Calcitonin gene‐related peptide immunoreactivity and afferent receptive properties of dorsal root ganglion neurones in guinea‐pigs

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Abstract
To establish the afferent receptive properties of lumbosacral dorsal root ganglion (DRG) neurones that express calcitonin gene‐related peptide (CGRP), intracellular recordings were made with fluorescent dye‐filled electrodes in deeply anaesthetised young guinea‐pigs. After determination of neuronal functional properties, dye was injected into the soma. CGRP‐like immunoreactivity (CGRP‐LI) was examined on histological sections of dye‐marked neurones. Fourteen of 34 C‐fibre neurones showed CGRP‐LI. These included 10/21 C‐fibre nociceptive neurones. All C‐polymodal nociceptors in glabrous (n= 4) but none in hairy skin (n= 4) were positive. Positive C‐fibre high threshold mechanoreceptive (HTM) units had receptive fields in dermal or deeper tissue. Four (n= 6) unresponsive or unidentified C‐fibre units were positive. Neither C‐fibre cooling sensitive (n= 4) nor C‐fibre low threshold mechanoreceptive (LTM) units (n= 3) had CGRP‐LI. Six of 23 A‐fibre nociceptive cells were positive including one Aα/β unit. Three of these positive cells had epidermal and three had dermal/deep receptive fields. Three of 36 A‐fibre LTM units exhibited CGRP‐LI; all were Aα/β‐fibre G hair units. All glabrous skin and muscle spindle units and in hairy skin slowly adapting and field units, and some G‐hair units lacked CGRP‐LI. CGRP‐LI stained fibres were found in tissues containing receptive fields of positive DRG neurones: glabrous skin, near hair follicles and in skeletal muscle. A few substance P‐labelled neurones did not exhibit CGRP‐LI and vice versa. Thus CGRP expression was detected in under half the nociceptive neurones, was not limited to nociceptive neurones and apart from receptive properties was also related to location/depth in the tissues of a DRG neurone's peripheral terminals.

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