Age-related electrophysiological changes in rat cerebellum

Aging of cerebellar Purkinje cells

Cerebellar Purkinje cells (PCs), the sole output neurons in the cerebellar cortex, play an important role in the cerebellar circuit. PCs appear to be rather sensitive to aging, exhibiting significant changes in both morphology and function during senescence. This article reviews such changes during the normal aging process, including a decrease in the quantity of cells, atrophy in the soma, retraction in the dendritic arborizations, degeneration in the subcellular organelles, a decline in synapse density, disorder in the neurotransmitter system, and alterations in electrophysiological properties. Although these deteriorative changes occur during aging, compensatory mechanisms exist to counteract the impairments in the aging PCs. The possible neural mechanisms underlying these changes and potential preventive treatments are discussed.

Age-related changes in the volume of somata and organelles of cerebellar granule cells

Because cerebellar granule cells are fixed post-mitotic cells, it is expected that they undergo age-related changes like other neurons. To examine this possibility, a stereological study on granule cells of rat neocerebellar cortex was performed for an age spectrum of 2 to 24 months using eight different age groups. The nucleator method, together with point and intersection counting, was used to obtain primary data; arithmetical calculations determined the secondary data. In the soma, the absolute surface area did not change significantly; the volume did, however, exhibit a significant negative linear trend with age. Excluding dense bodies, the absolute volumes of the cytoplasmic components did not vary significantly. The absolute volume of dense bodies displayed a significant positive linear trend with age. Significant positive correlations were detected between the somatic volume and the absolute volume of either mitochondria or ground substance. It was concluded that granule cells showed a fair degree of morphological stability through 18 months. However, the observed changes warn that accompanying physiological alterations may occur, with putative effects on motor coordination.

Caloric restriction enhances evoked DA overflow in striatum and nucleus accumbens of aged Fischer 344 rats

Previous studies have shown deficits in DA neuronal systems in senescence. Other studies indicate that prolonged dietary restriction can attenuate many of the detrimental effects of age. We have shown previously using in vivo electrochemistry that K+-evoked striatal DA overflow decreases as a function of age. This was a regional effect that appeared to be due to functional changes in DA neurons, rather than a decrease in the storage and synthesis of DA. In the present studies, we used in vivo electrochemistry to investigate the effects of caloric restriction on age related decreases in K+-evoked DA overflow along a dorsal to ventral axis in the striatum of aged female Fischer 344 rats. Aged (26-28-month-old) diet restricted animals (DRF) showed evoked DA overflow that was significantly greater in amplitude and duration compared to aged (26-28-month-old) ad lib fed animals (ALF). These results provide additional evidence that decreased DA neuronal function resulting from age is improved by caloric restriction.

Age-related subsensitivity of cerebellar Purkinje neurons to locally applied beta 1-selective adrenergic agonist

Previous electrophysiological studies in aged rats have revealed a number of deficits in noradrenergic neurotransmission in the central nervous system. Such deficits include subsensitivity to the depressant effects of norepinephrine on cerebellar Purkinje neurons, which has been attributed specifically to altered beta adrenergic receptor-mediated processes. The objective of this study was to determine which beta adrenergic receptor subtype, beta 1 or beta 2, is responsible for this age-related subsensitivity. The effects of beta 1 and beta 2 agonists on spontaneous activity of Purkinje neurons was first examined in young rats and the selectivity of these agents was validated using selective beta 1 and beta 2 antagonists. The effects of the selective beta 1 and beta 2 agonists were then compared in young (3-month-old) and aged (18- and 26-month-old) Fischer 344 rats. These agents were applied to Purkinje neurons by pressure microejection from multibarreled micropipettes and the change in neuronal action potential discharge rate was recorded. Both dobutamine, a beta 1-selective agonist, and zinterol, a beta 2-selective agonist, induced dose-dependent inhibitions of Purkinje cell firing rate. Dobutamine-induced inhibitions were blocked by the selective beta 1 antagonist, ICI 89406 and not by the beta 2-selective antagonist, ICI 118551; conversely, zinterol-induced inhibitions were not blocked by ICI 89406 but were blocked by the presence of ICI 118551. Purkinje neurons of both groups of aged rats were significantly less sensitive to locally applied dobutamine than Purkinje cells of young rats.(ABSTRACT TRUNCATED AT 250 WORDS)

The vestibular system in the elderly: clinical implications

Cell loss, changes in synapse morphology, electrophysiologic alterations, and changes in the supporting microenvironment have all been noted in portions of the vestibular systems of aged animals and humans. Increased variability with age is also a prominent finding in many studies. Quantitative vestibular testing in humans has shown alterations with age as well, but decline with aging is not a prominent feature of all measures, and many reported studies are methodogically flawed. The connection between these observed aging changes and the increased incidence of dizziness and falls in the elderly is unproven, however, and clinicians should search carefully for specific disease processes in their elderly patients who present with vestibular symptoms.

