Nicotinic receptor-mediated regulation of the dopamine transporter in rat prefrontocortical slices following chronic in vivo administration of nicotine

Low levels of dopaminergic activity in prefrontal cortex are thought to contribute to negative symptoms of schizophrenia. Negative symptoms are associated with the prefrontocortical area of the brain. Schizophrenic patients have a high rate of smoking, which by subjective as well as objective measures produces a cognitive benefit. We have previously shown that agonists at nicotinic receptors containing α4 and β2 subunits can enhance amphetamine-stimulated [3H]dopamine ([3H]DA) release via the dopamine transporter (DAT) from slices of rat prefrontal cortex. This effect is selective for prefrontal cortex; the enhancement does not occur in striatum or nucleus accumbens. The enhancement is dependent upon activation of protein kinase C (PKC). In the current study, we show that the enhancement of amphetamine-stimulated [3H]DA release is maintained after 10 days of chronic nicotine treatment, delivered subcutaneously twice daily. There are no significant changes in the ability of prefrontocortical brain slices to take up [3H]DA in tissue prepared from nicotine-treated vs. saline-treated rats. Nicotinic receptors mediating enhancement of amphetamine-stimulated [ 3H]DA release are at least partially localized to nerve terminals, as an enhancement in release is also observed in synaptosomal preparations. Finally, the sensitivity of the nicotine enhancement in release to the PKC inhibitor chelerythrine is also seen in synaptosomal preparations, suggesting that the signaling mechanism activated through α4β2 receptors is intact. © 2003 Elsevier Science B.V. All rights reserved

Acute and chronic effects of nicotine on serotonin uptake in prefrontal cortex and hippocampus of rats

We sought to investigate the effect of nicotine exposure (chronic and acute) on serotonin transporter (SERT) activity in two regions of the brain important for behavioral effects of nicotine. We first looked at the effects of chronic nicotine exposure (0.7 mg/kg nicotine, twice a day for 10 days) on [3H]5-HT uptake in prefrontal cortex (PFC) and hippocampus of rats. A significant increase in [3H]5-HT uptake was observed in synaptosomes prepared from both regions. To rule out the possibility that the increases were due to the last injection given, in a separate set of experiments a single injection of nicotine was administered the evening before sacrifice. No change in uptake occurred in either region, suggesting that the increases in uptake caused by nicotine was an effect of chronic exposure and not to an acute treatment. SERT binding studies, using prefrontocortical or hippocampal membrane preparations, revealed that chronic nicotine exposure significantly increased Bmax which correlated to an increase in SERT density. Lastly, we looked at the short-term effect of nicotine on [3H]5-HT uptake. Rats received a single nicotine injection 15-75 min before sacrifice. PFC synaptosomes displayed a time-dependent increase in uptake, whereas hippocampal synaptosomes showed an increase at only one time point. © 2003 Wiley-Liss, Inc

Lack of effects by sigma ligands on neuropeptide Y-induced G-protein activation in rat hippocampus and cerebellum.

It has been suggested that neuropeptide Y (NPY) and sigma (σ) receptor ligands may share a putative NPY/σ receptor in rat brain. To study whether NPY and σ receptor ligands have an inverse agonism at this putative NPY/σ receptor, we measured their effects on G-protein activity in rat brain. Using [35S]GTPγS autoradiography, we found that NPY-induced G-protein activation exhibited a discrete distribution pattern in rat brain. G-protein activation in superficial cortical layers and hippocampal CA1-3 region was mainly attributed to Y1 and Y2 receptors, respectively. In the presence of 10 μM σ-receptor agonist BD737 or 10 μM σ-receptor antagonist haloperidol, the distribution and density of [35S]GTPγS binding stimulated by 10 nM NPY was not significantly altered. In rat cerebellar membranes, NPY stimulated high-affinity GTPase activity in a dose-related manner, with maximal effects of 29% increase over basal level seen at 500 nM. This NPY-elicited GTPase activity was not significantly affected by micromolar concentrations of the σ-receptor antagonists Dup734 or haloperidol. Since no significant effects by σ-receptor ligands on NPY-induced G-protein activation were observed, we did not see an inverse agonism of NPY and σ-receptor ligands at the putative NPY/σ receptor measured at the level of G-protein activation, suggesting that σ receptors and NPY receptors do not represent a common population in rat hippocampus and cerebellum. It is also suggested that G-protein activation is not a convergent point for the signal transduction mechanisms of NPY receptors and σ receptors. © 2001 Elsevier Science B.V. All rights reserved

SH-SY5Y cells as a model for sigma receptor regulation of potassium-stimulated dopamine release.

Previous studies in our laboratory using rat brain tissue have shown that neuropeptide Y (NPY) can enhance NMDA- and potassium-stimulated dopamine release from various brain regions and that this enhancement is reversed by sigma (σ) receptor antagonists. In the current study, we sought to determine whether SH-SY5Y cells are suitable for investigating σ receptor effects and whether any σ receptors present are of the subtype responsive to NPY. We compare mechanisms by which the prototypical σ receptor agonist (+)- pentazocine, and the proposed endogenous σ receptor ligand NPY regulate potassium-stimulated [3 H]dopamine release from SH-SY5Y cells. Both (+)- pentazocine and NPY inhibit potassium-stimulated [3 H]dopamine release. Unlike our studies in rat brain tissue, the effect of NPY on [3 H]dopamine release is not reversed by σ receptor antagonists. SH-SY5Y cells appear to be an appropriate model to study the regulation of dopamine release by σ receptors or by NPY receptors, but this population is not identical to that population identified in brain slices. (C) 2000 Elsevier Science B.V