Effects of ipsapirone, a 5-HT1A agonist, on sleep/wakefulness cycles: probable post-synaptic action

The effects of ipsapirone, a ligand of the 5-HT1A receptors and a new potential anxiolytic, on sleep/wakefulness regulation were examined in the rat. Injected i.p. at 1, 3 and 5 mg kg-1, this compound induced a dose-dependent reduction of paradoxical sleep for 2 to 4 hours, followed, at a dose of 5 mg kg-1, by a secondary rebound. The other states of vigilance were not modified, except at the latter dose where the amounts of wakefulness were enhanced initially and decreased secondarily, while those of SWS were enhanced from 2 to 4 hours post-treatment. The effects of ipsapirone (3 mg kg-1) persisted after infusion of the neurotoxin 5,7-dihydroxytryptamine into the dorsal raphe nucleus which induced the sub-total destruction of the serotoninergic system. Thus, the action of the 5-HT1A agonist ipsapirone on sleep/wakefulness cycles probably involves the stimulation of the post-synaptic 5-HT1A receptors.     

A semi-quantitative atlas of 5-hydroxytryptamine-1 receptors in the primate brain

The regional distribution of 5-hydroxytryptamine-1 receptors in the primate brain was studied by semi-quantitative autoradiographic analysis of tritiated ligand binding. Areas showing the highest density of 5-hydroxytryptamine-1 receptors (greater than 200 fmol [3H]5-hydroxytryptamine bound per mg tissue), included the cerebral cortex (laminae I-II), claustrum, posterior cell group of the basal nucleus of Meynert, the infracommissural part of the globus pallidus, cortical amygdaloid nucleus, hippocampal formation (CA1-subiculum region, the anterior CA2, CA3 and CA4 regions and the molecular layer of the dentate gyrus), thalamic nuclei (parafascicular, parataenial, paraventricular and superior central lateral nuclei), substantia nigra pars reticulata, dorsal raphe nucleus and choroid plexus. The distribution of 5-hydroxytryptamine-1 receptors is compared to the distribution of both 5-hydroxytryptamine receptors and terminal fields of serotonergic projections as previously described in subprimates.     

Glucocorticoid receptor-dependent desensitization of 5-HT1A autoreceptors by sleep deprivation: studies in GR-i transgenic mice

Sleep deprivation for one night induces mood improvement in depressed patients, an action that probably involves the serotonergic (5-HT) system. In animals, sleep deprivation and pharmacologic treatment with antidepressants exert similar effects on 5-HT neurotransmission, notably functional desensitization of 5-HT1A autoreceptors located on 5-HT neurons in the dorsal raphe nucleus (DRN). However, in stressful conditions, corticosterone can also induce a desensitization of these autoreceptors. STUDY OBJECTIVES: To investigate the mechanisms of this adaptation during sleep deprivation and the possible involvement of corticosterone, we studied the effects of an 18-hour sleep deprivation, by forced locomotion, on 5-HT1A receptor-mediated firing response of DRN 5-HT neurons in transgenic mice with impaired glucocorticoid-receptor expression (GR-i) and in wild-type animals. We also examined the effects of chronic treatment with the antidepressant drug fluoxetine in the same paradigm. MEASUREMENTS AND RESULTS: In both wild-type and GR-i mice, the 18-hour sleep deprivation or fluoxetine treatment had no effect on the spontaneous firing of 5-HT neurons recorded under anesthesia. However, sleep deprivation decreased the potency of the 5-HT1A agonist 8-OH-DPAT to inhibit 5-HT neuronal firing in wild-type mice, whereas it had no effect in GR-i animals. Conversely, after chronic fluoxetine treatment, the induced reduction of this 5-HT1A autoreceptor-driven response was of larger amplitude in GR-i than in wild-type mice. CONCLUSIONS: These data suggest that glucocorticoid-receptor activation by corticosterone participates in the antidepressant-like adaptive changes in 5-HT1A autoreceptors in sleep-deprived mice. On the other hand, GR-i animals exhibited enhanced 5-HT1A autoreceptor desensitization induced by fluoxetine, in line with data in other animal models of depression.     

