Modulation of the function of presynaptic alpha7 and non-alpha7 nicotinic receptors by the tryptophan metabolites, 5-hydroxyindole and kynurenate in mouse brain

Abstract

Background and purpose: Two metabolites of tryptophan, 5-hydroxyindole and kynurenic acid (kynurenate) affect the function of alpha7 nicotinic acetylcholine receptors (nAChRs), as measured by electrophysiological and Ca2+ fluorescence techniques. To better understand the modulations by 5-hydroxyindole and kynurenate of the function of nAChR subtypes, we compared the effects of 5-hydroxyindole and kynurenate on the release of various transmitters evoked by nAChR activation.

Experimental approach: The function of alpha7nAChRs located on glutamatergic terminals was investigated by monitoring the release of [3H]D-aspartate or of endogenous glutamate from neocortical synaptosomes. We also comparatively considered non-alpha7 release-enhancing nAChRs localized on hippocampal noradrenergic or cholinergic terminals, as well as on striatal dopaminergic terminals.

Key results: Epibatidine or nicotine, inactive on their own on basal release, enhanced [3H]D- aspartate and glutamate efflux in presence of 5-hydroxyindole. The release evoked by nicotine plus 5-hydroxyindole was abolished by methyllycaconitine or alpha-bungarotoxin. Presynaptic nAChRs mediating the release of [3H]noradrenaline ([3H]NA), [3H]dopamine ([3H]DA), or [3H]ACh were inhibited by 5-OHi. The alpha7nAChR-mediated release of [3H]D-aspartate was reduced by kynurenate at concentrations unable to affect the non-alpha7 receptor-mediated release of tritiated NA, DA or ACh.

Conclusions and implications: (i) 5-hydroxyindole permits selective activation of alpha7nAChRs mediating glutamate release; (ii) kynurenate down-regulates the permissive role of 5-hydroxyindole on alpha7nAChR activation; (iii) the non-alpha7nAChRs mediating release of NA, DA or ACh can be inhibited by 5-hydroxyindole, but not by kynurenate. These findings suggest up the possibility of developing novel drugs able to modulate selectively the cholinergic-glutamatergic transmission.

Figure 1

Effects of 5-hydroxyindole (5-OHi) on the nAChR agonist-mediated overflow of [³H]D-aspartate ([³H]D-ASP) from mouse cortical synaptosomes. 5-OHi was added 8 min before agonists. Data are mean ±s.e.mean of 4–9 experiments run in triplicate (three superfusion chambers for each experimental condition). ^*P<0.05; ^**P<0.01 versus control (two-tailed student's t-test).

Figure 2

Antagonism by methyllycaconitine (MLA) and α-bungarotoxin (α-Bgtx) of the [³H]D-aspartate overflow evoked by nicotine plus 5-hydroxyindole (5-OHi) from mouse cortical synaptosomes. Data are means ±s.e.mean of 4–9 experiments run in triplicate. ^**P<0.01 versus nicotine plus 5-OHi; ^##P<0.01 versus nicotine (two-tailed student's t-test).

Figure 3

Effects of nicotine and 5-hydroxyindole (5-OHi), alone or in combination, on the release of endogenous glutamate from mouse cortical synaptosomes. 5-Hydroxyindole was added 8 min before nicotine. Data are mean ±s.e.mean of 5–9 experiments run in triplicate. ^*P<0.05; ^**P<0.01 versus nicotine (two-tailed student's t-test). Inset: effect of nicotine and 5-hydroxyindole alone or in combination on glutamate overflow (total minus basal release). ^*P<0.05; ^**P<0.01 versus nicotine (two-tailed student's t-test).

Figure 4

Concentration-response relationship of the effects of 5-hydroxyindole (5-OHi) on the nicotine-evoked [³H]D-aspartate overflow from mouse cortical synaptosomes. Data are means ±s.e.mean of three experiments run in triplicate. ^*P<0.05; ^**P<0.01 versus control (two-tailed student's t-test).

Figure 5

Effects of 5-hydroxyindole (5-OHi) or kynurenic acid (KYNA) on the nicotine-evoked overflow of [³H]NA from hippocampal synaptosomes (a) of [³H]DA from striatal synaptosomes (b) and of [³H]ACh from hippocampal synaptosomes (c). 5-Hydroxyindole or kynurenic acid were added 8 min before nicotine. Data are means ±s.e.mean of 4–9 experiments run in triplicate. ^*P<0.05; ^**P<0.01 (5-OHi versus nicotine); °P<0.05 (KYNA versus nicotine) (two-tailed student's t-test).

Figure 6

Effects of 5-hydroxyindole (5-OHi) on the overflow of [³H]NA evoked by AMPA or by NMDA plus glycine from mouse hippocampal synaptosomes. 5-Hydroxyindole, NMDA and AMPA were used at 100 μM; glycine at 1 μM. Data are means ±s.e.mean of 4–9 experiments run in triplicate.

Figure 7

Antagonism by kynurenic acid of the overflow of [³H]D-aspartate evoked by nicotine plus 5-hydroxyindole from mouse cortical synaptosomes. Kynurenic acid and 5-hydroxyindole were added 8 min before nicotine. Data are means ±s.e.mean of five experiments run in triplicate.