Immunocytochemical localization of calretinin-containing neurons in the rat periaqueductal gray and colocalization with enzymes producing nitric oxide: a double, double-labeling study

Intracallosal neuronal nitric oxide synthase neurons colocalize with neurokinin 1 substance P receptor in the rat

The corpus callosum (cc) contains nitric oxide (NO)-producing neurons. Because NO is a potent vasodilator, these neurons could translate neuronal signals into vascular responses that can be detected by functional brain imaging. Substance P (SP), one of the most widely expressed peptides in the CNS, also produces vasomotor responses by inducing calcium release from intracellular stores through its preferred neurokinin 1 (NK1) receptor, thus inducing NO production via activation of neuronal NO synthase (nNOS). Single- and double-labeling experiments were performed to establish whether NK1-immunopositive neurons (NK1IP -n) are found in the rat cc and the extent of NK1 colocalization with nNOS. NK1IP -n were seen to constitute a large neuronal population in the cc and had a distribution similar to that of nNOSIP neurons (nNOSIP -n). NK1IP -n were numerous in the lateral cc and gradually decreased in the more medial portions, where they were few or absent. Intracallosal NK1IP -n and their dendritic trees were intensely labeled, allowing classification into four morphological types: bipolar, round, polygonal, and pyramidal. Confocal microscopic examination demonstrated that nearly all NK1IP -n contained nNOS (96.43%) and that 84.59% of nNOSIP -n co-expressed NK1. These data suggest that the majority of intracallosal neurons can release NO as a result of the action of SP. A small proportion of nNOSIP -n does not contain NK1 and is not activated by SP; these neurons may release NO via alternative mechanisms. The possible mechanisms by which intracallosal neurons release NO are also reviewed.

Characterization of NO-producing neurons in the rat corpus callosum

INTRODUCTION

The aim of this study was to determine the presence and distribution of nitric oxide (NO)-producing neurons in the rat corpus callosum (cc).

MATERIAL AND METHODS

To investigate this aspect of cc organization we used nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry and neuronal NO synthase (nNOS) immunocytochemistry.

RESULTS

Intense NADPH-d-positive (NADPH-d+) neurons were found along the rostrocaudal extension of the cc (sagittal sections). They were more numerous in the lateral cc and gradually decreased in the more medial regions, where they were very few or absent. The Golgi-like appearance of NADPH-d+ intracallosal neurons allowed dividing them into five morphological types: (1) bipolar; (2) fusiform; (3) round; (4) polygonal; and (5) pyramidal. The number of NADPH-d+ neurons (both hemispheres) was counted in two brains using 50-μm thick sections. In the first brain, counts involved 145 sections and neurons were 2959; in the second, 2227 neurons were counted in 130 sections. The distribution and morphology of nNOS-immunopositive (nNOSIP) neurons was identical to that of NADPH-d+neurons. Some of these neurons were observed in the cc ependymal region, where they might be in contact with cerebrospinal fluid (CSF), monitoring its composition, pH, and osmolality changes, or playing a role in regulating the synthesis and release of several peptides. The somatic, dendritic, and axonal processes of many NADPH-d+/nNOSIP neurons were closely associated with intracallosal blood vessels.

CONCLUSIONS

Such close relationship raises the possibility that these neurons are a major source of NO during neural activity. As NO is a potent vasodilator, these findings strongly suggest that NO-positive neurons transduce neuronal signals into vascular responses in selected cc regions, thus giving rise to hemodynamic changes detectable by neuroimaging.

Differential distribution of parvalbumin- and calbindin-D28K-immunoreactive neurons in the rat periaqueductal gray matter and their colocalization with enzymes producing nitric oxide

