Loss of NMDA, but not GABA-A, binding in the brains of aged rats and monkeys

Age-dependent differences in survival of striatal somatostatin-NPY-NADPH-diaphorase-containing interneurons versus striatal projection neurons after intrastriatal injection of quinolinic acid in rats

Some authors have reported greater sparing of neurons containing somatostatin (SS)-neuropeptide Y (NPY)-NADPH-diaphorase (NADPHd) than projection neurons after intrastriatal injection of quinolinic acid (QA), an excitotoxin acting at NMDA receptors. Such findings have been used to support the NMDA receptor excitotoxin hypothesis of Huntington's disease (HD) and to claim that intrastriatal QA produces an animal model of HD. Other studies have, however, reported that SS/NPY/NADPHd interneurons are highly vulnerable to QA. We examined the influence of animal age (young versus mature), QA concentration (225 mM versus 50 mM), and injection speed (3 min versus 15 min) on the relative SS/NPY/NADPHd neuron survival in eight groups of rats that varied along these parameters to determine the basis of such prior discrepancies. Two weeks after QA injection, we analyzed the relative survival of neurons labeled by NADPHd histochemistry, SS/NPY immunohistochemistry, or cresyl violet staining (which stains all striatal neurons, the majority of which are projection neurons) in the so-called lesion transition zone (i.e., the zone of 40-60% neuronal survival). We found that age, and to a lesser extent injection speed, had a significant effect on relative SS/NPY/NADPHd interneuron survival. The NADPHd- and SS/NPY-labeled neurons typically survived better than projection neurons in young rats and more poorly in mature rats. This trend was greatly accentuated with fast QA injection. Age-related differences may be attributable to declines in projection neuron sensitivity to QA with age. Since rapid QA injections result in excitotoxin efflux, we interpret the effect of injection speed to suggest that brief exposure to a large dose of QA (with fast injection) may better accentuate the differential vulnerabilities of NADPHd/SS/NPY interneurons and projection neurons than does exposure to the same total amount of QA delivered more gradually (slow injection). These findings reconcile the discordant results found by previous authors and suggest that QA injected into rat striatum does reproduce the neurochemical traits of HD under some circumstances. These findings are consistent with a role of excitotoxicity in HD pathogenesis, and they also have implications for the basis of the more pernicious nature of striatal neuron loss in juvenile onset HD.

Age-related decrease in the N-methyl-D-aspartateR-mediated excitatory postsynaptic potential in hippocampal region CA1

Glutamatergic fast synaptic transmission is known to be altered with age in a region-specific manner in hippocampus of memory-impaired old rats. In the present experiment, presynaptic fiber potentials and non-N-methyl-D-aspartate (NMDAR) and NMDAR-mediated synaptic responses in CA1 were compared in three ages of behaviorally characterized male F-344 rats. In the CA1 region, old rats showed approximately equivalent reductions in non-NMDAR- and NMDAR-excitatory postsynaptic potential amplitudes for a given size of presynaptic fiber potential. There was no change in magnitude of the presynaptic response itself at any stimulus level. These results are consistent with the hypothesis that there is a reduction in the number of Schaffer collateral synapses per presynaptic axon. This pattern of results in CA1 is very different from what is known to occur at the perforant path-granule cell synapse. In fascia dentata the non-NMDAR-mediated excitatory postsynaptic potential is increased in amplitude, although the NMDAR-mediated excitatory postsynaptic potential is reduced for a given presynaptic input. These data suggest that age-related functional alterations in neurotransmitter receptor subtypes occur differentially between closely-related anatomical subregions.

Differential subcellular regulation of NMDAR1 protein and mRNA in dendrites of dentate gyrus granule cells after perforant path transection

Unilateral transection of the excitatory perforant path results in the acute deafferentation of a segregated zone on the distal dendrites of hippocampal dentate gyrus granule cells (i.e., outer molecular layer), followed by sprouting, reactive synaptogenesis, and a return of physiological and behavioral function. To investigate cellular mechanisms underlying NMDA receptor plasticity in response to such extensive synaptic reorganization, we quantitatively evaluated changes in intensity levels of NMDAR1 immunofluorescence and NMDAR1 mRNA hybridization within subcellular compartments of dentate gyrus granule cells 2, 5, and 9 d after perforant path lesions. There were no significant changes in either measure at 2 d postlesion. However, at 5 and 9 d postlesion, during the period of axonal sprouting and synaptogenesis, there was an increase in NMDAR1 immunolabeling that was restricted to the dendritic segments of the denervated outer molecular layer and the granule cell somata. In contrast, NMDAR1 mRNA levels at 5 and 9 d postlesion increased throughout the full extent of the molecular layer, including both denervated and nondenervated segments of granule cell dendrites. These findings reveal that NMDAR1 mRNA is one of a limited population of mRNAs that is transported into dendrites and further suggest that in response to terminal proliferation and sprouting, increased mRNA transport occurs throughout the full dendritic extent, whereas increased local protein synthesis is restricted to denervated regions of the dendrites whose afferent activity is perturbed. These results begin to elucidate the dynamic postsynaptic subcellular regulation of receptor subunits associated with synaptic plasticity after denervation.

Effect of age on pentylenetetrazol-kindling and kindling-induced impairments of learning performance

Epileptogenesis during ontogeny may not be linearly related to time. It is known that the behavioral manifestations of seizures are age-dependent, but more research is needed to clarify ontogenetic aspects of epilepsies and related alterations, including cognitive deficits. Kindling is an accepted animal model for the study of the convulsive component of epilepsy and its consequences on behavior. Recently, we demonstrated an impairment in acquisition of a conditioned reaction in young adult kindled rats, using pentylenetetrazol (PTZ) as the kindling stimulus. The present study was undertaken to investigate the dependence on age of alterations in the induction of PTZ kindling in rats. We started the kindling protocol in 4-, 6-, and 8-week- and 6-, 12-, 18-, and 24-month-old rats. The PTZ kindling showed an age-dependent decrease in expression of convulsions. The diminished kindling capacity was already seen in 6-month-old rats. In contrast, kindling-related impairment effects on cognitive functions increased with age. Thus, the correlation between learning impairment and occurrence of tonic-clonic seizures that we had demonstrated in 8-week-old rats was abolished in older rats. On the other hand, when the kindling procedure was started in 6-week-old rats, no impairment was found in fully kindled rats.

Effects of combined chronic nimodipine and acute metrifonate treatment on spatial and avoidance behavior

The present experiment was designed to elucidate whether chronic dietary treatment with nimodipine (3 months, 1000 ppm) enhances water maze spatial navigation, passive avoidance behavior and locomotor activity, and whether such a treatment with nimodipine would interact with the therapeutic effect of acute metrifonate treatment. In young medial septum-lesioned rats, nimodipine had no effect by its own on cognitive or motor behavior, and did not enhance the water maze and passive avoidance behavior improving action of metrifonate (3 and 10 mg/kg. p.o.). Nimodipine treatment of aged rats did not markedly affect the deficit in motor performance. Single and combined nimodipine and metrifonate (3 and 10 mg/kg, p.o.) treatment of aged rats resulted in shorter escape distance values to the hidden water maze escape platform compared to those of control aged rats. The passive avoidance performance of aged rats was more effectively facilitated by a combined nimodipine and metrifonate treatment than by either of the drugs on their own. Following a washout period of 2.5 months the rats that were treated previously with nimodipine no longer performed better than aged controls in the water maze test. Furthermore, after the washout period metrifonate 10 mg/kg was no longer effective in improving the water maze behavior of the now 26-month-old rats irrespective of their chronic pretreatment. Taken together, these findings indicate that chronic nimodipine and acute metrifonate treatment may more effectively stimulate cognitive functioning than either of the treatments on their own.

In vivo amino acid release from the striatum of aging rats: adenosine modulation

The release of glutamate, aspartate, GABA, and taurine from the striatum of young (3 months), mature (12 months), and old (22 months), freely moving male rats was investigated by using a microdialysis fiber inserted transversally in the striatum. In old rats basal extracellular glutamate and aspartate levels were decreased vs. young rats (-38 and -49%, respectively). GABA and taurine levels were unmodified by age. In the presence of the adenosine receptor antagonist 8-phenyltheophilline (8-pT) at the concentration of 50 microM, both K(+)-evoked releases of glutamate and aspartate were more than doubled in young, but not in mature and old rats. 8-pT at the concentration of 500 microM significantly decreased glutamate basal levels and K(+)-evoked aspartate release in old rats only. GABA and taurine releases were not affected by 8-pT at either dose. Our findings indicate a modified adenosine modulation on glutamate and aspartate release in aged rats, that could result from a change in the balance between A1 and A2a adenosine receptor density or an alteration of A1 and A2a receptor-effector coupling.

