>>7620090Norepinephrine interacts with postsynaptic α- and β-adrenergic receptor subtypes and presynaptic α2-autoreceptors. The α2-receptors include presynaptic autoreceptors which limit the neurophysiological activity of noradrenergic neurons in the central nervous system. Formation of norepinephrine is reduced by autoreceptors through the ratelimiting enzyme tyrosine hydroxylase, an effect mediated by decreased cyclic AMP-mediated phosphorylation-activation of the enzyme.[19] α2-receptors also cause decreased intracellular cyclic AMP expression which results in smooth muscle relaxation or decreased secretion.[20] TCAs activate a negative-feedback mechanism through their effects on presynaptic receptors. One probable explanation for the effects on decreased neurotransmitter release is that, as the receptors activate, inhibition of neurotransmitter release occurs (including suppression of voltage-gated Ca2+ currents and activation of G protein-coupled receptor-operated K+ currents). Repeated exposure of agents with this type of mechanism leads to inhibition of neurotransmitter release, but repeated administration of TCAs finally leads to decreased responses by α2-receptors. The desensitization of these responses may be due to increased exposure to endogenous norepinephrine or from the prolonged occupation of the norepinephrine transport mechanisms (via an allosteric effect). The adaptation allows the presynaptic synthesis and secretion of norepinephrine to return to, or even exceed, normal levels of norepinephrine in the synaptic clefts. Overall, inhibition of norepinephrine reuptake induced by TCAs, leads to decreased rates of neuron firing (mediated through α2-autoreceptors), metabolic activity, and release of neurotransmitters.[19]
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