Altered excitability and exocytosis in chromaffin cells from the R6/1 mouse model of Huntington's disease is linked to over-expression of mutated huntingtin

As the peripheral sympathoadrenal axis is tightly controlled by the cortex via hypothalamus and brain stem, the central pathological features of Hunting's disease, (HD) that is, deposition of mutated huntingtin and synaptic dysfunctions, could also be expressed in adrenal chromaffin cells. To test this hypothesis we here present a thorough investigation on the pathological and functional changes undergone by chromaffin cells (CCs) from 2-month (2 m) to 7-month (7 m) aged wild-type (WT) and R6/1 mouse model of Huntington's disease (HD), stimulated with acetylcholine (ACh) or high [K+] (K+). In order to do this, we used different techniques such as inmunohistochemistry, patch-clamp, and amperometric recording. With respect to WT cells, some of the changes next summarized were already observed in HD mice at a pre-disease stage (2 m); however, they were more pronounced at 7 m when motor deficits were clearly established, as follows: (i) huntingtin over-expression as nuclear aggregates in CCs; (ii) smaller CC size with decreased dopamine beta-hydroxylase expression, indicating lesser number of chromaffin secretory vesicles; (iii) reduced adrenal tissue catecholamine content; (iv) reduced Na+ currents with (v) membrane hyperpolarization and reduced ACh-evoked action potentials; (v) reduced [Ca2+](c) transients with faster Ca2+ clearance; (vi) diminished quantal secretion with smaller vesicle quantal size; (vii) faster kinetics of the exocytotic fusion pore, pore expansion, and closure. On the basis of these data, the hypothesis is here raised in the sense that nuclear deposition of mutated huntingtin in adrenal CCs of R6/1 mice could be primarily responsible for poorer Na+ channel expression and function, giving rise to profound depression of cell excitability, altered Ca2+ handling and exocytosis.