Serotonin-like actions of quipazine and CPP on spinal motoneurones

Evidence for an involvement of peripheral serotonin in p-chloroamphetamine-induced ejaculation of rats

The purpose of the present study was to determine the mechanism of the ejaculatory response induced by the 5-HT-releasing compound p-chloroamphetamine (PCA) in rats. The ejaculatory response was assessed by weighing the coagulated seminal materials accumulated over 1 h. Intraperitoneal injection of PCA (0.5-5.0 mg/kg) produced a dose-related increase in both the incidence of ejaculation and the weight of the accumulated seminal materials. The ejaculatory response induced by PCA (5.0 mg/kg) was abolished by pretreatment with the 5-HT synthesis inhibitor p-chlorophenylalanine, the 5-HT receptor antagonists methysergide and MDL72222, or by the selective 5-HT reuptake inhibitor citalopram, suggesting that the 5-HT-releasing property of PCA mainly involved the expression of ejaculation. Neither the section of the spinal cord at thoracic (T8-9) level nor the lumbosacral spinal 5-HT denervation by intrathecal (i.t.) injection of 5,7-dihydroxytryptamine affected the ejaculatory response induced by PCA. The i.t. injection of PCA (20-160 microg/rat) at lumbosacral spinal level did not exert the systemic PCA-like prominent effect on ejaculation, whereas i.t. injection of lidocain at the same site completely abolished the response to systemic PCA. Additionally, the peripherally administered 5-HT (0.1 and 0.25 mg/kg, i.p.) enhanced the proejaculatory effect at a threshold dose (1.0 mg/kg, i.p.) of PCA. From these observations, it is concluded that the ejaculatory response induced by PCA is mainly a spinally mediated reflex response that is triggered by the release of 5-HT in the peripheral sites.

A review and reevaluation of the role of serotonin in the modulation of lordosis behavior in the female rat

The role of serotonin (5-HT) in the modulation of sexual receptivity (lordosis) in the female rat is reviewed and reevaluated. The effects on lordosis of drug treatments that decrease or increase the activity and availability of central 5-HT are first discussed, and this is followed by an evaluation of the effects of drugs that act directly at 5-HT receptors. In order to shed light on the physiological significance of effects of serotonergic drugs on lordosis, there is also a review of what is known of changes in levels of serotonergic activity and densities of 5-HT receptors in the female rat brain that take place through the estrous cycle and in response to administration of behaviorally effective doses of gonadal steroids. Serotonin has generally been thought to have a tonic, inhibitory effect on lordosis. However, it is concluded that 5-HT can either inhibit or facilitate lordosis depending on which subtypes of central 5-HT receptors become activated. Because of a lack of consistent or compelling evidence of effects of ovarian hormones on serotonergic activity or 5-HT receptors in critical areas of the brain, it is stated that there is at present no basis to conclude that the effects of pharmacological manipulations of serotonergic activity on lordosis reflect an important, physiological role of 5-HT in the modulation of lordosis behavior in the female rat.

Serotonin mediates suppression of focal epileptiform activity induced by noxious stimulation

Noxious stimulation can suppress epileptic seizures in humans and epileptiform activity in laboratory animals. Using as a model system the focal epileptiform activity (FEA) induced by the pneumophoresis of penicillin, the role of 5-hydroxytryptamine (5HT) in suppression of this activity by noxious stimulation was investigated. Drugs known to depress dorsal raphe unit activity, (+/-)-8-hydroxydipropylaminotetralin (DPAT), imipramine, and fluoxetine prevented suppression of FEA induced by noxious stimulation. Desimipramine, which depresses locus ceruleus but not dorsal raphe unit activity, was ineffective in blocking the suppression. Quipazine, an agonist at 5-HT receptors, in part restored the suppression that had been blocked by DPAT or imipramine. Several serotonin antagonists effective at 5-HT1 and 5-HT2 receptors blocked suppression, but an unequivocal determination of the serotonin receptor subtype mediating suppression could not be made. We conclude that 5-HT mediates suppression of FEA induced by noxious stimulation.

Inhibition of tyrosine hydroxylase activity by serotonin in explants of newborn rat locus ceruleus

The long-term regulation of tyrosine hydroxylase (TH) by serotonin has been studied with cultures of newborn rat locus ceruleus explants. The presence of serotonin in the culture medium for a 24-h period was followed by an inhibition of TH activity in the explants. This effect lasted several days, with a maximal effect 2 days after treatment. Moreover, the decrease was reversible and dependent on the concentration of serotonin used (from 1 microM to 1 mM). The mechanisms of this regulation have been studied using drugs such as those known to act specifically on serotoninergic receptors and those known to interfere with protein synthesis. Thus, the action of serotonin (10(-5) M) on TH activity was suppressed with equimolar concentrations of serotoninergic antagonists such as metergoline or methiothepin. It was reproduced by quipazine, a drug capable of acting as a serotoninergic agonist. Inhibitors of protein synthesis acting either at the transcriptional or the translational levels can reproduce the inhibition of TH activity by serotonin alone. Furthermore, the effects of one or the other of these compounds and that of serotonin were not additive. This study confirms the hypothesis of an inhibitory control by serotonin on TH activity in the noradrenergic neurons of the locus ceruleus. Serotonin could regulate the synthesis of the enzyme through specific serotoninergic receptors.

