Pharmacological characterization of nicotine-induced acetylcholine release in the rat hippocampus in vivo: evidence for a permissive dopamine synapse

Abstract

In this study, the mechanism of nicotine-induced hippocampal acetylcholine (ACh) release in awake, freely moving rats was examined using in vivo microdialysis. Systemic administration of nicotine (0.4 mg kg(-1), s.c.) increased the levels of ACh in hippocampal dialysates. The nicotine-induced hippocampal ACh release was sensitive to the pretreatment of neuronal nicotinic acetylcholine receptor (nAChR) antagonists mecamylamine (3.0 mg kg(-1), s.c.) and dihydro-beta-erythrodine (DHbetaE; 4.0 mg kg(-1), s.c.) as well as systemic administration of the dopamine (DA) D1 receptor antagonist SCH-23390 (R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-benzaz epine; 0.3 mg kg(-1), s.c.). Local perfusion of mecamylamine (100 microM), DHbetaE (100 microM) or SCH-23390 (10 microM) through microdialysis probe did not increase basal hippocampal ACh release. Hippocampal ACh release elicited by systemic administration of nicotine (0.4 mg kg(-1), s.c.) was antagonized by local perfusion of SCH-23390 (10 microM), but not by MEC (100 microM) or DHbetaE (100 microM). Direct perfusion of nicotine (1 mM, but not 0.1 mM) increased hippocampal ACh levels; however, this effect was relatively insensitive to blockade by co-perfusion of either mecamylamine (100 microM) or SCH-23390 (10 microM). These results suggest that nicotine-induced hippocampal ACh release occurs by two distinct mechanisms: (1) activation of nAChRs outside the hippocampus leading to DA release and subsequent ACh release involving a permissive DA synapse, and (2) direct action of nicotine within the hippocampus leading to ACh release via non-DA-ergic mechanism.

Figure 1

Effects of saline and nicotine (NIC) on ACh levels in hippocampal dialysates. Nicotine (0.4 mg kg⁻¹, n=5) or saline (n=4) was administered subcutaneously at time=0 and the ACh levels in the dialysates determined as described in Methods. The data represent the mean±s.e.mean of the groups. ⁺P<0.05 vs pretreatment baseline (saline; one-way ANOVA followed by Dunnett's test), ^*P<0.05 for the effect of nicotine vs the saline control (two-way ANOVA). The arrow indicates the time of injection (zero min).

Figure 2

The pharmacology of nicotine (NIC)-induced hippocampal ACh release. Rats were given saline or various antagonists 20 min prior to s.c. injection of nicotine (0.4 mg kg⁻¹) or saline. Arrows (−20 and zero min) denote the injection times. Values represent mean±s.e.mean (n=6–7; saline+saline, n=7; saline+nicotine, n=6; mecamyalime (MEC; 3.0 mg kg⁻¹)+nicotine, n=6; DHβE (3.0 mg kg⁻¹)+nicotine, n=7; SCH-23390 (0.3 mg kg⁻¹)+nicotine, n=6). ^*P<0.05 for the effect of saline/nicotine vs the saline/saline response (two-way ANOVA); ⁺P<0.05 for the effect of antagonist/nicotine vs the saline/nicotine response (two-way ANOVA).

Figure 3

Summary of effects of mecamylamine (MEC), DHβE or SCH-23390 on nicotine (NIC)-induced ACh release from rat hippocampus. The area under curve values were determined using the trapezoidal rule and the values represent arbitrary units (mean±s.e.mean, n=6–7). ^*P<0.05 vs saline+saline response, ⁺P<0.05 vs saline+nicotine response (ANOVA followed by Neuman-Keul's test).

Figure 4

Effects of local perfusion of various antagonists on increases in ACh levels in hippocampal dialysates induced by s.c. injection of nicotine (NIC, 0.4 mg kg⁻¹). The perfusion of antagonists mecamylamine (MEC, 100 μM, n=5), DHβE (100 μM, n=4) or SCH-23390 (10 μM, n=4)) was initiated 20 min before the s.c. injection of nicotine (n=5). The arrows indicate the times of injection (−20 and zero min). Values represent mean±s.e.mean, n=5–6). ^*P<0.05 antagonist+nicotine vs nicotine+aCSF response. The horizontal line indicates the duration of perfusion and the arrows indicate the initiation and termination of the perfusion.

Figure 5

Summary of the effects of local perfusion of mecamylamine (MEC), DHβE or SCH-23390 on nicotine (NIC, 0.4 mg kg⁻¹, s.c.)-induced hippocampal ACh release. The area under curve values expressed as arbitrary units (mean±s.e.mean, n=5–6) were determined using the trapezoidal rule. ^*P<0.05 vs saline+aCSF response, ⁺P<0.05 vs nicotine+aCSF response.

Figure 6

The effects of local perfusion of various antagonists on ACh release elicited by local perfusion of nicotine (NIC, 1 mM, n=7). The antagonists mecamylamine (MEC, 100 μM, n=5), or SCH-23390 (10 μM, n=4) were co-perfused with nicotine in aCSF. Values represent mean±s.e.mean. The horizontal line denotes the duration of perfusion and the arrows indicate the initiation and termination of perfusion.