Behavioral characterization of serotonergic activation in the flatworm Planaria

Serotonin (5-hydroxytryptamine, 5-HT) receptors have been identified in Planaria, a model used for studying the pharmacology of behavioral phenomena. This study characterized the behavioral and locomotor effects of 5-HT, a 5-HT1A agonist, a 5-HT1B/2C agonist, and a 5-HT1A antagonist to examine the role of 5-HT receptor activation in this species. Planarians were video recorded individually in a clear plastic cube containing drug solution or vehicle. To quantify locomotor velocity (pLMV), planarians were placed individually into a dish containing drug solution or vehicle and the rate of gridline crossings was recorded. For the antagonist experiments, four conditions were studied: water alone, agonist alone, antagonist alone, and agonist plus antagonist. The decrease in pLMV induced by the5-HT1A agonist (8-OH-DPAT), and the 5-HT1B/2C agonist (mCPP), was antagonized by pretreatment with the 5-HT1A antagonist (WAY-100635) at a dose that had no effect of its own on pLMV. At a higher concentration of WAY-100635, further decreases in pLMV induced by 8-OH-DPAT were observed. Each agonist produced increased occurrences of 'C-like position' and 'screw-like hyperkinesia', 5-HT and mCPP produced 'writhing', and only mCPP produced a significant increase in duration of 'headswing' behavior. The results demonstrate that the 5-HT1A receptor identified in Planaria mediates behavioral responses to 5-HT receptor ligands, supporting the notion that planarians possess functional 5-HT receptors and might serve as a simple model for their study.     

Activation and blocking of 5-HT1A receptors influence on the immune response in CBA mice

5-HT1A type serotonin receptors influence the immunomodulating action of the selective preparations 8-OH-DPAT (5-HT1A receptor agonist) and WAY-100635 (5-HT1A receptor antagonist) in CBA mice. The activation of 5HT1A receptors with 8-OH-DPAT (1 mg/kg) decreased, while their blocking with WAY-100635 (1 mg/kg) increased the reaction intensity at the peak of response to immunization with ram erythrocytes. Preliminary blocking of the 5-HT1A receptors with WAY-100635 prevented the inhibiting action of 8-OH-DPAT.     

The stimulus effects of 8-OH-DPAT: evidence for a 5-HT2A receptor-mediated component

A previous investigation in our laboratory found that the stimulus effects of the 5-HT2A agonist, LSD, are potentiated by 5-HT1A receptor agonists including the prototypic agonist, 8-OH-DPAT. Also suggestive of behaviorally relevant interactions between 5-HT1A and 5-HT2A receptors are behavioral analyses of locomotor activity, head-twitch response, forepaw treading and production of the serotonin syndrome; in some instances effects are augmented, in other, diminished. These observations led us in the present investigation to test the hypothesis that stimulus control by 8-OH-DPAT [0.2 mg/kg; 15 min pretreatment time] is modulated by 5-HT2A ligands. Stimulus control was established with 8-OH-DPAT in a group of 10 rats. A two-lever, fixed ratio 10, positively reinforced task with saline controls was employed. As shown previously, stimulus control by 8-OH-DPAT and the generalization of 8-OH-DPAT to the 5-HT1A partial agonist, buspirone, was completely blocked by the selective 5-HT1A antagonist, WAY-100635. In contrast, antagonism by the selective 5-HT2A antagonist, M100907 [0.1 mg/kg; 30 min pretreatment time], of 8-OH-DPAT and of the generalization of 8-OH-DPAT to buspirone was statistically significant but less than complete. In light of our previous conclusions regarding the interactions of 5-HT1A agonists with LSD-induced stimulus control, the present data suggest that the interaction between 5-HT1A and 5-HT2A receptors is bidirectional in drug discrimination studies.     

