatal test days: 15, 16, 18, 20, 30, 45, and 60. The HTR testing involved administration of DOI (0.5 mg/kg, IP) and counting the frequency of the behavior for the next 20 min. Cocaine exposure caused bell-shaped, dose-dependent, enduring but reversible increase in DOI-induced HTR frequency (mean +/- SEM) during development. The developing pups were most sensitive to low and intermediate doses of cocaine (0.5-5 mg/kg). The greatest degree of increase in HTR frequency in response to DOI challenge occurred in the 1 mg/kg cocaine-exposure group on most test days. The onset of HTR supersensitivity varied from 48 h (5 mg/kg) to 144 h (0.5 mg/kg) following the termination of chronic cocaine exposure. Moreover, maximal supersensitivity for the latter doses of cocaine occurred 96 and 384 h postcocaine treatment, respectively. Other cocaine exposure groups attained their maxima sometime between the latter time periods. The duration of persistence of 5-HT2A receptor supersensitivity varied with different doses of cocaine: the 10-mg/kg group was supersensitive up to 384 h postcocaine treatment, the 1- and 5-mg/kg groups up to 744 h; and the 0.5-mg/kg group up to 1104 h. Although developmentally cocaine-exposed pups exhibit some similarities (i.e., exquisite sensitivity and bell-shaped dose-response) in 5-HT2A receptor adaptation to mature adult mice exposed to cocaine, they also differ from mature adult cocaine-exposed mice in the onset of appearance as well as the enduring persistence of the induced supersensitivity.

Serotonergic function in cocaine addicts: prolactin responses to sequential D,L-fenfluramine challenges

BACKGROUND

Preclinical studies have shown that cocaine produces alterations in serotonergic function but our knowledge of the serotonergic alterations due to cocaine abuse in humans is still fragmentary. We therefore assessed the central serotonergic responsivity of cocaine addicts and control subjects by neuroendocrine challenges with the serotonin releaser and reuptake inhibitor D,L-fenfluramine (FEN).

METHODS

Plasma prolactin levels following a 60 mg oral dose of FEN and placebo were studied in 25 hospitalized male cocaine addicts and 13 healthy male subjects. Control subjects underwent one set of FEN/placebo challenges and cocaine addicts two sets of challenges, during the first and third weeks following cocaine discontinuation. Patients were divided into two subgroups as a function of presence (FH+) and absence (FH-) of a paternal history of substance abuse. The following comparisons were made: 1) Control subjects versus entire patient group and versus patient subgroups; 2) entire patient group and patient subgroups responses to first versus second challenges; 3) FH+ versus FH- patients' early responses and FH+ versus FH- patients' late responses.

RESULTS

The prolactin responses to FEN increased significantly in the entire patient group as time following cocaine discontinuation increased. The FH+ patients had significantly blunted early responses by comparison with FH- patients and control subjects. There was a more pronounced rebound of the responses of FH+ patients by comparison with those of FH- patients. As a result, comparisons of the late responses of FH+ and FH- patients and of FH+ patients and control subjects became nonsignificant.

CONCLUSIONS

Cocaine use appears to have an effect on the serotonergic mechanisms mediating prolactin release in humans. In the present study, this effect was more pronounced in a subgroup of patients with a paternal history of alcoholism or drug abuse. The greater blunting of the prolactin response observed within days of cocaine discontinuation followed by a greater rebound of this response 2 weeks later could indicate an increased vulnerability to the disruptive effects of cocaine in these patients.

D-Fenfluramine induces serotonin-mediated Fos expression in corticotropin-releasing factor and oxytocin neurons of the hypothalamus, and serotonin-independent Fos expression in enkephalin and neurotensin neurons of the amygdala

