Functionally different neurons are organized topographically in the rostral ventrolateral medulla of rabbits

To examine whether the sympatho-excitatory neurons in the rostral ventrolateral medulla (RVLM) were divided into subgroups, gamma-amino-n-butyric acid (GABA) was injected into multiple sites of the medulla while simultaneous recordings of blood flows were made from the renal artery with an ultrasonic pulsed Doppler flowmeter and from the ear skin and muscles of fore- and hind-limbs with laser Doppler flow meters in urethane-anesthetized, vagotomized and immobilized rabbits. The magnitude of the responses of mean systemic arterial pressure (MAP), heart rate (HR) and conductance of each vascular bed, calculated by its blood flow and MAP, were represented as a contour map of the ventral surface of the medulla. Microinjection of GABA (50 mM, 9-27 nl) into the RVLM produced a decrease in MAP (-27 +/- 10 mmHg) and HR (-14 +/- 7 beat min-1) and an increase in the vascular conductance of the ear skin (ESC; 33 +/- 25 microliters min-1 100 g-1 (mmHg)-1), the fore-limb muscle (FLMC; 93 +/- 84 microliters min-1 100 g-1 (mmHg)-1), the hind-limb muscle (HLMC; 18 +/- 7 microliters min-1 100 g-1 (mmHg)-1) and the kidney (KC; 49 +/- 25 microliters min-1 (mmHg)-1). Comparing the sites into which the injection of GABA evoked the maximal response of MAP (the 'center' of the RVLM), the maximal responses of HR, ESC and KC were obtained from caudal, caudo-medial and slightly rostral sites, respectively. In more than half of cases, the maximal responses of FLMC and HLMC were obtained from the 'center' of the RVLM. These results indicated that the functionally different sympatho-excitatory reticulospinal neurons are located at different sites in the RVLM, although they considerably intermingle with each other.