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We have studied the responses to electrical and chemical stimulation of the ventrolateral medulla in the chloralose-anesthetized, paralyzed, artificially ventilated rat. Locations of most active pressor responses were compared to regions containing neurons labeled immunocytochemically for phenylethanolamine N-methyltransferase (PNMT), the enzyme catalyzing the synthesis of adrenaline. Elevations of arterial pressure (+81.6 +/- 2.5 mm Hg) and cardioacceleration (+73 +/- 13.6 bpm) were elicited with low current (5 times threshold of 9.5 +/- 1.1 microA) electrical stimulation in a region of rostral ventrolateral medullary reticular formation we have termed the nucleus reticularis rostroventrolateralis (RVL). Electrical stimulation of the RVL increased plasma catecholamines (16.8-fold for adrenaline, 5.3-fold for noradrenaline, and 1.9-fold for dopamine) and vasopressin (1.7-fold before spinal transection, 4.7-fold after). The location of the most active pressor region in the ventrolateral medulla corresponded closely with the location of C1 adrenaline-synthesizing (PNMT-containing) neurons. In addition, the location of the most active pressor region in the dorsomedial medulla corresponded with the location of a bundle of PNMT-containing axons. Unilateral injections into the RVL of the excitatory amino acid monosodium L-glutamate (50 pmol to 10 nmol), but not saline, caused transient dose-dependent and topographically specific elevations (maximum +71.6 +/- 4.9 mm Hg) of arterial blood pressure and tachycardia. Injections of the rigid structural analogue of glutamate, kainic acid, caused large, prolonged (at least 15 min) pressor responses and tachycardia. Unilateral injections of the inhibitory amino acid gamma-aminobutyric acid (GABA) into the RVL caused transient dose-dependent hypotension (maximum -40.8 +/- 6.6 mm Hg) and bradycardia, whereas the specific GABA antagonist bicuculline caused prolonged (10 to 20 min) elevations (+64.2 +/- 6.8 mm Hg) of arterial pressure and tachycardia. By contrast, injections of the glycine antagonist strychnine had no significant effect. Bilateral injections of the neurotoxin, tetrodotoxin, dropped arterial pressure to low levels (51.7 +/- 4.7) not changed by subsequent spinal cord transection at the first cervical segment (52.5 +/- 6.2). We propose the following. (1) Neurons within the RVL, most probably C1 adrenaline-synthesizing neurons, exert an excitatory influence on sympathetic vasomotor fibers, the adrenal medulla, and the posterior pituitary. (2) These neurons are tonically active and under tonic inhibitory control, in part via GABAergic mechanisms--perhaps via the nucleus of the solitary tract (NTS).(ABSTRACT TRUNCATED AT 400 WORDS)
Ross et al. (Wed,) studied this question.