Postsynaptic Surface Research Articles

Increases in pericellular proteolysis at developing neuromuscular junctions in culture

To determine whether localized changes in pericellular proteolysis contribute to synapse formation, we examined the degradative actions of developing Xenopus laevis nerve and muscle cells on films of extracellular matrix proteins adsorbed to the glass surface of a tissue culture chamber. Skeletal myocytes, growing neurites, and fibroblasts all removed fluorescent fibronectin and laminin from the culture substratum at regions of close cell-surface contact. In addition, however, motor neurites also displayed a particularly enhanced rate of gelatin elimination at developing neuromuscular junctions. It has already been shown (a) that there is a similar remodeling of organized muscle basal lamina proteoglycan accumulations along the path of nerve-muscle contact and (b) that this is the earliest detectable biochemical change specific to developing neuromuscular junctions. Our observations thus suggest that the establishment of motoneuron-muscle contact leads to a further activation of pericellular proteinases along both the pre- and the postsynaptic surfaces of the developing junction. We therefore consider whether site-specific proteinase-activation cascades could contribute to the inductive signals that direct synaptic differentiation.

Synaptic organization of globular bushy cells in the ventral cochlear nucleus of the cat: A quantitative study

The synaptic organization of globular bushy cells of the anteroventral cochlear nucleus was quantitatively analyzed in order to understand better their functional attributes. A method was devised to estimate the concentrations and relative proportions of synapses on the entire postsynaptic surface of Golgi-impregnated neurons, by sampling with limited series of sections for electron microscopy. This provided a characteristic synaptic profile which was homogeneous for the population measured. The total concentration of synaptic endings decreases with distance from the soma. The cochlear, presumably glutamatergic and excitatory, endings with large spherical vesicles (LS) account for most of this decrease. Of the noncochlear inputs, the putative glycinergic endings with flattened vesicles (FL) decrease slightly, and the presumed GABAergic terminals with pleomorphic vesicles (PL) maintain a relatively constant concentration, while endings with small spherical vesicles (SS) increase on the distal dendrites. LS endings have the largest proportion of synapses near the soma, while FL synapses maintain a constant proportion in all cell regions, and PL and SS proportions increase on higher-order dendrites. Excitatory and inhibitory synapses have significant inputs to the axon hillock and initial segment, as well as to the distal dendrites, where dual synapses may provide a way to sample the activity of surrounding neurons. These features must be considered in explanations of physiological properties, such as the synaptic security, level of spontaneous activity, and well-timed, rapid onset responses, as well as their potential for normalizing and synchronizing an important inhibitory pathway involved in binaural signal processing. Synaptic profile analysis should be useful for experimental studies and for developing realistic computational models.

Selective innervation of embryonic hippocampal transplants by adult host dentate granule cell axons

