Long-Term Potentiation Produces a Sustained Expansion of Synaptic Information Storage Capacity in Adult Rat Hippocampus

Long-term potentiation (LTP) has become a standard model for investigating synaptic mechanisms of learning and memory. Increasingly, it is of interest to understand how LTP affects the synaptic information storage capacity of the targeted population of synapses. Here, structural synaptic plasticity during LTP was explored using three-dimensional reconstruction from serial section electron microscopy. Storage capacity was assessed by applying a new analytical approach, Shannon information theory, to delineate the number of functionally distinguishable synaptic strengths. LTP was induced by delta-burst stimulation of perforant pathway inputs to the middle molecular layer of hippocampal dentate granule cells in adult rats. Spine head volumes were measured as predictors of synaptic strength and compared between LTP and control hemispheres at 30 min and 2 hr after the induction of LTP. Synapses from the same axon onto the same dendrite were used to determine the precision of synaptic plasticity based on the similarity of their physical dimensions. Shannon entropy was measured by exploiting the frequency of spine heads in functionally distinguishable sizes to assess the degree to which LTP altered the number of bits of information storage. Outcomes from these analyses reveal that LTP expanded storage capacity; the distribution of spine head volumes was increased from 2 bits in controls to 3 bits at 30 min and 2.7 bits at 2 hr after the induction of LTP. Furthermore, the distribution of spine head volumes was more uniform across the increased number of functionally distinguishable sizes following LTP, thus achieving more efficient use of coding space across the population of synapses.

Dually innervated dendritic spines develop in the absence of excitatory activity and resist plasticity through tonic inhibitory crosstalk

Dendritic spines can be directly connected to both inhibitory and excitatory presynaptic terminals, resulting in nanometer-scale proximity of opposing synaptic functions. While dually innervated spines (DiSs) are observed throughout the central nervous system, their developmental timeline and functional properties remain uncharacterized. Here we used a combination of serial section electron microscopy, live imaging, and local synapse activity manipulations to investigate DiS development and function in rodent hippocampus. Dual innervation occurred early in development, even on spines where the excitatory input was locally silenced. Synaptic NMDA receptor currents were selectively reduced at DiSs through tonic GABAB receptor signaling. Accordingly, spine enlargement normally associated with long-term potentiation on singly innervated spines (SiSs) was blocked at DiSs. Silencing somatostatin interneurons or pharmacologically blocking GABABRs restored NMDA receptor function and structural plasticity to levels comparable to neighboring SiSs. Thus, hippocampal DiSs are stable structures where function and plasticity are potently regulated by nanometer-scale GABAergic signaling.

Dendritic Spine Density Scales with Microtubule Number in Rat Hippocampal Dendrites

Microtubules deliver essential resources to and from synapses. Three-dimensional reconstructions in rat hippocampus reveal a sampling bias regarding spine density that needs to be controlled for dendrite caliber and resource delivery based on microtubule number. The strength of this relationship varies across dendritic arbors, as illustrated for area CA1 and dentate gyrus. In both regions, proximal dendrites had more microtubules than distal dendrites. For CA1 pyramidal cells, spine density was greater on thicker than thinner dendrites in stratum radiatum, or on the more uniformly thin terminal dendrites in stratum lacunosum moleculare. In contrast, spine density was constant across the cone shaped arbor of tapering dendrites from dentate granule cells. These differences suggest that thicker dendrites supply microtubules to subsequent dendritic branches and local dendritic spines, whereas microtubules in thinner dendrites need only provide resources to local spines. Most microtubules ran parallel to dendrite length and associated with long, presumably stable mitochondria, which occasionally branched into lateral dendritic branches. Short, presumably mobile, mitochondria were tethered to microtubules that bent and appeared to direct them into a thin lateral branch. Prior work showed that dendritic segments with the same number of microtubules had elevated resources in subregions of their dendritic shafts where spine synapses had enlarged, and spine clusters had formed. Thus, additional microtubules were not required for redistribution of resources locally to growing spines or synapses. These results provide new understanding about the potential for microtubules to regulate resource delivery to and from dendritic branches and locally among dendritic spines.

Medial femoral torsion and osteoarthritis

We investigated the hypothesis that medial femoral torsion is a predisposing factor to osteoarthritis of the hip. Anteversion was measured by biplane radiography in 44 hips (32 patients) with idiopathic osteoarthritis of the hip and a control group of 98 normal hips (49 adults). The mean and range of anteversion measures were similar in both groups. The mean was 22 degrees for osteoarthritic hips (range 3 degrees-49 degrees) and 19 degrees for normal controls (range -2 degrees-49 degrees). This difference was not significant. This study disagrees with others that propose an association between increased anterversion and osteoarthritis of the hip. Prophylactic operative correction of femoral torsion for osteoarthritis is not recommended.

Medial femoral torsion: experience with operative treatment

Seventy-eight patients who had undergone bilateral derotational osteotomy of the femur for medial femoral torsion were reviewed to evaluate the procedure and to determine the frequency of postoperative complications. Patients averaged 6 years of age at the time of the procedure; girls predominated. Before derotational osteotomy, the mean hip rotation measurements were 84 degrees medially and 16 degrees laterally; following the procedure, mean hip rotations were 41 degrees medially and 51 degrees laterally. Medial femoral torsion was reduced from 54 degrees mean to 14 degrees. Balancing these favorable results, however, was a 15% risk of complications; 12 of our patients had problems associated with loss of fixation, errors in amount of correction, fractures, heel ulcer, or infection. Derotational osteotomy is only justified for the child with a persistent severe disability from medial femoral torsion. The parents should be aware of the significant risks and the lack of evidence for any long-term adverse effects if moderate MFT is left uncorrected.

Fractures of the dorsal and lumbar spine

A study of 60 children with thoracic and lumbar spine injuries indicates that the child's response to trauma is unique. It is concluded that the potential for growth and development and the presence of healthy tissues separate these injuries from comparable injuries in adults. The epiphyseal growth partly or completely restores vertebral body height, leads to a mild spinal deformity in stable injuries, and can contribute to rapidly progressive spinal deformity in unstable injuries with or without paralysis. Healthy intervertebral discs account for the multiple compression fractures and lack of long term degenerative changes of the spine. On the basis of this review, a practical approach to patient care is presented.