Involvement of apamin-sensitive SK channels in spike frequency adaptation of neurons in nucleus tractus solitarii of the rat

We delineated the role of Ca²⁺-activated K⁺ channels in the phenomenon of spike frequency adaptation (SFA) exhibited by neurons in the caudal region of nucleus tractus solitarius (cNTS) using intracellular recording coupled with the current-clamp technique in rat brain slices. Intracellular injection of a constant depolarizing current evoked a train of action potentials whose discharge frequency declined rapidly to a lower steady-state level of irregular discharges. This manifested phenomenon of SFA was found to be related to extracellular Ca²⁺. Low Ca²⁺ (0.25 mM) or Cd²⁺ (0.5 mM) in the perfusing medium resulted in a significant increase in the adaptation time constant (_adap) and an appreciable reduction in the percentage adaptation of spike frequency (F_adap). In addition, the evoked discharges were converted from an irregular to a regular pattern, accompanied by a profound increase in mean firing rate. Intriguingly, similar alterations in _adap, F_adap, discharge pattern and discharge rate were elicited by apamin (1 µM), a selective blocker for small-conductance Ca²⁺-activated K⁺ (SK) channels. On the other hand, charybdotoxin (0.1 µM), a selective blocker for large-conductance Ca²⁺-activated K⁺ channels, was ineffective. Our results suggest that SK channels of cNTS neurons may subserve the generation of both SFA and irregular discharge patterns displayed by action potentials evoked with a prolonged depolarizing current.     

Endogenous GLP-1 acts on paraventricular nucleus to suppress feeding: Projection from nucleus tractus solitarius and activation of corticotropin-releasing hormone,nesfatin-1 and oxytocin neurons

Glucagon-like peptide-1 (GLP-1) receptor agonists have been used to treat type 2 diabetic patients and shown to reduce food intake and body weight. The anorexigenic effects of GLP-1 and GLP-1 receptor agonists are thought to be mediated primarily via the hypothalamic paraventricular nucleus (PVN). GLP-1, an intestinal hormone, is also localized in the nucleus tractus solitarius (NTS) of the brain stem. However, the role of endogenous GLP-1, particularly that in the NTS neurons, in feeding regulation remains to be established. The present study examined whether the NTS GLP-1 neurons project to PVN and whether the endogenous GLP-1 acts on PVN to restrict feeding. Intra-PVN injection of GLP-1 receptor antagonist exendin (9–39) increased food intake. Injection of retrograde tracer into PVN combined with immunohistochemistry for GLP-1 in NTS revealed direct projection of NTS GLP-1 neurons to PVN. Moreover, GLP-1 evoked Ca²⁺ signaling in single neurons isolated from PVN. The majority of GLP-1-responsive neurons were immunoreactive predominantly to corticotropin-releasing hormone (CRH) and nesfatin-1, and less frequently to oxytocin. These results indicate that endogenous GLP-1 targets PVN to restrict feeding behavior, in which the projection from NTS GLP-1 neurons and activation of CRH and nesfatin-1 neurons might be implicated. This study reveals a neuronal basis for the anorexigenic effect of endogenous GLP-1 in the brain.     

The use of neurotoxic dihydroxytryptamines as tools for morphological studies and localized lesioning of central indolamine neurons

Summary The usefulness of three neurotoxic dihydroxytryptamines — 5,6-dihydroxytryptamine, 5,7-dihydroxytryptamine and 4,5-dihydroxytryptamine — for fluorescence microscopical tracing and localized lesioning of central indolamine-containing axon bundles has been studied in the rat brain. The lesions produced by intraventricularly or intracerebrally administered dihydroxytryptamines were found to be much superior to mechanical or electrolytic lesions in producing extensive accumulations of fluorescence in the indolamine axon pathways. This greatly improves the possibilities for tracing of the normally non-fluorescent or weakly fluorescent indolamine axons from their cells of origin for long distances through the main fibre bundles and their branches. Much new information concerning the anatomy of the indolamine neuron systems is obtained with this technique, and some preliminary observations are presented.The efficiency of local, intracerebral injections of small amounts of dihydroxytryptamines for regional denervations in the CNS was also tested. It was found that local injections of 4 g of either of the three compounds into the ventromedial tegmentum and into the grey matter of the spinal cord produced extensive and probably rather selective damage to the ascending and descending indolamine fibre tracts and — although to a lesser and variable extent — the noradrenaline and dopamine systems. The denervating effects of the tegmental and the spinal cord injections were with respect to the serotonin-containing neurons comparable to those obtained by others after large lesions that destroy almost the entire midbrain raphe region, and after total transections of the spinal cord, respectively. The characteristics and the specificity of the dihydroxytryptamine-induced lesions are discussed.     