Fragments containing different cytoarchitectonic fields were dissected out of late embryonic rat hippocompal primordia and transplanted into the hippocompus or septum of adult syngeneic hosts. Field CA3 transplants contained clusters of large, angular (pyramidal) cell bodies surrounded by a radiating corona of dendrites. These cells stained selectively with our monoclonal antibody Py, and a proportion were labelled by [ 3H]thymicline administered on the 15th day of embryonic life. Field CA1 transplants contained smaller, angular, Py-negative cells, which formed elongated laminae rather than globular clusters. The ability of the host dentate granule cells to project to the transplants was examined by (1) the Timm stain for mossy fibres, (2) electron microscopy of Golgi-impregnated CA3 pyramidal neurons in the transplants, and (3) quantitative electron microscopic assessment of the proportions of large mossy fibre terminals in the synaptic population of the transplants. The Timm stain showed that CA3 transplants received a projection from host dentate granule cells when the transplants were placed in direct contact with the axons in the host mossy fibre pathway. As in the normal host field CA3, the ingrowing mossy fibres terminated selectively on the juxtacellular regions of the dendritic tree and ignored the major part of the dendrites in the radiating corona. The electron micrographs showed that within this territory the host mossy fibres formed synaptic terminals with all the complex features typical of normal mossy fibres, and were presynaptic to complex spines arising from the juxtacellular region of Golgi-impregnated donor CA3 pyramidal cells. The quantitative electron microscopic study demonstrated that the mossy fibre-innervated juxtacellular regions of the field CA3 transplants had up to 20% of the normal density of mossy fibre synapses found in the stratum lucidum of field CA3 in situ. CA3 transplants which were placed in the septum, remote from the host mossy fibres, had either trivial numbers of mossy fibre synapses or none. This confirmed that the abundant mossy fibre terminals in the intrahippocompal CA3 transplants were of host origin, and not due to donor dentate granule cells inadvertently included in the grafts. The selectivity of the host dentate projection for field CA3 transplants was demonstrated by the observation that CA1 transplants in the same locations received only slight mossy fibre projections in the Timm stain, and in electron micrographs their synaptic population had only insignificant numbers of large mossy fibre terminals. We have used the dentate granule → CA3 pyramidal cell system of the hippocompus to show that embryonic transplants containing a selected donor cell type can receive a correct and highly differentiated synaptic innervation arising from the appropriate host cell type, and terminating on the correct part of the postsynaptic surface. This specificity is expressed even when the spatial and temporal patterns of the confrontation are completely abnormal, and when the CA3 pyramidal cells have never been previously exposed to dentate granule cell axons.

A general model of stochastic neural processing

A model of neural processing is proposed which is able to incorporate a great deal of neurophysiological detail, including effects associated with the mechanics of postsynaptic summation and cell surface geometry and is capable of hardware realisation as a `probabilistic random access memory' (pRAM). The model is an extension of earlier work by the authors, which by operating at much shorter time scales (of the order of the lifetime of a quantum of neurotransmitter in the synaptic cleft) allows a greater amount of information to be retrieved from the simulated spike train. The mathematical framework for the model appears to be that of an extended Markov process (involving the firing histories of the N neurons); simulation work has yielded results in excellent agreement with theoretical predictions. The extended neural model is expected to be particularly applicable in situations where timing constraints are of special importance (such as the auditory cortex) or where firing thresholds are high, as in the case for the granule and pyramidal cells of the hippocampus.

Cytoskeletal architecture of neuromuscular junction: Localization of vinculin

Cytoskeletons underneath the postsynaptic membrane of neuromuscular junctions were studied by using a quick-freeze deep-etch method and immunoelectron microscopy of ultrathin frozen sections. In a quick-freeze deep-etched replica of fresh, unfixed muscles, 8.9 +/- 1.5-nm particles were present on the true postsynaptic membrane surface. Underneath this receptor-rich postsynaptic membrane, networks of fine filaments were observed. These cytoskeletal networks were more clearly observed in extracted samples. In these samples, diameters of the filaments which formed networks were measured. In the platinum replica, three kinds of filament were recognized--12 nm, 9 nm, and 7 nm in diameter. The 12-nm filament seemed to correspond to the intermediate filament. The other two filaments formed meshworks between intermediate filaments and plasma membrane. In ultrathin frozen sections vinculin label was localized just beneath the plasma membrane. Thirty-six percent of the label was within 18 nm from the cytoplasmic side of the plasma membrane and 50% was within 30 nm. Taking the size of the vinculin molecule into account, it was concluded that vinculin is localized just beneath the plasma membrane and might play some role in anchoring filaments which formed meshworks underneath the plasma membrane.

