Discriminative stimulus properties of quipazine

Effect of Hallucinogens on Unconditioned Behavior

Because of the ethical and regulatory hurdles associated with human studies, much of what is known about the psychopharmacology of hallucinogens has been derived from animal models. However, developing reliable animal models has proven to be a challenging task due to the complexity and variability of hallucinogen effects in humans. This chapter focuses on three animal models that are frequently used to test the effects of hallucinogens on unconditioned behavior: head twitch response (HTR), prepulse inhibition of startle (PPI), and exploratory behavior. The HTR has demonstrated considerable utility in the neurochemical actions of hallucinogens. However, the latter two models have clearer conceptual bridges to human phenomenology. Consistent with the known mechanism of action of hallucinogens in humans, the behavioral effects of hallucinogens in rodents are mediated primarily by activation of 5-HT2A receptors. There is evidence, however, that other receptors may play secondary roles. The structure-activity relationships (SAR) of hallucinogens are reviewed in relation to each model, with a focus on the HTR in rats and mice.

Early preclinical studies of discriminable sedative and hallucinogenic drug effects

RATIONALE

One important technique in behavioral pharmacology is to train laboratory animals to discriminate between a psychoactive drug effect and a nondrug condition. Tests with different drugs have identified several categories of drugs that have different discriminable effects.

OBJECTIVES

The two authors describe and discuss the early research on discriminable effects of sedative and hallucinogenic drugs and their acquaintance with each other at Yale University prior to their early and frequent publications on discriminable drug effects. Herb Barry studied sedative drugs primarily and Jim Appel studied hallucinogenic drugs.

RESULTS

Sedative drugs include ethyl alcohol, barbiturates, and benzodiazepines. Their discriminable effects are largely attributable to the activation of an inhibitory neurotransmitter, gamma-amino butyric acid. Alcohol has the most pervasive effect in accordance with the high dose required to alter behavior. Hallucinogenic drugs include lysergic acid diethylamide and mescaline. They increase the activity of the neurotransmitter 5-hydroxytryptamine and, perhaps, dopamine in the central nervous system (CNS). In spite of their relatively low concentrations in the brain, both of these neurotransmitters have many important behavioral effects.

CONCLUSIONS

Various sedative drugs cause a discriminable decrease in the function of the CNS. Different types of sedatives can be discriminated from each other. Indole and phenylethylamine hallucinogens have potent discriminative stimulus properties, which are related to the actions of biogenic amine neurotransmitters in the CNS.

Drug discrimination in neurobiology

Areas of neurobiological interest are identified towards which drug discrimination (DD) studies have made important contributions. DD allows ligand actions to be analyzed at the whole organism level, with a neurobiological specificity that is exquisite and often unrivalled. DD analyses have thus been made of a vast array of CNS agents acting on receptors, enzymes, or ion channels, including most drugs of abuse. DD uniquely offers access to the study of subjective drug effects in animals, using a methodology that also is transposable to humans and has generated unprecedented models of pathology (e.g., chronic pain, opiate addiction). Parametric studies of such independent variables as training dose and reinforcement provide refined insights into the dynamic psychophysiological mechanisms of both drug effects and behavior. Three different mechanisms have been identified by which discriminative, and perhaps other behaviors, can come about. DD also is superbly sensitive to small, partial activation of molecular substrates; this has enabled DD analyses to pioneer the unravelling of molecular mechanisms of drug action (attributing, f.ex., LSD's particular subjective effects to an unusual, partial activation of 5-HT, and perhaps other receptors). DD has both oriented and served as a tool to conduct drug discovery research (e.g., pirenperone-risperidone, loperamide). The DD response arguably constitutes a quantal, rather than graded, variable, and as such allows a comprehension of molecular, pharmacological, and behavioral mechanisms that would have been otherwise inaccessible. Perhaps most important are the following further contributions. One is the notion that particular, different levels of receptor activation are associated with qualities of neurobiological actions that also differ and are unique, this notion arguably constituting the most significant addition to affinity and intrinsic activity since the earliest theoretical conceptions of molecular pharmacology. Another contribution consists of studies that render redundant the notion of tolerance and identify fundamental mechanisms of signal transduction; these mechanisms account for apparent tolerance, dependence, addiction, and sensitization, and appear to operate ubiquitously in a bewildering array of biological systems.

