Sympathoexcitatory influence of a fast conducting raphe-spinal pathway in the rat

Experiments were carried out on 20 pentobarbitone sodium (alpha-chloralose supplemented)-anesthetized, artificially ventilated, and paralyzed rats. The possibility was explored that raphe-spinal neurons with myelinated axons arising in the rostral part of raphe obscurus provide excitatory drive to sympathetic neurons. Electrical stimulation within obscurus was observed to evoke an "early" sympathoexcitatory response compatible with its conduction over such a pathway. The effect of the microinjection of excitatory and inhibitory amino acids [DL-homocysteic acid (DLH) and gamma-aminobutyric acid (GABA), respectively] on the evoked response was studied at the sites of electrical stimulation. The size of the early response was increased by 91.7 +/- 24.4% (n = 7) and depressed by -48 +/- 4.8% (n = 7) by DLH and GABA, respectively. Saline was without effect (-14.5 +/- 12.2%, n = 6). The evoked responses were decreased when blood pressure was raised by administration of phenylephrine (2-6 micrograms/kg iv) and totally suppressed by an increase in blood pressure of 19.3 +/- 4.3 mmHg (baseline 89.1 +/- 2.5 mmHg, n = 7). It is concluded that some cell bodies located in rostral raphe obscurus that project to the spinal cord relay excitatory drive to sympathetic neurons.