K+ channel and 5-hydroxytryptamine1A autoreceptor interactions in the rat dorsal raphe nucleus: an in vitro electrophysiological study.

Extracellular recordings were made from serotonergic neurons of the rat dorsal raphe nucleus in a slice preparation. In the presence of phenylephrine (3 microM) to restore the pacemaker activity of otherwise silent serotonergic neurons, superfusion with the 5-hydroxytryptamine1A agonist ipsapirone depressed the firing of these neurons with an IC50 of approximately 50 nM. Complete inhibition was achieved with 100-300 nM of the drug. Concomitant superfusion with the 5-hydroxytryptamine1A antagonists spiperone (100 nM) or propranolol (10 microM) markedly reduced the inhibitory effect of ipsapirone (100 nM). Superfusion with K+ channel blockers such as apamin (50-100 nM), charybdotoxin (100 nM) or Ba2+ (1 mM) did not induce any changes in the electrical activity of serotonergic neurons. However, 4-aminopyridine (0.1-1 mM) disrupted the regularity of their discharge without affecting the mean firing rate. The ipsapirone-induced inhibition was unchanged by apamin and charybdotoxin, but was markedly reduced by Ba2+ and 4-aminopyridine. Thus the IC50 of ipsapirone was shifted to approximately 150 nM in the presence of 1 mM of 4-aminopyridine. These results indicate that, in serotonergic neurons within the dorsal raphe nucleus, the K+ channel opened through the stimulation of 5-hydroxytryptamine1A autoreceptors is 4-aminopyridine-sensitive.     

Effects of beta-amyloid protein on serotoninergic, noradrenergic, and cholinergic markers in neurons of the pontomesencephalic tegmentum in the rat

The effects on serotoninergic, noradrenergic and cholinergic markers on neurons of the pontomesencephalic tegmentum nuclei were studied in rats following local administration of fibrillar beta-amyloid peptide (Abeta1-40) into the left retrosplenial cortex. Focal deposition of Abeta in the retrosplenial cortex resulted in a loss of serotoninergic neurons in the dorsal and median raphe nuclei. The dorsal raphe nucleus showed a statistically significant reduction of 31.7% in the number of serotoninergic neurons and a decrease (up to 17.38%) in neuronal density in comparison with the same parameters in uninjected controls. A statistically significant reduction of 50.3%, together with a significant decrease of 53.94% in the density of serotoninergic neurons, was also observed in the median raphe nucleus as compared with control animals. Furthermore, a significant reduction of 35.07% in the number of noradrenergic neurons as well as a statistically significant decrease of 56.55% in the density of dopamine-beta-hydroxylase-immunoreactive neurons were also found in the locus coeruleus as compared with the corresponding hemisphere in uninjected controls. By contrast, a reduction of 24.37% in the number of choline acetyltransferase-positive neurons and a slight decrease (up to 22.28%) in the density of cholinergic neurons, which were not statistically significant, was observed in the laterodorsal tegmental nucleus in comparison with the same parameters in control animals. These results show that three different neurochemically defined populations of neurons in the pontomesencephalic tegmentum are affected by the neurotoxicity of Abeta in vivo and that Abeta might indirectly affect serotoninergic, noradrenergic and cholinergic innervation in the retrosplenial cortex.     

Immunohistochemical and behaviour pharmacological analysis of rats inoculated intranasally with vesicular stomatitis virus

A temperature-sensitive mutant of vesicular stomatitis virus was inoculated intranasally into infant Sprague Dawley rats aged 9 to 17 days. Rats receiving the virus at 9 days of age had an extensive spread of infection throughout the brain and the animals died after a few days. Rats inoculated at day 11 postnatally survived and the infection was limited to the olfactory pathways, hypothalamus, diagonal bands and the anterior raphe nuclei. Stereological measurements showed that the volume of infected neurons constituted 67 ± 10% of the total neuronal volume in the dorsal raphe nucleus. Double-labelling experiments revealed that both 5-hydroxytryptamine- and substance P-immunoreactive neurons contained the virus antigen. The motor stimulant effect of amphetamine was studied at 3 months post infection. The increase in amphetamine-induced frequency and duration of rearing was significantly attenuated in infected rats and the amphetamine-induced locomotion was slightly reduced.     

