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.

Nicotinic acetylcholine receptor antagonistic activity of monoamine uptake blockers in rat hippocampal slices.

The aim of our study was to investigate the effect of different monoamine uptake blockers on the nicotine-evoked release of [3H]noradrenaline ([3H]NA) from rat hippocampal slices. We found that desipramine (DMI), nisoxetine, cocaine, citalopram, and nomifensine inhibit the nicotine-evoked release of [3H]NA with an IC50 of 0.36, 0.59, 0.81, 0.93, and 1.84 microM, respectively. These IC50 values showed no correlation with the inhibitory effect (Ki) of monoamine uptake blockers on the neuronal NA transporter (r = 0.17, slope = 0.02), indicating that the NA uptake system is not involved in the process. In whole-cell patch clamp experiments neither drug blocked Na+ currents at 1 microM in sympathetic neurons from rat superior cervical ganglia, and only DMI produced a pronounced inhibition (52% decrease) at 10 microM. Comparison of the effect of DMI and tetrodotoxin (TTX) on the electrical stimulation- and nicotine-evoked release of [3H]NA showed that DMI, in contrast to TTX, inhibits only the nicotine-induced response, indicating that the target of DMI is not the Na+ channel. Our data suggest that monoamine uptake blockers with different chemical structure and selectivity are able to inhibit the nicotinic acetylcholine receptors in the CNS. Because these compounds are widely used in the therapy of depressed patients, our findings may have great importance in the evaluation of their clinical effects.

A possible role of nitric oxide in the regulation of dopamine transporter function in the striatum.

Brain microdialysis and high-performance liquid chromatography with electrochemical detection were used to study the effect of the nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyl ester (L-NAME) on striatal dopamine (DA) release in the anesthetized rat. Systemic administration of L-NAME (10 mg/kg, i.p.) significantly decreased the resting release of DA. The peak effect (23% decrease) was reached 45 min after injection. The inactive enantiomer D-NAME (10 mg/kg, i.p.) or the vehicle (saline, 5 ml/kg i.p.) had no effect on the striatal DA level. Neither treatment altered significantly the concentration of dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). To investigate the possible involvement of the DA uptake system L-NAME was injected also in the presence of the DA uptake inhibitor nomifensine. Local application of nomifensine (10 microM in the dialysate medium) increased the extracellular concentration of DA to about eight-fold of the basal value and stabilized it at this higher level. Under these conditions L-NAME (10 mg/kg, i.p.) was not able to alter the striatal DA level. Neither nomifensine nor L-NAME caused any change in the level of DOPAC and HVA. Our data suggest that endogenously produced nitric oxide may influence the activity of the DA transporter which effect may have special importance in the regulation of extracellular transmitter concentration in the striatum.

Analysis of deprenyl metabolites in the rat brain using HPLC-ES-MS.

Methylamphetamine and amphetamine, the two major metabolites of deprenyl in the rat brain were analyzed using HPLC method combined with electrospray-mass spectrometer. (-)-Deprenyl and (+)-deprenyl were orally administered to rats either in a single dose of 10 mg/kg, or three times a week for three weeks. The metabolites were determined in four different parts of the rat brain, such as in the frontal cortex, corpus striatum, hippocampus, and hypophysis. The ratio of methylamphetamine to amphetamine was also compared after (-)-deprenyl and (+)-deprenyl treatments.

Nicotinic acetylcholine receptor antagonist effect of fluoxetine in rat hippocampal slices.

We compared the effect of mecamylamine and fluoxetine on the hippocampal noradrenaline (NA) release evoked by nicotine in vitro. Nicotine (100 microM) increased the basal release of [3H]NA from rat hippocampal slices. This effect was blocked by the potent nicotinic antagonist mecamylamine in a dose-dependent manner (IC50 = 0.19 microM). The selective serotonin reuptake inhibitor (SSRI) fluoxetine also antagonised the response to nicotine in a dose-dependent manner with a similar strength (IC50 = 0.57 microM). Our data indicate that fluoxetine has nicotinic acetylcholine receptor antagonist effect in the central nervous system. The possible clinical significance of this finding is discussed.