Multiple changes in noradrenergic mechanisms in the coeruleo-hippocampal pathway during aging. Structural and functional correlates in intraocular double grafts

Age-related changes of the coeruleo-hippocampal noradrenergic system were investigated using intraocular double transplants. Pieces of fetal hippocampus were grafted into the anterior chamber of the eye and placed into contact with previously inserted locus coeruleus grafts. Ages of both transplants and hosts were varied to enable studies of intrinsic versus extrinsic determinants of aging in an isolated neuronal circuit. Four different experimental groups, with the approximate age in months of grafts/hosts at the time of recording given in parentheses, were studied; young grafts in the eyes of young hosts (3/7), young grafts in the eyes of old hosts (3/23), mature transplants in adult host rats (8/12) and aged transplants in the eyes of aged rats (21/25). Extracellular recordings from the hippocampal part of the double grafts were performed. Superfusion with alpha-adrenergic antagonists and the alpha 2-agonist clonidine elicited significant increases in the discharge rate of the grafted hippocampal neurons in all groups except the aged transplants in the aged hosts (21/25), where a small excitation was elicited with clonidine and no effect at all was seen with alpha-adrenergic antagonists. The host age did not seem to be important since young transplants in the old hosts (3/23) showed a similar increase in discharge rate as transplants in the young and adult hosts. Tyrosine hydroxylase immunohistochemistry and high-performance liquid chromatography revealed that hippocampal transplants remaining in oculo for a minimum of 6-10 months became permanently hyperinnervated by noradrenergic fibers from the locus coeruleus grafts. The density of noradrenergic fibers was significantly lower in young transplants.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related alterations in noradrenergic input to the hippocampal formation: structural and functional studies in intraocular transplants

Intrinsic versus extrinsic determinants of age-related alterations in hippocampal noradrenergic transmission were investigated using intraocular allografts in rats. Three groups of animals were examined: young hippocampal transplants in young hosts, old transplants in old hosts and young transplants in old hosts. Postsynaptic sensitivity to noradrenaline (NA) was measured by extracellular recordings of spontaneous activity and superfusion with known concentrations of catecholamines in the anterior chamber of the eye. Hill plots demonstrated that the dose-response relationships of NA-induced depressions were linear and parallel in the 3 groups. Aged hippocampal grafts displayed a highly significant subsensitivity to NA of one order of magnitude. The EC50 for this group was 203.1 microM as compared to 29.2 in young grafts. Young intraocular grafts in old hosts responded similarly to transplants in young hosts, with an EC50 of 32.4 microM for the depressant actions of NA. Collaterals of the host iris sympathetic ground plexus invaded the hippocampal grafts. The density of this noradrenergic innervation was estimated by immunohistochemistry for tyrosine hydroxylase. A slightly increased density and fluorescence intensity of the noradrenergic fibers were observed in the old transplants as compared to the young transplants in young and old hosts. This was correlated with a significantly (P less than 0.01) increased content of NA in old transplants, as measured with high performance liquid chromatography. The old transplants also contained a large number of autofluorescent lipofuchsin granules, which were absent in the young transplants, regardless of the recipient age. Taken together, these results suggest the existence of alterations in pre- as well as postsynaptic noradrenergic mechanisms in the aging hippocampus. These changes were dependent on transplant age rather than host age, thus suggesting an involvement of intrinsic rather than extrinsic determinants in this model system.

Age-related changes of catecholamines and their metabolites content in the visual system of the rat

The changes in the content of the catecholamines in each structure of the geniculate and extrageniculate visual system of the rat during the aging period (6-30 months) have been studied. Dopamine was found at lower levels than noradrenaline in all the structures. The dopamine and noradrenaline showed different developmental profiles. Dopamine and its metabolite levels decreased in the lateral geniculate and visual cortex and increased in superior colliculus and posterior thalamus. Noradrenaline and its metabolites increased in all structures during the aging period. However, 3-methoxy-4-hydroxyphenylglycol/noradrenaline and normetanephrine/noradrenaline ratios decreased in all structures except in superior colliculus. These results suggest age-related changes in the catecholamines in the visual system of the rat.