5-HT1A receptor activation counteracts c-Fos immunoreactivity induced in serotonin neurons of the raphe nuclei after immobilization stress in the male rat

The serotoninergic system and the 5-HT_1A receptors have been involved in the brain response to acute stress. The aim of our study was evaluate the role of the 5-HT_1A receptors in serotoninergic cells of rostral and caudal raphe nuclei under acute immobilization in rats. Double immunocytochemical staining of 5-hydroxy-tryptamine and c-Fos protein and stereology techniques were used to study the specific cell activation in the raphe nuclei neurons in five groups (control group, immobilization group (immobilization lasting 1 h), DPAT group (8-OH-DPAT 0.3 mg/kg, s.c.), DPAT + IMMO group (8-OH-DPAT 0.3 mg/kg, s.c., 30′ prior acute immobilization) and WAY + DPAT + IMMO group (WAY-100635 0.3 mg/kg, s.c. and 8-OH-DPAT 0.3 mg/kg, s.c., 45′ and 30′, respectively, before immobilization). Our results showed an increase in the number of c-Fos immunoreactive nuclei in serotoninergic cells in both dorsal and median raphe nuclei in the immobilized group. The 8-OH-DPAT pretreatment counteracted the excitatory effects of the acute immobilization in these brain regions. In addition, WAY-100635 administration reduced the effect of 8-OH-DPAT injection, suggesting a selective 5-HT_1A receptor role. Raphe pallidus and raphe obscurus did not show any differences among experimental groups. We suggest that somatodendritic 5-HT_1A receptors in rostral raphe nuclei may play a crucial role in both mediating the consequences of uncontrollable stress and the possible beneficial effects of treatment with 5-HT_1A receptor agonists.     

Effect of thalamic parafascicularis nucleus stimulation in regulation of serotoninergic transmission in the cat caudate nucleus: involvement of autoreceptors in the dorsalis raphe nucleus

The mechanisms involved in parafascicularis nucleus control on serotoninergic neurons projecting into the caudate nucleus were investigated in "encéphale-isole" cats. The effects of unilateral stimulation of the parafascicularis nucleus on the release of newly synthesized [3H]serotonin were simultaneously determined in the ipsilateral caudate nucleus and the dorsalis raphe nucleus using push-pull cannulae. The actions of various pharmacological treatments performed either in the caudate nucleus or in the dorsalis raphe nucleus were also examined. The electrical or chemical stimulation of the parafascicularis nucleus induced a decrease in striatal [3H]serotonin release and an increase in [3H]serotonin release in the dorsalis raphe nucleus. The blockade of cholinergic (mecamylamine) and glutamatergic (PK 26124) transmissions at the striatal level did not modify the thalamic stimulation-induced effect on serotonin release in the caudate nucleus or in the dorsalis raphe nucleus. However, a decrease induced by parafascicularis nucleus stimulation in serotonin release in the caudate nucleus could not be observed when the autoreceptors present on serotoninergic nerve cell bodies localized in the dorsalis raphe nucleus were blocked by a methiothepin perfusion within the nucleus. These results indicate that the parafascicularis nucleus controls striatal serotonin transmission by inducing changes in the nerve activity of serotoninergic neurons in the dorsalis raphe nucleus via somatodendritic serotonin release and autoreceptors.     

