Number Of Symmetric Synapses Research Articles

Decreased surface expression of the δ subunit of the GABAA receptor contributes to reduced tonic inhibition in dentate granule cells in a mouse model of fragile X syndrome

While numerous changes in the GABA system have been identified in models of Fragile X Syndrome (FXS), alterations in subunits of the GABAA receptors (GABAARs) that mediate tonic inhibition are particularly intriguing. Considering the key role of tonic inhibition in controlling neuronal excitability, reduced tonic inhibition could contribute to FXS-associated disorders such as hyperactivity, hypersensitivity, and increased seizure susceptibility. The current study has focused on the expression and function of the δ subunit of the GABAAR, a major subunit involved in tonic inhibition, in granule cells of the dentate gyrus in the Fmr1 knockout (KO) mouse model of FXS. Electrophysiological studies of dentate granule cells revealed a marked, nearly four-fold, decrease in tonic inhibition in the Fmr1 KO mice, as well as reduced effects of two δ subunit-preferring pharmacological agents, THIP and DS2, supporting the suggestion that δ subunit-containing GABAARs are compromised in the Fmr1 KO mice. Immunohistochemistry demonstrated a small but statistically significant decrease in δ subunit labeling in the molecular layer of the dentate gyrus in Fmr1 KO mice compared to wildtype (WT) littermates. The discrepancy between the large deficits in GABA-mediated tonic inhibition in granule cells in the Fmr1 KO mice and only modest reductions in immunolabeling of the δ subunit led to studies of surface expression of the δ subunit. Cross-linking experiments followed by Western blot analysis demonstrated a small, non-significant decrease in total δ subunit protein in the hippocampus of Fmr1 KO mice, but a four-fold decrease in surface expression of the δ subunit in these mice. No significant changes were observed in total or surface expression of the α4 subunit protein, a major partner of the δ subunit in the forebrain. Postembedding immunogold labeling for the δ subunit demonstrated a large, three-fold, decrease in the number of symmetric synapses with immunolabeling at perisynaptic locations in Fmr1 KO mice. While α4 immunogold particles were also reduced at perisynaptic locations in the Fmr1 KO mice, the labeling was increased at synaptic sites. Together these findings suggest that, in the dentate gyrus, altered surface expression of the δ subunit, rather than a decrease in δ subunit expression alone, could be limiting δ subunit-mediated tonic inhibition in this model of FXS. Finding ways to increase surface expression of the δ subunit of the GABAAR could be a novel approach to treatment of hyperexcitability-related alterations in FXS.

Increased anxiety‐like behavior in mice lacking the inhibitory synapse cell adhesion molecule neuroligin 2

Neuroligins (NL) are postsynaptic cell adhesion molecules that are thought to specify synapse properties. Previous studies showed that mutant mice carrying an autism-associated point mutation in NL3 exhibit social interaction deficits, enhanced inhibitory synaptic function and increased staining of inhibitory synaptic puncta without changes in overall inhibitory synapse numbers. In contrast, mutant mice lacking NL2 displayed decreased inhibitory synaptic function. These studies raised two relevant questions. First, does NL2 deletion impair inhibitory synaptic function by altering the number of inhibitory synapses, or by changing their efficacy? Second, does this effect of NL2 deletion on inhibition produce behavioral changes? We now show that although NL2-deficient mice exhibit an apparent decrease in number of inhibitory synaptic puncta, the number of symmetric synapses as determined by electron microscopy is unaltered, suggesting that NL2 deletion impairs the function of inhibitory synapses without decreasing their numbers. This decrease in inhibitory synaptic function in NL2-deficient mice correlates with a discrete behavioral phenotype that includes a marked increase in anxiety-like behavior, a decrease in pain sensitivity and a slight decrease in motor co-ordination. This work confirms that NL2 modulates inhibitory synaptic function and is the first demonstration that global deletion of NL2 can lead to a selective behavioral phenotype.

Effect of chronic olanzapine treatment on striatal synaptic organization.

