Further selection of rat lines differing in 5-HT-1A receptor sensitivity: behavioral and functional correlates

It was previously reported that selection for differences in the hypothermic effects to the selective 5-HT-1A agonist, 8-OH-DPAT, occurred rapidly, with very substantial differences present by the fourth generation. The present communication summarizes the findings from the next five generations of selection and from behavioral and other functional studies on these rats. The rats which were more sensitive to 8-OH-DPAT (High DPAT Sensitive-HDS) exhibited decreases in temperature of 4 degrees C or more and the distribution did not overlap with that of the rats which were less sensitive to 8-OH-DPAT (Low DPAT Sensitive-LDS) which exhibited decreases in temperature of 1.5 degrees C or less. The randomly bred control group (Random DPAT Sensitive-RDS) exhibited intermediate temperature decreases (means of 1.6-1.8 degrees C), with time overlap with the distributions of the selected groups. Pretreatment with pindolol, a 5-HT-1A antagonist, reduced the hypothermic response to 8-OH-DPAT, but pretreatment with ritanserin, a 5-HT-7 and 5-HT-2A/C antagonist, had no effect, confirming that the hypothermic response to 8-OH-DPAT is mediated predominantly by 5-HT-1A receptors. The HDS rats were less mobile in a forced swim test and drank more saccharin solution in a two-bottle choice paradigm than the LDS or RDS rats over several generations. In contrast, there were no consistent differences among the groups for open field activity or performance in an elevated plus maze. There were no differences among the groups for voluntary alcohol intake, but the HDS rats exhibited greater suppression of alcohol and saccharin intake after injection of 8-OH-DPAT (0.125 mg kg-1). The HDS rats were also found to have a higher number of 5-HT-1A binding sites in cortical regions than the LDS or RDS rats, but there were no 5-HT-1A binding site differences in the raphe nuclei. These findings clearly show that consistent behavioral differences do occur in the 8-OH-DPAT-selected lines of rats, but only for behaviors related to possible depression or reward, not anxiety. The pattern of binding results suggests that these behavioral correlates of 8-OH-DPAT selection may be related to changes in cortical 5-HT-1A receptors rather than raphe autoreceptors.

Antagonism by the 5-HT2A/C receptor agonist DOI of raclopride-induced catalepsy in the rat

It has been shown that the administration of 5-hydroxytryptamine (5-HT)1A receptor agonists will antagonize the catalepsy induced by dopamine D1 or D2 receptor blocking agents. In the present study, administration of the 5-HT2A/C receptor agonist, 1-(2,5-dimethoxy-4-iodo)-2-aminopropane (DOI) (1 mg kg-1 s.c.), counteracted the catalepsy produced by the dopamine D2 receptor antagonist, raclopride (16 mg kg-1 s.c.), but not by the dopamine D1 receptor antagonist (R)-(+)-8-chloro-2,3,4,5-tetra-hydro-3-methyl-5-phenyl-1H-3-benzazepine (SCH 23390) (0.2 mg kg-1 s.c.). The effects of DOI on raclopride-induced catalepsy were fully antagonized by pretreatment with the 5-HT2A/C receptor antagonist, ritanserin (2 mg kg-1 s.c.). The 5-HT precursor, 5-hydroxytryptophan (5-HTP) (6.25-25.0 mg kg-1 i.p.), in combination with the peripheral 5-HTP decarboxylase inhibitor, benserazide (25 mg kg-1 i.p.), and the selective serotonin reuptake inhibitor, zimeldine (10 mg kg-1 s.c.), enhanced the catalepsy produced by a low dose of raclopride (4 mg kg-1 s.c.). It is concluded that stimulation of (postsynaptic) 5-HT2 receptors results in antagonism of the catalepsy induced by treatment with a dopamine D2, but not a D1, receptor antagonist. The fact that 5-HTP, in the presence of benserazide and zimeldine, enhanced raclopride-induced catalepsy suggests the possibility of postsynaptic 5-HT receptors acting in opposition to the 5-HT1 and 5-HT2 receptors, as regards extrapyramidal motor functions in the rat.

Behavioural interactions between 5-hydroxytryptophan, neuroleptic agents and 5-HT receptor antagonists in modifying rodent responding to aversive situations

