Pharmacological characterization of nicotinic receptor-mediated acetylcholine release in rat brain—an in vivo microdialysis study

Abstract

In vivo microdialysis was used to investigate nicotinic receptor-mediated acetylcholine release in the hippocampus, frontal cortex, and striatum of freely moving rats. Intraperitoneal administration of (−)-nicotine increased the release of acetylcholine in the hippocampus and frontal cortex but not in the striatum. (−)-Nicotine exhibited a bell-shaped dose–response relationship, and showed attenuation of response at the highest dose (5.0 mg/kg i.p.) in both the hippocampus and frontal cortex. In the hippocampus, (−)-nicotine (1.0 mg/kg i.p.)-induced increase of acetylcholine release was blocked by pretreatment with the centrally acting nicotinic receptor channel blocker, mecamylamine (1.0 mg/kg i.p.), but not by hexamethonium (5.0 mg/kg i.p.), suggesting that the effects of (−)-nicotine were mediated by the central nicotinic receptor. (S)-3-methyl-5-(1-methyl-2-pyrrolidinyl)isoxazole (ABT-418, 1.0 and 5.0 mg/kg i.p.), reported to be a selective agonist for α4β2 nicotinic receptor subunits, also enhanced the release of acetylcholine in the hippocampus, while 3-(2,4-dimethoxybenzlidene)-anabaseine (GTS-21, 1.0 and 5.0 mg/kg i.p.), which has high affinity for the α7 nicotinic receptor subunit, was without effect. The natural alkaloids isolated from plants, (−)-cytisine and (−)-lobeline, had little effect on acetylcholine release from the hippocampus. A competitive antagonist for α4β2 subunits of the nicotinic receptor, dihydro-β-erythroidine, and a partial agonist for the β2 subunit-containing nicotinic receptor, (−)-cytisine, inhibited (−)-nicotine-induced increase of acetylcholine release from the hippocampus, whereas a selective antagonist for the α7 subunit, methyllycaconitine, and a partial agonist for the α3 subunit-containing nicotinic receptor, (−)-lobeline, did not. These results indicate that there are certain differences among brain regions in the response of nicotinic receptor-mediated acetylcholine release and that (−)-nicotine-induced acetylcholine release in the rat hippocampus may be attributed to activation of the α4β2 nicotinic receptor subunits.

Introduction

In the past decade, it has become evident that there is a great diversity of nicotinic receptors in the central nervous system (review articles, Sargent, 1993; Decker et al., 1995). Several neuronal nicotinic receptor subunits have been cloned, and eight types of α subunits (α2–α9) and three types of β subunits (β2–β4) (Boulter et al., 1987; Nef et al., 1988; Wada et al., 1988; Couturier et al., 1990; Elgoyhen et al., 1994) have been identified. The neuronal nicotinic receptors are thought to be composed of α and β subunits (Anand et al., 1991; Cooper et al., 1991) and the most abundant nicotinic receptor in the central nervous system consists of α4 and β2 subunits (Flores et al., 1992), while, in recombinant expression systems, α7 as well as α8 and α9 subunits can form functional homooligomeric receptors (Couturier et al., 1990; Gerzanich et al., 1994; Elgoyhen et al., 1994). However, the functional significance of the nicotinic receptor subtypes formed by these subunits remains obscure.

Recently, neuronal nicotinic receptor agonists have attracted much interest as potential therapeutic agents for the treatment of Alzheimer's disease. Clinical studies have revealed that (−)-nicotine is effective to ameliorate memory and attention deficits in Alzheimer's disease patients (Newhouse et al., 1986; Sahakian et al., 1989; Jones et al., 1992). In animals, (−)-nicotine has been reported to show beneficial effects on memory in aged monkeys and to reverse spatial memory deficits in rats with an experimental lesion of the medial septal nucleus (Levin, 1992; Decker et al., 1995). In addition, the centrally acting nicotinic receptor channel blocker, mecamylamine, produces significant cognitive impairment that mimics certain aspects of Alzheimer's disease in young and elderly volunteers (Newhouse et al., 1994). Postmortem studies of Alzheimer's disease brain tissue demonstrated marked reductions of nicotinic receptors in both neocortex and hippocampus, consistent with the Alzheimer's disease pathology of neuronal degeneration (Araujo et al., 1988b). These findings point to the functional importance of nicotinic acetylcholine systems in cognitive functions.

Previous in vitro studies have provided evidence that (−)-nicotine can enhance acetylcholine release by stimulating presynaptic nicotinic receptors in the cortex and hippocampus (Rowell and Winkler, 1984; Araujo et al., 1988a; Wilkie et al., 1996), but there are only a few studies that have addressed the in vivo ability of (−)-nicotine to increase acetylcholine release in these brain regions. In microdialysis studies, subcutaneous injection of (−)-nicotine significantly increased the release of acetylcholine from the rat cortex in a mecamylamine-sensitive manner (Summers et al., 1994; Summers and Giacobini, 1995). Since direct application of (−)-nicotine into the cortex through a microdialysis probe membrane increased acetylcholine release, it is likely that the stimulation of acetylcholine release by (−)-nicotine occurred via presynaptic nicotinic receptors (Summers and Giacobini, 1995). Thus, a role for nicotinic receptors in the modulation of acetylcholine release from cortex has been accepted, while the subunits of nicotinic receptors mediating acetylcholine release have not been definitively identified.

Regarding the several types of neuronal nicotinic receptor ligands recently discovered (see review, Brioni et al., 1996), extensive pharmacological and behavioral studies have been carried out on (S)-3-methyl-5-(1-methyl-2-pyrrolidnyl) pyrrolidinyl) isoxazole (ABT-418) (Garvey et al., 1994) and 3-(2,4-dimethoxybenzylidene) anabaseine (GTS-21) (Zoltewicz et al., 1993). ABT-418 selectively activates α4β2 subunits more than α3 and α7 subunits (Arneric et al., 1994) and shows potent cognition-enhancing and anxiolytic properties in animal models, with low side-effects (Decker et al., 1994). In contrast, GTS-21 has selectively for the α7 subunit, which is preponderant in the hippocampus, and exerts cognition-enhancing activity in rats (Meyer et al., 1994) and cytoprotective effects in cells (Martin et al., 1994; Kihara et al., 1997). These ligands' pharmacological and neurochemical activities may be associated with release-enhancing effects, although there is no direct evidence for acetylcholine release in vivo with these nicotinic receptor ligands.

In the present study, therefore, we examined the effects of (−)-nicotine on acetylcholine release in three brain regions and compared the abilities of several types of nicotinic receptor ligands to affect acetylcholine release from rat hippocampus by using an in vivo microdialysis technique.