Continuities between outer nuclear membrane and the rough endoplasmic reticulum increase in hippocampal neurons during seizure-induced protein synthesis

The soma and proximal dendrites of sympathetic preganglionic neurons innervating the major pelvic ganglion in female rats receive predominantly inhibitory inputs

Sympathetic preganglionic neurons (SPNs) in the intermediolateral (IML) and dorsal commissural nucleus (DCN) of the thoracolumbar segments of the spinal cord contribute to the autonomic control of the pelvic visceral organs. We examined the morphology of these neurons at the light and electron microscopic level and quantified the boutons apposing the soma and proximal dendrites of the SPNs innervating the major pelvic ganglion (MPG) in female rats. The majority of these cells resided in the DCN (61.6±6.2%) and IML (33.2±4.4%) nuclei. Measurements of cell volume and shape revealed no differences between SPNs sampled from the DCN and IML populations. Ultrastructural studies of DCN and IML SPNs revealed that coverage of SPNs by synaptic inputs is sparse, with an average of 11.60±2.41% of the soma membrane and 16.33±6.18% of proximal dendrites apposed by boutons, though some somata exhibited no synaptic coverage. Three distinct types of boutons were found to appose the SPN somata and dendrites. The putatively inhibitory F-type bouton covered a significantly greater percentage of membrane on the soma (8.48±2.12%) and dendrites (12.65±4.34%), than the S-type bouton, a putatively excitatory bouton, which only covered 2.94±0.70% of the somatic and 3.68±2.98% of the dendritic membranes. Boutons with dense-core vesicles were rare. Our results demonstrate that SPNs of the DCN and IML of female rats are similar morphologically, and that synaptic input on these cells, though sparse, is predominantly inhibitory.

Ultrastructure of retinal ganglion cell death after axotomy in chick embryos

Axotomy often leads to neuronal death, which occurs after a particularly short delay in immature animals. Tectal lesions were made in embryonic day (E) 12 chick embryos, thereby axotomizing the retinal ganglion cells of the contralateral eye, which then died within 3 days. We here describe the ultrastructural changes in the axotomized ganglion cells. The main changes were nuclear invagination and type 3B (cytoplasmic type) cell death characterized by dilation of the perinuclear space, endoplasmic reticulum, and Golgi apparatus. However, nuclear invagination was never seen in type 3B dying cells. All the axotomy-induced retinal ganglion cell death appears to have been of type 3B; apoptosis was not induced by axotomy, as was confirmed by additional light microscopic experiments showing that it did not increase the frequency of apoptotic markers revealed by terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (the TUNEL method) labeling and immunoreactivity for activated caspase-3. However, the latter methods did show small numbers of apoptotic cells dying naturally even in control retinas. After the death of the axotomized ganglion cells, they were phagocytosed mainly in Müller processes. The present findings open up the chick tectal lesion model as a system for analyzing type 3B neuronal death in vivo.

Changes in activating protein 1 (AP-1) composition correspond with the biphasic profile of nerve growth factor mRNA expression in rat hippocampus after hilus lesion-induced seizures

In adult brain, nerve growth factor (NGF) gene expression is generally upregulated by neuronal activity. However, a single episode of hilus lesion (HL)-induced limbic seizures stimulates a biphasic increase in NGF mRNA expression with peaks at 4-6 and 24 hr after lesion and an intervening return to control levels at 10-12 hr after lesion. In vitro studies suggest that NGF transcription is regulated via an activating protein 1 (AP-1) binding site in the first intron of the NGF gene. To examine the relationship between seizure-induced AP-1 binding and NGF gene expression in this paradigm, NGF mRNA levels and AP-1 binding were examined after HL seizures. Furthermore, to gain insight into the functional composition of the AP-1 complex, supershift analysis was performed to characterize which Fos and Jun family members are included in the AP-1-binding complex at the different time points analyzed. Solution hybridization analysis verified the biphasic increase in NGF mRNA content of the dentate gyrus after HL seizures. After an initial increase, AP-1 binding slowly declined in a stepwise manner that encompassed, but did not correspond with, the two phases of NGF mRNA expression. However, supershift analyses demonstrated that the relative contributions of JunD and JunB to the AP-1 complex exhibited positive and negative correlations, respectively, with the phases of increased NGF expression after HL. These results suggest that AP-1 complexes containing JunD promote NGF transactivation and that transient changes in the relative contributions of JunD and JunB to AP-1 binding underlie the biphasic increase in NGF gene expression induced by HL seizures.

Expression of agrin mRNA is altered following seizures in adult rat brain

Agrin mRNA is broadly distributed throughout the adult rat brain, consistent with its proposed role in synaptogenesis and the organization of synaptic proteins in the central nervous system. The present study examined the effect of neuronal activity on agrin mRNA expression in adult rat forebrain using the hilus lesion paradigm for seizure induction and in situ hybridization and polymerase chain reaction techniques for quantification and characterization of agrin mRNA content. Seizures induced rapid, prolonged, and region-specific changes in agrin mRNA expression with the most prominent alterations occurring in hippocampal and cortical neurons. However, there were no detectable perturbations in the relative abundance of alternatively spliced agrin transcripts in affected brain regions. Activity-dependent changes in agrin expression suggest a role for this protein in modifications of synaptic structure associated with functional synaptic plasticity.

