A novel cDNA clone, CR16, was isolated from a rat hippocampal cDNA library and characterized for responses to corticosteroids and regional expression. The 4-kb RNA was increased 3-fold by treatment of adrenalectomized (ADX) rats with corticosterone (CORT). Overlapping cDNA totaling 4,374 nt were used to define an open reading frame of 1,356 nt beginning 191 nt from the 5’-end and encoding a 45-kD protein containing 32% proline. CR16 has no obvious homologies to GenBank or protein databases. CR16 RNA was detected by in situ hybridization in neuron-rich layers of the hippocampal formation, layers II, III and VI of the cerebral cortex, thalamus, ventromedial nucleus of the hypothalamus, bed nucleus of the stria terminalis, lateral septal nucleus, nucleus accumbens, olfactory bulb, inferior colliculus, pons and inferior olive. The CR16 RNA has low prevalence in the hippocampus and cortex (<10 pg/µg total RNA) and is elevated 3-fold in both structures in a dose-dependent manner by CORT in ADX rats. Treatment of ADX rats with aldosterone (ALDO), CORT, or RU28362 increased CR16 RNA to similar levels in the hippocampus while ALDO had minimal effects on the level of CR16 RNA relative to CORT or RU28362 in the cortex. Neither shaking stress (2 h) nor 2 h CORT significantly elevated CR16 RNA in the hippocampus, suggesting that its response to elevated CORT is not rapid. ADX lowered CR16 RNA levels by 50% relative to intact rats while low-level CORT replacement (≧4 ng/ml serum CORT) significantly elevated CR16 RNA 2-fold in ADX rats. These results are consistent with both the mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) regulating the CR16 gene. This gene will be useful in dissecting the role of MR and GR in CNS neu