The nicotinic modulation of [H-3]D-aspartate outflow in primary cultures of rat neocortical neurons: effect of acute and long term nicotine treatment

The effect of nicotine 1 nM-10 muM on the efflux of [H-3]D-aspartate was te sted in primary cultures of rat conical neurons kept at rest and subjected to electrical field stimulation. Two trains of pulses at 20 Hz for 20 s wer e applied at the 60th (St(1)) and 90th (St(2)) min of perfusion. The drug s lightly and transiently increased the efflux of resting cells while, when g iven during St(2), it greatly enhanced the electrically evoked efflux estim ated as St(2)/St(1) ratio, EC50 bring 107 nM. The nicotinic receptors (nAChR) giving rise to this positive modulation wer e partly mecamylamine- and partly alpha -bungarotoxin-sensitive. They appea red to be located at the nerve endings since nicotine facilitation was only slightly prevented by tetrodotaxin during depolarisation with 15 mM KCl. P retreatment with glutamate antagonists did not reveal any interaction betwe en nAChR and ionotropic glutamate receptors. Membrane glutamate carrier inv olvement in the nicotine effect was ruled out. Long-term treatment with nic otine 1 muM (from the 3rd-4th to the 8th-9th day in vitro) reduced the maxi mal response to the drug but shifted its threshold concentration to the lef t (from 10 nM to 1 nM), leaving the contribution of the two receptor subtyp es unchanged. Reduced responsiveness to nicotine was also evident in long-t erm treated cerebellar granule cells. In conclusion, presynaptic nAChR's, b oth containing and lacking alpha (7) subunits, can contribute to enhance th e glutamatergic secretion in primary cultures of rat cortical neurons, chie fly during electrical stimulation. (C) 2000 Elsevier Science Ltd. All right s reserved.