SUMMARY1. Outward directed membrane currents have been studied in voltage clamp experiments on isolated neural somata of the marine gastropod Anisodoris.2. Stepping the membrane potential from a hyperpolarized level to a value in the neighbourhood of resting potential (-35 to-50 mV at 50 C) results in an outward current transient, IA, which is apparently carried by potassium ions.3. The peak amplitude of IA is dependent upon both the holding voltage level and the test step voltage while the time courses of development and decay are independent of, or only slightly dependent on, these parameters.4. The developing and decaying phases Of IA are approximated by exponentials, leading to time constants for development of 10-25 msec and for decay of 220-600 msec over the aggregate of cells studied (data at 5°C). Q10 for the processes is approximately 3.5. It is concluded that the transport mechanism for IA is at least operationally distinct from the mechanism underlying delayed outward current, IK
Pathfinding by growing nerve processes in the developing nervous system depends on the turning response of the growing tip, the growth cone, to extracellular guidance cues. There is evidence in vivo and in cell culture that some growth cones exhibit chemotropic behaviour, but the identity of endogenous chemoattractants remains elusive. Neurotransmitters appear early in the developing embryo and may have morphogenic roles in development. In cell culture a number of neurotransmitters were found to induce growth inhibition or retraction of neurites. Here we report positive turning responses of the nerve growth cone in a defined extracellular gradient of the neurotransmitter acetylcholine (ACh). The growth cone response depends on the activation of neuronal nicotinic ACh receptors, requires the presence of extracellular Ca2+, and appears to be mediated by Ca(2+)-calmodulin-dependent protein kinase II. Fluorescence imaging of cytosolic Ca2+ concentration ([Ca2+]i) at the growth cone showed a small but significant evaluation of [Ca2+]i within minutes of the onset of ACh application and before the turning of the growth cone. These findings suggest that neurotransmitters may serve as specific chemoattractants for growth cone guidance and that cytosolic Ca2+ may act as a second messenger in the cytoplasm of the growth cone to initiate the turning response.
Testing of 1701 water wells in northeastern Pennsylvania shows that methane is ubiquitous in groundwater, with higher concentrations observed in valleys vs. upland areas and in association with calcium-sodium-bicarbonate, sodium-bicarbonate, and sodium-chloride rich waters—indicating that, on a regional scale, methane concentrations are best correlated to topographic and hydrogeologic features, rather than shale-gas extraction. In addition, our assessment of isotopic and molecular analyses of hydrocarbon gases in the Dimock Township suggest that gases present in local water wells are most consistent with Middle and Upper Devonian gases sampled in the annular spaces of local gas wells, as opposed to Marcellus Production gas. Combined, these findings suggest that the methane concentrations in Susquehanna County water wells can be explained without the migration of Marcellus shale gas through fractures, an observation that has important implications for understanding the nature of risks associated with shale-gas extraction.
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