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The majority of massive disk galaxies in the local Universe show a stellar barred structure in their central regions, including our Milky Way 1,2 . Bars are supposed to develop in dynamically cold stellar disks at low redshift, as the strong gas turbulence typical of disk galaxies at high redshift suppresses or delays bar formation 3,4 . Moreover, simulations predict bars to be almost absent beyond z = 1.5 in the progenitors of Milky Way-like galaxies 5,6 . Here we report observations of ceers-2112, a barred spiral galaxy at redshift z phot  ≈ 3, which was already mature when the Universe was only 2 Gyr old. The stellar mass (M ★  = 3.9 × 10 9  M ⊙ ) and barred morphology mean that ceers-2112 can be considered a progenitor of the Milky Way 7-9 , in terms of both structure and mass-assembly history in the first 2 Gyr of the Universe, and was the closest in mass in the first 4 Gyr. We infer that baryons in galaxies could have already dominated over dark matter at z ≈ 3, that high-redshift bars could form in approximately 400 Myr and that dynamically cold stellar disks could have been in place by redshift z = 4-5 (more than 12 Gyrs ago) 10,11 ., (© 2023. The Author(s).)

Neural circuits in the cerebral cortex are shaped by experience during "critical periods" early in life. For example, visual cortex is immature at the time of eye opening and gradually develops its functional properties during a sensitive period. Very few reports have addressed the role of intrinsic neural activity in cortical maturation. Here we have exploited the bacterial enzyme botulinum neurotoxin E (BoNT/E) to produce a unilateral, reversible blockade of neural activity in rat visual cortex during the sensitive period. BoNT/E is a highly selective protease that interferes with transmitter release via cleavage of the synaptic protein SNAP-25 (synaptosomal-associated protein of 25 kDa). Unilateral, intracortical injections of BoNT/E were made at the time of eye opening and resulted in the silencing of the treated, but not contralateral, hemisphere for a period of 2 weeks. We found that visual acuity was permanently reduced in the blocked hemisphere, and the critical period for ocular dominance plasticity persisted into adulthood. Unexpectedly, these effects extended equally to the contralateral, uninjected side, demonstrating a fundamental role for interhemispheric connections in cortical maturation.

Neurogenesis continues to occur in the adult mammalian hippocampus and is regulated by both genetic and environmental factors. It is known that exposure to an enriched environment enhances the number of newly generated neurons in the dentate gyrus. However, the mechanisms by which enriched housing produces these effects are poorly understood. To test a role for neurotrophins, we used heterozygous knockout mice for brain-derived neurotrophic factor (BDNF+/-) and mice lacking neurotrophin-4 (NT-4-/-) together with their wild-type littermates. Mice were either reared in standard laboratory conditions or placed in an enriched environment for 8 weeks. Animals received injections of the mitotic marker bromodeoxyuridine (BrdU) to label newborn cells. Enriched wild-type and enriched NT-4-/- mice showed a two-fold increase in hippocampal neurogenesis as assessed by stereological counting of BrdU-positive cells in the dentate gyrus and double labelling for BrdU and the neuronal marker NeuN. Remarkably, this enhancement of hippocampal neurogenesis was not seen in enriched BDNF+/- mice. Failure to up-regulate BDNF accompanied the lack of a neurogenic response in enriched BDNF heterozygous mice. We conclude that BDNF but not NT-4 is required for the environmental induction of neurogenesis.

Botulinum neurotoxins (BoNTs) are metalloproteases which act on nerve terminals and cause a long-lasting inhibition of neurotransmitter release. BoNTs act by cleaving core proteins of the neurotransmitter release machinery, namely the SNARE (soluble NSF-attachment receptors) proteins. The action of BoNTs in the peripheral nervous system (PNS) has been extensively documented, and knowledge gained in this field laid the foundations for the use of BoNTs in human disorders characterized by hyperfunction of peripheral nerve terminals. Much less is known about the action of BoNTs on the central nervous system (CNS). In vitro studies have demonstrated that BoNTs can affect the release of several neurotransmitters from central neurons. Recent studies have provided the first characterization of the effects of BoNT/E on CNS neurons in vivo. It has been shown that BoNT/E injected into the rat hippocampus inhibits glutamate release and blocks spike activity of pyramidal neurons. Intrahippocampal injection of BoNT/E resulted in significant inhibition of seizure activity in experimental models of epilepsy, suggesting a potential therapeutic use of BoNTs in the CNS.

