The 5-HT syndrome in rats as tool for the screening of psychoactive drugs

Sex differences and serotonergic mechanisms in the behavioural effects of psilocin

Psilocybin has recently attracted a great deal of attention as a clinical research and therapeutic tool. The aim of this paper is to bridge two major knowledge gaps regarding its behavioural pharmacology - sex differences and the underlying receptor mechanisms. We used psilocin (0.25, 1 and 4 mg/kg), an active metabolite of psilocybin, in two behavioural paradigms - the open-field test and prepulse inhibition (PPI) of the acoustic startle reaction. Sex differences were evaluated with respect to the phase of the female cycle. The contribution of serotonin receptors in the behavioural action was tested in male rats with selective serotonin receptor antagonists: 5-HT1A receptor antagonist (WAY100635 1 mg/kg), 5-HT2A receptor antagonist (MDL100907 0.5 mg/kg), 5-HT2B receptor antagonist (SB215505 1 mg/kg) and 5-HT2C receptor antagonist (SB242084 1 mg/kg). Psilocin induced dose-dependent inhibition of locomotion and suppression of normal behaviour in rats (behavioural serotonin syndrome, impaired PPI). The effects were more pronounced in male rats than in females. The inhibition of locomotion was normalized by 5-HT1A and 5-HT2B/C antagonists; however, PPI was not affected significantly by these antagonists. Our findings highlight an important issue of sex-specific reactions to psilocin and that apart from 5-HT2A-mediated effects 5-HT1A and 5-HT2C/B receptors also play an important role. These findings have implications for recent clinical trials.

Serotonin receptor subtype mediation of the interoceptive discriminative stimuli induced by 5-methoxy-N,N-dimethyltryptamine

Male Wistar rats were trained to discriminate the interoceptive effects of 5-methoxy-N,N-dimethyltryptamine (5-OMe-DMT; 1.25 mg/kg, IP) from saline in a two-lever operant chamber. Following discrimination learning, the following drugs (with ED50 dose in mg/kg IP) dose-dependently generalized: lysergic acid diethylamide (LSD, 0.04), 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 0.11), 6-methoxy-4-(dipropyl-amino)-1,3,4,5-tetrahydrobenz(c,d)indole hydrochloride (BAY R 1531, 0.15), 5-OMe-DMT itself (0.63), ipsapirone (TVX Q 7821, 2.7), and buspirone (3.8). The potencies of these drugs in generalization tests were best correlated with their binding affinities for the 5-HT1A serotonin receptor subtype (as measured by displacement of 3H-ipsapirone in the hippocampus). Drugs not, or only partially generalizing included quipazine, bufotenin, m-trifluoromethylphenylpiperazine (TFMPP), 5-methoxy-3(1,2,3,6-tetrahydropyridine-4-yl)-1H-indole succinate (RU 24969), citalopram, clomipramine, 1,4-dihydro-2,6-dimethyl-3-nitro-4(2-trifluoromethylphenyl)-pyridine-5- carboxylate (BAY K 8644), the buspirone metabolite 1-pyrimidinyl-piperazine (1-PP), methysergide, metergoline, and metitepine. Of the last three compounds with antagonistic activity at 5-HT receptors, as well as ketanserin, pizotifen, and ritanserin, only metitepine and pindolol could fully block the 5-OMe-DMT stimulus. Pizotifen blocked the generalization of quipazine fully, that of 5-OMe-DMT only partially, and that of ipsapirone not at all. These data indicate that the 5-HT1A receptor subtype is strongly involved in the transduction of the interoceptive discriminative stimuli induced by 5-OMe-DMT, with 5-HT2 agonism also playing a possible role.

