The second FMRFamide-gated Na + channel (HtFaNaC), from Helisoma trivolvis, has been cloned. HtFaNaC has some different pharmacological properties to HaFaNaC, from Helix aspersa, which has enabled a rational approach to be made to start to identify the FMRFamide recognition site. Several chimeras were made by switching sections between the channels. The differences in sensitivity to FMRFamide, and amiloride, were assessed after expression in Xenopus oocytes. The data suggest that a recognition site for FMRFamide, and the potentiating action of amiloride, resides in a sequence of about 120 amino acids in the extracellular loop proximal to the first transmembrane segment. ß

1 Ca currents in rat and mouse sensory dorsal root ganglion (DRG) neurones were inhibited by concentrations of (-)-baclofen as low as 1^i . The proportion of neurones responding to baclofen was low (< 20%), except in young cultures of neonate rat DRG neurones (3 days in culture), where 86% of the neurones were responsive. 2 Three types of unitary Ca currents were observed in the rat DRG neurones, corresponding to the T-, N-and L-type currents of chick DRG neurones. 3 Baclofen produced two types of response on whole-cell currents of DRG neurones from both species. The first was on an early inactivating component of the Ca current. This early current was partially inactivated at a holding potential of -40mV. It was also reduced during the second of a pair of depolarizing command pulses. The results suggest that this action of baclofen is due to an action on an N-type component of the current. The second response to baclofen persisted throughout the command step and was not reduced during the second of a pair of command pulses, indicating that this effect is due to an action on the L-type current. 4 Unitary or ensemble Ca currents recorded in cell-attached patches, on neurones previously shown to respond to baclofen in whole-cell clamp mode, were generally not inhibited by baclofen application external to the patch electrode. This indicates that a readily diffusible internal second messenger substance is probably not involved in coupling the GABAB receptor to the ion channels.

1 5-Hydroxytryptamine (5-HT) activated a fast (70 ms to half maximum) and desensitizing inward current through non-selective channels conducting predominantly monovalent cations in neurones of Helix aspersa.2 a-Methyl-5-HT was equipotent with 5-HT in activating this current, but the known selective agonists at vertebrate 5-HT3 receptors, 2-methyl-5-HT and arylbiguanides were ineffective (< 100 gM). 5-Methoxytryptamine which is inactive on vertebrate 5-HT3 receptors was a very weak agonist. 3 The responses were antagonized by the specific vertebrate 5-HT3 receptor blocker MDL-72222 (IC50 = 1 gM), but were only weakly affected by ondansetron (10 gM). The 5-HT2-type antagonist, ketanserin (<5 gM), had no effect. The responses were also antagonized by the non-specific antagonists

No abstract