Role of serotonin in opiate-induced prolactin secretion and antinociception in the developing rat

Our laboratory has demonstrated previously that the ability of opiates to stimulate prolactin (PRL) release during ontogeny precedes the appearance of a PRL response to serotonergic drugs. The present study tests the hypothesis that opiates stimulate PRL secretion through a serotonergic mechanism in adult rats, but a nonserotonergic mechanism in neonatal rats. Morphine stimulated PRL secretion in adult and neonatal (10-day-old) rats and this increase was blocked with the opiate antagonist naloxone. Ten-day-old or adult rats were pretreated with the serotonin antagonist, cyproheptadine (CYPRO), or the neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT). Both CYPRO and 5,7-DHT attenuated the PRL response to morphine in adult but not neonatal rats. 5,7-DHT decreased serotonin and 5-hydroxyindoleacetic acid substantially in the hypothalamus. When rats were pretreated with 5,7-DHT several weeks before morphine challenge, serotonin depletion was more pronounced, but the PRL response to morphine was not decreased. In addition, the PRL response to 5-hydroxytryptophan was greatly potentiated, suggesting that functional supersensitivity developed in the 5,7-DHT-treated animals. The ability of CYPRO and 5,7-DHT to block the serotonergic component of a different morphine-induced behavior in the neonate was tested using the tail immersion test for analgesia. Morphine produced profound antinociception in the rat pup which was attenuated markedly by 5,7-DHT and CYPRO. These studies demonstrate that opiates mediate their stimulatory effects on PRL release, at least in part, through a serotonergic mechanism in adult rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Behavioral evidence for supersensitivity following destruction of central serotonergic nerve terminals by 5,7-dihydroxytryptamine.

Previous studies have established that a complex behavioral syndrome--consisting of tremor, rigidity, hindlimb abduction. Straub tail, lateral head weaving and reciprocal forepaw treading--is a specific reflection of the activity of central serotonin receptors. This syndrome was utilized in the present study to test for supersensitivity in the central serotonergic system. Specific destruction of central serotonin nerve terminals by intraventricular injection of 5,7-dihydroxytryptamine (5,7-DHT, 50 mug) in adult male rats pretreated with a catecholamine uptake blocking agent resulted in marked supersensitivity to serotonin precursors and agonists. The greatest degree of supersensitivity was observed in response to L-5-hydroxytryptophan, for which the ED50 for elicitation of the syndrome was 20% of the value for control rats. A lesser degree of supersensitivity was seen in response to L-tryptophan (following monoamine oxidase inhibition) and the direct-acting serotonin agonist, 5-methoxy-N,N-dimethyltryptamine, for which the ED50 was approximately 50% of the control value in both cases. Supersensitivity begins to develop within 24 hours and is relatively complete by 96 hours after 5,7-DHT. A marked subsensitivity to the serotonin releasing agent, fenfluramine, was found in 5,7-DHT-treated rats. In contrast to the marked supersensitivity to serotonin precursors and agonists which occurs following 5,7-DHT, chronic administration of a serotonin synthesis inhibitor, p-chlorophenylalanine (400 mg/kg every 3 days for a total of 24 days), did not produce supersensitivity to L-5-hydroxytryptophan or 5-methoxy-N,N-dimethyltryptamine. Possible pre- and postsynaptic mechanisms for the development of supersensitivity are discussed.     