Cytology and organization of rat cerebellar organ cultures

Roller tube cultures of parasagittal cerebellar slices were taken from young rats aged 9–11 days, and maintained in vitro for 1–2 weeks. Morphological aspects of cell types and synaptic relationships in such organ cultures were examined at light and electron microscopic levels. Some neurons were marked by intracellular injections of horseradish peroxidase for subsequent identification of their connection patterns. Cytoarchitecture of the cerebellar cortex was largely preserved in the organ cultures. Dendritic trees of Purkinje cells exhibited isoplanar organizations that often resembled their orientation at the time of explanation. Other cerebellar neurons, namely granule cells, Golgi cells, basket cells, stellate cells, all differentiated within the organ cultures. In addition, some neurons of the deep cerebellar nuclei remained viable during the period of culture. Mossy fibers most probably of cerebellar nuclear origin were found terminating on the dendrites of granule cells and Golgi cells. Quite unexpected were certain types of direct synapses of afferent fibers on short necked spines arising from Purkije cell smooth dendrites and somata. Such terminals resembled climbing fibers. They were most likely modified mossy fiber afferents, since the organ cultures did not include neurons of the inferior olive which are well spearated from the cerebellar mass at postnatal stages. These “ascending” mossy fibers presumably occupied postsynaptic surfaces that were either vacated by deafferentation or induced by the afferent fibers themselves. Intracellularly labeled Purkinje cells had widely distributed axonal collateral branches. Labeled axons were distributed within the Purkinje cell layer. Several recurrent Purkinje cell axon collaterals stained with reaction products of horseradish peroxidase tracer were followed at the ultrastructural level. In one case, labeled terminals were examined in an area of approximately 2 mm 2. Terminals of Purkinje cell collaterals formed symmetric synapses with somata of basket cells and dendrites of Golgi cells, but not Purkinje cell somata. Some large boutons of serially traced Purkinje cell axon collaterals formed asymmetric contacts with profiles interpreted as Golgi cell dendrites. In contrast to the apparent axonal sprouting in cerebellar organ cultures, maturation of dendritic processes remained static. Astroglia cells of diverse shapes were observed following immunocytdchemical staining with antisera to glia filament proteins. The distribution patterns of immunoreactive astrocytes changed dramatically in cerebellar slice cultures maintained for 3–6 weeks in vitro.

An immunocytochemical and biochemical study of the microtubule-associated protein MAP-2 during post-lesion dendritic remodeling in the central nervous system of adult rats

A monoclonal antibody against the microtubule-associated protein MAP-2 was used to examine the fate of this molecule during post-lesion dendritic remodeling in the hippocampus and septum of adult rats. Qualitative and quantitative immunocytochemical analyses were carried out in the dentate gyrus after unilateral destruction of the entorhinal cortex (EC). An increase in MAP-2 immunoreactivity was detected in dendritic processes located in the outer 2 3 of the ipsilateral molecular layer (ML) 2 days after the lesion, whereas dendritic staining decreased considerably in the inner 1 3 of the same ML. The increase of staining was also detected 4, 6 and 8 days after the lesion; it was accompanied by an increase in the immunoreactivity in the inner 1 3 of the ML. After that period, a progressive decrease in anti-MAP-2 staining toward control levels was detected along the whole extent of the ipsilateral ML. This was concurrent with alterations in dendritic orientation, and a decrease in stained dendrites in the inner 1 3 of the ML. By 30 days post-lesion anti-MAP-2 staining was almost identical to that of the contralateral ML, although the alterations in dendritic morphology were still present in the ipsilateral ML. Changes in MAP-2 levels were also evaluated by densitometry of Western blots or dot immunobinding of hippocampal extracts obtained at different post-lesion intervals. The results obtained revealed a pattern of change in MAP-2 levels identical to that observed with the immunohistochemical stain. A similar, immunocytochemical and biochemical, analysis conducted in the lateral septal nucleus after unilateral transection of the fimbria showed no changes in the distribution and/or content of MAP-2 at any post-lesion interval analyzed (2, 10 and 20 days post-lesion). The present observations show that post-lesion dendritic remodeling is concurrent with modifications in the levels and distribution of MAP-2. These modifications suggest that the dendritic cytoskeleton is dynamically changing in response to perturbation of the synaptic environment. In addition, our results indicate that these changes may only occur in those neurons which have the capability to remodel their post-synaptic surface in response to deafferentation.

