Major Neurotransmitters

Overview of Neurotransmitters

Neurotransmitters are chemical messengers that transmit signals across synapses in the nervous system. Their effects can be excitatory or inhibitory, depending on the receptor type and location. Each neurotransmitter may interact with multiple receptor subtypes and is classified by its chemical structure.

• Excitatory effects increase the likelihood of an action potential in the postsynaptic neuron.

• Inhibitory effects decrease the likelihood of an action potential.

• Neurotransmitters are essential for communication within the central and peripheral nervous systems.

Acetylcholine (ACh)

Acetylcholine is a major neurotransmitter found at cholinergic synapses, including the neuromuscular junction, brain, spinal cord, and autonomic nervous system (ANS). It is synthesized from choline and acetyl-CoA.

• Excitatory at neuromuscular junctions and some CNS synapses.

• Inhibitory at certain PNS synapses.

• Broken down by acetylcholinesterase (AChE) in the synaptic cleft into acetic acid and choline; choline is recycled by the neuron.

Biogenic Amines (Monoamines)

Biogenic amines are widely used in the CNS and PNS, with diverse functions including homeostasis, cognition, and mood regulation. They are implicated in psychiatric disorders and are common drug targets.

Catecholamines

• All synthesized from the amino acid tyrosine.

• Most synapses are excitatory, but some are inhibitory.

• Norepinephrine (Noradrenalin): Used by the ANS to regulate heart rate, blood pressure, digestion; in the CNS, regulates attention, feeding, and sleep/wake cycle.

• Epinephrine (Adrenalin): Similar effects to norepinephrine; more widely used as a hormone by the endocrine system.

• Dopamine: Used by the CNS; coordinates movement, emotion, and motivation. Dopamine receptors are drug targets (e.g., cocaine, amphetamine).

Other Biogenic Amines

• Serotonin: Synthesized from tryptophan in the brainstem; regulates mood, emotions, attention, cognitive functions, motor and feeding behaviors, and daily rhythms. Targeted in depression treatment.

• Histamine: Synthesized from histidine; regulates arousal and attention in the CNS. Outside the CNS, involved in allergic responses; antihistamines block histamine receptors, causing drowsiness as a side effect.

Amino Acid Neurotransmitters

• Glutamate: The most important excitatory neurotransmitter in the brain. Activates ionotropic and metabotropic receptors, leading to sodium or calcium channel opening and EPSP production.

• Glycine and Gamma (γ)-Aminobutyric Acid (GABA): Major inhibitory neurotransmitters. Induce IPSPs by opening ligand-gated chloride channels. GABA is predominant in the brain; glycine in the spinal cord.

Table: Major Neurotransmitters

Example: Synaptic Transmission at the Neuromuscular Junction

• Motor neuron releases acetylcholine into the synaptic cleft.

• ACh binds to receptors on the muscle cell membrane, causing sodium channels to open and initiating muscle contraction.

• ACh is broken down by acetylcholinesterase, terminating the signal.

Additional info:

• Neurotransmitter dysfunction is linked to various neurological and psychiatric disorders, such as depression (serotonin deficiency), schizophrenia (dopamine excess), and anxiety (GABA, norepinephrine, serotonin imbalance).

• Pharmacological agents often target neurotransmitter synthesis, release, receptor binding, or breakdown to treat these conditions.