(+/-)-3,4-Methylenedioxymethamphetamine administration to rats does not decrease levels of the serotonin transporter protein or alter its distribution between endosomes and the plasma membrane

We showed that the serotonin (5-HT) neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) reduces brain tissue 5-HT, decreases expression of 5-HT transporter (SERT) protein, and increases expression of glial fibrillary acidic protein (GFAP). In contrast, doses of (+/-)-3,4-methylenedioxymethamphetamine (MDMA) that decrease brain tissue 5-HT fail to alter expression of SERT or GFAP. Using a new and highly sensitive anti-SERT antibody, we determined whether MDMA alters the subcellular distribution of SERT protein by measuring SERT expression in endosomes and plasma membranes 2 weeks after MDMA administration. Rat brain tissues (caudate, cortex, and hippocampus) were collected 3 days and 2 weeks after MDMA (7.5 mg/kg i.p., every 2 h x 3 doses) or 5,7-DHT (150 microg/rat i.c.v.) administration. Representative results from cortex are as follows. At both 3 days and 2 weeks postinjection, MDMA decreased tissue 5-HT (65%) and had no effect on GFAP expression. MDMA increased heat shock protein 32 (HSP32; a marker for microglial activation) expression (30%) at 3 days, but not 2 weeks. MDMA did not alter SERT expression at either time point and did not alter SERT levels in either endosomes or plasma membranes (2 weeks). 5,7-DHT decreased tissue 5-HT (80%), increased HSP32 expression at both time points (about 50%), and increased GFAP expression at 2 weeks (40%). 5,7-DHT decreased SERT expression (33%) at 2 weeks, but not at 3 days. These findings indicate that a dosing regimen of MDMA that depletes brain 5-HT does not alter SERT protein expression or the distribution of SERT between endosomes and the plasma membrane and does not produce detectable evidence for neurotoxicity.     

Altered effects of desipramine on operant performance after 6-hydroxydopamine-induced depletion of brain dopamine or norepinephrine

Performance maintained by differential-reinforcement-of-low-rate operant schedules has been found to be sensitive to antidepressant drugs. Tricyclic antidepressants, monoamine oxidase inhibitors and atypical antidepressants reduce response rate and increase reinforcement rate under long differential-reinforcement-of-low-rate schedules. In order to study the neurochemical mechanism by which the tricyclic antidepressant desipramine alters differential-reinforcement-of-low-rate performance, the effect of desipramine was determined before and after brain catecholamine depletion was induced by 6-hydroxydopamine administration. Before lesioning, desipramine reduced response rate and increased reinforcement rate in a dose-dependent manner. Brain norepinephrine depletion (produced by 6-hydroxydopamine injection into the dorsal noradrenergic bundle) attenuated the ability of desipramine to reduce response rate, but did not alter its ability to increase reinforcement rate, but did not alter its ability to increase reinforcement rate. Brain dopamine depletion, (produced by i.c.v. 6-hydroxydopamine administration after pargyline and desipramine pretreatment) attenuated the ability of desipramine to increase reinforcement rate. These results suggest that the sedative effect of desipramine could be mediated by its interaction with central norepinephrine neurons and that the reinforcement rate-increasing effect may involve central dopamine neurons.     

The serotonin/norepinephrine-link in brain. II. Role of serotonin in the regulation of beta adrenoceptors in the low agonist affinity conformation

The present studies were undertaken to characterize further the role of serotonin (5-HT) in the regulation of the norepinephrine (NE) beta adrenoceptor coupled adenylate cyclase system in the rat cortex. Although 5-HT in vitro did not influence maximum binding and Kd values of [3H]dihydroalprenolol binding or the IC50 value for isoproterenol as estimated from competition binding curves in cortical tissue from control animals, 5-HT abolished the increase in beta adrenoceptor number and the marked elevation of the IC50 value for isoproterenol in cortical membrane preparations after selective lesions with 5,7-dihydroxytryptamine (5,7-DHT). Nonlinear regression analysis of competition binding curves revealed that the increase in the maximum binding of beta adrenoceptors after 5,7-DHT is due exclusively to an increase in beta adrenoceptors in the agonist low affinity conformation and that it is this receptor population that is reduced by nanomolar concentrations of 5-HT. The increase in the density of beta adrenoceptors in the low affinity conformation occurred approximately 11 days after the lesions and remained elevated throughout the experimental period of 28 days. Ritanserin in a dose that virtually abolished 5-HT2 receptor binding in cortex did not mimic the effect of 5,7-DHT.(ABSTRACT TRUNCATED AT 250 WORDS)     

Baroreflex changes produced by serotonergic or catecholaminergic lesions in the rat nucleus tractus solitarius.

