Mecp2-deficiency causes a neurodevelopmental disorder called Rett syndrome which is characterized by an imbalance between inhibitory and excitatory neurotransmission resulting in hyperexcitability. We observed an increased susceptibility of the Mecp2^-/y hippocampal formation to hypoxia-induced spreading depression which in part seems to be due to impaired K⁺-channel function. To investigate the underlying mechanisms in more detail, intracellular sharp electrode recordings as well as single-channel recordings were performed in CA1 pyramidal neurons of acute hippocampal tissue slices prepared from adult Mecp2-deficient and wildtype males. Intracellular recordings did not reveal any differences in pyramidal cell resting membrane potential and input resistance between wildtypes and Mecp2^-/y males. Chemical anoxia (1 mM NaCN) caused pronounced hyperpolarizations in wildtype neurons (-6 mV) but only mild hyperpolarizations in Mecp2^-/y neurons. In accordance, membrane resistance during cyanide-induced metabolic arrest decreased less in Mecp2-deficient hippocampal neurons. As these differences strongly suggest impaired potassium channel function in Mecp2-deficient neurons, further experiments were performed on the single channel level. Recordings in excised inside-out patches of CA1 neurons identified a large conductance (~260 pS), BK-type Ca²⁺-dependent potassium channel. Its basic biophysical properties (single-channel conductance, open-probability, voltage-dependence, Ca²⁺-dependence) were determined in symmetrical 145 mM K⁺-solution and did not differ significantly between wildtype and Mecp2^-/y males. Different responses were, however, observed in response to chemical anoxia. In cell-attached patches such metabolic arrest by 1 mM NaCN caused a pronounced activation of various K⁺-channels in wildtype which was less intense in Mecp2^-/y neurons. Despite inclusion of 200 mM tolbutamide into the pipette solution, an additional intermediate-conductance (~80 pS) K⁺-channel, which clearly differed from the BK-channel, became activated in cell-attached patches of wildtype and Mecp2^-/y neurons. These data suggest that the increased susceptibility to hypoxic spreading depression in Mecp2^-/y hippocampal neurons might be due to disturbed activation of different K⁺-channels during hypoxia. These results in a dampened hyperpolarization and input resistance decrease, i.e. a less efficient stabilization of the membrane potential and may therefore hasten the onset of hypoxic spreading depression.

Supported by the Deutsche Forschungsgemeinschaft (CMPB) and the BMBF (BCCN).