Serotonin syndrome and drug combinations: focus on MAOI and RIMA

Serotonin syndrome is a potentially life-threatening complication of psychopharmacological drug therapy. The syndrome is produced most often by the concurrent use of two or more drugs that increase brainstem serotonin activity and is often unrecognized because of the varied and nonspecific nature of its symptomatology. Serotonin syndrome is characterized by alterations in cognition, behavior, autonomic nervous system function and neuromuscular activity. The purpose of this study was to investigate the possibility that any serotomimetic substance alone or in combination may give rise to serotonin syndrome, that this condition is not confined to the use of newly introduced substances, and that the newer reversible inhibitors of monoamine oxidase type A (RIMAs) are at decreased risk for this phenomenon than older, classical (irreversible) monoamine oxidase inhibitors (MAOI). This is a hypothesis-generating study based on a review of all published cases of adverse effects arising in patients receiving serotomimetic substances or combinations. A wide range of substances were involved in 226 cases published worldwide since 1950 where there was any use of single or combined serotomimetic treatments. Of the 226 cases, 105 fulfilled the Sternbach criteria for serotonin syndrome. Some classes of drugs and individual substances were more commonly represented. This may arise from product utilization patterns or from the specific properties of the individual products. However, moclobemide, a RIMA, was represented in only 9 of the 226 published cases and 3 of the 105 defined serotonin syndromes, either in multi-drug combinations and/or in mixed drug overdose. One explanation for the small number of cases involving RIMAs could be the reversibility of these new products. In addition the small number of reports on moclobemide could be an effect of its short availability in routine use during the period of the literature review. We conclude that a spectrum of serotonergic hyperactivity, through to a defined serotonin syndrome, may arise from the use or combination of any serotomimetic substance, as this is a consequence of the mechanism involved, rather than the use of any specific product such as the new antidepressants. We further conclude that this condition is not confined to the use of newly introduced substances and that the newer reversible inhibitors of monoamine oxidase type A (RIMAs) may be at decreased risk for this phenomenon than older, classical (irreversible) (MAOI). Given that a spectrum of serotonergic hyperactivity was observed, this analysis prompts redefinition of the currently accepted criteria for serotonin syndrome.

Serotonin uptake inhibitors: uses in clinical therapy and in laboratory research

Fluoxetine, zimelidine, sertraline, paroxetine, fluvoxamine, indalpine and citalopram are the selective inhibitors of serotonin uptake that have been most widely studied. Some of these compounds are or have been used clinically in the treatment of mental depression, obsessive-compulsive disorder and bulimia, and therapeutic benefit has been claimed in additional diseases as well. By blocking the membrane uptake carrier which transports serotonin from the extracellular space to inside the serotonin nerve terminals, these compounds increase extracellular concentrations of serotonin and amplify signals sent by serotonin neurons. Because serotonin neurons are widespread in the central nervous system, the functional consequences of blocking serotonin uptake are diverse, but are generally subtle. Animals treated with serotonin uptake inhibitors look normal in gross appearance, but effects such as reduced aggressive behavior, decreased food intake and altered food selection, analgesia, anticonvulsant activity, endocrine changes and neurochemical changes have been demonstrated and characterized. Serotonin uptake inhibitors have helped in revealing some dynamics of serotonin neurons; for example, when uptake is inhibited and extracellular serotonin concentration increases, presynaptic as well as postsynaptic receptors for serotonin are activated to a greater degree. A consequence of increased activation of autoreceptors on serotonin cell bodies and nerve terminals is a reduction in firing of serotonin neurons and a decrease in serotonin synthesis and release. The result is a limit on the degree to which extracellular serotonin and serotonergic neurotransmission are increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of dorsal raphe nucleus stimulation on cerebral blood flow and flow-metabolism coupling in the conscious rat

In the present study, we have investigated the effects of an activation of the ascending serotonergic pathway on the cerebral blood supply to a number (63) of well-defined neuroanatomical structures. To this end, we have measured the local cerebral blood flow during electrical stimulation of the dorsal raphe nucleus. Measurement of regional blood flow was performed in the conscious rat through the use of the [14C]iodoantipyrine autoradiographic technique. Stimulation of the dorsal raphe nucleus induced increases (> 15% compared to control) in cerebral blood flow in 17 structures of which statistical significance (P < 0.05) was achieved in nine; raphe stimulation significantly decreased flow in three regions. The greatest increases (+71 and +46%) were found in the frontal sensorimotor and posterior parietal cortices. Other increases were noted in relay stations of the extrapyramidal and limbic systems. Stimulation induced a decrease in two regions of the primary auditory system and in the lateral habenular nucleus. These results show that activation of the serotonergic pathway in the conscious rat effects regional cerebral blood flow heterogeneously, differing from the widespread increase in glucose utilization that we previously observed using the same experimental paradigm. Statistical analyses indicated that activation of the dorsal raphe nucleus resulted in a global modification of the flow-metabolism ratio. Moreover, in 19 out of 31 regions analysed, this ratio is significantly altered as compared to control. The dichotomy between raphe-induced changes in flow and glucose-metabolism could be explained by one or both of two hypotheses; firstly there could be a direct serotonergic innervation of cerebral resistance vessels; secondly, during raphe stimulation it could be that glucose use is not the primary determinant of tissue perfusion.

