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

High-resolution tracking of unconfined zebrafish behavior reveals stimulatory and anxiolytic effects of psilocybin

Serotonergic psychedelics are emerging therapeutics for psychiatric disorders, yet their underlying mechanisms of action in the brain remain largely elusive. Here, we developed a wide-field behavioral tracking system for larval zebrafish and investigated the effects of psilocybin, a psychedelic serotonin receptor agonist. Machine learning analyses of precise body kinematics identified latent behavioral states reflecting spontaneous exploration, visually-driven rapid swimming, and irregular swim patterns following stress exposure. Using this method, we found that acute psilocybin treatment has two behavioral effects: [i] facilitation of spontaneous exploration ("stimulatory") and [ii] prevention of irregular swim patterns following stress exposure ("anxiolytic"). These effects differed from the effect of acute SSRI treatment and were rather similar to the effect of ketamine treatment. Neural activity imaging in the dorsal raphe nucleus suggested that psilocybin inhibits serotonergic neurons by activating local GABAergic neurons, consistent with psychedelic-induced suppression of serotonergic neurons in mammals. These findings pave the way for using larval zebrafish to elucidate neural mechanisms underlying the behavioral effects of serotonergic psychedelics.

Effect of a single psilocybin treatment on Fos protein expression in male rat brain

Psilocybin has received attention as a treatment for depression, stress disorders and drug and alcohol addiction. To help determine the mechanisms underlying its therapeutic effects, here we examined acute effects of a range of behaviourally relevant psilocybin doses (0.1-3 mg/kg SC) on regional expression of Fos, the protein product of the immediate early gene, c-fos in brain areas involved in stress, reward and motivation in male rats. We also determined the cellular phenotypes activated by psilocybin, in a co-labeling analysis with NeuN, a marker of mature neurons, or Olig1, a marker of oligodendrocytes. In adult male Sprague-Dawley rats, psilocybin increased Fos expression dose dependently in several brain regions, including the frontal cortex, nucleus accumbens, central and basolateral amygdala and locus coeruleus. These effects were most marked in the central amygdala. Double labeling experiments showed that Fos was expressed in both neurons and oligodendrocytes. These results extend previous research by determining Fos expression in multiple brain areas at a wider psilocybin dose range, and the cellular phenotypes expressing Fos. The data also highlight the amygdala, especially the central nucleus, a key brain region involved in emotional processing and learning and interconnected with other brain areas involved in stress, reward and addiction, as a potentially important locus for the therapeutic effects of psilocybin. Overall, the present findings suggest that the central amygdala may be an important site through which the initial brain activation induced by psilocybin is translated into neuroplastic changes, locally and in other regions that underlie its extended therapeutic effects.

The Effects of Psychedelics on Neuronal Physiology

Psychedelics are quite unique among drugs that impact the central nervous system, as a single administration of a psychedelic can both rapidly alter subjective experience in profound ways and produce sustained effects on circuits relevant to mood, fear, reward, and cognitive flexibility. These remarkable properties are a direct result of psychedelics interacting with several key neuroreceptors distributed across the brain. Stimulation of these receptors activates a variety of signaling cascades that ultimately culminate in changes in neuronal structure and function. Here, we describe the effects of psychedelics on neuronal physiology, highlighting their acute effects on serotonergic and glutamatergic neurotransmission as well as their long-lasting effects on structural and functional neuroplasticity in the cortex. We propose that the neurobiological changes leading to the acute and sustained effects of psychedelics might be distinct, which could provide opportunities for engineering compounds with optimized safety and efficacy profiles.

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.

