Impaired synaptic potentiation processes in the hippocampus of aged, memory-deficient rats

Stability of synaptic density and spine volume in dentate gyrus of aged rats

The number of synapses per unit volume and per granule cell and the size of dendritic spines were studied in the dentate gyrus of Sprague-Dawley rats 6, 24, and 30 months of age. Neither synaptic density nor mean spine volume showed any age-related trends. An increase in granule cell packing density at 24 months and concomitant stability of the height of the granule cell layer is consistent with the idea that postnatal generation of granule cells may continue late into life. Possible explanations for the discrepancies in the literature regarding synaptic loss in this area include differences in morphometric techniques, age of animals used, regional differences within dentate gyrus, and sampling variability. Generalized synapse loss in the senescent rodent brain remains to be established.

Eyeblink conditioning deficits in the old cat

Acquisition of a conditioned eyeblink reflex was studied in aged cats 10-23 years old and young cats one to three years old. All ten young cats became conditioned within 1,000 trials, after a mean of 270 trials, with a stimulus protocol including a 1500 msec 4 kHz tone CS+ followed by a shock unconditioned stimulus, and click CS-. Nine of 15 aged cats failed to become conditioned with 1,000 trials. Six old cats became conditioned after a mean of 522 trials. Four aged cats tested with a simpler protocol involving a 400 msec CS+ and no click CS- became conditioned with less than 500 trials. All retained the conditioned response when clicks were added to the 400 msec CS+. Three aged animals which failed to become conditioned with the 1500 msec CS+ and clicks were trained with an 800 msec CS+ without clicks. Only one became conditioned within 500 trials. Thus duration of the tone CS+ was critically important for acquisition of a conditioned eyeblink response by aged cats.

Monosynaptic activation of the hippocampus as a conditioned stimulus: behavioral effects

The aim of the present study with rats was to show that electrical stimulation of a monosynaptic pathway, whose high plastic potency is well-known, can serve as a conditioned stimulus (CS) in a learning paradigm. Using chronic rats, stimulation of the perforant pathway, which activates the entorhinal cortex input to the dentate gyrus of the hippocampus, was used as the CS in a footshock motivated two-way avoidance task (shuttle box). Among the different stimulation parameters tested, only trains of at least 15 Hz were shown to be effective as a CS, whereas the application of single impulses with a frequency of 1,7 Hz did not result in the establishment of conditioned behavior. Using the 15 Hz-paradigm, in "good learners," the development of reliable conditioned responses started at the end of the first training session (40 trials) and become fairly stable during a further training session, given on the following day. The application of stimulation trains with 100 Hz as a CS led to the development of a high rate of conditioned responses, however, there was also a high level of intertrial reactions. Considering the critical importance of the frequency parameters of the perforant pathway stimulation for the development of conditioned behavior the possible involvement of hippocampal long-term potentiation in behavioral plasticity is discussed. Additionally, it is concluded that this new learning paradigm offers the advantage of concurrent analysis of plastic processes both at the behavioral level and at the level of the synaptic population.

Rapid forgetting in aged rats

The retention performance of young (3-6 months) and aged (24-26 months) Sprague-Dawley rats was evaluated on three separate experimental paradigms. The three paradigms were sequenced so that the first, spontaneous alternation, tested for retention performance of short-term memory; the second, step-down inhibitory (passive) avoidance, tested for long-term memory; while the third, a shock-motivated spatial reversal problem, tested for both short- and long-term memory. The results suggest that aged rats experience significantly more rapid forgetting in both short- and long-term memory systems.

Aging of membrane transport mechanisms in the central nervous system. GABA transport in rat cortical synaptosomes

A kinetic study of sodium dependent GABA transport has been made in synaptosomes from 30-month old Long-Evans rats and compared to results from 2-month old animals. Initial velocity of uptake was measured as a function of both sodium and GABA concentration, and these data were than fitted to the model which was found to give best fit for the 2-month data. An excellent fit was also obtained for the 30-month data. Thus there has been no change with age in the fundamental mechanism by which carrier, sodium, and GABA interact in the process of transport. However, quantitative changes were found to occur with age both in initial velocity of uptake and in those constants which quantitate the model. The rate equation for the model was utilized along with the best fit constants to define and calculate certain parameters which were then used to quantitatively compare the transport mechanism in young and aged animals.

Age-related changes in classical (Pavlovian) conditioning in the New Zealand albino rabbit

The groups of New Zealand albino rabbits of approximately six and forty months of age respectively were subjected to Pavlovian conditioning in which a 1216 Hz, 75 dB tone served as the conditioning stimulus and a brief paraorbital electric shock train served as the unconditioned stimulus. Eyeblink and heart rate responses were assessed. Older animals revealed acquisition deficits in the somatomotor eyelid conditioning task compared to the young animals: however, the magnitude of the concomitantly occurring heart rate change was greater in older than in younger animals. These age-related changes appeared to be enhanced in males and were unrelated to general activity differences, or to the unconditioned response to unpaired tones or electric shocks.

