Synaptic Transmitters | Rapidly Acting and Slowly Acting

There are more than 50 chemical substances have been proved or postulated to function as synaptic transmitters. Many of them are listed in below, which give two groups of synaptic transmitters. One group comprises small-molecule, rapidly acting transmitters. The other is made up of a large number of neuropeptides of much larger molecular size that are usually much more slowly acting.

Small-Molecule, Rapidly Acting Transmitters

The small-molecule, rapidly acting transmitters are the ones that cause most acute responses of the nervous system, such as transmission of sensory signals to the brain and of motor signals back to the muscles.

Class I

  • Acetylcholine

Class II: The Amines

  • Norepinephrine
  • Epinephrine
  • Dopamine
  • Serotonin
  • Histamine

Class III: Amino Acids

  • Gamma-aminobutyric acid (GABA)
  • Glycine
  • Glutamate
  • Aspartate

Class IV

  • Nitric oxide (NO)

Neuropeptide, Slowly Acting Transmitters or Growth Factors

The neuropeptides, usually cause more prolonged actions, such as long-term changes in numbers of neuronal receptors, long-term opening or closure of certain ion channels, and possibly even long-term changes in numbers of synapses or sizes of synapses.

Hypothalamic-releasing hormones

  • Thyrotropin-releasing hormone
  • Luteinizing hormone–releasing hormone
  • Somatostatin (growth hormone inhibitory factor)

Pituitary peptides

  • Adrenocorticotropic hormone (ACTH)
  • β-Endorphin
  • α-Melanocyte-stimulating hormone
  • Prolactin
  • Luteinizing hormone
  • Thyrotropin
  • Growth hormone
  • Vasopressin
  • Oxytocin

Peptides that act on gut and brain

  • Leucine enkephalin
  • Methionine enkephalin
  • Substance P
  • Gastrin
  • Cholecystokinin
  • Vasoactive intestinal polypeptide (VIP)
  • Nerve growth factor
  • Brain-derived neurotropic factor
  • Neurotensin
  • Insulin
  • Glucagon

From other tissues

  • Angiotensin II
  • Bradykinin
  • Carnosine
  • Sleep peptides
  • Calcitonin

Synaptic transmission is mostly affected by enhancement or inhibition of neurotransmitter release, breakdown, and reabsorption, along with the blocking of neurotransmitter binding to receptors.

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