Stimulation of pontine reticular formation suppresses firing of serotonergic neuronses in the dorsal raphe

Chronic fluoxetine treatment desensitizes serotoninergic inhibition of GABA inputs and the intrinsic excitability of dorsal raphe serotonin neurons

Dorsal raphe serotonin (5-hydroxytryptamine, 5-HT) neurons are spontaneously active and release 5-HT that is critical to normal brain function such mood and emotion. Serotonin reuptake inhibitors (SSRIs) increase the synaptic and extracellular 5-HT level and are effective in treating depression. Treatment of two weeks or longer is often required for SSRIs to exert clinical benefits. The cellular mechanism underlying this delay was not fully understood. Here we show that the GABAergic inputs inhibit the spike firing of raphe 5-HT neurons; this GABAergic regulation was reduced by 5-HT, which was prevented by G-protein-activated inwardly rectifying potassium (Girk) channel inhibitor tertiapin-Q, indicating a contribution of 5-HT activation of Girk channels in GABAergic presynaptic axon terminals. Equally important, after 14 days of treatment of fluoxetine, a widely used SSRI type antidepressant, this 5-HT inhibition of GABAergic inputs was substantially downregulated. Furthermore, the chronic fluoxetine treatment substantially downregulated the 5-HT activation of the inhibitory Girk current in 5-HT neurons. Taken together, our results suggest that chronic fluoxetine administration, by blocking 5-HT reuptake and hence increasing the extracellular 5-HT level, can downregulate the function of 5-HT1B receptors on the GABAergic afferent axon terminals synapsing onto 5-HT neurons, allowing extrinsic, behaviorally important GABA neurons to more effectively influence 5-HT neurons; simultaneously, chronic fluoxetine treatment also downregulate somatic 5-HT autoreceptor-activated Girk channel-mediated hyperpolarization and decrease in input resistance and intrinsic excitability, rendering 5-HT neurons resistant to autoinhibition and leading to increased 5-HT neuron activity, potentially contributing to the antidepressant effect of SSRIs.

LC-derived excitatory synaptic transmission to dorsal raphe serotonin neurons is inhibited by activation of alpha2-adrenergic receptors

In the central nervous system, noradrenaline transmission controls the degree to which we are awake, alert, and attentive. Aberrant noradrenaline transmission is associated with pathological forms of hyper- and hypo-arousal that present in numerous neuropsychiatric disorders often associated with dysfunction in serotonin transmission. In vivo, noradrenaline regulates the release of serotonin because noradrenergic input drives the serotonin neurons to fire action potentials via activation of excitatory α1-adrenergic receptors (α1-AR). Despite the critical influence of noradrenaline on the activity of dorsal raphe serotonin neurons, the source of noradrenergic afferents has not been resolved and the presynaptic mechanisms that regulate noradrenaline-dependent synaptic transmission have not been described. Using an acute brain slice preparation from male and female mice and electrophysiological recordings from dorsal raphe serotonin neurons, we found that selective optogenetic activation of locus coeruleus terminals in the dorsal raphe was sufficient to produce an α1-AR-mediated excitatory postsynaptic current (α1-AR-EPSC). Activation of inhibitory α2-adrenergic receptors (α2-AR) with UK-14,304 eliminated the α1-AR-EPSC via presynaptic inhibition of noradrenaline release, likely via inhibition of voltage-gated calcium channels. In a subset of serotonin neurons, activation of postsynaptic α2-AR produced an outward current through activation of GIRK potassium conductance. Further, in vivo activation of α2-AR by systemic administration of clonidine reduced the expression of c-fos in the dorsal raphe serotonin neurons, indicating reduced neural activity. Thus, α2-AR are critical regulators of serotonin neuron excitability.

