The role of dopaminergic (DA) projections from the ventral tegmental area (VTA) in appetitive and rewarding behavior has been widely studied, but the VTA also has documented DA-independent functions. Several drugs of abuse, including nicotine, act on VTA GABAergic neurons, and most studies have focused on local inhibitory connections. Relatively little is known about VTA GABA projection neurons and their connections to brain sites outside the VTA. In this study, we employed viral-vector mediated cell-type specific anterograde tracing, classical retrograde tracing and immunohistochemistry to characterize VTA GABA efferents throughout the brain. We found that VTA GABA neurons project widely to forebrain and brainstem targets, including the ventral pallidum, lateral and magnocellular preoptic nuclei, lateral hypothalamus and lateral habenula. Minor projections also go to central amygdala, mediodorsal thalamus, dorsal raphe and deep mesencephalic nuclei, and sparse projections go to prefrontal cortical regions and to nucleus accumbens shell and core. Importantly, these projections differ from the major VTA DA target regions. Retrograde tracing studies confirmed results from the anterograde experiments and differences in projections from VTA subnuclei. Retrogradely-labeled GABA neurons were not numerous and most non-TH/retrogradely labeled cells lacked GABAergic markers. Many non-DA/retrogradely labeled cells projecting to several areas express VGluT2. VTA GABA and glutamate neurons project throughout the brain, most prominently to regions with reciprocal connections to the VTA. These data indicate that VTA GABA and glutamate neurons may have more dopamine-independent functions than previously recognized.

Functional neuroanatomy of Pavlovian fear has identified neuronal circuits and synapses associating conditioned stimuli with aversive events. Hebbian plasticity within these networks requires additional reinforcement to store particularly salient experiences into long-term memory. Here we have identified a circuit that reciprocally connects the ventral periaqueductal gray and dorsal raphe region with the central amygdala and that gates fear learning. We found that ventral periaqueductal gray and dorsal raphe dopaminergic (vPdRD) neurons encode a positive prediction error in response to unpredicted shocks and may reshape intra-amygdala connectivity via a dopamine-dependent form of long-term potentiation. Negative feedback from the central amygdala to vPdRD neurons might limit reinforcement to events that have not been predicted. These findings add a new module to the midbrain dopaminergic circuit architecture underlying associative reinforcement learning and identify vPdRD neurons as a critical component of Pavlovian fear conditioning. We propose that dysregulation of vPdRD neuronal activity may contribute to fear-related psychiatric disorders.

The related high molecular mass microtubuleassociated proteins (MAPs) MAP1A and MAP1B are predominantly expressed in the nervous system and are involved in axon guidance and synaptic function. MAP1B is implicated in fragile X mental retardation, giant axonal neuropathy, and ataxia type 1. We report the functional characterization of a novel member of the microtubule-associated protein 1 family, which we termed MAP1S (corresponding to sequence data bank entries for VCY2IP1 and C19ORF5). MAP1S contains the three hallmark domains of the microtubuleassociated protein 1 family but hardly any additional sequences. It decorates neuronal microtubules and copurifies with tubulin from brain. MAP1S is synthesized as a precursor protein that is partially cleaved into heavy and light chains in a tissue-specific manner. Heavy and light chains interact to form the MAP1S complex. The light chain binds, bundles, and stabilizes microtubules and binds to actin. The heavy chain appears to regulate light chain activity. In contrast to MAP1A and MAP1B, MAP1S is expressed in a wide range of tissues in addition to neurons and represents the non-neuronal counterpart of this cytolinker family.The classical microtubule-associated proteins (MAPs) 1 MAP1, MAP2, and tau were discovered more than 20 years ago by virtue of the fact that they copurified with microtubules from vertebrate brain (1). Because of the developmental regulation of expression of these proteins, their restricted localization in the axonal or somato-dendritic compartment of neurons, and the differential phosphorylation depending on the developmental stage and subcellular localization, it was soon proposed that MAPs are important regulators of neuronal microtubules during differentiation.The MAP1 family has two members: MAP1A and MAP1B. Both proteins are of high molecular mass (ϳ300 kDa, 2500 aa), are expressed predominantly in the nervous system, and consist of several subunits, one heavy chain (HC) and at least one light chain (LC) (1). In each case, heavy and light chains are the products of proteolytic cleavage of a common polyprotein precursor. MAP1A and MAP1B share three substantial regions of sequence homology (2), one in the NH 2 terminus of the heavy chains, one in the COOH terminus of the heavy chains, and one in the COOH-terminal half of the light chains. We termed these homologous hallmark domains of the MAP1 family MH1, MH2, and MH3, respectively. In MAP1B, the MH1 and MH3 domains mediate the interaction between heavy and light chains (3), and the MH3 domain of both proteins contains an actin binding site (4). MAP1A and MAP1B are conserved in vertebrates. A MAP1 ortholog termed Futsch has been identified in Drosophila (5).MAP1B function has been investigated by gene targeting in the mouse, and the original contention that it is important for neuronal differentiation and development of the nervous system has been confirmed (6 -9). Mice homozygous for hypomorphic or null alleles of MAP1B display defects in axonal guidance, neuronal migration, axon diameter, and ...

Inhibitory interneurons play a critical role in coordinating the activity of neural circuits. To explore the mechanisms that direct the organization of inhibitory circuits, we analyzed the involvement of TrkB in the assembly and maintenance of GABAergic inhibitory synapses between Golgi and granule cells in the mouse cerebellar cortex. We show that TrkB acts directly within each cell-type to regulate synaptic differentiation. TrkB is required not only for assembly, but also maintenance of these synapses and acts, primarily, by regulating the localization of synaptic constituents. Postsynaptically, TrkB controls the localization of a scaffolding protein, gephyrin, but acts at a step subsequent to the localization of a cell adhesion molecule, Neuroligin-2. Importantly, TrkB is required for the localization of an immunoglobulin superfamily cell adhesion molecule, Contactin-1, in Golgi and granule cells and the absence of Contactin-1 also results in deficits in inhibitory synaptic development. Thus, our findings demonstrate that TrkB controls the assembly and maintenance of GABAergic synapses and suggest that TrkB functions, in part, through promoting synaptic adhesion.