Cross-tolerance studies of serotonin receptors involved in behavioral effects of LSD in rats

Like hallucinogenic 5-HT₂ agonists, LSD (d-lysergic acid diethylamide) produces characteristic decreases in locomotor activity and investigatory behaviors of rats tested in a novel environment. Because LSD is an agonist at both 5-HT_1A and 5-HT₂ receptors, however, the respective influences of these different receptors in the behavioral effects of LSD remain unclear. In particular, the paucity of selective 5-HT_1A antagonists has made it difficult to assess the specific contribution of 5-HT_1A receptors to the effects of LSD. An alternative approach to the delineation of receptor-specific effects is the use of cross-tolerance regimens. In the present studies, rats were pretreated with saline, 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT) (0.5 mg/kg SC), 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (1.0 mg/kg SC), or LSD (60 µg/kg SC), every 12 h for 5 or 8 days. Thirty-six hours later, rats were tested in a behavioral pattern monitor 10 min after injection of saline, 0.5 mg/kg 8-OH-DPAT, 1.0 mg/kg DOI, or 60 µg/kg LSD. As expected, tolerance to the decreases in locomotor activity produced by acute administrations of 8-OH-DPAT, DOI, or LSD occurred when rats were pretreated chronically with 8-OH-DPAT, DOI, or LSD, respectively. Furthermore, pretreatment with either 8-OH-DPAT or DOI produced cross-tolerance to LSD. These results support the hypothesis that the effects of LSD in this model reflect a combination of 5-HT_1A and 5-HT₂ effects and support the view that there is an interaction between 5-HT_1A and 5-HT₂ receptors.     

Differential effects of 5-HT agonists and antagonists on the repeated acquisition and performance of response sequences in monkeys

As a means of characterizing the role of 5-HT1A and 5-HT2A receptors in learning, 5-hydroxytryptamine (5-HT) agonists and antagonists with selective affinities for each receptor subtype (i.e. 8-hydroxy-dipropylaminotetralin (8-OH-DPAT), (-)-4-(dipropylamino)-1,3,4,5-tetrahydrobenz-(c,d,)indole-6-carboxamide (LY228729), (+/-)-1-(4-iodo-2,5-dimeth-oxyphenyl)-2-aminopropane hydrochloride (DOI), 4-iodo-N-[2- [4-(methoxyphenyl)-1-piperazinyl] ethyl]-N-2-pyridinyl-benzamide hydrochloride (p-MPPI), N-[2- [4- (2-methoxyphenyl)-1-piperazinyl] ethyl] -N-2-pyridinyl-cyclohexanecarboxamide maleate (WAY-100635), 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyllpiperazine hydrobromide (NAN-190) and ritanserin) were administered to monkeys responding under a multiple schedule of repeated acquisition and performance. In addition, a selective 5-HT1A receptor agonist (8-OH-DPAT) was administered in combination with a 5-HT2A receptor antagonist (ritanserin) to examine any potential interactions between the two 5-HT receptor subtypes. When administered alone, 8-OH-DPAT (0.1-3.2mg/kg), LY228729 (0.32-3.2 mg/kg) and DOI (0.018-3.2 mg/kg) dose-dependently decreased overall response rate in both schedule components, and generally increased the percentage of errors in the acquisition components at doses lower than those that increased the percentage of errors in the performance components. At the doses of each drug tested (i.e. 0.1 or 0.32 mg/kg), both p-MPPI and WAY-100635 antagonized the disruptive effects of 8-OH-DPAT, by shifting the dose-effect curves for overall response rate and the percentage of errors to the right. In contrast, ritanserin (0.32 or 1mg/kg) had little or no effect on the disruptions produced by 8-OH-DPAT, but it effectively antagonized the rate-decreasing and error-increasing effects produced by the 5-HT2A agonist DOI. Administration of the 5-HT1A antagonists WAY-100635 and NAN-190 alone produced dose-dependent rate-decreasing effects, but the effects on accuracy of responding in the acquisition components differed from those of the 5-HT1A agonists (8-OH-DPAT and LY228729), in that they did not produce an increase in the percentage of errors. Together, these results suggest that 5-HT is capable of disrupting learning in monkeys through actions at both the 5-HT1A and 5-HT2A receptors, and that 5-HT2A receptor antagonism does not unilaterally modify the effects produced by 5-HTA1A receptor activation.     

Stimulation of 5-HT 1A receptors increases the seizure threshold for picrotoxin in mice

To evaluate the possible role of 5-HT 1A and 5-HT 2A receptors in the anticonvulsant effect of swim stress, mice were pre-treated with agonists and antagonists of these receptors prior to exposure to stress and the intravenous infusion of picrotoxin. 8-OH-DPAT ((+/-)-8-hydroxy-2-(di-n-propylamino) tetralin) and WAY-100635 (a selective agonist and antagonist of 5-HT 1A receptors), DOI (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) and ketanserin (a 5-HT 2A/2C receptor agonist and antagonist) were used. Results demonstrated that 1 and 3 mg/kg of 8-OH-DPAT increased the doses of picrotoxin producing running/bouncing clonus, tonic hindlimb extension and death in stressed and unstressed mice, respectively. Pre-treatment with WAY (0.3 mg/kg) prevented the effect of 8-OH-DPAT (3 mg/kg). DOI (2.5 mg/kg) and ketanserin (1 mg/kg) failed to affect the seizure threshold for picrotoxin. The results show that stimulation of 5-HT 1A receptors exerts anticonvulsant actions in stressed and unstressed mice, while stimulation of 5-HT 2A/2C receptors does not interfere with the effect of stress on picrotoxin-induced convulsions.     

