R. L. Commissaris

Comparison of anti-conflict drug effects in three experimental animal models of anxiety.

A novel form of experimentally-induced conflict behavior based on the conditioned suppression of drinking (CSD) is described and compared with two conventional animal models of human anxiety — a modified Geller-Seifter and an Estes-Skinner (Conditioned Emotional Response) procedure. The CSD procedure offered significant advantages over the two operant procudures in that the session duration was short (10 min) and the acquisition of stable behavioral baselines was rapid (approximately 2 weeks). Like the more conventional procedures, the CSD paradigm permitted the simultaneous determination of drug effects on shock-suppressed and nonsuppressed responding as estimates of antianxiety and sedative properties, respectively. With the CSD procedure, the anticonflict profiles for the benzodiazepines were highly correlated with their relative clinical antianxiety potency. Therefore, the CSD procedure appears to be a valuable tool in screening for possible antianxiety agents as well as in the behavioral testing of mechanism of action hypotheses regarding such agents.

Lack of central 5-hydroxytryptamine influence on the anticonflict activity of diazepam

This study examined the effects of various drug treatments (IP injections) proposed to modify central 5-hydroxytryptamine (5-HT) activity on a conditioned suppression of drinking behavior in water-deprived rats. The subjects were trained to drink their daily water requirement during a 10-min session. Intermittent tone periods of 7 s were then introduced, the last 5 s of which the drinking tube was electrified. The animals gradually suppressed tube contacts during the tone to a low constant level within 2 weeks of training. Diazepam increased punished responding dramatically. The 5-HT antagonists methysergide (1–18 mg/kg), cyproheptadine (1–18 mg/kg), metergoline (0,25–2.0 mg/kg) and cinanserin (10–100 mg/kg) failed to induce large, reliable increases in punished responding. When a low dose of diazepam was combined with 5-HT antagonists, only one treatment, methysergide at 3 mg/kg, potentiated the anticonflict activity of diazepam. Acute or chronic treatment with PCPA increased behavior suppressed by punishment, but this effect was weak, brief, and poorly related to the depletion of brain 5-HT. LSD (0.3–100 μg/kg) administered 1,10, or 30 min before the test was ineffective in overcoming suppression by punishment. Mescaline (6–30 mg/kg) had no significant effect on punished responding. 5-HTP (18 mg/kg) decreased the number of shocks accepted, but not after pretreating with carbidopa. Pretreatment with carbidopa plus 5-HTP potentiated the anticonflict effect of diazepam. The 5-HT agonist mCPP (0.25–2.0 mg/kg) enhanced suppression due to punishment, but only in doses that interfered with unpunished responding. The 5-HT-releasing agent fenfluramine (0.25–1.0 mg/kg) did not affect this behavior. Amitriptyline pretreatment in a dose not affecting unpunished behavior (5.6 mg/kg) potentiated the diazepam-induced increase in punished responding. These results are difficult to reconcile with the proposal that suppression of behavior consequent to punishment is related to brain 5-HT activity.

Interactions of metergoline with diazepam, quipazine, and hallucinogenic drugs on a conflict behavior in the rat

The effects of diazepam quipazine, lysergic acid diethylamide (LSD), and 2,5-dimethoxy-4-methylamphetamine (DOM) were examined on a conditioned suppression paradigm. Food-deprived rats were trained to drink a liquid diet from a tube. Subsequently, intermittent 7-s tones were presented during the daily 10-min sessions, the tube being electrified during the last 5 s of each tone. The subject gradually learned to suppress contact with the tube during the tone periods to a low stable level (punished responding) and consumed stable volumes of the liquid diet during the silent periods (unpunished responding). Treatment with diazepam caused large increases (1,000% of control) in punished responding. The hallucinogens produced only modest increases (200–300%), while quipazine did not significantly increase punished responding. Metergoline pretreatment (0.1–2.0 mg/kg, 180 min) had no effect on punished responding itself, and there was no significant alteration of the diazepam dose-response pattern. The weak increase in punished responding by LSD was antagonized by metergoline, but the interaction between metergoline and DOM was variable and inconsistent. Diazepam, quipazine, LSD, and DOM caused dose-dependent decreases in unpunished responding (fluid intake). Metergoline alone decreased unpunished responding only at 2.0 mg/kg. Metergoline pretreatment (1.0 mg/kg) only slightly antagonized the LSD effect on unpunished behavior, but shifted the dose-response curves of DOM and quipazine for decrease in fluid intake to the right approximately eight fold. On the contrary, the dose-response curve of diazepam to decrease fluid intake was shifted to the left by metergoline pretreatment. These data suggest that altered activity of brain serotonin (5-HT) neurons is not responsible for the dramatic increase in punished responding by diazepam. The hallucinogens, quipazine, and diazepam all produce a decrease in unpunished responding, but they appear to do so by different neuropharmacological mechanisms. In addition, there may be at least slight differences in the mechanism by which LSD produces its effects as compared with that of quipazine and DOM.

