Duchenne muscular dystrophy (DMD) is caused by deficient expression of the cytoskeletal protein, dystrophin. One third of DMD patients also have mental retardation (MR), likely due to mutations preventing expression of dystrophin and other brain products of the DMD gene expressed from distinct internal promoters. Loss of Dp71, the major DMD-gene product in brain, is thought to contribute to the severity of MR; however, the specific function of Dp71 is poorly understood. Complementary approaches were used to explore the role of Dp71 in neuronal function and identify mechanisms by which Dp71 loss may impair neuronal and cognitive functions. Besides the normal expression of Dp71 in a subpopulation of astrocytes, we found that a pool of Dp71 colocalizes with synaptic proteins in cultured neurons and is expressed in synaptic subcellular fractions in adult brains. We report that Dp71-associated protein complexes interact with specialized modular scaffolds of proteins that cluster glutamate receptors and organize signaling in postsynaptic densities. We then undertook the first functional examination of the brain and cognitive alterations in the Dp71-null mice. We found that these mice display abnormal synapse organization and maturation in vitro, altered synapse density in the adult brain, enhanced glutamatergic transmission and reduced synaptic plasticity in CA1 hippocampus. Dp71-null mice show selective behavioral disturbances characterized by reduced exploratory and novelty-seeking behavior, mild retention deficits in inhibitory avoidance, and impairments in spatial learning and memory. Results suggest that Dp71 expression in neurons play a regulatory role in glutamatergic synapse organization and function, which provides a new mechanism by which inactivation of Dp71 in association with that of other DMD-gene products may lead to increased severity of MR.

Extra- and intracellular records were made from rat acute hippocampal slices to examine the effects of partial inhibition of Na⁺-K⁺-ATPases (Na⁺-K⁺pumps) on neuronal hyperexcitability. Bath application of the low-affinity cardiac glycoside, dihydroouabain (DHO), reversibly induced interictal-like epileptiform bursting activity in the CA1 region. Burst-firing was correlated with inhibition of the pumps, which was assayed by changes in [K⁺]_ouptake rates measured with K⁺-ion-sensitive microelectrodes. Large increases in resting [K⁺]_odid not occur. DHO induced a transient depolarization (5–6 mV) followed by a long-lasting hyperpolarization (∼6 mV) in CA1 pyramidal neurons, which was accompanied by a 30% decrease in resting input resistance. Block of an electrogenic pump current could explain the depolarization but not the hyperpolarization of the membrane. Increasing [K⁺]_ofrom 3 to 5.5 mM minimized these transient shifts in passive membrane properties without preventing DHO-induced hyperexcitability. DHO decreased synaptic transmission, but increased the coupling between excitatory postsynaptic potentials and spike firing (E-S coupling). Monosynaptic inhibitory postsynaptic potential (IPSP) amplitudes declined to ∼25% of control at the peak of bursting activity; however, miniature TTX-resistant inhibitory postsynaptic current amplitudes were unaffected. DHO also reduced the initial slope of the intracellular excitatory postsynaptic potential (EPSP) to ∼40% of control. The conductances of pharmacologically isolated IPSPs and EPSPs in high-Ca/high-Mg-containing saline were also reduced by DHO by ∼50%. The extracellular fiber volley amplitude was reduced by 15–20%, suggesting that the decrease in neurotransmission was partly due to a reduction in presynaptic fiber excitability. DHO enhanced a late depolarizing potential that was superimposed on the EPSP and could obscure it. This potential was not blocked by antagonists of NMDA receptors, and blockade of NMDA, mGlu, or GABA_Areceptors did not affect burst firing. The late depolarizing component enabled the pyramidal cells to reach spike threshold without changing the actual voltage threshold for firing. We conclude that reduced GABAergic potentials and enhanced E-S coupling are the primary mechanisms underlying the hyperexcitability associated with impaired Na⁺-K⁺pump activity.