Age-dependent retinoic acid regulation of gene expression distinguishes the cervical, thoracic, lumbar, and sacral spinal cord regions during development.

We evaluated whether differences in the availability of retinoic acid (RA) establish distinct patterns of RA-dependent gene expression in the embryonic mouse thoracic/sacral versus cervical/lumbar spinal cord regions. Exogenous RA elicits ectopic expression of an RA-activated transgene and the RA receptor beta in the dorsal thoracic and sacral cord in mice at embryonic day (E) 12.5, but not E14.5. This age-dependent regulation is cell autonomous and is not accompanied by changes in expression patterns of several retinoid receptors, binding proteins, or the SMRT nuclear corepressor. Instead, this change apparently reflects the loss of endogenous RA in the dorsal thoracic and sacral cord between E12.5 and E14.5. Thus, chronic exposure to exogenous RA between E11.5 and E13.5 restores ectopic RA-mediated gene expression. These observations suggest that the local availability of RA establishes absolute differences in gene expression that distinguish the thoracic and sacral cord from the cervical and lumbar cord during midgestation.

Retinoid signaling and the generation of regional and cellular diversity in the embryonic mouse spinal cord.

Retinoid-dependent gene expression accompanies the emergence of distinct regions and cell classes in the mouse spinal cord around midgestation. We asked whether changes in the expression of retinoid signaling molecules and retinoid-responsive genes reflect the establishment of this regional and cellular diversity. At E10.5, retinoic acid (RA) receptors (RAR)alpha, RAR beta, the retinoid X receptor (RXR) gamma, cellular RA binding protein (CRABP)I, CRAPBII, and cellular retinol binding protein (CRBP)I mRNAs are found throughout the entire anterior-posterior (AP) axis of the cord, as is RA (Colbert et al. [1993] Proc. Natl. Acad. Sci. U.S.A. 90:6572-6576) and RA-sensitive transgene expression (Balkan et al. [1992] Proc. Natl. Acad. Sci. U.S.A. 89:3347-3351). At E12.5, RA, transgene expression, and RAR beta become restricted to the cervical and lumbar cord. RAR alpha, CRABPI, and RXR gamma, however, are found throughout the AP extent. CRABPII and CRBPI, although expanded within the cervical and lumbar regions, are also found throughout the AP axis. Thus, several retinoid signaling molecules continue to be expressed beyond distinct regions of the spinal cord where RA is available and some RA-responsive genes are either restricted or enhanced. Exogenous RA can activate a more widespread response resulting in ectopic transgene and RAR beta expression in the thoracic and sacral cord. Not all RA-sensitive genes, however, respond; CRABPII and CRBPI expression patterns are unchanged. Finally, not every cell within the normal or exogenously induced domains of RA-dependent gene expression responds to RA, nor does every cell express RA receptors or binding proteins. Thus, regional and cellular differences in the distribution of the known retinoid receptors and binding proteins do not predict absolutely where or whether retinoid sensitive genes will be expressed or where retinoids will be available in the developing spinal cord. Instead, retinoid-mediated gene expression in the cervical and lumbar cord seems to reflect retinoid responses that rely both on the local availability of retinoids, the identity of the responding gene, and an indeterminate array of retinoid signaling molecules.

Neuronal domains in developing neocortex: mechanisms of coactivation.

The mammalian neocortex consists of columnar circuits, whose development may be controlled by patterns of spontaneous activity. Columnar domains of spontaneously coactive neurons were previously described using Ca2+ imaging of slices from developing rat neocortex. We have now investigated the cellular mechanisms responsible for the coactivation of these domains. The activation starts in the center of a domain and spreads at speeds of approximately 100 microns/s. Domains occur in the presence of tetrodotoxin but are blocked by the gap junction blockers halothane and octanol. Simultaneous intracellular and optical recordings from dye-coupled cells reveal functional coupling between developing neocortical neurons. These data support the hypothesis that a neuronal domain results from the spontaneous excitation of one or a few trigger neurons that subsequently activate, either electrically or biochemically, the rest of the cells via gap junctions.

Corticosteroid induction of threat-induced behavioral inhibition in preweanling rats.

Termination of ongoing behavior and assumption of defensive postures when threatened are adaptive characteristics of vertebrates. Altricial rat pups develop these characteristics by 14 days of age. At this time, pups inhibit their ultrasonic vocalizations and freeze when threatened. This emergence of behavioral inhibition is impaired when rats are adrenalectomized (ADX) at 10 days of age. That is, 14-day-old ADX pups exhibit deficits in freezing and continue to emit ultrasounds when confronted by an adult male rat. Studies also showed that removal of adrenal hormones does not potentiate vocalizations or render pups incapable of reducing their ultrasounds. More important, 3.0 mg/kg of corticosterone (CORT), but not lower doses, administered daily to ADX pups restored freezing, with lesser effects on ultrasound inhibition. Disrupting the developmental action of endogenous CORT appears to impair the ontogenetic expression of behavioral inhibition.

Antagonism of corticotropin-releasing factor receptors in the locus coeruleus attenuates shock-induced freezing in rats.

Intracerebroventricularly administered alpha-helical CRF9-41, a corticotropin-releasing factor (CRF) receptor antagonist, is known to reduce a variety of stress-induced behavioral responses. This study examined in rats whether antagonism of CRF receptors in the region of locus coeruleus (LC) plays a role in reducing freezing induced by electric foot shock. Freezing is a well-characterized defensive response to stress and has been demonstrated to index an animal's degree of fear. A CRF-receptor antagonist, alpha-helical CRF9-41, bilaterally infused into the LC significantly reduced the duration of freezing at a dose as low as 0.20 micrograms. Additional experiments confirmed that 0.20 micrograms of alpha-helical CRF9-41 significantly reduced the duration of freezing only when cannulae were within the LC or in regions bordering the nucleus. Antagonist-treated rats with cannulae that did not impinge on the LC exhibited freezing at levels not different from vehicle-treated animals. These results strongly implicate CRF receptors located in the LC region in influencing the display of stress-induced behavior.