Electron-dense Terminals Research Articles

Responses of Rat Brain Interneuronal Synapses to Hypoxia in the Early Neonatal Period

The responses of forming synapses in the rat neocortex to the actions of hypoxia in the early period of neonatal life (day 2) were studied. Immunocytochemical studies were used to detect synaptophysin and these results, along with electron microscopic studies, addressed the sensorimotor cortex in rat pups at 3, 5, and 10 days of postnatal development (using groups of 6‐10 individuals) in an experimental group and a control group (intact animals). Immunocytochemical studies of control animal showed significant differences in the quantitative distribution of synaptophysin-positive structures in different layers of the neocortex during the early neonatal period of development (day 5). Perinatal hypoxia decreased the optical density of the immunocytochemical reaction product more than twofold, and this was accompanied by reductions in the density of synaptophysin-positive granules in all layers of the neocortex. In addition, electron-dense terminals, providing evidence of degenerative processes, were seen. The neuropil of the neocortex showed a sharp decrease in the number of growth cones, small processes, and forming synapses, along with a significant increase in the electron density of synaptic elements, especially postsynaptic membranes and densities. In experimental animals, increases in the numbers of growth cones and forming synaptic structures were seen only by postnatal day 10. Thus, the consequences of hypoxia during the early neonatal period, inducing impairments to synaptogenesis, persisted throughout the study period.

Electron microscopic analysis of lesion-induced changes in synaptic structure and immunogold labeling of neurotransmitters within the feline trigeminal nucleus.

This report uses lesion and postembedding immunogold protocols to examine the ultrastructural details of lesion-induced synaptic and neurotransmitter changes in the feline trigeminal nucleus. Electron microscopic (EM) analysis concentrated on lamina II (substantia gelatinosa) of the subnucleus pars caudalis (PC) which is one relay site or trigeminal fibers involved in nociception. Special attention was directed to analysis of reoccupation of synaptic sites vacated by primary afferent degeneration. Primary afferents are caused to degenerate by performing unilateral retrogasserian rhizotomy. After survival times of 1, 2, 6, and 7 days, sections of PC were processed for postembedding immunogold labeling with antibodies to the neurotransmitters gamma aminobutyric acid (GABA) and glutamate (Glu). The results show: (1) degenerating primary afferent terminals were easily identified in various stages of degeneration; (2) Glu immunoreactivity was observed in early forms of degenerated primary afferent terminals with clumped vesicles as well as in the highly distorted, electron dense terminals of later degeneration; and (3) some GABA immunoreactive terminals formed atypical synapses which exhibited both asymmetric (excitatory) and symmetric (inhibitory) synaptic densities. A possible model is presented of the progression of events following trigeminal nerve lesion which results in atypical synapse formation. Such altered synaptic relationships seen in PC following trigeminal rhizotomy may be related to hyperactivity that is seen in animals and to the atypical facial pain following nerve lesions in humans.

Neural grafting to ischemic lesions of the adult rat hippocampus.

The purpose of this study was to examine the structural and connective integration of developing hippocampal neurons grafted to ischemic lesions of the adult rat hippocampus. The 4-vessel occlusion model was used to cause transient cerebral ischemia which damages CA1 pyramidal cells in the dorsal hippocampus, but spares nonpyramidal neurons and afferents in the area. One week later, cell suspensions were made from the CA1 region of fetal (E18-20) rats and injected stereotaxically into the lesion. The recipient brains were examined 6 weeks to 6 months later for survival, morphology, and intrinsic and extrinsic connections of the grafts. The methods used included cell stains, histochemical staining for acetylcholinesterase (AChE), immunocytochemical staining for neuropeptides (cholecystokinin (CCK), somatostatin (SS), enkephalin (Enk) and an astrocytic marker, glial fibrillary acidic protein (GFAP), as well as tracing by retrograde axonal transport of fluorochromes and light and electron microscopy of anterograde axonal degeneration. The grafts survived well (80%) and were often quite large. They were well integrated in the lesioned host brain area, contained both pyramidal cells and neuropeptidergic neurons and displayed a near normal GFAP immunoreactivity for astrocytes. The latter contrasted the dense gliosis of the host ischemic lesion. Judged by the AChE staining the grafts were innervated by cholinergic host septohippocampal fibers. Ingrowth of host hippocampal commissural fibers was demonstrated by Fink-Heimer staining for degenerating nerve terminals following acute lesions of the hippocampal commissures. At the ultrastructural level degenerating, electron dense terminals of host commissural origin were found even deep inside the graft neuropil in synaptic contact with mainly dendritic spines. A transplant efferent connection to the host brain was demonstrated by retrograde fluorochrome tracing and consisted of a homotypic projection to more posterior levels of the ipsilateral host CA1 and subiculum. Minor abnormal, efferent projections to the host dentate molecular layer were shown in Timm staining. We conclude that fetal CA1 neurons grafted to one week old ischemic lesions of the dorsal CA1 in adult rats become structurally well incorporated and can establish nerve connections with the host brain.

