Homology modeling of histidine-containing phosphocarrier protein and eosinophil-derived neurotoxin: construction of models and comparison with experiment.

Homology modeling methods have been used to construct models of two proteins--the histidine-containing phosphocarrier protein (HPr) from Mycoplasma capricolum and human eosinophil-derived neurotoxin (EDN). Comparison of the models with the subsequently determined X-ray crystal structures indicates that the core regions of both proteins are reasonably well reproduced, although the template structures are closer to the X-ray structures in these regions--possible enhancements are discussed. The conformations of most of the side chains in the core of HPr are well reproduced in the modeled structure. As expected, the conformations of surface side chains in this protein differ significantly from the X-ray structure. The loop regions of EDN were incorrectly modeled--reasons for this and possible enhancements are discussed.

Computer simulations of EPSP-spike (E-S) potentiation in hippocampal CA1 pyramidal cells.

Long-term potentiation of hippocampal excitatory synapses is often accompanied by an increase in the probability of spiking to an EPSP of fixed strength (E-S potentiation). We used computer simulations of a CA1 pyramidal neuron to test the plausibility of the hypothesis that E-S potentiation is caused by changes in dendritic excitability. These changes were simulated by adding "hot spots" of noninactivating voltage-sensitive Ca2+ conductance to various dendritic compartments. This typically caused spiking in response to previously subthreshold synaptic inputs. The magnitude of the simulated E-S potentiation depended on the passive electrical properties of the cell, the excitability of the soma, and the relative locations on the dendrites of the synaptic inputs and hot spots. The specificity of the simulated E-S potentiation was quantified by colocalizing the hot spots with a subset (40 of 80) of the synaptic contacts, denoted "tetanized," and then comparing the effects of the hot spots on these and the remaining (untetanized) synaptic contacts. The simulated E-S potentiation tended to be specific to the tetanized input if the untetanized contacts were, on average, electrically closer to the soma than the tetanized contacts. Specificity was also high if the tetanized and untetanized contacts were segregated to different primary dendrites. The results also predict, however, that E-S potentiation by this mechanism will appear to be nonspecific (heterosynaptic) if the synapses of the untetanized input are sufficiently far from the soma relative to the tetanized synapses. Experimental confirmation of this prediction would support the hypothesis that changes in postsynaptic excitability can contribute to hippocampal E-S potentiation.

Quantitative analysis of the retinal ganglion cell layer and optic nerve of the barn owl Tyto alba.

The visual capacity of the common barn owl (Tyto alba) was studied by quantitative analysis of the retina and optic nerve. Cell counts in the ganglion cell layer of the whole-mounted retina revealed a temporal area centralis with peak cell density of 12,500 cells/mm2 and a horizontal streak of high cell density extending from the area centralis into the nasal retina. Integration of the ganglion cell density map gave an estimated total of 1.4 million cells for the ganglion cell layer. Electron microscopy of a single, complete section of the optic nerve revealed a bimodal fiber diameter spectrum (modes at 0.3 and 0.9 microns; bin width = 0.2 microns), with diameters ranging from 0.15 microns (unmyelinated) to 6.05 microns (myelinated, sheath included). The total axon count for the optic nerve was estimated from sample counts to be about 680,000 axons (25% unmyelinated). Therefore, roughly half of the cells in the retinal ganglion cell layer do not send axons into the optic nerve. With certain assumptions, the data predict a visual spatial acuity for barn owls on the order of 8 cycles/degree, a value similar to the known behaviorally measured acuities of masked owls (10 cycles/degree) and domestic cats (6 cycles/degree).

A double-label study of efferent projections from the Edinger-Westphal nucleus in goldfish and kelp bass.

The Edinger-Westphal nucleus in goldfish was identified by retrograde labeling from the ciliary ganglion. In the same animals a few neurons near this nucleus (perinuclear Edinger-Westphal neurons) were labeled by a different retrograde tracer injected into the cerebellum. No double-labeled cells were found. Similar results were obtained in kelp bass, except that in this species no cerebellar-projecting perinuclear neurons were observed. Cerebellar-projecting Edinger-Westphal neurons have previously been described in some mammals, but not in other vertebrates. Therefore the homology of cerebellar-projecting cells of the Edinger-Westphal region in mammals and teleost fishes is doubtful.

