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

LSD but not lisuride disrupts prepulse inhibition in rats by activating the 5-HT(2A) receptor

RATIONALE

Compounds that activate the 5-HT(2A) receptor, such as lysergic acid diethylamide (LSD), act as hallucinogens in humans. One notable exception is the LSD congener lisuride, which does not have hallucinogenic effects in humans even though it is a potent 5-HT(2A) agonist. LSD and other hallucinogens have been shown to disrupt prepulse inhibition (PPI), an operational measure of sensorimotor gating, by activating 5-HT(2A) receptors in rats.

OBJECTIVE

We tested whether lisuride disrupts PPI in male Sprague-Dawley rats. Experiments were also conducted to identify the mechanism(s) responsible for the effect of lisuride on PPI and to compare the effects of lisuride to those of LSD.

RESULTS

Confirming a previous report, LSD (0.05, 0.1, and 0.2 mg/kg, s.c.) reduced PPI, and the effect of LSD was blocked by pretreatment with the selective 5-HT(2A) antagonist MDL 11,939. Administration of lisuride (0.0375, 0.075, and 0.15 mg/kg, s.c.) also reduced PPI. However, the PPI disruption induced by lisuride (0.075 mg/kg) was not blocked by pretreatment with MDL 11,939 or the selective 5-HT(1A) antagonist WAY-100635 but was prevented by pretreatment with the selective dopamine D(2)/D(3) receptor antagonist raclopride (0.1 mg/kg, s.c).

CONCLUSIONS

The effect of LSD on PPI is mediated by the 5-HT(2A) receptor, whereas activation of the 5-HT(2A) receptor does not appear to contribute to the effect of lisuride on PPI. These findings demonstrate that lisuride and LSD disrupt PPI via distinct receptor mechanisms and provide additional support for the classification of lisuride as a non-hallucinogenic 5-HT(2A) agonist.

Dissociations between the effects of hallucinogens on behavior and raphe unit activity in behaving cats

The hypothesis that hallucinogenic drugs exert their behavioral effects by an action at pre- or postsynaptic serotonin receptors was evaluated by co-administering various drugs that possess either serotonin agonist or antagonist properties, while concurrently monitoring behavior and the electrophysiological activity of serotonin-containing dorsal and median raphe neurons in freely moving cats. Co-administration of the serotonin receptor blockers, metergoline or mianserin, with lysergic acid diethylamide (LSD) produced no change in the inhibitory effects of LSD on raphe neurons, but produced a dose-dependent blockade of the behavioral effects of LSD in the cat. The latter data suggest that perhaps LSD exerts its behavioral effects by an action at postsynaptic serotonin receptors. Co-administration of drugs that increase synaptic serotonin, L-5-hydroxytryptophan, tranylcypromine, fluoxetine or p-chloramphetamine with LSD greatly potentiated the inhibitory effect of LSD on raphe unit activity, but also produced dose-dependent decreases in these behavioral effects of LSD in the cat. Thus, both enhancing the activity at postsynaptic serotonin receptors and receptor antagonism blocked the behavioral effects of LSD. Co-administration of dopamine receptor blockers, haloperidol or chlorpromazine, produced no significant change in the response of raphe neurons to LSD, but these drugs also produced a dose-dependent blockade of the behavioral effects of LSD in the cat. Co-administration of the dopamine agonists, apomorphine or d-amphetamine, however, potentiated the behavioral effects of LSD, while producing a partial reversal of the inhibitory effects of LSD on raphe unit activity. The results are discussed in the context of using animal models to study the possible actions of hallucinogens in humans.

