Cardiovascular responses elicited by microinjection of monoamines into mesencephalic nucleus dorsalis raphe in cats

Summary The effect of monoaminergic agonists and antagonists microinjected into mesencephalic nucleus dorsalis raphe has been studied on blood pressure and heart rate to elucidate the nature and role of these monoaminergic receptors in cardiovascular regulation. Microinjection of monoamines, noradrenaline, phenylephrine and 5-hydroxytryptamine (5-HT) into nucleus dorsalis raphe elicited hypertension and tachycardia which could be blocked by local pretreatment with piperoxan (an -adrenoceptor blocker) and methysergide (a 5-HT receptor blocker) respectively. However, isoprenaline microinjections failed to evoke any response. Bilateral vagotomy did not prevent these cardiovascular responses evoked by monoamines microinjection, while cervical spinal cord (C₁) transection with bilateral vagotomy prevented these responses. These monoaminergic receptors seem to be localized in nucleus dorsalis raphe since microinjection of monoamines into neural structures adjoining nucleus dorsalis raphe, failed to induce any cardiovascular response. Monoaminergic receptors are present in nucleus dorsalis raphe which modulate cardiovascular activity by influencing sympathetic preganglionic neurons in the intermediolateral columns of the spinal cord.     

5-Hydroxytryptamine and antinociception

The considerable evidence supporting a role for 5-hydroxytryptamine (5-HT) in the modulation of nociceptive thresholds is reviewed. The postulate that an antinociceptive system of neurones projects from periaqueductal gray (PAG) to nucleus raphe magnus (NRM) and then to the dorsal horn, via cells containing 5-HT, has proved difficult to support experimentally. 5-Hydroxytryptamine, applied iontophoretically to dorsal horn neurones, does reduce the nociceptive responses of these neurones but it is not clear that activity in 5-HT cells of the raphe-spinal system is responsible for the descending inhibition of nociception. Although antagonists of 5-HT block some of the antinociceptive effects of both stimulation of the periaqueductal gray and systemically-applied morphine, they do not block the centrifugal inhibition of dorsal horn cells or the effects of iontophoretically applied 5-HT. Nor do they displace [³H]5-HT binding at low concentrations. Damage to, or selective depletion of 5-HT from the raphe-spinal 5-HT system has been reported not to alter nociceptive thresholds or the effects of stimulation of the periaqueductal gray.There is anatomical evidence for cells in the periaqueductal gray which contain 5-HT and project to the n. raphe magnus. Microinjected into the n. raphe magnus, 5-HT is antinociceptive in the tail-flick test and microinjection of an inhibitor of uptake of 5-HT or a 5-HT releasing agent, both cause antinociception. Furthermore, the effects of electrical stimulation of the periaqueductal gray are blocked by microinjection of a 5-HT antagonist into the n. raphe magnus. These observations suggest that 5-HT possibly mediates antinociceptive neurotransmission between the periaqueductal gray and the n. raphe magnus but its role in the dorsal horn is less clear.     

Analgesia produced by microinjection of baclofen and morphine at brain stem sites.

Microinjection of either baclofen (1.5 microgram) or morphine (2.5 microgram), in equimolar doses (14 mM), at sites located in the caudal periaqueductal gray (PAG) resulted in a delay in tail flick latency (analgesia). The relative analgesic potency of baclofen among caudal PAG sites, however, did not correlate with that of morphine. Application of either drug into the caudal aspect of the cerebral aqueduct also produced analgesia, but neither agent caused analgesia when applied at PAG sites rostral to the interaural line. Baclofen also produced analgesia when microinjected in the lower brain stem at sites lateral to the midline in or near the nucleus gigantocellularis, but did not produce analgesia when applied on the midline at sites located within or near the raphe magnus. Conversely, morphine produced analgesia when applied locally at midline sites but not at sites located lateral to the midline. These data suggest that the analgesia produced by systemic administration of baclofen and morphine involves activation of different neuronal substrates.     