1. The ability of 5-hydroxytryptophan, 5-HT2 receptor antagonists and typical and atypical neuroleptic agents to modify behavioural responding to aversive situations was investigated in the mouse light/dark test and rat social interaction. 2. The administration of 5-hydroxytryptophan inhibited rat social interaction and the exploratory behaviour of mice in the light/dark test. 3. The 5-HT2 receptor antagonists, ketanserin, ritanserin, MDL11939, methysergide and RP62203, the neuroleptic agents, spiperone, haloperidol and benperidol, and the atypical neuroleptic agent, clozapine, when administered alone failed to modify mouse or rat behaviour. In contrast, when administered alone, sulpiride in rats and mice and thioridazine in rats disinhibited behaviour. 4. Methysergide, RP62203, ketanserin, ritanserin and MDL11939 antagonized the inhibitory effects of 5-hydroxytryptophan or reversed the inhibitory effects to one of disinhibition. 5. Low doses of spiperone (but not haloperidol or benperidol) also antagonized the inhibitory effects of 5-hydroxytryptophan in the rat but not the mouse. Higher doses of the three neuroleptic agents caused locomotor depression in both rats and mice which obscured any specific changes in behavioural responding to the aversive situations. 6. The disinhibitory profile of sulpiride in both mice and rats and thioridazine in rats was evident during their interaction with 5-hydroxytryptophan. Thioridazine in the mouse and clozapine in rats and mice also reversed the inhibitory effects of 5-hydroxytryptophan to one of disinhibition. 7. In summary, we present evidence that the atypical neuroleptic agents, thioridazine and clozapine, with their known affinity for the 5-HT2 receptors, can mimic the actions of reference 5-HT2 receptor antagonists to antagonize the inhibitory effects of 5-hydroxytryptophan in rodent models of anxiety. The results are intepreted in terms of drug action on different 5-HT2 and other 5-HT receptor subtypes. In addition, thioridazine and sulpiride have disinhibitory effects in their own right which remain to be explained.

Evidence that 5-HT2A receptors are not involved in 5-HT-mediated thermoregulation in mice

To determine the role of 5-hydroxytryptamine2A (5-HT2A) receptors in 5-HT-mediated thermoregulation in mice, we studied the effects of a 5-HT2A receptor agonist and 5-HT2A receptor antagonists on the body temperature, and the effects of selective 5-HT2A receptor and nonselective 5-HT receptor antagonists on hypothermia induced by 5-hydroxytryptophan (5-HTP). (+/-)-1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a 5-HT2A receptor agonist, did not change body temperature in mice at doses of 1 and 5 mg/kg, intraperitoneally (IP), which induced head twitch response. Three 5-HT2A receptor antagonists, ketanserin (1 mg/kg, orally), ritanserin (1 and 10 mg/kg, orally), and DV-7028 (10 mg/kg, orally), also failed to alter body temperature, although these three 5-HT2A receptor antagonists at > or = 1 mg/kg, orally, inhibited head twitch response induced by 5-HTP (200 mg/kg, IP), a precursor of 5-HT. Ketanserin (1 mg/kg, orally), ritanserin (1 and 10 mg/kg, orally), and DV-7028 (10 mg/kg, orally) did not inhibit hypothermia induced by 5-HTP (200 mg/kg, IP). A nonselective 5-HT receptor antagonist, methysergide (1 mg/kg, subcutaneously), attenuated hypothermic response to 5-HTP. These results suggest that in mice, 5-HT2A receptors are unlikely to be involved in 5-HT-mediated thermoregulation.

Behavioural response to pharmacologic manipulation of serotonin receptors in the genetically dystonic hamster

The genetically dystonic (dtsz) hamster is an autosomal recessive mutant that shares several features with paroxysmal dystonia, i.e., a subcategory of inherited idiopathic dystonia in humans. Because the serotonin (5-HT) system has been suggested to be involved in dystonia, we examined the functional responsiveness of the 5-HT system in dystonic hamsters by administering various 5-HT agonists and antagonists selective for different receptor subtypes and observing the effects on dystonic attacks as well as the behavioural responses associated with drug administration. Paradoxically, marked prodystonic effects (i.e., increased severity and/or decreased latency of dystonic attacks) were seen with both the selective 5-HT1A receptor agonist 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT) and the selective and "silent" 5-HT1A receptor antagonist, N-tert-butyl-3[4-(2-methoxyphenyl)piperazin-1-yl]-2- phenylpropionamide [(+)-WAY-100135], whereas other 5-HT1A receptor antagonists, i.e., methyl 4[4-(4-[1,1,3-trioxo-2H-1,2-benzoiosothiazol-2-yl]butyl)-1- piperazinyl]1-H-indole-2-carboxylate (SDZ 216-525) and N1-bromoacetyl-N8-3'-(4-indolyloxy)-2'-hydroxypropyl-(Z)-1,8- diamino-p-methane (pindobind-5-HT1A) did not alter dystonia to any comparable extent. Because among these 5-HT1A receptor antagonists, (+)-WAY-100135 is the only drug known to be not only silent at postsynaptic but also presynaptic (somatodendritic) 5-HT1A receptors, the marked prodystonic effect of this drug could relate to increased 5-HT release as a result of the blockade of somatodendritic 5-HT1A receptors. The only 5-HT1A receptor antagonist that exerted antidystonic effects in hamsters was pindolol, which, however, could be related to its beta-adrenoceptor blocking action. The 5-HT1A receptor partial agonist ipsapirone exerted moderate prodystonic activity. Prodystonic activity was also determined for the mixed 5-HT1A/5-HT2 receptor agonist 5-methoxy-N,N-dimethyltryptamine, although this drug was less potent in this regard than 8-OH-DPAT. The 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) exerted prodystonic effects in mutant hamsters, which, however, were also seen after the administration of the 5-HT2 receptor antagonist ritanserin. Collectively, the results of this study demonstrate that dystonia in genetically dystonic hamsters can be affected by pharmacologic manipulation of 5-HT receptors. The data may also indicate that dystonia is not a potential clinical application for selective 5-HT1A or 5-HT2 receptor antagonists.

