Etomidate reduces glutamate uptake in rat cultured glial cells: involvement of PKA

Abstract

Background and purpose: Glutamate is the main excitatory neurotransmitter in the vertebrate CNS. Removal of the transmitter from the synaptic cleft by glial and neuronal glutamate transporters (GLTs) has an important function in terminating glutamatergic neurotransmission and neurological disorders. Five distinct excitatory amino-acid transporters have been characterized, among which the glial transporters excitatory amino-acid transporter 1 (EAAT1) (glutamate aspartate transporter) and EAAT2 (GLT1) are most important for the removal of extracellular glutamate. The purpose of this study was to describe the effect of the commonly used anaesthetic etomidate on glutamate uptake in cultures of glial cells. Experimental approach: The activity of the transporters was determined electrophysiologically using the whole cell configuration of the patch-clamp recording technique. Key results: Glutamate uptake was suppressed by etomidate (3-100 mM) in a time-and concentration-dependent manner with a half-maximum effect occurring at 2.4±0.6 mM. Maximum inhibition was approximately 50% with respect to the control. Etomidate led to a significant decrease of V max whereas the K m of the transporter was unaffected. In all cases, suppression of glutamate uptake was reversible within a few minutes upon washout. Furthermore, both GF 109203X, a nonselective inhibitor of PKs, and H89, a selective blocker of PKA, completely abolished the inhibitory effect of etomidate. Conclusion and implications: Inhibition of glutamate uptake by etomidate at clinically relevant concentrations may affect glutamatergic neurotransmission by increasing the glutamate concentration in the synaptic cleft and may compromise patients suffering from acute or chronic neurological disorders such as CNS trauma or epilepsy.

Astrocytic group I mGluR-dependent potentiation of astrocytic glutamate and potassium uptake

“…By inhibiting PKC, upregulation of astrocyte glutamate uptake is prevented, which may unmask the effects of signaling molecules activated in astrocytes by afferent stimulation that acutely depresses astrocyte glutamate uptake. Cultured studies suggest that Gs GPCRs and activation of PKA downregulate glutamate uptake (Adolph et al 2007;Rath et al 2008). In future studies, it will be interesting to determine the potential role of PKA in modulation of astrocyte glutamate uptake in intact tissue preparations.…”

Etomidate accurately localizes the epileptic area in patients with temporal lobe epilepsy

“…Studies in rats have shown that β 2 subunits seem to be preferentially located on GABAergic interneurons (Nishimura et al, 2005), and excessive activation of these receptors could cause disinhibition of cortical activity and seizures. Finally, in cultured astrocytes, etomidate can inhibit glutamate uptake, increasing the extracellular glutamate concentration such that it can spill over the synaptic cleft and activate extrasynaptic receptors, consequently increasing irritative activity (Räth et al, 2008).…”

Cambios en la perfusión cerebral inducidos por etomidato en pacientes con epilepsia del lóbulo temporal

“…Por último, como tercera posibilidad, el etomidato también actuaría disminuyendo la actividad de los transportadores de glutamato (EAAT, del inglés excitatory aminoacid transporter). En concreto, se ha observado que esta disminución de la actividad de recaptación se da en los EAAT del tipo 1 y 2, que se encuentran en los astrocitos [27,28]. La presencia de etomidato en concentraciones clínicas produce una disminución en la capacidad de recaptación de estos EAAT hasta del 40% [27].…”

“…En concreto, se ha observado que esta disminución de la actividad de recaptación se da en los EAAT del tipo 1 y 2, que se encuentran en los astrocitos [27,28]. La presencia de etomidato en concentraciones clínicas produce una disminución en la capacidad de recaptación de estos EAAT hasta del 40% [27]. Una de las funciones que realizan los astrocitos consiste en la recaptación de glutamato [3], y la disminución de esta actividad permitirá un aumento de la permanencia del glutamato en la hendidura sináptica, lo que permite incrementar la hiperexcitabilidad neuronal.…”