Structural components in the synaptic cleft captured by freeze-substitution and deep etching of directly frozen cerebellar cortex.

Structural components in the synaptic cleft were examined in cerebellar excitatory synapses by conventional electron microscopy and by rapid freezing followed by freeze-substitution or deep etching. Two transverse components and one parallel element were identified in the clefts of rapidly frozen and freeze-substituted synapses: (i) bridging fibrils, 4-6 nm in diameter, that span the cleft; (ii) columnar pegs, 4-6 nm wide and 8-15 nm high, projecting from the postsynaptic surface; and (iii) intervening fine fibrils running parallel to the apposed synaptic membranes. These were more clearly visible in deep-etched synapses, although the postsynaptic pegs were difficult to distinguish from intramembrane particles in the cross-fractured postsynaptic membranes. Deep etching also revealed other fibrils on the cytoplasmic surface of the postsynaptic membrane. These appear to contact the membrane surface or the intramembrane particles. Freeze-substituted materials also displayed the fibrillar components in the postsynaptic dense fuzz, but failed to display the presynaptic dense projections typically observed in thin sections or deep-etched replicas of the conventionally fixed materials. The bridging fibrils are likely to play a mechanical role in holding the synapse together, while the short pegs may be integral parts of the receptor molecules.

Ultrastructural analysis of the distribution of synaptic boutons from labeled preganglionic axons on rabbit ciliary neurons

The number of preganglionic inputs that innervate rabbit ciliary ganglion cells is directly correlated with the number of dendrites arising from each ganglion cell (Purves and Hume, 1981). In general, the innervation of multiply innervated ciliary neurons by individual preganglionic axons is regionally restricted to a portion of the postsynaptic surface that usually includes the cell body and some, but not all, of the dendrites (Forehand and Purves, 1984). These observations suggest that dendrites modulate convergence to each cell by providing relatively separate postsynaptic domains for individual inputs. To examine this possibility further, I have assessed the distribution of synaptic boutons from individually labeled preganglionic axons on ciliary ganglion cells at the ultrastructural level. The results show that at least a third of the dendrites of these neurons are contacted exclusively by synaptic boutons from a single preganglionic axon. However, at least half of the dendrites (and nearly all of the cell bodies) of multiply innervated ganglion cells are innervated by at least 2 different preganglionic axons. Moreover, synapses from 2 different inputs often coexist in close proximity on the postsynaptic surface. Thus, individual preganglionic axons do not require exclusive dominion over a particular part of a postsynaptic cell in order to maintain their connection with the cell. These results suggest that competitive interactions between the inputs to these cells occur between the sets of boutons arising from different inputs, rather than at the level of individual boutons.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential rearing effects on rat visual cortex synapses. I. Synaptic and neuronal density and synapses per neuron

The bulk of the evidence indicating that different experiences can lead to differences in synapse numbers involves inference from measures of postsynaptic surface (spines and dendrites) in Golgi impregnated tissue. The capriciousness of Golgi impregnation and the absence of direct evidence regarding changes in afferents mandate confirmation of synapse changes by electron microscopy. We calculated the ratio of synapses per neuron in layers I–IV of occipital cortex of rats reared in complex (EC), social (SC), or isolated (IC) environments. Synaptic density estimates were derived from electron micrographs of osmium-uranyl-lead stained tissue and neuronal density estimated were derived from toluidine blue stained semithin sections using stereological methods which correct for group differences in the sizes of synapses and neuronal nuclei. The ratio of these densities, synapses per neuron, was highest in complex environment rats, intermediate in socially reared rats and lowest in isolates, in accordance with predictions from prior Golgi studies. The bulk of the differences were attributable to neuronal density, which was highest in IC rats and lowest in ECs. Synaptic density did not differ statistically across groups. These results indicate, at least within this area and paradigm, that differences in dendritic measures in Golgi impregnated tissue reflect differences in the number of synapses per neuron.

Afferent influences on brain stem auditory nuclei of the chicken: time course and specificity of dendritic atrophy following deafferentation.

