High-affinity 3H-serotonin binding to caudate: inhibition by hallucinogens and serotoninergic drugs

Effects of early serotonin programming on behavior and central monoamine concentrations in an avian model

Serotonin (5-HT) acts as a neurogenic compound in the developing brain; however serotonin altering drugs such as SSRIs are often prescribed to pregnant and lactating mothers. Early agonism of 5-HT receptors could alter the development of serotonergic circuitry, altering neurotransmission and behaviors mediated by 5-HT signaling, including memory, fear and aggression. This study was designed to investigate the effects of early serotonin agonism on later behaviors. An extremely aggressive White leghorn strain (15I5) was used in the study. The chicks were injected with 5-MT (a serotonin agonist) at 2.5mg/kg (low dose), 10mg/kg (high dose) or saline (control) on the day of hatch and a second dose 24h later (n=9/sex/trt). Chicks' fear response and memory were tested at 2 weeks of age. In the fear test, chicks were subjected to a social isolation test for 20min, time to first vocalization and numbers of vocalizations were recorded. In the memory test, chicks were placed in a running wheel and presented with an imprinted object (white box with a red light) and a similar shaped novel object (blue box with a white light), respectively. The distance traveled in the wheel toward each object was measured. At 10 weeks of age birds were tested for aggression and concentrations of catecholamines were determined from the raphe nucleus and hypothalamus by HPLC (n=12). Expression of 5-HT1A and 5-HT1B receptor genes were measured by RT-PCR. Both high and low dose chicks tended to have shorter latency to first vocalization and a greater number of vocalizations compared with control chicks. Memory test showed that chicks from all groups traveled a similar distance toward a familiar object. However, control chicks walked the least toward a novel object, low dose chicks tended to walk further, and high dose chicks walked significantly further for a novel object. In aggression tests, both high and low dose males exhibited greater frequency of aggressive behaviors compared to controls, while no difference in aggression was evident in the females. Norepinephrine concentrations were also reduced in the low dose birds in the hypothalamus and in the raphe nucleus. Serotonin concentrations tended to be lower only in the both hypothalamus and raphe nucleus of the low dose birds. 5-HT1A expression was greatest in the hypothalamus and raphe nucleus of low dose birds. The agonism of the serotonin system during neural development of birds genetically predisposed to aggression alters both the dopaminergic and serotonergic systems further increasing their aggressiveness.

Developmental exposure to a serotonin agonist produces subsequent behavioral and neurochemical changes in the adult male prairie vole

Autistic spectrum disorders (ASDs) are classified as pervasive developmental disorders characterized by abnormalities in various cognitive and behavioral functions. Although exact underlying causes are still unknown, nearly 30% of autistic patients show elevated blood levels of serotonin (5-HT) and, therefore, various genetic and environmental factors that are known to elevate 5-HT levels may play a role in the development of ASDs. In the present study, we used the socially monogamous male prairie vole (Microtus ochrogaster) as an animal model to examine the effects of perinatal exposure to 5-methoxytryptamine (5-MT), a non-selective serotonin agonist, on subsequent behavioral and neurochemical changes in the brain. 5-MT treated males showed a decrease in affiliation and an increase in anxiety-related behavior, as well as a decrease in the density of 5-HT immunoreactive (ir) fibers in the amygdala and oxytocin-ir and vasopressin-ir cells in the paraventricular nucleus of the hypothalamus, compared to saline treated controls. These data indicate that exposure to 5-HT during early development can induce abnormalities in various neurochemical systems which, in turn, may underlie deficits in social and anxiety-related behaviors. In addition, these data will help to establish the prairie vole model to study the neurobiological underpinnings of complex neuropsychiatric disorders such as ASDs.

Behavioral and cellular consequences of increasing serotonergic activity during brain development: a role in autism?