(3H) glycine-accumulating neurons of the human retina.

Isolated human retinas were incubated in physiological saline containing micromolar (3H) glycine. The types, distributions, and synaptologies of glycine-accumulating neurons were determined by light and electron microscope autoradiography. Two types of amacrine cells were discriminated on the bases of number of processes descending into the inner plexiform layer, density of label in light-microscope autoradiographs, size, and synaptic features: (1) Gly1 amacrine cells have moderate labeling, several oblique dendrites arising from the soma, and electron lucent synaptic terminals containing large presynaptic specializations, nd (2) Gly2 amacrine cells have dense labeling, a single proximal dendrite, and moderately electron-dense terminals with small presynaptic specializations. Gly1 amacrine cells constitute approximately 15% and Gly2 amacrine cells approximately 38% of all cells in the amacrine cell layer. The laminar distribution of label in the inner plexiform layer was measured by scanning microdensitometry, which provided a format for categorizing types of synaptic contacts. Many features of glycine-accumulating amacrine cell contacts were similar to those of cat AII/Gly2 amacrine cells: a diffuse yet bisublaminar distribution of label, concentration of synaptic output in sublamina a, rod bipolar cell input in sublamina b and gap junctions in mid-inner plexiform layer involving labeled cells. The evidence seems to indicate that human Gly2 amacrine cells and cat AII/Gly2 amacrine cells are homologous cell types. finally, some cone bipolar cells were labeled.

Commissural fibers terminate on non-pyramidal neurons in the guinea pig hippocampus — a combined Golgi/EM degeneration study

The combined Golgi/EM method was applied to guinea pig hippocampi with acute anterograde degeneration of the commissural afferents in order to identify possible synaptic contacts between commissural terminals and non-pyramidal neurons. Degenerating, electron-dense terminals of commissural origin were found in synaptic contact with both perikarya and dendrites of two identified non-pyramidal neurons in regio superior, namely a basket cell and a bipolar neuron in stratum oriens. The observed connection may form the morphological basis for the physiologically observed feed-forward inhibition of the pyramidal cells.

Fine structure and fluoride resistant acid phosphatase activity of electron dense sinusoid terminals in the substantia gelatinosa Rolandi of the rat after dorsal root transection.

Fluoride resistant acid phosphatase (FRAP) activity of the rat substantia gelatinosa Rolandi is confined to electron dense sinusoid terminals under normal conditions. Transection of dorsal roots or removal of dorsal root ganglia results in a rapid degeneration of more than half of the electron dense sinusoid axon terminals. First signs of degeneration ensue 20 hours after surgery; at the 24 hours state osmiophilic degeneration bodies develop that are translocated into glial elements in the course of the second postoperative day. At the same time, light microscopically visible FRAP-activity of the Rolando substance disappears. Electron histochemical investigations reveal that decreased enzyme activity is due to degeneration of FRAP-positive terminals. It is concluded that FRAP-positive terminals, representing the majority of electron dense sinusoids in the Rolando substance, are dorsal root collaterals; the origin of non-degenerating FRAP-negative electron dense terminals remains unknown for the time being.