Early postnatal changes in presynaptic potassium sensitivity

Spontaneous muscle action potentials fail to develop without fetal-type acetylcholine receptors

In mammals, two combinations of muscle nicotinic acetylcholine receptors (AChRs) are used: alpha2betagammadelta (gamma-AChR) or alpha2betaepsilondelta (epsilon-AChR). After birth, gamma-AChRs are replaced by epsilon-AChRs (gamma/epsilon-switch). The two receptors have different conductances and open times. During perinatal period, the long open time gamma-AChRs generate random myofiber action potentials from uniquantal miniature end-plate potentials (mEPPs). epsilon-AChRs are suitable for strong adult muscle activities. Since the effect of the gamma/epsilon-switch on neuromuscular development was unclear, despite the many differences in channel characteristics, we carried out this study to generate gamma-subunit-deficient mice. Homozygotes born alive survived for 2 days in a stable condition, and were able to move their forelimbs. Endplate AChRs included epsilon-subunits, and muscle fibers had multiple neuromuscular junctions. Both pre- and postsynapses were abnormal and spontaneous action potentials generated from mEPPs were totally absent. Results suggest a requirement for gamma-AChRs in mediating synaptically-induced action potential activity critical for neuromuscular development.

Plasticity and stabilization of neuromuscular and CNS synapses: interactions between thrombin protease signaling pathways and tissue transglutaminase

The first association of the synapse as a potential site of neurodegenerative disease burden was suggested for Alzheimer's disease (AD) almost 30 years ago. Since then protease:protease inhibitor (P:PI) systems were first linked to functional regulation of synaptogenesis and synapse withdrawal at the neuromuscular junction (NMJ) more than 20 years ago. Confirmatory evidence for the involvement of the synapse, the rate-limiting or key unit in neural function, in AD did not become clear until the beginning of the 1990s. However, over the past 15 years evidence for participation of thrombin, related serine proteases and neural PIs, homologous and even identical to those of the plasma clot cascade, has been mounting. Throughout development a balance between stabilization forces, on the one hand, and breakdown influences, on the other, becomes established at synaptic junctions, just as it does in plasma clot proteins. The formation of protease-resistant cross-links by the transglutaminase (TGase) family of enzymes may add to the stability for this balance. The TGase family includes coagulation factor XIIIA and 8 other different genes, some of which may also influence the persistence of neural connections. Synaptic location of protease-activated, G-protein-coupled receptors (PARs) for thrombin and related proteases, their serpin and Kunitz-type PIs such as protease nexin I (PNI), alpha1-antichymotrypsin (alpha-ACT), and the Kunitz protease inhibitor (KPI)-containing secreted forms of beta-amyloid protein precursor (beta-APP), along with the TGases and their putative substrates, have all been amply documented. These findings strongly add to the conclusion that these molecules participate in the eventual structural stability of synaptic connections, as they do in coagulation cascades, and focus trophic activity on surviving terminals during periods of selective contact elimination. In disease states, this imbalance is likely to be shifted in favor of destabilizing forces: increased and/or altered protease activity, enhanced PAR influence, decreased and/or altered protease inhibitor function, reduction and/or alteration in tTG expression and activity, and alteration in its substrate profile. This imbalance further initiates a cascade of events leading to inappropriate programmed cell death and may well be considered evidence of synaptic apoptosis.

Changes in the endplate accumulation of acetylcholinesterase during synapse elimination in the mouse

Focal accumulations of acetylcholinesterase (AChE; EC 3.1.1.7), cholinesterase (ChE, EC 3.1.1.8) and total cholinesterase (TChE; AChE+ChE) were examined in developing mouse diaphragm by using a modified Karnovsky/Roots staining method. The lengths of TChE and AChE reaction product accumulations reached significant peaks on postnatal day (PD) 1 (P < 0.05), decreased to a minimum on PD 9 and then increased in proportion to muscle fiber diameter (PD 9 to adult). The normalized area of accumulation (area of accumulation/fiber diameter) for AChE and TChE also decreased by 19% (P < 0.05) between PD 3 and PD 7. In contrast, ChE focal accumulation did not decrease during the period of synapse elimination, but rather increased in proportion to the postnatal growth of the muscle fiber. These results suggest that AChE is more sensitive to neurotrophic influences than ChE; particularly during late embryonic and early postnatal periods of synapse elimination.

Calcium dependency and tetrodotoxin sensitivity of neostriatal dopamine release in 5-day-old and adult rats as measured by in vivo microdialysis

The calcium dependency and tetrodotoxin sensitivity of extracellular dopamine levels were assessed by microdialysis in the neostriatum of 5-day-old rat pups and were compared with those obtained in adult rats. The removal of calcium from the dialysate reduced spontaneous levels of extracellular dopamine to 20% of normal in the 5-day-old pups and to 10% of normal in the adults. Calcium-free dialysate also decreased potassium-evoked dopamine release to approximately 20% of baseline in both ages. Furthermore, the addition of tetrodotoxin to the dialysate decreased spontaneous levels of extracellular dopamine to 10% of baseline in both ages. The effects of calcium removal and the addition of tetrodotoxin on extracellular levels of the dopamine metabolite 3,4-dihydroxyphenylacetic acid were less pronounced. The results of this study demonstrate that extracellular levels of dopamine sampled by microdialysis in rats as young as 5 days of age are both calcium dependent and tetrodotoxin sensitive; thus, they are derived from neuronal activity and not from injury caused by acute implantation of the probe. Other age-related differences support the hypothesis that dopamine release and turnover is greater in immature rats and may represent a form of compensation for incomplete dopamine nerve terminal ingrowth.

The development of D2 autoreceptor-mediated modulation of K(+)-evoked dopamine release in the neostriatum

A within-subject dose-response analysis was conducted by locally perfusing increasing concentrations (0.1, 1, 10 and 100 microM) of the selective D2 agonist quinpirole via a microdialysis probe into the neostriatum of urethane-anesthetized rat pups 5, 10-11, 15-16 and 21-22 days of age and adult rats. In Expt. 1, K(+)-evoked dopamine release was significantly decreased by quinpirole relative to the vehicle control group for each age in a dose-dependent manner. The maximum effect of quinpirole was not influenced by acute tolerance or the length of the experiment (Expt. 2). Finally, the effect of quinpirole (10 microM) was blocked by the addition of the selective D2 antagonist (-)-sulpiride (100 microM) to the perfusion solution (Expt. 3). These results support and extend previous research that suggests that presynaptic D2 autoreceptors in the neostriatum are able to modulate K(+)-evoked dopamine release in vivo by postnatal day 5 in the rat.

The ontogeny of apomorphine-induced alterations of neostriatal dopamine release: effects on potassium-evoked release

The effects of apomorphine (0.05, 0.1, and 1.0 mg/kg, s.c.) on K(+)-evoked dopamine release were studied through the use of in vivo microdialysis in the neostriatum of developing and adult rats. Fifteen-minute samples were collected from urethane-anesthetized rats 5, 10-11, 21-22, 35-36 days of age, and adults, and quantified by high performance liquid chromatography with electrochemical detection. Apomorphine attenuated K(+)-evoked dopamine release in all age groups, suggesting that the dopamine autoreceptor modulating release in the neostriatum is functional by 5 days of age. A dose-response effect of apomorphine was observed in all age groups except at 5 and 10 days of age. Absolute levels of extracellular dopamine were significantly lower at 5 and 10 days of age compared with the other ages, and the effectiveness of a high-K+ artificial cerebrospinal fluid to evoke dopamine release increased with age.