To distinguish between catecholaminergic and serotonergic mechanisms for baroreflex regulation in the medulla, we compared rats with chemical lesions produced by injecting 6-hydroxydopamine (6-OHDA) or 5,7-dihydroxytryptamine (5,7-DHT) into the nucleus tractus solitarius (NTS) bilaterally at the caudal tip of the area postrema. After 2 weeks, basal blood pressure and heart rate were unaltered, but blood pressure lability, instead of being increased, was slightly reduced. Baroreflex tests in conscious rats showed that although phenylephrine-induced reflex bradycardia was unaffected, nitroprusside-induced reflex tachycardia was enhanced by 5,7-DHT. In anesthetized rats, drug-induced reflex chronotropic responses no longer differed between groups, but attendant decreases or increases in renal nerve activity were consistently reduced by 6-OHDA. On the other hand, upon afferent aortic nerve stimulation, particularly with low current frequencies, bradycardic and sympathoinhibitory responses were enhanced by 5,7-DHT, but the sympathoinhibitory responses were reduced by 6-OHDA. Despite the absence of demonstrable necrosis or cell loss at NTS injection sites, 6-OHDA reduced norepinephrine mainly and serotonin partly, whereas 5-7-DHT reduced serotonin content alone, thereby indicating that chemical lesions had indeed been produced. Because these cardiovascular changes probably reflect differences in catecholaminergic vs. serotonergic baroreflex regulation, our results are generally compatible with the interpretation that baroreflex modulation in the NTS involves catecholaminergic facilitation and serotonergic inhibition.     

The antinociceptive role of a bulbospinal serotonergic pathway in the rat brain

The antinociceptive role of spinal serotonin (5-HT) neurons descending from 5-HT cells near the ventrolateral surface of the medulla oblongata was investigated by stimulating these cells in normal rats, in rats with generalized or selective chemical ablation of 5-HT nerves, and in rats with postsynaptic blockade of 5-HT receptors. Electrical stimulation of the lateral medulla elicited analgesia in normal rats; the increase in pain threshold was proportional to the intensity and to the frequency of stimulation. In addition, microinjection of kainic acid or L-glutamate at the same sites also produced analgesia. However, generalized destruction of CNS 5-HT nerves produced by intraventricular injection of 5,7-dihydroxytryptamine (5,7-DHT) or selective destruction of spinal 5-HT nerves produced by intraspinal injection of 5,7-DHT reduced the magnitude of the antinociceptive responses to electrical stimulation. Postsynaptic blockade of CNS 5-HT receptors produced by intraventricular injection of cyproheptadine also reduced the stimulation-produced analgesia. The specificity of the lesions for 5-HT nerves is demonstrated by the lack of effect on the levels of noradrenaline in the same brain regions. The results indicate that the activity of 5-HT nerve cells adjacent to the ventrolateral surface of the medulla oblongata and projecting to the spinal cord serves to elevate pain threshold.     

Potentiation in phencyclidine-induced serotonin-mediated behaviors after intracerebroventricular administration of 5,7-dihydroxytryptamine in rats

