Heteroreceptors , presynaptic nerve terminal

Control of transmitter release is not limited to modulation by the transmitter itself. Nerve terminals also carry regulatory receptors that respond to many other substances. Such heteroreceptors may be activated by substances released from other nerve terminals that synapse with the nerve ending. For example, some vagal fibers in the myocardium synapse on sympathetic noradrenergic nerve terminals and inhibit norepinephrine release. Alternatively, the ligands for these receptors may diffuse to the receptors from the blood or from nearby tissues. Some of the transmitters and receptors identified to date are listed in Table 6-4. Presynaptic regulation by a variety of endogenous chemicals probably occurs in all nerve fibers. 

Compelling evidence indicates that a major function of GABAbRs is to mediate inhibition of neurotransmitter release from nerve terminals where they are localized as presynaptic auto- and heteroreceptors (see Bonanno and Raiteri 1993a Bowery et al. 2002 Raiteri 2006, for reviews). 

Both mianserin and mirtazapine are antidepressant drugs which possess central 0C2 adrenoceptor blocking properties (pA2 7.3). However, mirtazapine is much more potent at histamine Hi receptors (pA2 9.1) and at 5-HT2 and 5-HT3 receptors (pA2 8.2). Blocking of Hi receptors explains the main side effects of mirtazapine, which produces marked sedation and weight gain. Blockade of presynaptic inhibitory 0C2 autoreceptors increases the release of NA, while blockade of presynaptic 0C2 inhibitory heteroreceptors on serotonin nerve terminals (Table 2) is likely to increase the release of serotonin. 

The original observations made in the early 1970s led to the hypothesis that neurotransmitter release was regulated at the level of the nerve terminals by presynaptic auto- and heteroreceptors. These discoveries were confirmed and extended... 

When a stimulus depolarises the transmembrane potential in a spiking axon above the threshold level, an all-or-none action potential in a spiking axon is activated. The action potential propagates unattenuated to the nerve terminal where ion fluxes activate a mobilisation process leading to transmitter secretion.3 The neurotransmitter binds reversibly to receptor proteins embedded in the membrane of a neuron, which triggers a certain effect. There are two types of receptors known, presynaptic receptors or autoreceptors which are present on the neurotransmitter releasing neurons , and postsynaptic or heteroreceptors, which are present on the neurotransmitter receiving neuron . The former are supposed to perform a feed-back function, and slow down the release of neurotransmitter from these neurons when they are stimulated.4... 

Abstract Presynaptic receptors for dopamine, histamine and serotonin that are located on dopaminergic, histaminergic and sertonergic axon terminals, respectively, function as autoreceptors. Presynaptic receptors also occur as heteroreceptors on other axon terminals. Auto- and heteroreceptors mainly affect Ca2+-dependent exocytosis from the receptor-bearing nerve ending. Some additionally subserve other presynaptic functions. 

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