The hypothesis explored in this review is that the high levels of serotonin in the blood seen in some autistic children (the so-called hyperserotonemia of autism) may lead to some of the behavioral and cellular changes also observed in the disorder. At early stages of development, when the blood-brain Barrier is not yet fully formed, the high levels of serotonin in the blood can enter the brain of a developing fetus and cause loss of serotonin terminals through a known negative feedback function of serotonin during development. The loss of serotonin innervation persists throughout subsequent development and the symptoms of autism appear. A review of the basic scientific literature on prenatal treatments affecting serotonin is given, in support of this hypothesis, with an emphasis on studies using the serotonin agonist, 5-methoxytryptamine (5-MT). In work using 5-MT to mimic hyperserotonemia, Sprague-Dawley rats are treated from gestational day 12 until postnatal 20. In published reports, these animals have been found to have a significant loss of serotonin terminals, decreased metabolic activity in cortex, changes in columnar development in cortex, changes in serotonin receptors, and "autistic-like" behaviors. In preliminary cellular findings given in this review, the animals have also been found to have cellular changes in two relevant brain regions: 1. Central nucleus of the amygdala, a brain region involved in fear-responding, where an increase in calcitonin gene related peptide (CGRP) was found 2. Paraventricular nucleus of the hypothalamus, a brain region involved in social memory and bonding, where a decrease in oxytocin was found. Both of these cellular changes could result from loss of serotonin innervation, possibly due to loss of terminal outgrowth from the same cells of the raphe nuclei. Thus, increased serotonergic activity during development could damage neurocircuitry involved in emotional responding to social stressors and may have relevance to the symptoms of autism.

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.

High affinity serotonin binding sites in human brain: a comparison of cerebral cortex and basal ganglia

The high-affinity binding of 3H-serotonin and 3H-DPAT was studied in membrane preparations and tissue sections of cerebral cortex and basal ganglia of human brain. In tissue sections, 3H-serotonin bound to sites present at high density in the cerebral cortex, hippocampus and basal ganglia. 3H-DPAT bound predominantly to the outer layers of the cerebral cortex and the hippocampus, no significant binding was observed in the basal ganglia. In cortical membranes 3H-serotonin bound to a heterogeneous population of sites, one of which was similar to a binding site labelled by 3H-DPAT (the putative 5HT-1A receptor). 3H-serotonin binding to membranes prepared from the putamen displayed low affinity for DPAT, mesulergine and spiperone, RU24969 was considerably less potent than serotonin itself. This 3H-serotonin binding site does not have the properties of those sites described in rodent brain (5HT-1A, 5HT-1B, and 5HT-1C), and may represent a novel serotoninergic binding site.

Effects of prenatal 5-methoxytryptamine and parachlorophenylalanine on serotonergic uptake and behavior in the neonatal rat

Parachlorophenylalanine (pCPA) or 5-methoxytryptamine (5MT) was administered to pregnant Sprague Dawley rats from day 8 (D8) of gestation till D17 and from D12 until birth respectively. Birth weights of both drug groups of neonates were approximately 20% less than the saline-injected controls. 5MT neonates showed a significant reduction of high affinity 3H-5HT uptake in the brainstem at all three time points: D1, D15, D30, and a slight reduction in the forebrain reaching significance only on D30. The pCPA animals showed a significant reduction in the brainstem and forebrain on D1 and D30, but only a small nonsignificant reduction in both areas on D15. Behaviors measured on day 15 revealed that in general activity, spontaneous alternation, and passive avoidance both drug groups of neonates showed deficits: less activity, less alternation, and less avoidance.

Modulation of the vestibulo-ocular reflex by serotonin in the rat

The effects of intraventricular injection of serotonin (5-HT) and its agonists and antagonists on the amplitude of the vestibulo-ocular reflex were studied in chronic implanted rats. 5-HT (10(-5) M) triggers an increase of the amplitude of the reflex which lasts 30 min. Similar results are obtained when N,N-dimethyl-5-methoxytryptamine (10(-3) M) is introduced into the ventricular cannula. The increasing effects observed both with 5-HT and N,N-dimethyl-5-methoxy-tryptamine are abolished by methiothepin, a potent antagonist of 5-HT receptors. Injection of indirect agonists like pargyline, a monoamine oxidase inhibitor, or fluoxetine, a potent inhibitor of 5-HT reuptake, is followed by an increase of the amplitude of the vestibulo-ocular reflex. These results indicate that 5-HT can modulate the activity of the vestibulo-ocular pathway and muscular tone of extraocular muscles. Location and involvement of various modulating 5-HT sites are discussed.