N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline studies on the role of 5-HT1A and 5-HT2 receptors in mediating foot-shock-induced ultrasonic vocalisation in adult rats.

The role of 5-HT1A and 5-HT2 receptors in mediating foot-shock-induced ultrasonic vocalisation has been studied in rats. Furthermore, behavioural effects were correlated to receptor reserves in the brain by means of receptor inactivation with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). The dose-dependent inhibition of ultrasonic vocalisation by the 5-HT precursor, L-5-hydroxy-L-tryptophan (110-450 micromol/kg), was abolished by pretreatment with the 5-HT1A/1B antagonist, (-)-penbutolol (27 micromol/kg), and the 5-HT2A/2C antagonist, ritanserin (10 micromol/kg). The inhibitory actions of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and the 5-HT2A/5-HT2C. agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) were reversed by the 5-HT1A antagonist, (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide (WAY-100635), and the 5-HT2A antagonist, (+/-)alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorphenyl)ethyl]-4-pipe ridine-methanol (MDL 100151), respectively. Pretreatment with EEDQ (24 h, subcutaneous [s.c.]) inhibited foot-shock-induced ultrasonic vocalisation (effective dose50=0.95 micromol/kg) and decreased [3H]-8-OH-DPAT and [3H]-ketanserin binding in the brain. Pretreatment with WAY-100635 (0.3-20 micromol/kg) 20 min prior to EEDQ administration (1.3 micromol/kg, s.c.) did not reverse the EEDQ-induced inhibition of ultrasonic vocalisation but protected the 5-HT1A receptors against EEDQ inactivation. Pretreatment with MDL 100151 (0.83-54 micromol/kg) 20 min prior to EEDQ administration both reversed the EEDQ-induced inhibition of ultrasonic vocalisation and protected the 5-HT2A receptors against EEDQ inactivation. These findings demonstrate that 5-HT1A and 5-HT2 receptors are involved in the regulation of ultrasonic vocalisation in rats. However, the function of 5-HT1A and 5-HT2 receptors in this model seems to differ as vocalisation was preserved after protection of 5-HT2 but not 5-HT1A receptors.     

5-HT1A receptors in the dorsal hippocampus mediate the anxiogenic effect induced by the stimulation of 5-HT neurons in the median raphe nucleus.

We evaluated the involvement of dorsal hippocampus (DH) 5-HT1A receptors in the mediation of the behavioral effects caused by the pharmacological manipulation of 5-HT neurons in the median raphe nucleus (MRN). To this end, we used the rat elevated T-maze test of anxiety. The results showed that intra-DH injection of the 5-HT1A/7 agonist 8-OH-DPAT facilitated inhibitory avoidance, an anxiogenic effect, without affecting escape. Microinjection of the 5-HT1A antagonist WAY-100635 was ineffective. In the elevated T-maze, inhibitory avoidance and escape have been related to generalized anxiety and panic disorders, respectively. Intra-MRN administration of the excitatory amino acid kainic acid, which non-selectively stimulates 5-HT neurons in this brain area facilitated inhibitory avoidance and impaired escape performance, but also affected locomotion. Intra-MRN injection of WAY-100635, which has a disinhibitory effect on the activity of 5-HT neurons in this midbrain area, only facilitated inhibitory avoidance. Pre-administration of WAY-100635 into the DH blocked the behavioral effect of intra-MRN injection of WAY-100635, but not of kainic acid. These results indicate that DH 5-HT1A receptors mediate the anxiogenic effect induced by the selective stimulation of 5-HT neurons in the MRN.     

