Membrane properties of ameboid microglial cells in the corpus callosum slice from early postnatal mice

Abstract

Microglial cells in culture are distinct from neurons, macroglial cells, and macrophages of tissues other than brain with respect to their membrane current pattern. To assess these cells in the intact tissue, we have applied the patch-clamp technique to study membrane currents in microglial cells from acute, whole brain slices of 6-9-d-old mice in an area of microglial cell invasion, the cingulum. As strategies to identify microglial cells prior to or after recording, we used binding and incorporation of Dil-acetylated low-density lipoproteins, binding of fluorescein isothiocyanate-coupled IgG via microglial Fc-receptors, and ultrastructural characterization. As observed previously for cultured microglial cells, depolarizing voltage steps activate only minute if any membrane currents, while hyperpolarizing voltage steps induced large inward currents. These currents exhibited properties of the inwardly rectifying K+ channel in that the reversal potential depended on the transmembrane K+ gradient, inactivation time constants decreased with hyperpolarization, and the current was blocked by tetraethylammonium (50 mM). This study represents the first attempt to assess microglial cells in situ using electrophysiological methods. It opens the possibility to address questions related to the function of microglial cells in the intact CNS.

Physiology of Microglia

“…During postnatal development, invading microglial cells with amoeboid morphology can be studied in acutely isolated slices from 6-to 8-day-old mouse corpus callosum. The IK IR was the main component of K ϩ permeability of these invading microglia (86). The IK IR in these cells were inhibited by TEA (50 mM), but not by 4-AP, and were almost completely blocked by 2-5 mM Ba 2ϩ (86).…”

“…7). These currents exhibited properties of the inwardly rectifying K ϩ current in that the reversal potential depended on the transmembrane K ϩ gradient, inactivation time constants decreased with hyperpolarization, and the current was blocked by tetraethylammonium (86). The membrane potential varied considerably between Ϫ10 and Ϫ70 mV (with the mean value of Ϫ42 mV).…”

“…During postnatal development, they immigrate into the brain commonly until postnatal day 10 in rodents. These immigrating cells are amoeboid and can thus be easily recognized due to their morphology, for instance, in acute slices of the corpus callosum (86,342; see also supplementary videos 1 and 2). 1 In the adult animals there seems to be very little exchange between blood and brain parenchyma.…”

“…Voltageclamp experiments on ramified microglia in situ in acutely isolated brain slices failed to detect any signs of Na ϩ currents (77,81). The lack of sodium currents is also documented for microglia after pathology (81) or in amoeboid microglia during development (86). Finally, treatment with TTX (20 -50 M) does not affect the motility of microglial process in vivo (645).…”

The origin and differentiation of microglial cells during development

“…Second, some markers that label macroglial cells have been reported also to label microglia (Dickson and Mattiace, 1989;Wolswijk, 1995). And third, microglial cells have features di erent from other cells of the mononuclear phagocytic system: they display particular morphological features in vitro (Giulian et al, 1995), proliferate spontaneously in culture (Giulian et al, 1995), and have a distinct ion channel pattern di erent from that of monocytes and macrophages in non-nervous tissues (Kettenmann et al, 1990;Brockhaus et al, 1993).…”