The HCN4 channel modulates external tufted cell burst firing

Hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels are expressed widely in various brain regions and cardiac tissue. The HCN channel conductance, named as I_h (in neurons), plays a controlling role in neuronal excitability and rhythmic oscillatory activity in individual neurons and neuronal networks. Immunohistochemical studies in the literature show HCN channel isoforms are expressed in the olfactory bulb, but relative isoform expression levels are unknown. HCN channels are encoded by four genes (HCN1-4). The expression and function of those isoforms in the five types of mitral/tufted cells are not well known. Here, we examined HCN1-4 protein and channel conductance in olfactory bulb output neurons. HCN1, 2, 3 and 4 isoforms were expressed in the olfactory bulb, with HCN4 the most abundant. HCN channel mediated I_h conductance was present in the different classes of output neurons with highest level in external tufted cells and low to none in mitral cells. To isolate specific contributions of the isoforms to the rhythmic firing of external tufted cells, we used cell specific pharmacology infusing antagonists or isoform specific blocking antisera into single neurons. Only the HCN4 blocking antisera terminated external tufted cell burst firing, while other antisera had no significant effect on spiking. Those results indicate HCN4 in external tufted cells controls burst firing in these cells. Since burst firing of external tufted cells is a key regulator of activity in the olfactory bulb, this suggests HCN4 and the resulting I_h conductance are key circuit regulators.