Comparative effects of LSD and lisuride: clues to specific hallucinogenic drug actions

This review compares the effects of LSD and its nonhallucinogenic congener lisuride hydrogen maleate (LHM) on various biochemical, behavioral and electrophysiological indices of neuronal function. The underlying rationale is that any differences between the effects of LSD and LHM might be relevant to neuronal actions which are unique and specific to hallucinogenic drugs and thereby provide clues to the neurobiological substrates of hallucinogenesis. In biochemical studies, LHM appears to be very similar to LSD with respect to its actions on monoaminergic (5-HT, DA, NE) systems. The major difference between the two ergots appears quantitative in nature since LHM is more potent than LSD, especially on DA neurochemistry. Needed at the present time are additional comparative studies of LSD and LHM with respect to other biochemical measures, for example on the release of 5-HT and DA and comparisons at more molecular levels such as subcellular compartmentation. Also necessary are more intensive regional analyses on specific subpopulations of 5-HT and DA systems (mesolimbic, mesostriatal and mesocortical). Behavioral studies are relatively uniform in their characterization of the greater DA-ergic activity of LHM as compared to LSD. In particular, the drug discrimination (DD) procedure has indicated a more specific interaction of LSD with 5-HT neuronal systems as compared to LHM and has successfully differentiated the relative roles of 5-HT and DA systems in the behavioral effects of LSD and LHM. Electrophysiological studies have been consistent with both biochemical and behavioral findings with respect to the much greater effect of LHM on DA receptors. In fact, the effects of LSD on DA-containing neurons are both weak and heterogeneous, again indicating a need for more detailed analyses of specific DA projection systems. The greater potency of LHM than LSD on 5-HT containing dorsal raphe neurons has lessened the attractiveness of the once popular theory that hallucinogenic efficacy is related to diminution of impulse flow in 5-HT systems but has also spawned greater interest in the possible role of postsynaptic 5-HT receptors in hallucinogenic drug action. Thus far, the most interesting finding is the ability of LSD and other hallucinogens, but not LHM, to potentiate an excitomodulatory effect of 5-HT in the facial motor nucleus. If such a phenomenon occurs more generally in the CNS, the importance of this finding will be greatly enhanced. Preliminary data is presented which suggests that LSD may also induce such an effect in a limbic forebrain structure, the nucleus accumbens.(ABSTRACT TRUNCATED AT 400 WORDS)

A primate model for the study of hallucinogens

An animal model for studying the actions of hallucinogenic drugs using primate social colonies is presented. Although hallucinogens induce a number of behavioral changes in this paradigm, one emergent behavior, limb jerks, appears to be selectively induced by three classes of hallucinogens in doses which correlate with those reported to be hallucinogenic in humans. Several non-hallucinogenic congeners of hallucinogens failed to significantly elicit this response. Other behavioral changes induced by hallucinogens in monkeys such as ptosis and social withdrawal may be useful in studying aspects of hallucinogen intoxication other than hallucinations, or psychosis in general. Upon daily administration, tolerance developed to all hallucinogens tested except two, as is seen in humans. Moreover, cross-tolerance between hallucinogens could be demonstrated. Further experiments with the hallucinogen 5-methoxy N,N-dimethyltryptamine revealed that although certain individual behaviors could be antagonized by serotonin antagonists, dopamine antagonists, and physostigmine, no drug completely reversed the behavioral abnormalities induced by this hallucinogen. It is suggested that this paradigm, which offers an hallucinogen-induced behavior which correlates well with the human hallucinogen response and permits observation of a wide variety of other potentially relevant behaviors in primates, may be useful in developing and testing theories of hallucinogenic drug action. It may be especially valuable in view of the present difficulties of conducting hallucinogen research in humans.

Mechanistic studies on DOM as a discriminative stimulus

Ten rats were trained to discriminate racemic DOM (1.0 mg/kg, IP) from saline using a standard two-lever operant procedure. Once responding was stable, these animals were administered doses of lisuride and the purported 5-HT1 agonist 8-OH DPAT in tests of stimulus generalization. DOM-stimulus generalization occurred with lisuride, but not with 8-OH DPAT. These animals were also administered doses of LY-53,857, ritanserin, CP-52,215, and THT in tests of stimulus antagonism. Each of these agents possesses a significant affinity for 5-HT2 binding sites, and each effectively attenuated the DOM-stimulus. These results, coupled with our earlier findings, support the hypothesis that DOM may be producing its stimulus effects via a 5-HT2-related mechanism.

