Single molecule tracking of AMPA receptors shows the role of synaptic insertion during maintenance of chemical LTP

Long term potentiation (LTP) likely contributes to memory formation. Early expression of LTP involves insertion of AMPA receptors (AMPARs) to the extrasynaptic membrane followed by their lateral diffusion into the synaptic membrane. However, whether a similar mechanism mediates the maintenance of LTP is unclear. Using single-molecule microscopy, we quantified that 6 GluA1- and 11 GluA2-containing endogenous AMPARs were added per synapse in cultured hippocampal neurons at 20 min following chemical LTP (cLTP) induction for 10 min, resulting in a 54% increase for both subunits. Single molecular tracking of transfected subunits revealed that the number of exocytosed subunits at the synapse increased by 15-18% from 5 to 20 min following cLTP induction, but their lateral exchange between synaptic and extrasynaptic membranes was minimal. These findings suggest that cLTP maintenance is contributed largely by synaptic insertion of AMPARs rather than the surface diffusion of exocytosed AMPARs from extrasynaptic to synaptic regions.

Sarcospan reduces dystrophic pathology: stabilization of the utrophin–glycoprotein complex

Mutations in the dystrophin gene cause Duchenne muscular dystrophy and result in the loss of dystrophin and the entire dystrophin–glycoprotein complex (DGC) from the sarcolemma. We show that sarcospan (SSPN), a unique tetraspanin-like component of the DGC, ameliorates muscular dystrophy in dystrophin-deficient mdx mice. SSPN stabilizes the sarcolemma by increasing levels of the utrophin–glycoprotein complex (UGC) at the extrasynaptic membrane to compensate for the loss of dystrophin. Utrophin is normally restricted to the neuromuscular junction, where it replaces dystrophin to form a functionally analogous complex. SSPN directly interacts with the UGC and functions to stabilize utrophin protein without increasing utrophin transcription. These findings reveal the importance of protein stability in the prevention of muscular dystrophy and may impact the future design of therapeutics for muscular dystrophies.

Participation of voltage-gated conductances on the response succeeding inhibitory synaptic potentials in the crayfish slowly adapting stretch receptor neuron

1. We examined the contribution of voltage-gated conductances to inhibitory postsynaptic potential (IPSP) effects under current clamp in silent and spiking slowly adapting stretch receptor neurons (SN1s) in the slow receptor muscle of the crayfish Procambarus. The receptor exemplifies the simplest inhibitory neural circuit, with one presynaptic and one postsynaptic neuron. The effects of synaptic inhibition were compared with the outcome of hyperpolarizing current pulses. Because pulse effects were exclusively due to postsynaptic mechanisms, an estimation of the synaptic or extrasynaptic origin of the results of IPSP was possible. 2. Inhibition by single IPSPs increased gradually with the time elapsed from the preceding spike in 60% of the spiking SN1s. However, early IPSP arrivals were exclusively excitatory in the rest of the cases. Inhibition was restricted to a single expanded SN1 interspike interval, but the early excitation and the postinhibitory rebound lasted several intervals. Rebound was invariably present; it was the only consequence of IPSPs in silent receptors and could be extremely long lasting (> 25 s). 3. The membrane potential of the SN1 neuron was clamped at hyperpolarized values (greater than -65 mV) by prolonged IPSP barrages at high rate (> 20/s). A prominent depolarizing sag and a gradual reduction of the IPSP amplitude were observed with prolonged presynaptic stimulation. There were subthreshold IPSP amplitude oscillations consisting of gradual increases and decreases of the post-IPSP peak depolarization at lower presynaptic rates. IPSP amplitude variations (< or = 10 mV) were primarily due to larger local responses. 4. Essentially all IPSP effects were mimicked by hyperpolarizing pulses. Sag was also evoked by pulses and was accompanied by a gradual conductance increase preceded by a brief initial drop. Sag and rebound were markedly reduced by Cs+ (2 mM) and tetrodotoxin (1 microM) and less by Ba2+ (5 mM) or tetraethylammonium (25 mM) superfusion. Both were somewhat decreased by acetylcholine (30 microM), which also markedly depolarized and accelerated firings, results which were usually reduced by atropine (10 microM). 5. In conclusion, IPSP and hyperpolarizing pulse effects were essentially identical, implying that extrasynaptic membrane properties were decisive. Interestingly, net excitatory consequences were usual, effectively increasing sensitivity and reducing the sensory threshold. Pharmacological evidence is provided suggesting that the hyperpolarization-activated current, IQ, and also probably the K+ M-current, the A-current, and the low-threshold, persistent Na+ conductances participate in sag and rebound genesis.(ABSTRACT TRUNCATED AT 400 WORDS)