Functional and neurobiological similarities of aging in monkeys and humans

Aging in humans may be accompanied by alterations in several functional abilities. However, there is a great deal of individual variability in the functions that may be altered with age within and across aged people. One potential source of age-related behavioral variation may lie in a differential vulnerability of neurobiological systems to the aging process in particular individuals. Aged monkeys demonstrate behavioral and brain alterations that have many parallels with those observed in aged humans and are valuable animal models in which to investigate the interrelationships between age, behavior and neurobiological measures. This review outlines the similarities of functional and neurobiological aging in monkeys and humans, notes the variability that exists in both behavioral and neural systems in aging, and identifies some of the areas of aging that are in need of further investigation.

Effects of chronic treatment with a cyclic AMP-selective phosphodiesterase inhibitor, rolipram, on excitatory amino acid neurotransmission systems in young and aged rat brains

Rolipram selectively inhibits cyclic AMP-specific phosphodiesterase, and leads to an increase in cyclic AMP levels in the brain. In this study, we investigated the effects of chronic rolipram treatment on excitatory and inhibitory amino acid neurotransmission systems in young and aged Wistar rat brains. We used in vitro autoradiography with [3H]MK-801, [3H]glycine, D[3H]aspartate, and [3H]muscimol to label N-methyl-D-aspartate (NMDA) receptors, glycine modulatory sites, glutamate transport sites, and gamma-aminobutyric acid-A (GABA) receptors, respectively. Rolipram (0.01 or 0.1 mg/kg, per os) or its vehicle (distilled water) was administered once a day for 4 weeks. The highest binding of [3H]MK-801, [3H]glycine, and D-[3H]aspartate was seen in the hippocampus in vehicle-treated rats. No significant differences in these binding activities were seen between young and aged rat brains. [3H]Muscimol binding was the highest in the cerebellum, and decreased in many brain regions in aged rats. The chronic rolipram treatment resulted in (1) an increase in [3H]MK-801 binding in the dentate gyrus in both young and aged rats, (2) remarkable reductions in D-[3H]aspartate binding in many regions of both young and aged rats, and (3) no or minimal changes in [3H]glycine and [3H]muscimol binding. These results suggest that the chronic rolipram treatment modifies the excitatory amino acid neurotransmission system.

Increased susceptibility to induction of long-term depression and long-term potentiation reversal during aging

Homosynaptic long-term depression (LTD) and reversal of long-term potentiation (LTP) were examined extracellularly at CA3-CA1 synapses in stratum radiatum of slices from adult (6-9 months) and aged (20-24 months) Fischer 344 rats. Prolonged low-frequency stimulation (LFS) (900 pulses/1 Hz) of the Schaffer collaterals depressed the initial slope of the excitatory postsynaptic potential (EPSP) in aged but not adult rats. LTD at aged synapses was pathway-specific, persistent, and sensitive to the NMDA receptor antagonist DL-2-amino-5-phosphonopentanoic acid (AP5). Adult slices exhibited AP5-sensitive LTD in high [Ca2+] medium, whereas LTD in aged slices was blocked by high [Mg2+], suggesting that differences in Ca2+ regulation may underlie susceptibility to LTD. Despite age-related differences in LTD induction, no age difference in LTP magnitude was revealed. Additionally, LFS delivered 60 min after LTP induction resulted in similar LTP reversal for both age groups. Susceptibility differences to LTP reversal were indicated after multiple short-duration LFS bursts (30 pulses/1 Hz), with each burst separated by 10 min. Aged synapses exhibited significant reversal after a single burst and complete reversal after three LFS episodes. In adult slices, LTP reversal appeared after the fourth burst, and at no time was LTP depressed to initial baseline levels. This study provides the first characterization of homosynaptic LTD/LTP reversal in the aged animal and demonstrates that one form of plasticity, depression attributable to LFS, is increased during aging.

In vitro autoradiography of ionotropic glutamate receptors in hippocampus and striatum of aged Long-Evans rats: relationship to spatial learning

Using in vitro autoradiography, we investigated [3H] alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, [3H]kainate and [3H]N-methyl-D-aspartate binding in two forebrain regions, the hippocampus and striatum, of young (four months of age) and aged (24-25 months of age) Long-Evans rats that had previously been tested for spatial learning ability in the Morris water maze. Although there was substantial preservation of binding in the aged rats, reductions in binding were present in the aged rats that were specific to ligand and anatomical region. In the hippocampus of aged rats, [3H] alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate binding in CA1 and [3H]kainate binding in CA3 were reduced. In contrast, N-methyl-D-aspartate binding was not significantly different between age groups. There was evidence of sprouting in the dentate gyrus molecular layer of aged rats, indicated by changes in the topography of [3H]kainate binding. Binding density was analysed with respect to patch/matrix compartmentalization in the striatum. The most striking result was a large decrease in N-methyl-D-aspartate binding in aged rats that was not limited to any dorsal/ventral or patch/matrix area of the striatum. Additionally, [3H]kainate binding in striatal matrix was modestly reduced in aged rats. Of these age effects, only N-methyl-D-aspartate binding in the striatum and [3H]kainate binding in the CA3 region of the hippocampus were correlated with spatial learning, with lower binding in the aged rats associated with better spatial learning ability. Age-related alterations in ionotropic glutamate receptors differ with respect to the receptor subtype and anatomical region examined. The age effects were not necessarily indicative of cognitive decline, as only two age-related binding changes were correlated with spatial learning. Interestingly, in these instances, lower binding in the aged rats was associated with preserved spatial learning, suggesting a compensatory reduction in receptor binding in a subpopulation of aged rats.

Ionotropic glutamate receptor subtypes in the aged memory-impaired and unimpaired Long-Evans rat

The comparative quantitative autoradiographic distribution of ionotropic glutamate receptor subtypes were investigated in young adults (six months) and aged (24-25 months) cognitively impaired and unimpaired male Long-Evans rats. Aged rats were behaviorally characterized as either cognitively impaired or unimpaired based upon their performances in the Morris water maze task compared to the young adult controls. The status of the N-methyl-D-aspartate, [125I]dizocilpine maleate, [3H]kainate and amino-3-hydroxy-5-methylisoxasole-4-propionate (AMPA, [3H]AMPA) receptor binding sites were then established in these three subgroups of animals as a function of their cognitive performance in the Morris water maze task. The apparent densities of both N-methyl-D-aspartate/[125I]dizocilpine maleate and kainate binding sites were significantly decreased in various regions of the aged rat brain. Marked losses in [125I]dizocilpine maleate binding sites were observed in outer laminae of the frontal, parietal and temporal cortices, and the stratum radiatum of the CA3 subfield of the hippocampus. Interestingly, losses in [125I]dizocilpine maleate binding sites were generally most evident in the cognitively unimpaired aged subgroup, suggesting a possible inverse relationship between losses of this receptor subtype and cognitive performances in the Morris water maze task. The levels of [3H]kainate binding were most significantly diminished in various cortical and hippocampal areas as well as the striatum and septal nuclei of both groups of aged rats. In contrast, the apparent density of [3H]AMPA binding was increased in most hippocampal subfields and the superficial laminae of the occipital cortex of the cognitively impaired vs young adult rats. Changes in [3H]AMPA labeling failed to reach significance in the unimpaired cohort. Taken together, these results show that while losses in [3H]kainate binding were similar in both subgroups of aged rats, differences were seen with respect to cognitive status for both [125I]dizocilpine maleate/N-methyl-D-aspartate and [3H]AMPA binding sites. Decreases in [125I]dizocilpine maleate binding sites were mostly restricted to cortical areas of cognitively unimpaired rats, while increases in the AMPA binding subtype were seen in the memory-impaired subgroup. It would thus appear that changes in N-methyl-D-aspartate and AMPA receptor subtypes may be more critical than alterations in kainate binding sites for the emergence of the functional deficits seen in the aged cognitively impaired rat.

