Postsynaptic inhibition of cat medullary dorsal horn neurons by stimulation of nucleus raphe magnus and other brain stem sites

Sensory responses of neurons in the medullary and spinal cord dorsal horn can be inhibited by stimulation of a number of brain stem regions. These regions include the nucleus raphe magnus (NRM), the nucleus reticularis gigantocellularis (NGC), the nucleus reticularis magnocellularis (NMC), the periaqueductal gray (PAG), and the nucleus cuneiformis (CU). The purpose of this study was to determine whether or not this inhibition is mediated by postsynaptic processes. Experiments were carried out on chloralose-anesthetized cats. The responses of 29 medullary dorsal horn (trigeminal subnucleus caudalis) cells were recorded with carbon-fiber microelectrodes. Included were cells which responded to noxious stimulation (nine cells) as well as cells which responded only to nonnoxious input. The presence of postsynaptic inhibition was tested by two indirect techniques. We studied the effects of conditioning stimulation of the five regions on the latency of antidromically activated cells and also on the firing rate of neurons excited by iontophoretically applied glutamate. Conditioning stimulation was associated with a block or increased latency of antidromic activation in 15 of 18 nociceptive and nonnociceptive neurons. These effects reflect membrane hyperpolarization, presumably resulting from postsynaptic inhibition. Furthermore, conditioning stimulation of these regions inhibited the glutamate-evoked firing of all 11 cells tested, also indicating a postsynaptic type of inhibition of medullary dorsal horn cells. Thus these results indicate that at least part of the inhibition induced by stimulation of the NRM, NGC, NMC, PAG, and the CU probably results from postsynaptic inhibitory mechanisms.