In VivoIntracellular Recording Suggests That Gray Matter Astrocytes in Mature Cerebral Cortex and Hippocampus Are Electrophysiologically Homogeneous

Abstract

Previous anatomical and in vitro electrophysiology studies suggest that astrocytes are heterogeneous in physiology, morphology, and biochemical content. However, the extent to which this diversity applies to in vivo conditions is largely unknown. To characterize and classify the physiological and morphological properties of cerebral cortical and hippocampal astrocytes in the intact brain, we performed in vivo intracellular recordings from single astrocytes using anesthetized mature rats. Astrocytes were classified based on their glial fibrillary acidic protein (GFAP) immunoreactivity and cell body locations. We analyzed morphometric measures such as the occupied volume and polarity, as well as physiological characteristics such as the mean membrane potential. These measurements did not show obvious segregation into subpopulations, suggesting that gray matter astrocytes in the cortex and hippocampus are composed of a homogeneous population in mature animals. The membrane potential of astrocytes in both cortex and hippocampus fluctuated within a few millivolts in the presence of spontaneous network activity. These membrane potential fluctuations of an astrocyte showed a significant variability that depended on the local field potential state and cell body location. We attribute the variability of the membrane potential fluctuations to local potassium concentration changes due to neuronal activity.

Cortical energy demands of signaling and nonsignaling components in brain are conserved across mammalian species and activity levels

“…The constant E ns term for all activity states was 0.76 × 10 17 ATP/s per centimeter 3 , leading to average P ns,N and P ns,A values of 9.20 × 10 8 ATP/neuron per second and 6.85 × 10 8 ATP/astrocytes per second, respectively, for an OGI range of 5.1-6.0 (20). These values of P ns corresponded to an average R in value of 74 MΩ (SI Text, section 2), which is well within the in vivo range measured for neurons and astrocytes (13,14). As shown in Table 1, despite identical cortical densities of neurons and astrocytes, P ns,N and P ns,A were dissimilar, because slightly different Nernst potentials for Na + and K + and resting membrane potentials were used for neurons and astrocytes (SI Text, section 2), which has been done before (7,8).…”

“…This difference principally occurs, because Attwell and Laughlin (7) assumed R in,N and R in,A to be around 200 and 500 MΩ, respectively, based on in vitro recordings (30,31). However, our estimates of R in,N and R in,A values of 74 MΩ are in close agreement with in vivo measurements (13,14). Because P ns depends on the reciprocal of R in (SI Text, section 2), the higher R in,N and R in,A values by Attwell and Laughlin (7) are the main basis for their lower P ns,N and P ns,A estimates.…”

“…resistances (14). The in vivo results were fit well with the assumption of the major energetic changes with activity being in the neurons.…”

“…S1) and the average neuronal (η N ) and astrocytic (η A ) densities in the cerebral cortex (Table 1). Because recent R in,x and η x measurements in the rat show that values for neurons and astrocytes are quite similar (13)(14)(15)(16), the calculations were slightly simplified by assuming that R in,N ∼ R in,A and η N ∼ η A (Table 1 and SI Text, section 2). To determine P ns , we needed the metabolic demand for nonsignaling, which was possible for the rat, because state of deep pentobarbital anesthesia in Table S1 …”

Plasticity of astroglial networks in olfactory glomeruli

“…So far, V m fluctuations in astrocytes have mostly been reported from in vivo experiments (16,22) In KO Cx43 mice, dye coupling was diminished by 66% in the presence of TTX when injections were performed in glomerular astrocytes (n = 6 for both control and TTX conditions; P < 0.001), whereas in KO Cx30 animals, dye coupling was not affected by TTX treatment (78% of control, n = 10 and n = 6 for control and TTX conditions, respectively; P = 0.1462). Note that in the control condition, the number of coupled cells was reduced by 42% (n = 6, P < 0.001) and 29% (n = 10, P < 0.001) in KO Cx43 and KO Cx30 mice, respectively, compared with wild type (n = 26).…”