Neurohumoral Transmission in the CNS and Physiological Roles of Neurotransmitters

Neurohumoral transmission in the central nervous system (CNS) refers to the communication between neurons through chemical messengers called neurotransmitters. These substances are released from presynaptic nerve terminals in response to an action potential. Calcium influx triggers synaptic vesicle fusion and neurotransmitter release into the synaptic cleft. Neurotransmitters bind to specific postsynaptic receptors, producing excitatory or inhibitory responses. Their actions are terminated mainly through reuptake, enzymatic degradation, or diffusion.

Neurotransmitters play essential roles in regulating brain functions, including movement, memory, mood, sleep, cognition, and emotional behaviour. The major CNS neurotransmitters include GABA, glutamate, glycine, serotonin, and dopamine.

1. Gamma-Aminobutyric Acid (GABA): GABA is the principal inhibitory neurotransmitter in the brain. It reduces neuronal excitability and helps regulate anxiety, sleep, muscle tone, and seizure activity. GABA-A receptors are ligand-gated chloride channels, whereas GABA-B receptors are G-protein-coupled receptors. Drugs such as diazepam and phenobarbital enhance GABAergic activity.

2. Glutamate: Glutamate is the major excitatory neurotransmitter in the CNS. It plays a vital role in learning, memory, and synaptic plasticity. Its major receptors include NMDA, AMPA, and kainate receptors. Excessive glutamate stimulation can cause excitotoxicity and neuronal damage.

3. Glycine: Glycine is an important inhibitory neurotransmitter, particularly in the spinal cord and brainstem. It regulates motor activity and spinal reflexes. It also acts as a co-agonist at NMDA receptors. Strychnine blocks glycine receptors.

4. Serotonin (5-HT): Serotonin regulates mood, sleep, appetite, pain perception, and gastrointestinal functions. It is synthesised from tryptophan. Selective serotonin reuptake inhibitors (SSRIs), such as fluoxetine, increase serotonergic activity.

5. Dopamine: Dopamine is involved in movement, motivation, reward, cognition, and prolactin regulation. Its major pathways include nigrostriatal, mesolimbic, mesocortical, and tuberoinfundibular pathways. Dopaminergic dysfunction is associated with Parkinson's disease and schizophrenia.

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