Increased [3H]glutamate receptor binding in aged rats

Na-independent [3H]glutamate binding to rat hippocampal membranes increases progressively as a function of age. The increased binding represents an increased number of binding sites without changes in their apparent affinity for glutamate. However, [3H]glutamate binding, measured with a saturating concentration of calcium does not change at various ages. This does not reflect a change in the apparent affinity of calcium ions to stimulate [3H]glutamate binding, but a decrease in their maximal stimulatory effect. These results are discussed in relationship to age-related changes in certain physiological and behavioral functions.

Augmentation of short-term plasticity in CA1 of rat hippocampus after chronic ethanol treatment

The neurotoxic effects of chronic ethanol exposure were investigated in rat hippocampus by electrophysiological analysis of the Schaffer collateral-commissural input to stratum radiatum of CA1. Experimental animals were fed an ethanol-containing liquid diet for 20 weeks but were withdrawn from the special diet at least eight weeks prior to acute electrophysiological recordings. Ethanol treatment had no effect on input-output relationships for either the population EPSP or the population spike (PS). During paired-pulse stimulation, the ethanol group exhibited a greater facilitation of the test pulse PS relative to the control group, although potentiation of the EPSP was unchanged. In addition, the ethanol group showed a trend toward greater facilitation of the PS during 5 and 10 Hz tetani. No differences between groups were observed in the magnitude or duration of the long-term potentiation produced by 5, 10 or 100 Hz stimulus trains. Ethanol treatment did significantly reduce the transient spike depression after low frequency stimulation. This pattern of results is similar to that found for treatments which reduce hippocampal recurrent inhibition. Thus, chronic ethanol treatment may produce a lasting disruption of intrinsic inhibitory neurotransmission in the rat hippocampus.

Posttraining changes in excitability of the commissural path-CA1 pyramidal cell synapse in the hippocampus of mice

Mice were partially trained on a bar-press operant conditioning task on continuous reinforcement (CRF). The amplitude of population excitatory postsynaptic potentials (EPSPs) evoked by commissural stimulation of the Ca2 field of the dorsal hippocampus was found to significantly increase 1 h after training and then to decrease. This phenomenon was either not observed or much less evident in mice could not learn the task or which had already been trained on this task.

Quantification of synaptic vesicles in hippocampus of aging rats and initial studies of possible relations to neurophysiology

Synaptic vesicle populations were quantified in Schaffer-commissural synapses which terminate on CA1 pyramidal cell apical dendrites, in aging and young-mature rats. Vesicles were found to be reduced with age, and this effect was most pronounced in the oldest animals (e.g., 28 months) within the aged group. Numerical density of synaptic vesicles in aged rat hippocampus was reduced whether expressed as vesicles per terminal, vesicles per mu 2 of terminal, or vesicles per mu2 of terminal corrected for shrinkage or swelling as assessed by mitochondrial cross-sectional diameters. Counted synaptic terminal areas were not significantly different in the aged animals, although a trend toward reduced terminal size with aging was seen. The latter observation apparently rules out increased terminal swelling in aged rat synapses as a basis for reduced vesicle density. In some rats, neurophysiological studies were concomitantly performed. A significant correlation was found between the amplitude of the monosynaptic population spike after 20 min of 4 Hz stimulation of the Schaffer-commissural fibers and synaptic vesicle populations in terminals of these stimulated pathways. However, because of a low n this result must be viewed as preliminary.

Evoked EEG responses in several motor areas of brain to stimulation of hippocampus and caudate, alone and in combination

In curarized rats, electrical stimulation (6 Hz pulses in 6-sec trains) was delivered either to the dorsal hippocampus (Hip), the caudate nucleus (Caud), or to both. Hip and Caud stimulation evoked activity in the motor cortex and the Hip, and the responses commonly grew larger during the stimulation; to a lesser extent, responses occurred in the brainstem reticulum and cerebellar cortex. Combined Hip and Caud stimulation responses were often greater than the responses to stimulation of either Hip or Caud alone, but less than the sum of responses from each single-site stimulation, suggesting occlusive interference because of shared elements in the projection pathways. There were also some instances suggesting mutual facilitation, where the responses to combined stimulation were greater than the sum of the responses to each single-site stimulation. Both kinds of results seem to indicate that the hippocampus, as well as the caudate, has functional connections with motor control areas of the brain and that both areas may interact in motor control.

Hippocampal aging and adrenocorticoids: quantitative correlations

Altered neural-endocrine relations have been proposed as factors in mammalian aging. In the same rats from three age groups we quantified astrocyte reactivity in hippocampus, performed radioimmunoassays for plasma adrenocorticoids, and measured adrenal weight. These variables were correlated in individual animals and generally increased with age. The findings are consistent with recent hypotheses that endocrine levels are related to brain aging, either as cause or effect.