Stimulation within the rostral ventrolateral medulla can evoke monosynaptic GABAergic IPSPs in sympathetic...

The inhibitory responses of identified sympathetic preganglionic neurons (SPNs) to stimulation within the rostral ventrolateral medulla (RVLM) were studied to determine their nature and pharmacology. Whole cell patch-clamp recordings were made from 36 SPNs in the upper thoracic segments of the spinal cord in a neonatal rat brain stem-spinal cord preparation. Neurons were identified as SPNs on the basis of their antidromic activation after stimulation of the ipsilateral segmental ventral root and their morphology and location in the intermediolateral cell column and intercalated nucleus. In all SPNs, electrical stimulation of the RVLM evoked fast excitatory postsynaptic potentials (EPSPs) that were mediated by non-N-methyl-D-aspartate (NMDA) and NMDA receptors. These excitatory responses were the most prominent response in control artificial cerebrospinal fluid and have been studied previously. In 22 of the SPNs, RVLM stimulation also elicited fast inhibitory postsynaptic potentials (IPSPs), which increased in amplitude as the membrane was depolarized. Five of these neurons were not studied further as they responded occasionally with IPSPs that had highly variable onset latencies indicating the involvement of a polysynaptic pathway. In the remaining SPNs (n = 17), the evoked IPSPs persisted in the presence of the excitatory amino acid antagonists 6-cyano-7-nitroquinoxaline-2,3,-dione and D,L-2-amino-5-phosphonopentanoic acid. In eight of these SPNs, it was necessary to block the EPSPs to reveal the IPSPs. In the 7 SPNs tested, the onset latencies of the IPSPs were not significantly different from the onset latencies of the fast EPSPs. The low sweep-to-sweep fluctuations in onset latency of individual IPSPs (absolute average deviation: 0.4 ms) indicated that the IPSPs were elicited by activation of a monosynaptic pathway. The amplitudes of the IPSPs decreased in amplitude as the membrane was hyperpolarized and reversed in polarity at -70.3 +/- 1.7 mV (mean +/- SD), which was close to the equilibrium potential for chloride ions. In addition, in seven SPNs, bath applications of 5 microM bicuculline, a gamma-aminobuturic acid-A (GABAA) antagonist, abolished or reduced the evoked IPSPs. Five SPNs also were studied that displayed ongoing IPSPs. The amplitudes of these IPSPs increased with membrane depolarization and were blocked by bath applications of 5 microM bicuculline, suggesting that they also were mediated by activation of GABAA receptors. These results demonstrate the existence of a bulbospinal GABAergic pathway impinging directly onto SPNs. This pathway may be tonically active in the neonatal rat brain stem-spinal cord preparation.