Pigeon as a model to study peripheral projections from the horizontal semicircular canal vestibular apparatus to a brainstem target immunoreactive for AMPA

ABSTRACT Purpose: To evaluate whether the pigeon (Columba livia) is a good model for evaluating the vestibular system involved with postural maintenance during movement. Methods: This study maps the brainstem targets of the horizontal ampullary inputs from the vestibular periphery of the pigeon. We used biotin dextran amine (BDA) injection in horizontal semicircular canal (HSCC), immunohistochemistry for GluR2/3 and GluR4 AMPA and computerized histomorphology reconstruction. Results: Our results show the same distribution pattern with ipsilateral projections to vestibular nuclear complex (VNC) from the HSCC, with the majority of labeled fibers being, long, thin, with few varicosities and many ramifications. Horizontal semicircular canal projections achieve neurons belonging to all nuclei of the VNC with exception of dorsal portion of lateral vestibular nucleus and this area express GluR2/3 and GluR4 AMPA receptors reinforcing the idea of glutamate participation in these connections. Conclusions: Pigeon is an appropriated experimental model to study of projections of HSCC and reinforcing the information that the vestibular system has strong relation with the fast responses necessary for postural control. Moreover, its phylogenetic organization apparently conservation, also seems to be a fundamental characteristic for vertebrates.

Serotonin and circadian rhythms

All mammal behaviors and functions exhibit synchronization with environmental rhythms. This is accomplished through an internal mechanism that generates and modulates biological rhythms. The circadian timing system, responsible for this process, is formed by connected neural structures. Pathways receive and transmit environmental cues to the central oscillator, the hypothalamic suprachiasmatic nucleus, which mediates physiological and behavioral alterations. The suprachiasmatic nucleus has three major inputs: the retinohypothalamic tract (a direct projection from the retina), the geniculohypothalamic tract (an indirect photic projection originating in the intergeniculate leaflet), and a dense serotonergic plexus from the raphe nuclei. The serotonergic pathway, a source of non-photic cues to the suprachiasmatic nucleus, modulates its activity. The importance of raphe nuclei in circadian rhythms, especially in photic responses, has been demonstrated in many studies. Serotonin is the raphe neurotransmitter that triggers phase shifts, inhibits light-induced phase-shifts, and plays a role in controlling the sleep-wake cycle. All data to date have demonstrated the importance of the raphe, through serotonergic afferents, in adjusting circadian rhythms and must therefore be considered a component of the circadian timing system. The aim of this paper is to review the literature addressing the involvement of serotonin in the modulation of circadian rhythm