Separation of tolerance to the behavioral effects of LSD from changes in serotonin receptor binding in cats

Nearly complete tolerance to a test dose of 50 micrograms/kg of LSD occurred within 24 h following an initial dose of 50 micrograms/kg of the drug, using limb flicking and abortive grooming as as behavioral indices in the cat. No changes in serotonin receptor binding were observed at this time. However, chronic administration of LSD (50 micrograms/kg every 12 h for 6 days) produced a significant decrease in serotonin receptor binding in both the forebrain and brainstem plus spinal cord. These data suggest that the behavioral effects of LSD and tolerance to LSD as measured here are mediated predominantly by different sites.

Long-lasting behavioral effects of bromocriptine in cats

The objective was to determine the behavioral effects and duration of action of bromocriptine (BC) doses from 6 to 60 mg/kg i.p. Cats were housed in large outdoors cages designed for prolonged observation using an ethological approach. Baseline behavior was measured after a 3 month period of habituation. The frequency of 12 behaviors was then scored continuously over 12 h following BC administration. Cats were also observed for one h at 24, 30, 36 and 48 h post-drug. The behavioral effects of BC were remarkably similar to those of LSD and psilocybin but lasted much longer. Treated cats showed hallucinatory-like and escape behavior. Abnormal involuntary movements such as limb flicks, abortive grooming, and head and body shakes were also evoked with a dose-dependent frequency. Lower doses (6-20) produced generalized arousal, but larger doses (30-60) resulted in a decrease in arousal. After 30 mg/kg, hallucinatory-like behavior and general activity (rubbing, treading, kneading) were high at 48 h post-drug, but there were no longer abnormal movements. Behavioral suppression followed the 60 mg/kg dose and peak effects were at 96 h post-injection. This model may prove useful to further study the neural basis of the psychoactive effects of BC.

Ontogeny of the behavioral effects of lysergic acid diethylamide in cats

The ontogeny of the behavioral effects of lysergic acid diethylamide (LSD) was examined in cats between the ages of 4 and 112 days postpartum. The kittens showed little LSD-induced behavioral change prior to 14 days of age. By the age of 21 days, however, the kittens exhibited many of the behavioral signs characteristic of LSD-induced behaviors in adult cats. These behaviors include limb-flicking, abortive grooming, head-shakes, grooming, and investigatory responses. In general, these behaviors began at a low frequency of occurrence, then increased rapidly with advancing age, reaching adult values by approximately 35-40 days of age, and remained relatively constant through 112 days postpartum. The time course for the behavioral effects following an acute injection of LSD showed the adult pattern, i.e., persisting for approximately 8 hr post-injection, from their earliest appearance during ontogeny. Young kittens (21-42 days of age) were resistant to the development of tolerance following repeated administration of the drug. LSD was capable of eliciting certain behaviors, such as head-shakes and grooming, well in advance of the age at which they normally appear spontaneously. This indicates that the neuronal and musculature substrata are developed for the performance of these behaviors long before the kitten naturally employs them.

Some behavioral effects of hallucinogens are mediated by a postsynaptic serotonergic action: evidence from single unit studies in freely moving cats

Although central serotonergic systems appear to be linked importantly to the mechanism of action of a variety of hallucinogenic drugs, the nature of this interaction has remained unclear. In the present study, the question of whether the critical link is presynaptic or postsynaptic was examined in cats. Behaviorally inactive doses (1.0 mg/kg) of the serotonin receptor antagonists mianserin, ketanserin or metergoline effectively blocked behavior, as measured by the cat limb flick response, elicited by either LSD (50 micrograms/kg) or DOM (250 micrograms/kg) but not that resulting either from lisuride (50 micrograms/kg) or a high dose of apomorphine (4 mg/kg). Pretreatment with 1.0 mg/kg of mianserin, which completely attenuated LSD's behavioral effect, failed to alter LSD-induced depression of mesencephalic serotonergic neuron discharge. These results demonstrate that at least some of the behavioral effects of LSD can be blocked by pharmacological antagonism of postsynaptic serotonin receptors which leaves LSD's presynaptic effect unaffected. Thus, the behavioral, and possibly psychoactive, effects of hallucinogens appear to be attributable to an action at 5HT2 receptors, presumably located postsynaptically.