Experimental studies suggest that the delivery of antiepileptic agents into the seizure focus might be of potential utility for the treatment of focal-onset epilepsies. Botulinum neurotoxin E (BoNT/E) causes a prolonged inhibition of neurotransmitter release after its specific cleavage of the synaptic protein synaptosomal-associated protein of 25 kDa (SNAP-25). Here, we show that BoNT/E injected into the rat hippocampus inhibits glutamate release and blocks spike activity of pyramidal neurons. BoNT/E effects persist for at least 3 weeks, as determined by immunodetection of cleaved SNAP-25 and loss of intact SNAP-25. The delivery of BoNT/E to the rat hippocampus dramatically reduces both focal and generalized kainic acid-induced seizures as documented by behavioral and electrographic analysis. BoNT/E treatment also prevents neuronal loss and long-term cognitive deficits associated with kainic acid seizures. Moreover, BoNT/E-injected rats require 50% more electrical stimulations to reach stage 5 of kindling, thus indicating a delayed epileptogenesis. We conclude that BoNT/E delivery to the hippocampus is both antiictal and antiepileptogenic in experimental models of epilepsy.

The transverse tubular system (T-system) of skeletal muscle links surface membrane action potential and release of activator calcium from sarcoplasmic reticulum (SR). The spread of depolarization along this system has been studied in voltage-clamped frog muscle fibers by using the spread of contractile activation in a thin optical cross section through the fiber center as an index. In tetrodotoxin treated fibers as depolarization of the fiber is increased contraction spreads from superficial to axial myofibrils. In tetrodotoxin free fibers the radial gradient of activation is reversed indicating that normally the activating signal is propagated along the T-system. Activation across the SR-T tubule junction does not appear to trigger an all-or-none response from the SR,-Costantin, L.L. Electrical properties of the transverse tubular system.

Contractile activation was studied in frog single muscle fibers treated with tetrodotoxin to block action potentials. The membrane potential in a short segment of the fiber was controlled with a two-electrode voltage clamp, and the contractile response of superficial myofibrils in this segment was observed microscopically. The strength-duration relation for contractile activation was similar to that reported by Adrian, Chandler, and Hodgkin (1969); at 3.9 degrees C, the contraction threshold was -44 mV for long depolarizing pulses (100-ms) and increased to +64 mV for 2-ms depolarizations. Hyperpolarizing postpulses shifted the threshold for 2-ms pulses to more positive values, and a similar, but smaller, effect was produced by hyperpolarizing prepulses. The decay of excitability following subthreshold pulses showed two apparently distinct components; at 3.9 degrees C, excitability fell to 50% of its initial value within 4 ms, while the subsequent decline of excitability proceeded with a half-time of about 20 ms.

1. Skinned muscle fibres were prepared from the tonus bundle of the frog iliofibularis muscle and the contractile response elicited by applied calcium ions was studied. The fibre type was determined by electron microscopy.2. Fast fibres shortened many times more rapidly than slow fibres, indicating that the slow contraction of slow fibres is an inherent property of the contractile mechanism.3. The extent of spread of contraction following local calcium application was much greater in slow than in fast fibres, a difference which is consistent with the relative sparsity of the sarcoplasmic reticulum in slow fibres.4. The ability of the sarcoplasmic reticulum of slow fibres to accumulate calcium was demonstrated by the in situ immobilization of calcium when oxalate solutions were added to the skinned fibre.

Potassium-induced contractures were studied in single fibers from the semitendinosus muscle of Rana pipiens. Contractures elicited by solutions containing 60-117 mM potassium and 120 mM chloride were biphasic, consisting of a rapid initial contraction with a duration at 23 degrees C of less than 1 sec followed by a slow response with a duration of many seconds. At 13 degrees C, the initial response was greatly prolonged so that the two responses virtually fused into a single smooth contracture. Membrane potential in high potassium, high chloride solutions underwent a transient peak depolarization, probably as a result of time-dependent changes in membrane conductance during depolarization. It is proposed that this complex time course of depolarization gives rise to the biphasic contracture response.

The membrane potential of isolated muscle fibers was controlled with a two-electrode voltage clamp, and the radial extent of contraction elicited by depolarizing pulses of increasing magnitude was observed microscopically. Depolarizations of the fiber surface only 1-2 mv greater than the contraction threshold produced shortening throughout the entire cross-section of the muscle fiber. The radial spread of contraction was less effective in fibers exposed to tetrodotoxin or to a bathing medium with a greatly reduced sodium concentration. The results provide evidence that depolarization of a muscle fiber produces an increase in sodium conductance in the T tubule membrane and that the resultant sodium current contributes to the spread of depolarization along the T system.

