Neurotransmitter synthesis rates are regulated and modified by a variety of physiological and environmental factors. One of the best examples of this kind of biochemical neuroplasticity is the compensatory increase in the rate of catecholamine synthesis which occurs in the central and peripheral nervous system and in the adrenal medulla in response to conditions that cause catecholamines to be released. This enhanced rate of synthesis is accomplished primarily through modulation of the first and rate-limiting step in the formation of catecholamines, the hydroxylation of tyrosine to dihydroxyphenylalanine, catalyzed by the enzyme tyrosine hydroxylase. Acute, second-to-second regulation involves a rapid, calcium-dependent increase in tyrosine hydroxylase activity due to phosphorylation of pre-existing enzyme. Chronic, longer-term regulation involves a calcium-dependent increase in tyrosine hydroxylase gene expression that elevates the level of the enzyme. Research in this laboratory focuses on identification of the mechanisms responsible for these changes in catecholamine synthesis rate, with an emphasis on the mechanisms involved in modulating tyrosine hydroxylase expression. Primary cultures of adrenal chromaffin cells, which are catecholamine secretory cells of the adrenal medulla that are also equivalent to postganglionic sympathetic neurons, are currently being used as a model cell system for our studies. Release of catecholamines and ensuing changes in tyrosine hydroxylase gene expression and catecholamine synthesis rates are being examined in chromaffin cells in response to acetylcholine, a physiological stimulus, and in chromaffin cells exposed to an electromagnetic field, an environmental stimulus. These studies employ a variety of methods that include intracellular calcium imaging, biochemical and molecular biology techniques.
Chromaffin cell. Schematic illustration showing the binding of acetylcholine (Ach) to nicotinic cholinergic receptors {1} and the attendant influx of Ca2+ {2}. The resulting increase in intracellular Ca2+ level stimulates cellular processes that cause catecholamine (epinephrine) release {3} and an increase in tyrosine hydroxylase gene expression {4}. We are examining how intracellular Ca2+ level and catecholamine biosynthetic and secretory processes are affected by electromagnetic fields.
Selected Publications
G.L. Craviso, V.B. Hemelt and J.C. Waymire. (1992). Nicotinic cholinergic regulation of tyrosine hydroxylase gene expression and catecholamine synthesis in isolated bovine adrenal chromaffin cells. J. Neurochem. 59:2285-2296.
Craviso, G.L., V.B. Hemelt, J.C. Waymire, J. Larsen, S.T. Mihaylova-Todorova, D.P. Westfall and R.A. Bjur. (1993). Relationship between the nicotinic cholinergic mediated induction of tyrosine hydroxylase and release of catecholamines in bovine adrenal chromaffin cells. Proc. West. Pharmacol. Soc. 36:1-5.
Waymire, J.C. and G.L. Craviso. (1993). Multiple site phosphorylation and activation of tyrosine hydroxylase. Adv. Prot. Phosphatases 7:501-513.
Craviso, G.L., V.B. Hemelt and J.C. Waymire. (1995). The transient nicotinic stimulation of tyrosine hydroxylase gene transcription in bovine adrenal chromaffin cells is independent of c-fos gene activation. Mol. Brain Res. 29:233-244.
Craviso, G.L., V.B. Hemelt, J.C. Waymire, S.T. Mihaylova-Todorova, E. Fernandez and R.M. Moore. (1996). Posttranscriptional control of tyrosine hydroxylase induction during continuous nicotinic receptor activation of bovine adrenal chromaffin cells. In Stress: Molecular, Genetic and Neurobiological Advances, R. McCarty, G. Aguilera, E. Sabban and R. Kvetnansky, eds., Gordon and Breach Science Publishers. In press.