Neuropharmacology of drugs affecting food intake

The importance of the central monoamines NE, DA and 5-HT in ingestive behavior has inevitably resulted in considerable effort being expended in attempting to implicate these monoamines in the mechanism of action of anorectic drugs. The statements that amphetamine-induced anorexia is unlikely to be due to central serotoninergic systems and that central noradrenergic and dopaminergic systems are not implicated in the appetite suppressant effect of fenfluramine are in all probability correct. However, to attribute the ability of drugs to decrease food intake unequivocally to a specific effect on central monoaminergic systems is almost certainly an oversimplification, due to the fact that other putative neurotransmitters, such as GABA and peptides, play a critical role in eating. This can be achieved either directly or by modulating the release of other transmitters. An added complication in attempting to correlate a specific neurochemical process to a behavioral effect, such as anorexia, is the complexity of the central actions of the drug. At best, a predominant but not an exclusive process can be identified. Perhaps the in-built constraint of attempting to correlate a specific neurochemical effect to the desired action of a drug is accountable for the absence of a second generation of centrally acting anorectic drugs. Dramatic progress has been made in elucidating the factors involved in ingestive behavior over the last 5-10 years. This information should, and must, provide the catalyst for more efficacious anorectic drugs because obesity represents one of the few major diseases for which adequate drug therapy does not exist.

Autoregulation of fetal serotonergic neuronal development: role of high affinity serotonin receptors

A microculture technique was used to study the factors regulating the development of fetal rat serotonergic neurons. Mesencephalic raphe cells from E14 rats co-cultured with hippocampal cells from E18 were grown for up to 4 days in the presence of various agents known to alter serotonergic function in the mature brain. Pargyline (a non-specific monoamine oxidase inhibitor) alone and with serotonin (10(-8) to 10(-6) M) both inhibited growth of serotonergic neurons as assessed by uptake of [3H]serotonin. 5-methoxytryptamine (5-MT), a serotonin agonist with selectivity for serotonin autoreceptors, inhibited growth at low concentrations, but this inhibition was overcome at higher concentrations. Using immunocytochemistry with a primary anti-serotonin antibody, 5-MT was observed to produce stunted processes, increase autoinnervation and lead to neuronal death. A model is proposed whereby high affinity serotonin receptors in fetal brainstem tissue and in fetal forebrain tissue regulate direction and extent of growth. We have confirmed the presence of these receptors using a direct binding assay.

[3H]5-hydroxytryptamine binding to brain astroglial cells: differences between intact and homogenized preparations and mature and immature cultures

[3H]5-hydroxytryptamine ([3H]serotonin) binds with high affinity (KD 2-12 nM) to a finite number of sites on brain astroglial cells. The number of binding sites in the C6 glioma line is decreased significantly (Bmax = 315 versus 30 fmol/mg) by homogenization. In intact primary cultures, derived from newborn rat brain, the number of binding sites is far greater in cultures of immature astrocytes than in cultures treated with dibutyryl cyclic AMP (Bmax = 1,520 versus 580 fmol/mg). A role for these receptors in development is suggested.

Serotonin receptor involvement in the avoidance learning deficit caused by p-chloroamphetamine-induced serotonin release

The receptor involvement in the p-chloramphetamine (PCA, 2.5 mg kg-1) induced impairment of active avoidance acquisition was examined in the male rat. The avoidance deficit was blocked at low doses by serotonergic (5-HT)-receptor blocking agents but not by alpha-adrenergic-, beta-adrenergic-, opiate-, muscarinic- or dopamine D2-receptor antagonists. The potency of the 5-HT antagonists to block the PCA-induced deficit correlated with their affinity in displacing [3H]ketanserin but not [3H]5-HT binding in the frontal cortex. The potencies of the 5-HT antagonists to block the action of PCA could not be related to their action on muscarinic-, histaminergic H1- or dopaminergic D2-receptor binding in vitro. It is concluded that the avoidance learning deficit caused by PCA-induced 5-HT release is related to activation of 5-HT receptors in the frontal cortex having the characteristics of a 5-HT2 receptor.

