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Biomedical subjects

M Bruce MacIver

Publications and source records attributed to M Bruce MacIver.

4 recordsLinked to original sources
PubMed2006

Isoflurane depresses hippocampal CA1 glutamate nerve terminals without inhibiting fiber volleys.

BACKGROUND: Anesthetic-induced CNS depression is thought to involve reduction of glutamate release from nerve terminals. Recent studies suggest that isoflurane reduces glutamate release by block of Na channels. To further investigate this question we examined the actions of isoflurane, TTX, extracellular Ca2+, CNQX and stimulus voltage (stim) on glutamate-mediated transmission at hippocampal excitatory synapses. EPSPs were recorded from CA1 neurons in rat hippocampal brain slices in response to Schaffer-collateral fiber stimulation. RESULTS: Isoflurane (350 microM; 1 MAC) reversibly depressed EPSP amplitudes by ~60% while facilitation increased approximately 20%. Consistent with previous studies, these results indicate a presynaptic site of action that involves reduced excitation-release coupling. EPSPs were depressed to comparable levels by TTX (60 nM) or lowered stim, but facilitation was not changed, indicating a simple failure of axonal conduction. Similarly, partial antagonism of postsynaptic glutamate receptors with CNQX (10 microM) depressed EPSP amplitudes with no change in facilitation. However, EPSP depression by low external Ca2+ (0.8 mM) was accompanied by an increase in facilitation comparable to isoflurane. Isoflurane depression of EPSP amplitudes could also be partly reversed by high external Ca2+ (4 mM) that also decreased facilitation. Isoflurane or low Ca2+ markedly reduced the slopes of fiber volley (FV)-EPSP input-output curves, consistent with little or no effect on FVs. By contrast, TTX didn't alter the FV-EPSP curve slope, indicating that EPSP depression resulted from FV depression. FVs were remarkably resistant to isoflurane. Somatic spike currents were unaffected by 350 microM (1 MAC) isoflurane as well. The EC50 for isoflurane depression of FVs was approximately 2.8 mM (12 vol. %; 8 MAC). CONCLUSION: Isoflurane appears to depress CA1 synapses at presynaptic sites downstream from Na channels, as evident by the increased facilitation that accompanies EPSP depression. Fiber volleys did not exhibit depression by isoflurane, as has been reported for other brain regions.

2-Amino-5-phosphonovalerate↗
PubMed2004

Multiple synaptic and membrane sites of anesthetic action in the CA1 region of rat hippocampal slices.

BACKGROUND: Anesthesia is produced by a depression of central nervous system function, however, the sites and mechanisms of action underlying this depression remain poorly defined. The present study compared and contrasted effects produced by five general anesthetics on synaptic circuitry in the CA1 region of hippocampal slices. RESULTS: At clinically relevant and equi-effective concentrations, presynaptic and postsynaptic anesthetic actions were evident at glutamate-mediated excitatory synapses and at GABA-mediated inhibitory synapses. In addition, depressant effects on membrane excitability were observed for CA1 neuron discharge in response to direct current depolarization. Combined actions at several of these sites contributed to CA1 circuit depression, but the relative degree of effect at each site was different for each anesthetic studied. For example, most of propofol's depressant effect (> 70 %) was reversed with a GABA antagonist, but only a minor portion of isoflurane's depression was reversed (< 20 %). Differences were also apparent on glutamate synapses-pentobarbital depressed transmission by > 50 %, but thiopental by only < 25 %. CONCLUSIONS: These results, in as much as they may be relevant to anesthesia, indicate that general anesthetics act at several discrete sites, supporting a multi-site, agent specific theory for anesthetic actions. No single effect site (e.g. GABA synapses) or mechanism of action (e.g. depressed membrane excitability) could account for all of the effects produced for any anesthetic studied.

Action Potentials↗
PubMed2004

Major role for tonic GABAA conductances in anesthetic suppression of intrinsic neuronal excitability.

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 microM) 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 (< or =30 microM). 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.

Anesthetics, Intravenous↗