Lack of efficacy of 5-HT2A receptor antagonists to reduce brain damage after 3 minutes of transient global cerebral ischaemia in gerbils

Stimulation of the 5-HT2A receptors by serotonin has been reported to exert an excitatory effect on neocortical neurons in rats and mice, to facilitate ischaemia-induced release of excitatory amino acids and to mediate the vasomotor constrictor component of the response of blood vessels to 5-HT. 5-HT2A receptor antagonists have, therefore, been proposed as potential protectants against the effects of cerebral ischaemia. The aim of this study was to evaluate the effects of two relatively selective 5-HT2A receptor antagonists, ketanserin and ritanserin, on delayed hyperactivity and the ensuing neuronal degeneration induced by 3 minutes of bilateral carotid artery ligation in Mongolian gerbils. Effects were compared to that of flunarizine, which blocks calcium overload and served as a positive control in this paradigm. Temporal and/or rectal temperatures were measured and strictly controlled during the ischaemia and the early reperfusion phase. Locomotor activity was measured one day after the ischaemia and neuronal degeneration quantified 7 days later using an image analysis system (Quantimet 570, Leica). Global ischaemia in gerbils elicits hyperactivity associated with a delayed neuronal degeneration predominantly in the CA1 zone of the hippocampus. Ketanserin and ritanserin (3 and 10 mg/kg ip, twice daily for 3 days, pre- and postischaemia) did not protect the CA1 neurons against ischaemic damage. The postischaemic hyperactivity was inhibited only with the higher dose of ketanserin. As previously reported, flunarizine (30 mg/kg po) markedly reduced neuronal degeneration (-44.2%, p < 0.01) and totally abolished the ischaemia-induced hyperactivity. These data demonstrate that ketanserin and ritanserin are not effective protectants of the gerbil hippocampus against ischaemic damage when the body temperature of the animals is controlled, thus suggesting that 5-HT2A receptors are not directly implicated in the pathogenesis of global cerebral ischaemia in this model.

Single-dose ritanserin and alcohol in healthy volunteers: a placebo-controlled trial

A double blind cross-over design trial was carried out to investigate the effect of simultaneous administration of alcohol (0.5 g/kg) and ritanserin (10 mg) on biological and behavioral functioning. Twenty healthy volunteers were selected to participate in the study. To assess the effect of treatments the following evaluations were performed: psychomotor tests, vital signs, intoxication, euphoria, and mood. In addition, ritanserin and alcohol plasma concentration were measured. Psychomotor performance and vital signs during the ritanserin session did not differ significantly from the placebo session. Similar results were obtained in regard to alcohol intoxication, euphoria, and mood, except for tiredness and alertness, which were significantly different compared to placebo. Differences in blood alcohol concentration between the ritanserin and the placebo sessions did not attain significance. Plasma ritanserin concentration was 143 ng/ml 1 h after alcohol administration and decayed to 53 ng/ml 6 h after alcohol consumption in the active treatment session. Our findings tend to indicate that ritanserin neither enhances the central nervous system depressant effect of alcohol nor produces a pharmacokinetic interaction during acute alcohol ingestion.

Ritanserin potentiates the stimulatory effects of raclopride on neuronal activity and dopamine release selectivity in the mesolimbic dopaminergic system

The atypical profile of clozapine and some other new antipsychotic drugs has been attributed to a relatively selective effect on the mesolimbic dopaminergic system, as well as to their potent serotonin 5-HT2 receptor antagonism and high ratio of 5-HT2 to dopamine D2 receptor affinities. It is unclear, however, how concurrent 5-HT2 and D2 receptor antagonism specifically affects the mesoaccumbens and the mesocortical dopaminergic systems. The present study examined the effect of pretreatment with the 5-HT2 receptor antagonist, ritanserin, on changes in midbrain dopamine neuronal activity as well as in forebrain, extracellular concentrations of dopamine, induced by relatively low doses of the D2 receptor antagonist raclopride, utilizing in vivo extracellular single cell recording techniques and voltammetry in anesthetized rats, as well as microdialysis in freely moving rats. Raclopride alone (10-2560 microgram/kg, i.v.) induced a dose-dependent increase in three parameters of neuronal activity, i.e. burst firing, firing rate and variation coefficient, of midbrain DA neurons. This effect of raclopride was more pronounced in cells of the ventral tegmental area than in cells of the substantia nigra-zona compacta. Ritanserin alone (1.0 mg/kg, i.v.) also increased all three parameters of neuronal activity in dopamine cells of the ventral tegmental area, but only firing rate in the cells of the substantia nigra. Ritanserin pretreatment (30 min) significantly enhanced the stimulatory effects of low doses of raclopride (10-20 micrograms/kg, s.c.) increased extracellular concentrations of dopamine in the medial prefrontal cortex and the dorsolateral striatum by 75 and 110%, respectively, as measured by microdialysis. Ritanserin alone (1.5 mg/kg, s.c.) did not significantly affect cortical and striatal extracellular dopamine concentrations; however, pretreatment (40 min) with ritanserin elevated the raclopride-induced increase of dopamine concentrations in the medial prefrontal cortex of about 250%, but failed to affect the action of raclopride on striatal dopamine levels. Raclopride alone (10 and 320 micrograms/kg, i.v.) dose-dependently increased extracellular concentrations of dopamine in the nucleus accumbens and the dorsolateral striatum to about 500%, as determined by voltammetry. Ritanserin alone (1.0 mg/kg, i.v.) did not significantly affect the voltammetric dopamine signal in the nucleus accumbens or the dorsolateral striatum; however, ritanserin pretreatment (30 min) enhanced the raclopride-induced increase in accumbal but not striatal dopamine concentrations to about 1600%. The stimulatory effect of the combined ritanserin plus raclopride treatment on neuronal activity and DA release was more pronounced in the mesolimbic than the nigrostriatal dopaminergic system. The present data indicate that concurrent 5-HT2 and D2 receptor antagonism selectively affects the activity of the mesolimbic dopaminergic system. These findings provide an experimental basis for the notion that combined 5-HT2 and D2 receptor antagonism may underlie the limbic mode of action of at least some atypical antipsychotic drugs and consequently contribute to their unique therapeutic effects.