Indirect effects of apomorphine on serotoninergic neurons in rats

We have studied the effects of apomorphine on central serotoninergic system by using fluorescence histochemistry and high performance liquid chromatography coupled with electrochemical detection. Apomorphine has been shown selectively to elevate intracellular serotonin fluorescence in dorsal raphe neurons and serotonin and/or 5-hydroxyindoleacetic acid concentrations in the major terminal area of the dorsal raphe, the striatum. Apomorphine has no effect on serotonin neurons in the median raphe or its corresponding projection site, the hippocampus. In the present study, intraventricular 6-hydroxydopamine, when administered together with desimipramine and pargyline, antagonized the apomorphine-induced elevations of serotonin fluorescence in the dorsal raphe and serotonin and 5-hydroxyindoleacetic acid levels in the striatum. This antagonism was found when 6-hydroxydopamine was given either 3 or 10 days before 3 mg/kg apomorphine. Apomorphine also elevated extraperikaryal serotonin fluorescence and catecholamine fluorescence in the dorsal raphe selectively and these effects were also blocked by 6-hydroxydopamine. Additionally, 6-hydroxydopamine accelerated striatal serotonin turnover when it was given 10 days prior to death. This phenomenon probably reflects some compensatory change of mesostriatal serotonin neurons in response to the prolonged depletion of brain dopamine. The above results suggest that the observed effects of apomorphine on the serotoninergic system are mediated indirectly through dopaminergic neurons and that postsynaptic dopamine receptors are probably not responsible for apomorphine's effects.     

Distinguishing characteristics of serotonin and non-serotonin-containing cells in the dorsal raphe nucleus: electrophysiological and immunohistochemical studies

The membrane properties and receptor-mediated responses of rat dorsal raphe nucleus neurons were measured using intracellular recording techniques in a slice preparation. After each experiment, the recorded neuron was filled with neurobiotin and immunohistochemically identified as 5-hydroxytryptamine (5-HT)-immunopositive or 5-HT-immunonegative. The cellular characteristics of all recorded neurons conformed to previously determined classic properties of serotonergic dorsal raphe nucleus neurons: slow, rhythmic activity in spontaneously active cells, broad action potential and large afterhyperpolarization potential. Two electrophysiological characteristics were identified that distinguished 5-HT from non-5-HT-containing cells in this study. In 5-HT-immunopositive cells, the initial phase of the afterhyperpolarization potential was gradual (tau=7.3+/-1.9) and in 5-HT-immunonegative cells it was abrupt (tau=1.8+/-0.6). In addition, 5-HT-immunopositive cells had a shorter membrane time constant (tau=21.4+/-4.4) than 5-HT-immunonegative cells (tau=33.5+/-4.2). Interestingly, almost all recorded neurons were hyperpolarized in response to stimulation of the inhibitory 5-HT(1A) receptor. These results suggested that 5-HT(1A) receptors are present on non-5-HT as well as 5-HT neurons. This was confirmed by immunohistochemistry showing that although the majority of 5-HT-immunopositive cells in the dorsal raphe nucleus were double-labeled for 5-HT(1A) receptor-IR, a small but significant population of 5-HT-immunonegative cells expressed the 5-HT(1A) receptor. These results underscore the heterogeneous nature of the dorsal raphe nucleus and highlight two membrane properties that may better distinguish 5-HT from non-5-HT cells than those typically reported in the literature. In addition, these results present electrophysiological and anatomical evidence for the presence of 5-HT(1A) receptors on non-5-HT neurons in the dorsal raphe nucleus.     

多重脑震荡大鼠脑干中缝核团内5-羟色胺免疫阳性表达变化的研究

为了探讨多重脑震荡(multiple cerebral concussion,MCC)后大鼠中缝核团内5-羟色胺(5-HT)能神经元的变化规律,本实验采用自制单摆式机械打击装置复制MCC大鼠模型,研究伤后大鼠脑干中缝核团内5-HT及5-HT合成过程中的限速酶-色胺酸羟化酶(TPH)的表达。将56只大鼠随机分为7组:对照组、伤后1、2、4、8、16和24d组(n=8)。用免疫组织化学染色技术及图像分析法定量分析伤后大鼠脑干中缝核团内5-HT和TPH的表达变化。结果显示:(1)TPH免疫反应阳性产物在中缝背核、正中中缝核的表达在伤后2d时达到高峰,与正常对照组相比有显著性差异(P<0.05);中缝大核和中缝苍白核分别以伤后1d组和4d组阳性反应最强;(2)5-HT免疫反应阳性产物在中缝背核、正中中缝核的表达也在伤后2d时达到高峰,16、24d组基本恢复至正常水平;而中缝大核和中缝苍白核内5-HT的免疫反应性在各损伤组与正常对照组之间均无显著性差异(P>0.05)。以上结果表明,多重脑震荡后中缝核团内TPH和5-HT的表达增高,这为研究5-HT对MCC后认知障碍的影响提供了形态学依据。     