The behavioral pharmacology of hallucinogens

Until very recently, comparatively few scientists were studying hallucinogenic drugs. Nevertheless, selective antagonists are available for relevant serotonergic receptors, the majority of which have now been cloned, allowing for reasonably thorough pharmacological investigation. Animal models sensitive to the behavioral effects of the hallucinogens have been established and exploited. Sophisticated genetic techniques have enabled the development of mutant mice, which have proven useful in the study of hallucinogens. The capacity to study post-receptor signaling events has lead to the proposal of a plausible mechanism of action for these compounds. The tools currently available to study the hallucinogens are thus more plentiful and scientifically advanced than were those accessible to earlier researchers studying the opioids, benzodiazepines, cholinergics, or other centrally active compounds. The behavioral pharmacology of phenethylamine, tryptamine, and ergoline hallucinogens are described in this review, paying particular attention to important structure activity relationships which have emerged, receptors involved in their various actions, effects on conditioned and unconditioned behaviors, and in some cases, human psychopharmacology. As clinical interest in the therapeutic potential of these compounds is once again beginning to emerge, it is important to recognize the wealth of data derived from controlled preclinical studies on these compounds.

Hallucinogens

Hallucinogens (psychedelics) are psychoactive substances that powerfully alter perception, mood, and a host of cognitive processes. They are considered physiologically safe and do not produce dependence or addiction. Their origin predates written history, and they were employed by early cultures in a variety of sociocultural and ritual contexts. In the 1950s, after the virtually contemporaneous discovery of both serotonin (5-HT) and lysergic acid diethylamide (LSD-25), early brain research focused intensely on the possibility that LSD or other hallucinogens had a serotonergic basis of action and reinforced the idea that 5-HT was an important neurotransmitter in brain. These ideas were eventually proven, and today it is believed that hallucinogens stimulate 5-HT(2A) receptors, especially those expressed on neocortical pyramidal cells. Activation of 5-HT(2A) receptors also leads to increased cortical glutamate levels presumably by a presynaptic receptor-mediated release from thalamic afferents. These findings have led to comparisons of the effects of classical hallucinogens with certain aspects of acute psychosis and to a focus on thalamocortical interactions as key to understanding both the action of these substances and the neuroanatomical sites involved in altered states of consciousness (ASC). In vivo brain imaging in humans using [(18)F]fluorodeoxyglucose has shown that hallucinogens increase prefrontal cortical metabolism, and correlations have been developed between activity in specific brain areas and psychological elements of the ASC produced by hallucinogens. The 5-HT(2A) receptor clearly plays an essential role in cognitive processing, including working memory, and ligands for this receptor may be extremely useful tools for future cognitive neuroscience research. In addition, it appears entirely possible that utility may still emerge for the use of hallucinogens in treating alcoholism, substance abuse, and certain psychiatric disorders.

Evidence that central 5-HT2A and 5-HT2B/C receptors regulate 5-HT cell firing in the dorsal raphe nucleus of the anaesthetised rat

1. Systemic administration of phenethylamine-derived, 5-hydroxytryptamine(2) (5-HT(2)) receptor agonists inhibits the firing of midbrain 5-HT neurones, but the 5-HT receptors involved are poorly defined, and the contribution of peripheral mechanisms is uncertain. This study addresses these issues using extracellular recordings of 5-HT neurones in the dorsal raphe nucleus of anaesthetised rats. 2. The 5-HT(2) receptor agonists DOI ((+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride) and DOB ((+/-)-2,5-dimethoxy-4-bromoamphetamine hydrobromide), caused a dose-related (10-100 micro g kg(-1) i.v.) inhibition of 5-HT neuronal activity, with the highest dose reducing firing rates by >80%. 3. Pretreatment with the 5-HT(2) receptor antagonist ritanserin (1 mg kg(-1) i.v.) completely blocked the action of DOI. The 5-HT(2A) receptor antagonist MDL 100,907 (0.2 mg kg(-1) i.v.) blocked the action of both DOI and DOB. In comparison, the 5-HT(2B/C) receptor antagonist SB 206553 (0.5 mg kg(-1) i.v.) caused a small, but statistically significant, shift to the right in the dose response to DOI and DOB. 4. Pretreatment with the peripherally acting 5-HT(2) receptor antagonist BW 501C67 (0.1 mg kg(-1) i.v.) had no effect on the DOI-induced inhibition of 5-HT cell firing, but completely blocked the DOI-induced rise in mean arterial blood pressure. 5. These data indicate that the inhibition of 5-HT cell firing induced by systemic administration of DOI and DOB is mediated predominantly by the 5-HT(2A) receptor-subtype, but that 5-HT(2B/C) receptors also play a minor role. Moreover, central and not peripheral mechanisms are involved. Given evidence that 5-HT(2) receptors are not located on 5-HT neurones, postsynaptic 5-HT feedback mechanisms are implicated.

