Hallucinogens potentiate responses to serotonin and norepinephrine in the facial motor nucleus

Natural psychedelics in the treatment of depression; a review focusing on neurotransmitters

Natural psychedelic compounds are emerging as potential novel therapeutics in psychiatry. This review will discuss how natural psychedelics exert their neurobiological therapeutic effects, and how different neurotransmission systems mediate the effects of these compounds. Further, current therapeutic strategies for depression, and novel mechanism of action of natural psychedelics in the treatment of depression will be discussed. In this review, our focus will be on N, N-dimethyltryptamine (DMT), reversible type A monoamine oxidase inhibitors, mescaline-containing cacti, psilocybin/psilocin-containing mushrooms, ibogaine, muscimol extracted from Amanita spp. mushrooms and ibotenic acid.

Psychedelics in Psychiatry: Neuroplastic, Immunomodulatory, and Neurotransmitter Mechanisms

Mounting evidence suggests safety and efficacy of psychedelic compounds as potential novel therapeutics in psychiatry. Ketamine has been approved by the Food and Drug Administration in a new class of antidepressants, and 3,4-methylenedioxymethamphetamine (MDMA) is undergoing phase III clinical trials for post-traumatic stress disorder. Psilocybin and lysergic acid diethylamide (LSD) are being investigated in several phase II and phase I clinical trials. Hence, the concept of psychedelics as therapeutics may be incorporated into modern society. Here, we discuss the main known neurobiological therapeutic mechanisms of psychedelics, which are thought to be mediated by the effects of these compounds on the serotonergic (via 5-HT_2A and 5-HT_1A receptors) and glutamatergic [via N-methyl-d-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors] systems. We focus on 1) neuroplasticity mediated by the modulation of mammalian target of rapamycin-, brain-derived neurotrophic factor-, and early growth response-related pathways; 2) immunomodulation via effects on the hypothalamic-pituitary-adrenal axis, nuclear factor ĸB, and cytokines such as tumor necrosis factor-α and interleukin 1, 6, and 10 production and release; and 3) modulation of serotonergic, dopaminergic, glutamatergic, GABAergic, and norepinephrinergic receptors, transporters, and turnover systems. We discuss arising concerns and ways to assess potential neurobiological changes, dependence, and immunosuppression. Although larger cohorts are required to corroborate preliminary findings, the results obtained so far are promising and represent a critical opportunity for improvement of pharmacotherapies in psychiatry, an area that has seen limited therapeutic advancement in the last 20 years. Studies are underway that are trying to decouple the psychedelic effects from the therapeutic effects of these compounds. SIGNIFICANCE STATEMENT: Psychedelic compounds are emerging as potential novel therapeutics in psychiatry. However, understanding of molecular mechanisms mediating improvement remains limited. This paper reviews the available evidence concerning the effects of psychedelic compounds on pathways that modulate neuroplasticity, immunity, and neurotransmitter systems. This work aims to be a reference for psychiatrists who may soon be faced with the possibility of prescribing psychedelic compounds as medications, helping them assess which compound(s) and regimen could be most useful for decreasing specific psychiatric symptoms.

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.

Lysergic acid diethylamide (LSD) is a partial agonist of D2 dopaminergic receptors and it potentiates dopamine-mediated prolactin secretion in lactotrophs in vitro

The hallucinogenic effects of lysergic acid diethylamide (LSD) have mainly been attributed to the interaction of this drug with the serotoninergic system, but it seems more likely that they are the result of the complex interactions of the drug with both the serotoninergic and dopaminergic systems. The aim of the present study was to investigate the functional actions of LSD at dopaminergic receptors using prolactin secretion by primary cultures of rat pituitary cells as a model. LSD produced a dose-dependent inhibition of prolactin secretion in vitro with an IC50 at 1.7x10(-9) M. This action was antagonized by spiperone but not by SKF83566 or cyproheptadine, which indicates that LSD has a specific effect on D2 dopaminergic receptors. The maximum inhibition of prolactin secretion achieved by LSD was lower than that by dopamine (60% versus 80%). Moreover, the fact that LSD at 10(-8)-10(-6) M antagonized the inhibitory effect of dopamine (10(-7) M) and bromocriptine (10(-11) M) suggests that LSD acts as a partial agonist at D2 receptors on lactotrophs in vitro. Interestingly, LSD at 10(-13)-10(-10) M, the concentrations which are 10-1000-fold lower than those required to induce direct inhibition on pituitary prolactin secretion, potentiated the dopamine (10(-10)-2.5x10(-9) M)-mediated prolactin secretion by pituitary cells in vitro. These results suggest that LSD not only interacts with dopaminergic receptors but also has a unique capacity for modulating dopaminergic transmission. These findings may offer new insights into the hallucinogenic effect of LSD.

