The relationship between the afferent properties and substance P‐like immunoreactivity (SP‐LI) of L6 and SI dorsal root ganglion (DRG) neuronal somata was examined in anaesthetized guinea‐pigs. Glass pipette microelectrodes filled with fluorescent dyes were used to make intracellular recordings and to label DRG somata. The dorsal root conduction velocity (CV) and the afferent receptive properties of each unit were categorized according to criteria established in other species. Categories included a variety of low threshold mechanoreceptive classes, innocuous thermoreceptive and several nociceptive classes. Nociceptive units were further subdivided on the basis of CV and the locus of the receptive field (superficial cutaneous, deep cutaneous or subcutaneous). SP‐LI was determined using the avidin–biotin complex method and the relative staining intensity determined by image analysis. The possible significance of labelling intensity is discussed. Clear SP‐LI appeared in twenty‐nine of 117 dye‐labelled neurones. All SP‐LI positive units with identified receptive properties were nociceptive but not all categories of nociceptors were positive. The intensity of SP‐LI labelling varied, often systematically, in relation to afferent properties. There was a tendency for nociceptive neurones with slower CVs and/or smaller cell bodies to show SP‐LI. Nineteen of fifty‐one C fibre neurones showed SP‐LI. Fewer than half the C polymodal nociceptors (CPMs) were positive. The most intensely labelled units were the deep cutaneous nociceptors and some of the CPMs in glabrous skin. C low threshold mechanoreceptors and cooling‐sensitive units did not show SP‐LI. Ten of sixty‐six A fibre neurones exhibited SP‐LI, including eight of sixteen Aδ nociceptors and two of fifteen Aα/β nociceptors. A fibre neurones exhibiting SP‐LI included seven of eight deep cutaneous mechanical nociceptors and some superficial cutaneous mechano‐heat nociceptors of hairy skin. In contrast, none of twenty superficial cutaneous A high threshold mechanoreceptor units or the thirty‐five A fibre low threshold units (D‐hair and other units) showed detectable SP‐LI. We conclude that SP‐LI labelling in guinea‐pig DRG neurones is related to (a) afferent receptive properties, (b) the tissue in which the peripheral receptive terminals are located, (c) the CV and (d) the soma size.

Much attention has been given to the pelvic nerve afferent innervation of the urinary bladder; however, reports differ considerably in descriptions of afferent receptor types, their conduction velocities, and their potential roles in bladder reflexes and sensation. The present study was undertaken to do a relatively unbiased sampling of bladder afferent fibers of the pelvic nerve in adult female rats. The search stimulus for units to be studied was electrical stimulation of both the bladder nerves and the pelvic nerve. Single-unit activity of 100 L(6) dorsal root fibers, activated by both pelvic and bladder nerve stimulation, was analyzed. Sixty-five units had C-fiber and 35 units had Adelta-fiber conduction velocities. Receptive characteristics were established by direct mechanical stimulation, filling of the bladder with 0.9% NaCl at a physiological speed and by filling the bladder with solutions containing capsaicin, potassium, or turpentine oil. The majority (61) of these fibers were unambiguously excited by bladder filling with 0.9% NaCl and were classified as mechanoreceptors. All mechanoreceptors with receptive fields on the body of the bladder had low pressure thresholds (

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.