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)