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P2X2receptor currents are potentiated by acidic pH and zinc. To identify residues necessary for proton and zinc modulation, alanines were singly substituted for each of the nine histidines in the extracellular domain of the rat P2X2receptor. Wild‐type and mutant receptors were expressed in Xenopusoocytes and analysed with two‐electrode voltage clamp. All mutations caused less than a 2‐fold change in the EC50of the ATP concentration‐response relation. Decreasing the extracellular pH from 7.5 to 6.5 potentiated the responses to 10 μmATP of wild‐type P2X2and eight mutant receptors more than 4‐fold, but the response of the mutant receptor H319A was potentiated only 1.4‐fold. The H319A mutation greatly attenuated the maximal potentiation that could be produced by a drop in pH, shifted the pKa(‐log of dissociation constant) of the potentiation to a more basic pH as compared with P2X2and revealed a substantial pH‐dependent decrease in the maximum response with a pKanear 6.0. Substituting a lysine for H319 reduced the EC50for ATP 40‐fold. Zinc (20 μm) potentiated the responses to 10 μmATP of wild‐type P2X2and seven histidine mutants by ∼8‐fold but had virtually no effect on the responses of two mutants, H120A and H213A. Neither H120A nor H213A removed the voltage‐independent inhibition caused by high concentrations of zinc. The observation that different mutations selectively eliminated pH or zinc potentiation implies that there are two independent sites of action, even though the mechanisms of pH and zinc potentiation appear similar.

Basic helix-loop-helix (bHLH) transcription factors are known to function during mammalian neurogenesis. Here we show that transient transfection of vectors expressing neuroD2, MASH1, ngn1 or related neural bHLH proteins, with their putative dimerization partner E12, can convert mouse P19 embryonal carcinoma cells into differentiated neurons. Transfected cells express numerous neuron-specific proteins, adopt a neuronal morphology and are electrically excitable. Thus, the expression of neural bHLH proteins is sufficient to confer a neuronal fate on uncommitted mammalian cells. Neuronal differentiation of transfected cells is preceded by elevated expression of the cyclin-dependent kinase inhibitor p27Kip1 and cell cycle withdrawal. This demonstrates that the bHLH proteins can link neuronal differentiation to withdrawal from the cell cycle, possibly by activating the expression of p27Kip1. The ability to generate mammalian neurons by transient expression of neural bHLH proteins should create new opportunities for studying neurogenesis and devising neural repair strategies.

1The ATP receptor subunit P2X2was expressed in Xenopusoocytes and human embryonic kidney (HEK) 293 cells. ATP‐activated currents were studied with two‐electrode voltage clamp recordings from oocytes, whole‐cell recordings from HEK 293 cells, and outside‐out patch clamp recordings from both cell types. The steady‐state current‐voltage (I‐V) relation showed profound inward rectification in all recording configurations.2Recordings from outside‐out patches demonstrated that inward rectification does not require intracellular Mg2+or polyamines, and that inward rectification was present when the same solution was used on both sides of the patch.3Voltage jump experiments were performed to evaluate the voltage dependence of channel gating. After fast voltage jumps, instantaneous current jumps were followed by substantial relaxations to the steady state. The time course of the current relaxations could be fitted by single exponential functions. The instantaneous I‐Vrelation was less inwardly rectifying than the steady‐state I‐Vrelation; however, it was not linear.4Single channel recordings indicated that the single channel conductance became smaller when the membrane potential became more positive. This decrease could quantitatively account for inward rectification of the instantaneous I‐Vrelation.5We conclude that inward rectification of P2X2is due to two mechanisms: voltage‐dependent gating and voltage dependence of the single channel conductance.

