Surface forces between colloidal particles at high hydrostatic pressure.

It was recently suggested that the electrostatic double-layer force between colloidal particles might weaken at high hydrostatic pressure encountered, for example, in deep seas or during oil recovery. We have addressed this issue by means of a specially designed optical trapping setup that allowed us to explore the interaction of a micrometer-sized glass bead and a solid glass wall in water at hydrostatic pressures of up to 1 kbar. The setup allowed us to measure the distance between bead and wall with a subnanometer resolution. We have determined the Debye lengths in water for salt concentrations of 0.1 and 1 mM. We found that in the pressure range from 1 bar to 1 kbar the maximum variation of the Debye lengths was <1 nm for both salt concentrations. Furthermore, the magnitude of the zeta potentials of the glass surfaces in water showed no dependency on pressure.

Dynamic control of auditory activity during sleep: correlation between song response and EEG.

The song nucleus high vocal center (HVC) sends neural signals for song production and receives auditory input. By using electroencephalography (EEG) to objectively identify wake/sleep state, we show that HVC auditory responses change with physiological states. Comparison of EEG and HVC records revealed that HVC response to auditory stimuli is greatest during slow-wave sleep. During slow-wave sleep, HVC neurons responded preferentially to the bird's own song. Strikingly, both spontaneous and forced waking during sleep caused HVC auditory responses to cease within milliseconds of an EEG-measured state change. State-dependent phenomena in downstream nuclei, such as robustus archistriatalis, are likely to be derivatives of those in HVC.

Synaptic activity modulates presynaptic excitability.

Synaptic activity modulates synaptic efficacy and is important in learning and development. Here we show that development of excitability in presynaptic motor neurons required synaptic activation of postsynaptic muscle cells. Synaptic blockade broadened action potentials and decreased repetitive firing of presynaptic neurons. Consistent with these findings, synaptic blockade also decreased potassium-current density in the presynaptic cell. Application of neurotrophin-3, but not related neurotrophins, prevented these changes. Recordings from patches of somatic membrane indicated that modifications of presynaptic potassium and sodium currents occurred in a remote, nonsynaptic compartment. Thus, activity-dependent postsynaptic signals modulated presynaptic excitability, potentially regulating transmission at all synapses of the presynaptic cell.

Protein kinase C regulates a vesicular class of calcium channels in the bag cell neurons of aplysia.

Protein kinase C (PKC) acutely increases calcium currents in Aplysia bag cell neurons by recruiting calcium channels different from those constitutively active in the plasma membrane. To study the mechanism of PKC regulation we previously identified two calcium channel alpha1-subunits expressed in bag cell neurons. One of these, BC-alpha1A, is localized to vesicles concentrated primarily in somata and growth cones. We used antibodies to BC-alpha1A to analyze its expression in the bag cell neurons of juvenile Aplysia at a developmental stage at which PKC-sensitive calcium currents have previously been shown to be low. We find that vesicular BC-alpha1A staining is generally reduced in juvenile bag cell neurons but that its expression level can vary among juvenile animals. In 17 bag cell clusters examined, the percentage of neurons that displayed punctate alphaBC-alpha1A staining ranged from 0 to 85%. Sampling of calcium currents from cells of the same clusters by whole cell patch-clamp techniques revealed that the PKC-sensitive calcium current density is significantly correlated with the degree of vesicular staining. In contrast, no correlation of basal calcium current levels with aBC-alpha1A staining was found. These results strongly suggest that BC-alpha1A, a member of the ABE-subfamily of calcium channels, carries the PKC-sensitive calcium current in bag cell neurons. They are consistent with a model in which PKC recruits channels from the vesicular pool to the plasma membrane.

Ionic currents underlying developmental regulation of repetitive firing in Aplysia bag cell neurons.

We have investigated the developmental regulation of the ability to fire repetitively in the bag cell neurons of Aplysia californica, a neuronal system in which the behavioral effects of repetitive firing are well characterized. Adult bag cell neurons exhibit an afterdischarge, consisting of prolonged depolarization and repetitive firing, which causes the release of several peptides from these neurons that induce egg-laying behaviors. Afterdischarge can be triggered in vitro by a variety of stimuli, including electrical stimulation and exposure to the potassium channel blocker tetraethyl ammonium chloride (TEA). In contrast to adults, juvenile neurons did not exhibit afterdischarge in response to pleural-abdominal connective shock or TEA. Juvenile neurons did exhibit, however, prolonged depolarizations in the presence of TEA, perhaps reflecting the anlage of the mechanism responsible for afterdischarge in the adult. To investigate developmental mechanisms underlying the regulation of repetitive firing, we compared ionic currents in adult and juvenile bag cell neurons. We found that during the period in which these neurons acquire the capacity to fire repetitively, a number of currents are regulated: (1) three K+ currents decrease (Ca2+)-dependent K+ and two components of voltage-dependent delayed-rectifier K+ current); (2) A-type K+ current increases; and (3) two Ca2+ currents increase (basal and PKC-activated). This pattern is consistent with the increase in the ability to fire repetitively that we observe during maturation: our results indicate that developmental control of repetitive firing in this system is accompanied by selective regulation of specific ionic currents which, after maturation, play important roles in generating the afterdischarge and triggering egg-laying behaviors.

Developmental dissociation of excitability and secretory ability in Aplysia bag cell neurons.

1. Despite the considerable progress made in understanding the role of electrical activity in triggering secretion, the developmental relationships between excitability and secretion are not well understood. The well-characterized bag cell neurons of Aplysia provide an advantageous system in which to investigate developmental interactions of these two key properties of neurons. 2. A prolonged afterdischarge triggers egg laying hormone (ELH) secretion in mature bag cell neurons. To investigate secretion in the developmental framework of excitability, we first examined whether immature neurons, which are incapable of the mature form of excitability (afterdischarge), contain ELH and whether this hormone is packaged in vesicles. We used immunoelectron microscopy to compare vesicular localization of ELH and to compare the size and density of ELH-containing vesicles in neurons from adult and juvenile Aplysia. This comparison revealed that immature neurons contain ELH in vesicles in the size range of secretory vesicles. However, they lack a class of large vesicles (> 250 nm in diameter) that is characteristic of mature neurons. 3. To investigate whether the ELH contained in immature bag cell neurons could be secreted in response to electrical activity, we used the potassium channel blocker tetraethylammonium (TEA) combined with nerve stimulation to depolarize neurons from both juvenile animals (ovotestes do not contain eggs) and from adult Aplysia (ovotestes contain eggs). Using radioimmunoassay, we have found that the duration and amount of ELH secreted from bag cell neurons from juvenile Aplysia in response to TEA does not depend on whether or not the cells can be induced to afterdischarge, and the amount and duration of ELH secreted from bag cell neurons of juvenile Aplysia (whether or not they afterdischarged) differed from those secreted by adult neurons. However, by normalizing for body size, we found that the final estimated hemolymph concentration of ELH would be similar in juvenile and adult animals. 4. We investigated the potential functional significance of secretion of bag cell hormones in juvenile Aplysia by attempting to bypass the bag cell neurons and directly activate downstream elements with extract from adult bag cell neurons (BCE), known to contain ELH and other peptides. We found that juvenile Aplysia exhibit at least one component of egg-laying behavior, cessation of locomotion, in response to BCE during a developmental period (as measured by weight) in which they normally would possess neurons incapable of afterdischarge. Thus developmental regulation of excitability in the bag cell neurons may prevent inappropriate hormone release and subsequent premature expression of reproductive behaviors.