Antagonism of the behavioral effects of 2,5-dimethoxy-4-methylamphetamine (DOM) and quipazine by metergoline

Effects of serotonin agonists on operant behavior in the squirrel monkey: quipazine, MK-212, trifluoromethylphenylpiperazine, and chlorophenylpiperazine

The behavior of squirrel monkeys was studied under fixed-interval (FI) schedules with responding maintained either by food presentation or by termination of stimuli correlated with impending electric shock delivery (stimulus-shock termination). The 5-HT agonists m-trifluoromethylphenylpiperazine (TFMPP), m-chlorophenylpiperazine (mCPP), and 6-chloro-2(l-piperazinyl)pyrazine (MK-212) decreased responding under both the food and shock schedules (0.3-5.6 mg/kg). These decreases in responding were blocked by the nonselective 5-HT antagonists methysergide and mianserin (0.3, 1.0 mg/kg), but not by the selective 5-HT2 antagonists ketanserin (0.3-1.7 mg/kg) or pirenperone (0.001-0.1 mg/kg). Quipazine (0.3-5.6 mg/kg) decreased responding under the food schedule, and this effect was blocked by both the nonselective and selective 5-HT2 antagonists. This pattern of antagonism suggests that the decreases in responding produced by quipazine involve significant actions at 5-HT2 sites, whereas those produced by TFMPP, mCPP, and MK-212 do not. In contrast to the decreases in responding seen with the food schedules, quipazine produced moderate increases in responding under the shock schedules. Moreover, these increases in responding were not blocked by methysergide or mianserin, but instead were enhanced. The results with antagonists suggest that certain behavioral effects of quipazine are probably due to actions at 5-HT2 sites, whereas similar effects of TFMPP, mCPP, and MK-212 are more related to actions at other 5-HT receptor subtypes.

The 5HT2 antagonist pirenperone reverses disruption of FR-40 by hallucinogenic drugs

Indolealkylamine and phenethylamine hallucinogens disrupted responding maintained under a fixed-ratio 40 (FR-40) schedule of reinforcement. LSD, DMT, mescaline and DOM produced dose-dependent decreases in number of reinforcers and increases in 10-sec periods of non-responding (pause-intervals). The 5HT agonist quipazine, as well as the LSD congener lisuride, altered response patterns in a similar manner. The effects of these drugs were examined after pretreatment with pirenperone, an antagonist with specificity toward the 5HT2 receptor with reference to the 5HT1 receptor. The dose-response curves for the phenethylamine hallucinogens were shifted significantly to the right and to a greater degree than were those for the indolealkylamine hallucinogens. Pirenperone also antagonized the effects of quipazine to a degree similar to that observed with the phenethylamine-type hallucinogens. Pirenperone did not significantly shift the dose-response pattern to lisuride. These data suggest that the behavioral disruption induced by these hallucinogens and quipazine relates at least in part to an effect on 5HT2 receptors, while the effects of lisuride do not involve a significant interaction at the 5HT2 receptor.

Comparison of anorexia and motor disruption by cyclazocine and quipazine

The mixed narcotic agonist-antagonist cyclazocine and the 5-HT agonist quipazine disrupt food-rewarded fixed ratio-40 (FR-40) operant behavior in rats as a dose-dependent decrease in the number of reinforcers obtained and a reciprocal increase in the number of 10-second intervals between responding ("pausing"). This disruption has been shown to result in part from interaction with 5-HT neuronal systems, and may be a consequence of: (1) disruption of cognitive processes, (2) motivational impairment, or (3) motor deficits. To identify which of these components is (are) involved in the disruption of operant responding, female Sprague-Dawley rats were tested for food consumption, spontaneous locomotor activity, or rotarod performance following intraperitoneal injection of cyclazocine, quipazine, or both. Cyclazocine decreased food consumption at doses larger than those required to disrupt operant behavior, while quipazine decreased consumption at doses disruptive to operant responding. Little effect was exerted by either drug on spontaneous locomotor activity, while rotarod performance was disrupted only by very large doses of either drug relative to effects of FR-40 behavior. These data indicate that neither drug appears to disrupt operant behavior by causing gross motor deficits. Thus, cyclazocine may disrupt operant responding by impairing cognition, while quipazine may act through food satiation mechanisms.

