Central 5-hydroxytryptamine and the effects of hallucinogens and phenobarbital on operant responding in rats

Animal models of serotonergic psychedelics

The serotonin 5-HT(2A) receptor is the major target of psychedelic drugs such as lysergic acid diethylamide (LSD), mescaline, and psilocybin. Serotonergic psychedelics induce profound effects on cognition, emotion, and sensory processing that often seem uniquely human. This raises questions about the validity of animal models of psychedelic drug action. Nonetheless, recent findings suggest behavioral abnormalities elicited by psychedelics in rodents that predict such effects in humans. Here we review the behavioral effects induced by psychedelic drugs in rodent models, discuss the translational potential of these findings, and define areas where further research is needed to better understand the molecular mechanisms and neuronal circuits underlying their neuropsychological effects.

Neuroleptics and operant behavior: The anhedonia hypothesis

Neuroleptic drugs disrupt the learning and performance of operant habits motivated by a variety of positive reinforcers, including food, water, brain stimulation, intravenous opiates, stimulants, and barbiturates. This disruption has been demonstrated in several kinds of experiments with doses that do not significantly limit normal response capacity. With continuous reinforcement neuroleptics gradually cause responding to cease, as in extinction or satiation. This pattern is not due to satiation, however, because it also occurs with nonsatiating reinforcement (such as saccharin or brain stimulation). Repeated tests with neuroleptics result in earlier and earlier response cessation reminiscent of the kind of decreased resistance to extinction caused by repeated tests without the expected reward. Indeed, withholding reward can have the same effect on responding under later neuroleptic treatment as prior experience with neuroleptics themselves; this suggests that there is a transfer of learning (really unlearning) from nonreward to neuroleptic conditions. These tests under continuous reinforcement schedules suggest that neuroleptics blunt the ability of reinforcers to sustain responding at doses which largely spare the ability of the animal to initiate responding. Animals trained under partial reinforcement, however, do not respond as well during neuroleptic testing as animals trained under continuous reinforcement. Thus, neuroleptics can also impair responding (though not response capacity) that is normally sustained by environmental stimuli (and associated expectancies) in the absence of the primary reinforcer. Neuroleptics also blunt the euphoric impact of amphetamine in humans. These data suggest that the most subtle and interesting effect of neuroleptics is a selective attenuation of motivational arousal which is (a) critical for goal-directed behavior, (b) normally induced by reinforcers and associated environmental stimuli, and (c) normally accompanied by the subjective experience of pleasure. Because these drugs are used to treat schizophrenia and because they cause parkinsonian-like side effects, this action has implications for a better understanding of human pathology as well as normal motivational processes.

The effects of intracranial administration of hallucinogens on operant behavior in the rat. II. 2,5-Dimethoxy-4-methylamphetamine (DOM)

2,5-Dimethoxy-4-methylamphetamine (DOM) was infused into discrete brain regions of rats trained to press a bar for food reinforcement on a fixed ratio-40 (FR-40). Sites were chosen as major areas of the brain 5-hydroxytryptamine (5-HT) system: the dorsal and median raphe nuclei, dorsal hippocampus, lateral habenular nuclei and the prefrontal cortex. Following training in a fixed ratio-40 (FR-40) operant behavior rats were implanted with stainless steel cannulae into the brain area to be examined. Bilateral cannulae were implanted for the lateral habenular nuclei, dorsal hippocampus and the prefrontal cortex. Following recovery from surgery, DOM (20-300 micrograms) was tested on operant behavior by infusing the drug immediately before the operant session. Infusion of vehicle was inactive. DOM produced a dose-dependent decrease in reinforcements and a concomitant increase in 10-sec periods of non-responding (pause intervals). DOM was more potent when infused into the median raphe nucleus than following intracerebroventricular (ICV) administration. DOM was less potent when infused into the dorsal raphe, prefrontal cortex or dorsal hippocampus. Infusion of DOM into the lateral habenular nuclei produced a biphasic dose-response curve. ED50s for increases in pause intervals were 47, 77, 92, 103, and 114 micrograms for infusion into the median raphe, dorsal raphe, prefrontal cortex, lateral habenulae, and dorsal hippocampus, respectively. The ED50 for ICV administration in a previous study was 58 micrograms. The effects of DOM in the lateral habenulae could be divided into two curves; one curve had an ED50 of 69 micrograms, whereas the other had an ED50 of 176 micrograms. Furthermore, the dose-response curve for IP administration of DOM was shifted to the left in animals with cannulae placed into the lateral habenular nuclei. No change was seen in the response to IP administration of DOM in animals cannulated in the remaining sites or in animals with ICV cannulae. Therefore, the effects of DOM in disrupting operant behavior may be more critical with regard to its actions in the lateral habenulae and median raphe. Nonetheless, actions at multiple brain sites probably contribute to the total behavioral effects of the drug.

