Developmental changes in propagation patterns and transmitter dependence of waves of spontaneous activity in the mouse cerebral cortex.

Waves of spontaneous electrical activity propagate across many regions of the central nervous system during specific stages of early development. The patterns of wave propagation are critical in the activation of many activity-dependent developmental programs. It is not known how the mechanisms that initiate and propagate spontaneous waves operate during periods in which major changes in neuronal structure and function are taking place. We have recently reported that spontaneous waves of activity propagate across the neonatal mouse cerebral cortex and that these waves are initiated at pacemaker sites in the septal nucleus and ventral cortex. Here we show that spontaneous waves occur between embryonic day 18 (E18) and postnatal day 12 (P12), and that during that period they undergo major changes in transmitter dependence and propagation patterns. At early stages, spontaneous waves are largely GABA dependent and are mostly confined to the septum and ventral cortex. As development proceeds, wave initiation depends increasingly on AMPA-type glutamate receptors, and an ever increasing fraction of waves propagate into the dorsal cortex. The initiation sites and restricted propagation of waves at early stages are highly correlated with the position of GABAergic neurons in the cortex. The later switch to a glutamate-based mechanism allows propagation of waves into the dorsal cortex, and appears to be a compensatory mechanism that ensures continued wave generation even as GABA transmission becomes inhibitory.

Bimodal septal and cortical triggering and complex propagation patterns of spontaneous waves of activity in the developing mouse cerebral cortex.

Spontaneous waves of activity that propagate across large structures during specific developmental stages play central roles in CNS development. To understand the genesis and functions of these waves, it is critical to understand the spatial and temporal patterns of their propagation. We recently reported that spontaneous waves in the neonatal cerebral cortex originate from a ventrolateral pacemaker region. We have now analyzed a large number of spontaneous waves using calcium imaging over the entire area of coronal slices from E18-P1 mouse brains. In all waves, the first cortical region active is this ventrolateral pacemaker. In half of the waves, however, the cortical pacemaker activity is itself triggered by preceding activity in the septal nuclei. Most waves are restricted to the septum and/or ventral cortex, with only some invading the dorsal cortex or the contralateral hemisphere. Waves fail to propagate at very stereotyped locations at the boundary between ventral and dorsal cortex and at the dorsal midline. Waves that cross these boundaries pause at these same locations. Waves at these stages are blocked by both picrotoxin and CNQX, indicating that both GABA(A) and AMPA receptors are involved in spontaneous activity.

Differential targeting of nNOS and AQP4 to dystrophin-deficient sarcolemma by membrane-directed alpha-dystrobrevin.

alpha-Dystrobrevin associates with and is a homologue of dystrophin, the protein linked to Duchenne and Becker muscular dystrophies. We used a transgenic approach to restore alpha-dystrobrevin to the sarcolemma in mice that lack dystrophin (mdx mice) to study two interrelated functions: (1) the ability of alpha-dystrobrevin to rescue components of the dystrophin complex in the absence of dystrophin and (2) the ability of sarcolemmal alpha-dystrobrevin to ameliorate the dystrophic phenotype. We generated transgenic mice expressing alpha-dystrobrevin-2a linked to a palmitoylation signal sequence and bred them onto the alpha-dystrobrevin-null and mdx backgrounds. Expression of palmitoylated alpha-dystrobrevin prevented the muscular dystrophy observed in the alpha-dystrobrevin-null mice, demonstrating that the altered form of alpha-dystrobrevin was functional. On the mdx background, the palmitoylated form of alpha-dystrobrevin was expressed on the sarcolemma but did not significantly ameliorate the muscular dystrophy phenotype. Palmitoylated dystrobrevin restored alpha-syntrophin and aquaporin-4 (AQP4) to the mdx sarcolemma but was unable to recruit beta-dystroglycan or the sarcoglycans. Despite restoration of sarcolemmal alpha-syntrophin, neuronal nitric oxide synthase (nNOS) was not localized to the sarcolemma, suggesting that nNOS requires both dystrophin and alpha-syntrophin for correct localization. Thus, although nNOS and AQP4 both require interaction with the PDZ domain of alpha-syntrophin for sarcolemmal association, their localization is regulated differentially.