Bieda, Mark C. and M. Bruce MacIver. Major role for tonic GABA A conductances in anesthetic suppression of intrinsic neuronal excitability. J Neurophysiol 92: 1658 -1667, 2004. First published May 12, 2004 10.1152/jn.00223.2004. Anesthetics appear to produce neurodepression by altering synaptic transmission and/or intrinsic neuronal excitability. Propofol, a widely used anesthetic, has proposed effects on many targets, ranging from sodium channels to GABA A inhibition. We examined effects of propofol on the intrinsic excitability of hippocampal CA1 neurons (primarily interneurons) recorded from adult rat brain slices. Propofol strongly depressed action potential production induced by DC injection, synaptic stimulation, or high-potassium solutions. Propofol-induced depression of intrinsic excitability was completely reversed by bicuculline and picrotoxin but was strychnine-insensitive, implicating GABA A but not glycine receptors. Propofol strongly enhanced inhibitory postsynaptic currents (IPSCs) and induced a tonic GABA A -mediated current. We pharmacologically differentiated tonic and phasic (synaptic) GABA A -mediated inhibition using the GABA A receptor antagonist SR95531 (gabazine). Gabazine (20 M) completely blocked both evoked and spontaneous IPSCs but failed to block the propofol-induced depression of intrinsic excitability, implicating tonic, but not phasic, GABA A inhibition. Glutamatergic synaptic responses were not altered by propofol (Յ30 M). Similar results were found in both interneurons and pyramidal cells and with the chemically unrelated anesthetic thiopental. These results suggest that suppression of CA1 neuron intrinsic excitability, by these anesthetics, is largely due to activation of tonic GABA A conductances; although other sites of action may play important roles in affecting synaptic transmission, which also can produce strong neurodepression. We propose that for some anesthetics, suppression of intrinsic excitability, mediated by tonic GABA A conductances, operates in conjunction with effects on synaptic transmission, mediated by other mechanisms, to depress hippocampal function during anesthesia.

Sceniak, Michael P. and M. Bruce MacIver. Cellular actions of urethane on rat visual cortical neurons in vitro. J Neurophysiol 95: 3865-3874, 2006. First published March 1, 2006 doi:10.1152/jn.01196.2005. Urethane is widely used in neurophysiological experiments to anesthetize animals, yet little is known about its actions at the cellular and synaptic levels. This limits our ability to model systems-level cortical function using results from urethane-anesthetized preparations. The present study found that action potential discharge of cortical neurons in vitro, in response to depolarizing current, was strongly depressed by urethane and this was accompanied by a significant decrease in membrane resistance. Voltage-clamp experiments suggest that the mechanism of this depression involves selective activation of a Ba 2ϩ -sensitive K ϩ leak conductance. Urethane did not alter excitatory glutamate-mediated or inhibitory (GABA A -or GABA B -mediated) synaptic transmission. Neither the amplitude nor decay time constant of GABA A -or GABA B -mediated monosynaptic inhibitory postsynaptic currents (IPSCs) were altered by urethane, nor was the frequency of spontaneous IPSCs. These results are consistent with observations seen in vivo during urethane anesthesia where urethane produced minimal disruption of signal transmission in the neocortex.

Our results confirm earlier findings that clinically relevant concentrations of volatile anesthetics depress glutamate-mediated synaptic transmission. The observed increases in synaptic facilitation support recent findings from biochemical and electrophysiologic studies indicating presynaptic sites of action contribute to anesthetic-induced depression of excitatory transmission. This anesthetic-induced reduction in glutamate release would contribute to the central nervous system depression associated with anesthesia by adding to postsynaptic depressant actions on glutamate receptors.

Propofol does enhance inhibition in brain slices at a concentration of 0.63 microM in the superfusate, which produces brain concentrations corresponding with those achieved in vivo, but equilibration requires several hours. It is likely that slow diffusion to GABA receptors accounts for the high concentrations (>10 microM) that were needed to depress evoked responses in previous investigations.

An in vitro preparation of rabbit cornea was used to compare anatomical specialization and electrophysiological responses of A delta and C fiber sensory afferents which terminate as free nerve endings. Living nerve endings were visualized using epifluorescence microscopy and the vital dye 4-di2-ASP, and response properties were determined using microstimulation and recording of fiber discharge activity. Fiber type was determined based on conduction velocity measurement and preferred stimulus energy (modality) of each fiber. Four modality-specific fiber populations were identified: (1) slowly adapting C fiber cold receptors (conduction velocity of 0.25-1.6 m/sec), (2) C fiber chemosensitive units with mixed phasic and tonic activity (1.1-1.8 m/sec), (3) rapidly adapting mechanosensitive A delta fibers (1.5-2.8 m/sec), and (4) high-threshold mechano/heat (> 350 dyne or > 40 degrees C) phasic A delta afferents (3.5-4.4 m/sec). In addition to these physiological differences, anatomical specialization was also noted. A delta fiber nerve endings were distinguished from those of C fibers by thin, elongated sensory endings that ran parallel to the corneal surface; C fiber endings formed short, branching clusters that ran mostly perpendicular to the surface. The elongated structure of A delta nerve endings was associated with directional selectivity for mechanical stimuli. These results substantiate previous suggestions that free nerve endings can exhibit both structural and functional specialization.