Fenfluramine discrimination in obese and lean Zucker rats: serotonergic mediation of effect

The effect of N-methylation on fenfluramine's neurotoxic and pharmacologic actions

N-Methylation separates methamphetamine's neurotoxic and pharmacologic effects. In particular, N-methylation eliminates methamphetamine's neurotoxic activity while preserving its behavioral pharmacologic activity. The purpose of the present studies was to determine whether N-methylation could also be used to separate fenfluramine's neurotoxic and pharmacologic effects. Fenfluramine-induced serotonin neurotoxicity was assessed by measuring serotonin axonal markers 2 weeks after fenfluramine administration. Pharmacologic effects of fenfluramine were assessed by measuring fenfluramine-induced anorexia and fenfluramine discrimination. Both fenfluramine and its N-methylated analog, N-methylfenfluramine, produced dose-related effects in food intake, drug-discrimination and neurotoxicity studies. Although N-methylation reduced the neurotoxic potency of fenfluramine, it also reduced its pharmacologic activity. Neurotoxic potency was reduced 4- to 8-fold (depending on brain region), while pharmacologic potency was reduced 4- to 10-fold (depending on paradigm). Notably, N-methylation did not change the efficacy of fenfluramine as a serotonin neurotoxin, anorectic agent or discrimination stimulus. These results indicate that fenfluramine's behavioral and neurotoxic effects, unlike those of methamphetamine, are not dissociated by N-methylation. Further, the present results suggest that the effectiveness of side-chain nitrogen substitution in separating the behavioral and neurotoxic effects of amphetamine derivatives is strongly influenced by ring substitutions.

Monoamine systems in the discriminative effects of spiradoline, a kappa-opioid agonist

The results of studies on mice indicate that the antinociceptive effects of kappa-opioid agonists are due, in part, to activation of the 5-HT2 type of serotonin receptor. One objective of this study was to determine if the discriminative effects of spiradoline, a kappa-opioid agonist, are mediated by 5-HT2 receptors in rats also. A second objective was to confirm findings that dopamine receptor antagonists produce spiradoline-like discriminative effects (Ohno et al., 1992). Rats were trained to discriminate between spiradoline (3.0 mg/kg) and saline in a discrete-trial avoidance/escape procedure. In subsequent tests of stimulus generalization, the discriminative effects of spiradoline were not mimicked by fenfluramine (0.3-10 mg/kg) or fluoxetine (1.0-10 mg/kg), drugs that enhance serotonergically mediated neurotransmission, nor were they blocked by the 5-HT2 antagonists pirenperone (0.01-1.0 mg/kg) and ketanserin (0.1-10 mg/kg), or potentiated by fluoxetine pretreatment. Neither the dopamine receptor antagonists haloperidol (0.01-0.3 mg/kg) and sulpiride (3.0-100 mg/kg) nor the agonists apomorphine (0.03-0.3 mg/kg) and d-amphetamine (0.1-3.0 mg/kg) engendered spiradoline-like discriminative effects. These results demonstrate further the pharmacological specificity of the discriminative effects of spiradoline, but provide no evidence for mediation by serotonergic or dopaminergic systems.

Experimental obesity: a homeostatic failure due to defective nutrient stimulation of the sympathetic nervous system

The basic hypothesis of this review is that studies on models of experimental obesity can provide insight into the control systems regulating body nutrient stores in humans. In this homeostatic or feedback approach to analysis of the nutrient control system, we have examined the afferent feedback signals, the central controller, and the efferent control elements regulating the controlled system of nutrient intake, storage, and oxidation. The mechanisms involved in the beginning and ending of single meals must clearly be related to the long-term changes in fat stores, although this relationship is far from clear. Changes in total nutrient storage in adipose tissue can arise as a consequence of changes in the quantity of nutrients ingested in one form or another or a decrease in the utilization of the ingested nutrients. A change in energy intake can be effected by increased size of individual meals, increased number of meals in a 24-hour period, or a combination of these events. Similarly, a decrease in utilization of these nutrients can develop through changes in resting metabolic energy expenditure which are associated with one of more of the biological cycles such as protein metabolism, triglyceride for glycogen synthesis and breakdown, or maintenance of ionic gradients for Na+ + K+ across cell walls. In addition, differences in energy expenditure related to the thermogenesis of eating or to the level of physical activity may account for differences in nutrient utilization.

Norfenfluramine, the fenfluramine metabolite, provides stimulus control: evidence for serotonergic mediation

Nine male rats were trained to discriminate 1.4 mg/kg norfenfluramine (NF) from its vehicle using a two-lever, food-motivated, operant discrimination task. Once trained, the rats showed a dose-dependent decrease in responding on the NF-correct lever following decreased doses of NF (ED50 = 0.71 mg/kg). Administration of 2.0 mg/kg fenfluramine (FEN) produced 100% responding on the NF-correct lever and decreasing doses of FEN, likewise, produced a dose-dependent decrease in responding on the NF-correct lever (ED50 = 1.30 mg/kg). Time-course data indicated that NF has a fast onset and a peak effect at 20-60 min after administration. Analysis of the time-course data provided a half-life of approximately 8 hr. In contrast, FEN did not show the rapid onset that was observed with NF. However, NF had a similar peak effect and half-life. These results indicate a pharmacological similarity between NF and FEN. However, the difference in onset of action suggests a possible difference between the parent drug and its metabolite. The serotonergic agonists mCPP, DOI, 5-MeODMT and LSD generalized to 1.4 mg/kg NF, whereas neither TFMPP nor 8-OHDPAT generalized to NF. The dopaminergic agonist AMPH also did not generalize to NF. The implications of these findings are discussed.

Tetrahydro-beta-carboline may produce its stimulus effects via 5HT1B receptors

To further clarify the role of 5-hydroxytryptamine (5HT) in the behavioral effects of tetrahydro-beta-carboline, male rats were trained to discriminate either 20 mg/kg THBC from its vehicle (n = 10) or 2.0 mg/kg fenfluramine from saline (n = 5). THBC was observed to produce fenfluramine-like effects in the fenfluramine-trained rats while fenfluramine produced THBC-like effects in the THBC-discriminating rats. To investigate which of the serotonergic receptors may mediate the THBC-induced discriminative stimulus, various putatively specific 5HT agonists were administered to THBC-trained rats. Results indicate that the 5HT1B receptor agonists TFMPP and m-CPP substitute for THBC in a dose-response manner whereas 5HT1A agonists do not generalize to the THBC-induced discriminative stimulus. These observations support a role for the 5HT1B receptor site in the discriminative stimulus properties of THBC.