Evidence for projections from medullary nuclei onto serotonergic and dopaminergic neurons in the midbrain dorsal raphe nucleus of the rat

Summary The anterograde tracer Phaseolus vulgaris-leucoagglutinin was injected into the medial nucleus of the solitary tract and into the rostral dorsomedial medulla. A sequential two-color immunoperoxidase staining was accomplished in order to demonstrate the co-distribution of presumed terminal axons with chemically distinct neurons in the dorsal raphe nucleus of the midbrain central gray, i.e., B7 serotonergic and A10dc dopaminergic neurons. Black-stained efferent fibers from the medial nucleus of the solitary tract and the rostral dorsomedial medulla intermingled with brown-stained serotonergic (5-hydroxytryptamine-immunoreactive) or dopaminergic (tyrosine hydroxylase-immunoreactive) neurons. Light microscopy revealed that the black-stained efferent axons exhibited numerous en passant and terminal varicosities that were often found in close apposition to brown-stained serotonergic and dopaminergic somata, and to proximal and distal dendrites and dendritic processes. The close association of immunoreactive elements suggests the presence of axo-somatic and axodendritic synaptic contacts of medullary fibers with serotonergic and dopaminergic neurons in the dorsal raphe nucleus. These projections could be involved in the modulation of dorsal raphe neurons, depending on the autonomic status of an animal.     

Comparative effects on blood pressure and heart rate of dynorphin A(1–13) in anterior hypothalamic area, posterior hypothalamic area, nucleus tractus solitarius, and lateral cerebral ventricle in the rat

The objective of this study was to explore the effects of the endogenous opioid peptide dynorphin A(1–13) on the CNS regulation of blood pressure and heart rate. Wistar rats, anesthetized with pentobarbital and halothane, received dynorphin A(1–13) microinjected into the anterior hypothalamus area (AHA), the posterior hypothalamic area (PHA), the nucleus tractus solitarius (NTS), or the lateral cerebral ventricle (ICV). Dynorphin A(1–13), 20 (12 nmol) or 30 μg ICV, produced significant (p < 0.05) reductions in blood pressure and heart rate. Naloxone, 50 μg/kg ICV, completely prevented the blood pressure response and significantly (p < 0.05) blunted the heart rate response to the highest dynorphin concentration, 30 μg ICV (18 nmol). Dynorphin A(1–13), 5 μg, in the NTS significantly (p < 0.05) decreased systolic and diastolic blood pressure and heart rate with the response being evident 10 min and persisting for 30 min after injection. In contrast, the same dose of dynorphin A(1–13) in the AHA produced an immediate, marked, and significant (p < 0.05) decrease in systolic and diastolic blood pressure and heart rate that attained its maximum 1–3 min and returned rapidly towards baseline levels. Dynorphin A(1–13), 5 or 10 μg in the posterior hypothalamic area, was not associated with any change in blood pressure or heart rate. Injection of the diluent at any site was not associated with any changes in blood pressure or heart rate. The maximum change in blood pressure with dynorphin was greater in the AHA than NTS, and the maximum change in heart rate was greater in the NTS than AHA. These data indicate a potential role for dynorphin as a modulator of the CNS regulation of blood pressure and cardiac rate, and this is mediated in part through different areas in the brain that maybe localized to the anterior hypothalamic area and nucleus tractus solitarius but not the posterior hypothalamic area.     

Immunofluorescence study of LRH-producing neurons in prosimians (Tupaia and Galago)

Summary Reactive LRH neurons were characterized in prosimians (Tupaia and Galago) by immunofluorescence using rabbit immunesera against unconjugated synthetic LRH, or LRH conjugated with bovine serum albumin. These neurons, which vary individually in number in one species, are mainly concentrated in the rostral hypothalamus (medial preoptic area and anterior hypothalamic area) and in the lamina terminalis. In contrast to the simians and man, immunoreactive perikarya were not routinely found in the mediobasal hypothalamus of the prosimians investigated in the present study. Reactive axons of the hypothalamo-hypophyseal tract are more numerous and conspicuous in the retrochiasmatic area and in the postinfundibular eminence. They give rise to radiating collaterals ending mainly around the capillaries of the primary portal plexus of the median eminence and of the infundibular stem (where they are generally more numerous). Reactive axons of the preopticoterminal tract, originating from the perikarya of the lamina terminalis, end around the capillaries of the vascular organ or below and between the ependymal cells lining its ventricular side.In Galago a small but very distinct tract of reactive axons runs under the optic chiasma, between the lamina terminalis and the ventral labium of the infundibulum. Very fine reactive extrahypothalamic axons were observed in the posterior part of the habenular ganglia, along the preamygdaloid portion of the stria terminalis and along the blood vessels of the parolfactory area.This work was supported by a grant from the Foundation pour la Recherche Médicale Française. The author acknowledges the help of Miss D. Croix for the preparation of LRH-BSA conjugates and the radioimmunological study of the immunosera and A. Pillez (C.N.R.S.) for sectioning and staining the genital tracts