trans-metanicotine and methyllycaconitine

Several behavioral effects of nicotine are mediated by changes in serotonin (5-HT) release in brain areas that receive serotonergic afferents from the dorsal raphe nucleus (DRN). In vitro experiments have demonstrated that nicotine increases the firing activity in the majority of DRN 5-HT neurons and that DRN contains nicotinic acetylcholine receptors (nAChRs) located at both somata and presynaptic elements. One of the most common presynaptic effects of nicotine is to increase glutamate release. Although DRN receives profuse glutamatergic afferents, the effect of nicotine on glutamate release in the DRN has not been studied in detail. Using whole-cell recording techniques, we investigated the effects of nicotine on the glutamatergic input to 5-HT DRN neurons in rat midbrain slices. Low nicotine concentrations, in the presence of bicuculline and tetrodotoxin (TTX), increased the frequency but did not change the amplitude of glutamate-induced EPSCs, recorded from identified 5-HT neurons. Nicotine-induced increase of glutamatergic EPSC frequency persisted 10-20 min after drug withdrawal. This nicotinic effect was mimicked by exogenous administration of acetylcholine (ACh) or inhibition of ACh metabolism. In addition, the nicotine-induced increase in EPSC frequency was abolished by blockade of α4β2 nAChRs, voltage-gated calcium channels, or intracellular calcium signaling but not by α7 nAChR antagonists. These data suggest that both nicotine and endogenous ACh can increase glutamate release through activation of presynaptic α4β2 but not α7 nAChRs in the DRN. The effect involves long-term changes in synaptic function, and it is dependent on voltage-gated calcium channels and presynaptic calcium stores.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Acetylcholine; Aconitine; Animals; Animals, Newborn; Atropine; Bicuculline; Cadmium Chloride; Chelating Agents; Cholinergic Agonists; Dihydro-beta-Erythroidine; Egtazic Acid; Electric Stimulation; Enzyme Inhibitors; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; GABA Antagonists; Glutamic Acid; In Vitro Techniques; Indoles; Male; Muscarinic Antagonists; Nicotine; Nicotinic Antagonists; Patch-Clamp Techniques; Physostigmine; Presynaptic Terminals; Raphe Nuclei; Rats; Rats, Wistar; Receptors, Nicotinic; Ryanodine; Serotonergic Neurons; Serotonin; Sodium Channel Blockers; Tetrodotoxin

Dopaminergic (DAergic) neuronal activity in the ventral tegmental area (VTA) is thought to contribute generally to pleasure, reward, and drug reinforcement and has been implicated in nicotine dependence. nAChRs expressed in the VTA exhibit diverse subunit compositions, but the functional and pharmacological properties are largely unknown. Here, using patch-clamp recordings in single DAergic neurons freshly dissociated from rat VTA, we clarified three functional subtypes of nAChRs (termed ID, IID and IIID receptors) based on whole-cell current kinetics and pharmacology. Kinetic analysis demonstrated that comparing to ID, IID receptor-mediated current had faster activation and decay constant and IIID receptor-mediated current had larger current density. Pharmacologically, ID receptor-mediated current was sensitive to the alpha4beta2-nAChR agonist RJR-2403 and antagonist dihydro-beta-erythroidine (DHbetaE); IID receptor-mediated current was sensitive to the selective alpha7-nAChR agonist choline and antagonist methyllycaconitine (MLA); while IIID receptor-mediated current was sensitive to the beta4-containing nAChR agonist cytisine and antagonist mecamylamine (MEC). The agonist concentration-response relationships demonstrated that IID receptor-mediated current exhibited the highest EC(50) value compared to ID and IIID receptors, suggesting a relatively low agonist affinity of type IID receptors. These results suggest that the type ID, IID and IIID nAChR-mediated currents are predominately mediated by activation of alpha4beta2-nAChR, alpha7-nAChR and a novel nAChR subtype(s), respectively. Collectively, these findings indicate that the VTA DAergic neurons express diversity and multiplicity of functional nAChR subtypes. Interestingly, each DAergic neuron predominantly expresses only one particularly functional nAChR subtype, which may have distinct but important roles in regulation of VTA DA neuronal function, DA transmission and nicotine dependence.

Topics: Acetylcholine; Aconitine; Action Potentials; Alkaloids; Animals; Azocines; Brain; Choline; Dihydro-beta-Erythroidine; Dopamine; Dose-Response Relationship, Drug; Gene Expression; Kinetics; Mecamylamine; Neurons; Nicotine; Nicotinic Agonists; Nicotinic Antagonists; Patch-Clamp Techniques; Protein Subunits; Quinolizines; Rats; Rats, Wistar; Receptors, Nicotinic; Reverse Transcriptase Polymerase Chain Reaction; Tyrosine 3-Monooxygenase; Ventral Tegmental Area