LSD has high efficacy relative to serotonin in enhancing the cationic current Ih: intracellular studies in rat facial motoneurons

The effects of LSD (d-lysergic acid diethylamide) on rat facial motoneurons were compared to those of 5-hydroxytryptamine (5-HT) in brain slices by means of current clamp and single-electrode voltage-clamp recordings. As previously reported, 5-HT, in part by decreasing a resting potassium conductance, produced a reversible depolarization (approximately 5 mV), an increase in input resistance, and an enhancement in electrical excitability. LSD also produced an increase in electrical excitability, although with a much slower onset and longer duration. However, in contrast to 5-HT, LSD produced only a slight depolarization (1-2 mV). Moreover, in the presence of LSD the depolarizing effect of 5-HT was markedly attenuated. The 5-HT2/5-HT1C agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) produced effects intermediate between LSD and 5-HT. The LSD-induced increase in electrical excitability was completely reversed by spiperone, a 5-HT2/5-HT1A antagonist, and by ritanserin, a 5-HT2/5-HT1C antagonist; the effects of 5-HT were also reduced by these 2 antagonists, but complete blockade did not occur at the concentrations and durations tested. Surprisingly, LSD was found to enhance the hyperpolarization-activated nonspecific cation current Ih to a greater extent than did 5-HT; this enhancement was blocked by both spiperone and ritanserin. These results indicate that, despite having low efficacy relative to 5-HT in decreasing resting potassium conductance, LSD has high efficacy in enhancing the Ih current in rat facial motoneurons; possible mechanisms for this difference are discussed.

A review and reevaluation of the role of serotonin in the modulation of lordosis behavior in the female rat

The role of serotonin (5-HT) in the modulation of sexual receptivity (lordosis) in the female rat is reviewed and reevaluated. The effects on lordosis of drug treatments that decrease or increase the activity and availability of central 5-HT are first discussed, and this is followed by an evaluation of the effects of drugs that act directly at 5-HT receptors. In order to shed light on the physiological significance of effects of serotonergic drugs on lordosis, there is also a review of what is known of changes in levels of serotonergic activity and densities of 5-HT receptors in the female rat brain that take place through the estrous cycle and in response to administration of behaviorally effective doses of gonadal steroids. Serotonin has generally been thought to have a tonic, inhibitory effect on lordosis. However, it is concluded that 5-HT can either inhibit or facilitate lordosis depending on which subtypes of central 5-HT receptors become activated. Because of a lack of consistent or compelling evidence of effects of ovarian hormones on serotonergic activity or 5-HT receptors in critical areas of the brain, it is stated that there is at present no basis to conclude that the effects of pharmacological manipulations of serotonergic activity on lordosis reflect an important, physiological role of 5-HT in the modulation of lordosis behavior in the female rat.

Electrophysiology of the central serotonin system: receptor subtypes and transducer mechanisms

It is becoming evident that the diverse electrophysiological actions of 5-HT in the central nervous system can be best formulated in terms of receptor subtypes and their respective effector mechanisms. Based on the findings described in this review, the following pattern of central 5-HT electrophysiology is emerging: 1) inhibitory effects are mediated by 5-HT1 receptors linked to the opening of K channels via pertussis-toxin sensitive G proteins: 2) facilitatory effects are mediated by 5-HT2 receptors and involve the closing of K channels, an effect which appears to be negatively modulated by activation of the PI second messenger system: 3) fast excitations are mediated by 5-HT3 receptors, most likely involving a direct interaction with an ion channel rather than through coupling with a G protein or a second messenger. Further studies will be required in a wider range of brain areas to establish the generality of these conclusions.

