What the actions of anaesthetics on fast synaptic transmission reveal about the molecular mechanism of anaesthesia

Antidepressants modulate the in vitro inhibitory effects of propofol and ketamine on norepinephrine and serotonin transporter function

Norepinephrine transporter (NET) and serotonin transporter (SERT) proteins regulate norepinephrine (NE) and serotonin via their reuptake function and are targets of antidepressants action. Several intravenous (IV) anesthetics have been shown to inhibit NET and SERT. The interactions between antidepressants and anesthetics on transporter function, however, are not well studied. We examined the effect of different IV anesthetics on NET and SERT function, with and without chronic antidepressant pretreatment, by measuring NE or 5-hydroxytryptamine (5-HT) uptake and determined NET and SERT protein expression via immunoblotting. Both ketamine and propofol inhibited NET dose-dependently (propofol 10(-4)M -22%+/-5.6%, and propofol 10(-3)M -35%+/-5.7%; ketamine 10(-4)M -23%+/-4.1% and ketamine 10(-3)M -73%+/-2.9%); and SERT (propofol 10(-4)M -11%+/-4.3% and propofol 10(-3)M -23%+/-3.8%; ketamine 10(-4)M -29%+/-5.2% and ketamine 10(-3)M -63%+/-6.4%). Etomidate and thiopental had no effect on either NET or SERT function. Desipramine and fluoxetine, specific inhibitors of NET and SERT, respectively, both enhanced the inhibitory effects of propofol but reduced the inhibitory effects of ketamine on NET and SERT functions. IV anesthetics treatment did not change transporter protein expression in the presence of its respective inhibitor. Our results demonstrate that both ketamine and propofol inhibited SERT and NET function, but the inhibition was differentially modulated by antidepressants. Therefore, in the clinical context, this would suggest that patients receiving antidepressant treatments might have altered response to IV anesthetics in an agent-specific manner.

L-type calcium channel blockade modifies anesthetic actions on aged hippocampal neurons

Previous studies in our laboratory demonstrated a reversal of anesthetic actions on aged neurons by decreasing extracellular [Ca(2+)] in hippocampal slices. Such maneuver indirectly attenuated Ca(2+) influx, hence decreased exogenous intraneuronal Ca(2+) loads during neuronal activity and consequently improved intracellular Ca(2+) concentration homeostasis. Therefore, in the present study we hypothesized that decreasing exogenous Ca(2+) loads by blocking voltage-gated calcium influx in aged neurons would oppose isoflurane actions. Conversely, increasing endogenous Ca(2+) loads by suppressing calcium efflux during forced reversal of Na(+)/Ca(2+) exchanger function would enhance anesthetic effects. Hippocampal slices were prepared from young (2-4 months) and old (24-26 months) Fischer 344 rats. Isoflurane depressed the evoked dendritic field excitatory postsynaptic potentials by approximately 45% in slices taken from old animals. However, application of isoflurane in addition with CoCl(2) or nifedipine opposed the anesthetic actions, which then depressed the evoked dendritic field postsynaptic potentials by only 15%. Selective blockade of the N-type and P/Q-type calcium channels with omega-conotoxin GVIA and omega-conotoxin MVIIC respectively caused rapid but partial depression of synaptic transmission in slices taken from old Fischer 344 rats. However, isoflurane actions in these aged slices were not affected compared with slices perfused only with normal artificial cerebrospinal fluid. Young and aged slices were then exposed to a low sodium perfusate that forces the Na(+)/Ca(2+) exchanger protein into a reverse mode, thus increasing intracellular Ca(2+) concentration. Isoflurane actions under such conditions were profoundly potentiated in aged slices but were not altered in young hippocampi. The current results show that in aged central neurons, selectively blocking L-type calcium channels opposes anesthetic-induced depression of excitatory synaptic transmission. On the contrary, increasing calcium loads in aged neurons potentiates these actions.

Methodological considerations in rat brain BOLD contrast pharmacological MRI

RATIONALE AND OBJECTIVES

Blood oxygen level dependent (BOLD) contrast pharmacological magnetic resonance imaging (phMRI) is an increasingly popular technique that allows the non-invasive investigation of spatial and temporal changes in rat brain function in response to pharmacological stimulation in vivo. Rat brain BOLD contrast phMRI is, at present, established in few neuropharmacological laboratories, and various issues associated with the technique require attention. The present review is primarily aimed at psychopharmacologists with no previous experience of phMRI, who are interested in the practical aspects that phMRI studies entail.

RESULTS AND DISCUSSION

Experimental and analytical considerations, including anaesthesia, physiological monitoring, drug dose and delivery, scanning protocols, statistical approaches and the interpretation of phMRI data, are discussed.

Kinetics of etomidate actions on GABA(A) receptors in the rat spinal dorsal horn neurons

Electrophysiological properties of etomidate (ET)-induced current (I(ET)) at different concentrations and effects of ET at clinically relevant concentrations (1-10 microM) on postsynaptic GABA(A) receptor function were investigated using whole-cell patch-clamp technique in mechanically dissociated rat spinal dorsal horn neurons. The results showed that ET actions were concentration-dependent: low concentrations (10 microM) of ET potentiated GABA-activated current (I(GABA)), slowed activation, desensitization and deactivation of GABA(A) receptors; moderate concentrations (10-1,000 microM) of ET directly activated and desensitized GABA(A) receptors; high concentrations (>1,000 microM) of ET produced an inhibitory effect on I(ET). In addition, ET prolonged the duration of GABAergic miniature inhibitory postsynaptic currents (mIPSCs) in the mechanically dissociated rat dorsal horn neurons. These results suggest that general anesthetics-induced changes at spinal level could significantly contribute to analgesia and general anesthesia.

