SPINE(TRIGEMINO) PARABRACHIOHYPOTHALAMIC PATHWAY - ELECTROPHYSIOLOGICAL EVIDENCE FOR AN INVOLVEMENT IN PAIN PROCESSES

1. Parabrachiohypothalamic (PB-H) neurons (n = 71) were recorded with extracellular micropipettes in the parabrachial (PB) area and were ant idromically driven from the ventromedial nucleus (VMH) or the retrochi asmatic area (RCh) of the hypothalamus, in the anesthetized rat. The s pontaneous activity of these neurons was very low, (10th percentile < median frequency < 90th percentile were 0.01 < 0.2 < 7 Hz). The axons of these neurons exhibited a very slow conduction velocity in the rang e of 0.2-1.4 m/s, i.e., corresponding to thin unmyelinated fibers. 2. Most PB-H neurons (89%) were located in the mesencephalic division of the PB area (mPB) mainly in the superior lateral (mPBsl) and external lateral (mPBel) subnuclei. 3. These units were separated in three grou ps: 1) a group of nociceptive-specific (NS) neurons (49%) activated by mechanical and/or thermal (heat) cutaneous stimuli only in noxious ra nge; 2) a group of inhibited neurons (7%), not activated by any of the mechanical or thermal cutaneous stimuli but inhibited, by al least on e of these stimuli, which had to be in noxious range; and 3) a group o f nonresponsive neurons (44%). 4. The NS neurons responded exclusively to mechanical (pinch or squeeze) and/or thermal (waterbath or waterje t > 44 degrees C) noxious stimuli with a rapid onset, a marked and sus tained activation, and generally no afterdischarge. The magnitude of t he responses was between 2 and 30 Hz with a mean value of 14.8 +/- 1.4 Hz (mean +/- SE, n = 49). 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 mean curve increased up to a maximum (between 46 and 50 degrees C) then bey ond the slope decreased; and 3) the mean threshold was 44.3 +/- 2.2 de grees C. 5. The excitatory receptive fields of the NS neurons were oft en large including all (22% of the population) or several (67% of the population) parts of the body. In the few remaining cases (11%) they w ere restricted to one part of the body. In addition, in several cases, noxious stimuli applied outside the excitatory receptive field were f ound to strongly inhibit the discharge of NS neurons. 6. Most NS neuro ns responded to intense transcutaneous electrical stimulation with two peaks of activation. We determined, measuring the latency differences between the base and the tip of the tail (50 mm apart), that the earl y and the late peaks were triggered by the activation of peripheral fi bers with conduction velocities in the 5- to 17-m/s and 0.5- to 1.8-m/ s range, which could therefore be A delta- and C fibers, respectively. 7. In a subgroup of PB-H neurons (n = 32) tested in addition with col d stimuli (0 degrees C), 13% were strongly and exclusively excited by cold stimuli, 28% responded to both cold and mechanical or hot stimuli , and the remainder were nonresponsive to cooling. 8. It is concluded that this part of the mPB (primarily the mPBsl and mPBel) constitute a n anatomic and electrophysiological entity, which is implicated in the transmission of nociceptive messages in a spino (trigemino) parabrach iohypothalamic pathway. The role of such a pathway in pain is a matter of speculation. It is suggested that this system could be involved in motivational (affective-defensive, feeding, and sexual) reactions, en ergy (brown adipose tissues metabolism), and neuroendocrine (corticoad renal axis) adaptations to a noxious event. A role in thermoregulative integration must be also considered.