Evidence that noradrenaline reduces tonic descending inhibition of cat spinal cord nociceptor-driven neurones

To determine whether noradrenaline (NA) is involved in the powerful tonic descending inhibition which exists on dorsal horn nociceptor-driven neurones, their response to noxious radiant heat was tested with drugs that decrease (reserpine) or enhance (nisoxetine, desipramine) NA synaptic transmission. In animals that were depleted of NA by reserpine (1.0 mg/kg i.p.) the degree of tonic inhibition (as determined by comparing the response in the normal vs. cold block states of the cord) was greater when compared to controls. Conversely, the extent of tonic descending inhibition on these neurones was decreased after the intravenous administration of NA uptake blockers, nisoxetine HC1 and desipramine HC1 (6 mg/kg). Thus we conclude that NA is not involved in mediating, but rather appears to reduce this tonic descending inhibition.