Spinal 5-HT pathways and the antinociception induced by intramedullary clonidine in rats

Summary The possible involvement of spinal 5-hydroxytryptamine (5-HT) pathways in antinociception induced by microinjection of clonidine into the ventrolateral surface of the medulla oblongata was investigated in rats. Microinjection of clonidine (10-20 µg), but not yohimbine (1 µg) or 0.9% saline, into the lateral medulla prolonged the hot plate latency in rats. This clonidine-induced antinociception was abolished by intramedullary injection of the alpha₂-adrenoceptor antagonist, yohimbine. Selective destruction of spinal 5-HT neurons produced by intraspinal injection of 5,7-dihydroxytryptamine (5,7-DHT; 10 µg) or postsynaptic blockade of spinal 5-HT receptors produced by intrathecal injection of cyproheptadine (1 µg; a mixed 5-HT₁/5-HT₂ antagonist) also abolished clonidine-induced antinociception. Rats given 5,7-DHT intraspinally or cyproheptadine intrathecally showed a decrease in hot plate latency as compared with the controls. In anesthetized rats, the 5-HT release from the thoracic spinal cord was enhanced by microinjection of clonidine into the lateral medulla. This enhanced spinal 5-HT release evoked by intramedullary injection of clonidine was abolished by pretreatment of rats with intraspinal injection of 5,7-DHT. These results indicate that 5-HT pathways to the spinal cord mediate the antinociceptive effect induced by microinjection of clonidine into the ventrolateral surface of the medulla oblongata in rats.This study was supported by grants from the National Science Council (Taipei, Taiwan, Republic of China) Send offprint requests to M.T. Lin at the above address     

A study of the possible influence of central 5-HT function on clonidine-induced hypoactivity responses in mice

Administration of the ₂-adrenoceptor agonist clonidine (0.1 mg/kg) produces hypoactivity in mice. This sedation response was unaltered by pretreatment with either the 5-HT reuptake inhibitor zimeldine (1 or 10 mg/kg) or the 5-HT agonist quipazine (0.25 or 2.5 mg/kg). The 5-HT_1B agonist RU 24969 (0.2 or 1 mg/kg) enhanced hypoactivity responses at the higher dose. The non-selective 5-HT antagonists methysergide (1 or 10 mg/kg) and metergoline (0.2 or 1 mg/kg) potentiated clonidine-induced hypoactivity. However, the marked enhancement produced by metergoline may have been due to its potent action as a ₁-adrenoceptor antagonist. Nevertheless, the 5-HT₂ antagonists ritanserin (0.1 or 1 mg/kg) and ketanserin (0.1 or 1 mg/kg) both potentiated clonidine hypoactivity in a dose-dependent manner. -Adrenoceptor antagonists also inhibit 5-HT₁ receptors at high dose. Pindolol (10 mg/kg) had no effect on sedation, but [-]-propranolol (20 mg/kg) caused some attenuation. This latter effect was probably not due to inhibition of 5-HT₁ receptors, because this reduction also occurred at low dose (2 mg/kg). Destruction of 5-HT neurones by intracerebroventricular injection of 5,7-dihydroxytryptamine (50 g) resulted in a marginal increase in hypoactivity. In conclusion, these data shown that central 5-HT function can influence ₂-adrenoceptor-mediated hypoactivity responses. However, since these effects were usually only apparent after severe manipulation of 5-HT function, it suggests that while these interactions may be of pharmacological interest, they are probably not of physiological importance.     

The involvement of brain serotonin in the clonidine-induced hypothermia in rats

The effects of manipulation with brain serotonergic (5HT) activity on clonidine-induced hypothermia in rats were investigated. Lesion to the median raphe nucleus as well as p-chlorophenylalanine pretreatment significantly potentiated the temperature decrease after 0.2 mg/kg of clonidine. Pretreatment with a 5HT releasing agent--fenfluramine, or a 5HT receptor agonist--m-chlorophenylpiperazine (mCPP) antagonized the effect of clonidine. Additionally, both fenfluramine and mCPP given alone produced an elevation in body temperature. These results are discussed in terms of an involvement of the 5HT neuronal system in clonidine hypothermia, and the dissociation of the role of 5HT system in clonidine-induced behavioral and vegetative effects is suggested.     