Effects of fenfluramine on free-operant timing behaviour: evidence for involvement of 5-HT2A receptors

RATIONALE: Temporal differentiation in the free-operant psychophysical procedure is sensitive to the 5-hydroxytryptamine (5-HT)1A receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) and the 5-HT2 receptor agonist 2,5-dimethoxy-4-iodo-amphetamine (DOI); both drugs shift the psychophysical curve leftwards, reducing the indifference point, T50. We have examined the effect of the 5-HT releasing agent fenfluramine on temporal differentiation. OBJECTIVE: We examined whether fenfluramine's effect on temporal differentiation can be antagonised by the 5-HT1A receptor antagonist N-[2-(4-[2-methoxy-phenyl]-1-piperazinyl)ethyl]-N-2-pyridinylcyclohexane-carboxamide (WAY-100635) and the 5-HT2A receptor antagonist ketanserin, and compared the effects of fenfluramine, DOI and 8-OH-DPAT in intact rats and rats whose 5-HTergic pathways had been destroyed by 5,7-dihydroxytryptamine. METHODS: Rats were trained under the free-operant psychophysical procedure to press levers A and B in 50-s trials in which reinforcers were provided intermittently for responding on A in the first half, and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 5-s epochs of the trials; logistic psychophysical curves were fitted to the data for derivation of timing indices (T50, time corresponding to %B=50%, and Weber fraction). Experiment 1 examined the effects of acute treatment with fenfluramine, and the interaction between fenfluramine and the 5-HT1A and 5-HT2A receptor antagonists WAY-100635 and ketanserin; experiment 2 compared the effects of fenfluramine, 8-OH-DPAT and DOI in intact rats and rats whose 5-HTergic pathways had been destroyed by intra-raphe injection of 5,7-dihydroxytryptamine. Concentrations of 5-HT and catecholamines in the brain were measured by high-performance liquid chromatography. RESULTS: Experiment 1: fenfluramine (2 mg/kg) reduced T50; this effect was attenuated by ketanserin (1.0 mg/kg) but not by WAY-100635 (100 microg/kg). Experiment 2: 8-OH-DPAT (100 microg/kg) and DOI (250 microg/kg) reduced T50 in both groups; fenfluramine reduced T50 only in the sham-lesioned group. Levels of 5-HT were reduced by 80% in the lesioned group; catecholamine levels were not affected. CONCLUSIONS: The results suggest that fenfluramine affects temporal differentiation via the release of endogenous 5-HT which acts mainly on postsynaptic 5-HT2A receptors.     

5-HT1A receptors of the lateral septum regulate inhibitory avoidance but not escape behavior in rats

Serotonin in the lateral septum (LS) has been implicated in the modulation of defensive behaviors and in anxiety. However, it is currently unknown whether changes in 5-HT mechanisms in this brain area may selectively affect defensive responses associated with specific subtypes of anxiety disorders recognized in clinical settings. To address this question, we evaluated the effect of the intra-LS injection of the 5-HT(1A/7) receptor agonist 8-OH-DPAT (0.6, 3.0, 15.0 nmol) in male Wistar rats exposed to the elevated T-maze animal model of anxiety. This test allows the measurement of two behavioral defensive responses in the same rat: inhibitory avoidance and escape behavior. In clinical terms, these responses have been respectively related to generalized anxiety and panic disorder. The effects of 8-OH-DPAT were compared to those caused by a standard anxiolytic compound, the benzodiazepine receptor agonist midazolam (MDZ, 20 nmol). We also investigated whether the intra-LS injection of the 5-HT(1A) receptor antagonist WAY-100635 (0.37 nmol) was able to block the effects of 8-OH-DPAT. All animals were also tested in an open field for locomotor activity assessments. Results showed that whereas intra-LS administration of MDZ decreased avoidance latencies, suggesting an anxiolytic action, 8-OH-DPAT caused the opposite effect. Neither drug affected the escape performance. Intra-LS administration of WAY-100635 blocked the anxiogenic effect caused by 8-OH-DPAT. No changes to locomotion were detected in the open field. The data suggests that LS 5-HT(1A) receptors are involved in the control of inhibitory avoidance behavior and that a failure in this regulatory mechanism may be of importance to the physiopathology of generalized anxiety disorder.     

