Parallel nature of hippocampal synaptic plasticity.

This study demonstrates that the mechanisms involved in the production of long-term potentiation (LTP) in the hippocampus appear to be independent of those which generate shorter-lasting plasticity, but that both processes are activated concurrently following an LTP-inducing stimulus. Adult male Sprague-Dawley rats were anesthetized using either pentobarbital or secobarbital to record extracellular field potentials from the hippocampal CA1 pyramidal cell layer in response to stimulation of commissural afferents. Plasticity was generated by the delivery of a five-pulse patterned stimulus train, consisting of one priming pulse followed 170 milliseconds later by a burst of four pulses at 200 Hz. While similar LTP was observed in both groups, short-term plasticity was absent in the secobarbital-anesthetized animals. This result suggests that different plasticity mechanisms in the hippocampus are activated in parallel by the triggering stimulus.

LTP varies across the estrous cycle: enhanced synaptic plasticity in proestrus rats.

Previous studies have shown that the number of dendritic spines and synapses in hippocampal CA1 stratum radiatum decreases more than 30% between the proestrus (high estrogen) and estrus (low estrogen) phases of the rat estrous cycle [10,27]. In the present study, we investigated whether hippocampal synaptic plasticity, as measured by long-term potentiation (LTP), might also vary across the estrous cycle of the female rat. Male rats, and female rats at each phase of the estrous cycle were tested in either the morning or afternoon. There were no significant group differences in the pre-LTP I/O curves. However, females examined during the afternoon of proestrus, the phase during which prior studies indicate synapse number to be highest, demonstrated the greatest degree of potentiation. Diestrus, proestrus and estrus females tested in the morning demonstrated similar amounts of potentiation. There were also significant differences in post-LTP I/O curves between the afternoon proestrus females and males tested in the afternoon. These results suggest that gonadal hormones, interacting with the time of day, may regulate neural processes underlying learning and memory.

Enduring effects of chronic corticosterone treatment on spatial learning, synaptic plasticity, and hippocampal neuropathology in young and mid-aged rats.

Prolonged treatment with stress levels of corticosterone has been reported to produce changes in the hippocampus. In the experiments reported here, we examined for functional and morphological consequences of this treatment. First, young adult or mid-aged male Long-Evans rats were treated for either 1 or 3 months with corticosterone, at a dose sufficient to mimic the elevated hormone levels observed following exposure to mild stress. Two weeks following the termination of treatment, the animals were tested in the Morris water maze to assess spatial learning. No behavioral deficits were observed after 1 month of treatment. A 3 month treatment period also had no effect in young rats, but produced a learning impairment in the mid-aged rats. We then examined whether the effect of elevated corticosterone in mid-aged animals could be produced by a physiological stressor. Mid-aged rats were maintained for 6 months under conditions of low or high social stress. Six months of exposure to high social stress produced significant spatial learning impairments in the Morris water maze. These effects were absent in high social stress animals that had been previously adrenalectomized (with low-level corticosterone replacement), suggesting that elevated glucocorticoid levels mediate the effects of stress on spatial memory in older animals. In a final experiment, mid-aged rats were treated with corticosterone at levels that mimicked those naturally occurring at the diurnal peak (medium-B: 12-17 micrograms/dl) or in response to stress (high-B: 25-32 micrograms/dl). Only rats exposed to high levels of corticosterone demonstrated impaired performance in the Morris water maze.(ABSTRACT TRUNCATED AT 250 WORDS)