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A re-evaluation of the neurochemical and antinociceptive effects of intrathecal capsaicin in the rat
208 It is concluded that the newly appearing RFs are due to reinforcing or uncovering of existing but ineffective afferent terminals rather than to the distant sprouting within the spinal cord of new afferents.
STOMATOLOGY
An HRP study of the central projections of primary trigeminal neurons which innervate tooth pulps in the cat. - - J. Arvidsson and S. Gobel, Brain Res., 210 (1981) !-16. Following HRP application to the cut end of the inferior alveolar nerve or HRP injections into tooth pulps, trigeminal ganglia and the brain stem of adult cats were examined for labeled cells or terminals. Labeled cells in the ganglion only were found ipsilateral to HRP application sites. Terminal labeling in both experiments was present in all subdivisions of the trigerainal brain stem nuclei. In the medullary dorsal horn (trigemi~.~l nucleus caudalis), labeling was restricted to laminae I, Ila and V, following tooth pulp HRP injections. In contrast, terminal labeling was located throughout laminae I-V in the inferior alveolar nerve experiments. These findings do not support earlier reports of peripheral transmedian innervation of tooth pulps. In contrast to findings at spinal levels, these data also indicate that primary trigeminal neurons only have ipsilateral terminations. Excitation and inhibition of marginal layer and interstitial interneurons in cat nucleus caudalis by mechanical stimuli. - - J.l. Hubbard and R.F. Hellon, J. comp. Neurol., 193 (1980) 995-1007. Neuronal activity was recorded from the cat trigeminal nucleus caudalis, the trigeminal homologue of the spinal dorsal horn, following electrical or mechanical stimulation of the face. Recordings considered to arise from interneurons were often found in the descending spinal trigeminal tract above the marginal layer. Based on short latencies ( < 2.0 msec) to electrical stimulation and their ability to follow pairs of stimuli at 0.70 msec intervai~, these responses were thought to be monosynaptically driven. Many of th~se neurons responded to mechanoreceptive input. These findings suggest that myelinated mechanoreceptive afferents terminate in the marginal layer and the interstitial nucleus dorsal to trigeminal nucleus caudalis. The possible presence of low-threshold mechanoreceptive input terminating in this region is of considerable interest, since no such input has yet to be demonstrated in the spinal dorsal horn.
PHYSIOLOGY
A re-evaluation of the neurochemical and antinociceptive effects of intrathecal capsaicin in the rat. - - J.l. Nagy, P.C. Emson and L.L. Iversen, Brain Res., 211 ( 1981) 497- 502. The effect of intrathecai administration of capsaicin in the rat on thermal
209
nociceptive thresholds and on the content of substance P, somatostatin and glutamic acid decarboxylase in the dorsal horn of the spinal cord was determined. The results suggest that the depletion of spinal cord substance P induced by capsaicin may not by itself be sufficient to explain the observed changes in noxious thermal thresholds, which may be related instead to non-specific damage to the spinal cord.
The effect of peripheral nerve injury on dorsal root potentials and on transmission of afferent signals into spinal cord. - - P.D. Wall and M. Devor, Brain Res., 209 ( ! 981 ) 95-111. The response of the spinal cord to stimulation of the proximal part of the injured nerve was examined at various times after the lesion and compared to the effects of stimulating she intact nerve on the other side. The sciatic nerve of adult rats was either cut and ligated or was crushed on one side. During the first 10 days after nerve section the following measures were not affected: (i) the size of the input volley (compound action potential, CAP, measured on a dorsal root that carried sciatic nerve afferents (LS)); (ii) the volley running in the dorsal columns; (iii) the dorsal root potential (DRP) evoked on neighboring dorsal roots which do not contain sciatic afferents (L2 and L3); (iv) the postsynaptic volleys ascending in the spinal cord. H )wever, by the fourth day after nerve section, there was a decrease of the DRP evoked on the ipsilateral L5 dorsal root by stimulation of the cut nerve. By 10 days this DRP had decreased by 5070. There was also a decrease in the DRP on the L5 root evoked by stimulation of the contralateral intact nerve. Beginning 10-20 days after nerve cut, there was a decrease in the amplitude of the afferent CAP and of all the measures of central response to the afferent volley. The loss of the DRP may be associated with a disinhibition which results in novel receptive fields which were observed in cord cells deafferented by the peripheral nerve section. Tooth pulp input to the spinal trigeminal nucleus: a comparison of inhibitions following segmental and raphe magnus stimulation. - - A.H. Dicken:~on, R.F. Helion and C.J. Woolf, Brain Res., 214 (1981) 73-87. In rats and cats anesthetized with urethane a comparison was made of the inhibitory effects of raphe magnus (NRM) and segmental (facial skin) stimulation on neurons in nucleus caudalis excited by tooth pulp stimulation. The upper and lower ipsilaterai incisor teeth were used in rats (176 neurons) and the corresponding canine teeth in cats (34 neurons). The recording sites were located in all layers of nucleus caudalis and in the underlying reticular formation. Both the evoked responses and the conditioning effects were similar in the two species. Both forms of conditioning inhibited about half the neurons tested but only a small proportion was influenced from both sources. N R M stimulation had almost identical effects on neurons driven from upper teeth or from lower teeth and tended to ~ct on those cells with longer latencies. Segmental stimulation influenced the majority of shorter latency cells and produced greater inhibitions of upper tooth pulp neurons. Diffuse noxious inhibitory controls were also observed for certain neurons.
