Electrophysiological properties of neurons in the caudal ventrolateral medulla projecting to the paraventricular nucleus of the hypothalamus in rats

Extracellular recordings were made from neurons in the caudal ventrolateral medulla in urethane-chloralose-anesthetized rats. Stimulation of the paraventricular nucleus (PVN) in the hypothalamus evoked antidromic action potentials in 71 neurons. On the basis of antidromic spike latencies, these neurons could be divided into fast- (24 neurons) and slow-conducting cell groups (47 neurons). Slow-conducting cells showed irregular and slow spontaneous discharges, while a majority of the fast-conducting cells did not show spontaneous discharges. The spontaneous activity of slow-conducting cells was suppressed by i.v. clonidine administration. The effects of clonidine could be consistently reversed by administration of the alpha 2-adrenergic antagonist, yohimbine. The responses by clonidine and yohimbine remained unimpaired in baroreceptor-denervated rats. Vagus nerve stimulation produced an excitation in 80% of slow-conducting cells tested. Baroreceptor activation induced by i.v. administration of phenylephrine inhibited about half of slow-conducting cells tested. Similar elevation of blood pressure in baroreceptor-denervated rats did not show any effect. These physiological and pharmacological properties of slow-conducting cells were similar to those previously reported for catecholaminergic cells in other parts of the brain. The results show the existence of two different populations among neurons in the caudal ventrolateral medulla which project directly to the PVN, and suggest that the presumed A1 catecholaminergic cells are involved in the afferent pathway from cardiovascular baroreceptors and the vagus nerve to the PVN.     

Coexpression of VGLUT1 and VGLUT2 in trigeminothalamic projection neurons in the principal sensory trigeminal nucleus of the rat

VGLUT1 and VGLUT2 have been reported to show complementary distributions in most brain regions and have been assumed to define distinct functional elements. In the present study, we first investigated the expression of VGLUT1 and VGLUT2 in the trigeminal sensory nuclear complex of the rat by dual‐fluorescence in situ hybridization. Although VGLUT1 and/or VGLUT2 mRNA signals were detected in all the nuclei, colocalization was found only in the principal sensory trigeminal nucleus (Vp). About 64% of glutamatergic Vp neurons coexpressed VGLUT1 and VGLUT2, and the others expressed either VGLUT1 or VGLUT2, indicating that Vp neurons might be divided into three groups. We then injected retrograde tracer into the thalamic regions, including the posteromedial ventral nucleus (VPM) and posterior nuclei (Po), and observed that the majority of both VGLUT1‐ and VGLUT2‐expressing Vp neurons were retrogradely labeled with the tracer. We further performed anterograde labeling of Vp neurons and observed immunoreactivies for anterograde tracer, VGLUT1, and VGLUT2 in the VPM and Po. Most anterogradely labeled axon terminals showed immunoreactivities for both VGLUT1 and VGLUT2 in the VPM and made asymmetric synapses with dendritic profiles of VPM neurons. On the other hand, in the Po, only a few axon terminals were labeled with anterograde tracer, and they were positive only for VGLUT2. The results indicated that Vp neurons expressing VGLUT1 and VGLUT2 project to the VPM, but not to the Po, although the functional differences of three distinct populations of Vp neurons, VGLUT1‐, VGLUT2‐, and VGLUT1/VGLUT2‐expressing ones, remain unsettled. J. Comp. Neurol. 518:3149–3168, 2010. © 2010 Wiley‐Liss, Inc.     

Late postnatal shifts of parvalbumin and nitric oxide synthase expression within the GABAergic and glutamatergic phenotypes of inferior colliculus neurons