Complex stimulus properties of LSD: a drug discrimination study with alpha 2-adrenoceptor agonists and antagonists

The influence of several alpha 2-adrenergic agents on the discriminative stimulus (DS) properties of lysergic acid diethylamide (LSD) was studied in rats trained to discriminate 0.08 mg/kg (186 nmol/kg) of LSD from saline in a two-lever operant paradigm. Only yohimbine fully mimicked LSD with an ED50 of 2.05 mg/kg (5.24 mumol/kg). Yohimbine's 5-HT1A agonist properties may be responsible for this substitution. Other alpha 2-adrenoceptor antagonists, idazoxan with an agonist/antagonist profile at 5-HT1A receptors and RS 26026-197, a highly selective alpha 2-adrenoceptor antagonist, failed to produce substitution. Clonidine, an alpha 2-adrenoceptor agonist, did not substitute for LSD but the response rate was dose-dependently reduced. None of the alpha 2-adrenergic agents used for pretreatment before LSD inhibited the response to the LSD training dose. Coadministration of clonidine with LSD produced a leftward shift of the dose-response relationship of LSD without a significant change in the slope of the dose-response line. Simultaneous administration of alpha 2-adrenergic agents with LSD shifted the dose-response curve to the left only when the adrenergic agent also possessed at least moderate affinity for the 5-HT1A receptor. In addition, radioligand competition experiments were performed that showed LSD to have relatively high affinity (Ki = 37 nM) for [3H]clonidine-labeled sites in rat cortex with lower affinity for [3H]yohimbine labeled sites. While previous studies have suggested that the nature of the LSD cue may be essentially expressed by 5-HT2 receptor activation, the present data show that this cue can be modulated by effects of LSD at 5-HT1A and at other monoamine neurotransmitter receptors.

Three-choice discrimination among (+)-amphetamine, fenfluramine and saline in pigeons

Five pigeons were trained to discriminate among (+)-amphetamine (AMPH; 1.7 or 3.0 mg/kg), fenfluramine (FEN; 5.6 or 10 mg/kg), and saline using a three-choice drug discrimination procedure. The results of the study demonstrated that a reliable discrimination between AMPH and FEN could be obtained and the discriminative stimulus (DS) effects of these two drugs did not overlap, i.e., were mutually exclusive. Phenmetrazine produced a dose-related increase in AMPH-appropriate responding with no responding occurring on the FEN-appropriate key. Two serotonin agonists, quipazine (5-HT2) and MK 212 (5-HT1), produced FEN-appropriate responding in two of three pigeons, while a third pigeon responded predominantly on the AMPH-appropriate key following their administration. In contrast, phencyclidine produced predominantly (greater than 50%) saline-appropriate responding, indicating that the DS effect of phencyclidine was unlike either AMPH or FEN. Finally, compounds known to have multiple DS properties such as MDA and MDMA were tested. The results with these compounds confirmed that these drugs have complex DS effects both within and across individual pigeons.

Serotonergic-dopaminergic mediation of 3,4-methylenedioxymethamphetamine (MDMA, "ecstasy")

A series of three experiments were conducted to investigate the possible serotonergic and dopaminergic mediation of the discriminative stimulus properties of the "designer" drug MDMA. In Experiment 1, rats trained to discriminate 1.5 mg/kg (+/-)-MDMA from its vehicle at 20 min postadministration were shown to generalize to another drug of abuse, N-ethyl-3,4-methylenedioxyamphetamine (MDE) and to the serotonergically-active agents norfenfluramine and TFMPP. In contrast, testing of various dopaminergically-active agonists did not result in MDMA-like responding. In Experiment 2, dopaminergic and serotonergic antagonist were employed to observe their effect upon MDMA discrimination at 20 min postinjection. The serotonin antagonist pirenperone significantly decreased MDMA discrimination, whereas the dopamine decreasing drugs CGS 10746B and haloperidol had no effect. In Experiment 3, another group of rats were trained to discriminate MDMA at 105 min postadministration to investigate if, at this (later) time, the dopaminergic properties of MDMA may be more salient. Indeed, the dopaminergically-active drugs had a heightened effect upon MDMA at this later time, although the serotonergic component of the MDMA discriminative stimulus was predominant. The results suggest that the effects of MDMA at 20 min postadministration are solely serotonergic in nature. At 105 min postinjection there appears to be the presence of a weak dopaminergic component. This biphasic serotonergic-then-dopaminergic action of MDMA may explain the reported human experience with the drug, as well as the often controversial results in the literature.

