Using cats anesthetized with chloralose-Nembutal we have studied the effect of high frequency stimulation of the medial and lateral structures of the posterior, tuberal, and anterior hypothalamus, and also of the central gray matter of the midbrain on the nociceptive jaw-opening reflex induced by tooth pulp stimulation. We recorded the EMG response of the digastric muscle as the index of the nociceptive reflex. We have shown that the EMG response of the nociceptive reflex is effectively suppressed on stimulation of all hypothalamic structures and the central gray matter, the threshold of the suppressive action being lower on stimulation of the central gray matter. The effects of the hypothalamic suppression of the nociceptive reflex were eliminated almost completely after naloxone administration, with the exception of the central gray matter, which is slightly more resistant to the action of this agent. After bilateral electrolytic destruction of the central gray matter the antinociceptive effect of the hypothalamus was retained, decreasing only insignificantly, The effect of complete suppression of the amplitude of the EMG response, similar to the effect of stimulation of the hypothalamus and the central gray matter, was also observed after intravenous administration of phenapidine, an opiate agonist with a marked central analgesic action. The neuronal and neurochemical mechanisms of hypothalamic suppression of the nociceptive jaw-opening reflex are discussed.
The neuronal organization of the hypothalamobulbar system of regulation of vascular tone was studied in experiments on cats anesthetized with chloralose and nembutal and immobilized by dithylin [suxamethonium iodide]. It was established that the descending influence of the posterior, tuberal, and paraventricular structures of the hypothalamus on the activity of antidromically identified sympathicoactive reticulospinal neurons of the ventrolateral region of the medulla oblongata is realized by mono-, oligo-, and polysynaptic mechanisms. It was shown in a series of experiments carried out in rats that animals subjected to chemical desympathization, by contrast with the controls, do not develop chronic neurogenic hypertension during a six-week stressor influence on higher nervous activity. The central and peripheral neurohumoral mechanisms of the regulation of vascular tone are discussed.
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