Clozapine blocks disruptive and discriminative stimulus effects of quipazine

Clozapine was tested in two serotonin-dependent behavioral measures. One group of rats was trained to discriminate the serotonin agonist, quipazine, from saline in a two-lever operant choice task. Pretreatment with clozapine completely blocked the discrimination of quipazine. Another group of rats was trained to bar press for milk on a variable interval schedule of reinforcement. Quipazine decreased the response rate in these animals and pretreatment with clozapine completely reversed this effect. Thus, clozapine acted as a serotonin antagonist in both measures of serotonin function.

The effects of quipazine and fluoxetine on extinction of a previously-reinforced operant response in rats

Previous studies have shown that treatments that reduce serotonergic neurotransmission lead to enhanced responding during extinction. To evaluate the generality of this effect, the present study examined the effects of the serotonin agonists, quipazine and fluoxetine on responding in extinction. In Experiment 1, 72 rats were trained to lever press on a continuous reinforcement schedule for 5 30-min sessions. Four sessions of extinction followed; 30-min prior to each, 3 groups (n = 16) received quipazine (0, 1.0, 5.0 mg/kg) and 3 groups (n = 8) received fluoxetine (0, 1.0, 5.0 mg/kg). The 5.0 mg/kg dose of quipazine resulted in a significant reduction in responding on day 1; the lower dose of quipazine and both doses of fluoxetine were without significant effect. In Experiment 2, 3 similarly trained groups (n = 8) received either saline or quipazine (5.0 mg/kg) prior to each extinction session; additionally, one quipazine group was injected twice with the 5.0 mg/kg dose in its home cage several days before the beginning of extinction. The results of the drug-naive quipazine group replicated those of that group from Experiment 1 whereas the drug-experienced group showed no significant effect of quipazine in extinction. The results suggested that prior drug experience could modify the effects of quipazine on behaviour. Apart from this drug novelty effect the lack of significant effect of either quipazine or fluoxetine suggested that the effects of manipulations believed to increase and decrease serotonin functioning on responding in extinction may not be symmetrical. These results may be understood with reference to the hypothesis that serotonin plays a role in tuning out or reducing responsiveness to nonreinforced or irrelevant stimuli.

Naloxone alters the effects of LSD, DOM and quipazine on operant behavior of rats

Administration of the indolealkylamine hallucinogen d-lysergic acid diethylamide (LSD), the phenethylamine hallucinogen 2,5-dimethoxy-4-methylamphetamine (DOM) and the putative 5-hydroxytryptamine (5-HT) agonist quipazine all produced a dose-dependent decrease in fixed ratio (FR-40) response rates and a concomitant increase in the number of 10-second pause intervals. Although naloxone (4.0 mg/kg) had no effect on FR-40 responding per se, the pause-producing effects of LSD and, to a lesser extent, DOM were potentiated by pretreatment with naloxone. The action of quipazine on reinforcers was unaffected by combination with naloxone, while the effect on pause intervals was slightly attenuated by naloxone pretreatment. These data and previous studies suggest that the pause-producing effects of indolealkylamine and phenethylamine hallucinogens reflect their activation of a selective portion of brain 5-HT receptors. The potentiation of these effects by naloxone may relate to a modulation of central 5-HT systems by endogenous opioid mechanisms tending to restore an imbalance in various 5-HT pathways caused by the hallucinogenic 5-HT agonists. The more generalized disruptive effects of quipazine on brain 5-HT systems may be less susceptible to the endogenous opioid modulation or may actually combine with it to induce a greater disruption.

