Selective Excitation of Subtypes of Neocortical Interneurons by Nicotinic Receptors

Abstract

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...

α4* Nicotinic Acetylcholine Receptors Modulate Experience-Based Cortical Depression in the Adult Mouse Somatosensory Cortex

“…One clue from our data was that ␣4* nAChRs were highly expressed on GABAergic cortical interneurons and their relative levels of expression on these neurons increased following whisker trimming. This finding is consistent with previous work showing that mRNA for ␣4* nAChR are present in GABAergic cortical neurons (Porter et al, 1999;Couey et al, 2007). Furthermore, we show that activating ␣4* nAChRs in vivo by bath application or direct micro-injection of ␣4* nAChR agonist into layer 4 depressed whisker-evoked responses by 30 -50% in control mice, similar to that seen after 3-7 d of whisker deprivation.…”

“…Furthermore, we show that activating ␣4* nAChRs in vivo by bath application or direct micro-injection of ␣4* nAChR agonist into layer 4 depressed whisker-evoked responses by 30 -50% in control mice, similar to that seen after 3-7 d of whisker deprivation. Indeed, several electrophysiological studies using both human and rodent cortical tissue have shown that ␣4* nAChR activation rapidly depolarizes cortical interneurons, that can be blocked by the ␣4* nAChR antagonist DH␤E (Xiang et al, 1998;Porter et al, 1999;Alkondon et al, 2000;Christophe et al, 2002;Klaassen et al, 2006;Couey et al, 2007;Mann and Mody, 2008). Therefore one possible mechanism underlying the depression of whisker-evoked responses is that ␣4* nAChRs may be enhancing GABAergic inhibition in deprived barrel columns.…”

Seizures and enhanced cortical GABAergic inhibition in two mouse models of human autosomal dominant nocturnal frontal lobe epilepsy

“…Based on combined electrophysiological (15) and molecular studies (16), the observed increase in inhibitory I mean is very likely a result of an enhanced nicotine-evoked GABA release from Chrna4 S252F/wt or Chrna4 ϩL264/wt -expressing cortical interneurons that form synapses on the somata, axon initial segments, or dendrites of pyramidal cells. To determine whether nicotine acts on ␣4-subunit-containing nAChRs located on GABAergic interneuron terminals, we measured its effect on miniature IPSCs (mIPSCs) recorded in the absence of presynaptic Ca 2ϩ entry by blocking action potentials with tetrodotoxin (TTX) and voltage-gated Ca 2ϩ channels with cadmium.…”

“…Whereas rodent cortical layer II͞III and V pyramidal neurons are not directly depolarized by nicotinic cholinergic agonists (16,18,19), selected subtypes of cortical interneurons express functional nAChRs assembled from ␣4 and ␤2 subunits (with or without ␣5) or ␣7 subunits (16,20). Inhibitory interneurons release GABA and have been implicated in nicotine-induced hippocampal seizures (21), as well as in models of cortical or hippocampal pathophysiology that result from nAChR activation, modulation of GABAergic tone, and subsequent inhibition or disinhibition of neuronal networks (15).…”

“…However, it's unlikely that they play a direct role as nicotinic stimulation of layer I interneurons leads to an action potentialdependent increase in layer II͞III interneuron sIPSC frequency, with no effect on layer II͞III pyramidal cells (20). In layer II͞III, ␣4␤2 nAChRs (with or without ␣5) are found in a high percentage of regular (63%) and irregular-spiking (77%) interneurons, the majority of which (Ϸ70%) coexpress vasoactive intestinal peptide and cholecystokinin (16). Such interneurons innervate adjacent pyramidal cells and are candidates for the nicotine-evoked increase in inhibition.…”