A High-Tryptophan Diet Reduces Seizure-Induced Respiratory Arrest and Alters the Gut Microbiota in DBA/1 Mice

Background and Aims: Central 5-hydroxytryptamine (5-HT) defects are responsible for the occurrence of sudden unexpected death in epilepsy (SUDEP). The DBA/1 mouse is an animal model of SUDEP since the mouse exhibits audiogenic seizure-induced respiratory arrest (S-IRA). The synthesis of central 5-HT is closely related to the gut microbiota. Moreover, emerging studies suggest a possible role for the microbiota in mitigating seizure likelihood. Based on this, we aimed to explore the effect of a high-tryptophan diet (HTD) on SUDEP as well as the synthesis and metabolism of central 5-HT. Furthermore, we investigated the involvement of the gut microbiota in this process.

Methods: All DBA/1 mice were subjected to acoustic stimulation to induce seizures. Only those mice that exhibited S-IRA were randomly assigned to the normal diet (ND) group (n = 39) or HTD group (n = 53). After 1 month of dietary intervention, (1) S-IRA rates were evaluated, (2) the concentrations of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the plasma and brain were determined by ultra-high-pressure liquid chromatography, and (3) the fecal flora biodiversity and species composition were analyzed by 16S rDNA microbiota profiling.

Results: The S-IRA rate in DBA/1 mice was significantly reduced in the HTD group compared with that in the control group. HTD increased the levels of 5-HT and 5-HIAA in both the telencephalon and midbrain. HTD significantly elevated the species richness and diversity of the gut microbiota. Moreover, there was a significant difference in the gut microbiota composition between the two groups, and the intestinal flora was dominated by Proteobacteria and Actinobacteria after HTD.

Conclusions: HTD is efficient in lowering S-IRA rates and elevating the central 5-HT level in DBA/1 mice. The gut microbiota was altered after HTD intervention. The significant increase in Proteobacteria and Actinobacteria may be related to the SUDEP-protective effect of HTD. Our findings shed light on a candidate choice of dietary prevention for SUDEP.

Overexpression of O-GlcNAc by prostate cancer cells is significantly associated with poor prognosis of patients

BACKGROUND

O-linked β-N-acetylglucosamine (O-GlcNAc) is a glycan essential for fundamental cellular processes such as transcription/translation, nuclear transport, protein stability and protein-protein interactions. However, the role of O-GlcNAc in prostate cancer progression of patients remains poorly unknown. Here we investigated the clinicopathological significance of O-GlcNAc expression level in prostate cancer.

METHODS

O-GlcNAc expression level in prostate cancer cells was determined by immunohistochemistry of prostate biopsy specimens obtained from 56 patients later treated with hormone deprivation therapy comparing with adjacent normal prostate glands in the same sections. Overall survival was determined by the Kaplan-Meier and Cox proportional hazards methods with univariate and multivariate models. The effects of reduced O-GlcNAc expression level on proliferation and invasion of prostate cancer LNCaP cells were examined using small interfering RNA (siRNA) targeting O-GlcNAc transferase (OGT), the enzyme responsible for O-GlcNAc biosynthesis.

RESULTS

Defining cancer cells showing stronger cytoplasmic staining than normal prostate glands as overexpression of O-GlcNAc, 39% of prostate cancer patients were categorized as overexpression. The Kaplan-Meier and Cox proportional hazards methods with univariate model analysis revealed that O-GlcNAc overexpression was associated with overall survival (P=0.0012 for the Kaplan-Meier and P=0.0021 for Cox univariate hazard model analysis). Furthermore, O-GlcNAc was the only item in which a significant difference was observed at overall survival by multivariate analysis (P=0.0475). Finally, siRNA-mediated OGT knockdown in LNCaP cells resulted in decreased expression of O-GlcNAc and promoted decreased proliferation and tumor cell invasion compared with control siRNA-transfected LNCaP cells.

CONCLUSIONS

These results indicate that O-GlcNAc expression level in prostate cancer cells is associated with poor prognosis of prostate cancer patients and likely enhances tumor cell proliferation and invasion.

