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

Multiple receptors contribute to the behavioral effects of indoleamine hallucinogens

Serotonergic hallucinogens produce profound changes in perception, mood, and cognition. These drugs include phenylalkylamines such as mescaline and 2,5-dimethoxy-4-methylamphetamine (DOM), and indoleamines such as (+)-lysergic acid diethylamide (LSD) and psilocybin. Despite their differences in chemical structure, the two classes of hallucinogens produce remarkably similar subjective effects in humans, and induce cross-tolerance. The phenylalkylamine hallucinogens are selective 5-HT(2) receptor agonists, whereas the indoleamines are relatively non-selective for serotonin (5-HT) receptors. There is extensive evidence, from both animal and human studies, that the characteristic effects of hallucinogens are mediated by interactions with the 5-HT(2A) receptor. Nevertheless, there is also evidence that interactions with other receptor sites contribute to the psychopharmacological and behavioral effects of the indoleamine hallucinogens. This article reviews the evidence demonstrating that the effects of indoleamine hallucinogens in a variety of animal behavioral paradigms are mediated by both 5-HT(2) and non-5-HT(2) receptors.

Behavioral, sleep-waking and EEG power spectral effects following the two specific 5-HT uptake inhibitors zimeldine and alaproclate in cats

Sleep, waking and EEG power spectra were studied in cats for 15 h following peroral administration of placebo or 10 mg/kg and 20 mg/kg of the 5-HT reuptake inhibitors zimeldine and alaproclate. Behavior was also observed during the initial period following drug administration. Both drugs had effects on motor behavior and initiated hallucinatory like behavior. Zimeldine increased latency to stable sleep and to SWS-2. Alaproclate increased latency to SWS-1. Both drugs increased SWS (NREM sleep) and particularly SWS-2. REM sleep latency was increased and REM sleep was reduced following both drugs. EEG slow wave activity was increased following zimeldine. It is concluded that the 5-HT stimulation caused by the drugs yields complex effects on the sleep-waking axis, both sleep incompatible and sleep promoting effects.

Serotonin neuron transplants: electrophysiological unit activity of intrahippocampal raphe grafts in freely moving cats

Fetal serotonin (5HT)-containing neurons from the cat midbrain raphe were transplanted to the hippocampus of adult host cat brain. These neurons displayed spontaneous electrophysiological activity in freely moving cats, and showed the pacemaker-like discharge pattern characteristic of these cells. However, these neurons showed no significant change in activity across the sleep-waking cycle, nor were they responsive to sensory stimulation in the auditory and visual modalities. Grafted neurons were inhibited by 5HT agonists, but required higher doses than those in intact cats. These data are the first electrophysiological recordings of grafted 5HT neurons in freely moving animals, and question whether such tissue grafts can restore functional activity in the central 5HT system.

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.