Site-selective serotonin agonists as discriminative stimuli

Various direct- and indirect-acting serotonin (5-HT) agonists serve as training drugs in tests of stimulus control of behavior; such agents include: 5-hydroxytryptophan, 5-methoxy-N,N-dimethyltryptamine, and fenfluramine. However, with the recent discovery of multiple populations of central 5-HT binding sites, the concept of site-selective serotonergic agents needs to be addressed. Certain 4-substituted 1-(2,5-dimethoxyphenyl)-2-aminopropanes such as DOM (4-methyl), DOB (4-bromo), and DOI (4-iodo) appear to be 5-HT2-selective agonists and serve as effective training drugs in rats. Stimulus generalization occurs among these agents regardless of which is used as the training drug, although stimulus generalization does not occur with 5-HT1A-selective agonists [e.g., 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH DPAT)] or with 5-HT1B-selective agonists [e.g., 1-(3-trifluoromethylphenyl)piperazine (TFMPP)]. 8-OH DPAT and TFMPP also serve as training drugs; the 8-OH DPAT-stimulus generalizes to other 5-HT1A agonists, but not to 5-HT1B or 5-HT2 agonists, whereas the TFMPP-stimulus generalizes to other 5-HT1B agonists, but not to 5-HT1A or 5-HT2 agonists. Classical serotonin antagonists, most of which are rather selective for 5-HT2 sites, and 5-HT2-selective antagonists are able to block the stimulus effects of DOM, DOB, and DOI, but not those of 8-OH DPAT or TFMPP. The results of such studies reveal that, in rats, site-selective 5-HT agonists produce stimulus effects that are also selective; although generalization may occur with nonselective 5-HT agonists, animals trained to discriminate site-selective 5-HT agonists apparently do not recognize other 5-HT agonists that are selective for a different site. Animals trained to discriminate such agents from saline might be useful for the identification and/or investigation of novel site-selective agonists and antagonists (for example, the 8-OH DPAT-stimulus generalizes to members of a new class of anxiolytics that display high affinity for 5-HT1A binding sites), and might also aid in the overall understanding of central serotonergic mechanisms.

Effects of quipazine and of tryptamine on self-stimulation of median raphé nucleus and of lateral hypothalamus in rats

Separate groups of male Wistar rats were trained to lever press on a continuous reinforcement schedule under which behaviour was maintained by electrical stimulation of the median raphé nucleus (N = 6) or the lateral hypothalamus (N = 6). The effects of several doses of quipazine (2.5-7.1 mg/kg) and of tryptamine (10-80 mg/kg) were assessed with each group. Administration of quipazine resulted in a decrease of median raphé self-stimulation at 5.0 and 7.1 mg/kg. This compound had no statistically significant effect on lateral hypothalamic self-stimulation. Administration of tryptamine resulted in significant decreases in self-stimulation at both sites, however, whereas the effects of this drug were significant at 20, 40 and 80 mg/kg with median raphé self-stimulation, a significant decrease in lateral hypothalamic self-stimulation was only observed at 80 mg/kg. As baseline response rates differed in the two self-stimulation sites, a second group of animals with lateral hypothalamic sites (n = 6) were tested with quipazine (2.5-7.1 mg/kg) at an overall baseline response rate matched to that of the median raphé group. Although a tendency to decrease self-stimulation rates was found in this group, these results were not significant. These data suggest, therefore, that median raphé self-stimulation is more sensitive than lateral hypothalamic self stimulation to disruption by the effects of quipazine and tryptamine.

Drug-induced activation of the inferior olivary nucleus in young rabbits. Differential effects of harmaline and quipazine

Ontogenic evolution of behavioural and electrophysiological responses to the serotonergic agents, quipazine and harmaline, was studied in the maturing rabbit in normal and pretreated conditions. As regards behavioural effects, tremor induced by quipazine was present from the first postnatal day and was antagonized by methysergide, but not by p-chlorophenylalanine (PCPA) or pretreatment with 5,7-dihydroxytryptamine (5,7-DHT). In contrast, tremor induced by harmaline could not be elicited before the second postnatal week and was partially antagonized by methysergide and 5,7-DHT, but not by PCPA. Electrophysiological studies of cell activity in the inferior olivary nucleus revealed a similar dependency on age since rhythmic activation of the inferior olivary nucleus could be registered from the first postnatal day with quipazine and only from the 8th postnatal day with harmaline; drug interactions with methysergide, PCPA and 5,7-DHT were the same as for the behavioural observations. It is suggested that quipazine directly activates serotonin receptors which are already present at birth, whereas harmaline requires the presence of serotonergic fibres for such activation.