Skeletal muscles in developing chick embryos were tested for responsiveness to adenosine 5′-triphosphate (ATP), a substance known to depolarize chick skeletal muscle in culture. The sensitivity to extracellular ATP was tested at various stages of development in five different muscles; pectoralis superficia, anterior latissimus dorsi, posterior latissimus dorsi, sartorious, and gastrocnemius. At the earliest time that muscles were tested (Embryonic Day 6, stage 30 of Hamburger and Hamilton, 1951) application of ATP (50–100 μM) elicited vigorous contractions in all five muscles, but within a few days (Embryonic Day 17, stage 43) none of the muscles contracted in response to ATP. Sensitivity declined at approximately the same time in all five of these muscles. Intracellular recordings made from muscle fibers near the time of hatching (Embryonic Days 18–21 or Postnatal Days 1–2) indicated that the loss of the ability to contract in response to ATP was due to the total loss of responsiveness to ATP. Surgical denervation of the anterior latissimus dorsi and posterior latissimus dorsi was performed in a series of chicks 1–2 days after hatching, and the ability of these muscles to contract in response to ATP was tested 3–10 days after the surgery. Contractions in response to ATP were present in many of the muscles. Thus denervation of muscles in newly hatched chicks led to the reappearance of sensitivity to ATP. The disappearance of ATP responsiveness shortly after muscles become innervated and the reappearance of ATP responsiveness following denervation suggest that the expression of ATP responsiveness is regulated by motor neurons.

The time course of desensitization of P2X2receptors expressed inXenopusoocytes and HEK 293 cells was examined. We found that there was virtually no desensitization in response to 5 μM ATP, but that responses to 50 μM ATP desensitized with a highly variable time course. The time constant of desensitization varied 250fold among oocytes (from 4 to over 1000 seconds) and 10 fold among HEK cells (from 4 to 40 seconds). Mutation of D349, which lies at the cytoplasmic end of the second transmembrane domain (to either A, N, or R), resulted in a dramatic acceleration of the median rate of desensitization, and eliminated variation in the rate of desensitization. In contrast, mutation of four other charged residues in putative transmembrane domains had no effect on desensitization. These results suggest that P2X2receptor desensitization is strongly modulated by the intracellular environment, and that D349 is essential for this modulation.

When the euryhaline decapod Carcinus maenasis transferred from sea water to a medium more dilute than 75% sea water, the frequency of heart beat increases. The cardioacceleratory response is probably mediated by external rather than internal receptors, and the parameter that is sensed in neither the total osmotic strength of the medium, nor the concentration of any single ion. Although it is not yet determined whether the heart responds to a hormonal or nervous signal, it is likely that the control mechanisms are similar to those which mediate the water permeability changes in Carcinusand other euryhaline crabs.In somewhat more than half of the crabs tested, the frequency of scaphognathite beating is markedly reduced when the animal is exposed to dilute external medium.

Australia and New Zealand are adopting the Spatial Data Transfer Standard (SDTS) as their transfer standard for geographic data. The standard requires a number of modifications to suit Australia/New Zealand requirements. These modifications primarily involve coordinate reference systems for each country, references to those standards applicable to each country and new spatial feature dictionaries. For other countries adopting SDTS, future revisions to the standard should emphasize a framework for required modifications. Australia/New Zealand have established a support body to ensure the smooth introduction of the standard within these countries. This commercial venture has been successful in promoting the standard, in providing training and in related consulting work. The US Geological Survey has been the maintenance authority for the standard. It is essential that this function continues to be provided through this body to guarantee a single interpretation of the standard.

In the ciliary ganglion of adult rabbits, ganglion cells lacking dendrites are generally innervated by a single axon, whereas cells with one or more dendrites are innervated by a number of different axons that increases in proportion to the complexity of their dendritic arbor1(see also refs 2–4). We have now explored the basis of this correlation by comparing the dendritic arborizations of cells receiving different numbers of axons during and after the period of synapse elimination that occurs early in postnatal life5(see also ref. 6). Our results suggest that the geometry of neonatal neurones influences the competitive interaction between the several axons that initially innervate the same cell. This finding in turn implies that an important function of dendrites is to regulate the number of different axons that ultimately innervate each neurone.

Profiles Revlon's cover model and host of the television program `House of Style,' Daisy Fuentes. Background information on Fuentes' career; How Fuentes was introduced to television; Background information on Fuentes.