Aging reduces neostriatal responsiveness to N-methyl-D-aspartate and dopamine: an in vitro electrophysiological study

Excitatory amino acids and dopamine interact to control information flow in the neostriatum. The present study was designed to examine some of the age-induced alterations in the interaction of these two neurotransmitter systems. First, responsiveness of neostriatal neurons to glutamate and N-methyl-D-aspartate was compared in neurons from young and in aged animals. N-Methyl-D-aspartate function was chosen for emphasis because declines in cognitive processes during aging are thought to involve changes in this excitatory amino acid receptor. Second, the age-related changes in dopamine's ability to modulate responses mediated by excitatory amino acid receptors was examined. Specifically, the ability of dopamine to differentially modulate responses induced by N-methyl-D-aspartate and glutamate was assessed. There is considerable evidence for alterations in dopamine receptors and behavioral responses to dopamine in aged animals. It thus becomes important to determine how these alterations are reflected at an electrophysiological level. The responses to application of excitatory amino acid agonists and dopamine as well as changes in synaptic responses mediated by activation of N-methyl-D-aspartate receptors were assessed in 69 neurons obtained from young Fischer 344 rats (3-5 months) and young cats (3-4 years) and 69 neurons obtained from aged Fischer 344 rats (24-26 months) and aged cats (10-16 years) using an in vitro slice preparation. The results indicated that populations of aged neurons from both rats and cats displayed qualitative and quantitative alterations in responses to iontophoretic application of excitatory amino acid receptor agonists. These alterations included lack of response, unusual responses consisting of depolarizations without action potentials or combinations of prepotentials and full amplitude action potentials. Threshold currents for induction of responses were also significantly elevated in neurons from aged animals. Synaptic response components mediated by activation of N-methyl-D-aspartate receptors in aged rats were reduced as well. Exposure to Mg(2+)-free artificial cerebrospinal fluid resulted in marked increases in the size of responses evoked by local stimulation in young neurons from rats. These increases, which are mediated by activation of N-methyl-D-aspartate receptors, were significantly attenuated in aged neurons. The ability of dopamine to modulate responses mediated by activation of excitatory amino acid receptors was reduced in cells from both aged rats and cats. Subpopulations of cells were either unresponsive to dopamine or required higher iontophoretic current intensities to modulate excitatory amino acid-induced responses. The present findings further document age-induced changes in neostriatal electrophysiology indicating that interactions between excitatory amino acids and dopamine appear to be compromised during aging. They emphasize alterations in N-methyl-D-aspartate receptor function and suggest further than the ability of neostriatal neurons to integrate information is altered during aging. The present findings are supported by data from the literature indicating decreases in N-methyl-D-aspartate receptor function during aging. Furthermore, the decreases in excitatory amino acid function during aging suggest that therapeutic interventions designed to prevent or retard the deleterious effects of age in the neostriatum might be directed toward enhancing excitatory amino acid receptor function.

Regional age-related alterations in cholinergic and GABAergic receptors in the rat brain

The age-related changes of cholinergic and gamma-aminobutyric acid (GABA)ergic receptors were studied in 3-week- and 6-, 12-, 18- and 24-month-old Fisher 344 male rat brains using receptor autoradiography. [3H]Quinuclidinyl benzilate (QNB), [3H]hemicholinium-3 (HC) and [3H]muscimol were used to label acetylcholine receptors, acetylcholine reuptake sites and GABAA receptors, respectively. In immature rats (3-week-old), [3H]QNB and [3H]muscimol binding showed a significant increase in most brain areas, compared to adult young animals (6-month-old), whereas [3H]HC binding exhibited a significant increase only in the dentate gyrus, substantia nigra and cerebellum. In contrast, [3H]QNB and [3H]HC binding showed no significant changes in all brain areas during aging. On the other hands, [3H]muscimol binding showed a significant reduction in the substantia nigra and cerebellum of adult mature rats (12-month-old). Thereafter, the age-related reduction in [3H]muscimol binding was observed in all brain areas of aged rats (24-month-old). Our results demonstrate that the GABAergic system is susceptible to aging processes in the central nervous system, whereas the cholinergic system is unaltered by aging. Furthermore, our results suggest significant regional changes in both GABAergic and cholinergic systems in the brain even 3 weeks after birth. These findings suggest that the disturbance in GABAergic-cholinergic interactions may play a key role in age-related neurological deficits and cognitive dysfunction.

Effect of aging on the spatial learning deficit produced by diazepam in rats

1. After pretraining in an undrugged state, young (6 months) and aged (18-24 months) rats were trained on a spatial learning-set task after receiving one of four doses of diazepam (1, 2, 3 or 5 mg/kg) or the drug vehicle. The effects of 5 mg/kg of diazepam were also assessed on the spatial learning-set task one full hour after injection (delay condition) as well as on the visible platform task. 2. During pretraining (undrugged), aged rats demonstrated a transient impairment on the visible platform task but subsequently did not differ significantly from young rats on the submerged platform task. On the spatial learning-set task, aged rats performed as well as young rats under control conditions and diazepam produced a comparable dose-dependent impairment of spatial learning. However, when the 1 hr delay was interposed between diazepam administration and maze testing, only aged rats exhibited a spatial learning impairment. Diazepam did not impair performance on the visible platform task in either young or aged rats. 3. These results indicate that although the amnesic effect of diazepam is not initially greater in aged rats, it persists for longer periods.

Glutamate receptor binding in the human hippocampus and adjacent cortex during development and aging

Distinct patterns of age-related alterations in NMDA (MK801 binding) and non-NMDA, AMPA (CNQX), and kainate binding have been identified in human hippocampus and parahippocampal gyrus in normal individuals with no evidence of degenerative brain disease ranging in age from 24 gestational weeks to 94 years. Whereas MK801 binding did not alter substantially over this age range, CNQX binding rose from low levels in the fetus to maximum levels between neonate and middle age, and kainate binding declined extensively from the perinatal to adult stage. Following maturity, there were no significant changes in kainate binding, although MK801 binding increased in CA1 and CA3 and CNQX binding declined in several regions, particularly CA2 and subiculum. For each receptor binding the timing of these fluctuations ocurring during development and aging varied within different regions of the dentate gyrus, hippocampus proper, subicular complex, and entorhinal cortex examined. The transient peaks of receptor binding are likely to reflect processes of synaptogenesis and pruning and may provide clues regarding the role of the different glutamate receptor subtypes in various pathologies of the hippocampus and adjacent cortex associated with developmental disorders (of genetic origin or due to perinatal trauma or insult). The absence of substantial changes in any subtype examined from middle to old age suggests alterations in transmitter binding to these glutamate receptors are not involved in senescent neurodegeneration.

Effects of aging and chronic nimodipine on hippocampal binding of [3H]CGS 19755

Previous studies have suggested that aging is associated with impaired behavioral performance and with decrements of N-methyl-D-aspartate (NMDA) receptors in the rat hippo-campus. Other studies have indicated that chronic treatment with nimodipine, a Ca2+ channel antagonist, prevents the age-related decline in performance by rats in behavioral tasks. Therefore, we tested whether nimodipine altered binding of [3H]CGS 19755 to hippocampal NMDA receptors in rats whose performance on a 14-unit T maze had been tested previously (14). No significant age difference was observed in [3H]CGS 19755 binding in hippocampi from old Fischer-344 rats (27 months) as compared with mature but not senescent rats (9 months); however, old rats that received chronic treatment with a low dose of nimodipine (20 mg pellets implanted subcutaneously twice during 70 days of treatment) showed higher levels of binding. A high dose of nimodipine (40 mg pellets implanted by the same route and at the same times as the low dose) was without effect on [3H]CGS 19755 binding, although aged rats given this treatment performed better in the maze than rats that received no nimodipine or the low dose. In a second experiment comparing hippocampi of young (4 months) and old (24 months) rats, saturation studies confirmed the lack of an age difference in [3H]CGS 19755 binding. The findings suggest that neither the age-related decline in maze performance nor the enhancement of behavior by nimodipine depend upon changes in hippocampal NMDA receptors.