Involvement of opioid receptors in N-Methyl-d-aspartate-induced arterial hypertension in periaqueductal gray matter

Arterial hypertension induced by microinjections of N-methyl-d-aspartate (NMDA) (2 nmol/rat) into the midbrain periaqueductal gray matter was used to assess the involvement of opioid receptors (µ, and ) in modulating pressor periaqueductal gray neurons. Groups (n = 5 – 8) of urethane-anaesthetised rats received, 5 min before NMDA, microinjections of selective opioid receptor antagonists in the periaqueductal gray area and arterial blood pressure was monitored. Pretreatments with naloxone (5 nmol/rat), a non selective ,µ receptor antagonist, or naltrindole hydrochloride (5 nmol/rat), a selective receptor antagonist, significantly (P< 0.05) decreased by 31% and 37%, respectively, NMDA-induced hypertension. The latency for the maximum increase of NMDA-induced hypertension was also significantly (P<0.05) increased with naloxone. Pretreatment with nor-binaltorphimine (5 nmol/rat), a selective receptor antagonist, only increased the latency of NMDA-induced hypertension. Each opioid antagonist failed per se to alter arterial blood pressure. Microinjection of morphine (13 nmol/rat), a non selective ,µ receptor agonist, significantly decreased (P<0.05) by 57.5% NMDA-induced arterial hypertension and this effect was antagonised by naloxone. Combined pretreatments in the periaqueductal gray area with naloxone and the GABA_A antagonist bicuculline (2.5 nmol/rat; 5 min before naloxone) antagonised the effect of naloxone on NMDA induced hypertension. In contrast, bicuculline significantly (P<0.05) potentiated morphine-induced decrease of NMDA hypertension. Combined pretreatments in the periaqueductal gray area with naloxone and the glycine antagonist strychnine (8 nmol/rat; 5 min before naloxone) failed to prevent the effect of naloxone on the NMDA-induced cardiovascular changes.These data suggest that periaqueductal gray vasopressor neurons receive both direct opioid and GABAergic inhibitory inputs. The latter may be, in turn, negatively modulated by opioid fibres mainly through µ and subtype receptors.     

芬氟拉明合用电针对大鼠脑内中央灰质单胺类释放的影响

目的:研究5-羟色胺释放剂芬氟拉明加强针刺镇痛前后大鼠脑内中央灰质(PAG)腹侧部单胺类递质的变化。方法:运用微透析及高效液相电化学检测方法。结果:电针后Nor的释放减少,而5-HT,5-HIAA和DA,HVA在AG部位含量升高(P<0.05,vs NS组)。当芬氟拉明合用电针时,5-HT,5-HIAA含量进一步升高,但Nor,DA及其代谢产物却无明显变化(P<0.05 vs NS EA组)。结论:电针能促进DA和5-HT释放但抑制Nor释放。芬氟拉明合用电针能进一步加强5-HT的释放。芬氟拉明加强针刺镇痛可能与进一步激活5-羟色胺系统有关。     

Modulatory effect of p-chlorophenylalanine microinjected into the dorsal and median raphe nuclei on cocaine-induced behaviour in the rat.