The effect of the 5-HT1A receptor agonist, 8-OH-DPAT, on motion-induced emesis in Suncus murinus

In the present study we evaluated the role of 5-HT(1A) receptors in mediating the inhibitory action of 8-OH-DPAT, a 5-HT(1A) receptor agonist, in motion sickness in Suncus murinus. 8-OH-DPAT (0.1 mg/kg, i. p) attenuated motion-induced emesis which was associated with an increase in the latency of the onset to the first emetic episode. Pre-treatment with methysergide (a 5-HT(1/2/7) receptor antagonist, 1.0 mg/kg, i. p.), WAY-100635 (a 5-HT(1A) receptor antagonist, 1.0 mg/kg, i. p.), SB269970A (a 5-HT(7) receptor antagonist, 1.0 and 5.0 mg/kg, i. p.), ondansetron (a 5-HT(3) receptor antagonist, 1.0 mg/kg, i. p) or GR13808 (a 5-HT(4) receptor antagonist, 0.5 mg/kg, i. p) failed to modify the inhibitory action of 8-OH-DPAT on motion sickness. Furthermore, the application of either methysergide, WAY-100635, SB269970A, ondansetron or GR13808 alone had no effect on motion sickness in its own right. These data indicate that neither 5-HT(1A) nor any 5-HT(2) receptor subtypes, 5-HT(3), 5-HT(4) and 5-HT(7) receptors are likely to be involved in the inhibition of motion-induced emesis mediated by 8-OH-DPAT.     

The roles of 5-HT1A and 5-HT2 receptors in the effects of 5-MeO-DMT on locomotor activity and prepulse inhibition in rats

Rationale The hallucinogen 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT) is structurally similar to other indoleamine hallucinogens such as LSD. The present study examined the effects of 5-MeO-DMT in rats using the Behavioral Pattern Monitor (BPM), which enables analyses of patterns of locomotor activity and exploration, and the prepulse inhibition of startle (PPI) paradigm.Objectives A series of interaction studies using the serotonin (5-HT)_1A antagonist WAY-100635 (1.0 mg/kg), the 5-HT_2A antagonist M100907 (1.0 mg/kg), and the 5-HT_2C antagonist SER-082 (0.5 mg/kg) were performed to assess the respective contributions of these receptors to the behavioral effects of 5-MeO-DMT (0.01, 0.1, and 1.0 mg/kg) in the BPM and PPI paradigms.Results 5-MeO-DMT decreased locomotor activity, investigatory behavior, the time spent in the center of the BPM chamber, and disrupted PPI. All of these effects were antagonized by WAY-100635 pretreatment. M100907 pretreatment failed to attenuate any of these effects, while SER-082 pretreatment only antagonized the PPI disruption produced by 5-MeO-DMT.Conclusions While the prevailing view was that the activation of 5-HT₂ receptors is solely responsible for hallucinogenic drug effects, these results support a role for 5-HT_1A receptors in the effects of the indoleamine hallucinogen 5-MeO-DMT on locomotor activity and PPI in rats.     

Effects of 5-HT and 5-HT1A receptor agonists and antagonists on dorsal vagal preganglionic neurones in anaesthetized rats: an ionophoretic study.

1. Effects of ionophoretic administration of 5-hydroxytryptamine (5-HT) and selective 5-HT1A receptor agonists and antagonists on identified dorsal vagal preganglionic and dorsal raphe neurones were studied in pentobarbitone sodium or chloral hydrate-anaesthetized rats, respectively. 2. Extracellular recordings were made from 176 preganglionic neurones in the dorsal vagal nucleus (DVN). Application of 5-HT at low currents (< or = 10 nA) increased the activity of these neurones. However, at increased currents (10-60 nA), it had a predominantly depressant effect. Application of selective 5-HT1A receptor antagonists, (+/-)-pindolol or WAY-100635, attenuated the excitatory responses evoked by 5-HT. 3. Ionophoresis of the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (5-30 nA) increased the firing rate of 19 and decreased that of 67 of the 104 vagal neurones tested. Other 5-HT1A receptor agonists, flesinoxan and N,N-di-n-propyl-5-carboxamidotryptamine (DP-5-CT) also had predominantly depressant effects. 4. (+/-)-Pindolol attenuated excitations but not inhibitions evoked by 8-OH-DPAT. Surprisingly, WAY-100635 and 8-OH-DPAT produced the same effect on these neurones and when applied together, WAY-100635 failed to attenuate the 8-OH-DPAT responses. 5. Dorsal raphe neurones were identified by their low, regular firing rate and their subsequent histological localization. 8-OH-DPAT reversibly reduced the activity in all 7 neurones tested and this was antagonized by WAY-100635 in all 3 neurones tested. 6. In conclusion, 5-HT applied to vagal preganglionic neurones evokes excitatory and inhibitory responses. The excitatory, but not the inhibitory responses may be mediated, at least in part, by activation of 5-HT1A receptors.     