Ketanserin and ritanserin discriminate between recombinant human 5-HT1D alpha and 5-HT1D beta receptor subtypes

Compounds able to discriminate functionally between the closely related cloned human 5-HT1D alpha and 5-HT1D beta receptor subtypes have not been reported previously. In [3H]5-HT competition assays, the classical 5-HT2A receptor antagonists, ritanserin and ketanserin, displayed moderate affinity (pKi = 7.30 and 7.17, respectively) and marked selectivity (22- and 71-fold, respectively) for the recombinant human 5-HT1D alpha subtype relative to the 5-HT1D beta receptor. In contrast, the nonselective 5-HT1/2 receptor antagonist, methiothepin, exhibited similar binding affinities (pKi = 7.64-8.01) for both recombinant 5-HT1D subtypes. The antagonistic properties of these compounds were evaluated for their ability to block 5-HT-induced inhibition of forskolin-stimulated cAMP accumulation in intact cells stably expressing either 5-HT1D alpha or 5-HT1D beta receptors. All three compounds behaved as antagonists devoid of intrinsic activity in the functional assays. The apparent pKb values determined in functional assays closely matched their pKi values obtained in binding assays. Since ketanserin exhibits significant selectivity for the human 5-HT1D alpha receptor, this antagonist can be used as a pharmacological tool to discriminate between 5-HT1D alpha and 5-HT1D beta receptor-mediated responses in human tissues.

Differential effects of classical and newer antipsychotics on the hypermotility induced by two dose levels of D-amphetamine

The inhibitory effects of a variety of established and putative antipsychotic compounds on the hypermotility induced by D-amphetamine at two dose levels (0.5 and 2.0 mg/kg) have been studied. Classical antipsychotics (haloperidol, fluphenazine and cis(Z)-flupentixol) and the selective dopamine D2 receptor antagonist remoxipride inhibit hypermotility in the two conditions with similar potencies, whereas sertindole, clozapine, risperidone, ziprasidone and olanzapine preferentially inhibit the effect of the low dose of D-amphetamine (selectivity ratios between 6.5 and 18). Seroquel, amperozide and the selective 5-HT2A receptor antagonist MDL 100.151 ((+/-)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-p iperidine - methanol) have no effect on D-amphetamine 2.0 mg/kg, but inhibit the response to D-amphetamine 0.5 mg/kg. The alpha 1-adrenoceptor antagonist prazosin inhibits the motility response to D-amphetamine 0.5 mg/kg with slightly higher potency than that to D-amphetamine 2.0 mg/kg, whereas the 5-HT2A/2C receptor antagonist ritanserin selectively inhibits the effect of D-amphetamine 0.5 mg/kg. The histamine H1 receptor antagonist mepyramine is ineffective in both models. All compounds, except remoxipride, MDL 100.151 and ritanserin (which are ineffective) inhibit spontaneous locomotor activity at dose levels close to those inhibiting the response to D-amphetamine 2.0 mg/kg. Prazosin has partial inhibitory effect. In conclusion, dopamine antagonism has similar inhibitory effect on hyperactivity induced by low and high D-amphetamine dosages, alpha 1-adrenoceptor antagonism also contributes to both effects, whereas 5-HT2 receptor antagonism selectively interacts with the low D-amphetamine dose. This indicates that the responses to D-amphetamine 0.5 and 2.0 mg/kg are differently modulated by these neurotransmitters.(ABSTRACT TRUNCATED AT 250 WORDS)

Release of acetylcholinesterase from guinea-pig substantia nigra: effects of tryptaminergic drugs and dorsal raphé nucleus stimulation