Afferent modulation of dopamine neuron firing patterns

In recent studies examining the modulation of dopamine (DA) cell firing patterns, particular emphasis has been placed on excitatory afferents from the prefrontal cortex and the subthalamic nucleus. A number of inconsistencies in recently published reports, however, do not support the contention that tonic activation of NMDA receptors is the sole determinate of DA neuronal firing patterns. The results of work on the basic mechanism of DA firing and the action of apamin suggest that excitatory projections to DA neurons from cholinergic and glutamatergic neurons in the tegmental pedunculopontine nucleus, and/or inhibitory GABAergic projections, are also involved in modulating DA neuron firing behavior.

A key role for the caudoventral pontine tegmentum in the simultaneous generation of eye saccades in bursts and associated ponto-geniculo-occipital waves during paradoxical sleep in the cat

Ponto-geniculo-occipital waves and rapid eye movements (eye saccades) are two prominent phasic events of paradoxical sleep which occur in conjunction. Although they have been studied intensively, the neuronal link between these two events is still poorly understood. On the basis of our previous results, combining brainstem transections and carbachol microinjections, we postulated that the oculomotor and ponto-geniculo-occipital systems do not work in series, but in parallel, and that the caudoventral pontine tegmentum might represent a structure controlling and/or co-ordinating the simultaneous production of the two phenomena. This hypothesis was further supported by the demonstration that, during paradoxical sleep, the instantaneous velocity of eye saccades in bursts is higher than that of isolated ones which, in turn, are more rapid than waking saccades. This acceleration of eye saccades in bursts also seems to be under the cholinergic control of the caudoventral pontine tegmentum. In order to test the hypothesis that this area may be a prime mover leading to the simultaneous appearance of these two phasic events as a whole, we investigated, in the present study, the effects of pharmacological stimulation (with carbachol) and inhibition (with atropine) of the caudoventral pontine tegmentum on the production and the characteristics of eye saccades and ponto-geniculo-occipital waves. Cats' eye movements were recorded using the technique of the scleral search coil in a magnetic field, together with sleep-waking parameters. We found that: (i) unilateral microinjections of carbachol (0.4 microg) induced, during waking, a majority of long bursts of ponto-geniculo-occipital waves (i.e. bursts containing at least five waves) which had intra-burst intervals similar to natural ones (48-259 ms) and decreased the frequency of isolated ponto-geniculo-occipital waves; (ii) unilateral microinjections of atropine (2.4 microg) strongly decreased, during paradoxical sleep, the frequency (number/min) of eye saccades in bursts directed contralaterally to the side of the injection (by 48-54%) and reduced the velocity of these saccades to that of isolated eye saccades. Atropine also significantly reduced the frequency (by 60%) of all types of bursts of ponto-geniculo-occipital waves, with a maximal effect (80% reduction) on long bursts of ponto-geniculo-occipital waves, while it increased the frequency of isolated ponto-geniculo-occipital waves. However, atropine did not change the value of intra-burst intervals. These findings support the hypothesis that eye saccades in bursts and associated ponto-geniculo-occipital waves are generated as a whole by a common structure and that this structure is at least partly defined by the caudoventral pontine tegmentum.

