The HCN current in the rat substantia nigra pars compacta neurons: A reappraisal

The functional role of hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels in substantia nigra pars compacta (SNc) dopaminergic neurones is still somewhat elusive. We have re-examined this problem studying the h-current in thin slices in conditions that differed from most of the previous studies under three key aspects: i. the experiments were performed in perforated patch and ii. at 37°C, iii. we used a transgenic mice model expressing eGFP under the TH promoter in order to have a trusty identification of the dopaminergic neurones. First we have kinetically characterized the current, with a special attention to effect of temperature. The Q10 for I(h) amplitude in condition of open channel was 3.73, but, the Q10 for the time constant of activation was an amazing 10.8, implying an activation at physiological temperature much faster than previously assessed. The temperature affected significantly the midpoint of activation (V50), promoting a -9 mV shift going from 27 to 37 °C; furthermore, the duration of the conditioning step affected significantly the V50, with a further shift to depolarised potentials up to -77 mV once the steady-state conditions are attained, a potential much more positive than previously estimated, and implying a significant degree of opening already at potentials of physiological interest. The deactivation time constant is rather high (> 0.6 s), what entails an important activity-dependent memory effect evoked by trains of inhibitory synaptic inputs already at rest. Next, we analysed the modulation of the h-current by cyclic nucleotides, an effect that can be properly assessed only in perforated patches: forskolin at 37 °C increases the I(h) amplitude of +51.6% at -100 mV, due to a shift of V50 (+4.8 mV), and promotes a 31% decrease of the time constant of activation. The modulatory mechanism of I(h) mediated by cAMP has critical functional consequences as several neurotransmitters of interest for SNc neurons does affect the intracellular cAMP concentration, and in fact the I(h) amplitude is altered by D2 agonists (-15%), 5HT (-20%) and NA (+12%). Finally, we further investigated the role of the h-current in SNc neurones autorhythmicity. The block of the current with ivabradine hyperpolarises the cell for about -10 mV. This entails a blockage of the spontaneous activity, but the effect is only indirect, as if the membrane potential is brought back to the resting value the spontaneous activity is fully restored also in the absence of I(h). We conclude that the h-current has an central role in determining the excitability profile of SNc neurones, and that its fine tuning by several neurotransmitters coerces a reappraisal of its role.