Evolving concepts of sleep cycle generation: From brain centers to neuronal populations

As neurophysiological investigations of sleep cycle control have provided an increasingly detailed picture of events at the cellular level, the concept that the sleep cycle is generated by the interaction of multiple, anatomically distributed sets of neurons has gradually replaced the hypothesis that sleep is generated by a single, highly localized neuronal oscillator.

Cell groups that discharge during rapid-eye-movement (REM) sleep (REM-on) and neurons that slow or cease firing during REM sleep (REM-off) have long been thought to comprise at least two neurochemically distinct populations. The fact that putatively cholinoceptive and/or cholinergic (REM-on) and putatively aminergic (REM-off) cell populations discharge reciprocally over the sleep cycle suggests a causal interdependence.

In some brain stem areas these cell groups are not anatomically segregated and may instead be neurochemically mixed (interpenetrated). This finding raises important theoretical and practical issues not anticipated in the original reciprocal-interaction model. The electrophysiological evidence concerning the REM-on and REM-off cell groups suggests a gradient of sleep-dependent membrane excitability changes that may be a function of the connectivity strength within an anatomically distributed neuronal network. The connectivity strength may be influenced by the degree of neurochemical interpenetration between the REM-on and REM-offcells. Recognition of these complexities forces us to revise the reciprocal-interaction model and to seek new methods to test its tenets.

Cholinergic microinjection experiments indicate that some populations of REM-on cells can execute specific portions of the REM sleep syndrome or block the generation of REM sleep. This observation suggests that the order of activation within the anatomically distributed generator populations may be critical in determining behavioral outcome. Support for the cholinergic tenets of the reciprocal-interaction model has been reinforced by observations from sleep-disorders medicine.

Specific predictions of the reciprocal-interaction model and suggestions for testing these predictions are enumerated for future experimental programs that aim to understand the cellular and molecular basis of the mammalian sleep cycle.

Cessation of activity in red nucleus neurons during stimulation of the medial medulla in decerebrate rats

The pontine oral reticular nucleus, gigantocellular reticular nucleus (Gi) and dorsal paragigantocellular nucleus (DPGi) of the medulla are key elements of a brainstem-reticulospinal inhibitory system that participates in rapid eye movement (REM) sleep atonia. Our recent study has shown that excitation of these brainstem nuclei in decerebrate rats inhibits locus coeruleus cells and the midbrain locomotor region neurons related to muscle tone facilitation. In the present study we have examined the influences of electrical and chemical stimulation of Gi and DPGi inhibitory sites on the activity of neurons located in the magnocellular part of the red nucleus (RMC), a cell group that participates in both the tonic and phasic regulation of motor output. A total of 192 RMC neurons were recorded in precollicular-premammillary decerebrate rats with muscle rigidity and induced locomotion. Thirty-three RMC neurons were identified antidromically as rubrospinal (RMC-spinal) cells by stimulation of the contralateral dorsolateral funiculus at the L2 level. A total of 141 RMC neurons (88.7 %) and all RMC-spinal neurons were inhibited during electrical stimulation of Gi and DPGi inhibitory sites. This cessation of activity was correlated with bilateral muscle atonia or blockage of locomotion. Six RMC cells (3.8 %) were excited (224 +/- 50 %, n = 6, minimum = 98, maximum = 410, P < 0.05) and 11 cells (7 %) gave no response to Gi and DPGi stimulation. Microinjections of kainic acid (100 microM, 0.2 microl) into Gi and DPGi inhibitory sites, previously identified by electrical stimulation, produced a short-latency (35 +/- 3.5 s, n = 11) decrease of rigid hindlimb muscle tone and inhibition of all tested RMC (n = 7) and RMC-spinal (n = 5) neurons. These results, combined with our recent published data, suggest that inhibition of motor function during activation of the brainstem inhibitory system is related to both the descending inhibition of spinal motoneurons and suppression of activity in supraspinal motor facilitatory systems. These two mechanisms acting synergistically may cause generalized motor inhibition during REM sleep and cataplexy.

