Stimulus properties of the selective 5-HT reuptake inhibitor fluvoxamine in conditioned taste aversion procedures

Conditioned Taste Avoidance Drug Discrimination Procedure: Assessments and Applications

In the present chapter, we summarize much of the work on the taste avoidance drug discrimination procedure, presenting the logic for its initial introduction and the extension of the procedure in the investigation of the discriminative properties of various drugs. Results from these assessments parallel those from more traditional operant and maze designs in classifying and characterizing the discriminative properties of drug. At the same time, this design reveals a procedure that is sensitive in such assessments by indexing these stimulus properties more rapidly and at lower doses than in the more traditional procedures (in some cases for drugs heretofore resistant in their detection). Importantly, much remains to be learned about the taste avoidance procedure in that the nature of such learning remains unknown and the specific parameters under which it can be established and generalized and its neurochemical and neuroanatomical bases are largely unexplored. The application of drug discrimination learning to human drug abuse continues to be an important consideration for this specific design (as well as that of drug discrimination procedures in general), and recent parallels between drug use and food intake in terms of its regulation by interoceptive stimuli suggests a possible role of the loss of stimulus control in drug escalation and addiction (with possible therapeutic implications via the modulation of these interoceptive cues).

Dopamine mediates cocaine-induced conditioned taste aversions as demonstrated with cross-drug preexposure to GBR 12909

Although cocaine readily induces taste aversions, little is known about the mechanisms underlying this effect. It has been suggested that its inhibitory effects at one of the monoamine transporters may be mediating this suppression. Using the cross-drug preexposure preparation, the present series of studies examined a possible role of dopamine (DA) in this effect. Male Sprague-Dawley rats were exposed to cocaine (18 mg/kg; Experiment 1) or the selective DA transporter (DAT) inhibitor GBR 12909 (50 mg/kg; Experiment 2) prior to the pairing of a novel saccharin solution with injections of GBR 12909 (32 mg/kg), cocaine (18 mg/kg) or vehicle in a conditioned taste aversion (CTA) procedure. Preexposure to cocaine attenuated aversions induced by itself but not aversions induced by GBR 12909 (Experiment 1). Conversely, preexposure to GBR 12909 attenuated aversions induced by itself and cocaine (Experiment 2). This asymmetry suggests that cocaine and GBR 12909 induce CTAs via similar, but non-identical, mechanisms. These data are discussed in the context of previous work demonstrating roles for dopamine, norepinephrine and serotonin in cocaine-induced CTAs.

The effects of haloperidol on cocaine-induced conditioned taste aversions

Although the mechanism underlying the rewarding effects of cocaine has been well characterized, little is known about the mechanism underlying its aversive effects. Several reports have indicated a possible role of dopamine (DA) in the aversive effects; however, several procedural issues limit any conclusions regarding its specific role. In order to investigate a possible dopaminergic role in cocaine-induced CTAs using procedures that circumvented these possible issues, the present series of investigations assessed the aversive effects of the DA antagonist haloperidol alone (Experiment 1) and in combination with cocaine (Experiment 2). Haloperidol, at doses that were determined to be non-aversive, yet behaviorally active in a locomotor assessment, attenuated cocaine-induced taste aversions, suggesting that cocaine's aversive effects are mediated in part by dopaminergic activity. These findings were discussed in consideration with other evidence implicating DA and other neurotransmitter systems in cocaine-induced CTAs.

Regulation of dorsal raphe nucleus function by serotonin autoreceptors: a behavioral perspective

Neurotransmission by serotonin (5-HT) is tightly regulated by several autoreceptors that fine-tune serotonergic neurotransmission through negative feedback inhibition at the cell bodies (predominantly 5-HT(1A)) or at the axon terminals (predominantly 5-HT(1B)); however, more subtle roles for 5-HT(1D) and 5-HT(2B) autoreceptors have also been detected. This review provides an overview of 5-HT autoreceptors, focusing on their contribution in animal behavioral models of stress and emotion. Experiments targeting 5-HT autoreceptors in awake, behaving animals have generally shown that increasing autoreceptor feedback is anxiolytic and rewarding, while enhanced 5-HT function is aversive and anxiogenic; however, the role of serotonergic activity in behavioral models of helplessness is more complex. The prevailing model suggests that 5-HT autoreceptors become desensitized in response to stress exposure and antidepressant administration, two seemingly opposite manipulations. Thus there are still unresolved questions regarding the role of these receptors-and serotonin in general-in normal and pathological states.

