Ted L. Petit

Modulation of Presynaptic Plasticity and Learning by the H-ras/Extracellular Signal-Regulated Kinase/Synapsin I Signaling Pathway

Molecular and cellular studies of the mechanisms underlying mammalian learning and memory have focused almost exclusively on postsynaptic function. We now reveal an experience-dependent presynaptic mechanism that modulates learning and synaptic plasticity in mice. Consistent with a presynaptic function for endogenous H-ras/extracellular signal-regulated kinase (ERK) signaling, we observed that, under normal physiologic conditions in wild-type mice, hippocampus-dependent learning stimulated the ERK-dependent phosphorylation of synapsin I, and MEK (MAP kinase kinase)/ERK inhibition selectively decreased the frequency of miniature EPSCs. By generating transgenic mice expressing a constitutively active form of H-ras (H-rasG12V), which is abundantly localized in axon terminals, we were able to increase the ERK-dependent phosphorylation of synapsin I. This resulted in several presynaptic changes, including a higher density of docked neurotransmitter vesicles in glutamatergic terminals, an increased frequency of miniature EPSCs, and increased paired-pulse facilitation. In addition, we observed facilitated neurotransmitter release selectively during high-frequency activity with consequent increases in long-term potentiation. Moreover, these mice showed dramatic enhancements in hippocampus-dependent learning. Importantly, deletion of synapsin I, an exclusively presynaptic protein, blocked the enhancements of learning, presynaptic plasticity, and long-term potentiation. Together with previous invertebrate studies, these results demonstrate that presynaptic plasticity represents an important evolutionarily conserved mechanism for modulating learning and memory.

The pattern of dendritic development in the cerebral cortex of the rat.

The pattern of dendritic development of layer V pyramidal cells in the neocortex of the rat was studied using a variety of quantitative techniques in an attempt to determine what rules govern dendritic differentiation. Animals were sacrificed on postnatal days (P) 1, 3, 5, 7, 10, 15, 20, 25, 30 and 60, their brains impregnated with the rapid Golgi technique, and cells from the sensorimotor cortex examined for maximal apical and basilar dendritic field, number of dendritic branches at 20 micron intervals from the cell body, number of apical and basilar branch types (branching order), length of dendritic branch segments, and dendritic spine density. Primary dendrites are formed early in development, with no new ones formed after P7-10. Once a dendritic segment has bifurcated, all further development appears to occur at the tip, i.e. the trunk does not seem to undergo additional elongation, and new branches do not appear to form from the trunk. There is a plateau in dendritic differentiation close to the cell body after approximately P20; however, there is a continued increase in the length of terminal dendritic branches in the distal portions of the dendritic field into adulthood. During early development, dendrites bifurcate on reaching approximately 20-30 microns; however, during adulthood additional length is added to terminal dendrites without branching. Dendritic spines increase dramatically early in development, and then decline on proximal dendrites but continue to increase on terminal branches into adulthood. These results suggest that the terminal portion of the dendritic field remains plastic into adulthood, and that during development several general rules govern the pattern of dendritic differentiation.

Neonatal lead exposure alters the dendritic development of hippocampal dentate granule cells

Previous evidence suggests an association between hippocampal dysfunction and the behavioral deficiencies observed in experimental animals following neonatal lead (Pb) exposure. This study was thus conducted to discern the possible effects of Pb on the dendritic development of hippocampal dentate granule cells. Long-Evans hooded rat pups were exposed to Pb from postnatal days 1 to 25 via the maternal milk. Mothers were fed diets containing either 4.0% PbCO3 (high Pb), 0.4% PbCO3 (low Pb), or a 2.2% Na2CO3 control diet throughout this period. On postnatal day 30, pups from each group were randomly selected and their hippocampi processed with the rapid Golgi method. For analysis of dendritic development, granule cells from the infrapyramidal limb of the dentate gyrus were examined using the Scholl method. The maximal length and width of the dendritic field of these cells were also measured. The results of this study indicate not only increased dendritic branching at 20 μm from the cell body in low Pb-exposed animals, but also both a reduced length of the dendritic field and a reduction in the number of dendritic branches at distances greater than 160 μm from the cell body after both high or low Pb exposure. As such, these findings indicate that morphological changes in the development of the hippocampal formation may underlie many of the behavioral changes observed in experimental animals after neonatal Pb exposure.