Behavioral effects of serotonergic and dopaminergic drugs in cats following chronic amphetamine administration

Chronic administration of amphetamine to cats (twice daily, in doses increasing from 5 to 15 mg/kg over a 10-day period) elicited a number of behaviors e.g., limb flicking, abortive grooming, and excessive head shaking, which were originally proposed as an animal behavioral model for studying the actions of hallucinogens that depress central serotonergic neurotransmission. This drug treatment produced large decreases (approximately 50%) in central nervous system serotonin (5HT) and its major metabolite, 5-hydroxyindoleacetic acid, and even larger decreases (approximately 90%) in the levels of dopamine (DA) and norepinephrine. Administration of the 5HT precursors L-tryptophan (25 mg/kg i.p.) or L-5-hydroxytryptophan (12.5 mg/kg i.p.), a direct-acting 5HT agonist (quipazine, 1 mg/kg i.p.) or a monoamine oxidase inhibitor (tranylcypromine, 4 mg/kg i.p.) produced no significant changes in these behaviors in cats treated chronically with amphetamine. Administration of a 5HT reuptake blocker (fluoxetine, 5 mg/kg i.p.) produced a small, but significant, decrease in the frequency of occurrence of these behaviors in amphetamine-treated cats. L-Dihydroxyphenylalanine (L-DOPA, 20 mg/kg i.p.) greatly potentiated these behaviors in cats chronically treated with amphetamine, but L-DOPA was totally ineffective in eliciting these behaviors in naive animals. The behavioral effects of apomorphine (2 mg/kg i.p.) were also significantly potentiated by chronic amphetamine pretreatment. The amino acid precursor of DA, L-tyrosine (25 mg/kg i.p.), and a DA reuptake blocker, bupropion (5 mg/kg i.p.) were without significant effect on these behaviors in amphetamine-treated cats. The data suggest that these cat behaviors are elicited by an action at central DA receptors and that these receptors become supersensitive following chronic amphetamine administration. Furthermore, there may be a qualitative change in DA receptors, since L-DOPA is very effective in potentiating these behaviors in cats treated chronically with amphetamine, but is totally ineffective in naive cats.

Tolerance develops to LSD while the drug is exerting its maximal behavioral effects: implications for the neural bases of tolerance

Tolerance to a test dose of 50 mg/kg of LSD occurred within 0.5-1.0 h following an initial dose of 10 mg/kg of the drug, using limb flicking and abortive grooming as behavioral indices in the cat. These findings represent an example of very rapidly developing drug tolerance using a behavioral index. These data are discussed within the context of hypotheses concerning the neurochemical bases of tolerance to LSD.

Dissociations between the behavioral effects of LSD and tolerance development during ontogeny in cats: a novel approach to the study of tolerance mechanisms

The characteristic behavioral effects of d-lysergic acid diethylamide (LSD) in cats first appeared at approximately 25 days of age and increased rapidly in magnitude over the next 10 days. However, 25 day old kittens showed no tolerance to the repeated administration of the drug. While the behavioral response to the initial dose of LSD remained relatively constant between 35 and 112 days of age, the tolerance gradually became more pronounced throughout this time period, reaching an adult level of virtually complete tolerance at 112 days. These findings provide new insight into the nature of the relationship between the primary drug action and the development of tolerance, and suggest a new strategy for investigating the neural bases of tolerance, i.e., examining the neurochemical effects of repeated LSD administration in kittens during various stages of tolerance development.

Antagonism of a behavioral effect of LSD and lisuride in the cat

These experiments investigated the role of serotonin 5-HT) and dopamine (DA) receptors in the limb-flick (LF) response elicited by the hallucinogenic ergot LSD and its nonhallucinogenic structural congener lisuride. Pretreatment with either the 5-HT antagonist pizotifen (BC-105) or the DA antagonist haloperidol significantly attenuated LF elicited by either LSD or lisuride. Thus, the LF model failed to differentiate the neuropharmacological actions of LSD and lisuride. Cocaine also prevented LSD- and lisuride-elicited LF simply by reducing the activity of cats (response competition) suggesting the need for caution in interpreting 'antagonism' of the LF response.

