Nicotinic acetylcholine receptor antagonistic property of the selective dopamine uptake inhibitor, GBR-12909 in rat hippocampal slices.

Previously we found that inhibitors of noradrenaline (NA) and/or 5-HT reuptake are able to inhibit neuronal nicotinic acetylcholine receptors (nAChRs) in the CNS most probably by a channel blocker-type mechanism. The aim of our study was to clarify whether selective dopamine uptake inhibitors also possess this property, therefore we investigated the effect of GBR-12909 on the nicotine-evoked release of [3H]NA from rat hippocampal slices. GBR-12909, similar to selective NA and 5-HT uptake blockers, inhibited the nicotine-evoked release with an IC50 of 2.32 microM. The ability of monoamine uptake blockers to inhibit nicotine-evoked [3H]NA release (IC50) and NA reuptake (Ki) showed no correlation, indicating that the NA uptake system is not involved in the inhibition of the response to nicotine. Previously we have shown in whole cell patch clamp experiments, that GBR-12909, depending on the stimulation pattern, inhibits Na+-currents with an IC50 in the 6-35 microM concentration range [Mike A, Karoly R, Vizi ES, Kiss JP (2003) Inhibitory effect of the DA uptake blocker GBR-12909 on sodium channels of hippocampal neurons. Neuroreport 14:1945-1949]. To study whether the inhibition of Na+-channels is involved in the action of GBR-12909 on the nicotine-evoked [(3)H]NA release, we compared the effect of GBR-12909 and the Na(+)-channel blocker tetrodotoxin (TTX) on the electrical stimulation- and nicotine-evoked response. TTX prevented the release of [3H]NA induced by both types of stimulation, whereas GBR-12909 inhibited only the nicotine-induced response, indicating that under our experimental conditions the target of GBR-12909 is not the Na+-channel. These data indicate that the selective DA uptake inhibitor GBR-12909 is able to inhibit nAChRs, that is, the nAChR antagonistic property of monoamine uptake inhibitors is independent of their selectivity. The fact that monoamine uptake inhibitors with different chemical structure and selectivity are able to inhibit nAChRs may reveal some common properties of nicotinic receptors and monoamine uptake carriers.

The mechanism of activity-dependent sodium channel inhibition by the antidepressants fluoxetine and desipramine.

The effect of monoamine uptake inhibitor-type antidepressants on sodium channels of hippocampal neurons was investigated. Members of the tricyclic group of antidepressants are known to modify multiple targets, including sodium channels, whereas selective serotonin-reuptake inhibitors (SSRIs) are regarded as highly selective compounds, and their effect on sodium channels was not investigated in detail. In this study, a representative member of each group was chosen: the tricyclic antidepressant desipramine and the SSRI fluoxetine. The drugs were roughly equipotent use-dependent inhibitors of sodium channels, with IC(50) values approximately 100 microM at -150 mV holding potential, and approximately 1 microM at -60 mV. We suggest that therapeutic concentrations of antidepressants affect neuronal information processing partly by direct, activity-dependent inhibition of sodium channels. As for the mechanism of inhibition, use-dependent inhibition by antidepressants was believed to be due to a preferential affinity to the fast-inactivated state. Using a voltage and perfusion protocol by which relative affinities to fast-versus slow-inactivated states could be assessed, we challenged this view and found that the affinity of both drugs to slowinactivated state(s) was higher. We propose a different mechanism of action for these antidepressants, in which slow rather than fast inactivation plays the dominant role. This mechanism is similar but not equivalent with the novel mechanism of usedependent sodium channel inhibition previously described by our group (Neuroscience 125:1019-1028, 2004; Neuroreport 14:1945-1949, 2003). Our results suggest that different drugs can produce use-dependent sodium channel inhibition by different mechanisms.

Differential effect of nicotinic agonists on the [3H]norepinephrine release from rat hippocampal slices.

The aim of this study was to investigate the mechanisms involved in the effect of nicotinic agonists on the [3H]norepinephrine ([3H]NE) release from rat hippocampal slices. The stimulatory effect of nicotine, cytisine, epibatidine and anatoxin-A was completely blocked by the nicotinic antagonist mecamylamine (10 microM). In contrast, the effect of dimethylphenylpiperazinium (DMPP) was only partially inhibited by mecamylamine but was completely blocked by the NE uptake inhibitor desipramine (DMI, 10 microM). Finally, the effect of lobeline was not affected by mecamylamine and was only partially blocked by DMI. Our data indicate that the majority of nicotinic agonists increase the release of [3H]NE exclusively via stimulation of nicotinic acetylcholine receptors (nAChRs). DMPP, in addition to the stimulation of nAChRs, also evokes a carrier-mediated release. Lobeline has no stimulatory effect on nAChRs, induces a carrier-mediated release and has a further action of unidentified mechanism. Our results suggest that special caution is required for the interpretation of data, when DMPP or lobeline are used as nicotinic agonists.