EFFECTS OF SUBCUTANEOUS FORMALIN ON THE ACTIVITY OF TRIGEMINAL BRAIN-STEM NOCICEPTIVE NEURONS IN THE RAT

1. The subcutaneous injection of Formalin (5%, 50 mu l) into the receptive field of convergent (wide dynamic range) nociceptive neurons in the spinal dorsal horn has previously been reported to produce a prolonged biphasic response with a time course similar to that observed in behavioral experiments. However, conflicting data in other studies led us to examine the effects of Formalin on the activity of convergent nociceptive neurons at two levels of the trigeminal (V) brain stem complex, namely V subnuclei oralis (Sp5O) and caudalis (Sp5C). 2. Single neuron activity was extracellularly recorded in anesthetized rats. Each neuron was classified as convergent on the basis of its responses to both mechanical and transcutaneous electrical stimuli applied to its mechanoreceptive held (RF). All neurons responded to innocuous and noxious mechanical stimuli and had electrically evoked responses corresponding to both A and C fiber afferent inputs. Seventeen Sp5O and 32 Sp5C convergent neurons received an injection (50 mu l sc) of 5% Formalin into the center of their RF. 3. Three groups of neurons were distinguished: one group that was not activated by the Formalin injection (Sp5O, n = 1: Sp5C, n = 2), another group that responded only with an early and short-lasting response [monophasic neurons: Sp5O, n = 11 (65%); Sp5C, n = 10 (31%)], and a third group that responded with two phases separated by a short period of quiescence [biphasic neurons: Sp5O, n = 5 (29%); Sp5C, n = 20 (62.5%)]. The proportion of biphasic neurons was significantly higher in Sp5C than in Sp5O. 4. The first phases of the Sp5O neurons were not significantly different in terms of duration and frequency from those of the Sp5C neurons. However, duration and discharge frequency of the first phase of biphasic neurons were significantly greater than for monophasic neurons in Sp5O (nonsignificant in Sp5O). The second tonic period of excitation of all biphasic neurons was gradual in onset and offset, and long in duration (23-39 min). The duration of the second phase was significantly longer for the biphasic neurons in Sp5C than for those in Sp5O. 5. Our findings suggest that the mono- and biphasic neurons may have different roles in the transmission of nociceptive information induced by the peripheral injection of Formalin. Moreover, the difference in proportion of biphasic neurons between Sp5C and Sp5O could indicate that these structures serve distinct functions in nociceptive processing at the V brain stem level: the higher proportion of biphasic neurons in Sp5C support the view that this latter subnucleus may be more involved than Sp5O in tonic inflammatory nociceptive process and/or in central hyperalgesic states.