Evaluation of the direct actions of drugs with a serotonergic link in spinal analgesia on the release of [3H]serotonin from spinal cord synaptosomes

Morphine, ketamine, ethylketocyclazocine and quipazine, drugs with an apparent local spinal serotonergic action, which contributes to their analgesic effects, were tested for their ability to alter the release of [3H]serotonin ([3H]5-HT) from a synaptosomal preparation from the spinal cord of the rat. Related compounds including [D-Ala2, D-Leu5]enkephalin (DADLE), n-allylnormetazocine and phencyclidine were also examined. None of the drugs was found to be capable of inducing a direct release of [3H]5-HT or of facilitating potassium-induced release of 5-HT. However, quipazine inhibited the depressant action of exogenous 5-HT on overflow of 3H (mediated through the 5-HT autoreceptor), an action that should facilitate serotonergic neurotransmission. In contrast to the other drugs, DADLE was found to depress K+ stimulated release of 5-HT. The results suggests that the serotonergic mechanism involved in the antinociceptive action of some of these drugs (i.e. ketamine, morphine and ethyl-ketocyclazocine) is not related to direct presynaptic interactions to promote release of 5-HT. On the other hand, a small population of serotonergic nerves critical for analgesia may be involved and are not detected using tissue from the whole spinal cord, However, it seems equally plausible that these drugs may produce their antinociceptive action through interactions with other neurotransmitter systems that in turn interface with the serotonergic nerves, perhaps through interneurons or collateral connections.

Studies of the relationship between molecular structure and hallucinogenic activity

The nature of the stereochemistry and aromatic ring substituents and their importance to biological activity for phenethylamine-type hallucinogens is presented. The possibility of a hydrophobic site to bind to the 4-substituent and its likely geometry is described. A brief discussion of the structure-activity relationships for tryptamines such as psilocin and DMT is also given, with comments about the stereochemistry of alpha-methyltryptamines. Evaluation of a series of N(6)-alkyl-nor-LSD derivatives indicated that selected members such as N(6)-ethyl, allyl and propyl were as potent as, if not more potent than LSD, both in a two-lever drug discrimination assay in rats, and in man. N(6)-alkyl groups longer than n-propyl, such as n-butyl or 2-phenethyl, gave compounds that were greatly reduced in activity.

Correlation of [3H]5-hydroxytryptamine (5HT) binding to brain stem preparations and the production and prevention of myoclonus in guinea pig by 5HT agonists and antagonists

In guinea pig brain stem preparations [3H]5-hydroxytryptamine (5HT) bound specifically to both high and low affinity sites, but specific [3H]spiperone binding was low and could not be consistently detected. This indicates a prevalence of 5HT-1 type receptors in this tissue. High affinity-specific [3H]5HT binding was more potently displaced by indole-containing 5HT agonists than by piperazine-containing 5HT agonists. This agreed with the observation that indole-containing, but not piperazine-containing compounds induced dose-dependent myoclonus in guinea pigs which originates from brain stem. The capacity of indoleamine antagonists to displace [3H]5HT-specific binding from guinea pig brain stem was similar to their reported potency in displacing [3H]5HT from 5HT-1 receptors. The [3H]5HT-labelled binding site in guinea pig brain stem is a 5HT-1 receptor and appears to be responsible for the induction of indoleamine-dependent myoclonus.

