Mechanisms involved in the depolarization of cutaneous afferents produced by segmental and descending inputs in the cat spinal cord.

The relative contribution of specific and unspecific (potassium) components involved in the generation of primary afferent depolarization (PAD) of cutaneous fibres was analyzed in the spinal cord of the anesthetized cat. To this end we examined the correlation between the intraspinal threshold changes of single afferent fibres in the sural nerve produced by segmental and descending inputs and the negative DC potential shifts produced by these same stimuli at the site of excitability testing, the latter taken as indicators of the changes in extracellular concentration of potassium ions. Stimulation of the ipsilateral brain-stem reticular formation and of the contralateral red nucleus with 100-200 Hz trains reduced very effectively the intraspinal threshold of sural nerve fibres ending in the dorsal horn practically without producing any negative DC potential shifts at the site of excitability testing. However, negative DC potential shifts were produced more ventrally, in the intermediate nucleus and/or motor nucleus. Stimulation of the sural and superficial peroneus nerves with pulses at 2 Hz and strengths below 2 xT, also reduced the intraspinal threshold of single SU fibres without producing significant DC potential changes at the site of excitability testing. On the other hand, 100 Hz trains with strengths above 2 xT produced negative DC potential shifts and a proportional reduction of the intraspinal threshold of the SU fibres. The PAD of sural fibres produced by stimulation of rubro-spinal and reticulo-spinal fibres as well as by stimulation of sensory nerves with low frequency trains was unaffected or slightly increased, by i.v. injection of strychnine (0.2 mg/kg), but was readily abolished 5-10 min after the i.v. injection of picrotoxin (2 mg/kg). The results suggest that activation of reticulo-spinal and rubro-spinal fibres, as well as stimulation of cutaneous nerves with low frequencies and low strengths, produce PAD of cutaneous fibres involving activation of specific interneuronal pathways with interposed last-order GABAergic interneurons. The potassium component of the PAD produced by cutaneous fibres becomes dominant with high stimulus frequencies and strengths.