Behaviors induced by 5-hydroxytryptophan in neonatal, preweaning, postweaning, and adult Sprague-Dawley rats

The behaviors induced by the 5-hydroxytryptamine (5-HT) precursor 5-hydroxytryptophan (5-HTP) has been called the "5-HT (serotonin) syndrome." These behaviors and others identified in rat pups were observed following administration of 5-HTP (300 mg/kg, SC) on postnatal (PN) days 3, 14, and 28 and in adult rats. Certain 5-HT syndrome behaviors and other uniquely neonatal behaviors were present in PN3 pups treated with vehicle. 5-HTP-treated PN3 pups showed increased head-shakes, rollovers, vocalizations, and forepaw treading and decreased hindlimb abduction. No 5-HT syndrome or neonatal behaviors were present at PN14 or PN28 or in adults treated with vehicle. 5-HTP administered at PN14 stimulated circling, forepaw treading, and resting tremor; at PN28, stimulated head-shakes and resting tremor; and in adults produced only head-shakes. To determine if prior exposure to 5-HTP affected the sensitivity of 5-HT receptor subtypes, the 5-HT1A agonist (+/-)-8-hydroxy-dipropylaminotetralin (8-OH-DPAT) and the 5-HT2/1C agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) were administered to all rats as adults. 8-OH-DPAT (1 mg/kg, SC) produced flattened body posture unaffected by prior exposure to 5-HTP. Head-shakes induced by DOI (5mg/kg, IP) were decreased by prior exposure to 5-HTP at PN3 and adult, but increased by preexposure at PN28. Thus, serotonergic neural systems are implicated in some behaviors of neonates. The developmental patterns suggest changes in the sensitivity to these systems. Further, lasting changes in 5-HT2/1C receptor sensitivity occur due to exposure to 5-HTP.

Fluoxetine, a selective inhibitor of serotonin uptake

In summary, fluoxetine is a highly selective serotonin uptake inhibitor in vitro and in vivo. The conformation of fluoxetine, which resembles that of sertraline and other serotonin uptake inhibitors, appears to be a key feature that enables its high affinity and selective interaction with the serotonin transporter. The para-trifluoromethyl substituent, however, is also a pivotal structural element. The molecular pharmacology of fluoxetine has been well-defined, and its in vivo pharmacological effects appear to be mediated almost exclusively by serotonin uptake inhibition. Its selectivity for the serotonin transporter, lack of affinity for neurotransmitter receptors, and retention of selectivity following metabolism to norfluoxetine make fluoxetine a useful tool to explore pharmacologically induced increases in serotonin neurotransmission. Fluoxetine has found a variety of therapeutic application. Its use in treating depression has been most extensively studied, but controlled clinical studies also suggest the drug may have a role in treating obesity and bulimia. Moreover, a variety of other psychiatric disorders may be treatable with this drug. Regardless of the outcome of these clinical trials, it is apparent that fluoxetine has found a useful niche in therapy, and can be used as a probe to determine the role of serotonin in modulating human pathophysiologies.

