Pharmacological characterization of dopamine receptors in the corpus allatum of Manduca sexta larvae

Dopamine receptors previously identified in corpora allata (CA) of Manduca sexta last instars on the basis of dopamine effects on JH (juvenile hormone )/JH acid biosynthesis and cyclic AMP (cAMP) accumulation, were characteriz ed pharmacologically. For this study, a broad spectrum of agonists or antag onists of D1, D2, D3 or D4 dopamine receptors, together with the dopamine m etabolite N-acetyl-dopamine, other neurotransmitters and their agonists/ant agonists, were tested for their effects on gland activity and cAMP producti on. The lack of effect of other neurotransmitters supports the specificity of the effect of dopamine and the dopamine specificity of the receptors. On ly the D2 receptor antagonist spiperone had a potent effect on JH biosynthe sis and cAMP formation by CA taken on day 0 of the last stadium, when dopam ine stimulates both activities and thus appears to be acting via a D1-like receptor. Several other D2 receptor antagonists, and D1, D2/D1 and D4,3/D2 receptor antagonists were less effective. Thus, the D1-like receptor of the Manduca CA appears to be distinct pharmacologically from vertebrate D1 rec eptors. By contrast, a number of D2 agonists/antagonists had a significant effect on JH acid biosynthesis and cAMP production by the CA from day 6 of the last stadium, when dopamine inhibits both activities and thus appears t o be acting via a D2-like receptor. Certain D1-specific agonists/antagonist s were equally effective. The Manduca D2-like receptor therefore bears some pharmacological resemblance to vertebrate D2 receptors. N-acetyl dopamine acted as a dopamine agonist with day 6 CA, the first identified function fo r an N-acetylated biogenic amine in insects. Dopamine was found to have the same differential affect on the formation of cAMP in homogenates of day 0 and day 6 brains as it did with CA, and in the same concentration range. Do pamine receptor agonists/antagonists affecting cAMP formation by day 0 and day 6 CA homogenates had similar effects with brain homogenates. By contras t, dopamine only stimulated cAMP formation by homogenates of day 0 and day 6 abdominal or ventral nerve cord. These results suggest that D1- and D2-li ke dopamine receptors of Manduca are regionally as well as temporally local ized. (C) 2000 Elsevier Science Ltd. All rights reserved.