It has recently been shown that cooling-sensitive (CS) neurones express a different pattern of voltage-gated ion channels than cooling-insensitive (CI) ones (Viana et al. 2002). Along with these authors and others (McKemy et al. 2002), we found using calcium imaging that about half of the cold receptors are also capsaicin sensitive (19/39, 49 %), which is very similar to the proportion of capsaicin-sensitive cells among cold-insensitive neurones (123/241, 51 %). As capsaicin sensitivity has been considered a hallmark of nociception, we tried to identify nociceptor-like properties among cold-sensitive neurones.
Adult rats were killed by CO2 inhalation and decapitation, and dorsal root ganglion (DRG) neurones were dissociated and kept in culture for 1Ð3 days. Electrophysiological recordings were made from cells of diameter 20.0 ± 2.2 µm (n = 92) in the current-clamp and whole-cell voltage-clamp configurations. CS cells responded with bursts of action potentials on cooling to 18 °C. CI cells depolarised by less than 5 mV during the same cold stimulus. Data are presented as means ± S.E.M.
The following parameters were monitored: the magnitude of the hyperpolarisation-activated cation current (Ih), the inactivation time constant of the inward current and the presence of the fast-inactivating potassium current IA. IA was defined as the outward current elicited by a depolarising step, which inactivated with a time constant of less than 50 ms.
In agreement with Viana et al. (2002), CS neurones expressed a higher level of Ih than CI ones (114 ± 24.2 pA, n = 16, compared with 70.2 ± 9.6 pA, n = 39, P = 0.037, Student’s unpaired t test, significance level set at 0.05). In CS cells the inward current inactivated with a faster time course than in CI neurones (1.0 ± 0.1 ms, n = 16, compared with 2.2 ± 0.1 ms, n = 39, P < 0.001). Finally, IA appears to be present in fewer CS neurones (6 of 16) than in CI ones (32 of 39).
CS neurones had action potentials of shorter duration (3.6 ± 0.3 ms, n = 21 compared to 4.5 ± 0.2 ms, n = 38, for CI neurones, P = 0.02) and fewer had an inflexion (8/21 compared to 27/38), whereas nociceptors have action potentials of long duration with an inflexion.
All CS cells were type 3, according to the classification proposed by Petruska et al. (2000). We could not identify nociceptor-like cells (type 1 and 2) among the CS neurones. As a conclusion, cold-receptors comprise a distinct subpopulation of rat DRG neurones.
Financial support was from the Romanian Ministry of Education (World Bank grant C-326), and The Physiological Society.
All procedures accord with current local guidelines.