Second messenger enhancing

Modulation of second-messenger pathways is also an attractive target upon which to base novel antidepressants. Rolipram [61413-54-5] an antidepressant in the preregistration phase, enhances the effects of noradrenaline though selective inhibition of central phosphodiesterase, an enzyme which degrades cycHc adenosiae monophosphate (cAMP). Modulation of the phosphatidyl iaositol second-messenger system coupled to, for example, 5-HT,, 5-HT,3, or 5-HT2( receptors might also lead to novel antidepressants, as well as to alternatives to lithium for treatment of mania. Novel compounds such as inhibitors of A-adenosyl-methionine or central catechol-0-methyltransferase also warrant attention. 

In addition to the mechanism involving cycHc AMP, nonsugar sweeteners, eg, saccharin and a guanidine-type sweetener, have been found to enhance the production of another second messenger, inositol 1,4,5-trisphosphate (IP3), causing the closure of potassium channels and the release of... 

Interest in the PGs has recently reverted to their precursor arachidonic acid (AA), which seems to be able to act intracellulary as a second messenger, and also extra-cellularly. In this latter mode it may play a part in LTP. It is known that AA produces a long-lasting enhancement of synaptic transmission in the hippocampus that resembles LTP and in fact activation of NMDA receptors leads to the release of AA by phospholipase A2 (see Dumuis et al. 1988) and inhibition of this enzyme prevents the induction of LTP. AA has also been shown to block the uptake of glutamate (see Williams and Bliss 1989) which would potentiate its effects on NMDA receptors. This would not only prolong LTP but also cause neurotoxicity. 

The cAMP molecule serves as the second messenger, which carries out the effects of the hormone inside the cell. The primary function of cAMP is to activate protein kinase A. This kinase then attaches phosphate groups to specific enzymatic proteins in the cytoplasm. The phosphorylation of these enzymes enhances or inhibits their activity, resulting in the enhancement or inhibition of specific cellular reactions and processes. Either way, cellular... 

The first two antidepressants, iproniazid and imipramine, were developed in the same decade. They were shown to reverse the behavioural and neurochemical effects of reserpine in laboratory rodents, by inhibiting the inactivation of these monoamine transmitters (Leonard, 1985). Iproniazid inhibits MAO (monoamine oxidase), an enzyme located in the presynaptic neuronal terminal which breaks down NA, 5-HT and dopamine into physiologically inactive metabolites. Imipramine inhibits the reuptake of NA and 5-HT from the synaptic cleft by their transporters. Therefore, both of these drugs increase the availability of NA and 5-HT for binding to postsynaptic receptors and, therefore, result in enhanced synaptic transmission. Conversely, lithium, the oldest but still most frequently used mood stabiliser (see below), decreases synaptic NA (and possibly 5-HT) activity, by stimulating their reuptake and reducing the availability of precursor chemicals required in the biosynthesis of second messengers. 

Second messengers often involved Protein kinases activated Hormone-receptor complex binds hormone response elements (HRE, of enhancer regions) inDNA... 

The second messenger, cyclic adenosine monophosphate (cAMP), is thought to mediate the bronchodila-tor effects of the adrenomimetics. Adrenomimetics enhance the production of cAMP by activating adenylyl cyclase, the enzyme that converts adenosine triphosphate (ATP) to cAMP. This process is triggered by the interaction of the adrenomimetics with 2-(tdrenoceptors on airway smooth muscle. 

Antidepressant drugs, by contrast, require long-term administration for their therapeutic effects to become evident. Thus, it is clear that the acute actions of these drugs, which are most often to enhance synaptic levels of monoamines, are not sufficient in themselves to mediate their therapeutic effects. Neither is the effect of increased monoamines on postsynaptic second messenger systems, which occurs relatively quickly as well, adequate to account for the therapeutic actions of antidepressants. The most obvious explanation, then, is that molecular adaptations in response to chronic exposure to these drugs are what underlie their therapeutic effects. Consistent with this idea, several hypotheses as to the nature of the relevant molecular adaptations have been proposed. Two models focus on adaptations of the 5-HTia receptor. One theory proposes that desensitization of presynaptic somatodendritic 5-HTia autoreceptors is responsible for the therapeutic action... 

Dixon JF, Lee CH, Los GV, et al Lithium enhances accumulation of pH] inositol radioactivity and mass of second messenger inositol 1,4,5-tiiphosphate in monkey cerebral cortex slices. J Neurochem 59 2332-2335, 1992... 

Lithium carbonate and lithium citrate are the most commonly used compounds. Lithium has effects on cation transport, on individual neurotransmitters (including 5-HT) and on intracellular second messenger systems. Which of these is key to its therapeutic efficacy is not entirely clear but, as for the antidepressant drugs, the net effects seem to be to enhance serotonin function and to stabilise the noradrenergic system. Once lithium treatment is established it is very important that it is not suddenly stopped as this may result in rebound hypomania. 

The release of Ca2+ in response to such second messengers is known to activate the phosphorylation of a range of cytosolic proteins by Ca2+-dependent protein kinases, for example in hepatocytes,417 adrenal cortex418 and other cells.419 Ca2+ inhibits cAMP-activated protein kinase in parathyroid glands.420 Phosphorylation of proteins produced in the pancreatic /8-cell in response to enhanced [Ca2+] may involve calmodulin, while the stimulus produced by glucose is potentiated by cAMP 421 A calmodulin-activated NAD kinase is present in the outer mitochondrial membrane of com.422... 

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