Phencyclidine (PCP)-induced behaviors were compared with 5-methoxy-N,N-dimethyltryptamine (5-MeODMT)- and p-chloroamphetamine-induced behaviors in rats pretreated with ritanserin or 5,7-dihydroxytryptamine (5,7-DHT) in order to investigate whether PCP interacts with 5-hydroxytryptamine2 (5-HT2) receptors. Head-twitch and wet-dog shake induced by p-chloroamphetamine, a 5-HT releaser, and head-twitch induced by PCP were blocked completely by pretreatment with ritanserin, a specific 5-HT2 receptor blocker, but other behaviors induced by p-chloroamphetamine, PCP and 5-MeODMT, a 5-HT agonist, were not. The intensity of head-weaving, turning, backpedalling and hind-limb abduction induced by 5-MeODMT and the intensity of head-weaving, turning and head-twitch induced by PCP were markedly greater in the rats 2 weeks after the 5,7-DHT, a 5-HT neurotoxin-injection. Contrarily, 5-HT-mediated behaviors induced by p-chloroamphetamine were attenuated in the 5,7-DHT-treated rats. 5,7-DHT-treatment increased the number of 5-HT1 ([3H]-5-HT), 5-HT2 ([3H]ketanserin) and PCP ([3H]PCP) binding sites in the synaptic membrane of rat brain, but decreased the brain level of 5-HT (41% of control). These results may indicate that PCP as a 5-HT2 agonist induces head-twitch via 5-HT2 receptors, and that PCP induces head-weaving and turning via 5-HT1 receptors and/or some other mechanisms in rats.     

The effect of blood ingestion on brain serotonin synthesis in portacaval-shunted rats

In rats with a portacaval shunt (PCS), the effect on the serotonin metabolism in the brain after oral administration of blood, a mixed amino acid solution (Vamin 14; KabiVitrum, Sweden) or a 10% glucose solution was studied. One week after PCS, the animals were fed with a gastric tube for 8 h and thereafter tested for behavioral abnormalities before decapitation at 12 h. The concentration of 5-hydroxytryptophan (5-HTP), serotonin (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) were analyzed chromatographically (HPLC technique with electrochemical detection) in different regions of the brain. Estimation of synthetic rates of 5-hydroxyindoles was facilitated by aromatic aminoacid decarboxylase inhibition (m-hydroxybenzyl-hydrazine; NSD 1015). The brain concentrations of 5-HTP, 5-HT, and 5-HIAA were increased in all shunted rats as compared with sham-operated animals. Whether animals received blood, glucose, or aminoacid solution made no differences in the brain concentrations of 5-HTP and 5-HT. Concentrations of 5-HIAA were lower in those animals receiving blood as compared with the other shunted groups. No reproducible differences in the behavior of the animals were observed. These results suggest that massive blood administration 1 week after PCS in rats has no influence on the rate of brain indole synthesis. While alterations in serotonin metabolism may play a role in some forms of encephalopathy, this study implies that the behavioral and neurologic disorders which follow gastrointestinal tract hemorrhage in patients with liver failure may have other etiologies.     

Effects of pre- and postnatal haloperidol administrations to pregnant and nursing rats on brain catecholamine levels in their offspring

Effects of perinatal, maternal treatments with haloperidol on brain catecholamine and serotonin concentrations in maturing offspring were examined. The drug was administered orally, at three different dosages, each day from the 14th gestational day to postnatal day 12. At 14, 22, 53 and 90 days of age offspring were selected for regional assays of amine neurotransmitter concentrations. The results indicate that haloperidol can alter the developmental accumulation of central catecholaminergic neurotransmitters. The drug-induced decreases in cortical and spinal norepinephrine levels and in striatal dopamine content in the offspring persisted into adulthood. Regional serotonin concentrations in offspring were not affected by the maternal haloperidol treatments. These findings suggest that central catecholamine receptors are necessary for normal development of dopaminergic and noradrenergic innervation in the central nervous system.     

Effects of brain serotonin alterations on prostaglandin E1-induced bradycardia in rats.

