SUMMARY Bursts of spikes triggered by sensory stimuli in midbrain dopamine neurons evoke phasic release of dopamine in target brain areas, driving reward-based reinforcement learning and goal-directed behavior. NMDA-type glutamate receptors (NMDARs) play a critical role in the generation of these bursts. Here we report LTP of NMDAR-mediated excitatory transmission onto dopamine neurons in the substantia nigra. Induction of LTP requires burst-evoked Ca2+ signals amplified by preceding metabotropic neurotransmitter inputs in addition to the activation of NMDARs themselves. PKA activity gates LTP induction by regulating the magnitude of Ca2+ signal amplification. This novel form of plasticity is associative, input specific, reversible, and depends on the relative timing of synaptic input and postsynaptic bursting in a manner analogous to the timing rule for cue-reward learning paradigms in behaving animals. NMDAR plasticity may thus represent a potential neural substrate for conditioned dopamine neuron burst responses to environmental stimuli acquired during reward-based learning.

Drug addiction is driven, in part, by powerful and enduring memories of sensory cues associated with drug intake. As such, relapse to drug use during abstinence is frequently triggered by an encounter with drug-associated cues, including the drug itself. L-type Ca2+ channels (LTCCs) are known to regulate different forms of synaptic plasticity, the major neural substrate for learning and memory, in various brain areas. Long-term potentiation (LTP) of NMDA receptor (NMDAR)-mediated glutamatergic transmission in the ventral tegmental area (VTA) may contribute to the increased motivational valence of drug-associated cues triggering relapse. In this study, using rat brain slices, we found that isradipine, a general LTCC antagonist used as antihypertensive medication, not only blocks the induction of NMDAR LTP but also promotes the reversal of previously induced LTP in the VTA. In behaving rats, isradipine injected into the VTA suppressed the acquisition of cocaine-paired contextual cue memory assessed using a conditioned place preference (CPP) paradigm. Furthermore, administration of isradipine or a CaV1.3 subtype-selective LTCC antagonist (systemic or intra-VTA) before a single extinction or reinstatement session, while having no immediate effect at the time of administration, abolished previously acquired cocaine and alcohol (ethanol) CPP on subsequent days. Notably, CPP thus extinguished cannot be reinstated by drug re-exposure, even after 2 weeks of withdrawal. These results suggest that LTCC blockade during exposure to drug-associated cues may cause unlearning of the increased valence of those cues, presumably via reversal of glutamatergic synaptic plasticity in the VTA.

As a consequence, NMDAR-mediated transmission becomes more susceptible to LTP induction after repeated amphetamine exposure. We have also found that the magnitude of amphetamine-conditioned place preference (CPP) in behaving rats correlates with the magnitude of mGluR-dependent Ca 2ϩ signal facilitation measured in VTA slices prepared from these rats. Furthermore, the development of amphetamine CPP is significantly attenuated by intra-VTA infusion of the PKA inhibitor H89. We propose that enhancement of mGluR-dependent NMDAR plasticity in the VTA may promote the learning of environmental stimuli repeatedly associated with amphetamine experience.

SUMMARYHelisoma trivolvis has served as a model system to study the functions of serotonin (5-HT) from cellular, developmental, physiological and behavioural perspectives. To further explore the serotonin system at the molecular level, and to provide experimental knockout tools for future studies, in this study we identified serotonin receptor genes from the H. trivolvis genome, and characterized the molecular structure and expression profile of the serotonin receptor gene products. Degenerate oligonucleotide primers, based on conserved regions of the Lymnaea stagnalis 5-HT 1Lym receptor, were used to amplify G proteincoupled biogenic amine receptor sequences from H. trivolvis genomic cDNA, resulting in the cloning of two putative serotonin receptors. The deduced gene products both appear to be G protein-coupled serotonin receptors, with well-conserved structure in the functional domains and high variability in the vestibule entrance of the receptor protein. Phylogenetic analysis placed these receptors in the 5-HT 1 and 5-HT 7 families of serotonin receptors. They are thus named the 5-HT 1Hel and 5-HT 7Hel receptors, respectively. In situ hybridization and immunofluorescence studies revealed that these genes and gene products are expressed most heavily in the ciliated pedal and mantle epithelia of H. trivolvis embryos. In adults, widespread expression occurred in all ganglia and connectives of the central nervous system. Expression of both receptor proteins was localized exclusively to neurites when examined in situ. In contrast, when isolated neurons were grown in culture, 5-HT 1Hel and 5-HT 7Hel immunoreactivity were located primarily in the cell body. This is the first study to reveal a 5-HT 7 receptor in a molluscan species.Supplementary material available online at