The immunoreactivity for a gamma-aminobutyric acid (GABA) membrane transporter, GAT-1, was examined in the neocortex and hippocampal formation of developing rats from the day of birth (postnatal day 0, P0) to the adult stage. The immunolabeling was mainly localized to the neuropil, but was also in a select population of cell bodies during a limited time period. Layers I and VIb of neocortex exhibited relatively high reactivity at birth, but diminished their staining with development. In contrast, GAT-1 immunoreactivity in the neuropil in the cortical plate and its derivatives was light at birth, but increased rapidly during the first 2-3 postnatal weeks in an inside-out order. An adult pattern with immunoreactive puncta more densely distributed in layers II to IV than the deeper layers was completed by P30-45. The neuropil reactivity in the hippocampal formation at P0 was greater than that in the neocortex, densely localized in a supragranular band, and less densely in the hilus of the dentate gyrus and the strata radiatum and oriens of the hippocampus. This pattern was basically maintained at later stages except that the immunoreactivity in the supragranular band diminished, whereas that in the subgranular zone was enhanced. A population of cell bodies morphologically characteristic of cortical and hippocampal interneurons was substantially immunolabeled for GAT-1 by P5 and remained until P30. At the electron microscopic level, GAT-1 immunoreactivity was localized mainly to axon terminals and astrocytes between P5 and P45, but was also found in neuronal somata and their dendrites between P5 and P30. Our data show a differential postnatal development of GAT-1 immunoreactivity in the rat cerebral cortex, including a transient presence of immunoreactivity in the somata of a subpopulation of cerebral interneurons and a developmental downregulation of GAT-1 expression in the earliest generated cortical elements (layers 1 and VIb). The findings in the present study suggest that GAT-1 expression in the neocortex and hippocampus may relate to the functional maturation of the GABAergic system.

Corticotropin-releasing hormone (CRH) receptor type 1 (CRF(1)) is a member of the receptor family mediating the effects of CRH, a critical neuromediator of stress-related endocrine, autonomic, and behavioral responses. The detailed organization and fine localization of CRF(1)-like immunoreactivity (CRF(1)-LI) containing neurons in the rodent have not been described, and is important to better define the functions of this receptor. Here we characterize in detail the neuroanatomical distribution of CRF(1)-immunoreactive (CRF(1)-ir) neurons in the mouse brain, using an antiserum directed against the C-terminus of the receptor. We show that CRF(1)-LI is abundantly yet selectively expressed, and its localization generally overlaps the target regions of CRH-expressing projections and the established distribution of CRF(1) mRNA, with several intriguing exceptions. The most intensely CRF(1)-LI-labeled neurons are found in discrete neuronal systems, i.e., hypothalamic nuclei (paraventricular, supraoptic, and arcuate), major cholinergic and monoaminergic cell groups, and specific sensory relay and association thalamic nuclei. Pyramidal neurons in neocortex and magnocellular cells in basal amygdaloid nucleus are also intensely CRF(1)-ir. Finally, intense CRF(1)-LI is evident in brainstem auditory associated nuclei and several cranial nerves nuclei, as well as in cerebellar Purkinje cells. In addition to their regional specificity, CRF(1)-LI-labeled neurons are characterized by discrete patterns of the intracellular distribution of the immunoreaction product. While generally membrane associated, CRF(1)-LI may be classified as granular, punctate, or homogenous deposits, consistent with differential membrane localization. The selective distribution and morphological diversity of CRF(1)-ir neurons suggest that CRF(1) may mediate distinct functions in different regions of the mouse brain.