br Rapid nongenomic effects of

Rapid nongenomic effects of corticosterone in an amphibian model Studies in the model organism, the roughskin newt, Taricha granulosa, over 30 years ago led to the discovery that stress can rapidly (within minutes) suppress sexual behavior (amplectic clasping) of male T. granulosa, an effect that is mimicked by corticosterone injection (Boyd and Moore, 1990; Coddington and Moore, 2003; Coddington and Moore, 2001; Moore and Evans, 1999; Moore and Miller, 1984; Moore and Orchinik, 1994; Moore et al., 1995). Subsequent studies using this model system led to identification of a corticosteroid receptor in neuronal membranes. Importantly, steroids, including dexamethasone, that did not effectively compete with corticosterone for binding to the neuronal membrane receptor also failed to inhibit clasping behavior, while steroids that did compete (cortisol, aldosterone) did inhibit clasping behavior (Orchinik et al., 1991). These studies provided the first evidence for a membrane corticosteroid receptor involved in the rapid regulation of behavior.
Co-regulation of DA and 5-HT in the DMH; a clue to novel regulatory mechanisms? The rapid effects of corticosterone on behavior in T. granulosa led us to explore rapid effects (measured within 20 min) of corticosterone on catecholamines in microdissected till regions. These studies demonstrated that corticosterone increased both serotonin (5-HT) and dopamine (DA) concentrations within the dorsomedial hypothalamus (DMH) (Lowry et al., 2001). This effect was restricted to the DMH, a region that contains monoamine-accumulating cells, collectively referred to as the paraventricular organ, a common feature of the hypothalamus of all nonmammalian vertebrates (Lowry et al., 1996). A remarkable observation in studies of the effects of corticosterone on monoamines of the DMH of T. granulosa was that the tissue concentrations of DA in the DMH were positively correlated with the tissue concentrations of 5-HT within the DMH under a variety of experimental conditions (Lowry et al., 2001), including following injection of corticosterone. Similar increases in 5-HT, DA, and norepinephrine were observed in the DMH of rats exposed to a 30-min period of restraint stress (Lowry et al., 2003). Again, in this model, DA concentrations in the DMH were highly correlated with 5-HT concentrations in the DMH. It was recognized at the time that a clear precedent existed for a physiologically relevant co-regulation of DA and 5-HT. Synthesis of DA and 5-HT can occur within the same cellular compartment, e.g., within the epithelial cells of the renal cortical proximal tubules (Vieira-Coelho and Soares-Da-Silva, 1997; Vieira-Coelho and Soares-Da-Silva, 1998). This co-regulation of DA and 5-HT in the kidney was thought to be dependent on uptake of the precursors, l-3,4-dihydroxyphenylalanine (l-DOPA) and l-5-hydroxytryptophan (l-5-HTP), and possibly DA and 5-HT via the same transporter complex (Soares-Da-Silva and Pinto, 1996). In renal proximal tubule cells, DA and 5-HT excretion is mediated by specific transport proteins. Organic cation transporter type 1 (OCT1) is believed to mediate the uptake of organic cations including l-DOPA and l-5-HTP, as well as DA and 5-HT from the perivascular spaces around the basolateral cell surface (Breidert et al., 1998; Martel et al., 2000; Martel et al., 1999; Soares-Da-Silva and Pinto, 1996; Zhang et al., 1998). The organic cation transporter type 2 (OCT2) is believed to mediate the secretion of DA and 5-HT from the apical plasma membrane of renal proximal tubule cells (Grundemann et al., 1998a). Expression of OCT2 mRNA is found not only in the kidney, but also in DA-rich regions of the central nervous system (Grundemann et al., 1997). Interestingly, the transport of 3H-DA and 3H-5-HT by OCT2 in a stable transfection system is inhibited by corticosterone, with a Ki value of approximately 500 nM, a concentration that is well within the physiological range (Grundemann et al., 1998a). Additional corticosterone-sensitive members of the organic cation transporter family are expressed in brain and have been cloned (e.g., OCT3 (Wu et al., 1998); also called extraneuronal monoamine transporter, EMT (Grundemann et al., 1998b), as described above). Other members of this novel, corticosterone-sensitive family of transporters, belonging to the amphiphilic solute facilitator (ASF) family, exist (e.g., (Martel et al., 2000)), but have not been cloned. Likewise, additional members of this family have been cloned, but have not been functionally characterized (Schomig et al., 1998). Based on the observation that several members of this family of transporters are corticosterone-sensitive and are expressed in brain, it is possible that the rapid effect of corticosterone to induce site-specific accumulation of DA and 5-HT within the DMH is due to inhibition of DA and 5-HT release from the apical extension of DA- and 5-HT-accumulating cells associated with the paraventricular organ (cf. Discussion by (Grundemann et al., 1998a, Grundemann et al., 1998b)). Thus, paraneurons (a term originated by Fujita in 1975; i.e., groups of cells that have not been classified as neurons and yet share certain morphological and functional features with neurons) (Fujita, 1977; Fujita and Kobayashi, 1979) in the DMH may be functionally analogous to the “dopamine-handling” cells within the proximal tubules of the kidney, possibly clearing l-DOPA, catecholamines, 5-HTP, 5-HT and related cations from perivascular spaces within the central nervous system and ultimately excreting them into the lumen of the brain for clearance via the cerebrospinal fluid. If this is the case, the DMH may prove to be an excellent model system for studies of rapid effects of stress and glucocorticoid hormones on brain catecholaminergic and serotonergic function.