Age-related changes of the metabolic profile of rat cerebellar cortex: enzyme histochemical study

The influence of aging on the metabolic profile of cerebellar cortex was studied in young (3-month-old), adult (12-month-old) and aged (26-month-old) male Sprague-Dawley rats using enzyme histochemical techniques. The following enzymatic activities related to energy transduction were examined: lactate-(LDH) and succinate-(SDH) dehydrogenases; NADH2-tetrazolium reductase (NADHD) and alpha-glycerophosphate-dehydrogenase (GPDH). The intensity of enzymatic staining within the neuropil of molecular and granular layers as well as within the cytoplasm of Purkinje neurons of young, adult and aged animals was assessed microphotometrically. In the molecular layer LSH, SDH and NADHD levels were reduced in old rats; GPDH was decreased both in adult and old animals. In Purkinje neurons no age-related changes of the enzymatic activities under study were observed. In the granular layer LDH and GPDH showed an age-dependent loss; SDH and NADHD were unchanged. The possibility that age-related changes of the enzymatic activities under study may be due to impaired energy production mechanisms and/or represent the consequence of reduced energetic needs resulting from the documented age-dependent loss of synapses in the molecular or in the granular layers of cerebellar cortex is discussed.

Persistent reduction of Purkinje cell inhibition on neurones of the cerebellar nuclei after climbing fibre deafferentation

The long lasting effects of inferior olive (IO) destruction were studied in rats treated with 3-acetylpyridine two years before. The activities of the Purkinje cells (PCs) and of their target neurones in the cerebellar nuclei (ECNs), were investigated and compared to those of a group of non-treated rats. Our results show that long-term deafferented PCs recovered a mean firing frequency similar to that of the controls while the ECNs discharge is enhanced. Furthermore, the cryodestruction of the cerebellar cortex produces a significant release of the ECNs firing in the control rats but not in the poisoned animals. Thus, it appears that IO destruction induces a permanent impairment of the inhibitory control exerted by the PCs on the activity of their target neurones.

Age-related reduction in responses of rat hippocampal neurons to locally applied monoamines

Age-related changes in the responsiveness of hippocampal pyramidal neurons to norepinephrine (NE) and serotonin (5HT) were investigated using electrophysiological techniques. Local application of each monamine via pressure micro-ejection was employed to establish the dose which elicited a 50% change in spontaneous discharge rate of single pyramidal neurons; these data were used to construct dose response curves for the population of neurons tested in 3-6, 11-13, 18-20, and 27-30 month old rats. The percentage of cells responding in rats 18-20 and 27-30 months old decreased for both NE and 5HT. There was also a progressive increase with age in the population ED50 for 5HT starting at 18-20 months. For neurons which demonstrated a response to NE, no decrease in the population ED50 was observed. Taken together these data indicate that there is a progressive age-related decline in the postsynaptic response to NE and 5HT in the rodent hippocampus.

Desipramine and noradrenergic neurotransmission in aging: failure to respond in aged laboratory animals

Deficiencies in noradrenergic neurotransmission have been found in the central nervous system of aged laboratory animals. The purpose of the present study was to determine if tricyclic antidepressants, such as desipramine, can overcome the diminished noradrenergic neurotransmission found in these animals. Using electrophysiological techniques, noradrenergic neurotransmission was examined in the cerebellar cortex of rats, a model system which has been used extensively to characterize the effects of norepinephrine in the central nervous system. The discharge rate of cerebellar Purkinje neurons is very sensitive to changes in the noradrenergic input from the nucleus locus coeruleus. In this model system in young rats, treatment with desipramine slowly augments noradrenergic neurotransmission over several weeks. Similar treatment in aged animals caused no increase in the age-related deficient noradrenergic neurotransmission. The decline in efficacy of desipramine with age could not be accounted for by differences between young and old rats in the distribution of the drug. Failure of desipramine to be effective in older rats may reflect the insensitivity of aged neurons to norepinephrine itself, so that treatment strategies which increase the amount of nerepinephrine released onto these neurons may be ineffective. The findings may have implications for the use of tricyclic antidepressants in aged depressed patients.