The neurotransmitters expressed by neurons activated by D-fenfluramine (5 mg/kg, i.p.) were identified in the hypothalamus, amygdala and bed nucleus of the stria terminalis. Induction of Fos immunoreactivity following D-fenfluramine injection was used as an index of neuronal activation. To test whether D-fenfluramine activated neurons by releasing serotonin from the serotonergic nerve terminals, rats were pretreated with fluoxetine (10 mg/kg, i.p.), a serotonin reuptake inhibitor that prevents the release of serotonin stimulated by D-fenfluramine, 12 h before D-fenfluramine injection. The approximate percentages of peptidergic neurons that contained Fos immunoreactivity after D-fenfluramine administration were 94% of corticotropin-releasing factor and 22% of oxytocin cells in the paraventricular nucleus of the hypothalamus, 6% of oxytocin cells in the supraoptic nucleus of the hypothalamus, 36% of enkephalin and 15% of neurotensin cells in the central amygdaloid nucleus, and 19% of enkephalin and 9% of neurotensin cells in the bed nucleus of the stria terminalis. Fluoxetine pretreatment blocked Fos expression in corticotropin-releasing factor- and oxytocin-expressing cells in the hypothalamus, but not in enkephalin-and neurotensin-expressing cells located in the bed nucleus of the stria terminalis and central amygdaloid nucleus. D-Fenfluramine did not induce Fos immunoreactivity in vasopressin-, thyrotropin-releasing hormone-, somatostatin- and tyrosine hydroxylase-containing cells in the hypothalamus, and corticotropin-releasing factor-expressing cells in the central amygdaloid nucleus and bed nucleus of the stria terminalis. These results show that D-fenfluramine stimulates corticotropin-releasing factor- and oxytocin-expressing cells in the hypothalamus via serotonin release. The enkephalin- and neurotensin-expressing cells in the amygdala are activated by D-fenfluramine via non-serotonergic mechanisms. Induction of Fos expression by D-fenfluramine in restricted populations of cells suggests a selective activation of neuronal circuitry that is likely to be involved in the appetite suppressant effects of D-fenfluramine.

Functional consequences of central serotonin depletion produced by repeated fenfluramine administration in rats

Repeated administration of D,L-fenfluramine (FEN) is known to cause prolonged depletion of forebrain serotonin (5-HT) in animals. Ironically, few studies have evaluated functional consequences of such FEN-induced 5-HT loss. In the present work, we examined neuroendocrine and behavioral responses evoked by acute FEN injection in rats that had previously received a 4 d FEN-dosing regimen known to deplete forebrain 5-HT (D,L-FEN, 20 mg/kg, s.c., b. i.d.). Rats were fitted with indwelling jugular catheters before the study to allow for repeated intravenous challenge injections and stress-free blood sampling. At 1 and 2 weeks after the 4 d dosing regimen, acute FEN (1.5 or 3.0 mg/kg, i.v.) produced dose-related elevations in plasma corticosterone and prolactin; these hormonal responses were markedly attenuated in FEN-pretreated rats. Behavioral effects of acute FEN, namely flat body posture and forepaw treading, were also blunted in FEN-pretreated rats. Interestingly, rats exposed to repeated FEN did not display overt abnormalities in hormonal or behavioral parameters under basal (i.e., unprovoked) conditions, despite dramatic decreases in postmortem tissue levels of 5-HT in numerous brain areas. Our results suggest that FEN-induced 5-HT depletion is accompanied by multiple impairments in 5-HT function. Although the clinical relevance of our data are debatable, the findings clearly show the utility of the FEN challenge test for uncovering in vivo functional deficits that might otherwise go undetected. FEN should remain an important pharmacological tool for determining the role of 5-HT neurons in mediating diverse physiological and behavioral processes.

Downregulation of 5-HT1A receptors in rat hypothalamus and dentate gyrus after "binge" pattern cocaine administration

The effect of chronic cocaine exposure on the central serotonergic system in the rat was investigated using a selective 5-HT1A receptor agonist, [3H]8-hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT), and a 5-HT2A receptor antagonist, [3H]ketanserin, as tritiated ligands in a quantitative autoradiography study. Rats were administered cocaine in a "binge" pattern, 15 mg/kg/injection, three times a day, at 1-h intervals for 14 days to mimic the pattern often seen in human cocaine addicts. A significant decrease in the binding of [3H]8-OH-DPAT was found in the ventromedial hypothalamus (P < 0.001) and the dorsal dentate gyrus (P < 0.01) in rats administered cocaine as compared with rats injected with saline. No significant difference in the binding of [3H]ketanserin was found in frontal, parietal, agranular insular, and piriform cortices, caudate-putamen, olfactory tubercle, nucleus accumbens, thalamus, septohippocampal nucleus, and claustrum. Several studies have shown that 5-HT1A receptor agonists have antidepressant properties. Other studies, in animal models, have shown that 5-HT1A receptor agonists stimulate the hypothalamic-pituitary-adrenal axis, which is of interest, since chronic activation of this axis has been related to anxiety and depression. Our data show that the 5-HT1A component of the serotonergic system is altered following chronic "binge" pattern cocaine administration in an animal model and may be related to changes in the HPA axis and behavior.