Excitatory action of 5-HT on deglutitive substrates in the rat solitary complex

The excitatory effect of serotonin (5-HT) on the pharyngeal stage of swallowing was investigated in urethane anaesthetised rats with respect to the involvement of neural substrates located in the central and intermediolateral regions of the nucleus tractus solitarii (NTS). Micropneumophoretic ejection of 5-HT 5-50 pmol either produced deglutitory responses or selectively facilitated the S-glutamate-evoked pharyngeal responses when applied in 1-10 pmol prepulses. The excitatory/facilitatory effect of 5-HT was enhanced by intravenous threshold doses of the 5-HT-mimetic, quipazine (0.3-1 mumol/kg) and reversibly blocked by the 5-HT2-receptor antagonists, methysergide, metergoline and ketanserin. 5-HT doses exceeding 10-60 pmol gave rise to a non-selective reversible inhibition of glutamate- and acetylcholine (ACh)-evoked pharyngeal or oesophageal responses which was not prevented or reversed by 5-HT2-receptor antagonists, but was readily overcome by increasing the amount of glutamate or ACh ejected. Non-selective deglutitive inhibition after high doses of 5-HT could, therefore, result from neuronal desensitization secondary to excessive stimulation or activation of a different type of 5-HT receptor. These results corroborate an excitatory role of 5-HT in both reflex and automatic swallowing and demonstrate that the NTS is a major site of serotoninergic facilitation of swallowing.

3-Acetylpyridine lesions and four serotonergic behavioral syndromes in the rat

We studied the effect of 3-acetylpyridine (3-AP) lesions on the serotonergic-myoclonic syndromes evoked by quipazine (QP), 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT), fenfluramine (FF), and p-chloroamphetamine (PCA) in the adult rat. Eleven behaviors were scored from videotapes by an observer blind to drug status. In unlesioned rats, drugs could be differentiated by forelimb and axial myoclonus, pivoting and backing. All drugs significantly suppressed rearing. 3-AP produced a lasting action-enhanced body tremor which differed from axial myoclonus in its vertical direction and rhythmicity. 3-AP lesions modified the effect of drugs on several behaviors, increasing axial (QP, FF, PCA) and forelimb (5-MeO-DMT, FF, PCA) myoclonus and decreasing locomotor score. Prior lesions with 5,7-dihydroxytryptamine did not prevent the effect of 3-AP or any behaviors of the serotonin syndrome, but had a slight effect on the magnitude of forelimb myoclonus, head weaving, and hunching induced by some drugs. Neither lesion abolished or reduced myoclonus. These data suggest that intact 5-HT terminals are not requisite for the tremorogenic and cytotoxic effect of 3-AP. To the extent that chemical lesions with 3-AP are selective for the inferior olive (IO), the role of the IO in myoclonus in several 5-HT rodent myoclonic models appears to be regulatory rather than stimulatory.

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

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

Acrylamide neurotoxicity: altered spinal monosynaptic responses to quipazine, a serotonin agonist, in cats

The effects of the serotonin agonist quipazine on the spinal monosynaptic reflex (MSR) were investigated in acrylamide-treated cats. Cats were administered 30 mg/kg acrylamide (ACR) for 10 days, and the MSR and spontaneous ventral root discharge (VRD) were recorded on the tenth day (ACR 10) or 10 days subsequent to the last injection (ACR 20). Cumulative doses of quipazine were administered to increase the MSR and VRD. It was found that the MSR in ACR 10 cats was significantly depressed by ACR while the ACR 20 cats showed increased MSRs with double peaks. Dose-response relationships between quipazine and the area-under-the-MSR showed a significant shift to the right in the ACR 10 from control and no significant change in the ACR 20 from control. Dose-response curves of quipazine vs VRD showed no significant difference in either group from control. These data suggest that the alpha-motoneuron in ACR-treated cats was responding normally to quipazine while the primary afferent terminal was not.

Test-dependent antinociceptive effect of spinal serotonin release induced by intrathecal p-chloroamphetamine in mice

The effect of direct intrathecal injection of p-chloroamphetamine (PCA) into the lumbar subarachnoid space was investigated in mice. PCA (0.6 - 20 micrograms) induced a dose-related prolongation of response latencies in the tail-flick test, but failed to affect the hind-paw lick response in a hot-plate test employing slowly rising temperature. PCA (5 micrograms) given intracerebroventricularly did, however, significantly elevate the response temperature in the hot-plate test. The antinociceptive effect of PCA in the tail-flick test was prevented by spinalization, by pretreatment with the selective serotonergic re-uptake blocker zimelidine (20 mg X kg-1 i.p.) and by the serotonin synthesis inhibitor p-chlorophenylalanine (300 + 300 + 150 mg X kg-1 i.p. 72, 48 and 24 h before test). It is concluded that PCA given intrathecally releases serotonin from spinal terminals, which may under certain conditions induce antinociception.