03064522/92 $5.00 + 0.00
Neuroscience Vol. 48, No. 4, pp. 793-806, 1992 Printed in Great Britain
Pergamon Press Ltd 0 1992IBRO
ALTERATIONS IN THE PROPERTIES OF HIPPOCAMPAL PYRAMIDAL NEURONS IN THE AGED RAT B. POTIER,* 0. RAscoL,*t
F. JAZAT,* Y. LAMOVR*$ and P. DVTAR*$
‘Laboratoire de Physiopharmacologie
du Systeme Nerveux, INSERM U 161,2 rue d’Albsia, 75014 Paris, France iService de Pharmacologic Medicale et Clinique, INSERM U 317, Faculte de M&de&e, 37 all&es Jules Guesde, 31000 Toulouse, France $Service d’Exploration Fonctionnelle du Systeme Nerveux, Hopital Lariboisiere, 75010 Paris, France Abstract-The eleetrophysiological and pharmacological properties of CA1 hippocampal pyramidal neurons were studied in slices from young (three to four months) and aged (25-32 months) SpragueDawley rats having previously performed two behavioral tasks. About 20% of the aged rats were impaired in either the spontaneous alternation task or the water maze task. Electrophysiological parameters were measured and compared in young and aged animals using intracellular recordings. No age-related differences were observed in membrane potential, input resistance, amplitude of action potentials or amplitude of calcium spikes. The amplitude and duration of individual afterhyperpolarizations following a single spike were unchanged. In contrast, the neuronal excitability was significantly decreased and the spike duration significantly enhanced in aged rats as compared to young rats. The comparison of afterhyperpolarizations (which follow a burst of spikes) between young and aged rats was more complex. An increase in the amplitude and duration of afterhyperpolarizations generally occurred in aged animals. However, this increase was not consistent among animals and was dependent on the holding potential of the neuron and on the number of action potentials used to trigger the afterhyperpolarization. The depolarizing effect of bath-applied carbachol, as well as the associated increase in membrane resistance were reduced in neurons from aged rats. In contrast, the effects of carbachol on the depression of synaptic events and the blockade of the afterhyperpolarizations were similar in young and aged animals. In addition, the amplitude of the slow cholinergic excitatory postsynaptic potential induced by stimulation of cholinergic afferents in the presence of physostigmine was also decreased in aged rats. Excitatory postsynaptic potentials and inhibitory postsynaptic potentials following electrical stimulation of stratum radiatum were compared. The amplitude and duration of excitatory postsynaptic potentials were increased in aged rats. The amplitude and duration of the fast inhibitory postsynaptic potential were not significantly affected in aged animals. In contrast, the duration of the slow inhibitory postsynaptic potential was decreased in aged rats. Since the mean baclofen-induced hyperpolarization was only slightly reduced in aged rats, the most likely explanation is a decrease in the release of GABA rather than an alteration in the postsynaptic response mediated by GABA, receptors. A statistically significant correlation was found between the degree of impairment in the spontaneous alternation task and the amplitude of the carbachol-induced depolarization.
The limbic system and especially the hippocampus plays an important role in learning and memory. Age-related behavioral impairments might be due at least in part to alterations in the properties of the hippocampal pyramidal neurons and/or alterations in various neurotransmitter systems. As demonstrated by several studies,39 aged animals are not similarly affected by aging. Some are severely impaired in their behavioral performances and others are not. However, most of the electrophysiological and pharmacological studies in aged animals do not take these behavioral differences into account. In the present study, we compared §To whom correspondence should he addressed. Abbreuiutions:ACh, acetylcholine; AHP, afterhyperpolarization; EGTA, ethyleneglycolbis(aminoethylether)tetraacetate; EPSP, excitatory postsynaptic potential; IPSP, inhibitory postsynaptic potential; RMP, resting membrane potential; TEA, tetraethylammonium; ‘ITX, tetrodotoxin.
young rats previously ranked according to their ability to perform two different behavioral tasks: a spontaneous alternation test and a modified version of the water maze test initially designed by Morris.43 The rats were then processed for electrophysiological and pharmacological experiments. Recent experiments reported alterations in calcium-dependent mechanisms in aged animals (see Refs 23, 48) and in the distribution of a calciumbinding protein calbindin-28K.*r It has recently been suggested that calcium homeostasis is modified in hippocampal neurons in the aged rat.23,“,4” These in
alterations could be responsible for neuronal death and behavioral impairment. Therefore our electrophysiological experiments were focused on calciumdependent events which can be studied with intracellular recordings, e.g. afterhyperpolarization (AHP) and calcium spike. 793
B. POTIER CI ul
It is well known that pharmacological properties of central nervous system neurons deteriorate in aged animals’,3h and humans.s4 The noradrenergic, dopaminergic’ as well as the cholinergic systems6,3’ are affected. A decrease in the postsynaptic sensitivity of hippocampal neurons to acetylcholine (ACh) has been observed.‘h.53 However, several questions remain. Is the cholinergic alteration preferentially located on the pre- or the postsynaptic side? Is the alteration of the neuronal response to exogenous ACh also observed with the endogenous transmitter? Is there any relationship between behavioral impairments and pharmacological alterations? Are some other neurotransmitter pathways also altered during aging (particularly, the endogenous excitatory amino acid pathway and the GABAergic pathway)? The in vitro slice preparation provides an excellent model for the study of synaptic events identified in terms of transmitters and receptors involved, e.g. ACh, GABA and excitatory amino acids. EXPERIMENTAL
We have investigated the correlations between age-related changes in electrophysiological and pharmacological parameters and age-related changes in behavioral performances, using a combination of behavioral and intracellular in vitro electrophysiological techniques in three- to fourmonth-old (n = 15) and 25532-month-old (n = 25) male Sprague-Dawley rats. Rats were purchased from IFFACREDO (France). Rats were kept, three to the cage, in our animal house, under a 12/12 h illumination cycle, with free access to food and water. Rats were first submitted to the behavioral tests. They were then anesthetized with halothane and decapitated. One young or one aged rat was used each day. In the electrophysiological experiments, the experimenter was unaware of the behavioral score. In some experiments (n = 3), one aged rat and one young rat were processed simultaneously for the electrophysiological recordings to allow a better comparison of the neuronal properties. Behavioral studies The behavioral study was comprised of two different behavioral tests: spontaneous alternation, and a modified version of the Morris water maze test. Both tests were performed in the same room and were surrounded by visible spatial cues, always located at the same place. The spontaneous behavior of the aged rats (cage exploration, feeding behavior, etc.) indicated that they did not have any visual or motor impairment. Spontaneous alternation. Young and aged rats were first habituated to the experimental setting and to the experimenter by handling them once a day for three days. Then, on the fourth day, they were placed in the center of a Y maze (Y maze with three identical alleys, 39 x 13 x 30 cm, diverging at a 120” angle from a central point). An “entry” into an arm was recorded when the rat placed its four paws in a given arm. The entries were recorded over a period of five min, while the rat was allowed to explore the maze freely. The “correct” choice was considered to be the arm least recently visited. This procedure was repeated for three consecutive days. The mean percentage of spontaneous alternation (i.e. the ratio of the number of correct altemations to the total number of arm entries) was calculated for each animal in young and aged rats (for details see Ref. 29). Animals with a percentage of alternation below 60% (which was the mean percentage for the population of aged rats)
were considered as Impaired. and thoqc aho\c (pi? ~I I’ unimpaired. Water maze resr. A platform was placed in one