Inhibitory effect of antidepressants on the NMDA-evoked [(3)H]noradrenaline release from rat hippocampal slices.

We have previously shown that monoamine uptake blocker-type antidepressants with different chemical structure and selectivity are able to inhibit neuronal nicotinic acetylcholine receptors (nAChRs) in concentrations observed during antidepressant treatment. The mechanism of action of these drugs is similar to that of mecamylamine, a channel blocker-type antagonist of nAChRs. Since mecamylamine has been shown to block also NMDA receptors, our aim was to investigate whether the monoamine uptake blockers may affect the function of these ionotropic glutamate receptors. We studied, therefore the effect of the two most potent nicotinic antagonist antidepressants, the tricyclic desipramine and the selective serotonin reuptake inhibitor fluoxetine on the NMDA-induced [(3)H]noradrenaline ([(3)H]NA) release from rat hippocampal slices. The NMDA-induced hippocampal [(3)H]NA release was effectively blocked by the selective, non-competitive NMDA antagonist MK-801 (IC(50)=0.54 microM), indicating that the [(3)H]NA release was mediated through NMDA receptors. This response was also dose-dependently inhibited by desipramine (IC(50)=14.57 microM) and fluoxetine (IC(50)=41.06 microM). The Na(+)-channel blocker TTX equally inhibited both the electrical stimulation- and the NMDA-evoked [(3)H]NA release (the IC(50) was 55 nM and 66 nM, respectively), whereas the antidepressants inhibited only the NMDA-evoked response. These data suggest that the inhibitory effect of fluoxetine and desipramine on the NMDA-evoked [(3)H]NA release is exerted directly on NMDA receptors rather than indirectly on Na(+)-channels. Due to accumulation processes the concentration of desipramine and fluoxetine in the brain might be in the same range as the observed IC(50) values, thus our data indicate that monoamine uptake blocker-type antidepressants are able to influence the function of NMDA receptors during antidepressant treatment, and the inhibitory effect on NMDA receptors might contribute to the therapeutic effects of these drugs.

Neurotensin modulation of acetylcholine, GABA, and aspartate release from rat prefrontal cortex studied in vivo with microdialysis.

The effects of the peptide transmitter neurotensin (NT) on the release of acetylcholine (ACh), gamma-aminobutyric acid (GABA), glutamate (Glu), aspartate (Asp), and taurine from the prefrontal cortex (PFC) of freely moving rats were studied by transversal microdialysis. Neurotensin (0.2 and 1 microM) administered locally in the PFC produced a concentration-dependent increase in the extracellular levels of ACh, GABA, and Asp, but not of Glu or taurine. The increase produced by 1 microM NT reached a maximum of about 240% for ACh, 370% for GABA, and 380% for Asp. Lower doses of NT (0.05 microM) did not cause a significant change in ACh, GABA, or Asp output in the PFC. Higher concentrations of NT (2 microM) did not induce further increases in the level of neurotransmitters. A high-affinity selective neurotensin receptor (NTR1) antagonist SR 48692 (0.5 microM) perfused locally blocked neurotensin (1 microM)-evoked ACh, GABA, and Asp release. Local infusion of the sodium channel blocker tetrodotoxin (TTX) (1 microM) decreased the release of ACh, had no significant effect on GABA or Asp release, and prevented the 1 microM neurotensin-induced increase in ACh, GABA, and Asp output. Removal of calcium from the Ringer's solution prevented the peptide from having any effects on the neurotransmitters. Thus, in vivo NT plays a modulatory role in the PFC by interacting with cortical neurons releasing GABA and Asp and with ACh-containing neurons projecting to the PFC. The NT effects are of neural origin, as they are TTX-sensitive, and mediated by the NTR1 receptor, as they are antagonized by SR 48692.

Carrier-mediated release of monoamines induced by the nicotinic acetylcholine receptor agonist DMPP.

We have previously shown that dimethylphenylpiperazinium (DMPP) increases the release of noradrenaline (NA) from rat hippocampal slices via two distinct mechanisms: a nicotinic acetylcholine receptor (nAChR)-mediated exocytosis and a carrier-mediated release induced by the reversal of NA transporters. Our aim was to investigate whether other monoaminergic systems are also affected by the multiple actions of DMPP. In our experiments DMPP dose-dependently increased the release of dopamine (DA) and serotonin (5-HT) from rat striatal and hippocampal slices, respectively. The dual effect was observed, however, only in case of DA at a lower DMPP concentration (30 microM), where the response was partly inhibited by mecamylamine, TTX and Ca2+-free medium (nAChR-mediated exocytosis) while the other part of the response was blocked only by the DA uptake inhibitor nomifensine (carrier-mediated release). In contrast, the DMPP-evoked 5-HT release and the DA release induced by high concentration DMPP was not inhibited by nicotinic antagonists, TTX and Ca2+-free medium but only by selective uptake inhibitors. In addition, DMPP dose-dependently inhibited the [3H]DA and [3H]5-HT uptake in striatal and hippocampal synaptosome preparation with an IC50 of 3.18 and 0.49 microM, respectively. Our data show that DMPP interacts with monoamine transporters and induces a substantial carrier-mediated release of DA and 5-HT, therefore caution is needed for the interpretation of data, when this drug is used as a nAChR agonist.

Further evidence for the functional role of nonsynaptic nicotinic acetylcholine receptors.

The function of nicotinic acetylcholine receptors in the main central systems has been documented in the past decade. These studies focused mostly on the synaptic functions, although acetylcholine is released dominantly into the extrasynaptic space and the majority of nicotinic acetylcholine receptors on remote neurons are found on extrasynaptic membranes. Here, we show further evidence for the role of nonsynaptic nicotinic functions in the cognitive and the reward system. Dendrites of gamma-amino-n-butyric acid (GABA)-containing interneurons of the hippocampus are densely equipped with nicotinic acetylcholine receptors. These cells play an important role in memory processing. We analysed the effects of nicotinic acetylcholine receptor stimulation on the Ca(2+) dynamics of interneurons in different dendritic compartments. We also investigated the role of nicotinic receptors in the nucleus accumbens where nicotine stimulated vesicular dopamine release via activation of receptors located on varicosities. Nicotine produced comparable effects with 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) on dopamine release. These examples demonstrate that nonsynaptic nicotinic acetylcholine receptors can effectively influence activity pattern of neural networks in key structures of central systems.