L-glutamate mapping of cardioreactive areas in the rat posterior hypothalamus

The posterior hypothalamus has long been regarded as a CNS region that provides a sympatho-excitatory influence on the cardiovascular system and functions in thermoregulation as a heat-producing center. These ideas have been based on data derived from electrical stimulation and lesion experiments. These methods are now regarded as inadequate for accurate localization of CNS functions. In order to re-examine the function of the posterior hypothalamus, a chemical stimulation study was performed. Microinjections of the excitatory amino acid L-glutamate were made in the posterior hypothalamus of pentobarbital-anesthetized rats. This method was used in combination with autoradiography to localize [3H]glutamate, which was included with the injectate. No pressor responses were elicited from any site within the posterior hypothalamus. In contrast, chemical stimulation of the posterior periventricular hypothalamus produced large decreases in blood pressure (delta BP = 25 mm Hg) and in heart rate (delta HR = 30 bpm). Injections in the posterior hypothalamic nucleus elicited small reductions in blood pressure and heart rate. Injections in the dorsal hypothalamic area produced a similar small response. Injections ventral to the periventricular zone were also weakly reactive, but a significant elevation in rectal temperature was seen. To summarize, the most cardioresponsive area was within the periventricular zone caudal to the posterior hypothalamic nucleus and was situated near the fasciculus retroflexus.