Isoflurane differentially modulates inhibitory and excitatory synaptic transmission to the solitary tract nucleus.
BACKGROUND
Isoflurane anesthesia produces cardiovascular and respiratory depression, although the specific mechanisms are not fully understood. Cranial visceral afferents, which innervate the heart and lungs, synapse centrally onto neurons within the medial portion of the nucleus tractus solitarius (NTS). Isoflurane modulation of afferent to NTS synaptic communication may underlie compromised cardiorespiratory reflex function.


METHODS
Adult rat hindbrain slice preparations containing the solitary tract (ST) and NTS were used. Shocks to ST afferents evoked excitatory postsynaptic currents with low-variability (SEM <200 mus) latencies identifying neurons as second order. ST-evoked and miniature excitatory postsynaptic currents as well as miniature inhibitory postsynaptic currents were measured during isoflurane exposure. Perfusion bath samples were taken in each experiment to measure isoflurane concentrations by gas chromatography-mass spectrometry.


RESULTS
Isoflurane dose-dependently increased the decay-time constant of miniature inhibitory postsynaptic currents. At greater than 300 mum isoflurane, the amplitude of miniature inhibitory postsynaptic currents was decreased, but the frequency of events remained unaffected, whereas at equivalent isoflurane concentrations, the frequency of miniature excitatory postsynaptic currents was decreased. ST-evoked excitatory postsynaptic current amplitudes decreased without altering event kinetics. Isoflurane at greater than 300 mum increased the latency to onset and rate of synaptic failures of ST-evoked excitatory postsynaptic currents.


CONCLUSIONS
In second-order NTS neurons, isoflurane enhances phasic inhibitory transmission via postsynaptic gamma-aminobutyric acid type A receptors while suppressing excitatory transmission through presynaptic mechanisms. These results suggest that isoflurane acts through multiple distinct mechanisms to inhibit neurotransmission within the NTS, which would underlie suppression of homeostatic reflexes.
Peters
James H
JH
Department of Physiology and Pharmacolog, Oregon Health and Science University, Portland, Oregon 97239-3098, USA. petersj.ohsu@gmail.com
McDougall
Stuart J
SJ
Mendelowitz
David
D
Koop
Dennis R
DR
Andresen
Michael C
MC
D000818:Animals; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects* / Q000502:physiology; D053444:Inhibitory Postsynaptic Potentials / Q000187:drug effects* / Q000502:physiology; D007530:Isoflurane / Q000494:pharmacology*; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D017552:Solitary Nucleus / Q000187:drug effects* / Q000502:physiology; D009435:Synaptic Transmission / Q000187:drug effects* / Q000502:physiology
D007530:Isoflurane