ACETYLCHOLINE-RECEPTOR DESENSITIZATION INDUCED BY NICOTINE IN RAT MEDIAL HABENULA NEURONS

1. The activation and desensitization properties of nicotinic acetylch oline receptor (nAChR) channels were examined in acutely isolated medi al habenula (MHb) neurons using whole cell patch-clamp recordings. nAC hR-mediated currents were evoked by applying known concentrations of n icotinic agonists using rapid solution exchange techniques. 2. At a me mbrane potential of -60 mV, nAChR currents were observed above a conce ntration of approximate to 100 nM nicotine. The peak current amplitude at low doses of agonist was proportional to the square of the concent ration of nicotine, indicating that at least two molecules of agonist were required for channel opening. The concentration of nicotine requi red for half-maximal nAChR activation was estimated as 77 mu M from a complete concentration-response curve.3. During the continuous activat ion (2-5 s) of nAChRs by high concentrations of nicotine (300 mu M), t he current desensitized rapidly and extensively. The desensitization p hase was described by the sum of two exponentials, with time constants of 210 and 1,435 ms. The fast component comprised 74% of the desensit izing phase of the current. Recovery from desensitization induced by 2 -s applications of 300 mu M nicotine was also fast and could be reason ably well described by a single exponential with a time constant of ap proximate to 800 ms. Both the time courses of desensitization and reco very from desensitization were slightly slower at positive membrane po tentials. 4. Incubation of neurons with low concentrations of nicotine (100 nM-10 mu M) caused a slowly developing but pronounced desensitiz ation of the nAChRs. In these cases desensitization was assessed from the reduction in the amplitude of the peak nicotinic current induced b y repetitively applied pulses of a higher test concentration of agonis t. A 5-min continuous exposure to 1 mu M nicotine reduced the amplitud e of the acetylcholine (30 mu M, 1 s) test response to <30% of its con trol. value. As with higher concentrations of nicotine, the onset of t he desensitization induced by 1 mu M nicotine was biexponential, with fast and slow time constants of 15 s and 1.74 min, respectively. Recov ery from the desensitization induced by these longer applications of n icotine was much slower than that observed with the brief pulses of hi gh concentrations of nicotine. The concentration required for half-max imal desensitization after a 5-min incubation was approximate to 300 n M. 5. Peak nAChR currents were approximate to 85% smaller at +40 mV co mpared with -40 mV. The receptors that do not open at positive potenti als desensitize almost as well as they would at negative potentials af ter channel opening. Thus, under certain conditions, nAChR channels in MHb neurons may desensitize without opening. Desensitization from the open state, however, is likely to be the major transition at physiolo gically relevant membrane potentials. 6. The large difference between the effective concentrations of nicotine required for channel opening and desensitization has important implications for understanding how t he levels of nicotine associated with tobacco inhalation lead to long- term behavioral changes. nAChR channels in MHb neurons will be strongl y desensitized by smoking-related concentrations of nicotine. This eff ect occurs over several minutes and may result in acute tolerance to s ubsequent doses of nicotine. Long-term disruption of the endoge nous a ctivation of nAChRs due to desensitization could underlie the observed upregulation of receptors after chronic nicotine use.