Dorsal Horn Of Adult Rats Research Articles

Muscle afferents innervating skin form somatotopically appropriate connections in the adult rat dorsal horn.

We have studied the somatotopic reorganization in dorsal horn neurons after a disruption in the normal spatial arrangement of primary sensory axons in adult rats. Muscle afferents were redirected to skin by cutting and cross-anastomosing the hindlimb gastrocnemius nerve (GN) and sural nerve (SN). It has previously been shown that after 10-12 weeks GN afferents innervate the hairy skin of the lateral ankle and calf (previously innervated by SN afferents) and become potentially capable of relaying information on the location and intensity of stimuli applied to the skin. We determined the receptive field and response properties of dorsal horn neurons in the lumbar spinal cord, in regions where the lower hindlimb is normally represented. In control animals (with intact or self-anastomosed sural nerves) very few neurons (< 8%) received any synaptic input from the GN as assessed by electrical stimulation of the nerve. In contrast, when this nerve innervated skin, many cells responded to GN stimulation, and these nearly all had receptive field components in the former SN territory. Moreover, in animals with cross-anastomosed nerves, cells without GN inputs all had receptive fields outside the former SN skin territory. We have shown that in all likelihood GN afferents substituted for SN afferents in subserving the low and high threshold receptive fields of dorsal horn neurons. Furthermore, for many neurons, receptive fields were formed from inappropriately regrown GN afferents and adjacent intact cutaneous afferents (in the tibial or common peroneal nerves). Therefore, when GN afferents innervate skin in adult animals, they alter their central connectivity in an appropriate manner for their new peripheral terminations, so that an orderly somatotopic representation of the hind limb skin is maintained. We suggest that this plasticity of dorsal horn somatotopy is driven in part by activity-dependent mechanisms.

Deafferentation‐induced terminal field expansion of myelinated saphenous afferents in the adult rat dorsal horn and the nucleus gracilis following pronase injection of the sciatic nerve

We have previously demonstrated sprouting of small diameter saphenous afferents, labelled with wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) and (HRP), into the sciatic territory of the adult rat superficial dorsal horn following destruction of sciatic afferents by injection of the sciatic nerve with pronase (a combination of proteolytic enzymes). In the present experiments, we examined the response of myelinated saphenous axons, which terminate in lamina I and the deep dorsal horn (laminae III-V) under the same conditions, with the tracer B subunit of cholera toxin conjugated to HRP (B-HRP) which specifically labels myelinated primary afferents when injected into a peripheral somatic nerve. We also examined changes in the nucleus gracilis, another site of sciatic degeneration and a target of saphenous afferents. Four months after injection of the pronase, the area of label determined by measurement of the width of the saphenous territory in lamina III was expanded by 24% on the pronase side. Since there was also expansion throughout the deep dorsal horn, the area measured by tracing the labelled region in transverse sections was actually twice that of the control side, and the intensity of labelling within the traced area increased by 18%. There was no change in grey matter area due to the lesion. The traced area of labelling in the nucleus gracilis increased by 40%, and increased in intensity by 17%. The substantia gelatinosa is not normally supplied by B-HRP-labelled afferents, and there was no expansion of these sprouted saphenous afferents into the gelatinosa. These results indicate that myelinated afferents can sprout as vigorously in lamina I and the deep dorsal horn as the small diameter afferents do in the substantia gelatinosa; that there is no invasion of the substantia gelatinosa by the myelinated afferents at least as long as the small diameter afferents also have the opportunity to sprout; and that primary afferents have the potential to sprout at more than one site of termination, i.e., both the dorsal horn and the dorsal column nuclei.

Calcitonin gene-related peptide in primary afferent neurons of rat: Co-existence with fluoride-resistant acid phosphatase and depletion by neonatal capsaicin

Immunohistochemical and histochemical techniques were used to re-examine the extent to which neonatal capsaicin treatment depletes calcitonin gene-related peptide in the dorsal horn of the spinal cord, to determine the localization of calcitonin gene-related peptide in relation to that of fluoride-resistant acid phosphatase in lumbar dorsal root ganglia, and to compare the distribution of these primary afferent markers in the dorsal horn. A substantial depletion of calcitonin gene-related peptide was observed in the dorsal horn of adult rats treated neonatally with capsaicin suggesting that a large proportion of this peptide in the dorsal horn is contained within capsaicin-sensitive primary afferent fibers. In dorsal root ganglia 30% of all or 44% of small- and medium-sized calcitonin gene-related peptide-immunoreactive cells were positive for fluoride-resistant acid phosphatase. Conversely, 50% of cells positive for the phosphatase enzyme also displayed immunoreactivity for the peptide. In lamina II of the dorsal horn calcitonin gene-related peptide and fluoride-resistant acid phosphatase were found to have an overlapping distribution. The presence of fluoride-resistant acid phosphatase in a substantial proportion of neuropeptide-containing primary sensory neurons suggests a lack of segregation of sensory neuronal populations into peptide-and non-peptide-containing subgroups at least on the basis of non-peptide neurons defined as those containing fluoride-resistant acid phosphatase.

Deafferentation‐induced expansion of saphenous terminal field labelling in the adult rat dorsal horn following pronase injection of the sciatic nerve

We have examined the effect of the degeneration of sciatic nerve afferents on the distribution of saphenous terminals in the adult rat dorsal horn. Deafferentation was produced by injection into the sciatic nerve of pronase, a combination of proteolytic enzymes, which causes death of ganglion cells and degeneration of their terminal fields. The saphenous terminal fields were labelled by exposing the cut nerve to a combination of horseradish peroxidase (HRP) and wheat germ agglutinin-horseradish peroxidase (WGA-HRP). Terminals were mainly found in the superficial dorsal horn, indicating that small-diameter afferents were heavily labelled. In one group of control animals, the normal sciatic and normal saphenous terminal fields were shown to be bilaterally symmetrical. In the experimental group, the initial injection of one sciatic nerve with pronase was followed 4 months later by bilateral HRP/WGA-HRP labelling of both saphenous nerves. In each animal, the terminal field of the saphenous nerve on the lesioned side was expanded in the medial, lateral, and caudal directions. Medially and laterally, the expanded terminal field overlapped more of the sciatic territory than normal; caudally, saphenous terminals were found in the rostral portion of the L5 segment, in an area normally filled by sciatic terminals and devoid of saphenous terminals. The expansion resulted in a total saphenous area 26% larger than the control side. Electron microscopy demonstrated that the label in both the normal and expanded territories was primarily contained in axons and terminals, with minor transneuronal labelling. Labelled terminals in the expanded areas were both simple terminals with round, clear vesicles, and glomerular terminals with multiple synaptic contacts; these terminal types resemble those previously described for primary afferents in the superficial dorsal horn. Although the preexistence of "silent" synaptic terminals in the expanded areas cannot be disproven, the data support the hypothesis that primary afferents in the adult have the potential to sprout and establish synapses when the conditions of the deafferentation are favorable.

Distribution of sural nerve afferent fibres within the dorsal horn of adult rats treated at birth with capsaicin

The distribution of sural nerve afferent fibers within the spinal cord of normal adults rats and of adult rats treated at birth with capsaicin was examined using transganglionic transport of horseradish peroxidase (HRP). Labelled fibres were seen, in normal and in capsaicin-treated rats, in Laminae I-VI of the central third of the dorsal horn, extending rostrocaudally between the L 3 and L 5 segments. It is concluded that the changes in dorsal horn somatosensory systems induced by neonatal capsaicin are not due to anatomical redistribution of the areas of termination of peripheral nerves within the spinal cord.