Our previous work has shown that chronic haloperidol treatment decreases striatal symmetric synapses preferentially in rats which develop oral dyskinesias (vacuous chewing movements (VCMs)). The present experiment tests the hypothesis that olanzapine, which does not cause dyskinesia in humans or rats, would not cause the ultrastructural changes produced by haloperidol. After 6 months of treatment, VCM scores for the olanzapine group (5.1 +/- 4.5) were similar to those of controls (5.2 +/- 3.9), whereas rats in the haloperidol group were either nondyskinetic (4.3 +/- 2.2) or dyskinetic (16.9 +/- 6.7). The volume of the striatum (mm(3)), did not differ among the groups: control, 37.5 +/- 4.7; olanzapine, 36.4 +/- 4.3; haloperidol, nondyskinetic, 40.5 +/- 6.3; haloperidol, dyskinetic, 36.6 +/- 5.9. Synaptic density (per 1 microm(3)), obtained from the central region of the striatum, did not differ between the olanzapine (0.699 +/- 0.146) and control groups (0.652 +/- 0.108). The number of asymmetric synapses in the olanzapine group (0.624 +/- 0.136) was also similar to that of controls (0.550 +/- 0.090). The number of symmetric synapses in the olanzapine group (0.074 +/- 0.032) was not significantly different from that of controls (0.096 +/- 0.043). Thus, olanzapine, in contrast to haloperidol, did not produce dyskinesias or synapse loss. These results strengthen the correlation between the expression of VCMs and striatal synaptic changes and indicate that olanzapine has fewer behavioral and anatomical side effects than does haloperidol.

Cholinergic innervation of the mediodorsal thalamic nucleus in the monkey: ultrastructural evidence supportive of functional diversity.

The ultrastructural organization of association nuclei in the primate thalamus is largely unexplored. In the present study we have combined electron microscopy with immunocytochemistry for the acetylcholine synthesizing enzyme choline acetyltransferase (ChAT) to assess the cholinergic synaptic organization of the mediodorsal (MD) nucleus in macaque monkeys. The cholinergic innervation of the MD nucleus showed striking regional variations with the greatest density of immunoreactive axons and varicosities found within the parvicellular division. Electron microscopic examination revealed that these ChAT immunoreactive (ChAT-IR) axons were primarily small and unmyelinated. The majority of immunoreactive synaptic profiles were found within the extraglomerular neuropil (80.5%), with the remainder present in glomerular regions. Within the glomerular and extra-glomerular neuropil ChAT-IR profiles made contact with both conventional, presumably relay cell dendrites (CD), as well as with synaptic vesicle containing dendrites (SVCD) of local circuit neurons. In the glomeruli the frequency of synapses was approximately equal for CDs and SVCDs while in the extraglomerular areas 75% of the synaptic contacts were with CDs. ChAT-IR synaptic profiles had a diversity of junctional complex morphologies. Within glomeruli they made symmetric synapses with CDs and predominantly asymmetric with SVCDs. The majority of extraglomerular contacts (60%) were classified as asymmetric and these as well as the smaller number of symmetric synapses contacted both CDs and SVCDs. In accord with results of physiological studies, these anatomical data indicate that cholinergic input to thalamic nuclei influences relay cell activity both directly and indirectly via local circuit neurons.

Morphological evidence for altered synaptic organization and structure in the hippocampal formation of seizure-sensitive gerbils.