Effects of intraseptal injection of 192-IgG-saporin in mature and aged Long-Evans rats

In this study, the effects intraseptal injections of the selective cholinergic immunotoxin, 192-IgG-saporin, were investigated in mature (6-month-old) and aged (24-26-month-old) male Long-Evans rats. Ten days following intraseptal injection of either 192-IgG-saporin or saline, testing began in a battery of behavioral tests modulated by the septohippocampal system including two versions of the Morris water maze (i.e. submerged platform task, and 2-platform spatial discrimination), inhibitory avoidance, and pre-pulse inhibition of acoustic startle. In both mature and aged rats, intraseptal injection of 192-IgG-saporin selectively reduced ChAT activity in the hippocampus and posterior cingulate cortex, without affecting ChAT activity of amygdala or parietal cortex. In general, in all of the behavioral tests analyzed, intraseptal 192-IgG-saporin treatment had no effect in mature animals. Age-related deficits were observed in the spatial memory tasks, however this impairment was largely a function of the poor performance of aged rats treated with the toxin. In addition, an increase in the response to an acoustic startle was found in aged rats treated with 192-IgG-saporin. Thus, although intraseptal injection of 192-IgG-saporin produced similar reductions of ChAT activity, performance of mature and aged rats in tasks believed to be modulated by the septohippocampal pathway tended to be differentially affected in mature and aged rats.

Different age-related changes in NMDA and glycine receptors in the rat brain

We investigated the age-related changes of N-methyl-d-aspartate (NMDA) and strychnine-insensitive glycine receptors in the brain from Fischer rats aged 3 weeks (immature), 6 months (adult), 12 months (mature), 18 months (middle-aged) and 24 months (aged) using receptor autoradiography. [(3)H]MK-801 and [(3)H]glycine were used to label the NMDA receptor and the glycine receptor, respectively. In immature rats, [(3)H]MK-801 binding showed a significant decline only in the nucleus accumbens, whereas [(3)H]glycine binding exhibited a significant increase in the frontal cortex, parietal cortex, striatum and thalamus as compared with young rats. In mature, middle-aged and aged rats, [(3)H]MK-801 binding showed no significant change in the brain. In contrast, [(3)H]glycine binding showed a conspicuous reduction in the striatum and hippocampal CA3 sector and thalamus from mature rats. Thereafter, the age-related reduction in [(3)H]glycine binding was observed in all brain areas of middle-aged and aged rats. These results demonstrate that the glycine receptor in the rat brain is far more susceptible to aging processes than the NMDA receptor. Furthermore, they suggest the conspicuous differences in the developmental pattern between NMDA and glycine receptors in the rat brain after birth. These findings suggest that glycine receptor in the brain is primarily and severely affected in aging processes and this may lead to age-related neurological deficits.

Why Is Panic Disorder Less Frequent in Late life?

Epidemiological studies have found that few cases of panic disorder arise for the first time after the age of 40 years, and there is a steady decline in the prevalence of existing cases in the latter half of life. The authors review these epidemiological findings and explore various hypotheses that might explain the decreased frequency of panic disorder in old age. There is no available evidence to suggest that methodological factors have led to an underestimation of the prevalence of this disorder in older rather than younger age groups. However, there is evidence that disorder-associated mortality and age-related changes in brain neurochemistry may contribute to the decreased frequency of this illness in later life. A cohort effect also should be considered, although currently there are no data available to support or refute this idea.

Facilitation of glutamate receptors reverses an age-associated memory impairment in rats

The accuracy of memory for recent events is reported to decay between young adulthood and middle age in humans (Crook et al., 1990; Crook and West, 1990; Thomas et al., 1977) due to impairments in acquisition and/or retention (Craik, 1977; Huppert and Kopelman, 1989). Effects of this kind are also found in comparisons of middle-aged (12-18 months) vs. young adult (3 months) rats in tests requiring retention of recently sampled spatial cues (Kadar et al., 1990a; Kadar et al., 1990b; Goudsmit et al., 1990; Weiss and Thompson, 1991). The causes of such changes in memory processing are unknown but might be expected to involve age-related losses in forebrain glutamate receptors (Bahr et al., 1992; Magnusson and Cotman, 1993; Wenk et al., 1991); these receptors mediate fast excitatory transmission in many brain regions and play an essential role in the production of long-term potentiation (LTP), a form of synaptic plasticity that has been implicated in memory encoding (Landfield and Lynch, 1977; Moore et al., 1993). In the present communication we report results indicating that a drug that enhances AMPA-type glutamate receptors acts centrally to selectively increase hippocampal spatial cell firing and improves both acquisition performance and memory retention in middle-aged rats to levels equivalent to those found in young adult animals.

Effects of aging on light-induced phase-shifting of circadian behavioral rhythms, fos expression and CREB phosphorylation in the hamster suprachiasmatic nucleus

Aging is associated with a variety of alterations in circadian rhythms, including changes in the response to environmental stimuli. The underlying causes for these age-related changes in the circadian system remain unknown. Recent studies have demonstrated that light induces the expression of Fos and phosphorylation of the cyclic-AMP response element-binding protein in the rodent suprachiasmatic nuclei, the location of a master circadian pacemaker in mammals, suggesting that these transcription factors may mediate the effects of light on the circadian clock. The purpose of this study was to determine the effects of aging upon light-induced phase-shifting of circadian locomotor activity rhythms, Fos protein expression and cyclic-AMP response element-binding protein phosphorylation in the suprachiasmatic nuclei. Young (three to four months) and old (18-22 months) male golden hamsters free-running in constant darkness were exposed to 5-min monochromatic light pulses of different irradiance levels, at circadian time 19, after which either steady-state phase shifts of locomotor activity rhythms were measured, or else immunocytochemistry for Fos or for phospho-cyclic-AMP response element-binding protein was performed. Old hamsters were approximately 20 times less sensitive to the phase-shifting effects of light on the activity rhythm, and the photic irradiance threshold for Fos-like immunoreactivity induction in the suprachiasmatic nuclei was elevated when compared to young animals. Aging was also associated with a deficit in cyclic-AMP response element-binding protein phosphorylation by light. These data indicate that there are dramatic changes in light-activated molecular responses in the suprachiasmatic nuclei of old hamsters, and suggest that these molecular changes may underlie age-related changes in the effects of light on the circadian clock system.

Age-related decrease in vulnerability to excitatory amino acids in the nucleus basalis

The present study investigated the effects of nucleus basalis magnocellularis (NBM) lesions in young (3 months), adult (9 months), and aged (24 months) rats by injections of either NMDA or AMPA upon performance of a delayed alternation task on a T maze. During phase 1 of testing, the interchoice interval (ICI) was 5 s and each rat was given 10 trials per day during phase 2, the ICI was 30 s across 10 trials per day; during phase 3, the ICI was 5 s across 20 trials per day. Analyses of variance revealed (a) a significant effect of age during phase 1 (i.e., 24-month-old rats performed worse than 3-month-old rats); (b) a significant effect of age and lesion in phase 2 (i.e., the lesions impaired choice accuracy equally in all age groups when the ICIs were 30 s); (c) a significant effect of age and lesions, and a significant interaction in phase 3 (i.e., young rats were more impaired by the lesions than were aged rats.

Age-related changes in bindings of second messengers in the rat brain

Age-related alterations in bindings of major second messengers in the brain were studied in 3-week- and 6-, 12-, 18- and 24-month-old Fisher 344 rats using receptor autoradiography. [3H]Phorbol 12,13-dibutyrate (PDBu) and [3H]forskolin were used to label protein kinase C (PKC) and adenylate cyclase, respectively. In immature rats (3-week-old), [3H]PDBu binding showed a significant decrease only in the cerebellum as compared to adult rats (6-month-old), whereas [3H]forskolin binding exhibited a significant reduction in the neocortex, nucleus accumbens, thalamus and substantia nigra. In aged rats, [3H]PDBu binding showed no significant change in all brain areas. In contrast, [3H]forskolin binding showed a conspicuous reduction in various brain areas in 18-month-old rats as compared to adult animals. The age-related reduction was especially observed in the cerebral cortex, hippocampal CA3 pyramidal cell layer, dentate gyrus, thalamus and molecular layer of cerebellum of 24-month-old rats. The results indicate that adenylate cyclase system in the rat brain is more susceptible to aging processes than phosphoinositide cycle system. Furthermore, our data demonstrate that the change in the adenylate cyclase system is more pronounced than that in the phosphoinositide cycle system in immature rat brain. These findings suggest that the adenylate cyclase system is primarily affected in aging processes and this may lead to age-related neurological deficits.