GABAergic neurons of the cat dorsal raphe nucleus express c-fos during carbachol-induced active sleep

Serotonergic neurons of the dorsal raphe nucleus (DRN) cease firing during active sleep (AS, also called rapid-eye-movement sleep). This cessation of electrical activity is believed to play a 'permissive' role in the generation of AS. In the present study we explored the possibility that GABAergic cells in the DRN are involved in the suppression of serotonergic activity during AS. Accordingly, we examined whether immunocytochemically identified GABAergic neurons in the DRN were activated, as indicated by their expression of c-fos, during carbachol-induced AS (AS-carbachol). Three chronically-prepared cats were euthanized after prolonged episodes of AS that was induced by microinjections of carbachol into the nucleus pontis oralis. Another four cats (controls) were maintained 2 h in quiet wakefulness before being euthanized. Thereafter, immunocytochemical studies were performed on brainstem sections utilizing antibodies against Fos, GABA and serotonin. When compared with identically prepared tissue from awake cats, the number of Fos+ neurons was larger in the DRN during AS-carbachol (35.9+/-5.6 vs. 13.9+/-4.4, P<0.05). Furthermore, a larger number of GABA+ Fos+ neurons were observed during AS-carbachol than during wakefulness (24.8+/-3.3 vs. 4.0+/-1.0, P<0.001). These GABA+ Fos+ neurons were distributed asymmetrically with a larger number located ipsilaterally to the site of injection. There was no significant difference between control and experimental animals in the number of non-GABAergic neurons that expressed c-fos in the DRN. We therefore suggest that activated GABAergic neurons of the DRN are responsible for the inhibition of serotonergic neurons that occurs during natural AS.

Differential regulation of 5-hydroxytryptamine release by GABAA and GABAB receptors in midbrain raphe nuclei and forebrain of rats

1. Extracellular 5-hydroxytryptamine (5-HT) was determined in dorsal raphe nucleus (DRN), median raphe nucleus (MRN) and nucleus accumbens by use of microdialysis in unanaesthetized rats. 2. Infusion of the gamma-aminobutyric acid (GABA)A receptor agonist muscimol into DRN and MRN resulted in decreased 5-HT in DRN and MRN, respectively. Muscimol infusion into nucleus accumbens had no effect on 5-HT. 3. Infusion of the GABAA receptor antagonist bicuculline into DRN resulted in increased DRN and nucleus accumbens 5-HT. Bicuculline infusion into MRN had no effect on 5-HT. This suggests that endogenous GABA had a tonic, GABAA receptor-mediated inhibitory effect on 5-HT in DRN, but not in MRN. 4. Infusion of the GABAB receptor agonist baclofen into DRN produced a decrease in DRN 5-HT. Baclofen infusion into nucleus accumbens resulted in decreased nucleus accumbens 5-HT. This suggests that GABAB receptors are present in the area of cell bodies and terminals of 5-hydroxytryptaminergic neurones. 5. Infusion of the GABAB receptor antagonists phaclofen and 2-hydroxysaclofen had no effect on midbrain raphe and forebrain 5-HT. This suggests that GABAB receptors did not contribute to tonic inhibition of 5-HT release. 6. In conclusion, 5-HT release is physiologically regulated by distinct subtypes of GABA receptors in presynaptic and postsynaptic sites.

Fos and serotonin immunoreactivity in the raphe nuclei of the cat during carbachol-induced active sleep: a double-labeling study

The microinjection of carbachol into the nucleus pontis oralis produces a state which is polygraphically and behaviorally similar to active sleep (rapid eye movement sleep). In the present study, using double-labeling techniques for serotonin and the protein product of c-fos (Fos), we sought to examine whether immunocytochemically identified serotonergic neurons of the raphe nuclei of the cat were activated, as indicated by their expression of c-fos, during this pharmacologically-induced behavioral state (active sleep-carbachol). Compared with control cats, which were injected with saline, active sleep-carbachol cats exhibited a significantly greater number of c-fos-expressing neurons in the raphe dorsalis, magnus and pallidus. Whereas most of the c-fos-expressing neurons in the raphe dorsalis were small, those in the raphe magnus were medium-sized and in the raphe pallidus they were small and medium-sized. The mean number of serotonergic neurons that expressed c-fos (i.e. double-labeled cells) was similar in control and active sleep-carbachol cats. These data indicate that there is an increased number of non-serotonergic, c-fos-expressing neurons in the raphe dorsalis, magnus and pallidus during the carbachol-induced state.(ABSTRACT TRUNCATED AT 250 WORDS)

GABA receptors: are cellular differences reflected in function?