Morphologic and functional abnormalities that develop in kitten Purkinje neurons during maintenance for months after maturation in organotypic cultures

The morphologic and functional properties of the Purkinje cells (P-cells) grown for 10-11 weeks in organotypic cultures from newborn kitten cerebella were studied and compared to cultures which had been grown for 4-5 weeks under the same standard conditions. Electrophysiological and morphological data were obtained from HRP iontophoretically labeled neurons and were quantified by means of computerized techniques. Extracellular recordings of spontaneous activity showed that the 10-11-week-old P-cells had a pacemaker-like firing rate whereas the P-cells aged 4-5 weeks in vitro displayed a bursting activity. The qualitative morphological data evidenced abnormal swellings both on dendritic and axonal processes of the 10-11-week-old P-cells which were not present on the 4-5-week-old P-cells. The quantitative data revealed a significant decrease in the overall size of the dendritic network of the 10-11-week-old P-cells mainly due to a reduction in the total dendritic length and in the total number of dendritic segments, whereas the individual segment lengths remained almost unchanged. Dendritic spine counts showed no decrease in the dendritic density of these older P-cells. Such data suggest that the changes observed in 10-11-week-old cultured P-cells may be compared to the age-related changes occurring in vivo and that such in vitro models could be useful tools in the study of the pathology of aging. However, alternative factors other than senescence are discussed since they may account for some degenerative changes observed in the older cultured P-cells.

Electrophysiological assay and characterization of central adrenoceptors: techniques and neuropharmacology

Several single unit electrophysiological studies that have investigated central adrenoceptors are reviewed. The techniques and paradigms employed to electrophysiologically assay such adrenoceptors are discussed. Several regions of the brain, e.g., the nucleus locus coeruleus, the dorsal raphe nucleus, the lateral geniculate nucleus, and the cerebellar Purkinje neurons, are examined in detail, with reference to the nature of the adrenoceptor(s) located on these neurons. From the studies reviewed, it can be concluded that single unit electrophysiological recordings provide a valuable and powerful assay for adrenoceptors. Modification of this technique to study adrenoceptors from awake (behaving) or chronically treated animals is likely to result in significant advances in our understanding of mechanisms contributing to neuroreceptor plasticity.

Prolonged Ca2+-dependent afterhyperpolarizations in hippocampal neurons of aged rats

The possibility that calcium is elevated in brain neurons during aging was examined by quantifying afterhyperpolarizations induced by spike bursts in CAl neurons of hippocampal slices from young and aged rats. The afterhyperpolarizations result from Ca2+-dependent K+ conductance increases and are blocked in medium low in Ca2+ and prolonged in medium high in Ca2+. The afterhyperpolarization and associated conductance increases were considerably prolonged in cells from aged rats, although inhibitory postsynaptic potentials did not differ with age. Since elevated intracellular Ca2+ can exert deleterious effects on neurons, the data suggest that altered Ca2+ homeostasis may play a significant role in normal brain aging.

Senescent microstructural changes in rat cerebellum

Senescent changes in Sprague-Dawley rat cerebellar microstructure have been quantified, focusing on the dominant element of cerebellar information processing, the Purkinje cell. In Golgi-Kopsch sections, many 26-month-old Purkinje cells appear defoliated, with small distal dendrites and spiny branchlets being most affected. The mean Purkinje cell area (soma plus dendrites) in computer-oriented sagittal sections is significantly decreased from 20,675 +/- 1,355 micron2/cell in 6-month-old rats to 17,088 +/- 1,107 micron2/cell in 26-month rats. These morphologic changes may be the hallmark of dying cells: in hematoxylin and eosin (H + E)-stained sections from the same rats we also observe a significant senescent decrease in Purkinje neuron density in every vermis lobule examined (lobules II-VII). Overall, the mean number of Purkinje cells/mm of Purkinje cell layer (measured in 10 micron thick sagittal sections) declines from 16.6 +/- 0.8 cells/mm in young rats to 12.5 +/- 0.2 cells/mm in old rats. As Purkinje cells are lost, so too are ethanolic phosphotungstic acid-stained (EPTA) synapses in the upper molecular layer of the cerebellar vermis (lobules VIII-X), and there is a highly significant within-subjects correlation between Purkinje cell density and synaptic density. Overall, synaptic density (in sagittal, 842 micron2 thin sections) decreases significantly from an average of 150,485 +/- 3,641 synapses/mm2 in 6-month rats to an average of 125,000 +/- 4,849 synapses/mm2 in 26-month rats. These changes are consistent with previous electrophysiologic and biochemical data showing age pathology of the cerebellum.