The vasodepressor and bradycardia responses of saline control, serotonin-depleted and serotonin-potentiated rats to an intravenous dose of prostaglandin E1 (PGE1) were assessed under the urethane anesthesia. Elevation of serotonin concentration in brain with either 5-hydroxytryptophan (a serotonin precursor) or chlorimipramine (an inhibitor of serotonin reuptake), although causing no changes in vasodepressor reuptake), although causing no changes in vasodepressor response, did enhance the PGE1-induced bradycardia in contrast, depleting serotonin concentration in brain with either p-chlorophenylalanine or 5,7-dihydroxytryptamine greatly reduced the PGE1-induced bradycardia without changes in vasodepressor response. Moreover, the reduced PGE1 bradycardia induced by p-chlorophenylalanine treatment was readily reversed by the replacement of the depleted brain serotonin with 5-hydroxytryptophan in combination with a peripheral decarboxylase inhibitor Ro4-4602. The data indicate that brain serotonergic systems play a role in the elaboration or modulation of the PGE1-induced bradycardia. Specifically, brain serotonin seems to facilitate the PGE1-induced bradycardia since its depletion causes a decrease and its potentiation or elevation causes an increase in the PGE1-induced bradycardia.     

Effects of selective neurotoxic lesion of lumbosacral serotonergic and noradrenergic systems on autotomy behaviour in rats.

Male rats underwent unilateral ligation and transection of the sciatic and saphenous nerves 2, 7 or 14 days after being injected intrathecally (at the thoracolumbar junction) with 6-hydroxydopamine (6-OHDA), 5,6-dihydroxytryptamine (5,6-DHT) or vehicle, and the development of autotomy was monitored. The effects of both neurotoxins on cervicothoracic (C5-T1) and lumbosacral (L1-S1) norepinephrine (NE), dopamine (DA) and 5-hydroxytryptamine (5-HT) spinal cord levels were analysed by HPLC in separate groups of rats. 6-OHDA treatment (20 micrograms/10 microliters) produced a rapid (from day 2) and significant (90-95%) fall in NE content only at L1-S1. 5,6-DHT administration (20 micrograms/10 microliters) produced a gradual (68%, 90% and 94%, at 2, 7 and 14 days, respectively) and selective depletion of 5-HT only at L1-S1. DA levels remained essentially unchanged after both neurotoxins. No differences in monoamine levels were detected among groups injected with vehicle. The main effects of neurotoxins on autotomy were: (1) a significant delay in the onset of autotomy in the rats injected with 6-OHDA 2 days before neurectomy; (2) a trend to autotomize earlier and more severely in the rats injected with 5,6-DHT 7 days before neurectomy and (3) an almost complete suppression of autotomy in the rats injected with 5,6-DHT 14 days before neurectomy. These results revealed that the expression of autotomy in rats can be modulated by interfering with spinal cord serotonergic activity and suggest new possible avenues for the treatment of certain specific pain diseases, such a phantom limb pain, by using selective agents capable of activating or blocking spinal cord serotonergic receptor subtypes.     

Electrophysiological and functional effects of shock waves on the sciatic nerve of rats

Extracorporeal shockwave therapy (ESWT) has been applied in lithotripsy and treatments of musculoskeletal disorders over the past decade, but its effects on peripheral nerves remain unclear. This study investigated the short-term effects of shockwaves on the sciatic nerve of rats. The nerves were surgically exposed and then stimulated with shockwaves at three intensities. We evaluated the motor nerve conduction velocity (MNCV) of treated sciatic nerves before, immediately after (day 0) and at 1, 4, 7 and 14 d after shockwave treatment. Two functional tests-the sciatic functional index and the withdrawal reflex latency-were evaluated before and at 1, 4, 7 and 14 d after shockwave application. The rats were sacrificed on days 0, 1, 4, 7 and 14 for morphologic observation. The degassed treatment group received high-intensity shockwave treatment using degassed normal saline as the contact medium, and MNCV was measured before and on days 0, 1, 4, 7 and 14. The sham group received the same procedure as the treatment groups (i.e., the surgical operation to expose the sciatic nerve) but with no shockwave treatment. The control group received no surgical operation or shockwave treatment. The results showed moderate decrease in the MNCV after shockwave treatment and damage to the myelin sheath of large-diameter myelinated fibers. The effect was largest (reduction to 60.9% of baseline MNCV) and of longest duration (7 to 14 d) in the high-intensity group. There were no significant changes in functional tests. These results indicated that direct application of shockwaves can induce reversible segmental demyelination in large-diameter fibers, with the electrophysiological changes being positively correlated with the intensity of the shockwaves.     

