Dopamine release during reward-driven behaviors influences synaptic plasticity. However, dopamine innervation and release in the hippocampus and its role during aversive behaviors are controversial. Here we show that in vivo hippocampal synaptic plasticity in the CA3-CA1 circuit underlies contextual learning during inhibitory avoidance (IA) training. Immunohistochemistry and molecular techniques verified sparse dopaminergic innervation of the hippocampus from the midbrain. The long-term synaptic potentiation (LTP) underlying the learning of IA was assessed with a D1-like dopamine receptor agonist or antagonist in ex vivo hippocampal slices and in vivo in freely-moving mice. Inhibition of D1-like dopamine receptors impaired memory of the IA task and prevented the training-induced enhancement of both ex vivo and in vivo LTP induction. The results indicate that dopamine-receptor signaling during an aversive contextual task regulates aversive memory retention and regulates associated synaptic mechanisms in the hippocampus that likely underlie learning.

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 α4β2-nAChR agonist RJR-2403 and antagonist dihydro-β-erythroidine (DHβE); IID receptor-mediated current was sensitive to the selective α7-nAChR agonist choline and antagonist methyllycaconitine (MLA); while IIID receptor-mediated current was sensitive to the β4-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 α4β2-nAChR, α7-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.

Diverse nicotinic acetylcholine receptor (nAChR) subtypes containing different subunit combinations can be placed on nerve terminals or soma/dendrites in the ventral tegmental area (VTA). nAChR ␣6 subunit message is abundant in the VTA, but ␣6*-nAChR cellular localization, function, pharmacology, and roles in cholinergic modulation of dopaminergic (DA) neurons within the VTA are not well understood. Here, we report evidence for ␣6␤2*-nAChR expression on GABA neuronal boutons terminating on VTA DA neurons. ␣-Conotoxin (␣-Ctx) MII labeling coupled with immunocytochemical staining localizes putative ␣6*-nAChRs to presynaptic GABAergic boutons on acutely dissociated, rat VTA DA neurons. Functionally, acetylcholine (ACh) induces increases in the frequency of bicuculline-, picrotoxin-, and 4-aminopyridine-sensitive miniature IPSCs (mIPSCs) mediated by GABA A receptors. These increases are abolished by ␣6*-nAChR-selective ␣-Ctx MII or ␣-Ctx PIA (1 nM) but not by ␣7 (10 nM methyllycaconitine) or ␣4* (1 M dihydro-␤-erythroidine)-nAChR-selective antagonists. ACh also fails to increase mIPSC frequency in VTA DA neurons prepared from nAChR ␤2 knock-out mice. Moreover, ACh induces an ␣-Ctx PIA-sensitive elevation in intraterminal Ca 2ϩ in synaptosomes prepared from the rat VTA. Subchronic exposure to 500 nM nicotine reduces ACh-induced GABA release onto the VTA DA neurons, as does 10 d of systemic nicotine exposure. Collectively, these results indicate that ␣6␤2*-nAChRs are located on presynaptic GABAergic boutons within the VTA and modulate GABA release onto DA neurons. These presynaptic ␣6␤2*-nAChRs likely play important roles in nicotinic modulation of DA neuronal activity.