The putative involvement of GABAA and GABAB receptors in various behavioral and physiological effects is summarized in Table III. A division of function among the two types of GABA receptors appears to exist. GABAA receptors mediate feeding, cardiovascular regulation, anxiolytic effects, and anticonvulsive activity. GABAB receptors, on the other hand, are involved in analgesia, cardiovascular regulation, and depression. Although there is some overlap and shared functions among the receptor types, it is evident that GABAA and GABAB receptors have different behavioral and physiological profiles. Feeding, anticonvulsive activity and anxiety, for example, primarily involve GABAA receptors. Analgesia and depression, on the other hand, are GABAB effects. In those cases where GABAA and GABAB receptors mediate similar functions (e.g. cardiovascular regulation), they do so by affecting different transmitter systems and cellular mechanisms. It is proposed, therefore, that GABAA and GABAB receptors differ not only at the cellular level, but that they also have different functions in the mammalian central nervous system. The association of different subtypes of a receptor with different functions and mechanisms of action is not unique to the GABA system. D1 and D2 receptors in the dopamine system, for example, also exhibit some separation of function as do the mu, delta and kappa types of opiate receptors. Different subtypes of neurotransmitter receptors, therefore, appear to be a general organizing principle used by the brain to transduce chemical signals into different functional responses. A better understanding of the exact processes through which cellular signals are transformed into functional responses is a goal of future research.

A GABA cardiovascular mechanism in the dorsal raphe of the rat

An injection of gamma-aminobutyric acid (GABA) and the GABA agonist, muscimol, into the dorsal raphe reduced both heart rate and blood pressure in the urethane-anesthetized rat. Picrotoxinin (3.4 nmol) did not affect blood pressure and slightly reduced the heart rate when injected into the dorsal raphe, but it blocked the decrease in both cardiovascular responses produced by GABA. These decreases in heart rate and blood pressure most likely result from stimulation of a GABAA receptor, as the GABAB agonist, baclofen, appeared to elevate heart rate and blood pressure by a mechanism occurring outside the dorsal raphe area. The changes in blood pressure and heart rate induced by muscimol occurred whether or not respiration was supported. Finally, it can be inferred that these GABAergic actions on blood pressure and heart rate probably involve both an inhibition of central sympathetic outflow and an excitation of parasympathetic outflow, as the quaternary muscarinic blocker, atropine methyl nitrate, blocked the decrease in heart rate induced by muscimol, but not the decrease in blood pressure.

Effects of rapid eye movement sleep deprivation on regional brain metabolism as measured by 2-[14C]deoxyglucose autoradiography

Regional brain metabolic activity of six male Sprague-Dawley rats was investigated with 2-[14C]deoxyglucose autoradiography. Regional brain metabolic activity was determined bilaterally for 60 brain structures, using laser densitometry. After 5 days of uninterrupted rapid eye movement sleep deprivation, increased metabolic activity was found in the lateral habenula, the caudal and middle parts of the limbic system, and in the corpus callosum.