Alterations in serotonergic responsiveness during cocaine withdrawal in rats: similarities to major depression in humans

BACKGROUND

Withdrawal from long-term cocaine use is accompanied by symptoms resembling major depression. Because acute cocaine affects serotonin (5-HT) neurons, and 5-HT dysfunction is implicated in the pathophysiology of depression, we evaluated the effects to 5-HT agonists in rats withdrawn from repeated injections of cocaine (15 mg/kg i.p., b.i.d., 7 days) or saline.

METHODS

In the first study, prolactin (PRL) responses elicited by the 5-HT-releasing agent fenfluramine, the 5-HT1A agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), and the 5-HT2A/2C agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI) were examined as indices of postsynaptic 5-HT receptor function. In a second study, specific responses induced by 8-OH-DPAT, namely inhibition of brain 5-HT synthesis and stimulation of feeding, were examined as correlates of 5-HT1A autoreceptor function.

RESULTS

Prior treatment with cocaine did not modify fenfluramine-evoked PRL release; however, the PRL secretory response to 8-OH-DPAT was blunted and the PRL response to DOI was potentiated after chronic cocaine treatment. Cocaine exposure did not alter the inhibitory effect of 8-OH-DPAT on 5-HT synthesis. 8-OH-DPAT-induced feeding was influenced by prior cocaine, but this effect was secondary to pronounced baseline hyperphagia in the cocaine-treated group.

CONCLUSIONS

These data indicate that withdrawal from chronic cocaine renders specific subpopulations of postsynaptic 5-HT1A receptors subsensitive and 5-HT2A/2C receptors supersensitive. No evidence for cocaine-induced changes in 5-HT1A autoreceptor responsiveness was found. A survey of the literature reveals similarities in the profile of 5-HT dysfunction between rats withdrawn from cocaine and humans diagnosed with depression. We propose that withdrawal from chronic cocaine in rats may serve as a useful animal model of depressive disorders.

The 5-HT1A and 5-HT2A/2C receptor antagonists WAY-100635 and ritanserin do not attenuate D-fenfluramine-induced fos expression in the brain

D-Fenfluramine is a serotonin (5-hydroxytryptamine, 5-HT) releaser and reuptake inhibitor. It is used to study the neurochemical control of feeding and has been used to treat obesity. It has also been employed as a pharmacological tool to study changes in serotonergic function in psychiatric patients. Brain sites activated by D-fenfluramine via the release of 5-HT have been mapped via the expression of the immediate early gene c-fos. Studies in our laboratory have indicated that D-fenfluramine induces Fos in the hypothalamus and cortex through 5-HT release. The present study investigated whether 5-HT released by D-fenfluramine induces Fos expression in the brain by activating 5-HT1A or 5-HT2A/2C receptors. Rats were pretreated either with WAY-100635, a 5-HT1A antagonist, or ritanserin, a 5-HT2A/2C antagonist, prior to d-fenfluramine injection. Blockade of either 5-HT1A or 5-HT2A/2C receptors was not sufficient to suppress the Fos response to D-fenfluramine in any region of the brain examined, including the cingulate cortex, frontal cortex, caudate-putamen, paraventricular nucleus of the hypothalamus, amygdala, and brainstem. These results indicate that Fos response elicited by D-fenfluramine may be mediated by other receptors, in addition to the 5-HT1A or 5-HT2A/2C receptors.

Prolonged deficits in presynaptic serotonin function following withdrawal from chronic cocaine exposure as revealed by 5-HTP-induced head-twitch response in mice

