INSECT NEUROTRANSMISSION - NEUROTRANSMITTERS AND THEIR RECEPTORS

The roles of acetylcholine, dopamine, octopamine, tyramine, 5-hydroxyt ryptamine, histamine, glutamate, 4-aminobutanoic acid (gamma-aminobuty ric acid) and a range of peptides as insect neurotransmitters are eval uated in terms of the criteria used to identify transmitters. Of the b io genic amines considered, there is good evidence that acetylcholine, dopamine, octopamine, 5-hydroxytryptamine, and histamine should be co nsidered to be neurotransmitters, but the case for tyramine is less co nvincing at the moment. The evidence supporting neurotransmitter roles for glutamate and gamma-aminobutyric acid at specific insect synapses is overwhelming, but much work remains to be undertaken before the fu ll significance of these molecules in the insect nervous system is app reciated. Attempts to characterise biogenic amine and amino acid recep tors using pharmacological and molecular biological techniques have re vealed considerable differences between mammalian and insect receptors . The number of insect neuropeptides isolated and identified has incre ased spectacularly in recent years, bur genuine physiological or bioch emical functions can be assigned to very few of these molecules, Of th ese, only proctolin fulfills the criteria expected of a neurotransmitt er, and the recent discovery of proctolin receptor antagonists should enable the biology of this pentapeptide to be explored fully.