The nicotinic acetylcholine receptor subtypes and their function in the hippocampus and cerebral cortex

Abstract

Nicotinic acetylcholine receptors (nAChRs) are widely distributed in the central nervous system and have been implicated in multiple behavioral paradigms and pathological conditions. Nicotinic therapeutic interventions require an extensive characterization of native nAChRs including mapping of their distribution and function in different brain regions. Here, we describe the roles played by different nAChRs in affecting neuronal activity in the hippocampus and cerebral cortex. At least three distinct functional nAChR subtypes (α7, α4β2, α3β4) can be detected in the hippocampal region, and in many instances a single neuron type is found to be influenced by all three nAChRs. Further, it became clear that GABAergic and glutamatergic inputs to the hippocampal interneurons are modulated via different subtypes of nAChRs. In the cerebral cortex, GABAergic inhibition to the layer V pyramidal neurons is enhanced predominantly via activation of α4β2 nAChR and to a minor extent via activation of α7 nAChR. Such diversity offers pathways by which nicotinic drugs affect brain function.

Introduction

Nicotinic acetylcholine receptors (nAChRs) are diverse groups of membrane proteins present in muscle, ganglia and neurons of the central nervous system (CNS) (Lukas and Bencherif, 1992, Albuquerque et al., 1997). Earlier binding studies have detected only two subtypes in the brain: (i) a low-affinity receptor labeled by [¹²⁵I]α-bungarotoxin, and (ii) a high-affinity receptor labeled by [³H]nicotine or [³H]acetylcholine (Clark et al., 1985, Marks et al., 1986, Flores et al., 1992). However, molecular biological studies have identified at least nine α subunits (α2–α10) and three β subunits (β2–β4) (Lindstrom,1996, Elgoyhen et al., 2001) in the CNS. Although, these high number of nAChR subunits raise the possible existence of a variety of structurally divergent native nAChR subtypes, prior to 1990s, there were only a few studies on neuronal nAChRs (Aracava et al., 1987, Lipton et al., 1987). Soon thereafter, several electrophysiological studies surfaced in the literature, identifying and characterizing native nAChR subtypes in various brain regions such as rat hippocampus (Alkondon and Albuquerque, 1990, Alkondon et al., 1992, Zorumski et al., 1992, Alkondon and Albuquerque, 1993; see reviews by Albuquerque et al., 1995, Gray et al., 1996, Albuquerque et al., 1997), rat medial habenula and interpeduncular nucleus (Mulle and Changeux, 1990, Léna et al., 1993), porcine hypophyseal intermediate lobe cells (Zhang and Feltz, 1990), chick lateral spiriform nucleus (McMahon et al., 1994), rat midbrain neurons (Pidoplichko et al., 1997) and mouse amygdala neurons (Barazangi and Role, 2001). Pharmacological analysis of acetylcholine (ACh)-gated currents in cultured hippocampal neurons indicated the presence of at least three different nAChR subtypes giving rise to kinetically-distinct nicotinic whole-cell currents (Alkondon and Albuquerque, 1993, Castro and Albuquerque, 1995). A rapidly decaying nicotinic current (named as type IA current), sensitive to blockade by methyllycaconitine (MLA) or α-bungarotoxin (α-BGT), a slowly decaying current (named as type II current), sensitive to blockade by dihydro-β-erythroidine (DHβE), and a very slowly decaying current, sensitive to blockade by low concentration of mecamylamine (MEC), have been described (see review by Albuquerque et al., 1995, Albuquerque et al., 1997). In situ hybridization analysis and immunocytochemical studies substantiated the initial conclusion that type IA and type II currents are mediated, respectively, by α7 and α4β2 nAChR subunits (Alkondon et al., 1994, Barrantes et al., 1995, Zarei et al., 1999). It was inferred based on the sensitivity to MEC that the type III currents are mediated by α3β4 nAChRs, and recent studies by other investigators (Xiao et al., 1998, Papke et al., 2001) corroborate that MEC is more selective for α3β4 nAChR than for other known subunit combinations. Further, the studies by Zoli et al. (1998) extended the classification of nicotinic current types even further to include type IV currents in the mouse brain, and provided evidence for the involvement of other nAChR subunits.

Recently, choline, the product of hydrolysis of ACh, has received much attention as a nicotinic pharmacophore after the initial discovery that this endogenous agent can activate and desensitize some native and reconstituted nAChRs (Mandelzys et al., 1995, Papke et al., 1996, Alkondon et al., 1997b). These findings have rekindled the interest of the early attempts by Krnjević and others (see Krnjević and Reinhardt, 1979) to recognize the excitatory role of microiontophoretically injected choline in the cortical regions of the brain. Choline is an agonist like ACh at the α7 nAChR, and is a partial agonist at the α3β4 nAChR (Papke et al., 1996, Albuquerque et al., 1997, Alkondon et al., 1999). At low micromolar concentrations, choline desensitizes α7 nAChRs present in hippocampal neurons (Alkondon et al., 1997b, Alkondon et al., 1999). Choline-activated single channel α7 nAChR-mediated events have kinetics that are indistinguishable from those of the events induced by ACh (Mike et al., 2000). Yet, choline differs from ACh in being less potent (one-tenth potency), and allowing the receptors to recover from desensitization at a rapid rate (Mike et al., 2000).

There is ample evidence from various groups of investigators that α7 and α4β2 nAChRs are affected in various illnesses such as schizophrenia (Breese et al., 2000) and Alzheimer's disease (Whitehouse et al., 1988, Nordberg, 1999, Perry et al., 2000, Albuquerque et al., 2001), and such reports justify the need to characterize the properties and function of various nAChRs in the brain. The discovery of the allosteric potentiating ligand (APL) site in the nAChRs (Pereira et al., 1993, 2002) offers a unique advantage in that an impaired nicotinic function, as it occurs in Alzheimer's disease, can be corrected by the use of APLs. Galantamine, the first APL recommended for the treatment of Alzheimer's disease, has been shown recently (Santos et al., 2002) to strengthen glutamatergic and GABAergic synaptic transmission based on nAChR-dependent mechanisms.

In this brief study, we discuss recent findings from our laboratory on the effects of ACh, choline and nicotine on the nAChRs present in the neurons of brain slices from the cortex and the hippocampus, and additionally report some corroborative new findings. Several common principles emerge from the analysis of our results on the role of nAChRs in the brain neuronal activity.