Chapter 1: Chemical Neurotransmission

Chemical Neurotransmission presents chemical neurotransmission as the fundamental mechanism underlying psychopharmacological action, distinguishing it from the structural organization of the nervous system. While electrical signaling occurs within individual neurons, the transfer of information between neurons relies on chemical communication through neurotransmitter release. The classic neurotransmission sequence begins with excitation-secretion coupling, wherein arriving action potentials open voltage-gated calcium channels in the presynaptic terminal, allowing neurotransmitter vesicles to fuse with the membrane and release their contents into the synaptic cleft. Six major neurotransmitter systems form the primary targets of psychiatric medications: serotonin, norepinephrine, dopamine, acetylcholine, glutamate, and gamma-aminobutyric acid. Beyond traditional point-to-point synaptic transmission, the nervous system employs retrograde signaling, where postsynaptic neurons communicate back to presynaptic terminals using diffusible messengers such as endocannabinoids or nitric oxide, and volume transmission, in which neurotransmitters diffuse broadly throughout tissue to activate distant receptors in a spatially distributed manner. Upon binding to their receptors, neurotransmitters initiate signal transduction cascades, intracellular signaling sequences that convert brief chemical signals into prolonged cellular responses. Two principal cascade families operate through second messengers like cyclic adenosine monophosphate or intracellular calcium, which subsequently activate kinases and phosphatases—enzymes that phosphorylate or dephosphorylate target proteins, altering their activity. The most significant outcome of signal transduction is altered gene expression, achieved when transcription factors such as CREB become phosphorylated and enter the nucleus to modulate DNA transcription. Immediate early genes respond rapidly to neurotransmitter activation, and their protein products interact to regulate late genes responsible for enduring functional changes. Superimposed on this genetic regulation is epigenetic control, a system determining whether genes are actively expressed or transcriptionally silenced through chromatin modification. DNA methylation and histone deacetylation produce gene silencing, while demethylation and acetylation promote expression. The chapter concludes by highlighting post-transcriptional mechanisms including alternative splicing, which permits single genes to generate multiple protein variants, and RNA interference pathways that suppress protein production, emphasizing that neurotransmission fundamentally represents sustained, bidirectional genetic dialogue between neurons.