The present study examined whether a potentiation of cocaine-induced behaviour in rats following peripheral pretreatment with the 5-hydroxytryptamine (5-HT) biosynthesis inhibitor p-chlorophenylalanine may be due to depletion of 5-HT in the dorsal raphe nucleus and/or median raphe nucleus. Following peripheral pretreatment with p-chlorophenylalanine (100 mg/kg, i.p.) for 3 consecutive days, a potentiation of cocaine-induced locomotor activity and rears was observed. To investigate a possible involvement of serotonergic neurones arising in the midbrain raphe nuclei in the observed potentiation, p-chlorophenylalanine (0.5 microg) was microinjected in either the dorsal raphe nucleus or median raphe nucleus followed by behavioural testing 48 h later. Application of p-chlorophenylalanine in the dorsal raphe nucleus resulted in an enhancement of cocaine-induced locomotor activity and head bobs. In contrast, the stimulant effect of cocaine on behaviour was not altered by microinjection of p-chlorophenylalanine in the median raphe nucleus. Peripheral and central administration of p-chlorophenylalanine did not consistently alter the baseline behaviour of saline-treated animals. Biochemical results indicated only a moderate depletion of 5-HT in the midbrain raphe nuclei following peripheral p-chlorophenylalanine administration. Surprisingly, the central application of p-chlorophenylalanine in the dorsal raphe nucleus and median raphe nucleus did not alter the 5-HT levels in the midbrain raphe nucleus investigated. In addition, peripheral and central administration of p-chlorophenylalanine did not alter the 5-HT levels in the nucleus accumbens. In conclusion, the behavioural results suggest that the potentiation of cocaine-induced behaviour following peripheral p-chlorophenylalanine administration may be attributed to the dorsal raphe nucleus but not the median raphe nucleus suggesting that, serotonergic dorsal raphe nucleus neurones may normally mediate a tonic inhibitory effect on cocaine-induced behaviour. Furthermore, the biochemical data may indicate the existence of neurochemical resistance of the midbrain raphe nuclei to the 5-HT depleting effects of p-chlorophenylalanine.     

THE EFFECTS OF SEROTONIN PRECURSORS ON THE PRESSOR RESPONSE TO INTRAVENOUS CLONIDINE IN CONSCIOUS RATS

1. The interaction of the biosynthetic precursors of serotonin with the a-adrenoceptor-mediated pressor response to intravenous clonidine was investigated in unanaesthetized rats. 2. Pretreatment with 100 mg/kg of either L-tryptophan or 5-hydroxytryptophan (5-HTP) reduced the magnitude of the pressor response elicited by intravenous clonidine (25 μg/kg) to 15% and 11%, respectively, of that observed in control rats. 3. The depression by L-tryptophan of the clonidine-induced pressor response could be prevented by pretreatment with either the L-aromatic amino acid de-carboxylase inhibitor Rö-4– 4602 or the serotonin antagonist methysergide.     

Serotonin in the dorsal periaqueductal gray modulates inhibitory avoidance and one-way escape behaviors in the elevated T-maze

The dorsal periaqueductal gray has been implicated in the modulation of escape behavior, a defensive behavior that has been related to panic disorder. Intra-dorsal periaqueductal gray injection of serotonin or drugs that mimic its effects inhibits escape induced by electrical or chemical stimulation of this brainstem area. In this study, we investigate whether intra-dorsal periaqueductal gray injection of 5-HT receptor agonists attenuates escape generated by an ethologically based model of anxiety, the elevated T-maze. This test also allows the measurement of inhibitory avoidance, which has been related to generalized anxiety disorder. The effects of the 5-HT receptor agonists were compared in animals with or without a previous exposure to the open arms of the elevated T-maze. In these two test conditions, intra-dorsal periaqueductal gray injection of the endogenous agonist serotonin or the 5-HT(2B/2C) receptor agonist m-chlorophenylpiperazine (mCPP) enhanced inhibitory avoidance, suggesting an anxiogenic effect. The 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) impaired this response, suggesting an anxiolytic effect, and the preferential 5-HT2A receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) was ineffective. All these agonists inhibited escape behavior. Apart from mCPP, the effect on escape was detected only in animals pre-exposed to the open arm. None of the drugs tested affected locomotion in the open-field test. Taken altogether, our findings suggest that 5-HT1A and 5-HT2c receptors in the dorsal periaqueductal gray exert opposed control on inhibitory avoidance, implicating these receptors in anxiety conditioning. As previously observed in tests employing the aversive stimulation of the dorsal periaqueductal gray, 5-HT1A and 5-HT2A receptors in this brain area are involved in escape inhibition. Therefore, in different animal models, the activation of these two subtypes of receptors in the dorsal periaqueductal gray consistently attenuates the expression of a panic-related behavior.     