Recent in vivo microdialysis studies have indicated that presynaptic deficits occur in brain 5-HT neurochemistry during cocaine withdrawal. The purpose of the present study was to utilize the head-twitch response (HTR) produced by 5-hydroxytryptophan (5-HTP) to investigate the dose- and time-response effects of this deficit. The HTR is considered to be a sensitive model for activation of central postsynaptic 5-HT2A receptors in rodents. Thus, different groups of mice were injected with cocaine twice daily (0, 0.1, 0.5, 2.5, 5 or 10 mg/kg, i.p.) for 7 or 13 days. During HTR testing, at 24 h following last injection, the treated mice received either 1) no cocaine; 2) their corresponding daily dose as challenge injection; or 3) a 10 mg/kg challenge dose. In a second series of experiments, extended abstinence studies were performed under the conditions of experimental protocols 1 and 2 for both 7- and 13-day cocaine (0, 0.5 and 5 mg/kg, twice daily) exposure regimens at 24, 48, 72 and 96 h following last cocaine injection. In protocol 3, the effects of a 10 mg/kg challenge dose of cocaine were studied following prolonged withdrawal from chronic cocaine exposure (0, 0.5, 5 and 10 mg/kg, twice daily for 7 and 13 days) at 24, 96 and 240 h abstinence. In experimental protocol 1 at 24 h abstinence in the 7 day exposure group, only lower doses of cocaine (0.5-2.5 mg/kg) significantly attenuated the 5-HTP-induced HTR. The deficit in 0.5 mg/kg group persisted up to 72 h abstinence. Although in the 13 day cocaine exposure groups (experimental paradigm 1) mean HTRs were generally reduced, they however failed to attain statistical significance throughout the 96 h abstinence. In protocol 2 very low challenge doses of cocaine (0.1-0.5 mg/kg) in their corresponding pretreatment groups significantly reduced the behavior at diverse abstinence intervals in both 7- and 13-day exposure regimens relative to their chronically vehicle-treated controls which had received a vehicle challenge injection during HTR testing. Unlike small doses of cocaine, larger challenge doses (5-10 mg/kg) of the stimulant potentiated the HTR score at various abstinence periods. However, the degree of the potentiations are considerably less than the ability of acute cocaine administration in enhancing the 5-HTP-induced HTR. The 10 mg/kg challenge injection in experimental protocol 3 at 24 h abstinence in the 7-day exposed mice attenuated the 5-HTP-induced HTR in 0.5, 5 and 10 mg/kg cocaine-treated groups relative to their chronic vehicle-treated controls receiving a 10 mg/kg challenge cocaine injection. The deficit in chronic 10 mg/kg cocaine-exposed mice persisted up to 240 h postcocaine abstinence. On the other hand, in the 13-day regimen, the challenge 10 mg/kg dose exhibited significant potentiations at 24 h and at 96 h for 5 and 0.5 mg/kg chronic cocaine doses respectively, but it also produced significant deficits in 0.5 and 10 mg/kg chronic doses of cocaine at 240 h abstinence. Overall, the present results suggest that enduring deficits occur in presynaptic serotonin neurochemistry and serotonergic adaptive mechanisms are exquisitely sensitive to chronic administration of low- and high-doses of cocaine.

Preweaning cocaine administration alters the adult response to quipazine: comparison with fluoxetine

This study investigated whether exposure to cocaine during the preweaning period affects the behavioral response to administration of a challenge dose of quipazine, a relatively nonselective serotonin (5-HT) mixed agonist/antagonist, in adulthood. To determine whether selective inhibition of the 5-HT transporter during the preweaning period would produce a cocaine-like pattern of effects, another group of rats was given fluoxetine, a highly selective and potent inhibitor of the 5-HT transporter, and was tested along with the cocaine-treated rats. Male and female rats received 25 mg/kg cocaine HCl (82.5 mumol/kg), 25 mg/kg fluoxetine HCl (72.3 mumol/kg), or vehicle subcutaneous (s.c.) during postnatal days 11-20. Both treatments reduced weight gain during the injection period only. At 60 days of age, subjects were administered a single dose of quipazine (0, 0.4, or 1.0 mg/kg, s.c.) and placed in the Accuscan activity monitor for 1 h of behavioral recording. Overall, distance traveled, vertical activity, and time in the center of the chamber decreased during the initial time blocks of the session and vertical activity decreased with increasing doses of quipazine. Females in general showed greater overall activity levels than males as well as greater responsivity to quipazine. Preweaning cocaine exposure produced different effects in males and females. In males, cocaine enhanced the response to quipazine for vertical activity whereas it had no effect on quipazine-induced alterations on the other two behaviors. On the other hand, cocaine-treated females showed dampened dose-related quipazine responses across all behavioral measures. Fluoxetine administration produced a dampening of the quipazine effect for vertical activity and distance traveled in males and females. Therefore, these data indicate that cocaine administration during the preweaning period of development produced an increase in the effect of a serotonergic drug to alter vertical activity in males and a global dampening of the behavioral responses to that same drug in females. Preweaning fluoxetine treatment produced effects that resembled those produced by cocaine in females, a dampening of serotonergic responsivity, along with an overall decrease in locomotor activity. Because the majority of effects are seen during the initial portion of the behavioral session, a time of heightened activity in response to a novel environment, the data suggest that inhibition of the 5-HT transporter during the preweaning period alters serotonergic influences over novelty-induced activity but that brief periods of inhibition or other actions of cocaine, such as those at the catecholamine transporters, prevent this from happening, particularly in males.

