Regulation of two subfamilies of adenylyl cyclase by Gi-coupled receptors: a possible role during cAMP-dependent synaptic plasticity in the Hippocampus

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Regulation of two subfamilies of adenylyl cyclase by Gi-coupled receptors: a possible role during cAMP-dependent synaptic plasticity in the Hippocampus

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Title: Regulation of two subfamilies of adenylyl cyclase by Gi-coupled receptors: a possible role during cAMP-dependent synaptic plasticity in the Hippocampus
Author: Nielsen, Mark David, 1969-
Abstract: Adenylyl cyclases catalyze the synthesis of cAMP from ATP. Coupling of intracellular Ca$\sp{2+}$ to cAMP increases may be important for some forms of synaptic plasticity. The type 1 adenylyl cyclase (AC1) is a neural specific, Ca$\sp{2+}$-stimulated enzyme that couples intracellular Ca$\sp{2+}$ to cAMP increases. Here, I report that Ca$\sp{2+}$ stimulation of AC1 is inhibited by activation of G$\rm\sb{i}$-coupled somatostatin and dopamine D2L receptors. This inhibition is due primarily to G$\rm\sb{i\alpha}$ and not $\beta\gamma.$ $\beta\gamma$ released from G$\rm\sb{s}$ did inhibit AC1, indicating that the enzyme can be inhibited by $\beta\gamma$ in vivo. Type 8 adenylyl cyclase (AC8), was not inhibited by G$\rm\sb{i}$-coupled receptors. Thus AC1 and AC8 provide distinct mechanisms for interactions between the Ca$\sp{2+}$ and cAMP signal transduction systems.Next, I sought to determine if the G$\rm\sb{s}$-coupled serotonin receptors, 5HT$\sb6$ and 5HT$\sb7$ were capable of coupling to specific adenylyl cyclases. In brain, mRNA for 5HT$\sb6$ is found at high levels in the hippocampus, striatum, and nucleus accumbens while 5HT$\sb7$ is expressed most abundantly in hippocampus, neocortex, and hypothalamus. We coexpressed either the 5HT$\sb6$ or 5HT$\sb7$ receptors with specific isoforms of adenylyl cyclase in HEK 293 cells. Additionally, I tested whether stimulation of 5HT$\sb6$ or 5HT$\sb7$ was able to activate AC1 or AC8. The 5HT$\sb6$ receptor operates as a typical G$\rm\sb{s}$-coupled receptor, similar to $\beta$-adrenergic receptors, in HEK 293 cells in that it stimulated AC5 but not AC1 or AC8. We observed that 5HT$\sb7$ not only stimulated AC5, but also AC1 and AC8. Stimulation of AC1 and AC8 occurred primarily through 5HT$\sb7$-mediated increases of intracellular Ca$\sp{2+}.$In most tissues, adenylyl cyclase is stimulated by hormones which bind to G$\rm\sb{s}$-coupled receptors and is inhibited by G$\rm\sb{i}$-linked hormone receptors. Paradoxically, in lung and brain, activation of G$\rm\sb{i}$-coupled receptors potentiates G$\rm\sb{s}$-stimulated cAMP levels. Of the nine cloned mammalian adenylyl cyclases, the type 2 (AC2) and type 4 (AC4) isoforms demonstrate regulatory properties consistent with a role in G$\rm\sb{i}$-mediated potentiation of G$\rm\sb{s}$-stimulated cAMP levels. G$\rm\sb{s}$-coupled receptor stimulation of AC4, like that for AC2, was potentiated by activation of G$\rm\sb{i}$-coupled receptors in vivo.The pineal gland is the site of nighttime melatonin synthesis in a variety of organisms, including mammals. Melatonin feeds back to regulate, or entrain, the endogenous circadian clock in the suprachiasmatic nucleus of the hypothalamus, as well as affecting the functions of target tissues on a circadian basis. The retina also exhibits a circadian rhythm of melatonin synthesis, as well as expression of AC1. To probe the role of AC1 in pineal (and retinal) function, I have determined that mouse pineal Ca$\sp{2+}$/CaM-stimulated adenylyl cyclase activity undergoes a circadian oscillation in mice. On the other hand, Ca$\sp{2+}$/CaM-stimulated adenylyl cyclase activity in mouse eye cup preparations did not oscillate.I monitored the locomotor activity of wild-type and AC1 knockout mice. Both strains show a robust circadian oscillation in their locomotor activity, with activity being highest during the dark phase. First, the wild-type mice showed more activity and a shorter period of absolute inactivity than AC1 mutants during the light phase. Second, when subjected to a shift of the light/dark cycle, the wild-type nice adapted to the new rhythm, while the mutant mice remained entrained to the original light/dark cycle. (Abstract shortened by UMI.)
Description: Thesis (Ph. D.)--University of Washington, 1997
URI: http://hdl.handle.net/1773/6247

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