The electrophysiological properties of rat primary afferent neurones with carbonic anhydrase activity

1. Intracellular recordings of action potentials (APs) and after-hyperpolarizations (AHPs) were made from the L3, L4 and L5 dorsal root ganglia (DRGs) of 6- to 8-week-old anaesthetized female Wistar rats in vitro at 36.5 +/- 1 degree C. Neurones were classified by their conduction velocities (CVs) as A alpha/beta (> 12 m s-1), A delta (1.3-12 m s-1) or C fibre neurones (< 1.3 m s-1). 2. Following the recording, fluorescent dye was injected intracellularly. Sections of injected neurones were tested for carbonic anhydrase (CA) activity histochemically. Reaction product intensity and cell size were measured. Control experiments showed that intracellular dye, time in vitro, axotomy and electrical stimulation did not affect proportions of CA-positive neurones or their size distributions. 3. Approximately 28-30% of DRG neurones were CA positive. Their sizes were approximately normally distributed and covered the entire size range of DRG neurones with no correlation between size and CA intensity. A greater proportion of A alpha/beta cells (62%) than of A delta (32%) or C cells (38%) were CA positive, but CA intensity was not correlated with CV. 4. In A neurones mean AP duration was significantly shorter in CA-positive cells; for CA-positive and CA-negative cells, respectively, these values were 1.6 and 2.8 ms for A delta cells; 1.1 and 1.7 ms for A alpha/beta cells; and were 1.2 and 2.3 ms for all A cells. CA intensity was negatively correlated with AP duration at base in all these groups. 5. Again in A neurones, the mean AHP durations were significantly shorter in the CA-positive cells; the mean AHP durations to 80% recovery for positive and negative cells were 8.8 and 36 ms, respectively, for A alpha/beta cells and were 8.6 and 26 ms, respectively, for all A cells. CA intensity was negatively correlated with AHP duration in A alpha/beta cells and all A cells together. 6. A fibre cells with the longer AP and AHP durations were all CA negative, while cells with the shorter durations included both CA-positive and CA-negative cells. 7. CA-positive and CA-negative A fibre neurones therefore have different electrophysiological characteristics. It is suggested that CA-negative A fibre neurones may have slower somatic firing rates and different sensory functions from the CA-positive neurones.