Serotonin (5-HT) induces IPSPs in pyramidal layer cells of rat piriform cortex: evidence for the involvement of a 5-HT2-activated interneuron

In a slice preparation of rat piriform cortex, both intracellular and extracellular techniques were used to examine the pharmacological and electrophysiological actions of serotonin (5-HT). Bath application of 5-HT resulted in either depolarization (57%), hyperpolarization (34%) or no change (9%) in membrane potential of cells in the pyramidal cell layer (layer II) of piriform cortex. Additionally, when KCl-containing electrodes were used, 5-HT induced an increase in depolarizing synaptic potentials in 41% of these cells. It was concluded that these potentials were reverse inhibitory post-synaptic potentials (IPSPs) because they were blocked by bicuculline and tetrodotoxin. The induction of IPSPs by 5-HT was blocked by the 5-HT2-selective antagonist ritanserin. By recording extracellularly in the presence of 5-HT, a group of 5-HT-activated, putative interneurons was found at the border of layers II and III of piriform cortex, 5-HT but not norepinephrine activation was blocked by ritanserin. The actions of 5-HT were mimicked by the 5-HT2 agonist alpha-methyl-5-HT; the 5-HT2 partial agonist, 2,5-dimethoxy-4-methyl-amphetamine had a small agonist action of its own and blunted the effect of 5-HT. Activation of a larger group of putative interneurons by the more universal excitant N-methyl-D-aspartate showed that the 5-HT-activated interneurons represented 23% of the interneurons located on the border between layers II and III. We conclude that 5-HT induces IPSPs in layer II pyramidal cells by activating a subpopulation of interneurons at the border of layers II and III of piriform cortex.

The major hallucinogens and the central cytoskeleton: an association beyond coincidence? Towards sub-cellular mechanisms in schizophrenia

There appears to be a remarkably consistent structural and functional relationship between the phenylethylamine hallucinogens and the microtubule inhibitor colchicine. Such a relationship is not sustained in simple form through to the indoleamine hallucinogens and the indole based Vinca alkaloids. However, LSD and the more potent hallucinogens retain the full potential to disrupt the structure of the brain's cytoskeleton indirectly via serotonin and the raphe system. Serotonin appears to have a direct role in regulating and maintaining microtubules and microfilaments. It appears that a second receptor mediated action is required for full hallucinogenic activity. It is deduced that cytoskeletal restraints may have a role in governing central information processing. A theory for the cellular mechanisms of thought disorder and drug induced hallucinations is proposed. Schizophrenia may reflect a subtle disorder of central cytoskeletal function.

Serotonin excitation of facial motoneurons: receptor subtype characterization

The receptor subtype(s) mediating the enhancement of facial motoneuron excitability by serotonin (5-HT) was evaluated by means of single-cell recording in vivo (in the anesthetized rat) and in vitro in brain slices. In vivo, microiontophoretic application of the broad-spectrum 5-HT1 agonist 5-carboxamidotryptamine (5-CT), the 5-HT2/5-HT1C agonist 1-[2,5-dimethoxy-4-methylphenyl]-2-aminopropane (DOM), but not the selective 5-HT1A agonist 8-OH-2[di-n-propylamino]tetralin (8-OH-DPAT), produced a 5-HT-like enhancement of facial motoneuron excitability. Intravenous administration of the 5-HT2/5-HT1C antagonists ritanserin and LY 53857 in vivo blocked the facilitatory effects of 5-HT and DOM, but not norepinephrine (NE). Similarly, in brain slices, bath application of ritanserin blocked the effects of 5-HT, DOM, and 5-CT, but not NE on facial motoneurons. Intracellular recordings showed that DOM induced a slow depolarization and an increase in evoked spikes, but these effects were of lesser magnitude and longer duration than those produced by 5-HT. Taken together, these results indicate a role for 5-HT2 and/or 5-HT1C but not 5-HT1A receptors in serotonergic enhancement of facial motoneuron excitability since 5-HT's effect was 1) at least partially mimicked by the selective 5-HT2/5-HT1C agonist DOM, 2) mimicked by the broad-spectrum 5-HT1 agonist 5-CT but not the selective 5-HT1A agonist 8-OH-DPAT, and 3) blocked by the 5-HT2/5-HT1C antagonists ritanserin and LY 53857.

