P. Raboisson et al., EFFECTS OF SUBCUTANEOUS FORMALIN ON THE ACTIVITY OF TRIGEMINAL BRAIN-STEM NOCICEPTIVE NEURONS IN THE RAT, Journal of neurophysiology, 73(2), 1995, pp. 496-505
1. The subcutaneous injection of Formalin (5%, 50 mu l) into the recep
tive field of convergent (wide dynamic range) nociceptive neurons in t
he spinal dorsal horn has previously been reported to produce a prolon
ged biphasic response with a time course similar to that observed in b
ehavioral experiments. However, conflicting data in other studies led
us to examine the effects of Formalin on the activity of convergent no
ciceptive neurons at two levels of the trigeminal (V) brain stem compl
ex, namely V subnuclei oralis (Sp5O) and caudalis (Sp5C). 2. Single ne
uron activity was extracellularly recorded in anesthetized rats. Each
neuron was classified as convergent on the basis of its responses to b
oth mechanical and transcutaneous electrical stimuli applied to its me
chanoreceptive held (RF). All neurons responded to innocuous and noxio
us mechanical stimuli and had electrically evoked responses correspond
ing 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 i
nto the center of their RF. 3. Three groups of neurons were distinguis
hed: 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%); Sp
5C, n = 10 (31%)], and a third group that responded with two phases se
parated 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 disch
arge frequency of the first phase of biphasic neurons were significant
ly greater than for monophasic neurons in Sp5O (nonsignificant in Sp5O
). The second tonic period of excitation of all biphasic neurons was g
radual in onset and offset, and long in duration (23-39 min). The dura
tion of the second phase was significantly longer for the biphasic neu
rons in Sp5C than for those in Sp5O. 5. Our findings suggest that the
mono- and biphasic neurons may have different roles in the transmissio
n of nociceptive information induced by the peripheral injection of Fo
rmalin. Moreover, the difference in proportion of biphasic neurons bet
ween Sp5C and Sp5O could indicate that these structures serve distinct
functions in nociceptive processing at the V brain stem level: the hi
gher proportion of biphasic neurons in Sp5C support the view that this
latter subnucleus may be more involved than Sp5O in tonic inflammator
y nociceptive process and/or in central hyperalgesic states.