Nonstationary Internal Tides Observed Using Dual-Satellite Altimetry

Dual-satellite crossover data from the Jason-2 and Cryosat-2 altimeter missions are used in a novel approach to quantify stationary and nonstationary tides from time-lagged mean square sea surface height (SSH) differences, computed for lags from 1 to 1440 h (60 days). The approach is made feasible by removing independent estimates of the stationary tide and mesoscale SSH variance, which greatly reduces the sampling error of the SSH statistics. For the semidiurnal tidal band, the stationary tidal variance is approximately 0.73 cm2, and the nonstationary variance is about 0.33 cm2, or 30% of the total. The temporal correlation of the nonstationary tide is examined by complex demodulation and found to be oscillatory with first 0 crossing at 400 h (17 days). Because a significant fraction of the time-variable mesoscale signal is resolved at time scales of roughly 150 h by the present constellation of satellite altimeters, the results suggest that it may be feasible to predict the nonstationary tide from modulations of the resolved mesoscale, thus enhancing the efficacy of tidal corrections for planned wide-swath altimeters such as the Surface Water and Ocean Topography (SWOT) mission.

Contribution of Single-Channel Properties to the Time Course and Amplitude Variance of Quantal Glycine Currents Recorded in Rat Motoneurons

Contribution of single-channel properties to the time course and amplitude variance of quantal glycine currents recorded in rat motoneurons. The amplitude of spontaneous, glycinergic miniature inhibitory postsynaptic currents (mIPSCs) recorded in hypoglossal motoneurons (HMs) in an in vitro brain stem slice preparation increased over the first 3 postnatal weeks, from 42 ± 6 pA in neonate (P0–3) to 77 ± 11 pA in juvenile (P11–18) HMs. Additionally, mIPSC amplitude distributions were highly variable: CV 0.68 ± 0.05 (means ± SE) for neonates and 0.83 ± 0.06 for juveniles. We wished to ascertain the contribution of glycine receptor (GlyR)-channel properties to this change in quantal amplitude and to the amplitude variability and time course of mIPSCs. To determine whether a postnatal increase in GlyR-channel conductance accounted for the postnatal change in quantal amplitude, the conductance of synaptic GlyR channels was determined by nonstationary variance analysis of mIPSCs. It was 48 ± 8 pS in neonate and 46 ± 10 pS in juvenile HMs, suggesting that developmental changes in mIPSC amplitude do not result from a postnatal alteration of GlyR-channel conductance. Next we determined the open probability ( P open) of GlyR channels in outside-out patches excised from HMs to estimate the contribution of stochastic channel behavior to quantal amplitude variability. Brief (1 ms) pulses of glycine (1 mM) elicited patch currents that closely resembled mIPSCs. The GlyR channels’ P open, calculated by nonstationary variance analysis of these currents, was ∼0.70 (0.66 ± 0.09 in neonates and 0.72 ± 0.05 in juveniles). The decay rate of patch currents elicited by brief application of saturating concentrations of glycine (10 mM) increased postnatally, mimicking previously documented changes in mIPSC time course. Paired pulses of glycine (10 mM) were used to determine if rapid GlyR-channel desensitization contributed to either patch current time course or quantal amplitude variability. Because we did not observe any fast desensitization of patch currents, we believe that fast desensitization of GlyRs underlies neither phenomenon. From our analysis of glycinergic patch currents and mIPSCs, we draw three conclusions. First, channel deactivation is the primary determinant of glycinergic mIPSC time course, and postnatal changes in channel deactivation rate account for observed developmental changes in mIPSC decay rate. Second, because GlyR-channel P openis high, differences in receptor number between synapses rather than stochastic channel behavior are likely to underlie the majority of quantal variability seen at glycinergic synapses throughout postnatal development. We estimate the number of GlyRs available at a synapse to be on average 27 in neonate neurons and 39 in juvenile neurons. Third, this change in the calculated number of GlyRs at each synapse may account for the postnatal increase in mIPSC amplitude.