Psychomotor performance in the senescent rodent: reduction of deficits via striatal dopamine receptor up-regulation

In order to determine the relationship between striatal dopamine (DA) receptor density and psychomotor performance in senescent animals, two experiments were carried out. In the first, the age-related motor deficits were characterized using a battery of four psychomotor tests (rod walking, wire hanging, inclined screen, plank walking). These tests were administered to three groups of male Fischer rats (mature, 6-8 months; middle aged, 12-18 months; and senescent, 25 months) and performance measured. Age-related differences were observed on all the tasks, with the oldest animals showing the poorest performance. These animals were then used in a second experiment in which one-half of the group of animals from each age was administered 1.86 mg/kg/day of haloperidol for 14 days (via surgically implanted Alza Minipumps. Control groups of animals from each age were given pumps which contained only the vehicle (HCl diluted with distilled water, pH = 2.9). Following the 14 day drug administration, the pumps were surgically removed and 3 days later all the groups were retested on the psychomotor tests. Stereotypy (to 0.5 mg/kg of apomorphine, sniffing, licking, grooming and cage crossings) was also re-examined. Results show that haloperidol-treated animals from all three age groups display greater response times (i.e., better performance) than vehicle-treated animals on the battery of four motor tests and, the haloperidol-treated old animals exhibit more sniffing and grooming than the vehicle-treated old animals. Parallel increases in [3H]spiperone binding seen in all haloperidol-treated groups suggest a relationship between increases in the density of striatal DA receptors and improvement in motor performance.

Altered sensitivity of forebrain neurones to iontophoretically applied noradrenaline in aging rats

Responses of unidentified neurones in the rostral cerebral cortex to iontophoretically applied noradrenaline and GABA were compared in young (3-4 months old) and old (22-24 months old) albino rats. Depressant responses to noradrenaline were smaller in the older animals and this was manifested particularly as a decrease in response duration. Depressant responses to GABA did not differ significantly in the two groups. The results indicate that neurones in the cerebral cortex, as in the cerebellum, exhibit a decrease in postsynaptic responsiveness to noradrenaline with advancing age.

Cerebellar pathology in schizophrenia--cause or consequence?

Atrophy of the cerebellar vermal cortex has been reported to occur in 10% or more of patients with schizophrenia. Data from studies on experimental animals indicate that a functional relationship between the cerebellum and parts of the forebrain involved in emotion exists, and that the cerebellum may influence some types of behavior. Cerebellar abnormality in schizophrenic patients, although of uncertain cause, could contribute to the symptomatology of the disease.

Changes in noradrenergic neurotransmission in rat cerebellum during aging

This study compared the electrophysiological effects of locally applied cyclic adenosine-mono-phosphate (N6cAMP) between Purkinje neurons from young (4-month) and old rats (15-months and older). Purkinje neurons from young rats were significantly more sensitive to locally applied norepinephrine and N6cAMP than neurons from old rats. GABA sensitivity between the two groups was unaltered. Our results suggest that the locus of the adrenergic subsensitivity observed in older animals may reside to a large extent at or beyond the level of cAMP generation.

Senescent pathology of cerebellum: Purkinje neurons and their parallel fiber afferents

Two groups of naive male Sprague-Dawley rats, 5-7 and 24-26 months of age, were anesthetized with continuous intraperitoneal infusion of 4% chloral hydrate. Stimulation of the cerebellar vermis molecular layer permitted measurements of 12 different electrophysiological properties of parallel fiber Purkinje cell circuitry: parallel fiber conduction velocity, refractory period, threshold, and current dependent volley amplitude; slow negative wave threshold and current dependent amplitude; Purkinje cell activation threshold, latency, and current dependent spike driving; and Purkinje cell inhibitory threshold, latency, and current dependent duration of inhibition. Old subjects demonstrated deficits on all parameters except Purkinje cell inhibitory threshold. The relevance of these findings to our previous research on senescent changes in cell number, lipofuscin deposition, and spontaneous firing of Purkinje cells is discussed.