The guinea-pig substantia nigra receives a 5-hydroxytryptaminergic (5-HT ergic) projection from the dorsal raphé nucleus. In this study we have attempted to identify the 5-HT receptor subtype mediating release of acetylcholinesterase (AChE) from nigral neurones, measured by assay of perfusate obtained via chronically implanted push-pull cannulae. The effects of direct nigral application of 5-HT, 2-methyl-5-HT and 5-methoxytryptamine. Application of submicromolar concentrations of 5-HT, 2,5,-dimethoxy-4- iodoamphetamine and alpha-methyl-5-HT significantly enhanced release of AChE, whereas 5-carboxamidotryptamine, sumatriptan, 2-methyl-5-HT and 5-methoxytryptamine were ineffective at a similar concentration range. Electrical stimulation (50 Hz, 20-300 mu A) of the dorsal raphé nucleus evoked release of AChE from the substantia nigra, and induced a rotational behavioural effect for the duration of stimulation. Pretreatment with 5,7,-dihydroxytryptamine inhibited both DRN-evoked release of AChE and animal rotation. The 5-HT receptor antagonists ketanserin and ritanserin (10(-7)-10(-6)M, when applied to the substantia nigra, inhibited raphé-stimulated AChE release. Drugs which inhibited raphé-stimulated release of AChE had no effect on concomitant animal rotation, indicating that the behavioural events are mediated via distinct processes, unrelated to those mediating nigral AChE release. The data suggest that evoked release of AChE from the substantia nigra by stimulation of the dorsal raphé nucleus may be mediated in part via a 5-HT2 receptor type. The 5-HT1D agonists 5-carboxamidotryptamine (10(-6)M and sumatriptan (10(-5)M also inhibited raphé-evoked AChE release, suggesting a possible presynaptic autoinhibitory role for 5-HT1D receptors on raphé-nigral nerve terminals.

BIMT 17, a 5-HT1A receptor agonist/5-HT2A receptor antagonist, directly activates postsynaptic 5-HT inhibitory responses in the rat cerebral cortex

BIMT 17 (1-[2-[4-(3-trifluoromethyl phenyl) piperazin-1-yl] ethyl] benzimidazol- [1H]-2-one), a 5-HT1A receptor agonist/5-HT2A receptor antagonist (see Borsini et al., accompanying paper), in a dose range of 1-10 mg/kg i.v., dose-dependently inhibited the electrical activity of rat medial prefronto-cortical neurons, whereas buspirone, in a dose range of 0.1-1000 micrograms/kg, increased it. 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and 1-[2-(2-thenoylamino)ethyl]-4-[1-(7-methoxynaphthyl)] piperazine (S 14671) presented biphasic patterns of response; they increased electrical activity at doses in the range of 0.1-10 micrograms/kg and 0.1-3 micrograms/kg i.v. respectively, and reduced it at high doses, 30-300 micrograms/kg and 10-30 micrograms/kg i.v., respectively. The inhibitory effect of BIMT 17 on the firing rate of neurons in the frontal cortex was antagonized by the 5-HT1A antagonists tertatolol and WAY 100135, and was still present after destruction of serotonin (5-HT) containing neuronal endings by the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT; 150 micrograms/rat, given intraventricularly), which reduced the cortical 5-HT content by 85%. This destruction of 5-HT neurons, while suppressing the ability of 8-OH-DPAT to inhibit the firing rate at high doses, did not change the excitatory action of this compound at low doses. The addition of ritanserin, a 5-HT2A receptor antagonist, potentiated both the excitatory and inhibitory effects of 8-OH-DPAT on neuronal electrical activity. Direct microiontophoretic application (100 nA/20 s) of 5-HT and BIMT 17, but not that of 8-OH-DPAT, onto medial prefronto-cortical neurons, decreased the firing rate of these neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of captopril on the serotonin-induced vasopressor effect in tail artery isolated from two-kidney, one-clip renal hypertensive rats

The influence of captopril and ritanserin (5-HT2 receptor antagonist) on the serotonin-induced contraction of isolated tail artery of two-kidney, one clip hypertensive (2K, 1C-RHR) and normotensive rats was studied. Captopril, administrated in a single dose of 100 mg/kg, po or in a dose of 30 mg/kg/day, po for one week, diminished the systolic blood pressure in both groups of rats. Ritanserin (0.01 mg/kg/day, sc for one week) administrated alone or in combination with captopril (30 mg/kg/day) evoked such effect only in 2K, 1C-RHR. The reactivity of rat tail artery isolated from 2K, 1C-RHR to serotonin was significantly higher than that obtained from normotensive ones. Captopril given in a single dose or chronically did not change the serotonin-induced contraction of rat tail artery in normotensive rats. The attenuation of serotonin contractile effect was registered in hypertensive rats pre-treated with captopril. Administration ritanserin with or without captopril also inhibited the response to serotonin in both groups. The stronger effect was observed after administration of ritanserin with captopril in 2K, 1C-RHR than in control group. Our results indicate, that in 2K, 1C-RHR the reactivity of rat tail arteries to serotonin is enhanced and this effect can be reduced by chronic captopril administration.

