HALOTHANE-INDUCED SYNAPTIC DEPRESSION AT BOTH IN-VIVO AND IN-VITRO RECONSTRUCTED SYNAPSES BETWEEN IDENTIFIED LYMNAEA NEURONS

1. In the present study we tested the ability of the general anestheti c, halothane, to affect synaptic transmission at in vivo and in vitro reconstructed peptidergic synapses between identified neurons of Lymna ea stagnalis. 2. An identified respiratory interneuron, visceral dorsa l 4 (VD4), innervates a number of postsynaptic cells in the central ri ng ganglia of Lymnaea. Because VD4 has previously been shown to exhibi t immunoreactivity for FMRFamide-related peptides, it was hypothesized that these peptides may be utilized by VD4 during synaptic transmissi on. In the intact, isolated CNS of Lymnaea, we have identified novel c onnections between VD4 and the pedal A (PeA) cells. We demonstrate tha t VD4 makes inhibitory connections with the PeA neurons, in particular PeA4, and that these synaptic responses are mimicked by exogenous app lication of FMRFamide. 3. The synaptic transmission between VD4 and th e PeA cells in an intact, isolated CNS preparation was completely bloc ked in 2%, but not 1% halothane. Interestingly, the postsynaptic respo nses (PeA) to exogenous FMRFamide were maintained in the presence of b oth 1 and 2% halothane. 4. To determine the specificity of the observe d responses and to determine the precise synaptic site of anesthetic a ction, we reconstructed the VD4/PeA synapses in vitro. After isolation from their respective ganglia, both cell types extended processes and established neuritic contact. We demonstrated that not only did the p resynaptic neuron reestablish the appropriate inhibitory synapses with the PeA neurons, but that the PeA cells also maintained their respons iveness to exogenous FMRFamide. 5. Superfusion of the in vitro synapti cally connected VD4 and PeA cells with 2% halothane completely abolish ed 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% haloth ane enhanced the duration of the postsynaptic response to exogenously applied FMRFamide. These data suggest that the halothane-induced depre ssion of synaptic transmission most likely occurred at the presynaptic level. 6. This study provides the first direct evidence that peptider gic transmission in the nervous system may also be susceptible to the actions of general anesthetics. In addition, we utilized a novel appro ach of in vitro reconstructed synapses for studying the effects of gen eral anesthetics on monosynaptic transmission in the absence of other synaptic influences.