Feedback circuitry within a song-learning pathway.

The song system of birds consists of several neural pathways. One of these, the anterior forebrain pathway, is necessary for the acquisition but not for the production of learned song in zebra finches. It has been shown that the anterior forebrain pathway sequentially connects the following nuclei: the high vocal center, area X of lobus parolfactorius, the medial portion of the dorsolateral thalamic nucleus, the lateral magnocellular nucleus of anterior neostriatum (IMAN), and the robust nucleus of the archistriatum (RA). We now show in zebra finches (Taeniopygia guttata) that IMAN cells that project to RA also project to area X, forming a feedback loop within the anterior forebrain pathway. The axonal endings of the IMAN projection into area X form cohesive and distinct domains. Small injections of tracer in subregions of area X backfill a spatially restricted subset of cells in IMAN, that, in turn, send projections to RA that are arranged in horizontal layers, which may correspond to the functional representation of vocal tract muscles demonstrated by others. We infer from our data that there is a myotopic representation throughout the anterior forebrain pathway. In addition, we suggest that the parcellation of area X into smaller domains by the projection from IMAN highlights a functional architecture within X, which might correspond to units of motor control, to the representation of acoustic features of song, or both.

Protein export from the nucleus requires the GTPase Ran and GTP hydrolysis.

Nuclei of digitonin-permeabilized cells that had been preloaded with a model transport substrate in a cytosol-dependent import reaction were subsequently incubated to investigate which conditions would result in export of transport substrate. We found that up to 80% of the imported substrate was exported when recombinant human Ran and GTP were present in the export reaction. Ran-mediated export was inhibited by nonhydrolyzable GTP analogs and also by wheat germ agglutinin but was unaffected by a nonhydrolyzable ATP analog. Moreover, a recombinant human Ran mutant that was deficient in its GTPase activity inhibited export. These data indicate that export of proteins from the nucleus requires Ran and GTP hydrolysis but not ATP hydrolysis. We also found that digitonin-permeabilized cells were depleted of their endogenous nuclear Ran, thus allowing detection of Ran as a limiting factor for export. In contrast, most endogenous karyopherin alpha was retained in nuclei of digitonin-permeabilized cells. Unexpectedly, exogenously added, fluorescently labeled Ran, although it accessed the nuclear interior, was found to dock at the nuclear rim in a punctate pattern, suggesting the existence of Ran-binding sites at the nuclear pore complex.

Evidence for a hypothalamothalamocortical circuit mediating pheromonal influences on eye and head movements.

A method for simultaneous iontophoretic injections of the anterograde tracer Phaseolus vulgaris leukoagglutinin and the retrograde tracer fluorogold was used to characterize in the rat a hypothalamothalamocortical pathway ending in a region thought to regulate attentional mechanisms by way of eye and head movements. The relevant medial hypothalamic nuclei receive pheromonal information from the amygdala and project to specific parts of the thalamic nucleus reuniens and anteromedial nucleus, which then project to a specific lateral part of the retrosplenial area (or medial visual cortex). This cortical area receives a convergent input from the lateral posterior thalamic nucleus and projects to the superior colliculus. Bidirectional connections with the hippocampal formation suggest that activity in this circuit is modified by previous experience. Striking parallels with basal ganglia circuitry are noted.