Deficits in D-fenfluramine-sensitive pool of brain 5-HT following withdrawal from chronic cocaine exposure

Recent head-twitch response (HTR) studies in mice have indicated that withdrawal from chronic cocaine exposure produces deficits in CNS conversion of L-tryptophan to 5-HT. In the present study, the ability of 5-HT releaser, d-fenfluramine, was utilized to induce the HTR in mice following abstinence from chronic cocaine exposure. d-Fenfluramine-induced HTR, is a 5-HT2A receptor-mediated phenomenon and its induction frequency can be regarded as an indirect but in vivo measure of basal brain 5-HT concentration. Thus, different groups of mice were injected with cocaine twice daily (0, 0.1, 0.5, 2.5, 5 or 10 mg/kg, i.p.) for either 7 or 13 days. At 24 h after last cocaine injection, the treated mice received d-fenfluramine (5 mg/kg, i.p.) and the induced HTR (mean+/-SEM) was recorded for the next 30 min. Cocaine attenuated the d-fenfluramine-induced HTR frequency by 30-37% in the 13-day regimen and significant effects were observed from 0.5 mg/kg dose. At 24 h withdrawal in the 7-day cocaine exposure group, the mean HTR frequencies were attenuated, however, they did not achieve statistical significance. Extended abstinence studies (i.e. 24, 48, 72 and 96 h postwithdrawal) from chronic cocaine exposure (0, 0.5 and 5 mg/kg/day for either 7 or 13 days) indicated that in the 7-day exposure group, significant reductions (26, 39 and 22%) in HTR frequency occurred at 48, 72 and 96 h following withdrawal from 0.5 mg/kg cocaine, whereas its 5 mg/kg dose failed to induce a significant effect. In the 13-day exposure group significant reductions in HTR frequency were observed at 24 h abstinence (27%) for the 0.5 mg/kg cocaine dose and at 24 and 48 h for the 5 mg/kg. Overall, these results indicate that abstinence from chronic exposure to cocaine produces enduring deficits in basal 5-HT concentration. Lastly, serotonergic function appears to be uniquely sensitive to chronic administration of low doses of cocaine.

p-Chlorophenylalanine and fluoxetine inhibit D-fenfluramine-induced Fos expression in the paraventricular nucleus, cingulate cortex and frontal cortex but not in other forebrain and brainstem regions

D-Fenfluramine, a putative serotonin releaser and reuptake inhibitor, is commonly prescribed for the treatment of obesity. Brain sites activated by D-fenfluramine have been mapped via the expression of the immediate early gene Fos. However, it is not clear that serotonin release in the brain mediates the effects of D-fenfluramine on Fos expression. The present study determined whether D-fenfluramine induces the expression of Fos in the brain through the release of serotonin. Rats were pretreated either with the serotonin depleting drug p-chlorophenylalanine (PCPA) or with the serotonin reuptake inhibitor fluoxetine. Both drugs inhibited D-fenfluramine-induced Fos expression in the cingulate cortex, frontal cortex, and the parvocellular subdivision of the paraventricular nucleus of the hypothalamus. Neither drug reduced D-fenfluramine-induced Fos responses in several other brain areas, including the caudate-putamen, amygdala, and brainstem regions such as the lateral parabrachial nucleus and nucleus of the solitary tract. These results indicate regional specificity of mechanisms mediating D-fenfluramine-induced Fos expression. It is likely that D-fenfluramine-induced Fos expression at various sites in the brain is mediated via a combination of serotonin release and other, as yet unidentified, neurotransmitters.