Possible involvement of 5-hydroxytryptamine and cyclic-AMP in tolerance to tremorine analgesia in mice

The phenomenon of tolerance to the analgesic action of tremorine in mice was studied by the hot-plate and tail-clip methods. Reduction in 5-HT levels in brain by parachlorophenylalanine pretreatment decreased the ED₅₀ of tremorine analgesia in tremorine tolerant mice. 5-Hydroxytryptophan, l-Dopa or -methyl-para-tyrosine did not influence the analgesic response to tremorine in tremorine tolerant animals. However, theophylline was found to enhance the tolerance to tremorine analgesia.Brain 5-HT and cAMP are probably involved in tremorine tolerance, whereas neither noradrenaline nor dopamine is involved in the phenomenon.     

Presence of opioid receptors in mesencephalic nucleus dorsalis raphe concerned in cardiovascular regulation in cats

Summary The effects of microinjecion of opioid receptor agonist and antagonist into mesencephalic nucleus dorsalis raphe, were studied on mean arterial pressure and heart rate to elucidate the nature and role of these opioid receptors in cardiovascular regulation. Microinjection of morphine (5 g and 10 g) into nucleus dorsalis raphe elicited both inhibitory and excitatory cardiovascular responses respectively, while microinjection of opioid receptor antagonist, naloxone (10 g) failed to produce any significant cardiovascular responses. However, local pretreatment with naloxone blocked both inhibitory and excitatory responses of graded doses of morphine. These opioid receptors seem to be localised in the neurons of the nucleus since microinjection of morphine into neural structures adjoining nucleus dorsalis raphe failed to induce any cardiovascular responses. Furthermore, the dose or morphine (2 g) which was ineffective when microinjected into nucleus dorsalis raphe, produced inhibitory cardiovascular responses after pretreatment with LM 5008, a 5-HT uptake blocker. Similarly, the excitatory cardiovascular responses of morphine microinjection were blocked by spinal cord transection (C₁) and p-CPA, guanethidine and piperoxan pretreatments, while bilateral cervical vagotomy failed to do so. Thus, it is likely that the inhibitory cardiovascular responses of morphine are mediated directly through stimulation of opioid receptors present in the neurons of nucleus dorsalis raphe while the excitatory responses to higher dose of morphine, appear to be due to a release of noradrenaline which in turn modulates the activity of neurons by acting on a adrenoceptors. Send offprint requests to K. K. Tangri at the above address     

Dorsal raphe microinjection of 5-HT and indirect 5-HT agonists induces feeding in rats

Previous work has shown that 5-hydroxytryptamine (5-HT) receptor agonists such as 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) reduce 5-HT neurotransmission and induce feeding in rats. The effects of 8-OH-DPAT appear to be mediated in part in the dorsal raphe nucleus by serotonergic somatodendritic autoreceptors which normally regulate impulse flow in 5-HT dorsal raphe neurons. The present experiments sought to examine whether suppression of dorsal raphe serotonergic neural activity induced by exogenously applied, or endogenously released 5-HT would increase feeding. Free-feeding rats were microinjected in the dorsal raphe with 5-HT, the 5-HT releasing compound d-fenfluramine, the 5-HT re-uptake inhibitor zimelidine, or the type-A monoamine oxidase inhibitor brofaromine. Dose dependent increases in food intake over a 1 h period were found following treatment with 5-HT and the three indirectly acting compounds. Thus, increased serotonergic activity within the dorsal raphe increases feeding, presumably by inhibiting the activity of dorsal raphe 5-HT neurons. In addition the effects of 5-HT were blocked by pretreatment with haloperidol, indicating the involvement of a dopaminergic mechanism in mediating the effects of feeding of a suppression in dorsal raphe 5-HT neural activity. The results are discussed in terms of the general role which serotonergic neurons arising from the dorsal raphe may play in behavioural inhibition.     

