Distribuição da inervação da relaxina-3 no tectum e tegmentum no rato sugere envolvimento do núcleo incertus em redes defensivas centrais

Abstract

In mammals, tectal and tegmental divisions of the brainstem are involved in attentional mechanisms and responses to threatening stimuli such as predators. These centers are regulated by ascending connections, but the anatomical and neurochemical details of this drive are not fully known. The nucleus incertus (NI) in the pontine tegmentum is the source of ascending GABA projections to forebrain cognitive/emotional centers and NI neurons contain a number of neuropeptides, including relaxin-3 (RLN3). Tract-tracing studies have described NI projections within the tectum; and in this study we describe the distribution of relaxin-3 fibers within tectal and tegmental areas/nuclei of rat brain. RLN3-immunostained sections were also reacted with antisera against other neurochemical markers, as synaptophysin, nitric oxyde synthetase, tyrosine hydroxylase, calbindin, calretinin and 5-HT, to assist in demarcation of the area. RLN3-containing fibers were concentrated in the medial, olivary and ventrolateral pretectal nuclei; the medial intermediate grey layer of superior colliculus; and the pericentral area of inferior colliculus. Some labeled fibers were also detected in the cuneiform, parabigeminal and sagulum nuclei. RLN3 fibers were concentrated around the commissural bundles along the midline of the tectum, in the dorsal columns of the periaqueductal gray and in the dorsal raphe. In all areas, RLN3 and synaptophysin staining co-existed, indicating an association of the peptide with synapses. RLN3 projections target structures within the tectum and tegmentum that comprise the defensive system involved in detection of and response to unexpected threatening stimuli. NI neurons, which are a major source of RLN3 fibers and express corticotrophin-releasing factor receptors, may contribute to these responses following activation by stress-related stimuli.