The time course and specificity of the changes in dendritic morphology following deafferentation were examined in nucleus laminaris of young chickens. The dendrites of nucleus laminaris neurons are segregated into dorsal and ventral domains, which are innervated separately from the ipsilateral and contralateral nucleus magnocellularis, respectively. Transection of the crossed dorsal cochlear tract deafferents the ventral dendrites of nucleus laminaris bilaterally without interrupting the matching input to the dorsal dendrites. In 10-day-old chicks, atrophy of the ventral dendrites began immediately after transecting the tract; the ventral dendrites were 10% shorter by 1 hour and 16% shorter by 2 hours after deafferentation. The length of the ventral dendrites progressively decreased over the next 2 weeks, resulting in at least a 60% loss of ventral dendrite 16 days after surgery. The dorsal dendrites of the same cells, whose afferents remained intact, did not change in length during the time course of this study. However, 16 days after the lesion, spines appeared on the normally smooth dorsal and ventral dendrites. The time course of dendritic atrophy and its restriction to the deafferented postsynaptic surface are related to possible mechanisms by which afferents regulate and maintain their target neurons.

Comments on the structure of ancient wisdom

The awe-inspiring Structure of Ancient Wisdom sets the scenario for images of drowned cities of the past, cities that have sunk into the depth, while their dreamer looks through the lucid water of flickering lights refracted by passing ripples. Are these deep structures related to Chomsky’s deep structures, Durkheim’s collective representations, Jung’s archetypes, and the Freudian unconscious? The question is embedded into a feeling that the structure of ancient wisdom must satisfy some (cosmic) extracognitive needs of our nature and that the stunning quality of this wisdom may be due to its Pythagorean desire of expressing ‘being’ in numbers. Deciphering and interpreting this numerical language is very much like looking into the mirror and seeing a stranger whom we should have known better. Let me be more specific. Halving or doubling the string length of a harp and hearing the change in pitch by an octave is self-tuning. Whenever changes in a self-referential creature’s sensations are accounted for by its (voluntary) movements-and vice versaa sensory/motor closure ensues and recursion enters the picture (H. v. Foerster).+ The re-entrant form is the meaning of this eigenbehavior (F. Varela). Making a sensory/motor closure by halving or doubling the string brings the sensible world environment into existence-the sound of a higher or lower octave (seemingly from ‘out there’)-although ‘environment’ is for H. Maturana only a means for closure. Let me explain this in Maturana’s own words: ‘If an observer were to stand ln a synaptic cleft, and if while observing the preand post-synaptic surfaces he were to describe the transfer properties of the system thus obtained in terms of input and output relations, he would describe an open neuronal network (or environment), in which the postsynaptic membrane constitutes the sensory surface and the presynaptic membrane the effector (motor) surface. We create the world in which we live by living it and the logic of our description is isomorphic with the operation of the living system’. But we are a closed system, continues Maturana, ‘like a pilot in the cockpit who’s instrumental flight at zero visibility consists only in maintaining and adjusting dial readings’ for flight and fight, food, sex and sleep, the dial readings for repression, denial, sublimation, and so forth. Only those perceptions may be adjusted or controlled, I submit, for which there exists a corresponding proper behavior-in both man and plane.

Regional innervation of rabbit ciliary ganglion cells by the terminals of preganglionic axons

In the rabbit, ciliary ganglion neurons with dendrites maintain inputs from several different axons during the period of synaptic rearrangement that occurs in early postnatal life. Neurons without dendrites, on the other hand, lose the majority of their initial inputs and are innervated in maturity by the terminals of only one or two axons (Purves, D., and R.I. Hume (1981) J. Neurosci. 1: 441-452; Hume, R.I., and D. Purves (1981) Nature 293: 469-471). We have explored the basis of this phenomenon by individually marking preganglionic axons and the neurons they innervate with horseradish peroxidase. In general, the innervation of geometrically complex (multiply innervated) neurons by individual preganglionic axons is regional. That is, the synaptic contacts made by an axon on these neurons are limited to a portion of the postsynaptic surface that includes some, but not all, of the dendrites. This regional innervation of target neurons is consistent with the view that dendrites allow multiple innervation to persist by providing relatively separate postsynaptic domains for individual preganglionic axons. Such regional innervation may mitigate competitive interactions between the several axons which initially innervate the same neuron.