Modulation of rat neocortical high-voltage spindle activity by 5-HT1/5-HT2 receptor subtype specific drugs

To investigate the role of serotonin (5-hydroxytryptamine; 5-HT) receptors in the modulation of rat thalamocortical oscillations, we studied the effects of 5-HT1/5-HT2 receptor subtype specific drugs on neocortical high-voltage spindle activity in adult male rats. A 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (0.03, 0.1, 0.3 and 1.0 mg/kg s.c.), had no effect on neocortical high-voltage spindle activity. Furthermore, a mixed 5-HT1/5-HT2 receptor antagonist, methysergide (1.0, 5.0 and 15.0 mg/kg i.p.), had no effect, whereas a non-specific mixed 5-HT1/5-HT2 receptor antagonist, methiothepin (0.2, 1.0 and 5.0 mg/kg i.p.), significantly increased neocortical high-voltage spindles. Of the 5-HT2 receptor antagonists, ritanserin (0.1, 1.0 and 5.0 mg/kg s.c.) had no effect, whereas ketanserin (1.0, 5.0 and 20.0 mg/kg s.c.) increased neocortical high-voltage spindles, but only at the highest dose used. A 5-HT2 receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (0.5, 1.0 and 2.0 mg/kg s.c.), at the two highest doses significantly decreased neocortical high-voltage spindle activity, and this effect was blocked by the 5-HT2 receptor antagonists, ketanserin (1.0, 5.0 and 20.0 mg/kg s.c.) and ritanserin (1.0 and 5.0 mg/kg s.c.), as well as by methiothepin (0.2, 1.0 and 5.0 mg/kg i.p.) and methysergide (1.0, 5.0 and 15.0 mg/kg i.p.). Furthermore, unilateral intrathalamic infusions, but not intrahippocampal control infusions, of DOI (10 and 50 micrograms/1.0 microliters/rat) decreased neocortical high-voltage spindle activity and systemic administration of ketanserin (20.0 mg/kg s.c.) completely blocked this effect. The present results suggest that (1) the serotonergic system modulates rat thalamocortical oscillations as measured by neocortical high-voltage spindle activity, (2) activation of 5-HT2 receptors, possibly located in the thalamus, with a specific 5-HT2 receptor agonist, DOI, causes a reduction in rat neocortical high-voltage spindle activity.

Acute and subchronic effects of clozapine on licking in rats: tolerance to disruptive effects on number of licks, but no tolerance to rhythm slowing

In order to assess the effects of the atypical neuroleptic clozapine on orolingual competence in rats, tongue function was measured by quantitating the rhythm of tongue movements after acute (1.0, 3.0, 6.0 mg/kg) or subchronic intraperitoneal treatment (1.5, 3.0, 4.5 mg/kg, each dose for at least 7 days) with the drug. Thirsty rats were trained to lick water from a force-sensing disk by thrusting the tongue through a 12-mm-diameter hole to strike the horizontal disk located 5 mm below the hole. Number of licks in 2 min and rhythm of tongue movements (as determined by Fourier analysis of the force-time signal) were each dose dependently reduced in the acute dose-effect phase of the study. In the subchronic study number of licks exhibited tolerance, but the slowing of lick rhythm did not show tolerance. An acute dose range of the serotonin antagonist ritanserin (0.5, 1.0, 2.0, 4.0 mg/kg) was also studied in the same rats. Ritanserin had no effect on any of the measures of orolingual function. The clozapine result was replicated in a second study using younger, drug naive rats. The results for clozapine were contrasted with previous reports indicating that haloperidol has little effect on lick rhythm. Additional discussion evaluated the possible contribution of neurotransmitter receptors on motor neurons of the hypoglossal nucleus to the observed rhythm slowing induced by clozapine.

Ritanserin, a 5-HT2A/2C antagonist, reverses direct dopamine agonist-induced inhibition of midbrain dopamine neurons

Ritanserin (RIT), widely-used as a selective 5-HT2A/2C receptor antagonist, has been reported to produce significant therapeutic effects on the negative symptoms of schizophrenia and to improve extrapyramidal side effects induced by neuroleptics. Because midbrain dopamine (DA) systems are believed to be the major site of action for many antipsychotic drugs, the effect of RIT on substantia nigra DA neurons was examined in chloral hydrate-anesthetized rats using single unit recording techniques. Systemic injection of RIT (0.1-6.4 mg/kg, i.v.) had no consistent effect on basal firing rate but significantly reversed the inhibition induced by both direct and indirect DA agonists. However, our data suggest that this effect of RIT is largely mediated by a mechanism independent of 5-HT. Thus the 5-HT2A/2C agonist 1(2,5 dimethyoxy-4-iodophenyl)-2-aminopropane showed no effect on either basal firing rate or the inhibition induced by the direct DA agonist quinpirole. Neither the selective 5-HT2A antagonist MDL 100907 nor depletion of endogenous 5-HT using p-chlorophenylalanine mimicked the effect of RIT (i.e., attenuated quinpirole-induced inhibition). Furthermore, the effect of RIT persisted in animals pretreated with p-chlorophenylalanine. Because RIT is known to bind D2-like receptors and because the inhibition of DA neurons induced by low doses of a direct DA agonist is believed to be mediated by DA autoreceptors, these results suggest that RIT may act on DA autoreceptors directly as a DA antagonist. Since similar doses of RIT were reported to have no significant effect on postsynaptic D2 receptors in the striatum, it is possible that RIT at the doses used may selectively block DA autoreceptors.

