Blockade of astrocyte metabolism causes delayed excitation as revealed by voltage-sensitive dyes in mouse brainstem slices

Abstract.

Fluoroacetate is known to block cell metabolism and to change potassium conductances selectively in astrocytes. In a functional neuronal network with ongoing activity, we investigated the effects of such a blockade of the astrocytic metabolism by fluoroacetate on neuronal signal propagation. Transverse 400-µm slices were prepared from the caudal medulla of mice of postnatal day 3–8, which contained the hypoglossal nucleus receiving excitatory synaptic input from the ventral respiratory group. Propagation of excitation within this network was measured by optical imaging using the voltage-sensitive dye RH 795. A 464-element photodiode array allowed fast recordings of voltage changes within a small population of cells. The spatial and temporal resolution was advanced to 32 µm and 1.27 ms, respectively. Changes of cellular membrane potential levels were expressed as relative changes of fluorescence (ΔI/I). Stimulus-evoked excitation of neurons propagating from the ventral respiratory group to the hypoglossal nucleus peaked after 7.2±0.6 ms (n=6). The latency of this early excitatory response is consistent with the time course of stimulus-evoked EPSPs in whole-cell recordings. Mean changes of fluorescence in the hypoglossal nucleus were −2.1±0.5×10⁻³ (ΔI/I). After incubation in 1 mM fluoroacetate, the early depolarization was reduced to 69.1±9.8% of control (n=6, p=0.034). Additionally, fluoroacetate induced a delayed excitatory response, such that fluorescence intensity did not return to baseline within 1 s. Propagation velocity and spatial distribution of the voltage signal were not affected by fluoroacetate. Our results suggest that blockade of astrocyte metabolism impairs fast synaptic transmission and induces a delayed excitation, probably resulting from the combination of reduced repolarization of neurons and persistent depolarization of astrocytes.