Discriminative stimulus properties of pizotifen maleate (BC105): a putative serotonin antagonist

Rats were trained to discriminate the putative serotonin (5-HT) antagonist, pizotifen maleate (BC105), from saline using a two-lever drug discrimination paradigm. Pizotifen maleate (6 mg/kg, 14.6 mumol/kg, IP) or saline was administered 55 min prior to the operant training session. The pizotifen discriminative stimulus (DS) had a rapid onset (less than 7 min) and was of long duration. The pizotifen DS was dose dependent. The pizotifen DS did not generalize to the putative 5-HT antagonists, methiothepin, xylamidine, and cinanserin. Partial generalization was observed to methysergide and metergoline, and complete generalization to cyrproheptadine and the phenothiazine antihistamine, promethazine. The pizotifen DS failed to generalize to the antipsychotic chlorpromazine, the ethanolamine antihistamine diphenhydramine, the CNS stimulant, d-amphetamine, and the putative 5-HT agonists, LSD and quipazine. LSD and quipazine failed to antagonize the pizotifen DS. The results of this study suggest that different DS properties are associated with the different putative 5-HT antagonists and that pizotifen and cyproheptadine, in addition to their reported 5-HT antagonist properties, share a common property that is also associated with promethazine, probably involving antihistaminergic activity.

A characterization of LSD-antagonist effects of pirenperone in the rat

Rats were trained to discriminate between intraperitoneal injections of 0.16 mg/kg of d-lysergic acid diethylamide (d-LSD) and injections of saline in the two-bar (FR 10) food-reinforced drug discrimination procedure. The gradient for responses to LSD was established following pretreatment with saline or one of five doses of pirenperone. It was found that pretreatment with pirenperone caused a parallel shift to the right of the dose-effect curve of LSD. The magnitude of this shift was related to the dose of pirenperone, 0.006 mg/kg of the drug causing a 2-fold shift. A direct linear plot revealed that the curve fitting the data points passed through the origin, but that it was curvilinear rather than linear. The data did not, therefore, accommodate the requirements for reversible, competitive interaction. This finding is discussed in terms of the mixed agonist/antagonist activity of LSD that may occur at binding sites for 5-HT1 and 5-HT2 in the rat brain.

Dopaminergic activity of quipazine

Rats were trained to discriminate between the stimulus properties of intraperitoneal 0.16 mg/kg apomorphine and saline in a two-lever, food-motivated operant task. Administration of 1.0 mg/kg quipazine, a putative serotonin agonist, produced apomorphine-appropriate responding with a maximal effect occurring at 45 min post-injection. Pretreatment with either 2.0 mg/kg methysergide or 0.4 mg/kg haloperidol reduced quipazine-induced responding upon the apomorphine-appropriate lever to levels observed with methysergide or haloperidol administered alone. These results evidence a dopaminergic action for quipazine and suggest that central serotonergic and dopaminergic pathways may interact cooperatively to control behavior.

Psilocybin as a discriminative stimulus: lack of specificity in an animal behavior model for 'hallucinogens'

Fifteen rats were trained to discriminate between the tryptamine hallucinogen psilocybin (4-phosphoryloxy-N,N-dimethyltryptamine; 1.0 mg/kg) and saline in a two-lever choice task. Dose-response and time-response curves were obtained. The psilocybin cue generalized to psilocin (the dephosphorylated congener of psilocybin) and to the prototypical indoleamine hallucinogen LSD, but not to the phenylethylamine hallucinogen mescaline. These results indicate that the hallucinogenic effects of these drugs in humans may not be identical with their discriminative stimulus functions in animals, and that these four compounds may not be members of a single drug class. The term 'hallucinogen' may thus be a misnomer in the context of drug discrimination studies in nonhumans.