Neocortical synaptogenesis, aging, and behavior: Lifespan development in the motor-sensory system of the rat

Little evidence presently exists on the development and aging of synaptic contacts and their relationship to behavior, particularly in nonvisual brain areas. To investigate this interrelationship, rats at a series of developmental ages [postnatal day 1 (P1) to P90] were initially examined on a battery of motor tasks. The battery, ranging from simple reflexive tests to tests of complex locomotor capacities, consisted of tactile-induced forelimb placing, chin-induced placing, body righting, climbing an inclined plane, traversing a narrow beam, and keeping up with a revolving wheel. Following completion of the behavioral testing, the animals, together with an additional group of aged (28- to 29-month-old) rats, were killed and their motor-sensory cortex was removed, stained with osmium tetroxide or ethanol phosphotungstic acid (EPTA), and examined under electron microscopy for density of synaptic contacts. Simple motor abilities such as tactile-induced placing was present by the end of the first postnatal week, with locomotor performance reaching a mature level by the end of the third postnatal week, and intermediate task abilities maturing within this range. Paralleling the development of complex locomotor skills was a sharp increase in synaptic density in the molecular layer of the motor-sensory cortex, commencing in the second postnatal week and peaking at P30. After P30 there was a sharp decline in synaptic density as well as a decline in performance on some motor tasks, although these two functions seemed to be occurring independently. There was a continued, but less dramatic synaptic loss evident in the aged rats.

Neurofibrillary degeneration, dendritic dying back, and learning-memory deficits after aluminum administration: Implications for brain aging

Abstract Increased concentrations of aluminum have been observed in the brains of patients with senile dementia and have been implicated in the etiology of that disease. This study assessed the possible behavioral and neurocytologic effects of increased brain aluminum in experimental animals. An infusion of 5 μ m aluminum (as aluminum tartrate) was made into the lateral ventricles of young adult male New Zealand white rabbits. On the 10th postoperative day, all animals were trained on a step-down active avoidance task and retested 3 days later. The aluminum-treated rabbits showed deficits on both original learning and retention of this task. Electron microscopic examination of neocortical neurons revealed the presence of 10-nm neurofilamentous tangles in aluminum-treated animals. For analysis of dendritic morphology, layer V pyramidal cells from the sensorimotor cerebral cortex were examined using the Scholl method to determine the number of dendritic branches at 20-μm intervals from the cell body. Although there were no differences in the number of processes leaving the cell body, there was a sharp and progressive decrease in the number of branches with increasing distance from the cell body. This pattern is consistent with that expected from a dying-back process.

Sequential changes in the synaptic structural profile following long‐term potentiation in the rat dentate gyrus: I. The intermediate maintenance phase

Long‐term potentiation (LTP), one of the most compelling models of learning and memory, has been associated with changes in synaptic morphology. In this study, LTP was induced and animals were sacrificed 1 h after the stimulation of the LTP group (induction / early maintenance phase). Synapses in the directly stimulated middle third of the dentate gyrus molecular layer (MML) were examined while synapses from the inner third of the dentate molecular layer (IML) of the LTP animals and both the MML and the IML of implanted animals served as controls. The total number of synapses per neuron, synaptic curvature, the presence of synaptic perforations, and the maximum length of the synaptic contact and active zone were examined. No overall change in the number of synapses per neuron was observed in the LTP tissue. LTP was associated with a significant increase in the proportion of perforated and irregular‐shaped synapses compared to controls. The increase in perforated synapses was particularly apparent in the proportion of concave perforated synapses. Nonperforated concave synapses were found to be significantly larger in potentiated tissue. The total synaptic length per neuron of synapses in a concave configuration was also significantly higher following potentiation. These results suggest that the specific structural profile associated with 1‐h post‐LTP induction, which differed from the profile observed at 24 h post‐induction, may represent a unique early phase of synaptic remodeling in a series of changes observed during LTP induction, maintenance, and decay. Synapse 36:286–296, 2000.