Prenatal and postnatal development of synapses and acetylcholinesterase staining in the dentate gyrus of the rhesus monkey

Morphogenesis, distribution of cholinergic enzyme acetylcholinesterase and synaptogenesis in the dentate gyrus of the rhesus monkey during the pre- and postnatal periods of development were examined using histological, histochemical and ultrastructural methods. The pattern of neuronal differentiation in the dentate gyrus demonstrated distinct superficial-to-deep and lateral-to-medial gradients. The histochemical reaction for acetylcholinesterase was present on gestation day 120 as minimal staining in the supragranular band and in the inner one-third of the dentate molecular layer. At term, the laminar distribution of the enzyme assumed mature pattern although considerable enhancement in staining intensity was achieved postnatally. At term and at 9 months of postnatal age, the most pronounced enzyme activity was found in the supragranular band and in the inner one-third of the molecular layer. Synaptogenesis in the dentate molecular layer was characterized by the early formation of axo-dendritic contacts on dendritic trunks and branches followed by the appearance of synapses on simple and complex spines. Spines were detected infrequently on gestation day 132. On day 148, they ranged in morphology from short stubby protrusions to pedunculated, triangular processes. The majority of the spines exhibited flat postsynaptic surfaces. Complex, synapse-bearing U- and W-shaped spines were observed rarely at this age but appeared more frequently at term and at 15 months of postnatal age. However, at all ages, including 15 months postnatally, synapses on flat-surfaced simple spines predominated. Most synapses were of the asymmetric variety.With certain exceptions, these features of development of the rhesus dentate gyrus resemble the reported patterns of postnatal ontogenesis of this structure in the rat. However, the ingrowth of cholinergic afferents and the major modifications in synapse structure occur prenatally in the rhesus monkey during the second half of the gestation period. This temporal difference between the two species should receive consideration in the planning of neuroplasticity experiments designed to explore lesion-induced adaptations in afferent growth and synaptogenesis in the rhesus dentate gyrus.

The developmental morphology of Torpedo marmorata: electric lobe-electromotoneuron proliferation and cell death.

Electromotoneuron proliferation and cell death have been quantitatively studied in the electric lobe of Torpedo marmorata from an embryonic body-length stage of 26-mm to adult animals. These neurons project to the electric organ and form synapses with electrocytes which possess a remarkably large postsynaptic target surface. For this reason cell death would not be predicted to occur if synaptic competition were to be hypothesized as the cause. Isolated observations at the ultrastructural level suggested, however, that cell death was indeed taking place and therefore it seemed appropriate to examine this question in detail. Our findings show first that neuron production appears to be a continuous process throughout the period studied, generating totals of over 70,000 electromotoneurons per lobe by adulthood. Second, two waves of cell death were identified, one occurring early in embryogenesis (stage 30 mm), well before the onset of synaptogenesis, and a second coincident with the onset of synaptogenesis (stages 55--74 mm). It is difficult to reconcile this latter wave with the hypothesis of synaptic competition as the postsynaptic surface at this time of development is largely devoid of synaptic contacts. We conclude that in the electromotor system of Torpedo, synaptic competition is probably not the mechanism of cell death.

Axonal growth cones and developing axonal collaterals form synaptic junctions in embryonic mouse spinal cord.