Chloral hydrate anesthesia alters the responsiveness of central serotonergic neurons in the cat

The influence of chloral hydrate anesthesia on the spontaneous activity and responsiveness of serotonergic neurons was examined by administering chloral hydrate (300 mg/kg, i.p.) to freely moving cats from which serotonergic unit activity in the dorsal raphe nucleus (DRN) was being recorded. Although chloral hydrate administration produced a surgical level of anesthesia within 15 min following injection, it produced only a small decrease (approximately 20%) in the spontaneous activity of DRN serotonergic neurons. In contrast, the responsiveness of these same neurons was greatly altered by chloral hydrate administration. By examining the same neuron before and after chloral hydrate injection, it was found that chloral hydrate anesthesia completely abolished the excitatory responses of DRN serotonergic neurons to auditory and visual stimuli, as well as their excitatory response to electrical stimulation of the gigantocellular tegmental field (FTG) in the pontine reticular formation. On the other hand, the inhibition of serotonergic neuron firing resulting from systemic administration of WB 4101 (1.0 mg/kg, i.p.), a selective alpha 1 adrenergic receptor antagonist, was greatly potentiated by chloral hydrate anesthesia. Therefore, these data indicate that chloral hydrate anesthesia produces profound changes in the physiological and pharmacological responses of central serotonergic neurons which are not predictable by examination of spontaneous activity alone. Furthermore, as discussed, it it not clear to what extent these confounding influences might generalize to other anesthetized or immobilized preparations. Thus, beyond the obvious advantage which allows for the study of relationships between neuronal activity and behavior, single unit studies conducted in awake, freely moving animals also may be of greater value for basic physiological and pharmacological studies.

Monoamine involvement in the overeating caused by muscimol injection in the rat nucleus raphe dorsalis and the effects of d-fenfluramine and d-amphetamine

Injections of 5,7-dihydroxytryptamine in the nucleus raphe dorsalis did not significantly modify the eating caused by muscimol (100 ng) injected in the same area of freely fed rats 11 days later. Eating caused by muscimol, like food intake in starved rats, was significantly reduced by phenoxybenzamine (5, 10 and 20 mg/kg i.p.). Penfluridol (2.5 and 5 mg/kg p.o.), a dopamine receptor blocker, markedly reduced muscimol-induced eating, but had no effect on the food intake of starved rats. d-Fenfluramine (2.5 mg/kg i.p.), a releaser of serotonin from nerve terminals, significantly reduced eating in muscimol-injected and starved animals whereas d-amphetamine 1.25 and 2.5 mg/kg i.p. only inhibited the food intake of starved rats. Eating elicited by muscimol injected in the nucleus raphe dorsalis appears to constitute a catecholamine-mediated model of hyperphagia selectively inhibited by agents which increase serotonin transmission.

Immunohistochemical evidence for the presence of gamma-aminobutyric acid and serotonin in one nerve cell. A study on the raphe nuclei of the rat using antibodies to glutamate decarboxylase and serotonin

A specific and sensitive double immunocytochemical staining for the visualization of glutamate decarboxylase (GAD) and serotonin (5-HT) on the same brain section is developed. GAD is detected with specific GAD-antibodies by means of the unlabeled antibody enzyme, peroxidase anti-peroxidase, and serotonin with an antibody against the BSA-serotonin conjugate by an indirect immunofluorescent staining. The coexistence of GAD and 5-HT in the same perikaryon is demonstrated by a peroxidase reaction superimposed on fluorescent compounds. Cell bodies containing both antigens are observed in each raphe nuclei. However, the nucleus raphe dorsalis exhibits the largest number of cells containing either GAD alone or GAD and 5-HT together. An intracellular interaction between the metabolism of GABA and serotonin could be reasonably expected. The interactions between GABAergic and serotonergic systems must be thought of in terms of intracellular and/or transynaptic controls.

Enhancement of dorsal raphe discharge by medial pontine reticular formation stimulation depends on behavioral state

Single cell activity was recorded with microwire electrodes from the dorsal raphe nucleus (DRN) of cats across multiple cycles of sleep and wakefulness. Electrical stimulation of the medial pontine reticular formation (mPRF) produced a 10-fold enhancement of DRN discharge during wakefulness and slow wave sleep but not during desynchronized sleep. The neuronal mechanisms likely to mediate the state-dependent effects of mPRF stimulation are discussed with regard to the hypothesis that DRN plays a major role in behavioral state control.