Cellular mechanisms of rapid corticosteroid action in the vertebrate brain

Cellular and organismal responses to corticosteroid hormones are complex and integrative, involving immediate, short-term, and delayed components. While the cellular mechanisms of delayed responses to corticosteroids have been well characterized, those underlying immediate and short-term responses have not. This dissertation examines cellular mechanisms underlying rapid actions of the adrenal corticosteroid corticosterone (CORT) in an amphibian and a mammalian model. In Urodele amphibians, CORT interacts with the hypothalamic neuropeptide arginine vasotocin (AVT) in the acute regulation of reproductive behavior and its neural correlates. Western blot was used to investigate the effects of CORT and the AVT receptor agonist arginine vasopressin (AVP), alone and in combination, on the subcellular localization and proteolysis of protein kinase C-alpha (PKCalpha) in tiger salamander brain tissue. AVP treatment increased PKCalpha in membrane fractions and decreased PKCalpha in cytosolic fractions. AVP of AVT treatment also induced the appearance of PKMalpha, the proteolytically generated, free catalytic subunit of PKCalpha. Treatment with CORT alone had no effect on PKCalpha or PKMalpha. However, pretreatment with CORT blocked AVP-induced increases in PKMalpha. These data suggest a cellular mechanism by which CORT and AVT interact to regulate neuronal physiology and behavior.; In the second model system, CORT rapidly regulates monoamine levels in specific brain regions. Previous studies have shown that, within 20--30 minutes of exposure to a stressor, or injection of CORT, levels of serotonin (5-hydroxytryptamine, 5-HT) increase dramatically in the rat dorsomedial hypothalamus (DMH). To investigate the cellular mechanisms underlying this effect of CORT, the expression of organic cation transporters (OCTs) was examined, and bidirectional transport of two radiolabeled OCT substrates, N-methyl-4-phenylpyridinum (MPP +) and histamine was measured in vitro. RT-PCR revealed the presence of mRNA for OCT3 in the rat DMH. Acutely prepared minces of rat DMH tissue accumulated both substrates in a time dependent manner. CORT and the OCT inhibitor decynium22 partially inhibited uptake of both substrates. CORT, estradiol, 5-HT, and decynium22 also inhibited histamine uptake. Reserpine and desipramine enhanced net efflux of MPP+. Both CORT and decynium22 completely inhibited reserpine-/desipramine-enhanced MPP + efflux. Choline and tetramethylammonium also inhibited histamine uptake, but only at very high concentrations. The pharmacological properties of the transport of MPP+ and histamine suggest that rat DMH tissue contains active OCT3. These data support the hypothesis that CORT-induced inhibition of OCT3 mediates stress-induced accumulation of 5-HT in the rat DMH.