Axonal growth cones and developing axonal collaterals have been studied in electron microscopic and Golgi preparations of embryonic mouse cervical spinal cord. These structures are first observed on embryonic day 11 (E11), and by E12 both axonal growth cones and developing collaterals are observed to be the presynaptic elements of developing synaptic junctions. These relatively undifferentiated portions of axons comprise nearly one-half of the presynaptic components of the synaptic contacts observed on E12, and they continue to constitute about this same proportion during the rest of the embryonic period examined (E13–16). These early synaptic interactions of developing axonal collaterals may be involved in establishing the basic ramification patterns of mature axons by determining which developing collaterals will continue to grow and differentiate. Coated vesicles are observed fused with axonal collateral plasma membranes at the perimeters of developing synaptic contacts. Occasionally thin extensions of postsynaptic surfaces protrude into these coated vesicles, and similar relationships are also observed between presynaptic surfaces and coated vesicles fused with the membranes of postsynaptic elements. Such coated vesicle/ synaptic surface relationships may represent an early stage in the endocytosis of synaptic membranes by both the pre- and postsynaptic elements of developing synaptic contacts. This might represent a mechanism for the exchange of ‘molecular information’ between the components of protosynaptic contacts, and the results of such an exchange could be involved in determining whether protosynaptic contacts will differentiate into synaptic junctions. Developing axonal collaterals exhibit a preferential growth toward the sources of potential postsynaptic elements, and this growth implies that the axons present in the embryonic marginal zone are actively searching for appropriate postsynaptic processes rather than passively awaiting their arrival. Many of the collaterals weave around numerous intervening processes in order to reach their postsynaptic destinations, and unusual subaxolemmal lamellae are commonly observed where collaterals make acute curvatures around neurites which block their direct access to postsynaptic processes. The location of these lamellae suggests that they could be involved in producing changes in the direction of growth as developing axonal collaterals search for synaptic partnerships. Main axonal shafts commonly exhibit synaptic contacts adjacent to where they give rise to developing collaterals, and this observation hints that the formation of synapses by axonal shafts might stimulate collateral sprouting. However, other observations (see text) indicate that the formation of synaptic contacts by axonal shafts is not an obligatory prelude to collateral development. Therefore synaptic interactions of main axonal shafts do not appear to be a requirement for the primary induction of collateral development, but they may facilitate collateral growth by providing a confirmation that axons are located in appropriate synaptogenic fields.

Localization of acetylcholine receptors in central synapses.

The localization of cholinergic receptors in brain synaptosomes and in synapses of the midbrain reticular formation and hypothalamic preoptic nucleus has been demonstrated by means of a horseradish peroxidase-alpha-bungarotoxin (HRP-alpha-Btx) conjugate. Only a small proportion of the total number of synapses was reactive. Axon terminals of reactive synapses contained primarily small clear vesicles, while synapses characterized by large numbers of dense core vesicles were unreactive. Toxin-binding sites were found to occur in a thickened zone of the postsynaptic surface. This procedure can be employed to study the regional distribution and localization of nicotinic receptor sites in the central nervous system.

Protein differences associated with the absence of granule cells in the cerebella from the mutant weaver mouse and from X-irradiated rat

The development of synaptic connections in the nervous systems of vertebrates [l] and invertebrates [2,3] is subject to a stringent genetic determinism which governs, for instance, the differentiation and migration of neurons, the growth of their processes and the selective recognition between preand postsynaptic surfaces. Nevertheless, there exists some evidence that the final connectivity and the functional state of some particular synapses, in both central and peripheral nervous systems, might be modulated by their activity at critical stages of development (see ref. 14751). In spite of its complex structure, the cerebellum constitutes a particularly suitable region of the brain to study these processes and, in particular, to distinguish between genetic and ‘epigenetic’ [5] phenomena. Its anatomy is rather well understood [6,8] ; the electrophysiological activity of single neurons can be recorded [6,9] ; most of its development occurs postnatalIy and becomes therefore easily accessible to the experiment; it contains only a few classes of cells but these cells are repeated a large number of times and therefore amenable to direct biochemical analysis. In the mouse, a number of mutations have been identified and mapped which lead to profound alterations of cerebellum anatomy [lo] and therefore of its physiology [ 111. In addition, one can induce some phenocopies of these mutations, such as the lack of granule cells, by drug injection [ 12-151, infection with specific viruses [ 161 or X-irradiation [ 17,181. In this paper, we describe experiments done with agranular cerebella from both the mutant weaver (WV) mouse [ 19-241 and X-irradiated rats. The electrophoretie patterns of the proteins solubilised from subcellular fractions of these cerebella are compared with those obtained with normal mouse and rat. Striking differences are noticed and, by comparison with the patterns obtained with a preparation of purified rat granule cells, assigned to the lack of granule cells.