The effects of intracranial administration of hallucinogens on operant behavior in the rat. I. Lysergic acid diethylamide

Lysergic acid diethylamide (LSD) was infused in one microliter volumes into discrete brain regions of rats trained to press a bar for food reinforcement. The sites were chosen as major areas of the brain 5-hydroxytryptamine (5HT) system: the dorsal and median raphe nuclei, dorsal hippocampus, lateral habenular nuclei, and the prefrontal cortex. Following training in a fixed ratio-40 (FR-40) operant behavior rats were implanted for the lateral habenular nuclei, dorsal hippocampus and the prefrontal cortex. Following recovery from surgery, LSD (8.6 to 86 micrograms) or vehicle was infused immediately before a daily operant session. Infusion of vehicle was inactive. LSD produced a dose-dependent decrease in reinforcements and an increase in 10-sec periods of non-responding (pause intervals). LSD was significantly more potent when infused into the dorsal raphe nucleus than following intracerebroventricular (ICV) administration, whereas LSD was less potent when infused into the median raphe, lateral habenula or dorsal hippocampus. ED50s for increases in pause intervals were 9, 13, 23, 25, and 54 micrograms for infusion into the dorsal raphe, prefrontal cortex, dorsal hippocampus, median raphe, and lateral habenular nuclei, respectively. The ED50 for ICV administration in a previous study was 15 micrograms. The ED50 of LSD placed into the prefrontal cortex did not differ significantly from that of the ICV infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Action of a chronic administration of mescaline in dynamic behavioural situations

The modifications of the rat behaviour caused by a chronic administration of mescaline were studied in two schedules of operant conditioning. In the "periodic conditioning" test, the schedule of reinforcement was changed from a fixed ratio to a fixed interval schedule. Mescaline (4 mg/kg/day and 10 mg/kg/day) caused no modification of the ability of the rat to adapt its behaviour to the new experimental situation. In the "reversal test" the contingency for food delivery was switched from one lever, where responses were previously reinforced to the other lever where responses had no programmed consequences. A chronic administration of mescaline (4 mg/kg/day) caused a total incapacity of the rat to switch to the lever which became reinforced in the reversal trial. A chronic administration of 9 mg/kg/day of mescaline had an excitatory effect and the number of reinforced responses in the II and III reversals exceeded the unreinforced responses in a measure greater than in the controls.

Effects of valproic acid and ethosuximide on the responding of pigeons maintained under a multiple fixed-ratio fixed-interval schedule of food delivery

The effects of valproic acid and ethosuximide were examined in pigeons responding under a multiple Fixed-Ratio 50 Fixed-Interval 90-sec schedule of food delivery. When given acutely 30 min prior to behavioral testing, both valproic acid (40, 60, 80, 100, and 120 mg/kg) and ethosuximide (40, 60, 80, 100, and 120 mg/kg) produced generally dose-dependent decreases in responding under both the Fixed-Ratio and Fixed-Interval components. Detailed analysis of drug effects on the temporal distribution of responding under the Fixed-Interval failed to reveal rate-dependent effects for either drug. Varying the presession injection interval from 15 to 120 min indicated that both valproic acid and ethosuximide reduced responding to the greatest extent when given 30 or 60 min before behavioral testing. These results indicate that the anticonvulsants valproic acid and ethosuximide similarly affect schedule-controlled responding, although previous studies have revealed the drugs to have different effects under other procedures.

Effects of phenytoin, phenobarbital, and valproic acid, alone and in selected combinations, on schedule-controlled behavior of rats

The present study examined the effects of phenytoin (20, 30, 40, and 50 mg/kg), phenobarbital (10, 20, 30, and 40 mg/kg), and valproic acid (80, 120, 160, and 240 mg/kg), and those of phenobarbital (10 and 30 mg/kg) combined with phenytoin (20, 30, and 40 mg/kg) or valproic acid (80, 120, and 160 mg/kg), on the lever pressing of rats maintained under fixed-ratio and interresponse-time-greater-than-t schedules of food delivery. High doses of each individual drug significantly decreased mean group response (and reinforcement) rate under the fixed-ratio schedule. No dose of an individual agent significantly affected mean group response rate under the interresponse-time-greater-than-t schedule, although high doses of phenobarbital and valproic acid significantly reduced the mean group reinforcement rate under this schedule. When given in combination, phenobarbital and phenytoin and phenobarbital and valproic acid significantly reduced response (and reinforcement) rate under the fixed-ratio schedule and reinforcement rate under the interresponse-time-greater-than-t schedule. These reductions did not significantly differ in magnitude from those predicted by an additive model of drug interaction.