Analyzing mechanism(s) of hallucinogenic drug action with drug discrimination procedures

Some of the advantages of using drug discrimination (DD) procedure to analyze the mechanisms of action of hallucinogenic and related drugs were illustrated by reviewing research with lysergic acid diethylamide (LSD). Because they ensure that drug-induced alterations in interoceptive "state" become biologically meaningful "cues," these procedures are reliable, robust, sensitive and specific. With reference to LSD, many DD experiments suggest: (1) that while hallucinogens substitute for (mimic) LSD (in rats), such substitution does not predict hallucinogenic potency (in humans) but does predict similarities in mechanism(s) of action; (2) the behavioral (in vivo) effects of LSD, unlike those of some of its congeners, are mediated primarily by central, serotonergic (5-HT) neuronal mechanism although LSD may also have (secondary) dopamine (DA) agonist properties; (3) both the locus and the nature of these LSD-5-HT interactions are unclear: cells arising from B-7, B-8 and B-9 regions of the dorsal-medial raphe may be involved; pretreatment with agents that deplete 5-HT and increase the stereospecific binding of 3H-LSD in vitro (p-chlorophenylalanine; 5,7-dihydroxytryptamine) enhance sensitivity to LSD in vivo.

Lack of synergism and cross tolerance between tactile stimulus- and LSD-induced limb flicking in the cat

The hypotheses that LSD-induced limb flicking, as well as tolerance to this behavioral effect following repeated drug administration, are due to alterations in somatosensory thresholds were tested by examining the rate of limb flicking to LSD alone, saline plus water on the limbs, or LSD plus water on the limbs, and by comparing the limb flick rate with water on the limbs in drug tolerant versus non-tolerant conditions. Cats exhibited the same rate of limb flicking in response to water on the limbs regardless of whether they were pretreated with saline of LSD. Furthermore, there was no significant difference in the tactile stimulus-induced rate of limb flicking in the tolerant versus non-tolerant states. These data suggest that LSD-induced limb flicking is not simply a function of drug-induced altered somatosensory thresholds, but is apparently reflective of more complex neural processes.

Raphe unit activity in freely moving cats: effects of quipazine

Quipazine produced a dose-dependent decrease in the discharge rate of serotonin-containing neurons in the dorsal raphe nucleus of freely-moving cats. This ranged from a 10% decrease at 0.5 mg/kg, (i.p.), to a virtually complete depression of activity at 5.0 mg/kg. The effects of quipazine on raphe units occurred with a short latency (5--10 min) and its duration of action was dose-dependent and lasted from 1 to 6 hr. The degree of depression of raphe unit activity was directly related to the frequency of occurrence of a number of behaviors such as limb flicking and abortive grooming. There was a close temporal correlation between the depression of raphe unit activity and the occurrence of these behaviors. These data reveal that quipazine produces behavioral and raphe unit changes similar to those observed after administration of hallucinogens with an indole nucleus.

Behavioral and neurochemical effects of apomorphine in the cat

Administration of apomorphine (2-10 mg/kg i.p.) elicited a number of behaviors, such as limb flicking, abortive grooming, investigatory and hallucinatory-like responses, head and body shakes, and excessive grooming, which we have previously proposed as an animal model for studying the actions of LSD and related hallucinogens. Repeated administration of apomorphine resulted in a significant tolerance, which occurred within 2 h of the initial injection, and completely dissipated within 24 h. A pronounced LSD-apomorphine cross tolerance was observed; however, there was no significant apomorphine-LSD tolerance. Apomorphine-induced behavioral changes were blocked by prior treatment with haloperidol, but were unchanged by pretreatment with L-DOP[A. Administration of L-DOPA, in combination with a peripheral decarboxylase inhibitor, did not elicit these characteristic behavioral changes. Increasing synaptic serotonin levels by monoamine oxidase inhibition, precursor administration, or reuptake blockade in general did not alter the behavioral response to apomorphine. Similarly, pretreatment with serotonin receptor blockers produced no large changes in apomorphine-induced behaviors. Prior serotonin depletion with chronic p-chlorophenylalanine administration, however, potentiated certain apomorphine-induced behaviors. Neurochemical studies revealed that apomorphine administration increased striatal dopamine, and decreased dopamine metabolites. Norepinephrine levels were generally decreased throughout the CNS by apomorphine treatment. Administration of apomorphine increased CNS serotonin and 5-hydroxyindoleacetic acid levels, while tryptophan levels were unchanged. The biological bases of the limb flick model is discussed in the context of these pharmacological and neurochemical studies.