Further studies to examine the nature of dexfenfluramine-induced suppression of heroin self-administration

The present series of experiments sought to investigate further the mechanism by which dexfenfluramine, a selective 5-HT releaser/reuptake inhibitor, reduces heroin self-administration by male Wistar rats. In experiment 1, the effect of combined intravenous heroin and intraperitoneal dexfenfluramine injections on operant responding for food was examined. In experiment 2, the maintenance of dexfenfluramine suppression of heroin self-administration following chronic (7 day) treatment was evaluated. Finally, in experiment 3, the ability of various 5-HT antagonists to block the dexfenfluramine suppression was examined. The results from experiment 1 suggest that sensorimotor deficits/malaise potentially associated with heroin/dexfenfluramine combinations are unlikely to account for the reductions in heroin self-administration. Experiment 2 suggested that the suppressant effect of dexfenfluramine on heroin responding may diminish rapidly following chronic treatment. Finally, central 5-HT1 and/or 5-HT2, but not 5-HT3, receptors may underlie the suppressant effects of dexfenfluramine on heroin self-administration.

Serotonin receptors regulate canine regional vasodilator responses to burn

OBJECTIVE

To determine which serotoninergic receptor subtype(s) mediates the regional vasodilator response to scald injury.

DESIGN

Prospective, randomized trial.

SETTING

Microcirculation research laboratory.

SUBJECTS

Anesthetized dogs.

INTERVENTIONS

Mechanically ventilated dogs underwent cannulation of a brachial artery and placement of an ultrasonic flow probe around one femoral artery. All animals received a 2% to 3% body surface area partial thickness scald injury by immersing the paw ipsilateral to the instrumented femoral artery into 100 degrees C water for 5 secs. In one group of dogs, BMY 7378 (a serotoninergic1A receptor antagonist) was given by the peripheral intravenous route before burn. These results were compared with those findings obtained from a group of animals that received a burn only, and groups of animals given a peripheral intravenous injection of methysergide (a serotoninergic receptor antagonist) or ritanserin (a serotoninergic2 receptor blocking agent) before burn. Experiments were conducted for two postburn hours.

MEASUREMENTS AND MAIN RESULTS

Burn injury caused a marked and persistent increase in regional (e.g., femoral artery) blood flow, an effect that was significantly blunted by preburn administration of the serotoninergic receptor antagonist, methysergide. Preburn administration of BMY 7378 increased baseline femoral blood flow by 13%, reflecting its known serotonin agonist properties. However, when compared with the mean postscald increases in femoral blood flow over baseline seen in scald only dogs and in animals given the serotoninergic2 receptor blocking agent, ritanserin (before scald), the BMY 7378-treated group demonstrated a significant (p < .001 by analysis of variance) 2-hr-postscald blunting of this femoral vasodilator response.

CONCLUSION

These data suggest that serotoninergic1A-like receptors play an integral, albeit not an exclusive, role in blood flow regulation to the site of burn injury.

Attenuating effect of serotonin receptor antagonists on impairment of mealtime-associated activity rhythm in old rats

In the present study, we examined attenuating effect of serotonin (5-HT) receptor antagonists on the impairment of the time perception presented by daily scheduled feeding in old rats. When feeding was restricted to a single meal at a fixed time of day (1300-1700 h) for six consecutive days, young rats exhibited intense locomotor activity from 1-3 h before feeding time. Intense locomotor activity was observed between 1200 adn 1700 h in young animals even on the fasting day (day 7) (mealtime-associated activity). However, this mealtime-associated activity was impaired in 24-mo-old rats. Daily injections of 5-HT2 receptor antagonists, mianserin or ritanserin, or a 5-HT3 receptor antagonist, Y25130, at 1700 h for 6 consecutive days significantly and dose-dependently attenuated the impairment of mealtime-associated activity on the fasting day in old rats without affecting the food intake. Our results suggest that the blockade of 5-HT2 and/or 5-HT3 receptors attenuates impairment of the manifestation of mealtime-associated anticipatory activity related to temporal learning in old rats.

Effects of ritanserin, a specific serotonin-S2 receptor antagonist, on the release of anterior pituitary hormones during insulin-induced hypoglycemia in normal humans

The role of serotonin in the insulin hypoglycemia (IH) stimulated secretion of prolactin (PRL), growth hormone (GH), adrenocorticotropin (ACTH) and cortisol (F) was studied in a group of 12 normal subjects during the control period after placebo and a consecutive six-day treatment with 20 mg ritanserin (RIT) per day. RIT failed to affect the baseline levels of all the four hormones as well as the PRL response to IH (p > 0.5). The serum GH response to IH was moderately diminished after RIT, the reduction of integrated trapezoidal area under hormone curves (nAUC) being 50.7% +/- 6.9% (p < 0.005). Furthermore, RIT was found to slightly decrease the plasma ACTH response to IH, the reduction of nAUC being 36.3% +/- 2.6% (p < 0.005). Decrease in the corresponding plasma F response to IH was accompanied by 29.1% +/- 2.4% reduction of nAUC (p < 0.005). According to our results, serotonin-S2 receptors appeared to be moderately involved in IH-induced release of GH, but slightly in that of ACTH, leaving unaffected that of PRL.

