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.