Determination of lysergide (LSD) and phencyclidine in biosamples

Lysergic acid diethylamide (LSD) is difficult to detect and to quantify in biosamples because of its very low active dose. Although there are a number of tests available, routine analysis of LSD is rarely performed. Immunoassays largely vary in their specificity and cross-reactivities with other molecules often make these tests unreliable. Because of its low concentration and the instability of the derivatives (e.g. trimethylsilyl-LSD), routine gas chromatography-mass spectrometry (GC-MS) detection and quantitation of LSD remains a difficult task. The most promising procedures for LSD determination seems to be liquid chromatography-MS analysis using electrospray ionisation and selected ion monitoring (SIM). Extraction, derivatization, GC or high-performance liquid chromatography conditions and the different detection modes will be summarised. Phencyclidine (PCP) is an abused drug seldom found outside the United States. Well established detection and quantitation procedures include radioisotopic and nonradioisotopic immunoassays and GC-MS analysis using SIM mode with deuterated PCP as internal standard. Alternatively, GC with nitrogen-phosphorus detection or capillary electrophoresis has been used. Recent progress in PCP analysis will be summarised.

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.

Hallucinogenic drug interactions with neurotransmitter receptor binding sites in human cortex

The binding affinities of four hallucinogenic agents were analyzed at nine neurotransmitter binding sites in human cortex. d-Lysergic acid diethylamide (d-LSD), N,N-dimethyltryptamine (DMT), 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and 1-(2,5-dimethoxy-4-bromophenyl)-2-aminopropane (DOB) display highest affinity for the recently identified "DOB binding site" labeled by 77Br-R(-)DOB. The phenalkylamines, DOI and DOB, display subnanomolar affinity for the 77Br-R(-)DOB-labeled site, whereas the indolealkylamines, d-LSD and DMT, display nanomolar affinity for this site. d-LSD was the most potent of the four hallucinogens at six of the other eight sites analyzed in this study. All four hallucinogens also display high affinity for the 5-hydroxytryptamine2 (5-HT2) receptor subtype, with potencies ranging from 4 to 360 nM. Marked differences in relative affinities were observed between the indolealkylamines and the phenalkylamines at the 5-HT1A, 5-HT1D, and DOB binding sites. These rank-order differences in affinities are likely to account for the differing effects of these agents in various biochemical and physiological assays.

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.

Electrophysiological responses of serotoninergic dorsal raphe neurons to 5-HT1A and 5-HT1B agonists

A direct comparison was made of the effects of serotonin 5-HT1A and 5-HT1B selective compounds on the spontaneous firing rate of dorsal raphe serotoninergic neurons in chloral-hydrate-anesthetized rats. Following intravenous administration, the 5-HT1A selective compounds ipsapirone (TVX Q 7821) and LY 165163 potently inhibited single-unit activity in a dose-dependent manner whereas the 5-HT1B selective compounds, m-chlorophenylpiperazine (mCPP) and trifluoromethylphenylpiperazine (TFMPP), displayed only weak or irregular actions. Low microiontophoretic currents of ipsapirone and LY 165163 were also effective in suppressing spontaneous firing; dose-response relationships for the 5-HT1A compounds were indistinguishable from that of 5-HT itself. In contrast, dorsal raphe neurons were only weakly responsive to microiontophoretic application of mCPP and TFMPP; dose-response relationships for the 5-HT1B compounds were significantly displaced from that of 5-HT. In intracellular studies, ipsapirone and LY 165163, when added to the media bathing brain slices, mimicked the actions of 5-HT in hyperpolarizing dorsal raphe cell membranes and decreasing input resistance; however, the maximal effects of the 5-HT1A compounds on these membrane properties exceeded those of 5-HT. In summary, dorsal raphe 5-HT neurons appear highly responsive to 5-HT1A, but not to 5-HT1B compounds; these findings are discussed with regard to the 5-HT receptor subtypes as candidates for the somatodendritic autoreceptor of dorsal raphe neurons.