A comparative study of monoaminergic involvement in the antinociceptive action of E-2078, morphine and U-50,488E.

E-2078 ([N-methyl-Tyr1, N-alpha-methyl-Arg7, D-Leu8]dynorphin A(1-8) ethylamide) is a systemically active dynorphin analog. We examined monoaminergic involvement in the antinociceptive action of E-2078 compared with morphine and U-50,488E (trans-3,4-dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)- benzene-acetamide monohydrochloride). The antinociceptive effect of these drugs was determined using the mouse tail-flick test after depletion of norepinephrine or 5-HT by pretreatment with various neurotoxins. Reserpine (i.p.) depleted both norepinephrine and 5-HT. Selective degeneration of noradrenergic nerves was induced by N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4, i.p.) or intrathecal (i.t.) 6-hydroxydopamine (6-OHDA), whereas 5-HT was depleted by p-chlorophenylalanine (PCPA, i.p.) or 5,7-dihydroxytryptamine (5,7-DHT, i.t.). The antinociceptive action of E-2078 administered s.c. was significantly attenuated in mice treated with reserpine, DSP-4, 6-OHDA, PCPA or 5,7-DHT compared with that in non-neurotoxin animals. Antinociception induced by intracerebroventricular (i.c.v.) and i.t. injection of E-2078 was reversed by reserpine, DSP-4 or PCPA. The antinociceptive action of morphine (s.c.) was attenuated by reserpine, DSP-4, 6-OHDA and PCPA, but not by 5,7-DHT. Antinociception produced by i.c.v. morphine was antagonized by reserpine, DSP-4 and PCPA. In contrast, morphine given i.t. was not affected by these neurotoxins. U-50,488E (s.c.)-induced antinociception was attenuated by reserpine, DSP-4, 6-OHDA, PCPA and 5,7-DHT. Intracerebroventricular U-50,488E was antagonized by reserpine, DSP-4 and PCPA, whereas i.t. U-50,488E was reversed only by reserpine and PCPA(ABSTRACT TRUNCATED AT 250 WORDS)     

Potentiation of morphine antinociception by monoamine reuptake inhibitors in the rat spinal cord

Potentiation of the antinociceptive effects of morphine by the tricyclic antidepressants was assayed in awake restrained rats using the tail-flick test. Intrathecally administered amitriptyline, desipramine or sertraline at doses that had no effect themselves (25-30 micrograms) potentiated a subthreshold parenteral dose of morphine (0.5 mg/kg). The morphine potentiating effect of amitriptyline was prevented by prior administration of parachlorophenylalanine (PCPA). This effect of PCPA was not restored by 5-hydroxytryptophan (5-HTP) but was restored when the animals were left for 14 days to replete. The morphine potentiating effects of amitriptyline, desipramine and sertraline were blocked by intrathecal administration of low doses of the serotonin antagonist methysergide and the alpha-adrenergic antagonists yohimbine and phentolamine but not by the beta-adrenergic antagonist propranolol. The results are consistent with the hypothesis that the potentiation of morphine's antinociceptive effect by tricyclic antidepressants depends on activation of both spinopetal serotonin and adrenergic neurons.     

Fading: a new cytofluorimetric measure quantifying serotonin in the presence of catecholamines at the cellular level in brain.