STOMATOLOGY
An HRP study of the central projections of primary trigeminal neurons which innervate tooth pulps in the cat. - - J. Arvidsson and S. Gobel, Brain Res., 210 (1981) !-16. Following HRP application to the cut end of the inferior alveolar nerve or HRP injections into tooth pulps, trigeminal ganglia and the brain stem of adult cats were examined for labeled cells or terminals. Labeled cells in the ganglion only were found ipsilateral to HRP application sites. Terminal labeling in both experiments was present in all subdivisions of the trigerainal brain stem nuclei. In the medullary dorsal horn (trigemi~.~l nucleus caudalis), labeling was restricted to laminae I, Ila and V, following tooth pulp HRP injections. In contrast, terminal labeling was located throughout laminae I-V in the inferior alveolar nerve experiments. These findings do not support earlier reports of peripheral transmedian innervation of tooth pulps. In contrast to findings at spinal levels, these data also indicate that primary trigeminal neurons only have ipsilateral terminations. Excitation and inhibition of marginal layer and interstitial interneurons in cat nucleus caudalis by mechanical stimuli. - - J.l. Hubbard and R.F. Hellon, J. comp. Neurol., 193 (1980) 995-1007. Neuronal activity was recorded from the cat trigeminal nucleus caudalis, the trigeminal homologue of the spinal dorsal horn, following electrical or mechanical stimulation of the face. Recordings considered to arise from interneurons were often found in the descending spinal trigeminal tract above the marginal layer. Based on short latencies ( < 2.0 msec) to electrical stimulation and their ability to follow pairs of stimuli at 0.70 msec intervai~, these responses were thought to be monosynaptically driven. Many of th~se neurons responded to mechanoreceptive input. These findings suggest that myelinated mechanoreceptive afferents terminate in the marginal layer and the interstitial nucleus dorsal to trigeminal nucleus caudalis. The possible presence of low-threshold mechanoreceptive input terminating in this region is of considerable interest, since no such input has yet to be demonstrated in the spinal dorsal horn.
PHYSIOLOGY
A re-evaluation of the neurochemical and antinociceptive effects of intrathecal capsaicin in the rat. - - J.l. Nagy, P.C. Emson and L.L. Iversen, Brain Res., 211 ( 1981) 497- 502. The effect of intrathecai administration of capsaicin in the rat on thermal
209
nociceptive thresholds and on the content of substance P, somatostatin and glutamic acid decarboxylase in the dorsal horn of the spinal cord was determined. The results suggest that the depletion of spinal cord substance P induced by capsaicin may not by itself be sufficient to explain the observed changes in noxious thermal thresholds, which may be related instead to non-specific damage to the spinal cord.
The effect of peripheral nerve injury on dorsal root potentials and on transmission of afferent signals into spinal cord. - - P.D. Wall and M. Devor, Brain Res., 209 ( ! 981 ) 95-111. The response of the spinal cord to stimulation of the proximal part of the injured nerve was examined at various times after the lesion and compared to the effects of stimulating she intact nerve on the other side. The sciatic nerve of adult rats was either cut and ligated or was crushed on one side. During the first 10 days after nerve section the following measures were not affected: (i) the size of the input volley (compound action potential, CAP, measured on a dorsal root that carried sciatic nerve afferents (LS)); (ii) the volley running in the dorsal columns; (iii) the dorsal root potential (DRP) evoked on neighboring dorsal roots which do not contain sciatic afferents (L2 and L3); (iv) the postsynaptic volleys ascending in the spinal cord. H )wever, by the fourth day after nerve section, there was a decrease of the DRP evoked on the ipsilateral L5 dorsal root by stimulation of the cut nerve. By 10 days this DRP had decreased by 5070. There was also a decrease in the DRP on the L5 root evoked by stimulation of the contralateral intact nerve. Beginning 10-20 days after nerve cut, there was a decrease in the amplitude of the afferent CAP and of all the measures of central response to the afferent volley. The loss of the DRP may be associated with a disinhibition which results in novel receptive fields which were observed in cord cells deafferented by the peripheral nerve section. Tooth pulp input to the spinal trigeminal nucleus: a comparison of inhibitions following segmental and raphe magnus stimulation. - - A.H. Dicken:~on, R.F. Helion and C.J. Woolf, Brain Res., 214 (1981) 73-87. In rats and cats anesthetized with urethane a comparison was made of the inhibitory effects of raphe magnus (NRM) and segmental (facial skin) stimulation on neurons in nucleus caudalis excited by tooth pulp stimulation. The upper and lower ipsilaterai incisor teeth were used in rats (176 neurons) and the corresponding canine teeth in cats (34 neurons). The recording sites were located in all layers of nucleus caudalis and in the underlying reticular formation. Both the evoked responses and the conditioning effects were similar in the two species. Both forms of conditioning inhibited about half the neurons tested but only a small proportion was influenced from both sources. N R M stimulation had almost identical effects on neurons driven from upper teeth or from lower teeth and tended to ~ct on those cells with longer latencies. Segmental stimulation influenced the majority of shorter latency cells and produced greater inhibitions of upper tooth pulp neurons. Diffuse noxious inhibitory controls were also observed for certain neurons.