The patch-clamp technique was used to characterize the nicotinic currents in individual neurons acutely dissociated from the hippocampi of postnatal rats of different ages, and to investigate the effects of Pb2+ on these currents, both in acutely dissociated and in cultured hippocampal neurons. The effects of Pb2+ on nicotinic acetylcholine receptors (nAChR) expressed in frog muscle fibers were also investigated. Acetylcholine could activate fast- and slowly desensitizing whole-cell currents in neurons dissociated from the hippocampi of 4- to 20-day-old rats. Similar to the currents elicited in cultured hippocampal neurons, the rapidly desensitizing currents were blocked by methyllycaconitine, and were the nicotinic responses that could be elicited in most of the acutely dissociated neurons tested, whereas the slowly desensitizing currents could be evoked only in a few neurons. Although the peak amplitude of nicotinic currents recorded from acutely dissociated neurons increased with age of the neurons as it did in cultured neurons, the mean amplitude of the currents was at least an order of magnitude smaller in acutely dissociated neurons than in cultured neurons. Pb2+ could potently and specifically inhibit activation of fast-desensitizing nicotinic currents in hippocampal neurons. Pb2+ reduced the peak amplitude of fast-desensitizing currents with an IC50 of about 3 microM and an apparent Hill coefficient of 1.0, whereas only at 30 microM could Pb2+ decrease the peak amplitude of slowly desensitizing currents by about 50%. Inhibition of fast-desensitizing currents by Pb2+ was voltage independent and noncompetitive. However, Pb2+ had no effect on acetylcholine-activated single channels in frog muscle fibers. Our results indicate that the hippocampus of the developing rat expresses functional nAChRs of diverse types, and that methyllycaconitine-sensitive neuronal nAChRs, which are highly permeable to Ca2+, are much more sensitive to Pb2+ than other nAChR subtypes.