Serotonergic modulation of carbachol-induced rhythmic activity in hippocampal slices

Fast rhythmic activity in a frequency range between 20 and 40 Hz occurs in vitro in hippocampal area CA3 after activation of muscarinic receptors. Here we show that carbachol-induced rhythmic activity is modulated by serotonin (5-HT). Spectral analysis reveals that 5-HT (0.3-30 microM) decreases power, but not frequency, of rhythmic activity in a concentration-dependent and reversible manner. The 5-HT(1A) agonists 8-OH-DPAT and buspirone mimic the effect of 5-HT, whereas the selective 5-HT(1A) receptor antagonist WAY-100635 (1 microM) significantly prevents the effect of 5-HT. In contrast to the effect of 5-HT(1A) agonists, the 5-HT(2) agonist DOI increases spectral power and prevents the reduction of spectral power by 5-HT. Application of WAY-100635 alone has no effect on rhythmic activity. Likewise, the 5-HT(2) antagonist ritanserin (10 microM) does not affect rhythmic activity, or its reduction by 5-HT. Finally, the 5-HT re-uptake inhibitor fluoxetine significantly decreases rhythmic activity in the presence of a low concentration of 5-HT, suggesting that 5-HT released from terminals in the slice likely reduces rhythmic activity. These results strongly implicate 5-HT(1A) and 5-HT(2) receptors in the modulation of spectral power of carbachol-induced rhythmic activity and that 5-HT(1A) receptors are responsible for the prevailing effect of 5-HT.     

Involvement of central 5-HT1A receptors in the reflex activation of pulmonary vagal motoneurones by inhaled capsaicin in anaesthetized cats.

1. The aim of the present experiments was to determine whether 5-HT1A receptors play a role in the control of the reflex activation of pulmonary vagal motoneurones. This was carried out by investigating the effects of intracisternal injections (i.c.) of the 5-HT1A receptor ligands, 8-OH-DPAT (50 micrograms kg-1), buspirone (200 micrograms kg-1), WAY-100635 (100 micrograms kg-1), methiothepin (200 micrograms kg-1) and (-)-pindolol (100 micrograms kg-1) and the 5-HT2 receptor antagonist, cinanserin (200 micrograms kg-1), on the reflex bronchoconstriction evoked by inhaled capsaicin aerosol in alpha-chloralose anaesthetized, neuromuscularly blocked and artificially ventilated cats. Recordings were made of heart rate, blood pressure and upper tracheal pressure. 2. Central application of all the 5-HT1A receptor antagonists (methiothepin, WAY-100635 and (-)-pindolol) attenuated the reflex bronchoconstriction in the upper trachea. However, the same dose of WAY-100635 given i.v. had no effect on this reflex bronchoconstriction. The 5-HT1A receptor agonist, 8-OH-DPAT (50 micrograms kg-1) given i.c., potentiated the capsaicin-evoked reflex bronchoconstriction, whereas buspirone (200 micrograms kg-1) i.c. had no effect. The 5-HT2 receptor antagonist, cinanserin (200 micrograms kg-1) also had no effect. 3. It is concluded that the reflex excitation of pulmonary vagal motoneurones by inhaled capsaicin in alpha-chloralose anaesthetized cats involves the activation of central 5-HT1A receptors.     

Diabetes attenuates the antidepressant-like effect mediated by the activation of 5-HT1A receptor in the mouse tail suspension test.

