Involvement of Opioid and Endocanabinoid Receptors of the Substantia Nigra, Pars Reticulata, in the Modulation of the Activity of Nigro-Tectal GABAergic Pathways during the Organization of Defensive Behaviour of Mice Confronted with Venomous Snakes

Abstract

The study of the neuropsychopharmacological bases of the fear- and panic-related behaviour has been the focus of several scientific researches. The mesencephalic tectum (MT) has been considered as responsible for the control of defensive responses evoked during threatening situations. The deep layers of the superior colliculus (dlSC) and the dorsal columns of the periaqueductal grey matter (dPAG) have been considered important structures for the elaboration of innate fear and defensive behaviour. The substantia nigra, pars reticulata (SNpr), is known to send inputs to the intermediate layers of the superior colliculus (ilSC) that can work as a sensory channel to dorsal midbrain neurons responsible for the elaboration of defensive responses in more ventral regions of this neural substrate. The dorsal mesencephalon, however, is connected to both compacta and reticulata divisions of the substantia nigra, through inputs rich in endogenous opioid peptides. This mesencephalic structure is also rich in CB1 endocannabinoid receptors, with putative involvement in this neural system that is known for these interactive properties with the opioid system, and has been the focus of interest of scientific community. The purpose of the present work is to investigate the role of µ receptors, as well as CB1-endocannabinoid receptors of SNpr, rich in GABAergic neurons connected to the mesencephalic tectum, in the modulation of defensive behaviour related to instinctive fear and panic reactions. Thus, it will be studied the effect of microinjections of µ-opioid receptors selective antagonists, CB1-endocannabinoid receptors selective antagonists into the SNpr on the elaboration of the defensive behaviour elaborated by dorsal mesencephalon induced by GABAA receptor selective blockade in the dlSC/dPAG of Wistar rats confronted with Bothrops jararaca .