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.

Effect of mutual stimulation of the anterior and posterior hypothalamic nuclei on renal hemodynamics and sodium concentration in plasma

The effect of mutual stimulation of the anterior and posterior hypothalamic nuclei on renal hemodynamics and sodium concentration in plasma was studied in anaesthetized cats. Unilateral renal vasodilation induced by anterior hypothalamic nucleus stimulation resulted in an ipsilateral increase in renal flow and increase in plasma sodium Concentration. Stimulation of the posterior hypothalamic nucleus in the presence of unilateral renal vasodilated kidney resulted in a sustained marked increase in plasms sodium level. These results are consistent with the view that the anterior hypothalamic nucleus stimulation produces a cardiovascular depressor effect and that the posterior hypothalamic nuclei produce a cardiovascular pressor effect. The proper combinations of the two physiological important variables, arterial pressure, renal vascular resistance. can effect large changes in tubular reabsorption of sodium, probably through intrarenal mechanisms

Haemodynamic effects of hypothalamic stimulation on skin and muscle venous beds.

Twenty three points mainly located in the posterior hypothalamus were stimulated to study its effect on the pressures, flows and calculated segmental resistances of the skin and muscle venous beds of hind limbs in the dog. Stimulation of these points produced a uniform pattern of rise in pressures of the muscle veins consisting consisting of a steep rise during stimulation followed by a rapid decline to basal level on its cessation. Skin veins, on the other hand registered a gradual increase in pressure during stimulation followed by a secondary rise during post stimulatory period. Large veins of both muscle and skin exhibited comparatively smaller pressure increases than small vein. These pressure changes were accompanied by a similar marked rise in systemic arterial pressure. Out of 23 points, 21 points produced similar increases in the calculated resistances of skin and muscle veins. Two points produced greater increase of the skin vein resistance. Total venous resistance of the limb was therefore, raised by all the points stimulated. None of these points elicited any fall in the pressures or calculated resistances of either the muscle or skin venous bed. Muscle venous outflow always registered an increase while the skin venous outflow recorded either a small increase or decrease or at times no change during the hypothalamic stimulation. These findings demonstrate that hypothalamic stimulation can profoundly alter the haemodynamics of the hind limb venous beds and actively mobilize the post capillary venous sections of both skin and muscle venous beds.