Modulation of cocaine-induced locomotor activity, rears and head bobs by application of WAY100635 into the dorsal and median raphe nuclei of the rat

The present study investigated the role of somatodendritic 5-HT1A autoreceptors located in the dorsal and median raphe nuclei on the stimulant effect of cocaine on locomotor activity, rears and head bobs in female Glaxo Wistar rats. Cocaine was administered at a submaximal dose of 15 mg/kg i.p. to enable either a potentiation or attenuation to be observed. The selective 5-HT1A antagonist WAY100635 (0.21 ng or 21 ng) or saline was microinjected in the dorsal or median raphe nuclei followed by the peripheral administration of cocaine 60 min later. WAY 100635 microinjected in the dorsal or median raphe nuclei did not consistently alter the locomotor activity and the number of rears of saline-treated animals. Microinjection of WAY100635 in the dorsal raphe nucleus potentiated cocaine-induced locomotor activity and the number of head bobs. The number of rears induced by cocaine was not significantly altered by WAY100635 microinjected in the dorsal raphe nucleus. In contrast, microinjection of WAY100635 in the median raphe nucleus did not alter the stimulant effect of cocaine on locomotor activity, rears or head bobs. It may be suggested from these results that stimulation of somatodendritic 5-HT1A autoreceptors located in the dorsal raphe nucleus mediates an inhibitory effect on cocaine-induced locomotor activity and head bobs, whereas somatodendritic 5-HT1A autoreceptors in the median raphe nucleus are not involved in the inhibitory role of 5-HT on the stimulant effect of cocaine on locomotor activity and head bobs. A differential involvement of the midbrain raphe nuclei may exist controlling the stimulant effect of cocaine on locomotor activity and head bobs.     

Enhancement of clonidine-induced analgesia by lesions induced with spinal and intracerebroventricular administration of 5,7-dihydroxytryptamine

The role of serotonin (5-HT) in analgesia induced by clonidine was examined by determining the effect of intraspinal (i.s.) and intracerebroventricular (i.c.v.) injections of 5,7-dihydroxytryptamine (5,7-DHT) on analgesia produced by clonidine in the tail-flick and hot plate tests. Depletion of amines was verified by high performance liquid chromatography analysis. Intraspinal injections of 5,7-DHT potentiated the action of clonidine in both tests for analgesia and caused depletion of 5-HT in the spinal cord. Intracerebroventricularly injected 5,7-DHT also increased the action of clonidine and depleted 5-HT in brain as well as in the spinal cord. In the groups given intracerebroventricular injections, there appeared to be a biphasic increase in the action of the clonidine. Significant hyperalgesia from pretreatment with neurotoxin was observed only on a limited number of occasions. The present results indicate that 5-HT mechanisms in the CNS are important mediators of the analgesic action of clonidine. Interactions between clonidine and 5-HT systems at both spinal and supraspinal sites are considered.     

Y-maze behavior in the mouse after morphine or an enkephalin analog

The mechanism of clonidine-induced hyperphagia and weight gain in monkeys was studied in 11 Stumptail macaques. Clonidine induced a significant increase in food intake over baseline levels and a significant weight gain after the 3-day treatment period. Both changes induced by clonidine were antagonized by the ₂-noradrenergic antagonist yohimbine, but not by prazosin, an ₂-noradrenergic antagonist. These results suggest that clonidine-induced hyperphagia and weight gain in monkeys are mediated through ₂-noradrenergic receptors.     