Mescaline: excitatory effects on acoustic startle are blocked by serotonin2 antagonists

The ability of serotonin2 (5-HT2) antagonists to block the excitatory effects of mescaline on the acoustic startle reflex were analyzed. Mescaline (20 mg/kg) caused a consistent increase in the amplitude of the acoustic startle reflex. This effect was blocked in a dose-related fashion by the 5-HT2 antagonist ritanserin (ED50 dose = 0.25 mg/kg IP). In contrast, even a high dose of ritanserin (2.0 mg/kg) did not block the excitatory effects of amphetamine on startle. Other 5-HT2 antagonists (ketanserin, cinanserin, LY 53857) also blocked mescaline's effect, whereas the 5-HT1 antagonist pindolol (5 mg/kg) did not. These results support the hypothesis that the behavioral effects of hallucinogens are mediated by agonist actions at 5-HT2 receptors.

Effect of hallucinogens on spontaneous and sensory-evoked locus coeruleus unit activity in the rat: reversal by selective 5-HT2 antagonists

As previously reported, systemic administration of the hallucinogens D-lysergic acid diethylamide (LSD) (5-10 micrograms/kg) and mescaline (2 mg/kg) in the anesthetized rat produced a decrease in spontaneous activity but, paradoxically, facilitated activation of locus coeruleus (LC) neurons by sciatic nerve stimulation. In the present study, the hallucinogen 2,5-dimethoxy-4-methylamphetamine (DOM) (20-80 micrograms/kg) was found to have similar effects. Systemic administration of the selective 5-HT2 antagonists LY 53857 (0.02-0.8 mg/kg) and ritanserin (0.1-0.3 mg/kg) completely reversed both actions of the hallucinogens on the LC. In contrast, LY 53857 did not reverse the effects of (+)-amphetamine (0.5 mg/kg) on the spontaneous or sensory-evoked activity of the LC. These results suggest that the common actions of indoleamine and phenethylamine hallucinogens displayed in the LC are mediated via 5-HT2 receptors; however, these receptors appear to be located outside the LC itself.

Direct comparison of hallucinogenic phenethylamines and D-amphetamine on dorsal raphe neurons

We compared the effects of the hallucinogens 2,5-dimethoxy-4-methylamphetamine (DOM), mescaline and the simulant D-amphetamine, applied by microiontophoresis to rat dorsal raphe (DR) units. DR neuron firing rate was relatively insensitive to DOM and unaffected by mescaline, but was clearly inhibited by D-amphetamine. Intravenous DOM usually inhibited, but this effect was correlated with blood pressure changes; i.v. D-amphetamine produced inconsistent responses. These results suggest that most of the effects seen on i.v. administration of phenethylamines are not mediated directly on the serotonergic cell.

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)

Effects of hallucinogenic drugs on serotonergic neuronal systems

Studies indicate that hallucinogens markedly suppress the discharge of serotonin containing neurons in the dorsal raphe nucleus. Forebrain neurons receiving a major serotonergic input are relatively insensitive to hallucinogens. These actions of hallucinogens are not sufficient to explain the psychoactive effects of these drugs. Evidence is presented to indicate that hallucinogens sensitize serotonin and norepinephrine receptors in the facial nucleus. This receptor sensitizing effect is common to all, and specific for, hallucinogens. It is suggested that a mechanism of receptor sensitization might account for the altered perceptual reactivity produced by hallucinogens.

The 5HT2 antagonist pirenperone reverses disruption of FR-40 by hallucinogenic drugs

Indolealkylamine and phenethylamine hallucinogens disrupted responding maintained under a fixed-ratio 40 (FR-40) schedule of reinforcement. LSD, DMT, mescaline and DOM produced dose-dependent decreases in number of reinforcers and increases in 10-sec periods of non-responding (pause-intervals). The 5HT agonist quipazine, as well as the LSD congener lisuride, altered response patterns in a similar manner. The effects of these drugs were examined after pretreatment with pirenperone, an antagonist with specificity toward the 5HT2 receptor with reference to the 5HT1 receptor. The dose-response curves for the phenethylamine hallucinogens were shifted significantly to the right and to a greater degree than were those for the indolealkylamine hallucinogens. Pirenperone also antagonized the effects of quipazine to a degree similar to that observed with the phenethylamine-type hallucinogens. Pirenperone did not significantly shift the dose-response pattern to lisuride. These data suggest that the behavioral disruption induced by these hallucinogens and quipazine relates at least in part to an effect on 5HT2 receptors, while the effects of lisuride do not involve a significant interaction at the 5HT2 receptor.