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.

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.

Dopamine-containing ventral tegmental area neurons in freely moving cats: activity during the sleep-waking cycle and effects of stress

The activity of dopamine-containing ventral tegmental area (VTA) units was recorded by means of movable 32- and 64-microns-diameter insulated Nichrome wires in freely moving cats. The VTA units displayed a slow, somewhat irregular activity during quiet waking (mean 3.63 +/- 0.41 spikes/s) and showed no significant change in activity during slow-wave sleep or REM sleep. Although VTA unit activity was somewhat higher and more erratic during active waking, there was no relationship between unit discharge and phasic movement. These neurons were inhibited (-87%) by small doses of apomorphine (1.0 mg/kg, i.p.) and excited (+43%) by small doses of haloperidol (0.5 mg/kg, i.p.). The stress of a conditioned emotional reaction (CER) paradigm resulted in a significant increase in the discharge rate of VTA neurons (+39%), compared with the quiet-waking baseline. The CER paradigm increased plasma glucocorticoids by 74%. Neurochemical studies revealed that the CER paradigm resulted in a significant decrease of dopamine in the limbic forebrain (-31%), whereas both homovanillic acid (+47%) and dihydroxyphenylacetic acid (+43%) concentrations were increased. No significant changes in dopamine metabolism were observed in the striatum under the CER situation. These data have implications in relation to the role of stress and dopamine in mediating certain psychiatric disorders.

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.

Indolamine hallucinogens as MAO inhibitor agents--theoretical approach

A new theoretical hypothesis on the mode of action of LSD and other hallucinogens of the indolamine family is proposed. In view of the suggestions that LSD acts both on the 5-HT system as an antagonist and on the DA system as an agonist, we suggest that in addition to 5-HT blocking capacity, LSD and the other indolamine hallucinogens act as MAO inhibitors.

Differential effects of lysergic acid diethylamide, methysergide, and cyproheptadine on modality-specific and nonspecific sensory evoked potentials

The effects of lysergic acid diethylamide (LSD), methysergide, and cyproheptadine on activity in classical primary pathways of the visual and somatosensory systems were compared with their effects on activity in sensory convergent (association) regions in alpha-chloralose-anesthetized cats. Those effects were blocked by cyproheptadine whereas methysergide potentiated the actions of LSD on visual primary activity. In contrast, LSD depressed the primary somatic pathway, at small doses (25 to 50 micrograms/kg) and facilitated the response at larger doses (200 micrograms/kg). Cyproheptadine and methysergide did not agonize these actions of LSD. The anterior marginal cortex, nucleus central median-parafascicularis, nucleus lateral posterior, and the superior colliculus, all sites of heterosensory convergence, were depressed by LSD. The depression of responses at heterosensory sites by LSD was blocked by cyproheptadine. Methysergide potentiated the LSD-induced depression of visual-evoked activity but not somatosensory activity. These results suggest that LSD depresses sensory activity in regions which integrate multiple sensory modalities independently of actions on sensory-specific pathways. These effects appear to involve a cyproheptadine-sensitive system.

Harmaline effects on the sensory-motor reactivity: modifications of the acoustic startle pattern

The effects of harmaline, an indoleamine and a MAOI, were tested on the acoustic startle pattern. EMG measures of the startle reflex, the pinna reflex as well as the characters of the vertex evoked responses to brief intense tone bursts (60 msec, 110 dB, 8000 Hz) were simultaneously studied in 4 alert guinea-pigs. The basic experimental design was a 4 by 4 latin square, with the treatments being given at 2 day intervals. The four harmaline-HCl treatments were isotonic saline, 0.25, 5.0, and 10.0 mg/kg. Compared with saline baselines, all the doses resulted, throughout the 60 min session, in overall high significant depressions of the startle reflex, the pinna reflex and the initial wave of the acoustic evoked potential at the vertex. In contrast, harmaline had little or no influence on amplitude and latency of the late wave of the vertex response. The effects of harmaline on the general behavior of the guinea-pig are also reported. These results may support an involvement of serotonergic systems in the modulation of the sensory-motor reactivity at the brainstem level. Nevertheless, the probably more complex cortical processes involved in startle responsiveness do not appear univocally affected by the indoleamine drugs such as harmaline.

