Dopaminergic (DA) neurons in the ventral tegmental area (VTA) have the hyperpolarization activated cationic current (Ih) and receive serotonergic innervation from the raphe nuclei. We have previously shown that serotonin (5-HT, 50 - 500 µM) reduces the amplitude of Ih in a concentration-dependent manner (Liu et al. SFN Abstr. 25:439, 1999). In the present study, whole cell patch-in-the-slice recording from DA VTA neurons was used to measure Ih under voltage clamp conditions. Ih was evoked by hyperpolarizing voltage steps (duration 1 sec) from a holding potential of -60 mV to -130 mV (in 10 mV increments) and Current/Voltage (Ih/V) curves were constructed. Drug-induced changes in Ih were evaluated from Ih/V curves measured in control and in the presence of each drug, in the same VTA neuron. Bath application of the 5-HT2 receptor agonist α-methylserotonin (α-Me5-HT, 25 µM) reduced Ih over the entire voltage range in all 8 neurons tested. The mean percent reduction in Ih measured at -120 mV was -19.7 ± 2.8%, ...