Anorexia and brain serotonin: development of tolerance to the effects of fenfluramine and quipazine in rats with serotonin-depleting lesions

The acute and chronic effects of the "serotonergic anorectics" quipazine and dl-fenfluramine were examined in rats with substantial and specific depletions of brain 5-hydroxytryptamine (5-HT) induced by 5,7-dihydroxytryptamine (5,7-DHT). A "dessert" test which did not involve food deprivation was used to assess anorexia. Markedly increased sensitivity to the L-5-HTP-induced behavioral syndrome in 5,7-DHT-lesioned rats indicated postsynaptic 5-HT receptor supersensitivity. We found low (2 mg/kg) and intermediate (5 mg/kg) doses of fenfluramine, a putative presynaptic agent, were more effective in producing anorexia in lesion rats versus controls. A higher dose of fenfluramine (10 mg/kg) was less effective in lesion rats, suggesting that high dose and low dose fenfluramine anorexia are mediated by different mechanisms. We found quipazine, a putative 5-HT postsynaptic agonist, in a dose range of 2-10 mg/kg, to be no more effective in producing anorexia in lesion rats compared to controls. The development of tolerance to both fenfluramine and quipazine anorexia was similar for lesion and control rats showing that an intact brain 5-HT system is not necessary for tolerance. Tolerance to the "behavioral syndrome" induced by high doses of these agents developed rapidly in controls but not at all in lesion rats. This suggests that the behavioral syndrome and anorexia are independent effects of fenfluramine and quipazine. These results also challenge the popular notion that the primary anorectic action of fenfluramine is via brain serotonin.

5-Hydroxytryptamine (5-HT)-dependent myoclonus in guinea pigs is induced through brainstem 5-HT-1 receptors

Myoclonic jerking in guinea pigs originates from the brainstem. Indole-containing 5-hydroxytryptamine (5-HT) agonists, but not piperazine-containing 5-HT agonists, induced myoclonus in guinea pigs at pharmacologically relevant doses. Guinea pig brainstem preparations possessed specific binding sites for [3H]5-HT but specific [3H]spiperone binding was low and inconsistent. 5-HT-1 receptors appear to predominate in this tissue. High affinity [3H]5-HT binding was potently displaced by indole-containing 5-HT agonists but only weakly displaced by piperazine-containing 5-HT agonists. The [3H]5-HT specific binding site in guinea pig brainstem responsible for the induction of indoleamine-dependent myoclonus has the characteristics of a 5-HT-1 receptor.

Manipulations of synaptic serotonin: discrepancy of effects on serotonin S1 and S2 sites

The effect of repeated treatment (28 day) with D-fenfluramine, a serotonin (5HT) releaser, L-tryptophan, a 5HT precursor, or fluoxetine, a 5HT uptake inhibitor, on 3H-5HT and 3H-spiperone binding in the rat cerebral cortex was investigated. Treatment with fenfluramine and fluoxetine caused a significant decrease in the number of 3H-5HT binding sites (Bmax). Fenfluramine also decreased binding of 3H-spiperone in the cortex, but fluoxetine treatment increased this binding. Treatment with L-tryptophan produced no change in the binding of either 3H-5HT or of 3H-spiperone significantly. The data show that manipulation of synaptic 5HT concentration does not always result in parallel changes in S1 and S2 receptors. This suggests that the 5HT S1 and S2 receptors may be subject to different regulatory mechanisms.

Inhibition of synaptosomal neurotransmitter uptake by hallucinogens

The effect of 13 hallucinogens on the uptake of serotonin and norepinephrine into hippocampal synaptosomes and of serotonin and dopamine into caudate synaptosomes was found to be inhibitory, except for lysergic acid diethylamide and 2-bromolysergic acid diethylamide, which were inactive. The indolealkylamines were generally more potent than the phenylethylamines. The reported inhibition of uptake of serotonin by 5-methoxy-N,N-dimethyltryptamine and lysergic acid diethylamide into whole brain synaptosomes was not reproducible at concentrations 10(2) to 10(4) times higher than those stated in the literature.

Tryptamine: a neuromodulator or neurotransmitter in mammalian brain?