The inferior colliculus (IC) is partitioned into three subdivisions: the dorsal and lateral cortices (DC and LC) and the central nucleus (ICC), and serves as an integration center of auditory information. Recent studies indicate that a certain population of IC neurons may represent the non‐GABAergic phenotype, while they express well‐established cortical/hippocampal GABAergic neuron markers. In this study we used the optical disector to investigate the phenotype of IC neurons expressing parvalbumin (PV) and/or nitric oxide synthase (NOS) in C57BL/6J mice during the late postnatal period. Four major types of IC neurons were defined by the presence (+) or absence (–) of PV, NOS, and glutamic acid decarboxylase 67 (GAD67): PV⁺/NOS^?/GAD67⁺, PV⁺/NOS⁺/GAD67⁺, PV⁺/NOS^?/GAD67^?, and PV^?/NOS⁺/GAD67^?. Fluorescent in situ hybridization for vesicular glutamate transporter 2 mRNA indicated that almost all GAD67^? IC neurons represented the glutamatergic phenotype. The numerical densities (NDs) of total GAD67⁺ IC neurons remained unchanged in all subdivisions. The NDs of PV⁺/NOS^?/GAD67⁺ neurons and PV^?/NOS⁺/GAD67^? neurons were reduced with age in the ICC, while they remained unchanged in the DC and LC. By contrast, the NDs of PV⁺/NOS⁺/GAD67⁺ neurons and PV⁺/NOS^?/GAD67^? neurons were increased with age in the ICC, although there were no changes in the DC and LC. The cell body size of GAD67⁺ IC neurons did not vary according to the expression of PV with or without NOS. The present findings indicate that the expression of PV and NOS may shift with age within the GABAergic and glutamatergic phenotypes of IC neurons during the late postnatal period. J. Comp. Neurol. 525:868–884, 2017. © 2016 Wiley Periodicals, Inc.     

Changes in salt intake after lesions of the area postrema and the nucleus of the solitary tract in rats

Several reports have linked the area postrema (AP), a circumventricular organ in the dorsal medulla, to the control of sodium regulation. To clarify its role further we examined the effects of AP ablations in rats on sodium intake as well as on sodium output. Eighteen experimental rats received lesions to the AP while 6 control rats received sham lesions. After the lesions we gave the animals a two-bottle preference test between water and various molar concentrations of NaCl (0.03, 0.1, 0.3, 0.5), glucose (0.1, 0.3) or KCl (0.1, 0.3) solutions. Rats with AP lesions consumed supranormal amounts of NaCl solutions, but their intakes of glucose and KCl solutions were not significantly different from those of control rats. These changes in intake were apparently not secondary to changes in sodium output. Urinary sodium and potassium levels were the same for both groups of rats while on a normal, sodium-replete diet or on a sodium-free diet. Anatomical analyses revealed a significant correlation between the size of the lesion and the animals' salt intake (NaCl, KCl). Only when the lesions destroyed the AP without appreciable damage to the adjacent nucleus of the solitary tract (NST), a sensory relay for gustatory and visceral afferents, was there a significant tendency for rats to consume more salt solution. These changes in intake cannot be accounted for by lesioned produced deficits in gustatory function. The data were discussed with regard to possible hemodynamic effects of the lesions.     

Distribution of type 1 cannabinoid receptor-expressing neurons in the septal-hypothalamic region of the mouse: colocalization with GABAergic and glutamatergic markers

Type 1 cannabinoid receptor (CB1) is the principal mediator of retrograde endocannabinoid signaling in the brain. In this study, we addressed the topographic distribution and amino acid neurotransmitter phenotype of endocannabinoid-sensitive hypothalamic neurons in mice. The in situ hybridization detection of CB1 mRNA revealed high levels of expression in the medial septum (MS) and the diagonal band of Broca (DBB), moderate levels in the preoptic area and the hypothalamic lateroanterior (LA), paraventricular (Pa), ventromedial (VMH), lateral mammillary (LM), and ventral premammillary (PMV) nuclei, and low levels in many other hypothalamic regions including the suprachiasmatic (SCh) and arcuate (Arc) nuclei. This regional distribution pattern was compared with location of γ-aminobutyric acid (GABA)ergic and glutamatergic cell groups, as identified by the expression of glutamic acid decarboxylase 65 (GAD65) and type 2 vesicular glutamate transporter (VGLUT2) mRNAs, respectively. The MS, DBB, and preoptic area showed overlaps between GABAergic and CB1-expressing neurons, whereas hypothalamic sites with moderate CB1 signals, including the LA, Pa, VMH, LM, and PMV, were dominated by glutamatergic neurons. Low CB1 mRNA levels were also present in other glutamatergic and GABAergic regions. Dual-label in situ hybridization experiments confirmed the cellular co-expression of CB1 with both glutamatergic and GABAergic markers. In this report we provide a detailed anatomical map of hypothalamic glutamatergic and GABAergic systems whose neurotransmitter release is controlled by retrograde endocannabinoid signaling from hypothalamic and extrahypothalamic target neurons. This neuroanatomical information contributes to an understanding of the role that the endocannabinoid system plays in the regulation of endocrine and metabolic functions.