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

The present study was designed to examine the role of 5-hydroxytryptamine (5-HT) neurons in the behavioral effects of d-lysergic acid diethylamide (LSD), an indolealkylamine hallucinogen, 2.5-dimethoxy-4-methylamphetamine (DOM) and mescaline, phenethylamine hallucinogens, and phenobarbital, a non-hallucinogen. Male rats, maintained at 70-80% of their free-feeding weights, were trained to press a lever for food pellet reinforcement on a fixed ratio-40 operant schedule. When trained, these rats responded at a constant, rapid rate (approximately 100 responses/min) during daily 40 min test sessions. Administration of hallucinogens caused an abrupt cessation of responding (a pause), for some portion of the session. The duration of this pause was dose-dependent for LSD (12.5-100 micrograms/kg), DOM (0.125-1.0 mg/kg) and mescaline (7.1-14.2 mg/kg). On the other hand, phenobarbital (12.5-50 mg/kg) did not cause pausing, but resulted in slowed, erratic intrasession response rates. When the same tests were repeated in rats that had previously received an intracerebroventricular injection of 5,7-dihydroxyptamine (5,7-DHT) the dose-response curves for the pausing induced by all three hallucinogens were shifted to the left, while the behavioral disruption produced by phenobarbital was unaltered. In these animals the 5-HT but not the norepinephrine concentrations was markedly reduced in all brain regions examined. These results suggest that 5-HT neurons are involved with the behavioral effects of hallucinogens but not of phenobarbital.

Effects of acute and chronic interactions of diazepam and d-amphetamine on punished behavior of rats.

Drinking of water-deprived rats was punished by delivery of either 0.03 or 0.1 mA shocks through the drinking tube during the last 5 of 7-s tone components during a 15-min daily exposure to water. These sessions consisted of 66 alternating components marked by the presence or absence of a tone. Drinking was not punished during the 33 components without tones or during the first 2 s of each tone. The baseline number of shocks accepted by the rats at 0.1 mA was less than half that at 0.03 mA. Acute diazepam markedly increased shocks delivered from the baseline values for both shock intensities, while acute d-amphetamine either had no effect or decreased the number of shocks accepted. The combination of acute diazepam and d-amphetamine caused a decrease in shock rates as compared to diazepam alone. Daily treatment with the combination of 10 mg/kg diazepam and 1 mg/kg d-amphetamine caused a gradual increase in punished responding over 25 days under either shock intensity. Gross observation of these rats after daily treatments indicated the development of hyperreactivity and a pattern resembling stereotyped behavior. At the end of the chronic treatment with the drug combination the shock rates were significantly greater than those caused by diazepam alone. Nevertheless, neither the effect of diazepam nor that of d-amphetamine, when administered singly after the period of chronic treatment with the combination, differed significantly from initial effect of the respective drugs on punished responding.