Tryptamine synthesized by decarboxylation of L-tryptophan occurs as an endogenous constituent of mammalian brain albeit at very low concentrations (low ng/g range). It is primarily metabolized by oxidative deamination by MAO and possesses an extremely rapid turnover and half-life. Subcellular localization appears to be in nerve terminals and it is releasable by electrical or potassium evoked depolarization. Neuropharmacological and electrophysiological data strongly suggest the existence of post-synaptic receptors for tryptamine independent of those for 5HT. There may exist a rostrally projecting neuronal tryptamine containing system arising from cell bodies in or close to the nucleus raphé medianus. The demonstration of specific receptors for tryptamine in the CNS strongly indicates a transmitter role, although a strong case can be made for a role as a modifier of central 5HT systems. The possibility also exists that 5HT and tryptamine may be mediators of functionally opposite neuronal pathways. Whatever the role of tryptamine in the CNS it is clear that it not simply present as an accident of metabolism or a "biological artefact." The indications are that it possesses important functions in central neurotransmission.

Multiple serotonin receptors: regional distribution and effect of Raphe lesions

These studies confirm and extend the recent work suggesting that [3H]lysergic acid diethylamide (LSD) labels two distinct binding sites in rat brain resembling serotonin (5HT) receptors. Although Scatchard analyses of [3H]LSD binding to membranes prepared from cortex/hippocampus were linear, the heterogeneity of the [3H]LSD binding sites was clearly demonstrated in displacement studies. The displacement curves for both 5HT and spiperone were bisigmoidal with the concentration required to saturate the high affinity components nearly 3 orders of magnitude lower than the concentration necessary to saturate the low affinity components. Additivity studies suggested that the sites with high affinity for 5HT and spiperone are different, independent sites. These sites are referred to as 5HT, and 5HT2, respectively. Regional analyses showed, that in the frontal cortex, the density of the 5HT2 site was slightly greater than the 5HT1 site, whereas the 5HT1 site was predominant in all other brain areas, including the spinal cord. The pharmacological properties of the two sites have features in common with 5HT receptors; however, electrolytic lesions of the midbrain raphe nuclei did not change the densities or binding constants of the two apparent 5HT receptor subtypes, even though the number of high affinity 5HT uptake sites was markedly reduced.

A behavioural and biochemical study in rats of 5-hydroxytryptamine receptor agonists and antagonists, with observations on structure-activity requirements for the agonists

1 The effect of the putative 5-hydroxytryptamine (5-HT) receptor antagonists, methysergide, methergoline, mianserin, cyproheptadine, cinanserin (all at 10 mg/kg), methiothepin (5 mg/kg) and (-)-propranolol (20 mg/kg) on the behavioural responses to tranylcypromine (10 mg/kg) followed 30 min later by L-tryptophan (100 mg/kg) was examined.2 Methysergide, methergoline, methiothepin and (-)-propranolol inhibited head weaving, forepaw treading and hind-limb abduction. Methysergide and methergoline increased reactivity. In contrast, cypropheptadine, cinanserin and mianserin had no effects on the behaviour.3 Similar findings were obtained when the behaviours were elicited by administration of tranylcypromine (10 mg/kg) followed by the putative 5-HT receptor agonist, 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) (2 mg/kg).4 When the behaviours were elicited by the putative 5-HT receptor agonist, quipazine (50 mg/kg), all the drugs effectively inhibited head weaving and forepaw treading.5 When the dose of cypropheptadine was doubled to 20 mg/kg an inhibition of the tranylcypromine/L-tryptophan induced behaviours was seen.6 Methiothepin produced a marked inhibition of apomorphine-induced locomotor activity whilst all the others enhanced this response, suggesting that only methiothepin inhibits the 5-HT behaviours by dopamine antagonism and that the increased reactivity seen following tranylcypromine/L-tryptophan after pretreatment with methysergide or methergoline might be due to enhanced dopamine function.7 Pretreatment with p-chlorophenylalanine resulted in enhanced behavioural responses to both 5-MeODMT and quipazine.8 Both methergoline and methiothepin decreased the rate of 5-HT synthesis in whole brain but not spinal cord and methergoline decreased spinal cord 5-HIAA concentration. None of the other drugs had any significant effects on the concentration of 5-HT, 5-HIAA or 5-HT synthesis rate in brain or spinal cord.9 Experiments with compounds structurally related to quipazine and with molecular models suggested that quipazine produces behavioural changes probably by stimulating the 5-HT receptor in a similar way to 5-HT but that it would bind weakly, in agreement with ligand-receptor binding studies.10 It is suggested, therefore, that cyproheptadine, cinanserin and mianserin fail to inhibit 5-HT and 5-MeODMT-induced behaviours because they are weak antagonists whilst they are able to inhibit the same behaviours induced by quipazine because it is a weak agonist.11 These data indicate that extreme care should be taken in accepting or rejecting 5-HT as a mediator of behaviours or of other responses unless several antagonists or agonists have been examined.

