1. The activation and desensitization properties of nicotinic acetylcholine receptor (nAChR) channels were examined in acutely isolated medial habenula (MHb) neurons using whole cell patch-clamp recordings. nAChR-mediated currents were evoked by applying known concentrations of nicotinic agonists using rapid solution exchange techniques. 2. At a membrane potential of -60 mV, nAChR currents were observed above a concentration of approximately 100 mM nicotine. The peak current amplitude at low doses of agonist was proportional to the square of the concentration of nicotine, indicating that at least two molecules of agonist were required for channel opening. The concentration of nicotine required for half-maximal nAChR activation was estimated as 77 microM from a complete concentration-response curve. 3. During the continuous activation (2-5 s) of nAChRs by high concentrations of nicotine (300 microM), the current desensitized rapidly and extensively. The desensitization phase was described by the sum of two exponentials, with time constants of 210 and 1,435 ms. The fast component comprised 74% of the desensitizing phase of the current. Recovery from desensitization induced by 2- s applications of 300 microM nicotine was also fast and could be reasonably well described by a single exponential with a time constant of approximately 800 ms. Both the time courses of desensitization and recovery from desensitization were slightly slower at positive membrane potentials. 4. Incubation of neurons with low concentrations of nicotine (100 nM-10 microM) caused a slowly developing but pronounced desensitization of the nAChRs. In these cases desensitization was assessed from the reduction in the amplitude of the peak nicotinic current induced by repetitively applied pulses of a higher test concentration of agonist. A 5-min continuous exposure to 1 microM nicotine reduced the amplitude of the acetylcholine (30 microM, 1 s) test response to < 30% of its control value. As with higher concentrations of nicotine, the onset of the desensitization induced by 1 microM nicotine was biexponential, with fast and slow time constants of 15 s and 1.74 min, respectively. Recovery from the desensitization induced by these longer applications of nicotine was much slower than that observed with the brief pulses of high concentrations of nicotine. The concentration required for half-maximal desensitization after a 5-min incubation was approximately 300 nM. 5. Peak nAChR currents were approximately 85% smaller at +40 mV compared with -40 mV. The receptors that do not open at positive potentials desensitize almost as well as they would at negative potentials after channel opening.(ABSTRACT TRUNCATED AT 400 WORDS)