The human hyperpolarization-activated cyclic nucleotide-gated 1 (hHCN1) subunit was heterologously expressed in mammalian cell lines (CV-1 and CHO) and its properties investigated using whole-cell patch-clamp recordings. Activation of this recombinant channel, by membrane hyperpolarization, generated a slowly activating, noninactivating inward current. The pharmacological properties of hHCN1-mediated currents resembled those of native hyperpolarization-activated currents (I(h)), that is, blockade by Cs(+) (99% at 5 mm), ZD 7288 (98% at 100 microm) and zatebradine (92% at 10 microm). Inhibition of the hHCN1-mediated current by ZD 7288 was apparently independent of prior channel activation (i.e. non-use-dependent), whereas that induced by zatebradine was use-dependent. The VR1 receptor antagonist capsazepine inhibited hHCN1-mediated currents in a concentration-dependent (IC(50)=8 microm), reversible and apparently non-use-dependent manner. This inhibitory effect of capsazepine was voltage-independent and associated with a leftward shift in the hHCN1 activation curve as well as a dramatic slowing of the kinetics of current activation. Elevation of intracellular cAMP or extracellular K(+) significantly enhanced aspects of hHCN1 currents. However, these manipulations did not significantly affect the capsazepine-induced inhibition of hHCN1. The development of structural analogues of capsazepine may yield compounds that could selectively inhibit HCN channels and prove useful for the treatment of neurological disorders where a role for HCN channels has been described.