The effects of d-lysergic acid diethylamide (LSD), 2,5-dimethoxy-4-methylamphetamine (DOM), pentobarbital and methaqualone on punished responding in control and 5,7-dihydroxytryptamine-treated rats

The purpose of the present study was to determine the role of central 5-hydroxytryptamine (5-HT) neuronal systems in the effects of d-lysergic acid diethylamide (LSD), 2,5-methoxy-4-methylamphetamine (DOM), pentobarbital (PB) and methaqualone (MQ) on punished responding in rats. Water-deprived rats were trained to drink from a tube that was electrified at intervals (variable interval 21 sec; 0.03 mA current intensity), electrification being signalled by a tone. In daily 10-min control sessions, these animals accepted a relatively constant number of shocks; water consumption was also quite stable. At maximally effective doses PB, and to a lesser extent MQ, produced large (400-600 percent of control) increases in punished responding with little decrease in water intake. Higher doses of these agents produced a significant depression of unpunished responding (water intake). The hallucinogens, on the other hand, produced only moderate (125-175 percent of control) increases in the number of shock received, yet a similar depression of unpunished responding. Selective destruction of 5-HT neurons by intracerebroventricular administration of the neurotoxin 5,7-dihydroxytryptamine per se produced little change in the number of shocks received or water consumed in controls sessions. This destruction of 5-HT neurons failed to alter the effects of PB or MQ on punished or unpunished responding. The increase in punished responding produced by the hallucinogens, however, was blocked by this destruction of 5-HT neurons. Furthermore, the capacity of the hallucinogens to decrease water intake was significantly potentiated by the neurotoxin pretreatment. These data demonstrate that the effects of the hallucinogens LSD and DOM on conditioned suppression are quite different from those of PB and MQ, and that this difference may be due to the extent of 5-HT involvement in the effects of these agents.

Tolerance and cross-tolerance to central nervous system depressants after chronic pentobarbital or chronic methaqualone administration ☆

This study reports on tolerance and cross-tolerance to the rotarod (RR)-disrupting effects of various central nervous system (CNS) depressants. Female rats trained on the RR were fed ground chow containing pentobarbital (PB, 2.0 mg/g chow) or methaqualone (MQ, 1.0 mg/g chow) and were injected twice daily (PB) or daily MQ) with 30 mg/kg IP for 6 days. Control rats received ground chow and saline injections. On day 7 the subjects were tested with various doses of PB, MQ, diazepam (DZ), or ethanol (ET) for disruption of RR performance over the time-course of the drug effect (up to 12 hours). Control animals demonstrated a dose-dependent duration of impairment for all 4 agents. Both groups receiving chronic drug showed a prominent decrease in duration of RR impairment after PB, a less marked decrease after MQ, and even less of a decrease after DZ. However, neither chronic drug group showed an appreciable tolerance to the RR disruption of ET, relative to the control group. Based on the time of 50% recovery (RR performance recovering to 90 seconds or more), both chronic treatments resulted in a significant shift of the dose-response curves for PB, MQ and DZ to the right. Therefore, the degree of tolerance and cross-tolerance in rats chronically treated with PB or MQ was dramatic for PB and MQ, was significant for DZ, but was not demonstrable for ET.

The effects of d-lysergic acid diethylamide (LSD), 2,5-dimethoxy-4-methylamphetamine (DOM) and d-amphetamine on operant responding in control and 6-hydroxydopamine-treated rats

The purpose of the present study was to determine the role of central catecholaminergic neuronal systems in the effects of LSD, DOM and d-amphetamine on fixed ratio (FR) operant responding in rats. Food-deprived male rats were trained to press a bar for food reinforcement on a FR-40 schedule. Control responding on this schedule is characterized by a rapid, constant rate of responding (approximately 100 responses/min) throughout a 40 min test session. LSD and DOM, as with other hallucinogens, produced dose-dependent periods of nonresponding or pausing, followed by reinstatement of responding at or near the control rate. Administration of the non-hallucinogen, d-amphetamine, did not produce pausing, but caused the response rate to slow and become erratic. In animals pretreated intraventricularly with 6-hydroxydopamine (6-OHDA; 200 micrograms/10 microliter X 2), the response to LSD and DOM was unchanged, while the response to d-amphetamine was significantly diminished. The neurotoxin significantly decreased brain catecholamines to less than 25 percent of control in al regions examined, without altering 5-HT concentrations in these same regions. These data demonstrate that the effects of LSD and DOM on FR-40 responding are quite different from those of d-amphetamine, and that this difference may be due to the extent of catecholamine involvement in the effects of these agents.

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.