Presynaptic muscarinic receptor activation enhances striatal dopamine release evoked by depolarization but not that induced by non-depolarizing stimuli

Dopamine release mediated by the dopamine transporter, facts and consequences

Spontaneous and/or stimulated neural activity of the nigrostriatal dopamine (DA) pathway makes amines run out from the neurons. This DA dynamic follows a rather complex path, running in or out the terminals, and flushing or diffusing into the extracellular space. The location of this leakage is not limited to the axon terminals; it also occurs from the cell bodies and dendrites. This molecular release mechanism was, for a long time, considered as being produced, in part, by the exocytosis of previously stored vesicles. The DA carrier protein (DAT, DA transporter) embedded in the DA cell membrane is known to clear previously released amines through an inward DA influx. The DAT also appears to be an active vector of amine release. Particular local conditions and the presence of numerous psychostimulant substances are able to trigger an outward efflux of DA through the DAT. This process, delivering slowly large amounts of amine could play a major regulatory role in extracellular DA homeostasis.

Acute beta-amyloid administration disrupts the cholinergic control of dopamine release in the nucleus accumbens

The clinical presentation of Alzheimer's disease is characterized by memory deficits but it also involves the impairment of several cognitive functions. Some of these cognitive and executive functions are mediated by limbic areas and are regulated by dopaminergic neurotransmission. Furthermore, literature data suggest that beta-amyloid (Abeta) can influence synaptic activity in absence of neurotoxicity and in particular can impair cholinergic modulation of other neurotransmitter actions. In the present study, we evaluated whether small concentrations of Abeta could disrupt cholinergic control of dopamine (DA) release in nucleus accumbens using in vivo (brain dialysis) and in vitro (isolated synaptosomes) models. The cholinergic agonist carbachol (CCh) greatly enhanced DA release from dopaminergic nerve endings in nucleus accumbens both in vivo and in vitro. This effect was mainly exerted on muscarinic receptors because it was inhibited by the muscarinic antagonist atropine and it was unaffected by the nicotinic antagonist mecamylamine. Also the nicotinic agonists epibatidine and nicotine evoked a dopaminergic outflow in nucleus accumbens, which, however, was lower. Abeta 1-40 in absence of neurotoxicity fully inhibited the DA release evoked by CCh and only marginally affected the DA release evoked by epibatidine. The PKC inhibitor GF109203X mimicked the effect of Abeta on DA release and, in turn, Abeta impaired PKC activation by CCh. We can suggest that, in nucleus accumbens, Abeta disrupted in vivo and in vitro cholinergic control of DA release by acting on muscarinic transduction machinery.

Nitric oxide-evoked [3H] gamma-aminobutyric acid release is mediated by two distinct release mechanisms

Mechanisms underlying the release of [3H] gamma-aminobutyric acid (GABA) evoked by nitric oxide (NO) were investigated by use of primary cultured neurons prepared from the mouse cerebral cortex. NO generators such as sodium nitroprusside (SNP) and S-nitroso-N-a etylpenicillamine (SNAP) increased both [3H]GABA release from the neurons and [45Ca2+] influx into the neurons in a dose-dependent manner, which was significantly diminished by hemoglobin. The removal of Ca2+ significantly reduced the NO-induced [3H]GABA release by about 50%. Nipecotic acid and 1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1, 2, 5, 6-tetrahydro-3- pyridinecarboxylic acid (NO-711), GABA uptake inhibitors dose-dependently inhibited the NO-evoked [3H]GABA release in either the presence or absence of Ca2+. The concentration of these GABA uptake inhibitors to suppress the NO-induced release of [3H]GABA was sufficiently lower than that to exhibit the inhibition of [3H]GABA transport into the neurons. In addition, the NO-evoked [3H]GABA release was reduced by approximately 50% when total Na+ in incubation buffer was replaced with equimolar choline, and was also completely abolished by the removal of both Ca2+ and Na+. These results indicate that the release of [3H]GABA evoked by NO is mediated by two release mechanisms, a Ca2+ -dependent release system and the reverse process of the Ca2+ -independent and Na+ -dependent carrier-mediated GABA uptake system.

Carrier-mediated release of neurotransmitters

There is growing evidence that neurotransmitters can be released not only by exocytosis but also through the membrane carriers responsible for transmitter reuptake. Giulio Levi and Maurizio Raiteri review the in vitro and in vivo evidence supporting the existence of a carrier-mediated release for different classes of transmitters. While the physiological significance of carrier-mediated release remains speculative, widely used drugs such as sympathomimetic amines, the anorectic drug fenfluramine and some drugs of abuse act in part by stimulating monoamine carrier-mediated release. Moreover, antidepressants known to inhibit monoamine reuptake, can block carrier-mediated release. This mechanism may also come into play in pathological conditions such as ischaemia.

Presynaptic effects of glucocorticoids on dopaminergic and cholinergic synaptosomes. Implications for rapid endocrine-neural interactions in stress

Synaptosomal preparations from rat hippocampus were incubated with methylprednisolone or adrenocorticotropin. High affinity choline uptake was not affected by either hormones. Methylprednisolone however enhanced newly synthesized acetylcholine release in the presence of high potassium or acetylcholine concentrations, while adrenocorticotropin had no effect. Dopamine uptake was inhibited when synaptosomes from septum or striatum were incubated with methylprednisolone. We conclude: a) high glucocorticoid concentrations and not adrenocorticotropin can directly enhance acetylcholine release but only from stimulated cholinergic synaptosomes, and b) high glucocorticoids can reduce dopamine uptake by dopaminergic synaptosomes. The results imply that increased glucocorticoid levels during stress or disease, can directly modulate the neuronal activity of specific cholinergic and dopaminergic systems in the brain.

Is the muscarinic receptor that mediates potentiation of dopamine release negatively coupled to the cyclic GMP system?

Dopamine (DA) terminals in rat corpus striatum and frontal cortex possess muscarinic receptors that mediate enhancement of the depolarization-evoked release of the catecholamine. The effects of the membrane-permeating cyclic guanosine monophosphate (cyclic GMP) analog 8-Br-cyclic GMP and of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) on the muscarinic-induced increase of DA release were investigated in striatal synaptosomes prelabeled with [3H]DA and exposed in superfusion to 15 mM KCl and to acetylcholine (ACh). Preincubation of synaptosomes with 8-Br-cyclic GMP (10-200 microM) or with IBMX (200 microM) prevented the ACh-induced enhancement of [3H]DA release, without affecting the K+-evoked release of the [3H]amine. No significant decrease of the ACh effect was observed when 8-Br-cyclic GMP or IBMX were added concomitantly with ACh to the superfusion medium. The data suggest that stimulation of presynaptic muscarinic receptors on DA terminals may produce enhancement of 3H DA release through a decrease of the intraterminal cyclic GMP content.