Cold fibres: neurochemistry & anatomy

Takashima et al (2007) carried out one of the first investigations of the distribution of TRPM8-positive sensory nerve terminals in various peripheral structures, using transgenic mice which express enhanced green fluorescent protein under the control of the TRPM8 transcriptional promoter.

First, they confirmed that the transgene expression was neuron-specific, by showing that cultured DRG and TG neurons from the transgenic animals expressed both GFP and the pan-neuronal marker PGP-9.5. The correspondence of GFP and PGP-9.5 coexpression with TRPM8 immunoreactivity showed that the transgene expression was limited to TRPM8-positive sensory neurons. Furthermore, >80% of cultured GFP+ neurons responded to menthol by an increase in intracellular Ca²⁺ concentration, as determined by calcium microfluorimetry, and outwardly rectifying currents, as measured by whole-cell voltage clamp recordings.

Consistent with earlier studies, it was found that approximately 13% of DRG and TG cells expressed the transgene. The majority of these neurons had cell bodies of around 10µm diameter, and were assumed to be C-fibres, the remainder had cell bodies of up to 30µm diameter, and were assumed to be A delta fibres.

Antibody staining was then used to determine coexpression of the transgene with markers of primary sensory neurons. In DRG, approximately 25% of GFP+ cells were immunoreactive for the C-fibre marker peripherin, and ~14% stained with an antibody for NF200, an intermediate filament protein marker for A-delta fibres. In TG, >30% of GFP+ cells, expressed peripherin, and >25% were immunoreactive for NF200. Significantly, 60% of the DRG cells, and ~40% of TG cells, expressed neither peripherin nor NF200, and are, therefore characterized only by TRPM8 expression.

In the skin, GFP+ nerve terminals were found both superficially, in the stratum granulosum, and deeper, near the epidermis-dermis boundary. This is consistent with the hypothesis that innocuous cool stimuli are detected by nerve terminals located in the superficial skin layers, while noxious cold stimuli are detected terminals that are located deeper.

A complex distribution of TRPM8+ nerve terminals was also observed in other peripheral structures. In the teeth, GFP was found to be expressed in the dentin, which is known to contain A-delta fibres, and in the pulp, which contains C-fibres. In the palate, GFP+ nerve terminals were found in different layers of the epithelium.

Thus, in both DRG and TG, some TRPM8+ sensory neurons characteristics of nociceptors, but others do not. TG has a higher proportion of TRPM8+ cells than do the DRG, and, in the periphery, the terminals of TRPM8+ cells in both ganglia innervate regions of the skin and teeth that are associated with sensing different kinds of cold stimuli.