MECHANISMS UNDERLYING ANTINOCICEPTION PROVOKED BY HETEROSEGMENTAL NOXIOUS-STIMULATION IN THE RAT TAIL-FLICK TEST

Physiological studies were conducted to examine the effects of noxious stimulation of one hindpaw or one forepaw on the latency of the withd rawal reflex in the tail-flick test in lightly anesthetized spinally i ntact or transected rats. Male Sprague-Dawley rats were anesthetized w ith an intraperitoneal injection of a mixture of Na-pentobarbital (20 mg/kg) and chloral hydrate (120 mg/kg). After baseline readings were t aken in the tail-flick test, the effects of various noxious stimuli ap plied to remote body regions were assessed. The noxious stimuli includ ed unilateral or bilateral hindpaw or unilateral forepaw thermal (imme rsion in water at 55 degrees C for 90 s), unilateral or bilateral chem ical (subcutaneous hindpaw injection of 50 mu l of 5% formalin) and un ilateral or bilateral mechanical (pinch with clamp exerting a force of 14.75 or 27 N) stimulation. Bilateral chemical and thermal, and unila teral thermal stimulation induced an antinociceptive response, consist ing of an increase in tail-flick latency, peaking at 30 s after stimul ation. Recovery to baseline levels occurred over the next 3-6 min. The antinociceptive effect of noxious thermal stimulation was attenuated or absent in chronically spinalized animals (T6/7) following hindpaw o r forepaw immersion, respectively. Noxious mechanical stimulation had no effect on tail-flick latency. The data provide evidence that a noxi ous thermal or chemical stimulus produces a heterosegmental antinocice ptive effect which is mediated in part via a supraspinal mechanism and in part via a local spinal mechanism. As an effect of noxious thermal stimulation was seen in spinally transected rats and as noxious mecha nical stimulatiion failed to evoke an effect in spinally intact rats, the data indicate that the antinociceptive responses in this paradigm are different from those associated with DNIC (diffuse noxious inhibit ory controls).