The hyperpolarization activated current (I_h) is less prominent in isolated dorsal root ganglion (DRG) neurons with small than with large soma sizes (e.g. Scroggs et al. 1994). Slowly conducting neurons tend to have smaller somata (Harper & Lawson, 1985) and a high proportion are nociceptive, while a high proportion of fast conducting neurons are low threshold mechanoreceptive (LTM). The aim of this study was to determine the expression of I_h in DRG neurons with differing sensory receptor properties.

Functionally identified DRG neurons were studied in rats (L4/L5) and guinea-pigs (L6/S1) anaesthetised and maintained areflexic with sodium pentobarbitone (50 mg kg^-1, I.P.). Pancuronium (0.5 mg kg^-1 I.A.), always administered with 10 mg kg^-1 pentobarbitone, was used as a muscle relaxant. Arterial blood pressure was monitored throughout. Animals were humenaly killed with an overdose of the anaesthetic. Intracellular recordings were made using conventional sharp glass microelectrodes. Membrane voltage responses and currents were recorded either with a conventional bridge or a discontinuous single-electrode voltage-clamp amplifier. Under voltage-clamp conditions I_h was seen as a slowly activating inward current in response to hyperpolarizing voltage steps negative to -60 mV from about -50 mV. Consistent with its established characteristics in primary afferent neurons the reversal potential for I_h was -35 ± 5 mV (mean ± S.D., n = 5), and its activation kinetics were best described by the sum of fast and slow activating components. At a membrane potential of -100 mV the time constants were 28 ± 7 and 172 ± 58 ms (n = 5), respectively. In guinea-pig and rat DRG I_h was present in A-fibre LTM (6/6 and 12/12, respectively) and most nociceptive neurons (3/5 and 5/7), but not in C fibre nociceptive neurons (0/4 in guinea-pig). In rat A-fibre units, I_h current density was greater in slowly adapting (SA) than rapidly adapting (RA) LTM units, and was also high in nociceptive neurons (see Fig. 1). The differential expression of I_h may therefore contribute to differences in adaptation properties in LTM and nociceptive afferents.This work was supported by a Wellcome Trust grant to S.N.L. and a Dale/Rushton award to A.A.H.

Figure 1. Relationship between Ih current density and CV (at 30 °C) for rat LTM and nociceptive neurons. Cell capacitance was calculated from the time constant and input resistance of the neuron, measured from the voltage response to small hyperpolarizing current steps under current-clamp conditions. Ih density increased with CV for SA LTM units (r 2 = 0.71, P < 0.05, n = 9, Pearson correlation).

Harper, A.A. & Lawson, S.N. (1985). J. Physiol. 359, 31-46. abstract

Scroggs, R.S., Todorovic, S.M., Anderson, E.G. & Fox, A.P. (1994). J. Neurophysiol. 71, 271-279.