Neurotensin inhibition of the hyperpolarization-activated cation current (I-h) in the rat substantia nigra pars compacta implicates the protein kinase C pathway

1. Whole-cell patch-clamp recording was performed from principal neurones of the substantia nigra pars compacta (SNc). In 66% of these neurones, neurotensin (NT) induced, at -60 mV, an inward current associated with an increase in conductance. 2. Principal neurones displayed, in response to hyperpolarizing voltage steps, the voltage-dependent inward cationic current, I-h. This current activated at potentials more negative than -65 mV:and reached a maximum at -106 +/- 4 mV;, with a half-activation potential of -86 +/- 3 mV. Its estimated reversal potential was -43 +/- 7 mV and its activation curve was fitted with two exponentials, 3. In 41% of neurones showing the ina ard current, NT (0.5 mu M) also reversibly reduced the amplitude of I-h. The diminution was 48.5 +/- 12% when voltage steps were made from -60 to -95 mV. The decrease in I-h resulted from a reduction in the maximal current with no change in the voltage dependence of activation. 4. Forskolin (10 mu M), an activator of adenylate cyclase, increased I-h by shifting its activation range to more positive potentials, but it did not alter the NT inhibition of I-h. 5. The effect of NT was blocked by staurosporine (0.5 mu M) and by PKC-(19-31) (0.5 mu M), a specific protein kinase C (PKC) inhibitor, but was unaffected by Walsh's peptide (100 mu M), a specific inhibitor of protein kinase A. The reduction of I-h was mimicked by 1-oleoyl-2-acetyl-sn-glycerol (0.5-10 mu M), an analogue of diacylglycerol, an endogenous PKC activator. 6. These results suggest that the inhibition of I-h by NT involves a phosphorylation mechanism that implies activation of PKC.