KCa1.1 channel contributes to cell excitability in unmyelinated but not myelinated rat vagal afferents

Abstract

High conductance calcium-activated potassium (BK(Ca)) channels can modulate cell excitability and neurotransmitter release at synaptic and afferent terminals. BK(Ca) channels are present in primary afferents of most, if not, all internal organs and are an intriguing target for pharmacological manipulation of visceral sensation. Our laboratory has a long-standing interest in the neurophysiological differences between myelinated and unmyelinated visceral afferent function. Here, we seek to determine whether there is a differential distribution of BK(Ca) channels in myelinated and unmyelinated vagal afferents. Immunocytochemistry studies with double staining for the BK-type K(Ca)1.1 channel protein and isolectin B4 (IB4), a reliable marker of unmyelinated peripheral afferents, reveal a pattern of IB4 labeling that strongly correlates with the expression of the K(Ca)1.1 channel protein. Measures of cell size and immunostaining intensity for K(Ca)1.1 and IB4 cluster into two statistically distinct (P < 0.05) populations of cells. Smaller diameter neurons most often presented with strong IB4 labeling and are presumed to be unmyelinated (n = 1,390) vagal afferents. Larger diameter neurons most often lacked or exhibited a very weak IB4 labeling and are presumed to be myelinated (n = 58) vagal afferents. Complimentary electrophysiological studies reveal that the BK(Ca) channel blockers charybdotoxin (ChTX) and iberiotoxin (IbTX) bring about a comparable elevation in excitability and action potential widening in unmyelinated neurons but had no effect on the excitability of myelinated vagal afferents. This study is the first to demonstrate using combined immunohistochemical and electrophysiological techniques that K(Ca)1.1 channels are uniquely expressed in unmyelinated C-type vagal afferents and do not contribute to the dynamic discharge characteristics of myelinated A-type vagal afferents. This unique functional distribution of BK-type K(Ca) channels may provide an opportunity for afferent selective pharmacological intervention across a wide range of visceral pathophysiologies, particularly those with a reflexogenic etiology and pain.