Differential involvement of the norepinephrine, serotonin and dopamine reuptake transporter proteins in cocaine-induced taste aversion

Despite the impact of cocaine's aversive effects on its abuse potential, the neurochemical basis of these aversive effects remains poorly understood. By blocking the reuptake of the monoamine neurotransmitters dopamine (DA), norepinephrine (NE) and serotonin (5-HT) into the presynaptic terminal, cocaine acts as a potent indirect agonist of each of these systems. The following studies attempted to assess the extent of monoaminergic mediation of cocaine's aversive effects using conditioned taste aversion (CTA) learning [Garcia, J., Kimeldorf, D.J., Koelling, R.A., Conditioned aversion to saccharin resulting from exposure to gamma radiation. Science 1955;122:157-158.]. Specifically, Experiment 1 assessed the ability of selective monoamine transporter inhibitors, e.g., DAT (vanoxerine), NET (nisoxetine) and SERT (fluoxetine), to induce taste aversions (relative to cocaine). Only the NET inhibitor approximated the aversive strength of cocaine. Experiment 2 compared the effects of pretreatment of each of these transport inhibitors on the development of a cocaine-induced CTA. Pretreatment with nisoxetine and fluoxetine both attenuated cocaine-induced aversions in a manner comparable to that produced by cocaine itself. The DAT inhibitor was without effect. Combined, the results of these investigations indicate little or no involvement of dopaminergic systems in cocaine's aversive effects while NE appears to contribute most substantially, with a possible modulatory involvement by serotonin.

Selective and nonselective serotonin antagonists block the aversive stimulus properties of MK212 and m-chlorophenylpiperazine (mCPP) in mice

Serotonin(2C) (5-HT(2C)) receptors have been implicated to treat mood disorders such as depression and anxiety. In the present study, the capacities of two 5-HT(2C) agonists, MK212 and mCPP, to produce conditioned taste aversions in mice were evaluated. On two training days, Swiss-Webster male mice (19-34g) were trained to associate the flavor of a novel solution with the injection of various doses of MK212 or mCPP. On two alternate training days, mice were trained to associate a different flavored solution with an injection of saline. For testing, both flavored solutions were presented simultaneously and an avoidance of the MK212 or mCPP-paired solution indicated conditioned taste aversion. Robust conditioned taste aversions were observed to solutions paired with 1.0 or 10mg/kg MK212 or mCPP. Acquisition of conditioned taste aversions was blocked by nonselective serotonin antagonists cyproheptadine, bromo-LSD, metergoline, methysergide and mianserin. Selective 5-HT(2B/2C) antagonist SB206,553 blocked both MK212- and mCPP-induced conditioned taste aversion although selective 5-HT(2B/2C) antagonist SB200,646 only blocked mCPP-induced conditioned taste aversion. In a single-bottle procedure, MK212, bromo-LSD, and mianserin failed to alter acquisition rate of a LiCl-induced conditioned taste aversion. Taken together, these data indicate that the serotonin agonists MK212 and mCPP produce conditioned taste aversion and that these effects are mediated predominantly through 5-HT(2C) receptors.

Fluvoxamine-controlled release formulation for the treatment of generalized social anxiety disorder

BACKGROUND

Generalized social anxiety disorder is a highly prevalent anxiety disorder with deleterious effects on social and family relationships, as well as work performance. We report the results of a multicenter, randomized, placebo-controlled trial comparing the efficacy, safety, and tolerability of fluvoxamine controlled release (CR) to placebo in patients with generalized social anxiety disorder.

METHODS

A total of 279 adult patients meeting all inclusion/exclusion criteria was recruited at 23 United States sites and randomly assigned to receive either fluvoxamine CR (100-300 mg/d) or placebo for 12 weeks. The dose could be increased, based on efficacy and tolerability, in increments of 50 mg/d at weekly intervals. The dosage remained constant during weeks 6 to 12.

RESULTS

Treatment with fluvoxamine CR resulted in statistically and clinically significant improvements in symptoms associated with generalized social anxiety disorder as early as week 4 on the Liebowitz Social Anxiety Scale and the Clinical Global Impression Scale Global Improvement, and at week 6 on the Sheehan Disability Scale, Clinical Global Impression Scale Severity of Illness and the Patient Global Impression of Improvement Scale. The most frequent adverse events reported by patients on fluvoxamine CR were headache, nausea, somnolence, and insomnia. No weight gain was observed for either treatment group, and at end point, there were no differences between treatments on overall sexual function, as measured by the Arizona Sexual Experience Scale.

CONCLUSIONS

Both physician and patient-rated scales indicate that fluvoxamine CR is effective and safe for the treatment of generalized social anxiety disorder.

Conditioned taste aversion: modulation by 5-HT receptor activity and corticosterone

Two experiments were designed to elucidate the involvement of the hypothalamic-pituitary-adrenal axis and the 5-hydroxytryptamine (5-HT) system in the acquisition of lithium chloride-conditioned taste aversion. In Experiment 1, rats were administered either vehicle or 50 mg/kg nefazodone daily for 4 weeks. Rats were treated with 22 mg/kg of lithium chloride in order to produce conditioned taste aversion to a sucrose solution. Three days later, nefazodone completely blocked the lithium chloride-conditioned taste aversion. In Experiment 2, the effects of chronic corticosterone administration on lithium chloride-conditioned taste aversion were investigated. Twenty male rats received either corticosterone at a dose of (50 mg/kg) or vehicle injections over a period of 14 consecutive days. Lithium chloride-conditioned taste aversion was potentiated in rats treated with corticosterone. Additionally, corticosterone-treated animals required more trials to reach extinction. These results suggest the involvement of both the 5-HT system and the hypothalamic-pituitary-adrenal axis in lithium chloride-conditioned taste aversion.