The cellular mechanisms by which neuronal nicotinic cholinergic receptors influence many aspects of physiology and pathology in the neocortex remain primarily unknown. Whole-cell recordings and single-cell reverse transcription (RT)-PCR were combined to analyze the effect of nicotinic receptor agonists on different types of neurons in acute slices of rat neocortex. Nicotinic receptor agonists had no effect on pyramidal neurons and on most types of interneurons, including parvalbuminexpressing fast spiking interneurons and somatostatinexpressing interneurons, but selectively excited a subpopulation of interneurons coexpressing the neuropeptides vasoactive intestinal peptide (VIP) and cholecystokinin. This excitation persisted in the presence of glutamate, GABA, and muscarinic receptor antagonists and in the presence of tetrodotoxin and low extracellular calcium, suggesting that the depolarization was mediated through the direct activation of postsynaptic nicotinic receptors. The responses were blocked by the nicotinic receptor antagonists dihydro--erythroidine and mecamylamine and persisted in the presence of the ␣7 selective nicotinic receptor antagonist methyllycaconitine, suggesting that the involved nicotinic receptors lacked the ␣7 subunit. Single-cell RT-PCR analysis indicated that the majority of the interneurons that responded to nicotinic stimulation coexpressed the ␣4, ␣5, and 2 nicotinic receptor subunits. Therefore, these results provide a role for non-␣7 nicotinic receptors in the selective excitation of a subpopulation of neocortical interneurons. Because the neocortical interneurons expressing VIP have been proposed previously to regulate regional cortical blood flow and metabolism, these results also provide a cellular basis for the neuronal regulation of cortical blood flow mediated by acetylcholine.
Key words: single-cell PCR; neuropeptides; calcium-binding proteins; methyllycaconitine; dihydro--erythroidine; acetylcholine; mecamylamineNicotinic receptors are implicated in many important f unctions of the mammalian neocortex, including memory formation (Granon et al., 1995) and neuronal regulation of regional cerebral blood flow (Gitelman and Prohovnik, 1992;Uchida et al., 1997), as well as cortical pathologies such as Alzheimer's disease (Whitehouse et al., 1988;Newhouse et al., 1997), epilepsy (Steinlein et al., 1995, and Tourette's syndrome (Sanberg et al., 1997(Sanberg et al., , 1998. The cellular basis for the effects of nicotinic receptor stimulation is currently unknown but lies within its effects on the complex interactions between the excitatory glutamatergic pyramidal neurons and the inhibitory interneurons.A variety of different neuronal nicotinic receptor subunits have been cloned and named ␣2-␣9 and 2-4 (Elgoyhen et al., 1994;Le Novere and Changeux, 1995). C lassically, two broad subfamilies of nicotinic receptors have been characterized in neurons according to their sensitivity to ␣-bungarotoxin. Subunits ␣7 and ␣8 form a family of ␣-bungarotoxin-sensitive channels (Couturier...