1. The membrane potential of isolated muscle fibres in solutions containing tetrodotoxin (TTX) was controlled with a two-electrode voltage clamp. The striation pattern in the region of the electrodes was observed microscopically.2. With square steps of depolarization of increasing magnitude, contraction occurs first in the myofibrils just beneath the surface membrane, and then spreads inwards towards the axis of the fibre as the depolarization is increased.3. From the depolarizations which make the superficial and axial myofibrils contract it is possible to estimate a space constant (lambda(T)) for electrotonic spread in a transverse tubular network.4. lambda(T) was found to vary with fibre radius; for a 50 mu fibre it was about 60 mu. lambda(T) was not greatly affected by tetraethylammonium (TEA) chloride (111 mM), or by sucrose substitution of most of the sodium chloride in the Ringer solution.5. The ratio of the depolarization threshold for contraction of surface myofibrils and of central myofibrils was smaller for short (3 msec) than for long depolarization.6. Action potentials, recorded from a sartorius fibre, were used as the command signal for the voltage-clamped fibre in tetrodotoxin. The central myofibrils of this fibre did not appear to contract unless the imposed ;action potentials' were of normal size.7. The passive electrical characteristics of the transverse tubular system will just allow an action potential, at room temperature, to activate the myofibrils at the centre of a frog muscle fibre. An active potential change would be required to achieve a safety factor appreciably greater than one for this process.

When frog muscle fibers from which the sarcolemma had been dissected away were perfused with a calcium solution and then treated with oxalate, electron-opaque material, probably calcium oxalate, accumulated in the terminal sacs of the sarcoplasmic reticulum. These regions of calcium accumulation were identified with the intracellular calcium sink that controls the relaxation phase of the contraction-relaxation cycle; their proximity to tubules implicated in intracellular stimulus conduction suggests that they might also be regions from which calcium is released to trigger contraction.

1. The effect of extracellular calcium and magnesium on the contraction threshold and on the thresholds for an increase in sodium and potassium conductance with depolarization was studied in voltage-clamped frog muscle fibres.2. A larger depolarization was required to reach each of the three thresholds when the concentration of divalent cation was increased.3. The contraction and potassium conductance thresholds appeared to shift in parallel with alterations in calcium over the concentration range 0.2-10.0 mM and in magnesium over the concentration range 5.4-90.0 mM. The shift amounted to about 4 mV for a threefold change in concentration of divalent cation.4. The sodium conductance threshold was much more sensitive to alterations in divalent cation concentration than was either the contraction or the potassium conductance threshold.

The membrane potential of frog single muscle fibers in solutions containing tetrodotoxin was controlled with a two-electrode voltage clamp. Local contractions elicited by 100-ms square steps of depolarization were observed microscopically and recorded on cinefilm. The absence of myofibrillar folding with shortening to striation spacings below 1.95 microm served as a criterion for activation of the entire fiber cross section. With depolarizing steps of increasing magnitude, shortening occurred first in the most superficial myofibrils and spread inward to involve axial myofibrils as the depolarization was increased. In contractions in which the entire fiber cross section shortened actively, both the extent of shortening and the velocity of shortening at a given striation spacing could be graded by varying the magnitude of the depolarization step. The results provide evidence that the degree of activation of individual myofibrils can be graded with membrane depolarization.

"Skinned" muscle fibers, single fibers from the frog semitendinosus muscle in which the sarcolemma had been removed, could be reversibly activated by electrical stimulation. Electrical responsiveness was abolished when the skinned fiber was prepared from a muscle exposed to a cardiac glycoside, and the development of responsiveness was delayed when the muscle was bathed in high potassium solution. The findings were taken as evidence that active sodium-potassium exchange across the internal membranes restored electrical excitability, after the sarcolemma had been removed, by establishing a potential gradient across the internal membranes. In general, the contractions were graded with the strength of the applied current. On occasion, however, "all-or-none" type responses were seen, raising the possibility that the internal membranes were capable of an electrically regenerative response. Activation could also be produced by an elevation of the intracellular chloride ion concentration or a decrease in the intracellular potassium, ion concentration, suggesting that depolarization of some element of the internal membrane system, that is, a decrease in the potential of the lumen of the internal membrane system relative to the potential of the myofibrillar space, was responsible for activation in these experiments. The distribution of both the electrically induced contractions and those produced by changes in the intracellular ion concentrations indicated that the responsive element of the internal membrane system was electrically continuous over many sarcomeres.