Low threshold mechanical stimulation of the vagina depresses dorsal horn unit activity in the spinal cat

Low threshold (0.5-15 g) mechanical punctate stimulation of the labia and vagina depressed dorsal horn neurones in the cat with spinal transection at L1. Similar stimulation 2 mm or more from the outer margin of the labia was without effect. The depression was observable within 1-2 sec, and usually required 4-6 min for full recovery, although a smaller more prolonged depression may also have occurred. Stronger stimuli evoked greater inhibition, requiring longer times for recovery. Vaginal stimulation depressed 9 of 10 sensory dorsal horn neurones (including non-nociceptive, wide dynamic range and nociceptive specific). Depression included a decrease in spontaneous activity and a decrease of naturally elicited responses to noxious and innocuous cutaneous stimuli; therefore the effect is not modality related. One unit without sensory input was unaffected by vaginal stimulation. In the one case tested, vaginal stimulation depressed glutamate-evoked excitation, suggesting that the depression of naturally, elicited responses was via a postsynaptic mechanism.