Effect of stimulation of brain serotonergic system on mouse-killing behavior in rats

Mouse-killing behavior was induced in male Wistar rats due to 6 weeks isolation. The selective stimulation of the dorsal raphe nucleus markedly reduced muricide behavior. On the other hand rats with electrodes situated outside of the dorsal raphe (within the substantia grisea centralis) showed no changes in muricide behavior when compared with control (non-stimulated) animals. Pharmacological stimulation of whole serotonergic brain neurons by quipazine, an agonist of serotonin receptors, and compound CGP 6085-A, a selective inhibitor of serotonin uptake, strongly decreased mouse-killing behavior. Our data indicates that serotonergic neurons inhibit predatory aggression in rats.     

Release of endogenous 5-hydroxytryptamine in rat ventral hippocampus evoked by electrical stimulation of the dorsal raphe nucleus as detected by microdialysis: sensitivity to tetrodotoxin, calcium and calcium antagonists

We have utilized the brain microdialysis technique in an attempt to measure excitation-secretion coupled release of endogenous 5-hydroxytryptamine in rat brain in vivo and investigated the pharmacology of the voltage-sensitive calcium channel involved in this process. All experiments were carried out using chloral hydrate anaesthetized rats. Ascending serotoninergic neurons were electrically stimulated using an electrode implanted into the dorsal raphe nucleus. A dialysis probe was implanted into the ventral hippocampus and continuously perfused with artificial cerebrospinal fluid containing the selective 5-hydroxytryptamine uptake inhibitor citalopram (1 microM). Twenty-minute perfusates were analysed for endogenous 5-hydroxytryptamine using high performance liquid chromatography with electrochemical detection. Electrical stimulation (cathodal monophasic 1 ms pulses, 300 microA, 2-10 Hz) of the dorsal raphe nucleus for 20 min induced an immediate release of 5-hydroxytryptamine which lasted for the duration of the stimulus and was frequency-dependent. The calculated amount of 5-hydroxytryptamine release per electrical impulse was constant over the frequency range used. Addition of tetrodotoxin (10 microM) to, or omission of calcium from, the perfusion medium reduced the spontaneous output of 5-hydroxytryptamine by 60-70% and caused a near complete inhibition of the effect of low frequency (3 Hz) electrical stimulation of the dorsal raphe nucleus. Local perfusion with cadmium (30 and 300 microM), which is reported to antagonize both N- and L-type voltage-sensitive calcium channels, also caused a pronounced decrease of basal output of 5-hydroxytryptamine and a marked, but not complete inhibition of the effect of nerve stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dual effects of spinally delivered 8-bromo-cyclic guanosine mono-phosphate (8-bromo-cGMP) in formalin-induced nociception in rats

Interactions are known to occur in the brain between serotonin (5-HT) and substance P (SP). To investigate whether SP can directly influence serotonergic neurons, double immunohistochemical labelings were performed on rat brain sections with NK1 or NK3 affinity-purified antibodies and a 5-HT monoclonal antibody. It was found that the vast majority of serotonergic cell bodies do not colocalize NK1 or NK3 labeling. Only in the central linear nucleus and ventral part of the dorsal raphe nucleus were a few serotonergic neurons double-labeled for NK1 receptors (15 and 0.8% of serotonergic neurons, respectively). It is suggested that serotonergic neurons are not major direct targets for SP in the rat brain.     

