Release of the neurotransmitter dopamine in the mesolimbic system of the brain mediates the reinforcing properties of several drugs of abuse, including nicotine. Here we investigate the contribution of the high-affinity neuronal nicotinic acetylcholine receptor to the effects of nicotine on the mesolimbic dopamine system in mice lacking the beta2 subunit of this receptor. We found that nicotine stimulates dopamine release in the ventral striatum of wild-type mice but not in the ventral striatum of beta2-mutant mice. Using patch-clamp recording, we show that mesencephalic dopaminergic neurons from mice without the beta2 subunit no longer respond to nicotine, and that self-administration of nicotine is attenuated in these mutant mice. Our results strongly support the idea that the beta2-containing neuronal nicotinic acetylcholine receptor is involved in mediating the reinforcing properties of nicotine.

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Prolonged exposure of the brain to ethanol is a prerequisite for developing ethanol dependence, but the underlying neural adaptations are unknown. Here we demonstrate that rats subjected to repeated cycles of intoxication and withdrawal develop a marked and long-lasting increase in voluntary ethanol intake. Exposure-induced but not spontaneous alcohol intake is antagonized by acamprosate, a compound clinically effective in human alcoholism. Expression analysis of cingulate cortex and amygdala reveals a set of long-term up-regulated transcripts in this model. These include members of pathways previously implicated in alcohol dependence (glutamatergic, endocannabinoid, and monoaminergic neurotransmission), as well as pathways not previously thought to be involved in this disorder (e.g., members of the mitogen-activated protein kinase pathway). Thus, alternating periods of ethanol intoxication and withdrawal are sufficient to induce an altered functional brain state, which is likely to be encoded by long-term changes in gene expression. These observations may have important implications for how alcoholism is managed clinically. Novel clinically effective treatments may be possible to develop by targeting the products of genes found to be regulated in our model.

New neurons in the adult dentate gyrus are widely held to incorporate into hippocampal circuitry via a stereotypical sequence of morphological and physiological transitions, yet the molecular control over this process remains unclear. We studied the role of brain-derived neurotrophic factor (BDNF)/TrkB signaling in adult neurogenesis by deleting the full-length TrkB via Cre expression within adult progenitors in TrkB lox/lox mice. By 4 weeks after deletion, the growth of dendrites and spines is reduced in adultborn neurons demonstrating that TrkB is required to create the basic organization of synaptic connections. Later, when new neurons normally display facilitated synaptic plasticity and become preferentially recruited into functional networks, lack of TrkB results in impaired neurogenesis-dependent long-term potentiation and cell survival becomes compromised. Because of the specific lack of TrkB signaling in recently generated neurons a remarkably increased anxiety-like behavior was observed in mice carrying the mutation, emphasizing the contribution of adult neurogenesis in regulating mood-related behavior.BDNF ͉ LTP ͉ neurogenesis ͉ plasticity ͉ dendritogenesis T he life-long generation of new neurons is well documented in the subgranule zone of the dentate gyrus in the hippocampus (1-4). Dentate gyrus neurons result from local self-replicating radial glia-like stem cells (5). Once generated, the vast majority of neurons remain located on the hilar side of the granule layer and attempt to connect into the existing neuronal network, finally receiving afferent input from perforant path fibers (6, 7) and providing efferent output to CA3 cells (8, 9). While new neurons are held to incorporate into the preexisting circuitry via a stereotypical sequence of morphological transitions (10), the molecular mechanisms regulating the functional integration and/or survival of newborn neurons are not yet fully understood. There is growing evidence of the role of neuronal activity in this process. New neurons sense neuronal activity through ambient ␥-aminobutyric acid (GABA) before receiving, in sequence, GABAergic and glutamatergic inputs. Defects in the GABA responsiveness of newborn neurons, such as that obtained by inducing the conversion of GABA-mediated depolarization into hyperpolarization, lead to marked deficits in dendritic arborization and synapse formation (11), suggesting that network activity controls key morphological transitions required for the connectivity of adult-born neurons. At the initiation of connectivity glutamatergic inputs control newborn neuron survival (12). Indeed, access to afferent inputs may be the key for their life and death decisions. A central hypothesis arising from this regulation is that adult-born neurons could contribute to the formation of new circuits in tune with network needs, which, in turn, relates to the functional incorporation of adult-born neurons into hippocampal circuits. Hence, around connectivity time and later adult-born neurons become preferentially recruited into...

␣6* nicotinic acetylcholine receptors (nAChRs) are highly and selectively expressed by mesostriatal dopamine (DA) neurons. These neurons are thought to mediate several behavioral effects of nicotine, including locomotion, habit learning, and reinforcement. Yet the functional role of ␣6* nAChRs in midbrain DA neurons is mostly unknown. The aim of this study was to determine the composition and in vivo functional role of ␣6* nAChR in mesolimbic DA neurons of male rats. Immunoprecipitation and immunopurification techniques coupled with cell-specific lesions showed that the composition of ␣6* nAChR in the mesostriatal system is heterogeneous, with (non-␣4)␣6␤2* being predominant in the mesolimbic pathway and ␣4␣6␤2* in the nigrostriatal pathway. We verified whether ␣6* receptors mediate the systemic effects of nicotine on the mesolimbic DA pathway by perfusing the selective antagonists ␣-conotoxin MII (CntxMII) (␣3/␣6␤2* selective) or ␣-conotoxin PIA (CntxPIA) (␣6␤2* selective) into ventral tegmental area (VTA). The intra-VTA perfusion of CntxMII or CntxPIA markedly decreased systemic nicotine-elicited DA release in the nucleus accumbens and habituated locomotion; the intra-VTA perfusion of CntxMII also decreased the rate of nicotine infusion in the maintenance phase of nicotine, but not of food, self-administration. Overall, the results of these experiments show that the ␣6␤2* nAChRs expressed in the VTA are necessary for the effects of systemic nicotine on DA neuron activity and DA-dependent behaviors such as locomotion and reinforcement, and suggest that ␣6␤2*-selective compounds capable of crossing the blood-brain barrier may affect the addictive properties of nicotine and therefore be useful in the treatment of tobacco dependence.