Evidence for a serotonergically mediated sympathoexcitatory response to stimulation of medullary raphe nuclei

The cardiovascular role of serotonin (5-HT) containing neurons in the midline medullary raphe nuclei was studied in anesthetized cats. High frequency electrical stimulation of nucleus (n.) raphe (r.) pallidus, n.r. obscurus and n.r. magnus produced both pressor and depressor responses. Single shock stimulation of pressor sites produced an excitatory evoked potential of sympathetic nervous discharge (SND) recorded from the inferior cardiac nerve. Conversely, single shock stimulation of vasodepressor sites resulted in a computer-summed inhibition of SND. The mean conduction velocity in the sympathoexcitatory medullo-spinal pathway to sympathetic preganglionic neurons was calculated to be 1.24 m/s. The 5-HT antagonists methysergide and metergoline blocked the excitation of sympathetic activity evoked from medullary raphe nuclei. In contrast, these agents failed to alter the sympathoexcitatory response to electrical stimulation of lateral medulla pressor sites or the sympathoinhibitory response elicited by raphe stimulation. The 5-HT uptake inhibitor chlorimipramine increased the duration of the sympathoexcitatory response evoked from the raphe but not from the lateral medulla. Finally, mid-collicular transection did not effect the excitation of sympathetic activity elicited by stimulation of medullary raphe nuclei. These data suggest that serotonergic neurons in the midline medullary raphe nuclei provide an excitatory input to sympathetic neurons in the spinal cord.

Hyperpolarization of serotonergic neurons by serotonin and LSD: studies in brain slices showing increased K+ conductance

Serotonin and LSD hyperpolarized serotonergic dorsal raphe neurons in rat midbrain slices; the hyperpolarizations were accompanied by a decrease in input resistance, suggesting an increase in potassium conductance as one possible mechanism. Reversal potentials for serotonin and LSD-induced hyperpolarizations showed a shift of approximately 18 mV for a two-fold change in extracellular potassium concentration; this shift was close to that predicted by the Nernst equation for a potassium-dependent conductance.

Mescaline elicits behavioral effects in cats by an action at both serotonin and dopamine receptors

The characteristic behavioral effects of mescaline in cats were nearly completely blocked by pretreatment with low doses of either a specific serotonin antagonist (methysergide) or a dopamine specific antagonist (haloperidol). These blocking effects were not due to non-specific actions, since methysergide did not block the behavioral effects of apomorphine, and haloperidol did not block the behavioral effects of 5-methoxy-N,N-dimethyltryptamine. Thus, it appears that the behavioral effects of mescaline are dependent upon the simultaneous action of the drug at both serotonin and dopamine receptors.

Neurophysiological effects of hallucinogens on serotonergic neuronal systems

Low intravenous doses of the hallucinogen d-lysergic acid diethylamide (LSD) markedly suppress the discharge of serotonin (5-HT)-containing neurons in the dorsal raphe nucleus of the rat. Microiontophoretically applied LSD also inhibits the firing of 5-HT neurons, indicating that the inhibitory effect is mediated directing on 5-HT neurons. Forebrain neurons receiving a major serotonergic input are relatively insensitive to LSD. Other indole hallucinogens (i.e., psilocin, dimethyltryptamine, and 5-methoxydimethyltryptamine) also preferentially inhibit raphe firing as compared to postsynaptic forebrain neurons. These observations led to the hypothesis that hallucinogens produce their psychoactive effects by acting preferentially upon 5-HT autoreceptors in the dorsal raphe allowing postsynaptic neurons to escape from the tonic inhibitory action of 5-HT neurons. However, problems exist with the concept that hallucinogens produce their psychoactive effects by disinhibiting postsynaptic neurons. First, the time course of the behavioral and neuronal effects of LSD do not correlate. Second, 5-HT neurons do not become tolerant to the inhibitory actions of LSD. Third, the hallucinogen mescaline fails to directly inhibit 5-HT neurons. Finally, the nonhallucinogen lisuride markedly suppresses the discharges of 5-HT neurons. These observations suggest that postsynaptic actions of hallucinogens may be of prime importance in producing their psychedelic effects. Evidence is presented to suggest that the hallucinogens may act postsynaptically to sensitize both serotonergic and noradrenergic receptors. It is suggested that a mechanism of receptor sensitization, in distinction to disinhibition, might account for the altered perceptual reactivity produced by these drugs.

