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Although a few regulators of memory and addiction have been identified, the biochemical pathways that mediate the development of addiction and memory remain poorly understood. In addition, important questions remain as to how these two phenomena can persist for so long, sometimes for the entire life of an individual.Signaling molecules and transcription factors are activated in response to stimuli that induce long-term neuronal plastic changes. The transcription factor CREB (cAMP-responsive element binding protein) is clearly involved in triggering processes of addiction and memory, but its sustained activation following a course of chronic drug exposure (or learning) returns to baseline within days [1]. Even the enduring increased levels of deltaFosB (a Fos family transcription factor that couples with other proteins in the AP-1 family to form transcriptional activator/repressor complexes) observed in regions of the mammalian brain following chronic drug exposure, persists for only weeks or months. Thus, although CREB and deltaFosB probably initiate the very stable behavioral changes seen with addiction and memory, their alterations cannot mediate those behavioral changes per se [1]. Long-term up- or down-regulation of molecules downstream of these transcription factors, or others, must be responsible for the enduring modifications in synaptic connectivity and structure believed to be required for the maintenance of these durable behavioral states [2].Many believe that more rapid progress will be made toward understanding the molecular basis of addiction if research efforts proceed hand-in-hand with, rather than in isolation from, the overlapping neurobiological study of learning and memory [1, 2]. The importance and utility of using simple model systems such as Drosophila and Aplysia to identify and characterize genes involved in long-term synaptic plasticity, and hence memory formation, is well documented [3-5]. Identification and functional analyses of neuronal genes transcriptionally regulated by AP-1 and CREB in Drosophila would elaborate on molecular mechanisms of long-term plasticity and hence help us understand, and perhaps manipulate, processes that underlie addiction and memory.

Nitric Oxide (NO) regulates cardiovascular homeostasis by binding to soluble guanylate cyclase (sGC), leading to cGMP production, reduced cytosolic calcium concentration ([Ca²⁺]ᵢ) and vasorelaxation. Thrombospondin-1 (TSP-1), a secreted matricellular protein, was recently discovered to inhibit NO signaling and sGC activity. Inhibition of sGC requires binding to cell-surface receptor CD47. Here, I show that a TSP-1 C-terminal fragment (E3CaG1) readily inhibits sGC in Jurkat T cells, and that inhibition requires an increase in [Ca²⁺]ᵢ. Using digital imaging microscopy on live cells, I further show that E3CaG1 binding results in a substantial increase in [Ca²⁺]ᵢ, up to 300 nM. Addition of angiotensin II, a potent vasoconstrictor known to increase [Ca²⁺]ᵢ, also strongly inhibits sGC activity. sGC isolated from calcium-treated cells or from cell-free lysates supplemented with Ca²⁺ remains inhibited, while addition of kinase inhibitors staurosporine, genistein, PP1 or PP2 reverse inhibition, indicating inhibition likely involves a tyrosine kinase, more specifically, a src family kinase. Rat sGC is also inhibited by lysates supplemented with Ca²⁺, suggesting that the site of modification is at an evolutionarily conserved residue. Inhibition is through an increase in K(m) for GTP, which rises to 834 μM for the NO-stimulated protein, a 13-fold increase over the uninhibited protein. Compounds YC-1 and BAY 41-2272, allosteric stimulators of sGC that are of interest for treating hypertension, overcome E3CaG1-mediated inhibition of NO-ligated sGC. Taken together, these data suggest that sGC not only lowers [Ca²⁺]ᵢ in response to NO, inducing vasodilation, but is also inhibited by high [Ca²⁺]ᵢ, providing a fine balance between signals for vasodilation and vasoconstriction.

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