Chloral hydrate anesthesia alters the responsiveness of dorsal raphe neurons to psychoactive drugs

The effects of several psychoactive drugs on raphe unit activity in freely moving cats was compared with drug-induced effects in chloral hydrate anesthetized cats. The anesthesia greatly potentiated the depressant effects of LSD, phenoxybenzamine, clonidine, methiothepin, clozapine, and chlorimipramine on raphe units, but partially antagonized the depressant effects of diazepam. These results demonstrate that apparently discrepant reports of the affects of these drugs on raphe neurons in anesthetized rats versus freely moving cats are attributable to the use of anesthesia in rat studies. These data underscore the importance of conducting such drug studies in awake, freely moving animals, for which the results would be far more relevant to the issue of human drug use.

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.

Interaction of synthetic opioid metenkephalin peptide analogs, Lilly 127623 and FK 33-824 with indole hallucinogens: antagonism of N,N-dimethyltryptamine- and LSD-induced disruption of food-rewarded bar pressing behavior in the rat

The selected opioid metenkephalin synthetic peptide analogs Lilly (LY) 127623 and FK 33-824 were tested for behavioral dose effects and potential interaction with N,N-dimethyltryptamine (DMT) and lysergic acid diethylamide-25 (LSD) in adult male Holtzman rats trained on a positive reinforcement fixed-ratio 4 (FR-4) behavioral bar pressing schedule, i.e., a reward of 0.01 ml sugar-sweetened evaporated milk was earned on every fourth bar press. DMT (3.2 mg/kg) and LSD (0.1 mg/kg), administered IP following a 0.9% NaCl 15-20-min control pretreatment, disrupted established food-rewarded FR-4 bar pressing in a consistent and reproducible manner. Animals pretreated IP with predetermined behaviorally noneffective doses of LY 127623 (0.01-0.32 mg/kg) and FK 33-824 (0.001-0.01 mg/kg) 15-20 min prior to receiving DMT demonstrated significant antagonism to DMT-induced disruption of FR-4 bar pressing, while doses of 0.10-0.32 mg/kg LY 127623 and 0.00032-0.0032 mg/kg FK 33-824 significantly antagonized LSD-induced behavioral effects.

Medullary serotonergic neurons are insensitive to 5-MeoDMT and LSD

A comparison was made of the effects of 5-MeoDMT or LSD on serotonergic unit activity in the dorsal raphe nucleus (DRN) and nucleus raphe pallidus (NRP) of freely moving cats. NRP neurons were substantially less responsive than DRN neurons to both drugs. NRP neurons were unresponsive to behaviorally effective low doses of these drugs whereas the activity of DRN neurons was strongly depressed. These data are discussed in terms of autoregulatory control of serotonergic neurons.

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.

Activity of nucleus raphe pallidus neurons across the sleep-waking cycle in freely moving cats

The activity of serotonin-containing nucleus raphe pallidus (RPA) units was recorded by means of movable 32 or 62 microns diameter insulated nichrome wires in freely moving cats. RPA units displayed a slow, rhythmic discharge rate during waking (mean = 5.3 spikes/s) and showed no significant change in activity during slow-wave sleep. However, these neurons showed a large decrease in activity during REM sleep (mean = 1.2 spikes/s). In contrast with results from studies on serotonin-containing nucleus raphe dorsalis units, RPA neurons showed no relationship to the occurrence of sleep spindles, and were not inhibited by low doses of LSD.