General anesthetic actions on norepinephrine, dopamine, and gamma-aminobutyric acid transporters in stably transfected cells

The effects of general anesthetics on neurotransmitter uptake by plasma membrane transporters are controversial. We analyzed the effects of representative volatile and IV general anesthetics on recombinant transporters for norepinephrine (human NET), dopamine (rat DAT), or gamma-aminobutyric acid (rat GAT-1) stably expressed in a porcine kidney cell line (LLC-PK(1)). This approach avoids complicating factors associated with neuronal preparations, such as the involvement of multiple transporters and the indirect effects of membrane potential. At clinical concentrations, human NET was inhibited only by halothane (50% inhibitory concentration [IC(50)] = 0.54 mM), rat DAT was sensitive to both halothane and isoflurane (IC(50) = 0.60 and 0.64 mM, respectively), and rat GAT-1 was insensitive to both volatile anesthetics. Human NET was inhibited in a dose-dependent fashion by propofol (IC(50) = 41 micro M), ketamine (IC(50) = 150 micro M), and etomidate (IC(50) > 200 micro M), but not by pentobarbital. Only propofol inhibited NET at a clinically relevant concentration (5 micro M). Rat DAT was inhibited in a dose-dependent fashion by propofol (IC(50) = 120 micro M), etomidate (IC(50) = 100 micro M), and ketamine (IC(50) = 210 micro M), but not by pentobarbital. None of these anesthetics was predicted to inhibit DAT at concentrations that produce anesthesia. Propofol inhibited rat GAT-1, but only at the largest concentration tested. General anesthetics have drug- and subtype-selective actions on neurotransmitter transporters. We conclude that effects on catecholamine, but not gamma-aminobutyric acid, transporters may contribute to secondary synaptic actions of certain anesthetics but are unlikely to be essential to their anesthetic properties.

IMPLICATIONS

Previous studies have implicated neurotransmitter transporters as targets for general anesthetic effects on synaptic transmission. Recombinant transporters for norepinephrine and dopamine were sensitive to certain volatile and IV anesthetics, whereas gamma-aminobutyric acid transporters were insensitive. These anesthetic- and neurotransmitter-specific effects may underlie some of the secondary effects of general anesthetics.

The actions of propofol on gamma-aminobutyric acid-A and glycine receptors in acutely dissociated spinal dorsal horn neurons of the rat

The spinal cord plays an important role in modulating anesthetic-induced suppression of nociceptive transmission. To gain some insight into the anesthetic mechanisms of propofol at the spinal level, we investigated the direct action of propofol and its modulation on the gamma-aminobutyric acid-A receptor (GABA(A)R) and the glycine receptor (GlyR) in acutely dissociated rat spinal dorsal horn neurons by using whole-cell patch-clamp electrophysiology. Propofol induced Cl(-) currents (I(Cl)), which were sensitive to bicuculline and, to a lesser extent, to strychnine. The activation, desensitization, and deactivation of propofol-induced I(Cl) were slower than those of GABA- and glycine-induced I(Cl). In addition, this study revealed similar modulatory actions of propofol on GABA(A)R and GlyR. Propofol potentiated both GABA- and glycine-induced I(Cl) at small con-centrations and inhibited both GABA- and glycine-induced I(Cl) at large concentrations. The potentiation of propofol on I(Cl) was caused by slowing current desensitization and deactivation, whereas the inhibition actions might be involved in the cross-desensitization between GABA- and propofol-induced I(Cl) and the cross-inhibition between the GABA(A)R and GlyR. The results suggest that propofol facilitation of GABA(A)R and GlyR at the spinal level could contribute significantly to general anesthetic-induced analgesia and anesthesia.

IMPLICATIONS

The actions of propofol on the gamma-aminobutyric acid-A receptor (GABA(A)R) and the glycine receptor (GlyR) were investigated in acutely dissociated rat spinal dorsal horn neurons by using whole-cell patch-clamp electrophysiology. Propofol was found to potentiate the functions of GABA(A)R and GlyR at the spinal level, which might contribute to propofol-induced analgesia and anesthesia.

Total neuroenergetics support localized brain activity: implications for the interpretation of fMRI

In alpha-chloralose-anesthetized rats, changes in the blood oxygenation level-dependent (BOLD) functional MRI (fMRI) signal (DeltaS/S), and the relative spiking frequency of a neuronal ensemble (Deltanu/nu) were measured in the somatosensory cortex during forepaw stimulation from two different baselines. Changes in cerebral oxygen consumption (DeltaCMR(O2)/CMR(O2)) were derived from the BOLD signal (at 7T) by independent determinations in cerebral blood flow (DeltaCBF/CBF) and volume (DeltaCBV/CBV). The spiking frequency was measured by extracellular recordings in layer 4. Changes in all three parameters (CMR(O2), nu, and S) were greater from the lower baseline (i.e., deeper anesthesia). For both baselines, DeltaCMR(O2)/CMR(O2) and Deltanu/nu were approximately one order of magnitude larger than DeltaS/S. The final values of CMR(O2) and nu reached during stimulation were approximately the same from both baselines. If only increments were required to support functions then their magnitudes should be independent of the baseline. In contrast, if particular magnitudes of activity were required, then sizes of increments should inversely correlate with the baseline (being larger from a lower baseline). The results show that particular magnitudes of activity support neural function. The disregard of baseline activity in fMRI experiments by differencing removes a large and necessary component of the total activity. Implications of these results for understanding brain function and fMRI experiments are discussed.