The compact division of the posterior pallial amygdala (PoAc) and lateral part of the bed nucleus of the stria terminalis (BSTL) are components of the limbic system in the pigeon brain. In this study, we examined the position and fiber connections of these two nuclei by using Nissl staining and tract-tracing methods. PoAc occupies a central division in the posterior pallial amygdala. BSTL faces the ventral horn of the lateral ventricle and extends between A 7.25 and A 10.50. PoAc and BSTL connect bidirectionally by the stria terminalis. PoAc connects reciprocally with two nuclear groups in the cerebrum: 1) a continuum consisting of the caudoventral nidopallium, lateral part of the caudoventral nidopallium (NCVl), subnidopallium beneath NCVl, and piriform cortex and 2) rostral areas of the hemisphere, including the frontolateral and frontomedial nidopallium and the densocellular part of the hyperpallium. Extratelencephalic projections of PoAc terminate in the dorsomedial thalamic nuclei and reach the lateral hypothalamic area via the hypothalamic part of the occipito-mesencephalic tract. BSTL also connects reciprocally with two main regions: 1) the same continuum as for PoAc projections, except the piriform cortex and 2) rostral areas of the hemisphere, including the olfactory tubercle and nucleus accumbens. Extratelencephalic reciprocal connections are with the substantia nigra, nucleus subceruleus dorsalis, parabrachial nucleus, locus coeruleus, and nucleus of the solitary tract. The dorsomedial subdivision of the hippocampal formation projects massively to PoAc and BSTL. These findings indicate that PoAc and BSTL are important components of an interconnected neural circuit involving widespread regions of the neuraxis and mediating limbic-visceral functions.

In the course of evolution, the vomeronasal organ (VNO) first appeared in amphibians. To understand the relationship between the VNO and the vomeronasal receptors, we isolated and analyzed the expression of the vomeronasal receptor genes of Xenopus laevis. We identified genes of the Xenopus V2R receptor family, which are predominantly expressed throughout the sensory epithelium of the VNO. The G-protein Go, which is coexpressed with V2Rs in the rodent VNO, was also extensively expressed throughout the vomeronasal sensory epithelium. These results strongly suggest that the V2Rs and Go are coexpressed in the vomeronasal receptor cells. The predominant expression of the Xenopus V2R families and the coexpression of the V2Rs and Go imply that V2Rs play important roles in the sensory transduction of Xenopus VNO. We found that these receptors were expressed not only in the VNO, but also in the posterolateral epithelial area of the principal cavity (PLPC). Electron microscopic study revealed that the epithelium of the PLPC is more like that of the VNO than that of the principal and the middle cavity. These results suggest that in adult Xenopus the V2Rs analyzed so far are predominantly expressed in the vomeronasal and vomeronasal-like epithelium. The analysis of V2R expression in Xenopus larvae demonstrates that V2Rs are predominantly expressed in the VNO even before metamorphosis.

The renal glomerular podocyte exhibits a highly arborized morphology. In comparison with the neuron, which is the best studied process-bearing cell, the podocyte major processes share many cell biological characteristics with neuronal dendrites. Both podocytes and neurons develop microtubule-based thick processes with branching morphology and both have thin actin-based projections (i.e. podocyte foot processes and dendritic spines). Formation of podocyte processes and neuronal dendrites depends on the assembly of microtubules. Because the assembly of microtubules is regulated by phosphorylation of microtubule-associated proteins, inhibition of protein phosphatases abolishes and inhibition of protein kinases promotes process formation. Podocytes and dendrites also share the machinery of intracellular traffic of membranous vesicles, as well as cytoskeletal elements, which is indispensable for the elongation of these processes. Furthermore, these two cell types share expression of various molecules working for signal transduction, transmembranous transport and intercellular contacts. Such common gene expression implies a similar transcriptional regulation in these cells. Concerning the formation of podocyte foot processes and dendritic branches, actin filaments are thought to play a central role in orchestrating the function of various molecules and the regulation of actin assembly is necessary to establish and maintain such sophisticated cellular architecture. The molecular mechanism of foot process formation seems to include Rho family small GTP-binding proteins, which are known to be responsible for the establishment of dendritic branching morphology.

The songbird brain has a system of interconnected nuclei that are specialized for singing and song learning. Wada et al. (2004; J. Comp. Neurol. 476:44-64) found a unique distribution of the mRNAs for glutamate receptor subunits in the song control brain areas of songbirds. In conjunction with data from electrophysiological studies, these finding indicate a role for the glutamatergic neurons and circuits in the song system. This study examines vesicular glutamate transporter 2 (VGLUT2) mRNA and protein expression in the zebra finch brain, particularly in auditory areas and song nuclei. In situ hybridization assays for VGLUT2 mRNA revealed high levels of expression in the ascending auditory nuclei (magnocellular, angular, and laminar nuclei; dorsal part of the lateral mesencephalic nucleus; ovoidal nucleus), high or moderate levels of expression in the telencephalic auditory areas (cudomedial mesopallium, field L, caudomedial nidopallium), and expression in the song nuclei (HVC, lateral magnocellular nucleus of the anterior nidopallium, robust nucleus of the arcopallium), where levels of expression were greater than in the surrounding brain subdivisions. Area X did not show expression of VGLUT2 mRNA. Nuclei in the descending motor pathway (dorsomedial nucleus of the intercollicular complex, retroambigual nucleus, tracheosyringeal motor nucleus of the hypoglossal nerve) expressed VGLUT2 mRNA. The target nuclei of VGLUT2 mRNA-expressing nuclei showed immunoreactivity for VGLUT2 as well as hybridization signals for the mRNA of glutamate receptor subunits. The present findings demonstrate the origins and targets of glutamatergic neurons and indicate a central role for glutamatergic circuits in the auditory and song systems in songbirds.

ABSTRACT. Xenopus laevis has three distinctive olfactory neuroepithelia. We examined the axonal projection from each of these epithelia to the olfactory bulb by Di