Behavioral effects of quipazine in the cat

Administration of quipazine to cats elicits a number of behaviors, such as limb flicking abortive grooming, investigatory behavior and hallucinatory-like behavior, which we have previously proposed as an animal behavioral model for studying the actions of LSD and related hallucinogens. While recent studies have indicated that these model behaviors may not be totally specific for hallucinogenic drugs, the model can still be useful for studying drug action. Quipazine (0.5-5.0 mg/kg i.p.) produced significant increases in limb flicking, abortive grooming, investigatory behavior, hallucinatory-like behavior grooming, head and body shakes, staring and yawning. These behavioral changes persisted for 1-6 h, depending on the dose of quipazine employed. Administration of quipazine (5.0 mg/kg per day) for 5 consecutive days produced no significant tolerance effect on any of these model behaviors. These quipazine induced behavioral changes were potentiated by pretreatment with apomorphine, and partially blocked by pretreatment with haloperidol. Quipazine-induced behavioral changes were potentiated by prior serotonin depletion with p-chlorophenylalanine, and completely blocked by pretreatment with a monoamine oxidase inhibitor or the serotonin precursor, L-5-hydroxytryptophan. These quipazine-induced behavioral changes were also blocked by pretreatment with the serotonin receptor blockers, cinnanserin, methysergide or cyproheptadine. The mechanism of action of quipazine, as well as the neuropharmacology of the limb flick model, is discussed in the content of these studies with serotonergic and dopaminergic drugs.

Pilocarpine, a non-hallucinogenic cholinergic agonist, elicits limb flicking in cats

The centrally- and peripherally-acting muscarinic cholinergic agonist pilocarpine (PILO, 0.125-1.0 mg/kg, IP) elicited a significantly increased frequency of occurrence of limb flicking at 0.25-1.0 mg/kg, and significantly increased the frequency of occurrence of other grooming behaviors, in 4-6 cats in the 90 min following its administration. These effects of PILO at 0.5 mg/kg were antagonized by the peripherally-acting antimuscarinic agent, N-methylscopolamine (MESCO, 0.5 mg/kg, IP), when MESCO was administered 15 min before PILO. The same lisuride (LIS, 0.05 mg/kg, IP). These results provide further evidence that a cat behavior model for LSD-like hallucinogens which employs limb flicking and similar grooming behaviors is not specific for hallucinogens; indicate that important "model behaviors" may be elicited by a peripheral mechanism; and show that a peripheral muscarinic cholinergic mechanism is not responsible for LSD- and LIS-elicited limb flicking. The results also suggest that the increased frequency of occurrence of the model behaviors after PILO reflects their function as grooming behaviors, elicited by PILO's intense cholinergic effects, including salivation or sialorrhea and emesis.

Serotonergic and dopaminergic effects of yawning in the cat

The serotonergic agents LSD (0.01-0.05 mg/kg) and lisuride (0.025 and 0.05 mg/kg) elicited a high frequency of limb flicking in the cat after IP doses; LSD, but not lisuride, elicited a significantly increased frequency of yawning as well. In combination, LSD plus lisuride (0.025 mg/kg each) gave additive frequencies of limb flicking, but the frequency of yawning was half that after LSD alone. The dopamine agonist apomorphine had no significant effect on either yawning or limb flicking over the dose range 0.006 to 3.2 mg/kg. Pretreatment of cats with 1.0 mg/kg of apomorphine (but not with 0.05 mg/kg) significantly reduced the frequency of yawning elicited by 0.01 or 0.025 mg/kg of LSD, but had no effect on limb flicking. The dopamine antagonist haloperidol had no effect on limb flicking at doses from 0.008 to 0.512 mg/kg, but produced a significantly increased frequency of yawning at 0.256 mg/kg, an effect antagonized by lisuride administration. Given that lisuride has more potent dopamine agonist properties than LSD, these results are consistent with serotonergic elicitation of yawning, dopaminergic inhibition of yawning, and with their concomitant interaction in the expression of drug-induced yawning in the cat. The behavioral pharmacologies of limb flicking and yawning are different in this species.