Sympathetic modulation of cold-receptive neurones in the trigeminal system of the rat.

The effects of cervical sympathetic electrical stimulation on the activity of cold-receptive neurones in the trigeminal system of the rat have been studied. Sympathetic stimulation caused excitation and/or suppression of cold-receptive cells in the trigeminal nucleus caudalis. The responses were complex but within any one cell low-frequency stimulation (below 5 Hz) usually (81% of cells) caused excitation, while high-frequency stimulation (above 10 Hz) caused excitation and suppression. The temperature of the cutaneous receptive fields was 'clamped' with a thermode. Measurements of surface and intradermal temperatures suggested that the temperature fall caused by sympathetic stimulation was not sufficient to account for the increased firing rate observed. The suppression was blocked by a low dose of phentolamine; it was mimicked by carotid occlusion and may be a consequence of vasoconstriction. The excitation was resistant to beta-blockade; it was best mimicked by the alpha-agonist phenylephrine. Similar frequency-dependent excitations and suppressions, and responses to pharmacological agonists, were obtained in recordings from cold-receptive primary afferent neurones in the trigeminal ganglion. The excitation may be a complex secondary effect, or there may be a direct alpha-receptor-mediated excitation of cold-receptive primary afferent fibres by the sympathetic system.