Several lines of evidence have indicated that the prevalence of depression in diabetic subjects is higher than that in the general population, however, little information is available on the effects of antidepressants in diabetes. In the present study, the antidepressant-like effect mediated by the activation of 5-HT(1A) receptors was examined using the tail suspension test in streptozotocin-induced diabetic mice. Long-lasting increases in 5-HT turnover rates were observed in the diabetic mouse midbrain and frontal cortex, but not in the hippocampus. Duration of immobility was significantly longer in diabetic than in nondiabetic mice in the tail suspension test. The 5-HT(1A) receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) (3-30 microg/kg, i.p.) reduced the duration of immobility in nondiabetic mice, and this effect was completely antagonized by pretreatment with N-[2-[4-(2-methoxyphenil)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY-100635) (30 microg/kg, s.c.), a selective 5-HT(1A) receptor antagonist. In contrast, 8-OH-DPAT (3 microg/kg-3 mg/kg, i.p.) was ineffective in diabetic mice. The selective 5-HT reuptake inhibitor fluoxetine (3-56 mg/kg, i.p.) reduced the duration of immobility in both nondiabetic and diabetic mice. However, fluoxetine was less effective in diabetic mice than in nondiabetic mice. WAY-100635 (30 microg/kg, s.c.) reversed the suppression of the duration of immobility by fluoxetine (30 mg/kg, i.p.) in nondiabetic mice. On the other hand, the anti-immobility effect of fluoxetine (56 mg/kg, i.p.) was not antagonized by WAY-100635 (30 microg/kg, s.c.) in diabetic mice. The selective 5-HT(2) receptor antagonist 6-methyl-1-(1-methylethyl)-ergoline-8beta-carboxylic acid 2-hydroxy-1-methylpropyl ester (LY53,857) (30 microg/kg, s.c.) reversed the anti-immobility effect of fluoxetine in both nondiabetic and diabetic mice. Spontaneous locomotor activity in diabetic mice was not different from that in nondiabetic mice. 8-OH-DPAT (30 microg/kg, i.p.), but not fluoxetine, increased the spontaneous locomotor activity in both nondiabetic and diabetic mice. The number of 5-HT(1A) receptors in the mouse frontal cortex was unaffected by diabetes. Plasma corticosterone levels in diabetic mice were significantly higher than that in nondiabetic mice. These results suggest that the antidepressant-like effect mediated by 5-HT(1A) receptors may be attenuated by diabetes.     

Prenatal stress influences 8-OH-DPAT modulated startle responding and [3H]-8-OH-DPAT binding in rats

The present study was to investigate some aspects of the 5-HT1A receptor system in adult-aged rats (50-60 days) that were either exposed to prenatal stress (PS) or not exposed to prenatal stress (CON). In the first series of experiments, rats were pretreated with vehicle, the 5-HT1A agonist 8-OH-DPAT or the 5-HT1A antagonist, WAY-100635 and exposed to 120 acoustic startle stimuli (95 dB) using a 30 s inter-trial interval. 8-OH-DPAT produced a dose-dependent increase in acoustic startle responding in CON and PS rats, with the PS rats exhibiting greater responding than CON rats. WAY-100635 depressed startle amplitudes only in the CON group. Finally, radioligand binding studies using [3H]-8-OH-DPAT indicated a significant decrease in receptor density in hippocampal homogenates from PS rats but no difference in [3H]-8-OH-DPAT binding from homogenates of the amygdala. Our results are consistent with earlier reports indicating that prenatal stress alters the serotonergic system. Specifically, our results indicate that gestational exposure to chronic mild stress enhances startle amplitudes following 8-OH-DPAT administration, prevents the depression in startle amplitudes following WAY-100635 administration and reduces [3H]-8-OH-DPAT binding in hippocampal preparations.     

GR46611 potentiates 5-HT1A receptor-mediated locomotor activity in the guinea pig.

