Heterotopic activation of A delta and C fibres triggers inhibition of trigeminal and spinal convergent neurones in the rat.

1. Extracellular recordings were made from fourteen non-noxious only and nineteen convergent neurones in trigeminal nucleus caudalis of halothane-anaesthetized rats. All the neurones studied were excited by the continuous micro-electrophoretic ejection of an excitatory amino acid, DL-homocysteic acid (DLH), with mean currents of 38.0 +/- 7.2 and 39.8 + 6.5 nA producing steady discharges of 35.0 +/- 3.3 and 31.8 +/- 1.3 spikes/s from the non-noxious only and convergent neurones respectively. 2. The repeated percutaneous application (100 trials; 0.66 Hz) of single square-wave stimuli (10 mA; 2 ms) to the tail always induced a biphasic depression of the activity of the convergent, but never of the non-noxious only, neurones. Both the early and late components of this inhibition occurred at shorter latencies when the base rather than the tip of the tail was stimulated. Differences in latencies from the two sites of stimulation (100 mm apart) were used to estimate the conduction velocities of the peripheral fibres which were triggering the inhibitions. 3. The cumulated results showed that, for the onset of the earlier component of the inhibition, the mean difference between the latencies from the two sites of stimulation was 13.6 +/- 1.9 ms, corresponding to a peripheral conduction velocity of 7.3 +/- 0.3 m/s, which is in the A delta-fibre range. For the onset of the late component of inhibition, the mean difference was 147.7 +/- 14.9 ms, corresponding to a peripheral conduction velocity of 0.68 +/- 0.07 m/s, which is in the C-fibre range. 4. When currents of different intensities were applied percutaneously to the two stimulation sites, the thresholds for obtaining the A delta component were in the range 0.25-0.5 mA whereas the C component appeared with currents 1-2 mA. A clear relationship between current intensity and magnitude of inhibition was observed in the 0.25-2 mA range for the A delta component and in the 1-5 mA range for the C component. 5. In an additional series of experiments recordings were made from eleven convergent neurones in the dorsal horn of the lumbar spinal cord. By using essentially the same experimental procedure the effects of repetitive application (100 trials, 0.66 Hz) of percutaneous electrical stimuli (1 or 10 mA, 2 ms) applied to the muzzle, were studied on the steady discharges (42.3 +/- 5.4 spikes/s) induced by DLH. The application of the 10 mA stimuli induced a biphasic depression of activity, whereas only an early component was observed following 1 mA stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)