The role of synaptic morphology in neural plasticity: structural interactions underlying synaptic power

The study of synaptic plasticity has revealed a common cascade of ultrastructural events across several paradigms. Most notable of these paradigms are development, long-term potentiation (LTP), and adult reactive synaptogenesis (RS). These plastic neural events are discussed in terms of major categories of synaptic morphological change--synaptic density, curvature, and perforations, as well as the size of synaptic elements. The potential functional implications of these morphological changes are reviewed, along with considerations based on recently developed mathematical models of synaptic function. These considerations are then incorporated into the common structural alterations observed during multiple forms of synaptic activation, producing a sequential model supporting increased efficacy associated with neural plasticity. The data suggest that during a plastic challenge, synapses move through a continuum of morphological change, dependent upon the interaction of structural parameters and their effect on various aspects critical to synaptic efficacy. This complex interplay of morphological alterations and synaptic types over time and location may form a critical aspect of neural plasticity.

Abnormal electric membrane properties of Down's syndrome DRG neurons in cell culture.

Cell cultures were prepared from normal and Downs syndrome dorsal root ganglia (DRG). Both pre- and postnatal specimens were utilized; 8 normal and 4 Downs. Cultures were maintained in medium with normal (4 mM) and elevated (20 mM) potassium (K) since the latter was found to enhance neuron survival. After various period of incubation, cultures were transferred to normal K medium and their electrical membrane properties (EMP) determined using intracellular recording techniques. An analysis of variance was performed with 5 covariates: developmental stage, culture duration, K concentration, type of action potential, and neuronal surface area. This analysis indicated that the Downs neurons had abnormal EMP, the most affected being the after hyperpolarization (-41%), membrane time constant (+30%), threshold rheobasic depolarization (-22%), rate of falling phase of action potential (-20%), specific membrane resistance (+18%) and absolute refractory period (+12%). All differences were also observed when samples of normal and Downs neurons were matched for the 5 covariates mentioned above, take separately. If the abnormal EMP observed in the present study for Downs DRG neurons in culture occurred for CNS neurons in situ they would disrupt the normal function of the nervous system and could therefore constitute the neurobiological basis of the mental retardation observed in Downs syndrome.

Behavioral effects of postnatal lead exposure: Possible relationship to hippocampal dysfunction

A review of previous evidence suggested the possibility of hippocampal involvement in the behavioral changes observed following postnatal lead (Pb) exposure. To further assess this possibility, Long—Evans hooded rat pups were exposed to inorganic Pb from postnatal Days 1 to 25 via the maternal milk. Mothers were fed diets containing either 4.0% PbCO 3 , 0.4% PbCO 3 , or a 2.2% Na 2 CO 3 control diet throughout this period. Animals were tested at maturity in four situations considered sensitive to hippocampal dysfunction. Exposure to Pb resulted in delayed acquisition of the radial eight-arm spatial maze, but produced no changes in either the susceptibility to audiogenic seizures or the acquisition and performance of a two-way active avoidance response. Pb-exposed animals were also observed to perform deficiently on a DRL-20 schedule of operant reinforcement, although significant variation was observed between litters in the Control group on this task. The results of this study are discussed in terms of a relationship between postnatal Pb exposure and hippocampal dysfunction. However, other forms of central nervous system alterations have been observed to result from early Pb exposure and alternative explanations for these behavioral changes are also suggested.

Effects of lead exposure during development on neocortical dendritic and synaptic structure

Abstract The effect of lead (Pb) on neocortical dendritic and synaptic development was examined in rats. Newborn pups were indirectly exposed to Pb by placing 4% Pb carbonate in their mothers diet from postnatal days 1 to 25. The mean brain weight of the Pb-treated animals was reduced 13.2%; neocortical thickness was reduced 13.9%. For analysis of dendritic development, layer V pyramidal cells from area 3 of the sensorimotor cerebral cortex were examined using the Scholl method to determine the number of dendritic branches at 20-μm intervals from the cell body. Although there were no differences in the number of processes leaving the cell body, reductions in dendritic branches were observed at distances greater than 40 μm, reaching significance at 80 and 100 μm. In addition the length of the primary apical dendrite was reduced 5.6% in Pb-treated animals. Synaptic parameters were examined in the molecular layer of the occipital cortex in ethanol phosphotungstic acid-stained tissue. There was a 22.7% reduction in the number of synapses per 15,000× field. No significant differences were observed in the following synaptic parameters measured at 300,000×: presynaptic length and thickness, postsynaptic length and thickness, and cleft width.