An l-DOPAergic relay from the posterior hypothalamic nucleus to the rostral ventrolateral medulla and its cardiovascular function in anesthetized rats

Abstract

We have proposed that l-3,4-dihydroxyphenylalanine ( l-DOPA) is a neurotransmitter in the central nervous system [Misu Y. et al. (1996) Prog. Neurobiol. 49, 415–454]. Herein, we attempt to clarify whether lesions in the posterior hypothalamic nucleus decrease the tissue content of l-DOPA in the rostral ventrolateral medulla. We also attempt to clarify whether or not endogenous l-DOPA is evoked by electrical stimulation of the posterior hypothalamic nucleus. It is possible that evoked l-DOPA functions as a transmitter candidate to activate pressor sites of the rostral ventrolateral medulla in anesthetized rats. Electrolytic lesions were made in the bilateral posterior hypothalamic nucleus by a monopolar direct current of 2 mA for 10 s, 10 days before measurements. The effect of the lesions was to selectively decrease the tissue content of l-DOPA by one-half in the right rostral ventrolateral medulla. Decreases in the amounts of dopamine, noradrenaline or adrenaline were not observed. Decreases were also not evident in the right caudal ventrolateral medulla. During microdialysis of the right rostral ventrolateral medulla, extracellular basal levels of l-DOPA and three types of catecholamine were consistently detectable by high-performance liquid chromatography with electrochemical detection. Tetrodotoxin (1 μM) perfused into the right rostral ventrolateral medulla gradually decreased basal levels of l-DOPA by 25%; it decreased basal levels of noradrenaline and adrenaline by 25–30% and dopamine levels by 40%. Intensive electrical stimulation of the ipsilateral posterior hypothalamic nucleus (50 Hz, 0.3 mA, 0.1 ms duration, twice for 5 min at an interval of 5 min) selectively caused the release of l-DOPA in a repetitive and constant manner. The stimulation was accompanied by hypertension and tachycardia. However, catecholamines were not released. Tetrodotoxin suppressed the release of l-DOPA, but partially inhibited hypertension with only a slight inhibition of tachycardia evoked by stimulation of the posterior hypothalamic nucleus. l-DOPA methyl ester, a competitive l-DOPA antagonist, was bilaterally microinjected into pressor sites of the rostral ventrolateral medulla at 1.5 μg×2 and 3 μg×2. The antagonist dose-dependently and consistently antagonized pressor and tachycardiac responses to mild transient stimulation of the unilateral posterior hypothalamic nucleus (33 Hz, 0.2 mA, 0.1 ms duration, for 10 s). In addition, the antagonist alone (3 μg×2) elicited hypotension and bradycardia. These results show that an l-DOPAergic relay may project from the posterior hypothalamic nucleus directly to pressor sites of the rostral ventrolateral medulla and/or indirectly to certain neurons near pressor sites in microcircuits of the same region. When released, l-DOPA appears to function tonically to activate pressor sites; it also appears to be involved in the maintenance and regulation of blood pressure and heart rate.