Stimulus-function, wind-up and modulation by diffuse noxious inhibitory controls of responses of convergent neurons of the spinal trigeminal nucleus oralis

Extracellular unitary recordings were made from 53 spinal trigeminal nucleu s oralis (Sp5O) convergent neurons in halothane-anaesthetized rats. The neu rons had an ipsilateral receptive field including mainly oral or perioral r egions. They responded to percutaneous electrical stimulation with two peak s of activation. The first had a short latency (4.3 +/- 0.3 ms) and low thr eshold (0.35 +/- 0.04 mA), whereas the second had a longer latency (68.1 +/ - 3.4 ms) and higher threshold (7.3 +/- 0.5 mA). Intracutaneous injection o f capsaicin (0.1%) produced a strong and rapid reduction of the long-latenc y responses of Sp5O convergent neurons with little effect on the short-late ncy responses. In most cases (73%), the long-latency responses exhibited a wind-up phenomenon during repetitive (0.66 Hz) suprathreshold electrical st imulation. These results suggest that C-fibres mediate the long-latency res ponse of Sp5O convergent neurons. Regarding the C-fibre-evoked responses, a linear relationship between the intensity of the applied current and the m agnitude of the response was found within the one to three times threshold range. The Sp5O convergent neurons also encoded the intensity of mechanical stimuli applied to the skin or mucosa in the 5-50 g ranges. The evoked act ivity of Sp5O convergent neurons could be suppressed by noxious heat applie d to the tail (52 degrees C) and long-lasting poststimulus effects followed this. These findings show that convergent neurons in the Sp5O resemble tho se in the deep laminae of the spinal dorsal horn and spinal trigeminal nucl eus caudalis, and further support that the Sp5O plays a part in the process ing of nociceptive information from the orofacial region.