The distribution, colocalization with enzymes producing nitric oxide (NO), and the synaptic organization of neurons containing two calcium-binding proteins (CaBPs) - parvalbumin (Parv) and calbindin-D28K (Calb) - were investigated in the rat periaqueductal gray matter (PAG). Parv-immunopositive (ParvIP) neurons were detected in the mesencephalic nucleus and rarely in the PAG. CalbIP neurons were found both in the dorsolateral (PAG-dl) and ventrolateral PAG (PAG-vl); their size ranged from 112.96 μm(2) (PAG-dl) to 125.13 μm(2) (PAG-vl). Ultrastructurally Parv and Calb immunoreactivity was mostly found in dendritic profiles. Axon terminals containing each of the two CaBPs formed symmetric synapses. Moreover both Parv and Calb were used to label a subpopulation of NO-producing neurons. Colocalization was investigated using two protocols: (i) a combination of Calb and Parv immunocytochemistry (Icc) with nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry (Hi) and (ii) neuronal NO synthase-Icc (nNOS) (immunofluorescence). Both techniques demonstrated a complete lack of colocalization of Parv and NADPH-d/nNOS in PAG neurons. Double-labeled (DL) neurons (Calb-NADPH-d; Calb-nNOS) were detected in PAG-dl. NADPH-d-Hi/Calb-Icc indicated that 41-47% of NADPH-d-positive neurons contained Calb, whereas 17-23% of CalbIP cells contained NADPH-d. Two-color immunofluorescence revealed that 53-66% of nNOSIP cells colocalized with Calb and 24-34% of CalbIP neurons contained nNOS. DL neuron size was 104.44 μm(2); neurons labeled only with NADPH-d or Calb measured 89.793 μm(2) and 113.48 μm(2), respectively. Together with previous findings (Barbaresi et al. [2012]) these data suggest that: Therefore the important aspect of the PAG intrinsic organization emerging from this and previous double-labeling studies is the chemical diversity of NO-synthesizing neurons, which is likely related to the different functions in which these neurons are involved.

Extending the socio-sexual brain: arginine-vasopressin immunoreactive circuits in the telencephalon of mice

Quantitative analysis of the immunoreactivity for arginine-vasopressin (AVP-ir) in the telencephalon of male (intact and castrated) and female CD1 mice allows us to precisely locate two sexually dimorphic (more abundant in intact than castrated males and females) AVP-ir cell groups in the posterior bed nucleus of the stria terminalis (BST) and the amygdala. Chemoarchitecture (NADPH diaphorase) reveals that the intraamygdaloid AVP-ir cells are located in the intra-amygdaloid BST (BSTIA) rather than the medial amygdala (Me), as previously thought. Then, we have used for the first time tract tracing (combined with AVP immunofluorescence) and fiber-sparing lesions of the BST to analyze the projections of the telencephalic AVP-ir cell groups. The results demonstrate that the posterior BST originates the sexually dimorphic innervation of the lateral septum, the posterodorsal Me and a substance P-negative area in the medioventral striato-pallidum (mvStP).The BSTIA may also contribute to some of these terminal fields. Our material also reveals non-dimorphic AVP-ir processes in two locations of the amygdala. First, the ventral Me shows dendrite-like AVP-ir processes apparently belonging supraoptic neurons, whose possible functions are discussed. Second, the Ce shows sparse, thick AVP-ir axons with high individual variability in density and distribution, whose possible influence on stress coping in relation to the affiliative or agonistic behaviors mediated by the Me are discussed. Finally, we propose that the region of the mvStP showing sexually dimorphic AVP-ir innervation is part of the brain network for socio-sexual behavior, in which it would mediate motivational aspects of chemosensory-guided social interactions.

Immunocytochemical profiles of inferior colliculus neurons in the rat and their changes with aging

The inferior colliculus (IC) plays a strategic role in the central auditory system in relaying and processing acoustical information, and therefore its age-related changes may significantly influence the quality of the auditory function. A very complex processing of acoustical stimuli occurs in the IC, as supported also by the fact that the rat IC contains more neurons than all other subcortical auditory structures combined. GABAergic neurons, which predominantly co-express parvalbumin (PV), are present in the central nucleus of the IC in large numbers and to a lesser extent in the dorsal and external/lateral cortices of the IC. On the other hand, calbindin (CB) and calretinin (CR) are prevalent in the dorsal and external cortices of the IC, with only a few positive neurons in the central nucleus. The relationship between CB and CR expression in the IC and any neurotransmitter system has not yet been well established, but the distribution and morphology of the immunoreactive neurons suggest that they are at least partially non-GABAergic cells. The expression of glutamate decarboxylase (GAD) (a key enzyme for GABA synthesis) and calcium binding proteins (CBPs) in the IC of rats undergoes pronounced changes with aging that involve mostly a decline in protein expression and a decline in the number of immunoreactive neurons. Similar age-related changes in GAD, CB, and CR expression are present in the IC of two rat strains with differently preserved inner ear function up to late senescence (Long-Evans and Fischer 344), which suggests that these changes do not depend exclusively on peripheral deafferentation but are, at least partially, of central origin. These changes may be associated with the age-related deterioration in the processing of the temporal parameters of acoustical stimuli, which is not correlated with hearing threshold shifts, and therefore may contribute to central presbycusis.