Serotonin release estimated by transcortical dialysis in freely-moving rats

The transcerebral dialysis method has been utilized for measuring extracellular brain concentrations of serotonin and 5-hydroxyindolacetic acid. Dialysis fibres were implanted transversally in the rat frontal cortex and perfused by Ringer. Serotonin and 5-hydroxyindolacetic acid were quantified by reverse phase high performance liquid chromatography with electrochemical detection. Experiments were performed in freely-moving rats 20-24 h after the implant of the fibre. Basal output of serotonin and 5-hydroxyindolacetic acid was 0.12 and 22.8 pmol in 20 min, respectively. The output of serotonin was calcium-dependent and tetrodotoxin-sensitive (1 micron in the Ringer) while was stimulated by veratridine (50 microns) and by high concentrations of K+ (60 and 100 mM). Serotonin output was increased in a concentration-dependent manner by chlorimipramine (1-10 microM) in the Ringer; this drug stimulated serotonin release also when administered s.c. (20 mg/kg) in a tetrodotoxin-sensitive manner. The irreversible monoamine-oxidase inhibitor pargyline (75 mg/kg, i.p.) strongly stimulated serotonin output while reduced 5-hydroxyindolacetic acid output. A proposed serotonin releaser, fenfluramine (25 mg/kg, s.c.), stimulated serotonin release and this effect was strongly potentiated by local application of tetrodotoxin (1 microM). Agonists of serotonin receptors such as lisuride (0.03 mg/kg, s.c.), 8-hydroxy-2-(di-n-propilamino)tetraline (0.25 mg/kg, s.c.) and 5-methoxy 3(1,2,3,6-tetrahydro-4-pyridinil)-1H indole succinate (1 mg/kg, s.c.) reduced serotonin release. It appears that brain dialysis is a suitable method for the study of serotonin release in the cortex of freely-moving rats.

Interactions between serotonergic agonists and antagonists in rats trained with LSD as a discriminative stimulus

Drugs purported to have selective affinities for 5-HT1A, 5-HT1B, and 5-HT2 receptors were tested in rats trained with 0.1 mg LSD versus saline. Included were 5-methoxy-dimethyltryptamine (MDMT), 2,5-dimethoxy-4-methyl-amphetamine (DOM), 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), m-trifluoromethylphenyl-piperazine (TFMPP), and 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole (RU-24969). Tests were then repeated in the presence of either pizotyline or pirenperone. DOM substituted for LSD and both were blocked by pizotyline and pirenperone. MDMT, 8-OH-DPAT, TFMPP, and RU-24969 substituted less completely and were variably affected by the antagonists. An unexpected result was potentiation of the stimulus or disruptive effects of certain doses of 8-OH-DPAT and TFMPP by pizotyline and pirenperone. The present findings suggest more complex interactions between these drugs than has previously been assumed.

Evidence that mCPP may have behavioural effects mediated by central 5-HT1C receptors

1. The effects of 1-(3-chlorophenyl)piperazine (mCPP) and 1-[3-(trifluoromethyl)phenyl] piperazine (TFMPP) on activity of rats in a novel cage, and on the rotorod and elevated bar co-ordination tests was examined. 2. Peripherally administered mCPP and TFMPP dose-dependently reduced locomotion, rearing, and feeding scores but not grooming of freely fed rats placed in a novel observation cage. Yawning behaviour was increased. Similar effects were also observed after injection of mCPP into the 3rd ventricle. 3. Co-ordination on a rotating drum of both untrained and trained rats was impaired following mCPP but co-ordination on an elevated bar was not. 4. The hypoactivity induced by mCPP was opposed by three antagonists with high affinity for the 5-hydroxytryptamine (5-HT1C) site; metergoline, mianserin, cyproheptadine and possibly also by a fourth antagonist mesulergine. Metergoline, mianserin and cyproheptadine also opposed the reduction in feeding scores. However, neither effect of mCPP was antagonized by the 5-HT2-receptor antagonists ketanserin or ritanserin, the 5-HT3-receptor antagonist ICS 205-930, the 5-HT1A and 5-HT1B-receptor antagonists (-)-pindolol, (-)-propranolol and (+/-)-cyanopindolol or the 5-HT1A-, 5-HT2- and dopamine receptor antagonist spiperone. The specific alpha 2-adrenoceptor antagonist idazoxan was also without effect. 5. Hypoactivity induced by TFMPP was similarly antagonized by mianserin but unaffected by (+/-)-cyanopindolol. 6. These results suggest that the hypoactivity is mediated by central 5-HT1C-receptors and that mCPP and possibly TFMPP may be 5-HT1C-receptor agonists. 7. As mianserin, cyproheptadine and mesulergine in the absence of mCPP did not increase locomotion but increased the number of feeding scores, the activation of 5-HT1C-receptors may be of physiological importance in the control of appetite. The possible relevance of these results to the therapeutic and side-effects of clinically used antidepressants (particularly trazodone and mianserin) and anorexigenic drugs is discussed.