Glutamergic transmission of neuronal responses to carbachol in rat dorsal cochlear nucleus slices

This study found that glutamate receptor antagonists block the excitatory effects of carbachol, a cholinergic agonist, on bursting neurons in the dorsal cochlear nucleus of rat brain slices. Among antagonists for glutamate receptor subtypes, those for non-N-methyl-D-aspartate ionotropic glutamate receptors were more potent than those for N-methyl-D-aspartate receptors. The glutamate receptor antagonists did not block the effects of carbachol on regularly firing neurons in the dorsal cochlear nucleus of the same slices. Antagonists for GABA or glycine receptors did not alter the effects of carbachol on bursting neurons. Effects of carbachol on bursting activity could be mimicked by application of glutamate or its agonist, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, whose effects were not blocked by synaptic blockade. During carbachol application, increased release of glutamate and glycine from the dorsal cochlear nucleus part of brain slices was measured using high-performance liquid chromatography. Release of other amino acids showed no significant change. The results suggest that, in rat dorsal cochlear nucleus, cholinergic effects on regular and bursting spontaneous firing occur through different mechanisms. Cholinergic effects on regular neurons (which include fusiform cells) are direct, through muscarinic receptors. Cholinergic effects on bursting neurons (which include cartwheel cells) are indirect and involve glutamatergic neurotransmission, mostly via non-N-methyl-D-aspartate ionotropic receptors. The granule cell-parallel fiber pathway may be involved in this glutamatergic transmission.     

Structure and neurochemistry of the raphe nuclei in teleosts

NO is an important modulator of the brain serotoninergic function as it is co-localized with 5-hydroxytryptamine in the neurons of raphe nuclei in the brain of all the higher vertebrates. In the brain of fishes this property of NO is poorly studied. The purpose of this work was to study the neuromorphological peculiarities and distribution of NO-ergic neurons in raphe nuclei of Teleost species--carps and perch-like fishes. Using standard neuromorphological methods and the histochemical reaction demonstrating NADPH-diaphorase activity, the comparative analysis of NO-ergic neurons was performed and their localization was studied in the raphe complex of five teleost species. Within the raphe complex in fishes two main nuclei--nucleus raphe superior and nucleus raphe inferior were demonstrated. NADPH-diaphorase activity was observed in practically all the neurons of raphe niclei in species examined. Some intergroup morpho-adaptational neurochemical differences were found in the of the ventro-medial reticular formation adjacent to the raphe complex.     

Transneuronal labelling of nerve cells in the CNS of female rat from the mammary gland by viral tracing technique.

Using the viral transneuronal tracing technique, the cell groups in the CNS transneuronally connected with the female mammary gland were detected. Lactating and non-lactating female rats were infected with pseudorabies virus injected into the mammary gland. The other group of animals was subjected to virus injection into the skin of the back. Four days after virus injection, infected neurons detected by immunocytochemistry, were present in the dorsal root ganglia ipsilateral to inoculation and in the intermediolateral cell column of the spinal cord. In addition, a few labelled cells could be detected in the dorsal horn and in the central autonomic nucleus (lamina X) of the spinal cord. At this survival time several brain stem nuclei including the A5 noradrenergic cell group, the caudal raphe nuclei (raphe obscurus, raphe pallidus, raphe magnus), the A1/C1 noradrenergic and adrenergic cell group, the nucleus of the solitary tract, the area postrema, the gigantocellular reticular nucleus, and the locus coeruleus contained virus-infected neurons. In some animals, additional cell groups, among others the periaqueductal gray and the red nucleus displayed labelling. In the diencephalon, a significant number of virus-infected neurons could be detected in the hypothalamic paraventricular nucleus. In most cases, virus-labelled neurons were present also in the lateral hypothalamus, in the retrochiasmatic area, and in the anterior hypothalamus. In the telencephalon, in some animals a few virus-infected neurons could be found in the preoptic area, in the bed nucleus of the stria terminalis, in the central amygdala, and in the somatosensory cortex. At the longer (5 days) survival time each cell group mentioned displayed immunopositive neurons, and the number of infected cells increased. The pattern of labelling was similar in animals subjected to virus inoculation into the mammary gland and into the skin. The distribution and density of labelling was similar in lactating and non-lactating rats.The present findings provide the first morphological data on the localization of CNS structures connected with the preganglionic neurons of the sympathetic motor system innervating the mammary gland. It may be assumed that the structures found virus-infected belong to the neuronal circuitry involved in the control of the sympathetic motor innervation of the mammary gland.     