Identification of the teleost Edinger-Westphal nucleus by retrograde horseradish peroxidase labeling and by electrophysiological criteria.

A homolog of the Edinger-Westphal nucleus of other vertebrates is described in two species of serranid basses of the genus Paralabrax, a group possessing a wide range of ocular accommodation but lacking a pupillary reflex to light. The nucleus was found by retrograde labeling from the ciliary ganglion and lies dorsolateral to the ipsilateral oculomotor nucleus. The nucleus consists of 60 to 100 neurons with an average soma diameter of about 20 microns in animals weighing 70 to 150 g. Electrophysiological experiments support the identification. Microstimulation of the nucleus evokes contraction of the ipsilateral lens retractor muscle and slight constriction of the caudal ipsilateral iris. Multi- and single-unit recordings in the nucleus reveal spontaneous firing (about 30 spikes/s in single units), the rate of which decreases during visually-evoked lens retractor relaxations (accommodation to near stimuli). Recordings of muscle fiber activity in the lens retractor show essentially the same behavior, which suggests that the ciliary ganglion and neuromuscular junctions simply relay impulses with little if any synaptic integration. The existence of a discrete Edinger-Westphal nucleus devoted largely to accommodation makes Paralabrax a good model system for the further tracing of central accommodation control pathways.

Accommodation motor neurons in the foveate teleost Paralabrax clathratus: horseradish peroxidase labeling and axonal morphometry, with comparisons to other ciliary nerve components.

Although much is known about the optics and mechanism of ocular accommodation in teleost fishes, there is to date no description of the neurons innervating the muscle of accommodation, the lens retractor. I have identified accommodation motor neurons in the kelp bass, Paralabrax clathratus, by backfilling the lens retractor nerve (a branch of the short ciliary nerve) with the retrograde tracer horseradish peroxidase. These neurons comprise a subpopulation of relatively large unipolar neurons in the ciliary ganglion. Backfilling either of the remaining ciliary nerve branches (which innervate mainly cornea and iris) labels smaller cells in the ciliary ganglion as well as sensory neurons in the profundus ganglion and sympathetic neurons in the trigeminal sympathetic ganglion. No labeled cells were found in the brain in any of these experiments. I have also examined the lens retractor nerve and the corneal-iridal branch of the short ciliary nerve by electron microscopy. Counts of axons in these nerves from animals of different sizes suggest postembryonic growth of axon number in the corneal-iridal branch but not in the lens retractor nerve. The latter comprises approximately 100 myelinated and a few unmyelinated axons. Its diameter spectrum shows a preponderance of large-diameter axons, but the myelin sheaths are unusually thin (mean axon diameter: 7.5 micron; mean ratio axon diameter/fiber diameter: g = 0.81 for 830-gram animal). The results indicate that kelp bass accommodation motor neurons lie primarily if not entirely within the ciliary ganglion. Some of their axons are the largest in the short ciliary nerve, but their sheath thicknesses are apparently not optimal with respect to conduction velocity.

Numerical reconstruction of the quantal event at nicotinic synapses.

To test our present quantitative knowledge of nicotinic transmission, we reconstruct the postsynaptic conductance change that results after a presynaptic nerve terminal liberates a quantum of acetylcholine (ACh) into the synaptic cleft. The theory assumes that ACh appears suddenly in the cleft and that is subsequent fate is determined by radial diffusion, by enzymatic hydrolysis, and by binding to receptors. Each receptor has one channel and two ACh binding sites; the channel opens when both sites are occupied and the rate-limiting step id the binding and dissociation of the second ACh molecule. The calculations reproduce the experimentally measured growth phase (200 microseconds), peak number of open channels (2,000), and exponential decay phase. The time constant of the decay phase exceeds the channel duration by approximately equal to 20%. The normal event is highly localized: at the peak, two-thirds of the open channels are within an area of 0.15 micrometer 2. This represents 75% of the available channels within this area. The model also simulates voltage and temperature dependence and effects of inactivating esterase and receptors. The calculations show that in the absence of esterase, transmitter is buffered by binding to receptors and the postsynaptic response can be potentiated.