THE SPINO(TRIGEMINO)PONTOAMYGDALOID PATHWAY - ELECTROPHYSIOLOGICAL EVIDENCE FOR AN INVOLVEMENT IN PAIN PROCESSES

1. Neurons were recorded in the parabrachial (PB) area, located in the dorsolateral region of the pons (with the use of extracellular micropipette), in the anesthetized rat. Parabrachioamygdaloid (PA) neurons (n = 67) were antidromically identified after stimulation in the centralis nucleus of the amgydala (Ce). The axons of these neurons exhibit a very low conduction velocity, between 0.26 and 1.1 m/s, i.e., in the unmyelinated range. 2. These PA neurons were located in a restricted region of the PB area: the subnuclei external lateral (PBel) and external medial (PBem). A relative somatotopic organization was found in this region. 3. These units were separated into two groups: 1) a group of nociceptive-specific (NS) neurons (69%), which responded exclusively to noxious stimuli, and 2) a group of nonresponsive (NR) neurons (31%). 4. The NS neurons exhibited low or lacked spontaneous activity. They responded exclusively to mechanical (pinch or squeeze) and/or thermal (waterbath or waterjet >44°C) noxious stimuli with a marked and sustained activation with a rapid onset and generally without afterdischarge. Noxious thermal stimuli generally induced a stronger response than the noxious mechanical stimuli. These neurons exhibited a clear capacity to encode thermal stimuli in the noxious range: 1) the stimulus-response function was always positive and monotonic; 2) the slope of the curve progressively increased up to a maximum where it was very steep, then the steepness of the slope decreased close to the maximum response; and 3) the mean threshold was 44.1 ± 2°C, and the point of steepest slope of the mean curve was around 47°C. 5. The excitatory receptive fields of the NS neurons were large in the majority (70%) of the cases and included several areas of the body. A more marked activation was often obtained from stimuli applied to one part of the body, denoted as the preferential receptive field (PRF). In the other cases (30%), the excitatory receptive field was relatively small (SRF) and restricted to one part of the body (the tail, a paw, a hemiface, or the tongue). Both the PRF and SRF were more often located on the contralateral side. In addition, noxious stimuli applied outside the excitatory receptive field were found to strongly inhibit the responses of NS neurons. 6. All the NS neurons responded to intense transcutaneous electrical stimulation applied to the PRF or SRF with two peaks of activation. We found that the early and the late peak were triggered by the activity of peripheral fibers with conduction velocities in the 8-20 m/s and 0.5-0.8 m/s range, i.e., Aδ- and C-fibers, respectively. In numerous cases, the response due to C-fibers exhibited a wind-up phenomenon during repetitive stimulation (0.66 Hz) of the PRF or SRF and a wind-down phenomenon during repetitive stimulation applied to other parts of the excitatory field. In some cases an excessive depolarization was observed for the response to intense and repetitive stimulation in the PRF and SRF. 7. It is concluded that the PBel and the PBem constitute an anatomic and electrophysiological entity, which is implicated in the transmission of nociceptive messages in the spino(trigemino)pontoamygdaloid pathway. The role of such a pathway in pain is a matter of speculation. It is suggested that this system could be involved in the emotional-affective (fear and memory of the aggression); behavioral (vocalization, flight, freezing, defense, and offense); and autonomic (pupil dilatation, cardiorespiratory and adrenocortical responses, and micturition) reactions to noxious events.