Role of 5-HT and NA in spinal dopaminergic analgesia

Spinal rats and rats with an intact neuraxis given the dopamine (DA) agonist R-apomorphine (0.31-1.75 mumol/kg) in the lumbar subarachnoid space by intrathecal injection were tested 5 and 10 min later for spinal analgesia (increased tail-flick response latency). Apomorphine produced analgesia in spinal rats but not in rats with an intact neuraxis. However, pretreatment of intact rats with the serotonergic (5-HT) receptor antagonist methysergide, the noradrenergic (NA) receptor antagonist phentolamine or the two antagonists together led to a dose-dependent analgesia following apomorphine. Intact rats pretreated with the monoamine depleting drug reserpine, the 5-HT synthesis inhibitor p-chlorophenylalanine (PCPA) or the NA synthesis inhibitor FLA 63 also showed analgesia to apomorphine. On the other hand, pretreatment with the catecholamine depleting agent alpha-methyl-p-tyrosine (AMPT), the beta-adrenergic receptor blocker propranolol or the opioid receptor antagonist naloxone failed to produce DA analgesia. The present findings suggest that both 5-HT and NA descending fiber systems exert tonic inhibitory effects on spinal DA nociceptive processes.     

8-OH-DPAT in the median raphe, dorsal periaqueductal gray and corticomedial amygdala nucleus decreases, but in the medial septal area it can increase maternal aggressive behavior in rats

 The purpose of the present study was to analyze the role of somatodendritic autoreceptors and postsynaptic 5-HT_1A receptors in the modulation of maternal aggressive behavior. The 5-HT_1A receptor agonist 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) was microinjected (0.2, 0.5 and 2.0 μg/0.2 μl) in different brain areas of female Wistar rats: median raphe nucleus (MnR); medial septal area (MS); anterior corticomedial amygdaloid nucleus (ACoM); and dorsal periaqueductal gray (DPAG). The behaviors of lactating female rats with pups against a conspecific male intruder were recorded on day 7 post-partum. Results showed that in the median raphe nuclei, in the dorsal periaqueductal gray and in the corticomedial amygdaloid nucleus 8-OH-DPAT decreased maternal aggression; while in the medial septum, the intermediate dose (0.5 μg/0.2 μl) of the 5-HT_1A receptor agonist increased the aggressive behavior of the lactating female rat. It is concluded that the main role of the 5-HT_1A somatodendritic autoreceptors and the postsynaptic receptors of the brain areas studied is to decrease maternal aggression, however, at a specific dosage, 8-OH-DPAT acting on postsynaptic receptors of the medial septal area can increase aggressiveness. Received: 4 February 1997 / Final version: 27 June 1997     

Effects of 5-hydroxytryptamine agonists and antagonists on the responses of rat spinal motoneurones to raphe obscurus stimulation.

1. The excitability of lumbar spinal motoneurones was studied in halothane-anaesthetized rats by recording with microelectrodes the amplitude of the population spike evoked antidromically by stimulation of the cut ventral roots. 2. Electrical stimulation of the nucleus raphe obscurus for 1 min at 20 Hz increased the population spike amplitude and, as shown by intracellular recording, depolarized motoneurones. This response could be mimicked by microinjection of DL-homocysteic acid into raphe obscurus but the response was not present in animals pretreated with the 5-hydroxytryptamine (5-HT) neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). 3. Microiontophoretically applied 5-HT had very similar effects on the extracellularly recorded population spike to those caused by stimulation of the raphe obscurus. These responses to 5-HT were larger in 5,7-DHT-pretreated animals. 4. The effects of 5-HT were potently mimicked by iontophoretically applied 5-carboxamidotryptamine but 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) was without effect. 5. Antagonists were applied by microiontophoresis and also by intravenous injection. Ketanserin, the selective 5-HT2 antagonist, did not antagonize the effects of 5-HT. Neither did the 5-HT3-receptor antagonist MDL 72222 or the selective 5-HT1 binding ligand cyanopindolol. 6. The non-selective 5-HT1/5-HT2-receptor antagonist methysergide was an effective antagonist of both the effects of 5-HT and the response to raphe obscurus stimulation. Methysergide did not reduce the excitatory effects of noradrenaline. 7. It is concluded that 5-HT application and stimulation of raphe obscurus increase the excitability of motoneurones by an action on a 5-HT1-like receptor which appears to be different from the 5-HT1A-and the 5-HT1B-binding sites characterized by others.