Seizure-sensitive (SS) and seizure-resistant (SR) Mongolian gerbils were used for three experiments. In the first experiment, GABAergic neurons and terminals in the dentate gyrus were localized with GAD immunocytochemistry. GAD-positive puncta adjacent to cell bodies of GABAergic pyramidal basket cells were counted in light microscopic preparations. The pyramidal basket cells of SS gerbils displayed a significant threefold increase in the number of GAD-positive puncta associated with their cell bodies as compared to those from SR gerbils. These data indicate that the number of GABAergic synapses with pyramidal basket cell bodies in the dentate gyrus was greater in SS gerbils. An electron microscopic (EM) analysis of GAD immunocytochemical preparations showed GAD-positive axon terminals forming symmetric synapses with GAD-positive basket cell bodies. However, numerous terminals forming symmetric axosomatic synapses with basket cells were not immunopositive, and other synapses formed by terminals were not classified because reaction product in the cell bodies obscured postsynaptic densities. Therefore, routine EM preparations were analyzed for symmetric and asymmetric axosomatic synapses on pyramidal basket cells and granule cells of SS and SR gerbils. The data obtained from these preparations showed that the pyramidal basket cells of SS gerbils had a selective increase in the number of symmetric synapses per 10 microns of soma as compared to those of the SR gerbils. In contrast, the granule cells did not show any significant difference in the number of either symmetric or asymmetric axosomatic synapses between SS and SR gerbils. These results indicate that pyramidal basket cell bodies of SS gerbils have more inhibitory synapses than do those of SR gerbils. The third experiment used SS gerbils with lesions of the perforant pathway that stopped seizure activity (Ribak, C. E., and S. U. Khan (1987) The effects of knife cuts of hippocampal pathways on epileptic activity in the seizure-sensitive gerbil. Brain Res. 418:251-260). The percentage of axon terminal area occupied by synaptic vesicles and their packing density was determined in CA3 mossy fiber boutons and compared for lesioned and nonlesioned SS gerbils. The mossy fibers of nonlesioned SS gerbils showed a depletion of synaptic vesicles consistent with the previous results of Peterson et al. (Peterson, G. M., C. E. Ribak, and W. H. Oertel (1985) A regional increase in the number of hippocampal GABAergic neurons and terminals in the seizure-sensitive gerbil. Brain Res. 340:384-389).(ABSTRACT TRUNCATED AT 400 WORDS)

A preferential loss of GABAergic, symmetric synapses in epileptic foci: a quantitative ultrastructural analysis of monkey neocortex

Previous immunocytochemical results from five monkeys with cortical focal epilepsy produced by alumina gel showed a severe decrease at seizure foci of axon terminals that contained glutamic acid decarboxylase (GAD), the synthesizing enzyme for the inhibitory neurotrasmitter, GABA. These data indicated a functional loss of GABAergic terminals but did not show: (1) whether this loss was caused by GABAergic nerve terminal degeneration or by a lack of GAD immunoreactivity within these terminals and (2) if this loss of GABAergic terminals was selective for only this terminal type. To resolve these issues, cortical tissue from three of the five monkeys used in the previous study was reexamined using electron microscopy, and a quantitative morphological analysis of cortical structures was made to compare profiles of terminals and glia in the nonepileptic cortex with those in the focus and parafocus. The following statistically significant changes were observed: (1) the number of axosomatic symmetric synapses with layer V pyramidal cells was decreased 80% at the focus and 50% at the parafocus, (2) in the neuropil adjacent to these pyramidal somata, the number of terminals forming symmetric synapses was reduced 50% at the epileptic focus but was unchanged at the parafocus, while the number of asymmetric synapses was reduced 25% at the focus and 15% at the parafocus, and (3) a 50% increase of glial profiles occurred at epileptic foci both in the neuropil and at sites apposed to pyramidal cell somata. The quantitative results also showed that terminals which form symmetric synapses had twice the number of mitochondria per terminal as those that form asymmetric synapses. Axon terminals which form symmetric synapses with somata and dendrites in the neocortex have been shown previously to contain GAD. Therefore, the large reduction in the number of symmetric synapses at epileptic foci and the increased gliosis indicate that the previously observed loss of GABAergic terminals at sites of focal epilepsy is caused by terminal degeneration. Since such terminals are reduced more severely at epileptic foci than other terminals, their loss could be the basis for seizure activity due to a preferential decrease of inhibitory function at epileptic foci. Hypoxia has been shown to cause a selective degeneration of terminals with the same morphology as GABAergic terminals in the cortex, and the basis for this loss could be related to higher physiological and/or metabolic activities of GABAergic cortical cells which may inhibit other cells tonically. The fact that increased numbers of mitochondria occur in GABAergic terminals supports this idea.