5-HT(1B/D) receptor agonists such as GR46611 (3-[3-(2-Dimethylaminoethyl)-H-indol-5-yl]-N-(4-methoxybenzyl)acrylamide ) are known to lower body temperature in guinea pigs. Although stimulation of their functional analogs in rats, the 5-HT1B receptor induces hyperlocomotion, this effect has yet to be demonstrated with 5-HT(1B/D) receptor agonists in the guinea pig. Previous studies have shown that 5-HT1A agonists increase locomotor activity in guinea pigs. The current study set out to examine the effects of 5-HT(1B/D) receptor stimulation on locomotor activity in the guinea pig and to examine the interaction between 5-HT1A and 5-HT(1B/D) receptor stimulation on locomotor activity in that species. The full agonist at 5-HT1A receptors, 8-OH-DPAT (R(+)-8-Hydroxy-dipropylaminotetralin HBr) dose-dependently increased locomotor activity in guinea pigs (0.3-1.25 mg kg(-1) s.c.), as to a lesser extent, did the partial agonist, buspirone (8-[4-[4-(2-Pyramidinyl)-1-piperazinyl]butyl]-8-azaspiro[4.5 ]decane-7,9-dione HCl) (5.0-20.0 mg kg(-1) s.c.). The 5-HT(1B/D) receptor agonist GR46611 had no effect on locomotor activity in guinea pigs at doses up to 40 mg kg(-1) s.c. 8-OH-DPAT-induced behavioural activation was reversed by the selective 5-HT1A receptor antagonist WAY100635 (N-[-2-[4-(-methoxyphenyl)-1-piperazinyl]ethyl]-N-(pyrinidyl) cyclo hexanocarboxamide trihydro-chloride), with a minimum effective dose of 0.006 mg kg(-1), but not by the 5-HT(1B/D) receptor antagonist GR127935 (2'-methyl-4-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carboxyli c acid [4-methoxy-3-(4-methyl-piperazin-1-yl)phenyl]-amide) (0.25-1.0 mg kg(-1)). GR46611, at doses that were without effect given alone (0.5-2.5 mg kg(-1)), significantly enhanced the locomotor response to subthreshold doses of 8-OH-DPAT (0.5 mg kg(-1)) and buspirone (10 mg kg(-1)). The effect of GR46611 on 8-OH-DPAT-induced hyperactivity was reversed by pretreatment with GR127935 and with WAY 100635 indicating that activation of both receptors was required for the expression of locomotor hyperactivity. These findings suggest that activation of 5-HT(1B/D) receptors alone may not stimulate locomotor activity but it does potentiate the locomotion induced by 5-HT1A receptor stimulation in guinea pigs.     

Brown adipose tissue sympathetic nerve activity is potentiated by activation of 5-hydroxytryptamine (5-HT)1A/5-HT7 receptors in the rat spinal cord

In urethane-chloralose anesthetized, neuromuscularly blocked, ventilated rats, microinjection of NMDA (12 pmol) into the right fourth thoracic segment (T4) spinal intermediolateral nucleus (IML) immediately increased ipsilateral brown adipose tissue (BAT) sympathetic nerve activity (SNA; peak +492% of control), expired CO2 (+0.1%) heart rate (+48 beats min(-1)) and arterial pressure (+8 mmHg). The increase in BAT SNA evoked by T4 IML microinjection of NMDA was potentiated when it was administered immediately following a T4 IML microinjection of 5-hydroxytryptamine (5-HT, 100 pmol) or the 5-HT1A/5-HT7 receptor agonist, 8-OH-DPAT (600 pmol), (area under the curve: 184%, and 259% of the NMDA-only response, respectively). In contrast, T4 IML microinjection of the 5-HT2 receptor agonist, DOI (28 pmol) did not potentiate the NMDA-evoked increase in BAT SNA (101% of NMDA-only response). Microinjection into the T4 IML of the selective 5-HT1A antagonist, WAY-100635 (500 pmol), plus the 5-HT7 antagonist, SB-269970 (500 pmol), prevented the 5-HT-induced potentiation of the NMDA-evoked increase in BAT SNA. When administered separately, WAY-100635 (800 pmol) and SB-269970 (800 pmol) attenuated the 8-OH-DPAT-induced potentiation of the NMDA-evoked increase in BAT SNA through effects on the amplitude and duration of the response, respectively. The selective 5-HT2 receptor antagonist, ketanserin (100 pmol), did not attenuate the potentiations of the NMDA-evoked increase in BAT SNA induced by either 5-HT or 8-OH-DPAT. These results demonstrate that activation of 5-HT1A/5-HT7 receptors can act synergistically with NMDA receptor activation within the IML to markedly increase BAT SNA.     

Corticosterone selectively attenuates 8-OH-DPAT-mediated hypothermia in mice

The 5-HT1A agonist 8-OH-DPAT produces a hypothermia in mice mediated by somatodendritic 5-HT1A receptors, that is attenuated by antidepressants and corticosterone. The present study investigated if the effect of corticosterone is specific to the serotonergic system or a non-specific effect on thermoregulation. Administration of corticosterone for 3 d had no effect on dopaminergic (apomorphine) or adrenergic (clonidine) hypothermic challenges. However in addition to 8-OH-DPAT, nicotine-induced hypothermia was attenuated by corticosterone. Administration of the selective nicotinic antagonist mecamylamine had no effect on 8-OH-DPAT-induced hypothermia, although nicotine-induced hypothermia was attenuated by the selective 5-HT1A antagonist WAY-100635. This demonstrates a serotonergic-nicotinic interaction in the generation of hypothermia in mice and is consistent with corticosterone selectively attenuating somatodendritic 5-HT1A receptor function.