Speculations on the mechanism of action of hallucinogenic indolealkylamines

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

Differential effect of chronic desipramine and amitriptyline treatment on rat brain adrenergic and serotonergic receptors

Serotonergic and adrenergic receptors were examine in rat brains by direct binding assays after chronic treatment with tricyclic antidepressants. Chronic amitriptyline (AMT) treatment (10 mg/kg/day i.p. for 21 days) decreased specific 3H-spiperone binding in the cortex and not in the caudate nuclei. Specific 3H-dihydroalprenolol (3H-DHA) binding was reduced in cerebellar but not in cerebral cortex. Chronic desipramine (DMI) treatment given in the same dose schedule, on the other hand, decreased specific 3H-DHA binding in both cerebellar and cerebral cortex. Specific 3H-spiperone binding in the cerebral cortex was also reduced but to a lesser extent than that in the AMT treated group. Scatchard analysis showed that reductions in 3H-spiperone or 3H-DHA binding in all cases were due to decreases in number of binding sites (Bmax) and not to changes in dissociation constants (KD). No change was observed in 3H-serotonin (3H-5HT), 3H-clonidine, or 3H-WB-4101 binding. The results show that there is no single common change in brain adrenergic and serotonergic receptors after chronic AMT and DMI treatment.

Inhibition of neurotransmitter receptor binding by ergot derivatives

Bromocriptine, lergotrile, lisuride, metergoline, and the Sandoz ergot derivatives 25-397, 29-712, and 29-717 have been tested for their ability to inhibit the synaptic receptor binding of spiroperidol, 5-hydroxytryptamine (5-HT), d-lysergic acid diethylamide (LSD), quinuclidinyl benzilate (QNB), WB.4101, and gamma-aminobutyric acid (GABA). Only GABA binding was not affected, and QNB binding was decreased only by lergotrile and metergoline at high concentrations. The most potent inhibitors of the other ligands were bromocriptine and lisuride for spiroperidol (1-2 nMM), metergoline for 5-HT (29 nM), lisuride for LSD (15 nM), and lergotrile for WB.4101 (17 nM). The direct receptor effects of the ergot derivatives in vitro may contribute to understanding their in vivo effects on behavior and in predicting their therapeutic potential in neurological and neuroendocrine disorders.

Serotonin receptors in hippocampus and frontal cortex

The inhibition by various serotonin agonists and antagonists of the binding of 3 nM 3H-d-LSD, 1.7 nM 3H-serotonin and 0.22 nM 3H-spiperone to homogenates of calf hippocampus and frontal cortex was studied. The 50% inhibitory concentration (IC50) for these drugs versus 3H-d-LSD binding had similar values to and correlated with corresponding IC50 values versus 3H-serotonin binding in the hippocampus, suggesting that 3H-LSD and 3H-serotonin label similar sites in this region. In the calf frontal cortex, serotonin revealed a biphasic inhibition against 3H-d-LSD binding and the tryptamines inhibited over a concentration range of 10 000-fold. The IC25 values of various drugs versus 3H-d-LSD binding correlated with the IC50 values versus 3H-serotonin, but did not correlate with the IC50 values versus 3H-spiperone. These data suggest that 3H-d-LSD bound to more than one serotonin site in the calf frontal cortex and that 3H-spiperone bound to a separate serotonergic site. Scatchard analyses of the binding for these three 3H-ligands indicated that in the calf frontal cortex the density of 3H-d-LSD sites was approximately equal to the sum of the densities for 3H-serotonin (S-1 sites) and 3H-spiperone (S-2 sites). Two weeks after serotonin-depleting radiofrequency heat lesions of the midbrain dorsal and median raphe nuclei in rats, both 3H-serotonin and 3H-LSD showed enhanced binding in the hippocampus. These data support previous suggestions that supersensitivity develops specifically in serotonin receptors following afferent denervation.