Discriminative stimulus properties of L-5-hydroxytryptophan: behavioral evidence for multiple serotonin receptors

Rats were trained to discriminate the stimulus properties of L-5-hydroxytryptophan (L-5-HTP) (30 mg/kg SC), the immediate precursor of serotonin (5-HT). The peripheral decarboxylase inhibitor R04-4602, administered prior to L-5-HTP, greatly attenuated the disruptive effects observed on responding when L-5-HTP alone was injected. Following acquisition, the discrimination was dose-dependent and generalized to fenfluramine, a 5-HT-releasing drug, but not to amphetamine, a catecholamine-releasing agent. Further evidence for the involvement of 5-HT receptor stimulation in mediating the discrimination was that pretreatment with fluoxetine, a highly specific 5-HT uptake inhibitor, markedly potentiated the cue. Nevertheless, the classical 5-HT antagonists methysergide, cyproheptadine, metergoline, and methiothepin did not block the L-5-HTP-related discriminative stimulus. This finding suggested that the cue properties of L-5-HTP might be mediated by a population of 5-HT receptors previously identified electrophysiologically in limbic structures. As in the present experiment, the putative 5-HT antagonists did not block the synaptic effects of 5-HT in these structures.

Speculations on the mechanism of action of hallucinogenic indolealkylamines

In this review we attempt to develop a fluid theoretical model which is being used as a strategy-base for future experimentation. The first two sections (A and B) describe how we have conducted our research, and present the perspective value of each. This is important because the research strategies developed in these laboratories over the last 5 years combine in vitro and in vivo pharmacological techniques as a means of understanding mechanisms of drug action. Sections C and D attempt to describe how we interpret our data and how we have utilized these data to formulate hypotheses concerning drug mechanisms. The last section of this review sets forth our own ideas on how we believe hallucinogenic agents produce their effects and presents some original data, which we feel, allows us to develop the overall hypotheses presented.

Lack of specificity of an animal behavior model for hallucinogenic drug action

It has been proposed recently that the occurrence of drug-induced limb-flicking (LF) and abortive grooming (AG) in cats can serve as a viable animal behavior model for the actions of hallucinogens in humans. If this is the case, such behaviors should occur reliably following the administration of drugs that produce hallucinations in humans and should not occur after administration of other, non-hallucinogenic drugs--a hypothesis that was examined in the present experiment. The frequency of LF and AG were observed in 12 cats which were given a wide range of doses of the potent hallucinogen, d-LSD (0.01-0.16 mg/kg), as well as several other compounds. The results showed that three non-hallucinogenic agents which are related to LSD in various ways, the ergot derivative lisuride, the serotonin agonist, quipazine, and the dopamine agonist, apomorphine, significantly increased LF frequency. Lisuride and quipazine also caused AG. Cocaine did not elicit either behavior. Thus, it was concluded that the proposed model cannot be regarded as specific to hallucinogenic drugs. In addition, the frequency of these behaviors, as well as their reliability and robustness, were shown to be party dependent on the environment in which observation occurs.

A neuropharmacological analysis of the discriminative stimulus properties of fenfluramine

Rats were trained to discriminate fenfluramine (1.0 mg/kg) from saline in a two-lever drug discrimination task. The dose-response curve for this discrimination was orderly with an ED50 of about one-half of the training dose (0.52 mg/kg). In substitution tests, indirect (p-chloroamphetamine) and direct (quipazine, MK-212, lisuride) serotonin (5-HT) agonists substituted for fenfluramine. Since none of these compounds have been reported to be hallucinogenic and the potent hallucinogen LSD did not substitute completely, it was suggested that the discriminative stimulus properties of fenfluramine are not related to its ability to produce hallucinations in humans. The fenfluramine cue, like the quipazine cue, was antagonized by the 5-HT antagonists cyproheptadine and methiothepin. Unlike quipazine, fenfluramine was also partially antagonized by the 5-HT uptake inhibitor, fluoxetine, and the 5-HT synthesis inhibitor, p-chlorophenylalanine. Thus, the fenfluramine cue differs from that of quipazine in that it is mediated via indirect actions on 5-HT receptors. Since the indirect dopamine (DA) agonist d-amphetamine failed to substitute and the DA antagonist haloperidol failed to block the fenfluramine cue, a mediating role for DA was not indicated. Another indirect DA agonist, cocaine, substituted partially for fenfluramine, a result which paralleled that seen with fluoxetine. Both of these partial substitutions were reduced by cyproheptadine; therefore, it was concluded that these effects may be due to the common ability of cocaine, fluoxetine, and fenfluramine to inhibit 5-HT uptake.

