Panic-like defensive behavior but not fear-induced antinociception is differently organized by dorsomedial and posterior hypothalamic nuclei of Rattus norvegicus (Rodentia, Muridae)

The hypothalamus is a forebrain structure critically involved in the organization of defensive responses to aversive stimuli. Gamma-aminobutyric acid (GABA)ergic dysfunction in dorsomedial and posterior hypothalamic nuclei is implicated in the origin of panic-like defensive behavior, as well as in pain modulation. The present study was conducted to test the difference between these two hypothalamic nuclei regarding defensive and antinociceptive mechanisms. Thus, the GABA A antagonist bicuculline (40 ng/0.2 µL) or saline (0.9% NaCl) was microinjected into the dorsomedial or posterior hypothalamus in independent groups. Innate fear-induced responses characterized by defensive attention, defensive immobility and elaborate escape behavior were evoked by hypothalamic blockade of GABA A receptors. Fear-induced defensive behavior organized by the posterior hypothalamus was more intense than that organized by dorsomedial hypothalamic nuclei. Escape behavior elicited by GABA A receptor blockade in both the dorsomedial and posterior hypothalamus was followed by an increase in nociceptive threshold. Interestingly, there was no difference in the intensity or in the duration of fear-induced antinociception shown by each hypothalamic division presently investigated. The present study showed that GABAergic dysfunction in nuclei of both the dorsomedial and posterior hypothalamus elicit panic attack-like defensive responses followed by fear-induced antinociception, although the innate fear-induced behavior originates differently in the posterior hypothalamus in comparison to the activity of medial hypothalamic subdivisions.

Primary role of posterior hypothalamic cholinergic receptors in central regulation of blood pressure and heart rate in rats

The purpose of the present study is to determine the role of muscarinic cholinergic receptors of posterior hypothalamus in the central blood pressure regulation when respiration is controlled. In anesthetized and artificially ventilated rats, vasodepressor response was evoked by injection of L-glutamate (10 nmol) neuroexcitatory amino acid into the posterior hypothalamic area. The injection of carbachol (0.5 ~ 8 nmol) into the same area induced dose-dependent vasodepressor and bradycardic responses. Pretreatment with atropine (4 nmol) completely blocked the vasodepressor response to carbachol (2 nmol). In contrast, in spontaneously breathing rats, the injection of carbachol (8 nmol) into the posterior hypothalamic area induced the vasopressor and tachycardic responses. These results suyggest that the muscarinic cholinergic receptors in the posterior hypothalamic area primarily play an inhibitory role in the central regulation of blood pressure and heart rate.