Regulation of phosphatidylinositide transduction system in the rat spinal cord during aging

Age-related functional alterations in a variety of neurotransmitter systems result in modulation of interneuronal communications which has some relevance in neurological deficits observed in the aging process. The synergistic interactions between protein kinase and inositol 1,4,5-trisphosphate (insP3)/Ca2+ pathways underlie a variety of cellular responses to external stimuli. To determine whether age-dependent changes occur in the regulation of protein kinase C and inositol 1,4,5-trisphosphate/Ca2+ pathways, insP3 contents as a marker for the release of intracellular calcium, saturation binding analysis of Ins P3 receptor using [3H]inositol 1,4,5-trisphosphate, slot/northern blot analysis of Ins P3 receptor-encoding mRNA transcripts, and the activities of Ca2+/phospholipid-dependent protein kinase C isozymes were investigated in the rat spinal cord. Inositol 1,4,5-trisphosphate content and [3H]inositol 1,4,5-trisphosphate binding site density (Bmax) were quantified in the spinal cords of young (three months old), adult (12 months old) and senescent (25 months old) male Fischer 344 rats. Spinal cord content of inositol 1,4,5-trisphosphate was increased (P < 0.01) in the 25-month old compared to the three- and 12-month old animals. The density of Ins P3 receptor in particulate membranes derived from the 25-month old rats was reduced (P < or = 0.01), but the binding affinity (Kd) was increased (P < or = 0.04) by a factor of 2.2 and 3.2 at 25 months of age when compared with three- and 12-month old animals, respectively. Young and middle-aged animals showed no differences in both inositol 1,4,5-trisphosphate contents and [3H]inositol 1,4,5-trisphosphate binding site density. The quantity of Ins P3 receptor mRNA was significantly increased with age in the order 25 >> 12 > 3 months of age. Total functional cytosolic and membrane-associated PKC activities were decreased (P < or = 0.05) in the 25-month compared to the three- and 12-month old rats in which activity remained unchanged. Total membrane/cytosolic activity ratios were unchanged by the aging process. In all cases, the activities of membrane-associated conventional protein kinase C isozymes (alpha, beta and gamma), determined by immunoprecipitation followed by in situ quantification of protein kinase C activities in the immunoprecipitates, showed age-dependent decline. The activities of protein kinase C-alpha and beta were significantly decreased in age-related manner. However, the activity of the gamma-isozyme was not significantly changed at 12- and 25-months of age, although it was higher (P < or = 0.03) in young rats. Western blot analyses using affinity purified polyclonal antibodies specific for each isozyme indicated a single protein with an apparent molecular mass of approximately 80 x 10(3) molec. weight for all isozymes except for the beta isozyme that also had an appreciable immunoreactive band at approximately 36 x 10(3) molec. weight. Overall, the aging process did not affect the electropheretic mobility of each isozyme. With decreased protein kinase C activity, the present data suggest that the aging process would decrease protein kinase C-induced phosphorylation of membrane proteins including Ins P3 receptor. A significant change in Ins P3 receptor affinity combined with increased levels of Ins P3 receptor mRNA-encoding transcripts in senescent rats suggests not only a modification (possibly by phosphorylation) of Ins P3 receptor protein but also the existence of multiple (spliced) variants of Ins P3 receptor in spinal neurons with increasing age. The present data indicate that the spinal contents of inositol 1,4,5-trisphosphate increased with age, but with decreased efficacy and number of inositol 1,4,5-trisphosphate-activatable Ca2+ channels in the spinal cord of senescent rats. These age-related changes may contribute to the attenuated responsiveness of spinal cord neurons by phosphoinositide-coupled receptors during the aging process.

Differential effects of aging on binding sites of the activated NMDA receptor complex in mice

The NMDA receptor site has been shown to be vulnerable to the effects of aging. Decreases in binding to the receptor site of up to 50% have been reported in aged animals. The present study was designed to quantitate and compare the effects of aging on multiple binding sites of the NMDA receptor complex in various brain regions. Autoradiography with [3H]glutamate, [3H]CPP, [3H]glycine, [3H]MK801 and [3H]TCP was performed on brain sections from 3, 10 and 28-30 month old C57B1/6 mice. The percent declines between 3 and 28-30 months of age in [3H]-glutamate (15-35% declines) and [3H]CPP (20-42% declines) binding were similar within most cortical regions and the caudate nucleus but [3H]glutamate binding showed less change (0-11% declines) than [3H]CPP (13-27% declines) in the occipital/temporal cortex and hippocampal regions. [3H]MK801 and [3H]TCP binding, stimulated by 10 microM glutamate, exhibited intermediate aging changes between the glycine and NMDA sites, both in percent decline (3-28% and 0-26%, respectively) and in the number of brain regions involved. [3H]Glycine binding, stimulated by 10 microM glutamate, showed no significant overall effect of age (declines ranged from 0-34%). [3H]CPP binding was significantly more affected than [3H]glycine binding in many regions. These results suggest that aging has heterogeneous effects on different sites on the NMDA receptor complex throughout the brain and on NMDA receptor agonist versus antagonist binding in selected brain regions.

Reduced hippocampal CA1 Ca(2+)-induced long-term potentiation is associated with age-dependent impairment of spatial learning

Expression of Ca(2+)-induced CA1 long-term potentiation (LTP) was analysed in hippocampal slices obtained from (1) 3-month-old and (2) 18-20-month-old Sprague-Dawley rats selected for their performances in the Morris water maze task. In all slices, a transient (10 min) increase of extracellular Ca2+ concentration (4 mM) caused a long-lasting enhancement of potentials evoked by electrical stimulation of radiatum fibers. However, a significant difference was found in the degree of potentiation among groups. In particular, increases of the CA1 response amplitudes were significantly lower in old rats impaired in spatial learning than in young at 30 (P < 0.05), 60, 90 and 120 min (P < 0.01) after restoring the normal Ca2+ concentration. On the contrary, no differences were observed between young animals and the old ones with good performances in spatial learning. The data suggest that amplitude of CA1 Ca(2+)-induced LTP in old rats is related to spatial learning abilities.

Experimental stroke and neuroprotection in the aging rat brain

BACKGROUND AND PURPOSE

Experimental stroke research has for the most part incorporated the use of young animals despite the importance of aging in cerebrovascular disease in humans. We hypothesized that age-related reductions in the density and function of cortical N-methyl-D-aspartate (NMDA) receptors might limit neuroprotective potential in the elderly. In this study, a model of occlusive stroke in the aging rat brain has been developed and used to establish the effects of age on cerebral infarction and to evaluate the scope for protecting the aging brain during ischemia.

METHODS

Focal cerebral ischemia was produced by thermocoagulation of the left middle cerebral artery in adult (11 to 17 months) and aged (28 to 36 months) male Wistar rats. Infarcts were assessed histologically with volumetric analysis of infarct size, hemodynamically by serial cerebral blood flow measurement using the hydrogen clearance technique, and by analysis of specific gravity as an index of brain edema. Neuroprotective potential was assessed using the competitive NMDA receptor antagonist 3-(2-carboxy piperazin-4-yl)propyl-1-phosphonate (D-CPPene).

RESULTS

Aging was associated with a significant increase in infarct size, with a mean infarct volume of 40.5 +/- 2.6% of the hemisphere volume in aged rats compared with 30.9 +/- 0.7% in adult rats (P < .01). D-CPPene reduced the mean infarct volume to 33 +/- 1.8% and 20.7 +/- 3.2% in aged and adult rats, respectively (P < .05). Cerebral blood flow fell markedly after infarction, but thereafter D-CPPene-pretreated rats maintained higher cerebral blood flow than untreated animals throughout the duration of the experiment (22.8 +/- 3.2 and 30.1 +/- 5.5 mL.100 g-1.min-1 in treated aged and adult rats, respectively, compared with 11.3 +/- 2.7 and 16.5 +/- 3.2 mL.100 g-1.min-1 in untreated aged and adult groups, 90 minutes after infarction [P < .05]). Pretreatment also reduced cortical edema; mean cortical specific gravity 4 hours after infarction was 1.0381 +/- 0.0013 in untreated aged rats and 1.0391 +/- 0.0014 in untreated adults compared with 1.0458 +/- 0.0031 in treated aged rats and 1.0442 +/- 0.0014 in treated adult rats (P < .05).

