Ultrastructure of synaptosomes from fetal rat brain

Glutamate transporter coupling to Na,K-ATPase

Deactivation of glutamatergic signaling in the brain is mediated by glutamate uptake into glia and neurons by glutamate transporters. Glutamate transporters are sodium-dependent proteins that putatively rely indirectly on Na,K-ATPases to generate ion gradients that drive transmitter uptake. Based on anatomical colocalization, mutual sodium dependency, and the inhibitory effects of the Na,K-ATPase inhibitor ouabain on glutamate transporter activity, we postulated that glutamate transporters are directly coupled to Na,K-ATPase and that Na,K-ATPase is an essential modulator of glutamate uptake. Na,K-ATPase was purified from rat cerebellum by tandem anion exchange and ouabain affinity chromatography, and the cohort of associated proteins was characterized by mass spectrometry. The alpha1-alpha 3 subunits of Na,K-ATPase were detected, as were the glutamate transporters GLAST and GLT-1, demonstrating that glutamate transporters copurify with Na,K-ATPases. The link between glutamate transporters and Na,K-ATPase was further established by coimmunoprecipitation and colocalization. Analysis of the regulation of glutamate transporter and Na,K-ATPase activities was assessed using [(3)H]D-aspartate, [(3)H]L-glutamate, and rubidium-86 uptake into synaptosomes and cultured astrocytes. In synaptosomes, ouabain produced a dose-dependent inhibition of glutamate transporter and Na,K-ATPase activities, whereas in astrocytes, ouabain showed a bimodal effect whereby glutamate transporter activity was stimulated at 1 microm ouabain and inhibited at higher concentrations. The effects of protein kinase inhibitors on [(3)H]D-aspartate uptake indicated the selective involvement of Src kinases, which are probably a component of the Na,K-ATPase/glutamate transporter complex. These findings demonstrate that glutamate transporters and Na,K-ATPases are part of the same macromolecular complexes and operate as a functional unit to regulate glutamatergic neurotransmission.

Degeneration of the amygdala/piriform cortex and enhanced fear/anxiety behaviors in sodium pump alpha2 subunit (Atp1a2)-deficient mice

The sodium pump is the enzyme responsible for the maintenance of Na+ and K+ gradients across the cell membrane. Four isoforms of the catalytic alpha subunit have been identified, but their individual roles remain essentially unknown. To investigate the necessary functions of the alpha2 subunit in vivo, we generated and analyzed mice defective in the alpha2 subunit gene. Mice homozygous for the alpha2 mutation died just after birth and displayed selective neuronal apoptosis in the amygdala and piriform cortex. In these regions, high expression of c-Fos before apoptosis indicated neural hyperactivity, and re-uptake of glutamic acid and GABA into P2 fraction containing crude synaptosome was impaired. These results indicate that the alpha2 subunit plays a critical role regulating neural activity in the developing amygdala and piriform cortex. Further supporting a role of the alpha2 subunit in the function of the amygdala, heterozygous adult mice showed augmented fear/anxiety behaviors and enhanced neuronal activity in the amygdala and piriform cortex after conditioned fear stimuli.

Ontogenesis of neurons immunoreactive for nitric oxide synthase in rat forebrain and midbrain

We studied the immunohistochemical localization of neuronal nitric oxide synthase (nNOS) in the developing rat brain on embryonic days 13 (E13), 15 (E15), 17 (E17) and 19 (E19) and postnatal days 0 (P0), 7 (P7) and 14 (P14). A few neurons positive for nNOS were first detected at E15 in the hypothalamus and pons. At E17, many positive cells became detectable in the thalamus. At E19, the positive cells in these three regions were rapidly increased in number, and a few positive neurons were also observed in such regions as the cerebral cortex and striatum. Positive cells in the hypothalamus tended to locate ventrolaterally. Positive neurons, stained very intensely as in adult rats, were seen in the pedunculopontine tegmental nucleus, laterodorsal tegmental nucleus and parafascicular nucleus. Two weeks after birth, positive neurons of larger somata with many processes were distributed widely in the cerebral cortex and hippocampus. The present study indicates that, in the forebrain and midbrain, the distribution pattern of nNOS-containing neurons is fundamentally completed by E19.

Na+ channel changes in the growth cone and developing nerve terminal

Previous saxitoxin binding studies indicated two forms of the sodium channel in the fetal rat brain; a low-affinity precursor located in an internal membrane compartment, present exclusively in growth cones and a high-affinity mature form present in the plasmalemma of growth cones and characteristic of synapses. This raises the questions (1) of the presence or absence of the beta2 subunit in these channel forms and (2) of the developmental regulation of the the beta2 subunit. Antibodies against the alpha and beta2 channel subunits were used to probe Western blots of subcellular fractions from rat brains at embryonic day 18 (E18), pups at postnatal (P) days 7-25, and adults, as well as purified sodium channels from adult brain. In both synaptosomes and the purified sodium channel the beta2 antibody recognized the expected band at 38 kDa under reducing conditions. However, in contrast to the alpha subunit, this band was absent at E18 and became apparent only from P7 onwards. At the earlier time intervals a very prominent immunoreactive band of unknown identity was evident at 260--300kDa, which declined in intensity concomitant with the appearance of the 38 kDa beta2 band. These data indicate that beta2 subunits are regulated independently from alpha subunits, are absent in differentiating neurons, and hence are not necessary for insertion of the sodium channel into the plasmalemma, at least during early development of the neuron.

Nerve growth cones isolated from fetal rat brain: subcellular fractionation and characterization

The biochemical and functional characterization of the nerve growth cone is of major interest for studies on mechanisms involved in nervous system development. We describe the isolation from fetal brain of membrane-bound fragments of nerve growth cones by density gradient fractionation. These so-called growth cone particles are highly uniform and identifiable on the basis of their organelle complement. Furthermore, they co-purify in mixing experiments with fragments of radiolabeled and light microscopically identified nerve growth cones from primary cultures. The possibility of isolating growth cone fragments in quantity renders feasible the analysis of molecular mechanisms involved in growth cone function.