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.

Behavioral effects of intracerebroventricular administration of LSD, DOM, mescaline or lisuride

The effects on a fixed ratio-40 (FR-40) operant behavior of intracerebroventricular (ICV) administration of the hallucinogens lysergic acid diethylamide (LSD), 2,5-dimethoxy-4-methylamphetamine (DOM), mescaline or the non-hallucinogenic LSD-analogue lisuride were compared with intraperitoneal (IP) administration. Infusion of LSD (8.5 to 34 micrograms) into the left lateral ventricle produced a dose-dependent decrease in reinforcers and an increase in 10-sec periods of non-responding (pause intervals). The time-course of LSD showed a shorter latency to onset after ICV than IP administration. The ED50 for doses increasing pause intervals by ICV administration was 15 micrograms. This disruption was greater than that produced by IP administration of equivalent doses of LSD (IP ED50: 19 micrograms). DOM (40 to 120 micrograms) infused into the lateral ventricle also produced a dose-dependent disruption of FR-40 behavior. ICV DOM also showed a rapid onset to peak effects, but a slower offset than LSD, and was 3 times more potent than systemic administration (ED50s: 58 micrograms ICV vs. 153 micrograms IP). Mescaline was much more potent in disrupting FR-40 behavior by the ICV route than by IP administration. The ICV ED50 for doses of mescaline increasing pause intervals was 74 micrograms, in contrast to an ED50 following systemic administration of 2251 micrograms, demonstrating a 30-fold difference in potency. Lisuride administered via the ICV route was no more potent than by IP administration with ED50s of 4 micrograms ICV and 4 micrograms IP. Lower doses of lisuride administered by both routes had a similar effect over time on pause intervals.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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

The present study examined the disruptive effects of the phenethylamine hallucinogen 2,5-dimethoxy-4-methylamphetamine (DOM) and the putative 5-hydroxytryptamine (5-HT) agonist quipazine on fixed ratio-40 (FR-40) operant responding alone or after pretreatment with putative 5-HT antagonist metergoline. Food-deprived male rats were trained to bar press on a FR-40 schedule for food reinforcements; control responding under this schedule is characterized by a rapid, constant rate of responding (approximately 100 response/min). In control animals, both DOM and quipazine produced dose-dependent disruptions of FR-40 performance characterized by periods of non-responding or "pausing". Following pretreatment with 1.0 mg/kg, and to a lesser extent 0.1 mg/kg, metergoline (180 min prior to the session) the dose-response curves for the "pausing" produced by both DOM and quipazine were shifted significantly to the right. Moreover, increasing the dose of DOM about 16-fold and that of quipazine about 8-fold appears to completely override the antagonism by 1 mg/kg metergoline. These results suggest that the "pausing" produced by DOM or quipazine is the result of activation of 5-HT receptors.

The behavioral effects of hallucinogens in rats following 5,7-dihydroxytryptamine administration into the medial forebrain bundle

The hypothesis that 5-hydroxytryptamine (5-HT) neurons and/or receptors are involved in the mechanism of action of hallucinogens is supported by the fact that intraventricular administration of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) selectively destroys central 5-HT neurons in the brain and potentiates the behavioral effects of lysergic acid diethylamide (LSD), 2,5-dimethoxy-4-methylamphetamine (DOM) and mescaline. The locus in the brain where this potentiation might occur is not known. In the present experiment, the medial forebrain bundle (MFB) was studied because it is the primary tract containing fibers from the cell bodies in the raphe nuclei to forebrain structures receiving 5-HT input. Male rats received 5,7-DHT (6 micrograms/2 microliter) or vehicle injections bilaterally into the MFB; this procedure caused a significant reduction of 5-HT in the cortex, hippocampus and hypothalamus of lesioned rats, but not in the striatum. Regional dopamine and norepinephrine concentrations were not affected by this treatment. The behavioral effects of the hallucinogens were tested in a situation in which the animals pressed a bar under a fixed ratio-40 (FR-40) schedule of food reinforcement. The disruptive effects of LSD on responding were enhanced in the 5,7-DHT-treated animals, while the effects of DOM were diminished; there was no change in the response to mescaline. These data suggest that, while 5-HT neurons are involved in the behavioral effects of hallucinogens, the precise sites and/or mechanisms of action of LSD, DOM and mescaline may differ.