CONCLUSIONS

Under similar experimental conditions, there was an age-related increase in cerebral infarct size. However, NMDA receptor antagonism was neuroprotective in the aging brain and resulted in a significant reduction in cerebral ischemic damage, less cortical edema, and preservation of cerebral blood flow.

Effects of excitatory amino acid lesions upon neurokinin B and acetylcholine neurons in the nucleus basalis of the rat

The nucleus basalis magnocellularis (NBM) contains cholinergic neurons that project to the neocortex and is densely innervated by excitatory amino acid-containing terminals. A dysfunction in the balance of excitatory inputs or an alteration in the sensitivity of NBM cells to glutamate may underlie the selective vulnerability to aging. Some large NBM neurons contain neurokinin B (NKB) mRNA. The present study investigated whether alpha-2-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) or N-methyl-D-aspartate (NMDA) differentially destroy NKB-containing, NKB-receptive, or cholinergic NBM cells, and whether this vulnerability is altered by aging. Injections of AMPA or NMDA significantly decreased neocortical ChAT activity, as compared to control levels, across all three age groups, with no interaction between lesion and age group. The results of in situ hybridization histochemistry and NKB receptor studies suggest that NKB-containing neurons in the NBM, and the neurons they innervate, are not vulnerable to NMDA or AMPA in either young or old rats. While NKB mRNA-positive cells were diffusely distributed throughout the basal forebrain, only a small proportion of the large NBM cells contained NKB mRNA. The results suggest that NKB does not extensively colocalize with acetylcholine within the basal forebrain of rats and that NBM NKB neurons do not directly innervate cholinergic cells.

Age-related changes in the NMDA receptor/nitric oxide/cGMP pathway in the hippocampus and cerebellum of freely moving rats subjected to transcerebral microdialysis

The N-methyl-D-aspartate (NMDA) receptor/nitric oxide synthase/guanylate cyclase pathway was studied during aging by monitoring extracellular cGMP in the rat hippocampus and cerebellum during in vivo microdialysis. In the hippocampus the basal cGMP efflux decreased by 50% from 3 to 12 months of age, whereas it remained constant with age in the cerebellum. Locally perfused NMDA (1 mM) evoked remarkable cGMP responses in 3-month-old rats; in the hippocampus the cGMP production was already dramatically reduced at 12 months, whereas in the cerebellum a similar impairment occurred much later (24 months). The nitric oxide donor S-nitroso-N-penicillamine (1 mM) elicited cGMP responses which slightly decreased from 3 to 12-24 months in the hippocampus, while no significant decrement with age could be seen in the cerebellum. Local perfusion of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX, 1 mM) produced large increases in hippocampal cGMP levels. The response decreased at 12 and 24 months, apparently in parallel with the fall in the basal level of cGMP. No significant differences across ages were observed following IBMX infusion in the cerebellum. The decreases in basal outflow and in the NMDA-evoked cGMP response seen in the aged hippocampus were not compensated for by supplying L-arginine. Infusion of D-serine (1 mM) enhanced (150-200%) extracellular cGMP in the cerebellum with no age-related differences. The activity in vitro of hippocampal nitric oxide synthase at 24 months was 33% lower than at 3 months, whereas the cerebellar enzyme did not show any age-related decay.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional analysis of the proximal 5'-flanking region of the N-methyl-D-aspartate receptor subunit gene, NMDAR1

The NMDAR1 receptor subunit is a common subunit of N-methyl-D-aspartate receptors. We have previously characterized 3 kilobases (kb) of 5'-flanking sequence of the NMDAR1 gene and now report on the ability of this region to direct transcription of a reporter gene and on its interaction with nuclear proteins. The sequence 356 base pairs (bp) 5' of the first nucleotide of codon 1 was sufficient to express a luciferase reporter gene in rat PC12 pheochromocytoma cells. Additional sequences upstream of nucleotide -356 influenced the activity approximately 2-fold. A labeled 112-bp fragment (position -356 to -245) formed six complexes (C1A and -B, C2A and -B, and C3A and -B), grouped as three double bands, with nuclear extracts from PC12 cells. Competition with Sp1 oligonucleotides abolished formation of C2A and -B and C3A and -B complexes. Sp1 antibody recognized the C3A complex in supershift experiments. Prior immunoprecipitation of nuclear extracts with Sp1 antibody abolished formation CA2 and -B and C3A and -B complexes. Purified Sp1 protein alone did not form a C3A complex but potentiated its formation when PC12 nuclear extract was added. A GC-rich sequence in this fragment was protected from DNase I digestion by nuclear extract. These results suggest that a 356-bp sequence comprises the NMDAR1 basal promoter, and that NMDAR1 gene expression may be regulated by Sp1-like nuclear factors.

Age-related levels of GABA/benzodiazepine binding sites in cerebrum of F-344 rats: effects of exercise

We examined gamma-aminobutyric acid (GABA), benzodiazepine and convulsant sites of postsynaptic GABA/benzodiazepine receptors (GBZR) in cerebral membranes of inbred Fischer 344 male rats as a function of age. In aged rats (23 to 24 months), the benzodiazepine binding site as determined by [3H]flunitrazepam was 47% and 43% lower than corresponding values in young adult (3 to 4 months) and mature (10 to 12 months) rats, respectively. The decrease was due to the loss of binding density rather than a change in affinity. No statistically significant age-related changes in [3H]muscimol binding were observed when 5 nM or 40 nM labeled muscimol were used. GABA produced a dose-dependent stimulation of flunitrazepam binding in all age groups, but the maximum stimulation in aged animals was significantly higher (24%) than in young and mature animals. The [35S]TBPS binding site, the convulsant site of GABA/benzodiazepine receptors, was unaffected with age. We also studied the effects of exercise on GBZR binding sites of aged rats. The decline of flunitrazepam binding sites and the high sensitivity of flunitrazepam binding to regulation by GABA in aged animals were reversed by 8 to 10 weeks of endurance exercise. Endurance exercise did not have any significant effect on muscimol or TBPS binding sites. Results suggest that there are aged-related alterations of GBZR binding sites and that these modifications can be reversed by exercise.

Age-related changes in parvalbumin- and GABA-immunoreactive cells in the rat septum

The calcium binding protein parvalbumin is present in GABAergic neurons of the medial septum-diagonal band of Broca (MS-DBB) region that project to the hippocampal formation. We examined the distribution pattern, the number, and the morphological features of the parvalbumin-containing cells (parv+) in the MS-DBB region of 2- to 3-, 8- to 9-, 15- to 16-, and 26- to 27-month-old Sprague-Dawley rats. A significant reduction in the number of parv+ cells was observed as a function of age. The mean somal area of the parv+ cells was significantly reduced in the 26- to 27-month-old rats. A significant reduction in the number of parv+ cells was also observed in the 26- to 27-month-old rats in the cingulate cortex, but not in the striatum or the hippocampal formation. No significant age-related changes were observed in the number of the GABA-immunoreactive cells in the MS-DBB region nor in the cingulate cortex. In conclusion, there is an age-related decrease in the number of parv+ cells, with no change in the number of GABA-immunoreactive cells in the MS-DBB region of the rat. Because GABA and parvalbumin are colocalized in the MS-DBB neurons, the results suggest that the level of parvalbumin is decreased, but that the cells are not lost.

Age-related decrease in GABAB receptor binding in the Fischer 344 rat inferior colliculus

Quantitative receptor autoradiography was used to assess GABAB receptor binding in three primary subdivisions of the inferior colliculus (IC): dorsal cortex (DCIC), external cortex (ECIC), and the central nucleus (CIC) of 3-, 18-20-, and 26-month-old Fischer 344 rats. GABAB binding sites were localized using [3H]GABA in the presence of a saturating concentration of isoguvacine, a selective GABAA receptor agonist, to displace [3H]GABA bound to GABAA receptor sites. In the three IC subdivisions examined, GABAB receptor binding was significantly reduced in 26-month-old rats when compared to 3-month-old rats (DCIC, -44%; ECIC, -36%; CIC, -32%; p < 0.05). For comparison, GABAB binding was determined in the portion of cerebellum located in the recess of the IC. In the molecular layer of this region, there was no statistically significant differences in receptor binding between 3, 18-20-, and 26-month-old rats. In addition, there was not a significant age-related change in the cross-sectional area of the IC. These findings provide additional evidence to support the existence of selective age-related changes in GABA neurotransmitter function in the rat IC.