Ultrastructural plasticity of the dentate gyrus granule cells following recurrent limbic seizures: I. Increase in somatic spines

Various paradigms have been used to assess the capacity of the adult brain to undergo activity-dependent morphological plasticity. In this report we have employed recurrent limbic seizures as a means of studying the effects of this form of enhanced neuronal activity on cellular morphology and, in particular, on the incidence of somatic spines on the dentate gyrus granule cells. Seizure activity was induced by the placement of focal, unilateral electrolytic lesions in the dentate gyrus hilus of adult rats. At various intervals postlesion, rats with behaviorally verified seizures were sacrificed, and the hippocampi contralateral to the lesions were removed and prepared for electron microscopy. Quantitative analysis showed that as early as 5 hours postlesion there was a dramatic increase in the density and morphological complexity of spines on the perikarya of the granule cells in rats that received seizure-producing hilus lesions when compared to granule cells from control rats. Many of the somatic spines received asymmetric synapses. The increase in somatic spines was dependent on seizure activity and persisted for at least 1 month following a single recurrent seizure episode. CA1 pyramidal neurons, which exhibit changes in gene expression in response to hilus lesion-induced seizures but do not normally possess somatic spines, did not exhibit an activity-dependent elaboration of somatic spines. Thus, the seizure-induced elaboration of somatic spines represents an amplification of an existing feature of the granule cells and not an effect occurring throughout hippocampus. These data provide evidence for very rapid and long-lasting structural plasticity in response to brief episodes of seizure activity in the adult brain.

Hippocampal epileptogenesis produced by electrolytic iron deposition in the rat dentate gyrus

Anodal current passed through a stainless-steel electrode, positioned unilaterally in the rat dentate gyrus hilus, will produce recurrent motor seizures and significant changes in the neuronal expression of several messenger RNAs (mRNAs) throughout the full bilateral extent of the hippocampus. The present study quantitatively analyzed electroencephalograms (EEGs) from rats receiving this electrolytic treatment in order to characterize the resultant hippocampal seizure activity. To examine the epileptogenic role of ferric ion deposition to that of current-induced tissue destruction, we compared steel to platinum electrodes. Adult male rats were surgically implanted with a chronic recording electrode in the CA3 region of the hippocampus, and then (contralaterally) with either an insulated steel electrode in the hilus, platinum electrode in the hilus, or steel electrode in the medial entorhinal cortex. Each rat received an anodal current through the nonrecording treatment electrode while connected to a polygraph. Currents ranged from 0.8 mA, 7 s for hilus electrodes to 2.0 mA, 20 s for entorhinal cortex electrodes. EEGs were collected from alert, unrestrained rats for up to 50 consecutive hours, and additional EEGs were recorded periodically over a 4-day period. Subjects were sacrificed and brain sections were microscopically examined for evidence of neuropathology. The results demonstrate that electrolytic deposition of iron ions in the hilus, and not merely hilus tissue destruction, produce electrographic seizure activity within 1-2 h of current passage. Seizures recurred most intensely for 2-3 h, and sporadic epileptiform activity was detected for up to 12 h. Motor seizures of class 4 or 5 were observed in all seizing rats, and were always coincident with hippocampal seizure discharges. Histological examination of brain sections from all subjects found no evidence of cell death in the contralateral hippocampus. The dentate gyrus appeared to be the most epileptogenic site tested because hippocampal iron deposition that did not include the dentate gyrus, or iron deposition in the entorhinal cortex, was significantly less epileptogenic.

Ultrastructural changes in contralateral vestibulo-ocular neurons following vestibular neurectomy in the cat

Twenty contralateral superior vestibular vestibulo-ocular neurons (SVON) from 4 cats were studied morphologically 8 weeks after a right vestibular neurectomy. Nine SVON demonstrated a 35% loss of somal synaptic profiles (SP), normal nuclear and cytoplasmic size, but a slight decrease in organelles responsible for protein synthesis (rough endoplasmic reticulum and polyribosomes). Eleven SVON showed an 82% loss of SP, decrease in cell and nuclear size, and significant reduction in rough endoplasmic reticulum (RER) and polyribosomes. These transneuronal SVON changes are presumed to be the result of commissural pathway degeneration caused by the vestibular neurectomy.

Levels of mRNA for a putative kainate receptor are affected by seizures

In situ hybridization and RNA blot-hybridization techniques were used (i) to examine the regional distribution of mRNA for a putative kainate receptor in adult rat brain and ii) to test the possibility that seizures affect expression of the receptor gene. The highest densities of hybridization were distributed within hippocampal pyramidal and granule cells, medial habenula, Purkinje cells and the molecular layer of cerebellum, and olfactory bulb. Recurrent limbic seizures caused a massive, delayed, and reversible reduction in levels of the kainate receptor mRNA in dentate gyrus; lesser decreases were found in pyramidal cell fields of hippocampus and superficial cortex. These findings provide evidence that unusual patterns of physiological activity can alter genomic expression for a subclass of glutamate receptors in brain.