Veterinary Neuropharmacology

SYNTHESIS AND RELEASE OF NEUROTRANSMITTER

• Depending on the type of innervation the specific neurotransmitter is synthesized and stored (in  granules / vesicles ) in the nerve terminal.
• Release of the neurotransmitter substance is triggered by the arrival of axonal action potential. Refer for neurotransmitter release...
• Ca⁺⁺ acts to link or couple the excitation of the membrane with discharge of neurotransmitter from the nerve terminal.
• The sources of Ca⁺⁺ are believed to be the interstitial space and/or superficial membrane binding sites at the axonal terminal.
• The action potential initiates an inward movement of Ca⁺⁺ into the nerve terminal from these sites.
• Inward movement of Ca⁺⁺ somehow triggers exocytotic discharge of neurotransmitter from the vesicles into the junctional cleft.
• In contrast the neurotransmitter, nitric oxide is not stored in synaptic vesicles. Nitric oxide is highly lipophilic and it rapidly diffuses to effector cells. This nitric oxide formation is also activated by Ca⁺⁺ through the enzyme nitric oxide synthase.