The effects of long-term ethanol on brain receptors for dopamine, acetylcholine, serotonin and noradrenaline

Rats were treated with ethanol for 11-15 days, after which the brain neurotransmitter receptors for dopamine, acetylcholine, serotonin and noradrenaline were measured. The rats were intubated at 12 h intervals with 4 g/kg ethanol, and sacrificed 10 h after the last intubation. After ethanol treatment, the specific binding of 3H-haloperidol to dopamine receptors was significantly decreased in the mesolimbic areas by 20 +/- 6%, but was unchanged in the striatum. Specific binding of 3H-serotonin was increased by 63 +/- 29% in the striatum and by 32 +/- 9% in the brainstem, but decreased by 20 +/- 7% in the hippocampus. The specific binding of 3H-quinuclidinyl benzilate to muscarinic acetylcholine receptors increased by 7 +/- 2% in the striatum, but decreased by 5 +/- 2% in the cerebral cortex. The specific binding of 3H-WB-4101 to alpha-adrenergic receptors was unchanged in all brain areas assayed, namely the striatum, mesolimbic areas, hippocampus, hypothalamus, brainstem and cerebral cortex. These changes may be related to some aspects of tolerance to, and dependence on, ethanol.

Effects of 6-methoxy-1,2,3,4-tetrahydro-beta-carboline (6-MeO-THbetaC) on audiogenic seizures in DBA/2J mice

It was found previously that 6-methoxy-1,2,3,4-tetrahydro-beta-carboline (6-MeO-THbetaC) increased brain concentration of the neurotransmitter serotonin (5-HT) and decreased the concentration of its metabolite 5-hydroxyindole acetic acid (5-HIAA) at the same time the compound attenuated audiogenic seizures (AGS) in DBA/2J mice. In the present study we determined the time-course and dose-response effects of 6-MeO-THbetaC for blockade of AGS. Drugs sharing common effects with 6-MeO-THbetaC were also tested. At a dose of 100 mg/kg, 6-MeO-THbetaC blocked AGS between 10 min and 12 hr after injection, with maximal inhibition at 1 hr at which time a dose-related decrease in AGS was also demonstrated. All of the drugs tested which blocked AGS, including 6-MeO-THbetaC, THbetaC, 5-Hydroxytryptophan, chlorimipramine and pargyline, have biochemical similarities suggesting that facilitating serotonin function may be responsible for seizure-attenuating effects.

In vitro and in vivo disposition of 3H-methiothepin in brain tissues. Relationship to the effects of acute treatment with methiothepin on central serotoninergic receptors

A single treatment with a large dose of methiothepin (20 mg/kg, i.p.) induced, as early as the 2nd day after injection, a significant increase (+20--35%) in the number of specific binding sites for 3H-5-HT in forebrain areas, particularly the hippocampus. Experiments with 3H-methiothepin indicated that the drug remained firmly bound to brain membranes thus maintaining a local concentration high enough to effectively block 5-HT receptors for 10--12 h after its peripheral administration. Accordingly, it can be concluded that the occupancy of central 5-HT receptor sites by methiothepin for several hours was sufficient to induce a supersensitivity phenomenon within the two following days. Although 3H-methiothepin was a useful marker for analyzing the disposition and the kinetics of the 5-HT antagonist in brain tissues, it could not be used as a specific ligand of 5-HT receptors in brain since under in vitro as well as in vivo conditions most of 3H-methiothepin bound to non-specific sites, especially to the lipid component of the membranes.