Halothane-induced synaptic depression at both in vivo and in vitro reconstructed synapses between identified Lymnaea neurons. Spencer, Gaynor E., Naweed I. Syed, Ken Lukowiak and William Winlow. Department of Physiology, University of Leeds, Leeds. United Kingdom. LS2 9NQ and Departments of Anatomy and Medical Physiology, Health Sciences Centre, 3330, Hospital Drive N.W. Calgary, Alberta, Canada. T2N 4N1.
APStracts 2:0217N, 1995.
SUMMARY AND CONCLUSIONS
1. In the present study, we tested the ability of the general anesthetic, halothane, to effect synaptic transmission at in vivo and in vitro reconstructed peptidergic synapses between identified neurons of Lymnaea stagnalis. 2. An identified respiratory interneuron Visceral Dorsal 4 (VD4), innervates a number of post-synaptic cells in the central ring ganglia of Lymnaea . As VD4 has previously been shown to exhibit immunoreactivity for FMRFamide-related peptides, it was hypothesized that these peptides may be utilized by VD4 during synaptic transmission. In the intact, isolated CNS of Lymnaea , we have identified novel connections between VD4 and the pedal A (PeA) cells. We demonstrate that VD4 makes inhibitory connections with the PeA neurons, in particular PeA4, and that these synaptic responses are mimicked by exogenous application of FMRFamide. 3. The synaptic transmission between VD4 and the PeA cells in an intact, isolated CNS preparation was completely blocked in 2%, but not 1% halothane. Interestingly, the post-synaptic responses (PeA) to exogenous FMRFamide were maintained in the presence of both 1% and 2% halothane. 4. In order to determine the specificity of the observed responses and to determine the precise synaptic site of anesthetic action, we reconstructed the VD4 / PeA synapses in vitro . Following isolation from their respective ganglia, both cell types extended processes and established neuritic contact. We demonstrated that not only did the pre-synaptic neuron re-establish the appropriate inhibitory synapses with the PeA neurons, but that the PeA cells also maintained their responsiveness to exogenous FMRFamide. 5. Superfusion of the in vitro synaptically connected VD4 and PeA cells with 2% halothane completely abolished the synaptic transmission between these cells. However, even higher concentrations of 4% halothane, failed to block the responsiveness of the PeA neurons to exogenous FMRFamide. Moreover, both 1% and 2% halothane enhanced the duration of the post-synaptic response to exogenously applied FMRFamide. These data suggest that the halothane- induced depression of synaptic transmission most likely occurred at the pre- synaptic level. 6. This study provides the first direct evidence that peptidergic transmission in the nervous system may also be susceptible to the actions of general anesthetics. In addition, we utilized a novel approach of in vitro reconstructed synapses for studying the effects of general anesthetics on monosynaptic transmission in the absence of other synaptic influences. 

Received 25 April 1995; accepted in final form 25 July 1995.
APS Manuscript Number J274-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 10 August 1995.