Influence of ascending serotonergic pathways on glucose use in the conscious rat brain. II. Effects of electrical stimulation of the rostral raphé nuclei

Although lesions of the rostral raphé nuclei have minimal effects on integrated functional activity, as studied by the 2-deoxyglucose technique, the repercussions of activating the ascending serotonergic pathways have yet to be reported in the literature. To examine this question, we studied the consequences of the electrical stimulation of the rostral (median or dorsal) raphé nuclei on local cerebral glucose use in the conscious rat. Glucose use was determined by quantitative autoradiography in 105 defined brain structures. Raphé stimulation increased glucose utilization in a number of well-defined structures and pathways, dorsal raphé stimulation being systematically more effective than median raphé stimulation. Of all the neocortical regions studied, only the somatosensory cortex displayed a columnar and laminar pattern of increased glucose use that was restricted to the somatotopic delineation of the rat's head and face. Increased glucose use was seen in almost all key elements of the extrapyramidal system with the notable exception of the caudate-putamen. The thalamic nuclei that were activated by rostral raphé stimulation included those that subserve the processing of somesthetic, accessory visual and limbic information. Raphé stimulation-induced decreases in local cerebral glucose use were never observed. Almost all of the induced changes could be prevented or obtunded by prior intraventricular administration of the serotonergic neurotoxin 5,7-dihydroxytryptamine, suggesting that the majority of the raphé-induced changes in integrated functional activity were mediated via the activation of serotonergic neurones. The magnitude and pattern of the increases in glucose use could not always be correlated with the regional density of serotonergic innervation nor with the distribution of 5-hydroxytryptamine receptor subtypes in the adult brain. However, the pattern of increased cortical glucose use closely matches the selective serotonergic innervation of the somatosensory cortex found in early postnatal development. Thus, it would appear that the 2-deoxyglucose technique reveals functional units in the cortex that are innervated at an early ontogenic stage. We postulate that the discrete and highly organized changes in integrated functional activity that follow raphé stimulation are due to serotonin acting in a phasic manner on restricted, possibly specialized, postsynaptic structures.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of 8-hydroxy-2-(di-n-propylamino)tetralin and other serotonergic agonists on performance in a radial maze: a possible role for 5-HT1A receptors in memory

A group of ten rats was trained to obtain food pellets in an 8-arm radial maze. The effects of pretreatment with (+)-Lysergic acid diethylamide (+)-tartrate (LSD), m-trifluoromethylphenylpiperazine (TFMPP), 5-methoxy-N,N-dimethyltryptamine oxalate (5-MeO-DMT), racemic 8-hydroxy-2-(di-n-propylamino)tetralin HBr (8-OH-DPAT), and 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole succinate (RU 24969) were then evaluated. All drugs were administered IP 15 min before testing. With the exception of an increased rate of responding at a dose of 0.1 mg/kg of 8-OH-DPAT, all drugs produced a dose-related decline in response rate. In addition, LSD, RU 24969, and 8-OH-DPAT caused a statistically significant decrease in efficiency of responding. Of the three, 8-OH-DPAT was clearly the most active. Doses of 0.3, 1.0, and 3.0 mg/kg resulted in efficiencies of 61%, 53%, and 44%, respectively. The present results taken in light of 8-OH-DPAT's preferential binding to 5-HT1A receptors, the high density of these receptors in hippocampus, and the observation that the number of 5-HT1A receptors is decreased in Alzheimer's disease, suggest a possible role for this serotonergic receptor subtype in memory.

Serotonergic neuron stimulation modulates thalamocortical glucose use in the conscious rat

We have studied the effects, in the conscious rat, of electrical stimulation of the dorsal or median raphe nuclei on integrated functional activity, as assessed by the quantitative 2-deoxyglucose autoradiographic technique. Stimulation of serotonergic neurons elicits metabolic changes in cortical and thalamic regions that are not limited to those structures known to receive the densest serotonergic innervation. The thalamic nuclei that are activated by raphe stimulation include those that subserve the processing of somesthetic, accessory visual, and limbic information. Raphe stimulation increased cortical glucose use in a laminar and columnar pattern, but only in a highly circumscribed region that corresponds to the somatotopic representation of the rat's face and head. These findings indicate that ascending serotonergic neurons play an important modulatory role in the regulation of thalamocortical glucose use, observations that may be of value in the understanding of the etiology and expression of classic migraine.

