Veterinary Neuropharmacology

ADRENOCEPTORS

• Pharmacological effects of adrenergic drugs are mediated by activation of adrenergic receptors.
• Two types of adrenergic receptors were proposed to explain the dissimilar effects of sympathomimetic agents in different tissues.
• They are α and β receptors.
• The α and β receptors are differentiated pharmacologically, initially based on the relative properties of catecholamines namely epinephrine, norepinephrine and isoproterenol.

Effects of adrenoceptors

• Released catecholamines interact with the  α-adrenergic or  β-adrenergic receptors on the postjunctional membrane to produce specific effects in the postjunctional cell through  activation of specific G-proteins.
• In some systems released catecholamines interact with a α₂ -adrenergic receptors, which are on the prejunctional nerve cell. These receptors mediate an inhibition of the release of norepinephrine thus allowing the neurotransmitter to modulate its own release. This has been termed as auto-inhibitory feedback mechanism and may play an important role in the regulation of catecholamine release.
• In general, α receptors mediate excitatory/contractile response and β receptors mediate stimulation of the heart, relaxation of smooth muscles and metabolic effects. 
• In addition to these receptors, dopamine receptors are available in periphery, which mediate vasodilatation in the kidney, heart, mesentry and increase the force of contraction of the heart. Dopamine also has a weak agonistic activity on the α adrenergic receptors.