There is a change in the expression of sodium–potassium chloride transporters and potassium chloride cotransporters in later stages of human development that modify GABA action from excitatory to inhibitory. Because the inhibitory action of GABA is well pronounced at later stages of development, it is widely considered in psychopharmacology that low levels of GABA mediate the excitability of neurons. On the other hand, the inhibitory actions of GABA are mediated by two specific receptors: GABAA (ionotropic) and GABAB (metabotropic). Compared to glutamate neurons, there are fewer GABA neurons in the central nervous system (Arbring, 2020). However, they play a primary role in maintaining the balance between the inhibitory and excitatory transmissions required for optimal brain functioning. To a large extent, this explains why GABA neurons and their corresponding regulatory mechanisms are implicated in different behavioral disorders, neuronal malfunctions, and sleep disorders. Based on information from different research submissions in modern psychopharmacology, alterations in GABA homeostasis have been linked to the development of different neurological impairments, including epilepsy (Müller et al., 2020), autism spectrum disorders (Kolodny et al., 2020); neurobehavioral disorders, including schizophrenia; and neurodegenerative disorders such as multiple sclerosis, Parkinson’s disease, and Huntington’s disease. Glycine, another amino neurotransmitter, performs the major inhibitory neuronal functions in the spinal cord. As a biological co-agonist with glutamate and NMDA receptors, Glycine exerts neurotransmitter effects on the brainstem and the medulla. It appears the functionalities of glycine in neurobehavioral science mimic those of GABA. In the early stages of human development, glycine acts as an excitatory neurotransmitter, with primary signal control in neuronal differentiation, proliferation, and communication. At the later stages of human development, glycine’s neuronal influences changes, with its core functionalities changing to inhibitory action in voluntary motor control, respiration, and auditory and sensory processing (Shimizu- Okabe et al., 2022). D-serine, another neurotransmitter in this class, performs primary roles in neurological development with a direct influence on behavior. It is produced from L-serine in a conversion process catalyzed by serine racemase and subsequently released by the glial cells. Vesicles housing D-serine are particularly localized in the protoplasmic astrocytes on the grey matter sheathing the synapses. The neurotransmitter actions of D-serine are pronounced in the rostral cerebral cortex, hippocampus, anterior olfactory nuclei, olfactory tubercule, corpus striatum, and amygdala (Wang, Serratrice et al., 2021). Amines Amine has long been studied in human psychology for possible psychoactive functions. In modern psychopharmacology, the research on this class of compounds is focused on their neurotransmitter actions and on their primary action in regulating the mechanisms controlling behavioral manifestations. As a representative class of neurotransmitters, amine actions seem to be dominant in emotional responses, motivations, behavior, and motor functions. This class of neurotransmitters is generally produced from presynaptic neurons before they are transmitted into the postsynaptic neurons for neuronal transmission in the synapse. The excess quantity transferred into the synapse is degraded by monoamine oxidase (EC 1.4.3.4.) or catechol- O-methyltransferase in a feedback mechanism that regulates amine production and degradation. Excess amines can also undergo reuptake into the presynaptic terminals by monoamine transporters. The events that disrupt the synthesis, release, and reuptake of amine neurotransmitters have been linked with different neurological pathologies, including schizophrenia, Parkinson’s disease, and Huntington’s disease (Ju & Tam, 2022). One of the most important amine neurotransmitters in psychopharmacology is 4-(2-aminoethyl)-1,2-benzenediol, which is also known as dopamine. Its clinical relevance is
found in its role in regulating and controlling CNS functions in humans. These actions are especially dominant in motivations, behavior, and motor activities (Swamy et al., 2020). Dopamine is produced by the dopaminergic neurons that are abundantly expressed in the substantia nigra pars compacta and the ventral tegmental area. These amine neurotransmitters also function in maintaining homeostasis and double as precursor molecules for other catecholamines, including epinephrine and norepinephrine. Since dopamine show considerable involvement in many neurobiological processes, it is logical that alterations in dopamine synthesis, release, and conversion have been linked with many psychiatric conditions, including addiction and schizophrenia (Franco et al., 2021). Another amine neurotransmitter usually studied with dopamine is serotonin (5-hydroxytryptamine). In human medicine, serotonin is significant for its primary functions in respiration, peristalsis, modulating the sleep–wake cycle, aggressive behavior, and the regulation of gastrointestinal secretions. In psychopharmacology, the functions of serotonin in aggressive behavior, feeding, and mood regulation are the most popular serotonin research foci (Dicks, 2022). About 95% of the body’s total serotonin secretion occurs from the enterochromaffin cells of the gut with inputs from the tryptophan hydroxylase enzyme. However, there are reports of other serotonin production sites in the adult human body. Studies have reported serotonin production by the rostral and caudal group of neurons in the raphe nuclei, the cerebral cortex, the thalamus, the hypothalamus, and the basal ganglia. There are also reports of this occurring in the spinal cord and brainstem. The direct influence of serotonin on other neurotransmitters also commands huge research interests in psychopharmacology. Research in this regard has explained the inhibitory actions of serotonin on dopamine release, the modulatory action of serotonin on glutamate and GABA transmission, and its inhibitory actions on glutamate release in the frontal cortex. Serotonin also appears to enhance glutamate transmission in the prefrontal cortex (Murley et al., 2020). Epinephrine and norepinephrine are other amine neurotransmitters with important physiological influences on behavior in humans. These neurotransmitters are primarily involved in the fight or flight response of the autonomic nervous system. Epinephrine neurons populate different regions of the brain, including the medulla and the lateral tegmental system. Its impact on the sympathetic and parasympathetic nervous systems is mediated through increased heart rate, pupil dilation, increased blood sugar levels, and blood vessel dilation. Beyond these functions, the roles of norepinephrine as an amine neurotransmitter are still under investigation. Epinephrine, on the other hand, has been reported to be important in sensory signal detection, regulation of emotions, and learning. In behavioral science, this neurotransmitter is recognized as exerting considerable influence on attention, alertness, memory, and arousal. Histamine, another amine molecule, acts as a signaling molecule and a neurotransmitter in the central nervous system. It is produced by a group of histaminergic neurons in the tuberomammillary nucleus of the hypothalamus. Histaminergic neurons in the amygdala, cerebral cortex, substantia nigra, striatum, thalamus, and spinal cord also produce histamine. In psychopharmacology, alterations in histamine functioning have been linked to the development of neurological disorders, including schizophrenia. Other Canonical Neurotransmitter Molecules In addition to the amines and amino acids, there are other neurotransmitter molecules of various importance in neurology. ACh, one such molecule, is widely regarded as the first substance to be characterized and identified as a neurotransmitter in the peripheral nervous system. It is released by the post-ganglion neurons in the parasympathetic system and plays primary roles in the control of muscle contraction. In the central nervous system, ACh exerts a principal function
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