The effect of serotonin depletion on the discriminability of LSD

Nine groups of rats were trained to discriminate LSD (0.12 mg/kg) from saline in a two-lever, water-reinforced, drug discrimination procedure. After stable discriminative performance was obtained (>95% correct), groups were administered one of several treatments which lower the concentration of serotonin (5-HT) in brain: (1) 12.5, 25, 50, 100 or 200 microgram of 5,7-dihydroxytryptamine (5,7-DHT) intraventricularly (IVT); (2) 3 X 100 mg/kg of p-chlorophenylalanine (PCPA) intraperitoneally (IP); or (3) 20 mg/kg of p-chloroamphetamine (PCA) IP. Control rats received either IVT injections of 5,7-DHT vehicle or IP injections of PCA or PCPA vehicles. Beginning 12 days after treatment, lever preference following various doses of LSD was determined. The results indicated that only the 200 microgram dose of 5,7-DHT and PCPA caused a significant potentiation of LSD-lever responding at the 0.03 mg/kg dose of LSD while all treatments except 12.5 and 25 microgram of 5,7-DHT resulted in significant depletion of 5-HT. Moreover, amount of 5-HT and percent LSD responding following 0.03 mg/kg LSD were not significantly correlated. It was concluded that 5-HT depletion, per se, cannot account for supersensitivity to the behavioral effects of LSD.

Effects of the phenethylamine derivatives, BL-3912, fenfluramine, and Sch-12679, in rats trained with LSD as a discriminative stimulus

Six rats were trained to discriminate the effects of LSD (100 micrograms/kg) and saline in a two-lever choice task. They were then tested with each of three phenethylamine derivatives, BL-3912 (2,5-dimethoxy-4-methyl-alpha-ethyl-phenethylamine), fenfluramine (N-ethyl-alpha-methyl-m-(trifluoro-methyl)phenethylamine), and Sch-12679 (N-methyl-1-phenyl-7,8-dimethoxy-2,3,4,5-tetra-hydro-3-benzazepine maleate). Fenfluramine and Sch-12679 yielded intermediate results, i.e., responding was not fully appropriate for either training condition while BL-3912 substituted completely for LSD. The LSD-like effects of each of the drugs were antagonized by pretreatment with BC-105, a serotonergic antagonist known to block the stimulus effects of indole and phenethylamine hallucinogens. The present data together with consideration of the known clinical effects of BL-3912, fenfluramine, and Sch-12679 are consistent with the following conclusions: (1) a variety of drugs may substitute in whole or in part for LSD in LSD-trained rats, and (2) even complete substitution of a drug for LSD in the rat is not necessarily associated with the production by that drug of hallucinations in man.

Drug effects under automaintenance and negative automaintenance procedures

Three food-deprived pigeons were initially exposed to an automaintenance procedure in which brief periods of response key illumination were followed by food delivery without regard to the subject's behavior. Keypecking occurred at a high rate while the key was illuminated and was reduced in dose-dependent fashion by acute administration of LSD (0.05--0.45 mg/kg), quipazine (1.0--8.0 mg/kg), haloperidol (0.08--0.32 mg/kg), and pentobarbital (4.0--16.0 mg/kg). The animals were then exposed to a negative automaintenance procedure in which food delivery followed key illumination only if the lighted key was not contacted. Keypecking occurred at a low rate under this procedure, with no responses occurring during the majority of key illuminations and was decreased or unaffected by LSD, quipazine, and haloperidol; pentobarbital increased responding at doses of 4.0 mg/kg and 8.0 mg/kg and reduced responding at a dose of 16.0 mg/kg.

Effects of quipazine on behavior under a multiple schedule of reinforcement

The effects of three doses of quipazine (1.0, 5.0 and 10.0 mg/kg) on the performance of rats under a multiple fixed-ratio 15 fixed-interval 60-sec schedule of food reinforcement were examined. In the absence of drug, response rates under the fixed-ratio component were much higher than response rates under the fixed-interval component. Rates under the fixed-ratio component were decreased by quipazine in dose-dependent fashion, while response rates under the fixed-interval component were increased by the lowest dose and decreased by the two higher doses of the drug.