Evidence for peripheral, but not central modulation of trigeminal cold receptive cells in the rat.

The effects of locus coeruleus (LC), periaqueductal grey (PAG) and segmental stimulation (all of which are known to inhibit convergent nociceptive cells), were tested on the activity of cold receptive cells in the trigeminal system of the rat. LC and PAG stimulation from sites which inhibited convergent nociceptive cells had no effect on cells with cold receptive input in the trigeminal nucleus caudalis. Electrical or mechanical segmental stimulation caused suppression of activity in cold receptive trigeminal nucleus neurons. Recording from the trigeminal ganglion showed this suppression to be a property of the primary afferent cold receptors themselves and therefore it is not analogous to the proposed mechanism for the segmental inhibition of convergent nociceptive neurons.