Monoaminergic and cholinergic modulation of REM-sleep timing in rats

The effects on sleep structure of systemic administration of benchmark cholinergic, serotonergic, and noradrenergic antagonists (QNB, ritanserin, metergoline, and prazosin) were characterized in rats using a new technique for identifying transitions (NRTs) from non-REM (NREM) sleep to REM sleep. In agreement with previous studies, all agents tested reduced REM-sleep expression (by 36-86%). In addition, the serotonergic and noradrenergic antagonists reduced NRT frequency (by 58-81%). The cholinergic antagonist QNB had no effect on NRT frequency. These findings suggest that blockade of serotonergic or noradrenergic receptors increases the interval between REM-sleep episodes, perhaps reducing the rate of accumulation of REM-sleep propensity. Blockade of cholinergic receptors, by contrast, decreases REM-sleep expression by interfering with REM-sleep maintenance, not by modulating REM-sleep timing. These conclusions are contrary to the predictions of a number of published models of REM-sleep timing.

Low responsiveness to agents evoking 5-HT2 receptor-mediated behaviors in Sardinian alcohol-preferring rats

The selective NK3 tachykinin agonist senktide evokes in rodents 5-HT mediated behaviors, including 5-HT2 receptor-mediated wet dog shakes (WDS) and head shakes (HS). It was observed previously that genetically selected Sardinian alcohol-preferring (sP) rats show a small number of WDS and HS following intracerebroventricular (ICV) injection of senktide. The present study was aimed at confirming these observations and at providing information on the reasons accounting for the anomalous response of sP rats. Senktide (500-2000 ng/rat, ICV) produced a much lower number of WDS and HS in sP rats than in nonselected Wistar (nsW) rats. Both behaviors were suppressed by the 5-HT2 antagonist ritanserin (1 mg/kg, subcutaneously), confirming that 5-HT2 receptors mediate the response. HS induced by the ICV injection of 5-HT agonists endowed with marked activity at 5-HT2 receptors, such as quipazine (1500-6000 ng/rat) or DOI (500-3500 ng/rat), were much less pronounced in sP rats than in nsW rats. Moreover, WDS following peripheral injection of 5-hydroxytryptophan, 25-100 mg/kg, and carbidopa, 12.5 mg/kg, were less intense in sP and in ethanol-naive sP rats than in nsW and in Sardinian alcohol-nonpreferring rats. These findings suggest that sP rats have an inherent different regulation of central 5-HT2 mechanisms.

Antipsychotic drugs induce Fos protein in the thalamic paraventricular nucleus: a novel locus of antipsychotic drug action

Monitoring expression of c-fos and other immediate-early genes has proven a useful method for determining potential sites of action of antipsychotic drugs. Most studies of the effects of antipsychotic drugs on immediate-early gene expression have focused on the basal ganglia and allied cortical regions. We now report that clozapine administration markedly increases both the number of cells expressing Fos protein-like immunoreactivity and the amount of Fos protein in the thalamic paraventricular nucleus, but not the contiguous mediodorsal thalamic nucleus. Comparable doses of several dopamine D2-like antagonists, including raclopride, sulpiride, remoxipride and haloperidol, did not induce Fos expression in the paraventricular nucleus. However, loxapine and very high doses of haloperidol resulted in a small but significant increase in paraventricular nucleus Fos expression. The dopamine D1 receptor antagonist SCH23390 did not induce Fos in the paraventricular nucleus or alter the magnitude of the clozapine-elicited increase in Fos expression. The serotonergic 5-hydroxytryptamine2a/2c antagonist ritanserin, alone or in combination with sulpiride, did not increase Fos expression in the paraventricular nucleus. Similarly, the 5-hydroxytryptamine2:D2 antagonist risperidone did not change the amount of Fos protein in the paraventricular nucleus. Neither the alpha 1 adrenergic antagonist prazosin nor the muscarinic cholinergic antagonist scopolamine mimicked the effect of clozapine. The key placement of the paraventricular nucleus as an interface between the reticular formation and forebrain dopamine systems suggests that this thalamic nucleus may be an important part of an extended neural network subserving certain actions of antipsychotic drugs.

Neither the 5-HT1A- nor the 5-HT2-receptor subtype mediates the effect of fluvoxamine, a selective serotonin reuptake inhibitor, on forced-swimming-induced immobility in mice

The effect of fluvoxamine, a selective serotonin (5-HT) reuptake inhibitor, was studied in the forced-swimming test, a model of depression, in mice. Fluvoxamine at 60 mg/kg, p.o. significantly decreased the immobility time in the forced-swimming test. A similar effect was observed by the selective norepinephrine reuptake inhibitor desipramine at the same dose. Furthermore, the suppression of immobility time was slightly potentiated by repeated administration of fluvoxamine, and a significant effect was observed at 30 mg/kg, p.o. The effect of fluvoxamine on forced-swimming was unaffected by the 5-HT2 antagonist ritanserin. On the other hand, the 5-HT1A antagonist NAN-190 (1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl] piperazine) potentiated the effect of fluvoxamine on forced-swimming. It is expected, however, that a 5-HT1A antagonist should antagonize the effect of fluvoxamine when 5-HT1A mediates the suppressive effect of fluvoxamine on the immobility time in forced-swimming. From these results, neither the 5-HT1A- nor the 5-HT2-receptor subtype is involved in the suppressive effect of fluvoxamine on the immobility associated with forced-swimming.