GABAB binding sites in early adult and aging rat brain

The effect of aging on GABAB binding was investigated in rat brain. Receptor autoradiography was used to investigate both GABAB and GABAA binding at 2 months, 3 months, 13 months, and 23 months. GABAB binding decreases significantly between 2 months and 23 months of age, as does GABAA binding, with was investigated in rat brain. Receptor autoradiography was used to investigate both GABAB and GABAA binding at 2 months, 3 months, 13 months, and 23 months. GABAB binding decreases significantly between 2 months and 23 months of age, as does GABAA binding, with the greatest decrease between 2 and 3 months. The decrease in GABAB binding appears to be due to a decrease in binding site affinity rather than a decrease in receptor density. The noncompetitive GABAB antagonist zinc, the competitive GABAB antagonist CGP 35348, and the guanyl nucleotide analogue GTP-gamma-S all inhibit GABAB binding identically in 2 month and 23 month brain. These data indicate subtle age-related changes in the GABAB binding in early adult life but little change with senescence.

Reduced bicuculline response and GABAA agonist binding in aged rat hippocampus

Extracellular field recordings from CA1 pyramidal cells in the rat hippocampal slice preparation were used to examine the effects of age on gamma-aminobutyric acid (GABA)-mediated recurrent inhibition. The actions of bicuculline (1-100 microM), a GABAA antagonist, were assessed in slices from young (1-3 months) and aged (26 months) Fischer 344 rats. Pre-drug population spike amplitudes were smaller in slices from aged rats. Bicuculline increased population spike amplitudes in slices from both age groups, but slices from young rats were more sensitive to the antagonist. Bicuculline also produced multiple population spikes in slices from both age groups, however the increase in population spike burst durations was much greater in slices from young rats than in slices from aged rats. Agonist radiolabeled GABAA binding site density was significantly decreased in hippocampal tissue from aged rats. Our results suggest there is a reduction in GABAergic inhibition in hippocampal slices from aged rats, possibly mediated by a decrease in GABAA receptors.

Responses of cerebral arterioles to N-methyl-D-aspartate and activation of ATP-sensitive potassium channels in old rats

The first goal of this study was to examine the hypothesis that aging impairs dilator responses of cerebral arterioles to N-methyl-D-aspartate (NMDA). The second goal was to determine whether aging impairs vasodilatation in response to activation of ATP-sensitive K+ channels. Diameter of cerebral arterioles was measured through a cranial window in anesthetized adult (5 +/- 1 (mean +/- S.E.) months old) and old (26 +/- 1 months old) Fischer 344 rats. In adult rats, NMDA (10 and 100 microM) dilated pial arterioles by 14 +/- 5 and 72 +/- 9%, respectively, from a control diameter of 36 +/- 5 microns. Aprikalim (1 and 10 microM), a direct activator of ATP-sensitive potassium channels, dilated cerebral arterioles in adult rats by 14 +/- 3 and 49 +/- 6%, respectively. Vasodilatation in response to NMDA and aprikalim were similar in old and adult rats. Thus, in contrast to impaired cerebral vasodilator responses to some stimuli, responses of cerebral arterioles to NMDA and activation of ATP-sensitive potassium channels are preserved in old Fischer 344 rats.

Age-related changes in expression of AMPA-selective glutamate receptor subunits: is calcium-permeability altered in hippocampal neurons?

Age-related decline of cognition and memory, in humans and other animals, appears to be associated with neuronal loss. Experimental and clinical evidence has shown that the hippocampal formation is one of the brain regions most vulnerable to the ageing process. Because excess of glutamate is neurotoxic to hippocampal neurons, abnormalities in glutamate neurotransmitter function may play a crucial role in neurodegenerative disorders, especially in conjunction with brain ageing. We have used in situ hybridization to study the expression of the two major alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-selective glutamate receptor subunits, involved in the control of calcium permeability in the young adult and aged rat hippocampus. We show that the levels of messenger RNA encoding the AMPA-selective glutamate receptor subunit-1 (GluR1 or GluRA) and AMPA-selective glutamate receptor subunit-2 (GluR2 or GluRB) are highest in the dentate gyrus, followed by the CA1 and CA3 hippocampal subfields. We also show that the levels of both messenger RNAs decrease differentially with age in all subfields of the hippocampus. Finally, the GluR1/GluR2 messenger RNA ratios increase in the aged hippocampus, particularly in the CA3 subfield, suggesting that altered calcium homeostasis may contribute to age-related neuronal death.

New pharmacological strategies for cognitive enhancement using a rat model of age-related memory impairment

We have developed the Stone maze paradigm for use as a rat model of memory impairment observed in normal aging and in Alzheimer's disease. Evidence produced thus far clearly implicates both the cholinergic and glutamatergic systems in acquisition performance in this complex maze task. Although results have been very inconsistent regarding the cognitive enhancing abilities of cholinomimetics for use in Alzheimer's disease, new classes of cholinesterase inhibitors may offer greater therapeutic efficacy. The use of glycine and polyamine agonists appears to be a viable strategy for positive modulation of the NMDA receptor. In addition, an approach that combines stimulation both of cholinergic and glutamatergic systems may have greater potential than agonism of either separately. Manipulation of signal transduction events might also have potential for cognitive enhancement. The influx of Ca2+ through the NMDA receptor stimulates production of NO via the action of NOS. By using NARG to block NOS activity, we have demonstrated in rats that NO production appears to influence learning in the Stone maze. We are currently exploring the age-related changes in NOS activity in specific brain regions of rats to determine if loss in the NO generating system is related to age-related memory impairment observed in the Stone maze. In addition, we are exploring pharmacological strategies for inducing NO production; however, because of the potential neurotoxicity for NO overstimulation, this strategy will present some obstacles. The identification of NO as a simple molecule serving vital physiological functions but representing potential for neurotoxicity presents an important unifying area for neurobiological investigations searching for mechanisms of normal brain aging and of age-related neuropathology, as observed in Alzheimer's disease.

Melanocortins as factors in somatic neuromuscular growth and regrowth

Melanocortins, non-corticotropic fragments of adrenocorticotropic hormone, accelerate growth of the developing neuromuscular system and regrowth of damaged neurons, both in the adult and neonatal rat. Morphological, electrophysiological and behavioral characteristics are all improved by melanocortins, which, however, vary in potency, with alpha-MSH being the most effective. Tissue substrate, dosage, critical time periods and pattern of neuropeptide administration are all important variables. Melanocortins protect central neurons affecting motor behavior during development or following neuronal damage in the adult brain. Possible mechanisms of melanocortin action are discussed.

Acetyl-L-carnitine affects aged brain receptorial system in rodents

Acetyl-L-carnitine (ALCAR), the acetyl ester of carnitine, is regarded as a compound of considerable interest because of its capacity to counteract several physiological and pathological modifications typical of brain ageing processes. In particular, it has been demonstrated that ALCAR can counteract the age-dependent reduction of several receptors in the central nervous system of rodents, such as the NMDA receptorial system, the Nerve Growth Factor (NGF) receptors, those of glucocorticoids, neurotransmitters and others, thereby enhancing the efficiency of synaptic transmission, which is considerably slowed down by ageing. The present review thus postulates the importance of ALCAR administration in preserving and/or facilitating the functionality of carnitines, the concentrations of which are diminished in the brain of old animals.