Correlation between postsynaptic activity and surface potentials in the gigantopyramidal area of the cat cortex.

Cats were anesthetized with chloralose and pentobarbital and immobilized with muscle relaxants. Parallel macroelectrode recordings of the spontaneous electrocorticogram (ECoG) and cortical surface potentials — primary and association responses (PRs and ARs) — evoked by peripheral stimuli and microelectrode intracellular recordings of unit activity in the gigantopyramidal area were obtained. Both spontaneous and evoked changes in membrane potential in many cases correlated clearly with certain components of the ECoG, PR and AR. The most obvious and stable correlation was that between postsynaptic depolarization in the neurons of all three groups isolated (pyramidal tract, unidentified corticofugal neurons, and interneurons) and the first positive component of the PR and AR. A tendency was observed for synchronous development of the inhibitory postsynaptic potential (IPSP) and negative phase of the evoked potentials. No specific correlates were found in the ECoG for spike discharges in single neurons. The possibility of assessing cellular mechanisms of the genesis of PR and AR components is discussed on the basis of correlation observed between PSPs and global cortical surface potentials. The factor deciding the probability of appearance of an ECoG wave is evidently the degree of synchronization of PSP generation in large populations of cortical neurons.

Retinal structure in the smooth dogfish, Mustelus canis: general description and light microscopy of giant ganglion cells.

AbstractGiant ganglion cells (GGC) were demonstrated in retinas of adult dogfish by Golgi impregnation, reduced silver and vital methylene blue staining. All GGC have large, flattened perikarya, and dendrites which radiate in a single horizontal plane of the inner synaptic (plexiform) layer. They are identifiable as ganglion cells by their axons, which after a tortuous initial course enter the nerve fiber layer. We divide the GGC into three varieties: (1) ordinary, with perikarya in the layer of ganglion cells and nerve fibers, and with dendrites radiating in the inner (proximal) one‐fourth of the inner synaptic layer; (2) displaced or Dogiel's cells, with perikarya in the amacrine cell layer and dendrites radiating in the outer (distal) one‐fourth of the inner synaptic layer; and (3) intermediate, with both perikarya and dendritic trees at intermediate levels in the inner synaptic layer.Only the ordinary and displaced GGC in the ventral portion of retinas stained with methylene blue were studied in detail. Cells of both types are arranged in irregular patterns with perikarya about 0.8–1.0 mm apart. Dendrites of the ordinary GGC spread within an elliptical field having a major axis of about 1.7 mm (2.0 mm max) and a minor axis of about 1.5 mm (1.7 mm max). The major axis is vertical in the eye. Dendrites of the displaced GGC spread within a circular field having a diameter of about 1.9–2.0 mm (2.2 mm max). Quantitative comparison of the dendritic trees of ordinary and displaced GGC shows that they differ also in details of dendritic morphology, the dendrites of the ordinary GGC being on the average more numerous and highly branched but shorter than those of the displaced GGC. Ordinary and displaced GGC, therefore, comprise distinct populations which must be presumed to differ functionally.For both ordinary and displaced GGC, the dendritic density and, therefore, the total available postsynaptic surface per unit retinal area decline exponentially with distance from the center to the edge of the dendritic tree. In contrast, the sensitivity to light is constant throughout the receptive field centers of comparable diameter which have been analysed in parallel electrophysiological studies. While the diameters of the largest ganglion cell dendritic fields and receptive field centers are similar, therefore, more detailed correlations of structure and function cannot be made at present.