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