A new histopharmacological approach to the study of serotonergic neurons is described. Using a computerized microspectrofluorimeter, we demonstrated that a measure of fluorescence fading reliably detects changes in serotonin even in the presence of catecholamines. This "fading measure" was validated with model droplets containing serotonin, norepinephrine or mixtures of the two, and with in vivo studies using pargyline to increase brain serotonin. After various doses of pargyline, a correlation of 0.927 was found between our intraperikaryal fading measure and a standard florimetric measure of serotonin in dissected samples of the raphe. Further experiments were designed to test the feasibility of the cytofluorimetric fading measure for quantifying differential changes in serotonin in intraperikaryal and extraperikaryal regions. Both LSD (75 and 150 microgram/kg) and a low dose of the monoamine oxidase inhibitor pargyline (25 mg/kg) increased intraperikaryal serotonin without affecting extraperikaryal fluorescence. Conversely, the serotonin reuptake inhibitor fluoxetine produced the opposite effect of selectively increasing extraperikaryal serotonin. Another inhibitor of serotonin reuptake, chlorimipramine, which also blocks reuptake of norepinephrine in vivo, markedly increased both intraperikaryal and extraperikaryal serotonin. These results confirm the utility of cytofluorimetric measures of serotonin within raphe cell bodies from untreated rats, and indicate that changes in the intracellular and extracellular concentrations of serotonin can be differentiated.     

Central presynaptic receptors

Experiments on two different inhibitory presynaptic receptor systems are presented. 1. Superfused and electrically stimulated brain slices are a widely used experimental model to study the release of noradrenaline and its modulation by inhibitory alpha-2 adrenoceptors. By using a minisuperfusion chamber we succeeded in studying the simplest case of autoinhibition, i.e. the release of transmitter induced by a single pulse and two consecutive pulses, respectively. When electrical stimulation is performed using a single pulse, no autoinhibition is possible, whereas following stimulation with two pulses the transmitter released by the first pulse will inhibit the effect of the second pulse. By systemically varying the time interval between the two pulses the minimal time requirement for development of autoinhibition was determined to be 100 ms. Short pulse trains of high frequency such as 4 pulses within 30 ms circumvent autoinhibition and cause inhibition-free release by each applied pulse. The release of transmitter evoked in this way is not only free from autoinhibition but, in addition, easily measurable, which makes this method of stimulation very suitable for analyses at presynaptic receptors. By using this approach it became possible, for the first time, to determine dissociation constants of antagonists and agonists at the central presynaptic alpha-2 adrenoceptor without the distortion introduced by autoinhibition occurring during release. 2. There is a substantial body of evidence for a role of medullary serotonergic nerve cells in the regulation of blood pressure and heart rate. It is hypothesized that the serotonergic neurons project to the thoracic spinal cord exerting a tonic excitatory influence on presynaptic sympathetic neurons of the intermediolateral cell column. Experiments were performed in pentobarbital anaesthetized rats to reduce this excitatory tone by activating inhibitory autoreceptors which are located on the perikarya and dendrites on the serotonergic cells and which have been shown to belong to the 5-HT1A subtype. Local stereotactic injection of the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) caused a decrease in mean arterial blood pressure (MAP) and heart rate (HR). The effects were blocked by pretreatment of the animals with the 5-HT1A antagonist spiroxatrine. Moreover, neurochemical lesioning of serotonergic neurons by intracisternal injection of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) abolished the effects of 8-OH-DPAT. Bilateral intraspinal injection of 5,7-DHT, which interrupts the medullo-spinal serotonergic pathway, markedly attenuated the effects of local intramedullary injection of 8-OH-DPAT.(ABSTRACT TRUNCATED AT 400 WORDS)     

Analgesic antidepressants promote the responsiveness of locus coeruleus neurons to noxious stimulation: implications for neuropathic pain

