Neuro-Transmitters in the Central Nervous System & their Implication in Learning and Memory Processes

Abstract

This review article gives an overview of a number of central neuro-transmitters, which are essential for integrating many functions in the central nervous system (CNS), such as learning, memory, sleep cycle, body movement, hormone regulation and many others. Neurons use neuro-transmitters to communicate, and a great variety of molecules are known to fit the criteria to be classified as such. A process shared by all neuro-transmitters is their release by excocytosis, and we give an outline of the molecular events and protein complexes involved in this mechanism. Synthesis, transport, inactivation, and cellular signaling can be very diverse when different neuro-transmitters are compared, and these processes are described separately for each neuro-transmitter system. Here we focus on the most well known neuro-transmitters: acetyl-choline, catechol-amines (dopamine and nor-adrenalin), indole-amine (serotonin), glutamate, and gamma-amino-butyric acid (GABA). Glutamate is the major excitatory neuro-transmitter in the brain and its actions are counter-balanced by GABA, which is the major inhibitory substance in the CNS. A balance of neuronal transmission between these two neuro-transmitters is essential to normal brain function. Acetyl-choline, serotonin and catechol-amines have a more modulatory function in the brain, being involved in many neuronal circuits. Apart from summarizing the current knowledge about the synthesis, release and receptor signaling of these transmitters, some disease states due to alteration of their normal neuro-transmission are also described.

Low frequency of bipolar disorder, dopamine dysregulation syndrome, and punding in Brazilian patients with Parkinson's disease

“…1 Parkinson's disease (PD), a degenerative condition resulting in predominantly dopamine depletion at pars compacta from substantia nigra, may provide relevant insights into this matter. Dysfunction of the dopaminergic neurotransmission may underlie the pathophysiology of some psychiatric disorders which have their frequency increased in PD.…”

Biomarkers of the cholinergic and dopaminergic signaling pathways in Arctic beluga whales (Delphinapterus leucas): relationship to methylmercury and selenium

“…Messenger RNA (mRNA) transcription levels may provide insight into effects on organisms, prior to effects on the whole organism; however, mRNA abundance is not necessarily related to protein function (Feder and Walser 2005). Cholinergic signaling pathways have been linked to essential physiological processes including learning, memory, stress response, and modulation of sensory information; the disturbance of dopaminergic neurotransmission is related to psychiatric symptoms in humans (Reis et al 2009). The muscarinic acetylcholine receptor (mAChR) from the cholinergic signaling pathway and monoamine oxidase B (MAO-B) from the dopaminergic signaling pathway are known to be sensitive to MeHg exposure (Chakrabarti et al 1998;Coccini et al 2000;Basu et al 2005aBasu et al , 2006aBasu et al , 2006bBasu et al , 2007aBeyrouty et al 2006;Stamler et al 2006;Coccini et al 2007).…”

Assessing the neurotoxicological risk of methylmercury exposure for beluga whales (delphinapterus leucas) harvested in the Mackenzie Delta Estuary.

“…Cholinergic signaling pathways have been linked to essential physiological processes including learning, memory, stress response and modulation o f sensory information (Reis et al, 2009). The muscarinic acetylcholine receptor (mAChR) may play a critical role in physiological processes including thermoregulation, motor function and feeding (Bymaster et al, 2003;Wess, 2004).…”

“…Decreased GABAa receptor levels occur in neurons following exposure to GABA and various GABAa receptor agonists (Barnes, 1996). Inhibition of the GABAa receptor could potentially lead to an excitatory effect (Sunol et al, 2008); therefore, receptors and enzymes that mediate GABAergic signaling are tightly regulated to protect neurons from excitotoxicity (Reis et al, 2009). Regulation o f GABAa receptors may include desensitization o f the receptor, endocytosis and degradation o f subunit polypeptides and the repression o f subunit gene expression (Barnes, 1996).…”

“…In wildlife studies, MeHg exposure was associated with significant reductions in y-aminobutyric acid type A receptor (GABAa-R) binding in lab-exposed mink (Mustela vison) and decreased N-methyl-D-aspartate receptor (NMDA-R) binding in several brain regions of MeHg-exposed bald eagles (Haliaeetus leucocephalus) (Rutkiewicz et al, 2011;Scheuhammer et al, 2008), mink (Basu et al, 2007c), and polar bears ( Ursus maritimus) (Basu et al, 2009). It is believed that such MeHg-associated neurochemical changes may have consequences to the whole organism as these receptors are components o f the main inhibitory and excitiatory pathways in the central nervous system, and they play important roles in animal behaviour, memory and motor function (Popescu, 2005;Reis et al, 2009;Vicini and Ortinski, 2004).…”