Lysergic acid diethylamide (LSD) and lisuride: differentiation of their neuropharmacological actions

The nonhallucinogenic ergot derivative lisuride exerts many pharmacological effects that are similar to those of its hallucinogenic congener, lysergic acid diethylamide (LSD). Animals trained to discriminate between the presence of one drug and the other can be used to differentiate the actions of these compounds on a neuronal level. The discriminative stimulus effect of LSD (the LSD cue) is similar to that of the serotonin agonist quipazine, whereas the lisuride cue is similar to that of the dopamine agonist apomorphine. These data support the hypothesis that serotonin is intricately involved in the hallucinogenic effects of LSD.

LSD mescaline and serotonin injected into medial raphe nucleus potentiate apomorphine hypermotility

Microinjections of LSD (0.05 microgram), mescaline (0.5 microgram) and serotonin (10 microgram) into the medial raphe nucleus of rats resulted in a strong potentiation of apomorphine (1 mg/kg i.p.)-induced hypermotility. The potentiating effect of LSD or serotonin was suppressed by simultaneous injections of methysergide (0.05 microgram) or cyproheptadine (0.05 microgram) into the medial raphe nucleus. The same doses of LSD injected into the dorsal raphe nucleus and of LSD and mescaline injected into the nucleus accumbens failed to influence locomotor activity, whereas injections of higher doses of LSD and mescaline into the nucleus accumbens inhibited spontaneous and apomorphine-stimulated locomotor activity. It is concluded that the potentiating effect of systemically administered low doses of hallucinogens was triggered by preferential actions on the serotonergic system in the medial raphe nucleus.

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.

Dissociations between the effects of hallucinogenic drugs on behavior and raphe unit activity in freely moving cats

The hypothesis that the action of hallucinogenic drugs is mediated by a depression of the activity of brain serotonergic (raphe) neurons was tested by examining the behavioral effects of several hallucinogenic drugs while concurrently monitoring the activity of raphe neurons in freely moving cats. LSD produced a dose-dependent decrease in raphe unit activity and a dose-dependent increase in certain behaviors (e.g. limb flick and abortive groom), and the peak of the behavioral and unit changes were temporally correlated. However, there were three important dissociations between the behavioral and electrophysiological effects of LSD. Firstly, low doses of LSD produced only small decreases in raphe unit activity but significant behavioral changes. Secondly, the duration of LSD-induced behavioral changes significantly outlasted the depression of raphe unit activity. And thirdly, raphe neurons were at least as responsive to LSD during tolerance as they were in the nontolerant condition. Psilocin produced a dose-dependent decrease in raphe unit activity, while the behavioral changes were not dose-related. However, the peak behavioral changes corresponded to the maximal depression of raphe unit activity. The phenylethylamine hallucinogens, DOM and mescaline, both produced large behavioral changes but no overall effect on raphe neurons. Following administration of DOM or mescaline, some raphe units showed a significant increase, while some showed a significant decrease, and others showed no change in activity. Therefore, the phenylethylamine hallucinogens may exert a depressant effect upon a subset of serotonin-containing neurons, and an amphetamine-like excitatory effect upon another subset of these neurons. Consistent with previous studies, all hallucinogens produced a high concentration of slow waves in the cortical EEG. Following administration of LSD or psilocin, the appearance of slow waves in the EEG was often associated with a transitory decrease in unit activity, while this was not observed for the phenylethylamine hallucinogens. The present data, in conjunction with recent data from other laboratories, suggest that the serotonin hypothesis of hallucinogenic drug action should be re-evaluated.