Withdrawal from chronic cocaine administration causes prolonged deficits in L-tryptophan-induced head-twitch response in mice

Withdrawal from chronic cocaine exposure potentiates the ability of direct 5-HT2A agonists to induce the head-twitch response (HTR) in rodents. This supersensitivity is assumed to be a consequence of cocaine-induced deficits in presynaptic serotonin neurochemistry. The present study utilized the HTR produced by L-tryptophan (TP) to investigate the dose- and time-response effects of cocaine-induced 5-HT deficit. Thus, different groups of mice were injected with cocaine twice daily (0, 0.1, 0.5, 2.5, 5 or 10 mg/kg, i.p.) for 7 or 13 days. During HTR testing procedure, at 24 h after last chronic injection, treated-mice received either: 1) no cocaine; 2) their corresponding daily dose; or 3) a 10 mg/kg dose. In paradigm 1, the frequency of TP-induced HTR was attenuated in a dose-dependent manner in both chronic cocaine regimens. In paradigm 2, small challenge doses (0.1-2.5 mg/kg) of cocaine in their respective pretreatment groups failed to alter HTR, but larger challenge doses (5 and 10 mg/kg) potentiated the behavior. In paradigm 3, the 10 mg/kg challenge dose potentiated the HTR to a similar degree in both chronically exposed vehicle and various cocaine-treated groups in both treatment regimens. Extended withdrawal studies from cocaine exposure (0, 0.5 and 5 mg/kg twice daily for 7 or 13 days) indicated attenuations in HTR persisted up to 96 h postcocaine abstinence in paradigm 1, whereas paradigm 2 revealed significant attenuations between 48-72 h for 0.5 mg/kg dose; and potentiations for the 5 mg/kg dose persisted throughout the 96 h abstinence. In paradigm 3, no significant effect was observed at 96 h abstinence, but the 10 mg/kg challenge dose significantly potentiated HTR in chronically exposed 10 mg/kg cocaine group 10 days following cocaine abstinence in both exposure regimens. Overall, these results support the notion that chronic cocaine exposure produces prolonged deficits in presynaptic serotonin neurochemistry. Furthermore, serotonergic mechanisms appear to be exquisitely sensitive to chronic administration of both low and high doses of cocaine.

Evidence for alterations in presynaptic serotonergic function during withdrawal from chronic cocaine in rats

The effects of repeated cocaine administration on serotonin (5-hydroxytryptamine, 5-HT) function were investigated by comparing the corticosterone response to 5-HT receptor agonists in cocaine-treated and vehicle-treated rats. Male rats were fitted with indwelling jugular catheters and received cocaine (15 mg/kg i.p., b.i.d.) or saline for 7 days. Rats were challenged with either saline, the 5-HT releaser fenfluramine (1.2 mg/kg i.v.), the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 50 micrograms/kg i.v.), or the 5-HT2A/2C receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI; 100 micrograms/kg i.v.) 42 h and 8 days after the final chronic treatment. Repeated blood samples were withdrawn immediately before and at 15, 30 and 60 min after acute challenge injections. All 5-HT receptor agonists increased plasma corticosterone, but the fenfluramine-induced rise in corticosterone was significantly attenuated in cocaine-treated rats withdrawn for 42 h. This blunted response to fenfluramine exhibited only partial recovery when examined at 8 days postchronic treatment. Corticosterone responses to 8-OH-DPAT and DOI were not affected by cocaine exposure. Our data suggest that chronic cocaine produces deficits in presynaptic 5-HT function, and alterations in 5-HT neurotransmission may underlie the dysphoria experienced by abstinent cocaine users. Neuroendocrine challenge tests should be performed in human addicts to evaluate potential 5-HT dysfunction associated with cocaine abuse.

Repeated cocaine administration reduces 5-HT1A-mediated prolactin secretion in rats

The prolactin and behavioral responses elicited by the 5-HT1A agonist 8-OH-DPAT (8-hydroxy-2-[di-n-propylamino]tetralin) were examined in male rats previously exposed to chronic cocaine (15 mg/kg, i.p., b.i.d., 7 days) or saline. After 42 h of withdrawal, cocaine-treated rats exhibited a reduced prolactin response to 8-OH-DPAT challenge (50 micrograms/kg, i.v.). A 5-fold higher dose of 8-OH-DPAT stimulated maximal prolactin secretion that was similar in cocaine- and saline-treated rats. Prior cocaine treatment had no effect on the 5-HT syndrome induced by 8-OH-DPAT. Our data agree with the findings of others and suggest that 5-HT1A receptors mediating neuroendocrine secretion become subsensitive after repeated cocaine administration.