Primate raphe- and reticulospinal neurons: effects of stimulation in periaqueductal gray or VPLc thalamic nucleus

Abstract

Recordings were made from 132 raphe- and reticulospinal tract neurons in the medial part of the lower brain stem in 32 anesthetized monkeys. Recording sites were in the nucleus raphe magnus, the rostral nucleus raphe obscurus, and the reticular formation adjacent to the raphe. The neurons were identified by antidromic activation from the upper lumbar spinal cord. Of the population sampled, 83 cells were activated antidromically from the left dorsal lateral funiculus (DLF), 32 from the right DLF, and 17 from both sides. The mean latency for antidromic activation was 8.2 +/- 7.1 ms, corresponding to a mean conduction velocity of 22.8 m/s. No conduction velocities characteristic of unmyelinated axons were observed. Collision tests indicated that raphe-spinal axons that bifurcated to descend in both DLFs branched within the spinal cord. The effects of stimulation in the periaqueductal gray (PAG) or adjacent midbrain reticular formation were tested on 102 spinally projecting neurons in the medial medulla. Of these, 60 cells were excited, 9 cells were inhibited, 8 showed mixed excitation and inhibition, and 25 cells were unaffected. The mean latency for excitation was 11.6 ms and for inhibition, 17.8 ms. Threshold for excitation of raphe- and reticulospinal neurons ranged from 50 to 400 microA. Raphe- and reticulospinal tract cells could often (31/46 cells tested) be excited following stimulation in the ventral posterior lateral nucleus of the thalamus. The mean latency of excitation was 35.6 ms (range, 6-112 ms). Receptive fields could be demonstrated for 80 raphe- and reticulospinal cells, while 48 neurons possessed no demonstrable cutaneous receptive field. Most cells had large excitatory receptive fields, often encompassing the surface of the entire body and face. Some neurons had complex excitatory and inhibitory receptive fields, whereas other cells had large inhibitory receptive fields over much of the surface of the body and face. For most cells (52/55) with excitatory receptive fields, the only effective stimuli were noxious mechanical or noxious heat stimuli. Nonnoxious mechanical stimuli, such as brushing the skin, were capable of activating only a few (3/55) raphe- and reticulospinal neurons, and these were more effectively excited by noxious stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

Pathways involved in thalamic ventrobasal stimulation for pain relief: evidence against the hypothesis VB stimulation -> rostroventral medulla excitation -> dorsal horn inhibition

“…However, no significant change in LCGU was detected in the spinal dorsal horn, nucleus raphe magnus and nucleus raphe dorsalis. Gerhart et al 7 demonstrated that, in monkeys, the latency for VB stimulation induced inhibition of dorsal horn neurons in the lumbosacral enlargement is 33.7 ms. Willis et al 19 , working with monkeys also, showed that the latency for VB stimulation excitation of RVM neurons is 35.6 ms and that the latency for antidromic activation of RVM neurons by stimulation of the DLF in an upper lumbar level is 8.2 ms. One can easily see, contrarily to what would be expected from Tsubokawa's hypothesis, that the latency for inhibition of dorsal horn neurons by VB stimulation is less than the sum of the latencies for VB stimulation excitation of RVM neurons and DLF stimulation antidromic excitation of RVM neurons.…”

Evidence for two classes of nociceptive modulating neurons in the periaqueductal gray

“…First, anatomical studies reveal a heavy projection from the PAG (particularly its dorsal and ventrolateral aspects) to the RVM, as well as a projection from the RVM to the PAG (Gallagher and Pert, 1978;Abols and Basbaum, 198 1;Beitz, 1982a;Mantyh, 1982a;Carlton et al, 1983). Second, activation of PAG neurons, either by electrical stimulation or by microinjection of the potent neuroexcitant glutamate, has an excitatory effect on neurons in RVM (Fields and Anderson, 1978;Behbehani and Fields, 1979;Pomeroy and Behbehani, 1979;Mohrland and Gebhart, 1980;Vanegas et al, 1984b;Willis et al, 1984). Microinjection of opiates in the PAG also alters the activity of RVM neurons (Behbehani and Pomeroy, 1978;Mohrland and Gebhart, 1980;Cheng et al, 1986).…”

Thalamic ventrobasal stimulation for pain relief: probable mechanisms, pathways and neurotransmitters

“…Besides, there is also experimental evidence against the discussed hypothesis: A) Benabid et al 6 were not able to demonstrate any inhibition of DH nociceptive neurons following VB stimulation; the number of DH cells tested (8 cells), however, was too small, B) Aiko et al 2 , using the deoxyglucose method, could not detect any significant change in local glucose utilization in the nucleus raphe magnus and in the spinal dorsal horn following VB stimulation and C) The latency for VB induced inhibition of DH nociceptive neurons -33.7 ms n ' 2 7 is less than the sum of the latencies for VB induced excitation of RVM neurons -35.6 ms 27 - 29 and DLF antidromic excitation of RVM neurons -8.2 ms 27 -29 .…”

“…The mechanism of pain relief induced by VB stimulation, however, is still unknown. Many hypotheses have been proposed 1,2,6 ' 11 ' 16 ' 23 " 25,29 . Unfortunately, none of them seems to completely explain this mechanism.…”