Neurotransmitters

Introduction

  • Neurotransmitters
    • More than 100 different ones in animals
    • Categorized by size or structure
  • Excitatory and inhibitory neurotransmitters
    • Like brakes and accelerators on a car
  • All nervous systems operate with combined excitatory and inhibitory neurotransmitters

Five Classes of Neurotransmitters

  1. Acetylcholine
    • One of most widespread neurotransmitters
    • Released at neuromuscular junctions
    • Excitatory in brain and skeletal muscles but inhibitory in cardiac muscles
  2. Biogenic amines
    • Widespread physiological effects and psychoactive
    • Abnormally high or low levels associated with a variety of mental illnesses (schizophrenia, depression)
  3. Amino Acids
    • Glutamate most widespread excitatory neurotransmitter
    • GABA (gamma aminobutyric acid) most common inhibitory neurotransmitter
  4. Neuropeptides
    • Often called neuromodulators – can alter response of postsynaptic neuron to other neurotransmitters
    • Example: Opiate peptides
  5. Gaseous Neurotransmitters
    • Nitric oxide, NO
    • Carbon monoxide, CO
    • Not sequestered into vesicles
    • Produced locally as required
    • Short-acting – influence other cells by diffusion
    • Drugs for male sexual dysfunction enhance erections by increasing or mimicking action of NO on smooth muscle
    • Function of CO uncertain

Otto Loewi Discovered Acetylcholine

  • He was interested in how nerves communicate with muscles
  • A certain nerve attached to the heart increased contraction rate while another nerved decreased it
  • Placed two frog hearts in separate, connected chambers
  • Stimulated vagus nerve on first heart and rate slowed
  • Transferred sample of saline solution from first to second heart - rate also slowed
  • Acetylcholine was the first neurotransmitter discovered

Postsynaptic Receptors

  • In some cases, same neurotransmitter can have excitatory or inhibitory effects
  • Response of postsynaptic cell depends on receptor type
  • Ionotropic receptors: ligand-gated ion channels open in response to neurotransmitter
  • Metabotropic receptors: G-protein coupled receptors (GPCRs) initiate changes in postsynaptic cell