Apart from the recently cloned metabotropic glutamate receptor (DmGluRA) from Drosophila melanogaster head cDNA library, the characterisation of the native neuronal mGluRs in insect is still in its infancy. Whole cell patch-clamp studies performed on the cockroach Periplaneta americana cultured adult dorsal unpaired median (DUM) neurones revealed that bath application of glutamate (10 µM) decreased the action potential amplitude. Under voltage-clamp conditions, both glutamate and the group III-selective mGluR agonist L-AP4 also reduced in a dose-dependent manner the voltage-dependent inward sodium current (I_Na) amplitude (EC₅₀ = 16.7 nM and 760 nM, respectively). Because glutamate-mediated I_Na reduction was blocked by the group III mGluR antagonist MSOP (IC₅₀ = 45 µM) and not by group I (AIDA) and group II (MCCG) mGluR antagonists, we concluded that a mGluR sharing pharmacological properties with the vertebrate mGluRIII was involved. Consistent with this, neither t-ADA nor LY354740, group I and group II-selective mGluR agonists respectively, induced any significant effect on I_Na. It is worth noting that the maximal inhibition of I_Na induced by glutamate and L-AP4 only reached 48.1 ± 3.9 % (mean ± S.E.M., n = 14) and 33.5 ± 4.1 % (n = 6), respectively. These results together with the deactivation tail current analysis showed that two sub-populations of voltage-gated sodium channels (Na1, Na2) were present in DUM neurones and only one (Na1) was sensitive to glutamate. We have also characterized the intracellular messenger pathway coupling this mGluRIII to Na1. The pertussis toxin A-protomer (10 nM), applied alone, decreased I_Na1 by 32 ± 4.5 % (n = 5). In this condition, we never observed any effect of L-AP4. Increasing intracellular cAMP level, by perfusing the cell body with 0.3 mM cAMP through the patch pipette, also reduced I_Na1 (28.44 ± 4 %, n = 19) like L-AP4. In this case, glutamate still produced an additional effect on I_Na1 as observed in control condition. Finally, because both glutamate and L-AP4 effects were completely suppressed in the presence of the PKA inhibitor H89 (0.3 mM), it was strongly suggested that glutamate regulated I_Na1 via a cAMP/PKA cascade. This was confirmed by immunocytochemistry using cAMP antibodies. cAMP immunoreactivity exhibited by DUM neurone cell body was observed before and after stimulating mGluRIII by glutamate and L-AP4. The intensity of the cAMP-immunofluorescence was higher with glutamate compared to L-AP4 demonstrating that the stronger effect observed with glutamate occurred via a higher elevation in cAMP level and suggested the implication of a second intracellular regulatory pathway.

C. Lavialle is supported by a doctoral fellowship from Region Pays de la Loire