Physiological properties of the lamina I spinoparabrachial neurons in the rat

Single-unit extracellular recordings of spino-parabrachial (spino-PB) neuro ns (n = 53) antidromically driven from the contralateral parabrachial (PB) area were per for med in the lumbar cord in anesthetized rats. All the spin o-PB neurons were located in the lamina I of the dorsal hem. Their axons ex hibited conduction velocities between 2.8 and 27.8 m/s, in the thin myelina ted fibers range. They had an extremely low spontaneous activity (median = 0.064 Hz) and a small excitatory receptive field (less than or equal to 2 t oes or pads). They were all activated by both peripheral A (mainly A delta) and C fibers after intense transcutaneous electrical stimulation. Their di scharge always increased in response to noxious natural stimuli of increasi ng intensities. The great majority (75%) of spino-PB neurons were nocicepti ve specific, i.e., they were excited only by noxious stimuli. The remaining (25%) still were excited primarily by noxious stimuli but also responded m oderately to innocuous stimuli. Almost all spino-PB neurons (92%, 49/53) we re activated by both mechanical and heat noxious stimuli. Among them, 35% w ere in addition moderately activated by noxious cold (thresholds between +2 0 and -10 degrees C). Only (8%, 4/53) responded exclusively to noxious heat . Spino-PB neurons clearly encoded the intensity of mechanical (n = 39) and thermal (n = 38) stimuli in the noxious range, acid most of the individual stimulus-response functions were monotonic and positive up to 40/60 N . cm (-2) and 50 degrees C, respectively. For the mechanical modality, the mean threshold was 11.5 +/- 1.25 N . cm(-2) (mean +/- SE), the response increase d almost linearly with the logarithm of the pressure between 10 and 60 N . cm(-2), the mean p(50) (pressure evoking 50% of the maximum response) and t he maximum responsiveness were: 30 +/- 2.4 N . cm(-2) and 40.5 +/- 5 Hz, re spectively. For the thermal modality, the mean threshold was 43.6 +/- 0.5 d egrees C, the mean curve had a general sigmoid aspect, the steepest portion being in the 46-48 degrees C interval, the mean t(50) and the maximum resp onsiveness were: 47.4 +/- 0.3 degrees C and 40 +/- 4.4 Hz, respectively. Mo st of the spino-PB neurons tested (13/16) had their noxiously evoked respon ses clearly inhibited by heterotopic noxious stimuli. The mean response to noxious stimuli during heterotopic stimuli was 31.7 +/- 6.1% of the control response. We conclude that the nociceptive properties of the lamina I spin o-PB neurons are reflected largely by those of PB neurons that were suggest ed to be involved in autonomic and emotional/aversive aspects of pain.