Antidepressants that block the reuptake of noradrenaline and/or serotonin are among the first-line treatments for neuropathic pain, although the mechanisms underlying this analgesia remain unclear. The noradrenergic locus coeruleus is an essential element of both the ascending and descending pain modulator systems regulated by these antidepressants. Hence, we investigated the effect of analgesic antidepressants on locus coeruleus activity in Sprague-Dawley rats subjected to chronic constriction injury (CCI), a model of neuropathic pain. In vivo extracellular recordings of locus coeruleus revealed that CCI did not modify the basal tonic activity of this nucleus, although its sensory-evoked response to noxious stimuli was significantly altered. Under normal conditions, noxious stimulation evokes an early response, corresponding to the activation of myelinated A fibers, which is followed by an inhibitory period and a subsequent late capsaicin-sensitive response, consistent with the activation of unmyelinated C fibers. CCI provokes an enhanced excitatory early response in the animals and the loss of the late response. Antidepressant administration over 7 days (desipramine, 10mg/kg/day or duloxetine, 5mg/kg/day, delivered by osmotic minipumps) decreased the excitatory firing rate of the early response in the CCI group. Moreover, in all animals, these antidepressants reduced the inhibitory period and augmented the late response. We propose that N-methyl-d-aspartate and alpha-2-adrenoceptors are involved in the analgesic effect of antidepressants. Antidepressant-mediated changes were correlated with behavioral effects indicative of analgesia in healthy and neuropathic rats.     

Serotonin depletion potentiates gastric secretory and motor responses to vagal but not peripheral gastric stimulants

Vagal stimulation is known to release gastrointestinal serotonin. The effect of depletion of serotonin stores on vagally stimulated gastric acid secretion and motility was studied in rats. Pretreatment of rats with parachlorophenylalanine (p-CPA) did not alter basal gastric acid and serotonin secretion but produced a 57% reduction in the intraluminal gastric release of serotonin and a 43 to 100% potentiation of the gastric acid secretory response elicited by intracisternal injection of the stable thyrotropin-releasing hormone analog, RX 77368, in conscious pylorus-ligated rats or urethane-anesthetized rats with an acute gastric fistula. Dose-response studies revealed that the gastric acid secretion induced by submaximal but not high doses of RX 77368 was elevated significantly by p-CPA pretreatment. p-CPA also enhanced the gastric acid output produced by submaximal, but not high doses of the vagal stimulant baclofen, [beta-(p-chlorophenyl)-gamma-aminobutyric acid]. In contrast, p-CPA pretreatment had no effect on gastric acid secretion stimulated by bethanechol, histamine or pentagastrin. Selective depletion of central serotonin stores by pretreatment with the neurotoxin 5,7-dihydroxytryptamine given alone, or combined with parachloroamphetamine did not alter RX 77368-stimulated gastric acid secretion. In addition, gastric contractility stimulated by intracisternal injection of RX 77368 was significantly enhanced by p-CPA but not by 5,7-dihydroxytryptamine pretreatment, whereas the contractile response to carbachol was not altered by p-CPA pretreatment. These results suggest that depletion of peripheral but not central serotonergic stores potentiate gastric acid secretion and contractility induced by vagally, but not peripherally acting gastric stimulants. Thus, peripheral serotonin may exert an inhibitory tone on vagally stimulated gastric acid secretion and motility in the rat.     

急性一氧化碳中毒大鼠迟发性神经元损伤与记忆功能改变

目的：观察急性一氧化碳中毒（CO）中毒大鼠脑内迟发神经元损伤和记忆功能的改变，探讨CO中毒导致的迟发性神经损伤和记忆功能改变两者之间的关系。方法：SD大鼠暴露在空气或CO（3451ppm）60min，暴露后1、3、5、7d处死大鼠，大脑经处理制成石蜡切片，行HE染色以观察脑内病理损伤程度，通过被动回辟跳台实验评估CO中毒对大鼠记忆保持巩固能力的影响。结果：大鼠海马CA1区发生迟发性神经元损伤。锥体细胞从CO暴露后第3天开始显著减少，被动回避跳台实验中，CO中毒大鼠从染毒后第5天开始跳台潜伏期明显缩短，记忆力部分丧失，出现迟发性健忘症（delayed amnesia,DA)。结论：急性CO中毒导致迟发性神经元损伤，迟发性神经元损伤引起DA。