Regulatory Influences on Crustacean Neurosecretory Cells

During the last decade, new evidence has been produced on the subtle mechanisms by which invertebrate neurosecretory cell activity is regulated. Multiple synaptic and humoral mechanisms regulate the endogenous activity of secretory neurons. Specific cellular interactions and ionic mechanisms have been disclosed, and new insights are now available on the integrative features of invertebrate neurosecretory systems.

Blockage of synaptic release by brief hyperpolarizing pulses in the neuromuscular junction of the crayfish

1. Synaptic currents were evoked at the neuromuscular junction of the deep extensor abdominal muscle of the crayfish by direct depolarization of motor nerve endings. 2. Quantal content and time course of neurotransmitter release were determined from delay histograms of unitary release events recorded with a macropatch clamp technique. 3. Synaptic facilitation was elicited by pairing depolarizing pulses at intervals ranging from 10 to 200 ms. At 14 degrees C the duration of facilitation was about 50 ms. Reducing activity of the Nao(+)-Cai2+ exchange by lowering [Na+]o by 50% resulted in prolonged facilitation, which lasted approximately 150 ms. 4. Normalized synaptic delay histograms at normal [Na+]o and 50% [Na+]o were the same for the first and the facilitated second response, indicating that activity of the Na(+)-Ca2+ exchange does not determine the time course of release. 5. The application of a hyperpolarizing post-pulse after the first depolarizing stimulus reduced release and altered its time course to a similar extent both in normal and in 50% [Na+]o. However, it did not affect the level and the time course of release of the facilitated response. 6. A hyperpolarizing post-pulse given after the first and second pulses of a pair reduced release to the same extent for the two depolarizing pulses. 7. These results indicate that whereas manipulations thought to increase [Ca2+]i (i.e. reducing activity of the Nao(+)-Cai2+ exchange or facilitation) affect the quantal content, they do not influence the time course of release. However, changes of membrane potential do affect the quantal content, and more importantly the time course of release, thus suggesting a contributory role of membrane potential in the control of synaptic release.

Optical recording of calcium and voltage transients following impulses in cell bodies and processes of identified leech neurons in culture

Optical methods were used to examine the spread of electrical potentials and the distribution and time course of calcium transients in individual identified nerve cells isolated from the leech. A photodiode array detected voltage transients by measuring absorbance changes of cells stained with the voltage-sensitive dye RH155 added to the bath. Calcium transients were recorded by measuring absorbance changes of the dye arsenazo III, which had been injected into the cells. In addition, Lucifer yellow was injected to outline the some and processes. Calcium changes resulting from individual action potentials were recorded from N, P, and Retzius cells without averaging. Signals from T cells and anterior pagoda (AP) cells were weaker but could be detected with averaging. These results are in accord with previous studies on calcium contributions to action potentials in these cells. For all cells, larger or wider action potentials gave bigger signals. Calcium changes from each of a train of action potentials were of equal amplitude, showing no sign of facilitation. Calcium transients from Retzius cells that had formed chemical synapses with P cells had properties similar to those of isolated cells. We were also able to detect responses from prolonged subthreshold depolarizations to -40 mV from a hyperpolarized membrane potential (-65 mV). These signals rose throughout the duration of the pulse (1-2 sec). With the photodiode array we mapped the distribution of the calcium signals. The amplitudes from each pixel are proportional to the amount of calcium entering that element in response to the stimulating pulse, if the simplifying assumption is made that the calcium buffering of the cytoplasm is uniform throughout the cell. The largest signals were detected over the axon stump left from the cell isolation procedure. Large signals were also detected from the soma. Weak signals were detected from the processes of some cells. From many Retzius cells, no signals at all were detected from the newly formed processes. Using the photodiode array, we also recorded voltage transients from the cells. Signals were recorded from all over the arborization of the neuron, with no obvious variation in time course, showing that the entire cell, including fine slender processes and broad growth cones, was essentially isopotential. Combining these observations with the measured distribution of calcium transients in the same cell suggests that the density of calcium channels in most cells is less in the outgrowing processes than in the soma or axon stump.

Isolation and purification of a neurodepressing hormone from the eyestalk of Procambarus bouvieri (Ortmann)

1. A neurodepressing hormone has been isolated and purified to homogeneity from aqueous extracts of 2000 eyestalks of the Mexican crayfish Procambarus bouvieri (Ortmann). 2. Purification was achieved by gel filtration on Sephadex G-25 and G-15, and preparative paper electrophoresis at four pH valueimately 1200 molecular weight and composed of neutral amino acids. 5. No N-terminal group could be found. From its electrophoretic behavior it is concluded that the C-terminal group is also blocked.