Neuropharmacological studies on the receptors mediating responses to carbachol, amino acids and octopamine on Limulus and Hirudo central neurons.

Intracellular recordings were made from central neurons of Limulus polyphemus and from Retizu cells of the leech, Hirudo medicinalis. The effects of carbachol, amino acids and octopamine were examined on these neurons. Octopamine was found to have a mainly inhibitory effect on a few Limulus neurons. The effects of octopamine were mimicked by clonidine and napthazoline but not by xylazine. Both compounds were slightly more potent than octopamine. Yohimbine, metoclopramide, chlorpromazine and chlordimeform failed to antagonize this octopamine response. The excitatory effect of carbachol was blocked by alpha-bungarotoxin, 10(-7)M. Neither this concentration nor higher concentrations of alpha-bungarotoxin had any effect on L-glutamate excitation. m-Carboxyphenyl derivatives of alanine and glycine acted differentially on Limulus neurons responding to L-glutamate. m- Carboxyphenylglycine only inhibited neurones which showed a biphasic response to L-glutamate while m- carboxyphenylalanine only excited these neurons. Both compounds excited leech Retzius cells, with m- carboxyphenylalanine being about 20 times more potent than m- carboxyphenylglycine . The actions of alpha- ketokainate and allo-alpha- ketokainate were compared to kainate, dihydrokainate and L-glutamate on leech Retzius cells. The equipotent molar ratios for kainate, dihydrokainate , alpha- ketokainate and allo-alpha- ketokainate were 0.0029 +/- 0.0004, 0.021 +/- 0.047, 0.029 +/- 0.005 and 0.14 +/- 0.0093 respectively with L-glutamate as one. All the analogues were more potent than L-glutamate. Quinolinic acid had no glutamate-like activity on either Limulus or Hirudo neurons. Methyltetrahydrofolate was inactive on Limulus neurons but excited leech Retzius cells, being slightly less potent than L-glutamate. Dibutyl cAMP terminated the excitatory actions of kainate on both Limulus and Hirudo neurons. Anisatin , a putative GABA antagonist, was a potent antagonist of GABA inhibition on Limulus neurons.