Diffuse noxious inhibitory controls (DNIC) in animals and in man.

Some neurones in the dorsal horn of the spinal cord are strongly inhibited when a nociceptive stimulus is applied to any part of the body, distinct from their excitatory receptive fields. This phenomenon was termed "Diffuse Noxious Inhibitory Controls" (DNIC). DNIC influence only convergent neurones: the other cell types which are found in the dorsal horn, including specific nociceptive neurones, are not affected by this type of control. In normal conditions, these inhibitions can be triggered only by conditioning stimuli which are nociceptive. The inhibitions are then extremely potent, affect all the activities of the convergent neurones and persist, sometimes for several minutes, after the removal of the conditioning stimulus. In fact, only activity of A delta- or A delta- and C-peripheral fibres can trigger DNIC. DNIC are sustained by a complex loop which involves supraspinal structures since, unlike segmental inhibitions, they can not be observed in animals in which the cord has previously been transsected at the cervical level. The ascending and descending limbs of this loop travel respectively through the ventro-lateral and dorso-lateral funiculi respectively. We proposed that DNIC result from the physiological activation of some brain structures putatively involved in descending inhibition. However, lesions of the following structures did not modify DNIC: Periaqueductal grey (PAG), Cuneiform nucleus, Parabrachial area, locus coeruleus/subcoeruleus, rostral ventromedial medulla (RVM) including Raphe Magnus, Gigantocellularis and Paragigantocellularis nuclei. By contrast, lesions of Subnucleus Reticularis Dorsalis (SPD) in the caudal medulla strongly reduced DNIC. Both electrophysiological and anatomical data support the involvement of SRD neurones in spin-bulbo-spinal loop(s). Indeed, they are unresponsive to visual, auditory or proprioceptive stimulation but are preferentially or exclusively activated by nociceptive stimuli with a "whole-body receptive field"; they encode precisely the intensity of cutaneous and visceral stimulation within the noxious range and are exclusively activated by cutaneous A delta- or A delta- and C-fibre peripheral volleys; they send descending projections through the dorsolateral funiculus that terminate in the dorsal horn at all levels of the spinal cord. In man, exactly analogous results have been obtained by means of combined psychophysical measurements and recordings of nociceptive reflexes. Electrical stimulation of the sural nerve at the ankle simultaneously induces a nociceptive reflex in a flexor muscle of the knee (the RIII reflex) and a painful sensation from the territory of the nerve. Painful heterotopic conditioning stimuli, no matter whether thermal, mechanical or chemical in nature, depress both the reflex and the associated painful sensation, with stronger effects being observed with more intense conditioning stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)