Serotoninergic neurons: long-lasting activation by a possible antidepressant dihydroergosine

The duration of the stimulating effect of one single i.p. injection of a possible antidepressant dihydroergosine (50 mg/kg) on the 5-HT syndrome was studied in rats. Dihydroergosine produced a very pronounced stimulation of the 5-HT syndrome which was still present after 1-144 h (6 days). The results suggest that dihydroergosine produces a long-lasting stimulation of serotoninergic neurons.

Central beta-adrenoceptors can modulate 5-hydroxytryptamine-induced tremor in rats

The effects of two beta 2-adrenoceptor agonists with different lipophilicities were studied on tremor induced by L-5-hydroxytryptophan (L-5-HTP) in pargyline- and carbidopa-pretreated rats. Tremor was recorded and analysed by an objective method based on accelerometry. Clenbuterol, a lipophilic beta 2-selective agonist, dose-dependently enhanced tremor intensity, whereas the hydrophilic beta 2-agonist terbutaline had no effect. The clenbuterol-induced enhancement of tremor was completely abolished by the beta 2-selective antagonist ICI 118,551 but unchanged by the beta 1-selective antagonist metoprolol. The results suggest that centrally located beta 2-adrenoceptors can mediate a modulation of 5-hydroxytryptamine-induced tremor in rats.

Effect of dihydroergosine (DHESN) on the serotoninergic system and behaviour: is DHESN a new antidepressive agent?

Acute (50.0 mg/kg) and repeated (0.1-10.0 mg/kg) administration of dihydroergosine (DHESN) to rats over 5 days lowered the concentration of 5-HIAA in the brain. DHESN given acutely increased the brain 5-HT in p-CPA-treated animals and diminished the probenecid-induced increase in brain 5-HIAA. In pargyline-treated rats DHESN enhanced the 5-HT/5-HIAA ratio. DHESN administered to rats repeatedly over 5 days decreased the level of 5-HT in blood platelets, and in vitro at concentrations of 10(-4) M and 10(-3) M inhibited the uptake of [14C]-5-HT in platelets. DHESN (10.0-100.0 mg/kg) potentiated the 5-HT syndrome produced in rats by pargyline and 5-HTP. This potentiation was blocked with cyproheptadine but not with haloperidol. DHESN (1.0 and 10.0 mg/kg) lowered the locomotor activity of rats and 10.0 mg/kg DHESN also reduced the duration of immobility in rats forced to swim in a restricted space. The results indicate that DHESN, like antidepressants, decreases the turnover of serotonin in the brain and potentiates the 5-HT-mediated behaviour. This might suggest that the drug should be further investigated for its potential antidepressive properties.

Effects of tranylcypromine treatment on neuroendocrine, behavioral, and autonomic responses to tryptophan in depressed patients

Effects of intravenous administration of the serotonin precursor tryptophan (TRP) on serum prolactin, neuromotor function, subjective mood, and blood pressure and pulse were determined in nine depressed patients before and during placebo-controlled treatment with the monoamine oxidase inhibitor (MAOI) tranylcypromine. Tranylcypromine significantly increased the prolactin response to TRP. Four patients developed a distinctive neuromotor syndrome following TRP during tranylcypromine, but not placebo, treatment. Symptoms included hyperreflexia, ankle clonus, nystagmus, incoordination, tremor, myoclonic jerks, and nausea. There were no differences in peak prolactin, mood, or autonomic responses between patients with and without the syndrome, but those with the syndrome had received active tranylcypromine for a significantly shorter duration. Tranylcypromine had little effect on TRP-induced changes in mood or autonomic function, except for a modest enhancement of the TRP-induced rise in diastolic blood pressure. These results suggest that tranylcypromine treatment may enhance serotonin function in depression.