Rodent models of memory dysfunction in Alzheimer's disease and normal aging: moving beyond the cholinergic hypothesis

The Stone maze paradigm has been developed for use as a rat model of memory impairment observed in normal aging and in Alzheimer's disease. Results from several studies have demonstrated the involvement of both cholinergic and glutamatergic systems in acquisition performance in this complex maze task. Although results of clinical studies on the cognitive enhancing abilities of cholinomimetics for treatment of memory impairment in Alzheimer's disease have been inconsistent, new classes of cholinesterase inhibitors offer greater potential for therapeutic efficacy. The physostigimine derivative, phenserine, appears to have marked efficacy for improving learning performance of aged rats or of young rats treated with scopolamine in the Stone maze. Declines in markers of glutamatergic neurotransmission in Alzheimer's disease and in normal aging suggest that pharmacological manipulation of this system might also prove beneficial for cognitive enhancement. Treatment with glycine and/or polyamine agonists is suggested as a strategy for activating the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor. In addition, the use of combined pharmacological activation of cholinergic and glutamatergic systems is suggested. Manipulation of signal transduction events should also be considered as a strategy for cognitive enhancement. The influx of Ca2+ through the channel formed by the NMDA receptor stimulates the production of the oxyradical, nitric oxide (NO*), via the action of nitric oxide synthase (NOS). Compounds that inhibit NOS activity impair acquisition in the Stone maze, suggesting an involvement of NO*. Thus, strategies for inducing NO* production to enhance cognitive performance may be beneficial. Because of the potential neurotoxicity for NO*, this strategy is not straightforward. Although many new directions beyond the cholinergic hypothesis can be suggested, each has its potential benefits which must be weighed against its risks. Nonetheless, an important unifying area for neurobiological research examining mechanisms of normal brain aging and of age-related neuropathology, as observed in Alzheimer's disease, might emerge from the identification of NO* as a simple molecule serving vital physiological functions but representing potential for neurotoxicity.

Chronic nimodipine treatment in aged rats: analysis of motor and cognitive effects and muscarinic-induced striatal dopamine release

Nimodipine is a calcium channel blocker reported to have beneficial effects on treatment of ischemic damage as well as the potential for retarding aspects of brain and behavioral aging when provided chronically to rats. We treated aged male F-344 rats (24 months) with nimodipine in SC pellets in the following doses: 0 (controls), 20 mg (low-dose), or 40 mg (high-dose) replenished after 6 weeks. After 3 months of treatment, surviving rats and a group of young controls (6 months) were tested in a behavioral battery involving exploratory activity in an open field and in a runwheel cage as well as motor abilities required for remaining on an inclined screen, suspended from a wire, and balanced on a rotorod. Rats were also pretrained for one-way active avoidance in a straight runway before being trained in a 14-unit T maze. During 20 trials rats were required to negotiate each of 5 maze segments within 10 s to avoid foot shock (0.8 mA). Nimodipine treatment produced no significant effects on body weight, food intake, or survival of aged rats. Analysis of behavioral results indicated significant age-related decline in performance of all tasks except in open-field behavior. Nimodipine treatment had no significant effects on behavioral performance of aged rats except in maze learning. Rats on the high-dose regimen performed significantly better than aged controls in the maze. The results indicate that chronic nimodipine treatment of aged rats had no toxic effects and might be beneficial for preventing age-related decline in learning performance.

The effect of age on cerebral oedema, cerebral infarction and neuroprotective potential in experimental occlusive stroke

A model of occlusive stroke in the aging brain has been developed and used to evaluate the effects of age upon cerebral infarction, cerebral oedema and neuroprotective potential. Focal ischaemia following left middle cerebral artery occlusion has been compared in aged (30 month) and adult (< 17 month) rats, with histological assessment of infarct volume and analysis of specific gravity as an index of cerebral oedema. Aging was associated with a significant increase in cerebral infarct size. The mean infarct volume in aged rats was 40.5% +/- 2.6% of the hemisphere volume, compared to 30.9% +/- 0.7% in adults (p < 0.01). Pre-treatment with the competitive N-Methyl-D-Aspartate (NMDA) receptor antagonist 3-(2-Carboxy Piperazin-4-yl)Propyl-l-Phosphonate (D-CPP-ene) reduced infarct volumes in both age groups to 33.0% +/- 1.8% and 20.7% +/- 3.2% in aged and adult animals, respectively (p < 0.05). There was significantly less oedema of the cerebral cortex in D-CPP-ene pre-treated rats; mean cortical specific gravity 4 hours post-infarction was 1.0381 +/- 0.0013 in untreated aged rats and 1.0391 +/- 0.0014 in untreated adults, compared to 1.0458 +/- 0.0031 in treated aged rats and 1.0442 +/- 0.0014 in treated adults (p < 0.05). At 24 hours post-infarction, D-CPP-ene pre-treated aged rats had a mean cortical specific gravity of 1.0403 +/- 0.0006 compared to 1.0361 +/- 0.0014 in untreated aged animals (p < 0.05). This study has demonstrated an age-related increase in cerebral infarct size, but has shown that the aging brain is amenable to neuroprotection by NMDA receptor antagonism.

Release of 3H-noradrenaline by excitatory amino acids from rat mediobasal hypothalamus and the influence of aging

The present study was designed to analyze the effects of glutamate (GLU) and its agonists on the release of noradrenaline (NA) from the mediobasal region of rat hypothalamus (MBH). Slices from hypothalamus were loaded in vitro with 3H-NA and thereafter exposed to GLU and the glutamate agonists N-methyl-D-aspartic acid (NMDA) and kainate (KA), in superfusion chambers. GLU evoked a significant 3H-NA release in a concentration-dependent manner. The EC50 was 35 mM. 6-Cyano-7-nitro-quinoxaline-2,3-dione (CNQX), a non-NMDA selective antagonist, and amino-7-phosphonoheptanoic acid (AP 7), a NMDA selective antagonist, both decreased the GLU-evoked response to about 50% of its value. NMDA, superfused in Mg(2+)-free Krebs-Ringer, exhibited a greater potency than GLU with an EC50 = 124 microM. KA was also able to evoke 3H-NA release, although overall responses to KA were lower than those of NMDA. The maximal response to KA was a 36% increase of release at a concentration of 200 microM. The effect of KA was blunted by CNQX. NMDA-induced 3H-NA release was progressively altered with age. In old rats (16-18 months) and middle-aged rats (10 months), responses to 200 microM NMDA were decreased respect to young (4 months) male rats. These results show that NMDA and KA receptors mediate the excitatory effects of GLU on NA release from nerve terminals in the MBH and suggest that GLU, in association with NA, participates in the complex mechanisms that regulate neuroendocrine functions.(ABSTRACT TRUNCATED AT 250 WORDS)

Normal aging: regionally specific changes in hippocampal synaptic transmission

Results of electrophysiological investigations of aging in the rodent hippocampus contradict the popular conception of the aging process as one of general deterioration. Such studies have revealed a selective pattern of both degenerative change and functional sparing in different physiological parameters of the same cells. In synaptic transmission, changes have been observed that might even be considered compensatory. The selectivity of the aging process is further demonstrated by the fact that it exhibits clear regional specificity, even among the different subfields of the hippocampus. The future challenges will be to understand both how these specific patterns of age-related neurobiological change arise, and how they lead to the cognitive changes that arise during normal aging.

Selective changes of neurotransmitter receptors in middle-aged gerbil brain

Age-related alterations in major neurotransmitter receptors and voltage dependent calcium channels were analyzed by receptor autoradiography in the gerbil brain. [3H]Quinuclidinyl benzilate (QNB), [3H]cyclohexyladenosine (CHA), [3H]muscimol, [3H]MK-801, [3H]SCH 23390, [3H]naloxone, and [3H]PN200-110 were used to label muscarinic acetylcholine receptors, adenosine A1 receptors, gamma-aminobutyric acidA (GABAA) receptors, N-methyl-D-aspartate (NMDA) receptors, dopamine D1 receptors, opioid receptors, and voltage dependent calcium channels, respectively. In middle-aged gerbils (16 months old), the hippocampus exhibited a significant elevation in [3H]QNB, [3H]MK-801, [3H]SCH 23390, [3H]naloxone, and [3H]PN200-110 binding, whereas [3H]CHA and [3H]muscimol binding showed a significant reduction in this area, compared with that of young animals (1 month). On the other hand, the cerebellum showed a significant alteration in [3H]QNB, [3H]CHA, and [3H]naloxone binding and the striatum also exhibited a significant alteration in [3H]SCH 23390 and [3H]CHA binding in middle-aged gerbils. The neocortex showed a significant elevation only in [3H]CHA binding in middle-aged animals. The nucleus accumbens and thalamus also showed a significant alteration only in [3H]muscimol binding. However, the hypothalamus and substantia nigra exhibited no significant alteration in these bindings in middle-aged gerbils. These results demonstrate the age-related alterations of various neurotransmitter receptors and voltage dependent calcium channels in most brain regions. Furthermore, they suggest that the hippocampus is most susceptible to aging processes and is altered at an early stage of senescence.