Nicotine cue in rats analysed with drugs acting on cholinergic and 5-hydroxytryptamine mechanisms

The nicotine discriminative stimulus (cue) has been used to characterize further the underlying receptor mechanisms. Rats were trained to discriminate the effects of nicotine in a standard, two-bar operant conditioning procedure with food reinforcement. Mecamylamine blocked both the discriminative effect of nicotine and the reducing effect on the response-rate. The block of the discriminative effect could not be overcome by increasing the dose of nicotine, whereas the block of the reducing effect on the response-rate could be reversed. Mecamylamine was effective when administered by either the subcutaneous or the intraventricular route, but hexamethonium was inactive by both routes. Mecamylamine, but not hexamethonium, blocked the discriminative effect of the nicotinic cholinergic agonist, cytisine. Methergoline did not block the discriminative effects of nicotine, even in doses considerably larger than those which blocked the discriminative effects of the 5-HT agonist, quipazine. Mecamylamine did not block the effects of quipazine. The results are consistent with the view that the nicotinic cue is mediated primarily through cholinergic receptors, and that 5-HT mechanisms are not involved. The block of the quipazine cue supports the view that the discriminative effects of this drug are mediated through 5-HT receptors.

Cyclazocine disruption of operant behavior is antagonized by naloxone and metergoline

Male Sprague-Dawley rats were trained to press a lever on a fixed ratio-40 (FR-40) schedule for food reinforcement. Doses ranging from 0.5 to 16 mg/kg of the mixed narcotic agonist-antagonist cyclazocine (30-min pretreatment) resulted in a dose-dependent decrease in the number of reinforcements obtained and a reciprocal increase in "pausing" (IRT's greater than 10 sec). A 5-min pretreatment with 4 mg/kg of the narcotic antagonist naloxone attenuated the cyclazocine disruption. The 5-HT antagonist metergoline (1 mg/kg; 180-min pretreatment) also blocked cyclazocine effects to approximately the same degree as did naloxone. However, the shift of the dose response pattern of cyclazocine was not parallel for either antagonist. A greater degree of attenuation of the cyclazocine effects was observed when naloxone (4 mg/kg) and metergoline (0.1 mg/kg) were given together, indicating that cyclazocine disruption may be antagonized by either a narcotic antagonist or a 5-HT antagonist, and that these antagonists may operate synergistically. Thus, the behavioral effects of cyclazocine may relate to both opioid and serotonergic components.

Neurotransmitter basis of the behavioral effects of hallucinogens

Indole and phenethylamine-type hallucinogenic drugs were studied in an FR-40 operant behavioral procedure programmed to quantify "pausing,"-a behavioral disruption somewhat specific to hallucinatory drug effects. LSD, DOM, DMT and mescaline showed a potency ratio to produce pausing that is well correlated with the hallucinatory potencies of these agents in man. Furthermore, combinations of the hallucinogens interact with potentiation to cause FR-40 pausing, whereas a variety of non-hallucinogenic psychoactive drugs failed to shift the dose-response patterns of pausing for DOM or LSD. Depletion of brain catecholamines by pretreatment with intraventricular 6-OHDA reduced baseline FR-40 rates and attenuated the disruptive effects of d-amphetamine, but failed to modify the dose-response patterns of indole and phenethylamine hallucinogens. On the other hand, pretreatment with intraventricular 5,7-DHT to deplete brain 5-HT potentiated the pause-producing effects of the hallucinogens, although the disruptive effects of phenobarbital were not altered by this pretreatment. Injection of 5,7-DHT into the medial forebrain bundle at the hypothalamic level slightly potentiated LSD, attenuated DOM, and did not affect the pausing produced by mescaline. Metergoline pretreatment shifted the LSD and DMT dose-response curves for pausing to the right by a factor of 2--3, but shifted the DOM and mescaline dose-response patterns to a much greater extent. Metergoline alone slightly increased FR-40 response rates and decreased pausing from baseline levels. The patterns of imparied FR-40 performance induced by d-amphetamine and phenobarbital were unaltered by pretreatment with metergoline. The indole and phenethylamine classes of hallucinogens appear to disrupt this behavior by an agonistic effect at central 5-HT receptors. However, the two classes of drugs may interact with brain 5-HT systems by somewhat different mechanisms.