Diffuse Noxious Inhibitory Controls (DNIC) were investigated in anaesthetized intact
rats, with or without p-chlorophenylalanine (pCPA) pretreatment. Dorsal horn convergent
neurones responding to both noxious and non-noxious stimuli applied to their excitatory
receptive field located on the distal part of the hindlimb, were recorded in the lumbar
spinal cord. These cells received A alpha and C fibre inputs as shown by electrical
stimulation of their receptive field. In control animals, the evoked responses to
C fibre inputs could be strongly inhibited by various noxious stimuli applied to widespread
areas of the body: the inhibitory effects induced by intraperitoneal administration
of bradykinin, pinch applied to the tail or muzzle and noxious heat applied to the
tail were of 77%, 87%, 83% and 61% respectively. Long-lasting post-effects were seen
in most cases after cessation of the application of the conditioning stimulus. Pretreatment
with pCPA (300 mg/kg, i.p., 3 days) resulted in a strong reduction of DNIC. The inhibitory
effects induced by intraperitoneal administration of bradykinin, pinch applied to
the tail or muzzle and noxious heat applied to the tail were reduced by 47%, 63%,
87% and 63%, respectively. The post-effects were also reduced both in terms of magnitude
and duration. These results strongly suggest that serotonergic pathways partially
involved in DNIC. They are discussed with reference to the descending control systems,
originating from the caudal raphé, which modulate the transmission and/or the integration
of nociceptive messages at the spinal level. The possible involvement of DNIC and
5-HT mechanisms to the hypo-algesic phenomena induced by hyper-stimulation is also
suggested.