Behavioural deficits and serotonin depletion in adult rats after transient infant nasal viral infection.

Dysfunction of subcortical serotoninergic neurons has been implicated in some behaviour disturbances. The serotoninergic neurons in the dorsal and median raphe project widely in the brain. They innervate the olfactory bulbs and can be targets for exogenous agents attacking the olfactory epithelium and bulbs. We report here an injury to the serotoninergic neurons after intranasal infection in 12-day-old rats with a temperature-sensitive mutant of vesicular stomatitis virus. The brain infection was focal and transient. Viral antigens could no longer be detected 13-15 days after infection. In spite of this the animals, as adults, had a severe serotonin depletion in the cerebral cortex and hippocampus, and showed abnormal locomotor and explorative behaviour as well as learning deficits. The neocortex was histologically intact and parameters related to other neurotransmitters such as dopamine, noradrenaline, GABA and acetylcholine showed no marked changes. A relatively selective damage to serotoninergic nuclei as a result of virus neuroinvasion through a natural portal of entry, may constitute a new pathogenetic mechanism for cortical dysfunction and behavioural deficits.     

Chronic morphine increases GABA tone on serotonergic neurons of the dorsal raphe nucleus: association with an up-regulation of the cyclic AMP pathway

Major adaptations after chronic exposure to morphine include an up-regulation of the adenosine 3',5'-monophosphate pathway. Acute opioids, via mu-opioid receptors, disinhibit midbrain serotonergic neurons by suppressing inhibitory GABAergic transmission in the dorsal raphe nucleus and adjacent periaqueductal gray. This study examined whether chronic morphine induces a compensatory increase in GABA inputs to 5-hydroxytryptamine neurons and whether this was associated with an up-regulation of the adenosine 3',5'-monophosphate pathway. The firing rate of serotonergic neurons was reduced in brain slices from morphine-dependent rats, an effect reversed by the GABA(A) antagonist bicuculline. The reduction in firing rate was accompanied by an increased frequency of spontaneous GABAergic inhibitory postsynaptic currents, indicating increased GABA tone in the slice. The increase in GABA tone in brain slices from dependent rats was associated with increased induction of inhibitory postsynaptic currents by the adenylyl cyclase activator forskolin, suggesting an up-regulation of the adenosine 3',5'-monophosphate pathway. Indeed, chronic morphine increased levels of adenylyl cyclase VIII (but not of adenylyl cyclase I, III or V) immunoreactivity in the dorsal raphe nucleus area. Two adenosine 3',5'-monophosphate-mediated mechanisms for the increase in GABA tone were discerned. The first, which predominated when impulse-flow was blocked by tetrodotoxin, involves protein kinase A since it was sensitive to protein kinase A inhibitors. The second, seen when impulse-flow was intact (i.e. absence of tetrodotoxin), was insensitive to protein kinase A inhibitors but was suppressed by ZD7288, a blocker of hyperpolarizing-activated Ih channels which are directly activated by adenosine 3',5'-monophosphate. We conclude that chronic morphine induces an up-regulation of the adenosine 3',5'-monophosphate pathway in GABAergic inputs to serotonergic cells, resulting in an increase in spontaneous and impulse-flow dependent GABA release. These changes would lead to an increase in GABA tone and subsequently to the reported decrease in serotonergic activity during opiate withdrawal.     

豚鼠中脑中缝背核和中缝正中核5-羟色胺能神经元向视交叉上核的直接投射

实验借助HRP逆行定位和免疫细胞化学定性相结合的双重标记技术,探讨豚鼠中脑中缝背核和中缝正中核向下丘脑视交叉上核的直接投射.结果首次确定了:(1)中缝背核/中缝正中核内5-羟色胺和非5-羟色胺能神经元对视交叉上核的直接投射和分布特点;(2)投射神经元主要集中在中缝背核;(3)中缝背核对视交叉上核的投射神经元几乎均为5-羟色胺能,而中缝正中核中5-羟色胺和非5-羟色胺能投射神经元约各占半数.