In vivo acetylcholine release as measured by microdialysis is unaltered in the hippocampus of cognitively impaired aged rats with degenerative changes in the basal forebrain

Functional activity of intrahippocampal septal grafts is regulated by catecholaminergic host afferents as studied by microdialysis of acetylcholine

Previous microdialysis experiments have shown that acetylcholine (ACh) release from septal grafts in the hippocampus of awake rats is influenced by the behaviour of the animals, which strongly suggests that the host brain can exert a regulatory control over the activity of the grafted neurons. Since the activity of the normal septo-hippocampal cholinergic system is likely to be regulated, in part, by brainstem catecholaminergic afferents, we wished to study the effect of catecholaminergic drugs on ACh release in the hippocampus reinnervated by septal grafts. Rats were subjected to a unilateral aspirative fimbria-fornix (FF) transection and grafted with tissue from the fetal septal-diagonal band area, either as a cell suspension injection into the depth of the hippocampus or as a solid implant in the FF lesion cavity. Microdialysis of ACh release was carried out 17-20 months after transplantation in awake, freely-moving animals. The reduction in steady-state ACh overflow induced by the FF lesion (-81%) was restored to normal or above normal levels in rats with either solid or suspension grafts. In normal rats, systemic administration of apomorphine (2.0 mg/kg, s.c.) or amphetamine (2.5 mg/kg, i.p.) caused a 3.7 (+189%) or 7.8 (+301%) pmol/15 min increase in ACh overflow compared to the previous baseline level, respectively. The drug-induced increases in ACh levels in the FF-lesioned controls was substantially lower than normal (86-89% reduction). Both apomorphine and amphetamine resulted in an approximately two-fold increase in hippocampal ACh release in rats with suspension grafts. These responses were significantly increased over those seen in rats with FF lesions only, but they tended to be lower and more variable than normal. Rats with solid septal grafts responded significantly stronger than FF lesion controls to amphetamine with two-fold increased ACh overflow, whereas the response to apomorphine was less clear-cut. Pretreatment with the catecholamine synthesis blocker alpha-methyl-p-tyrosine (AMPT; 200 mg/kg x 3) did not affect steady-state or apomorphine-stimulated release of ACh in any of the groups, whereas the effect of amphetamine was abolished in both normal and grafted rats. The results suggest that ACh release derived from septal grafts in the hippocampus, similar to the normal septo-hippocampal system, can be affected by manipulations of the host catecholaminergic systems. This mechanism may, at least in part, underlie the ability of the host brain to influence and control the activity of grafted cholinergic neurons.

Effects of aging on spatial learning and hippocampal protein kinase C in mice

C57BL/6Nia and F1(B6xD2)Nia mice were tested on the Morris water maze task for 5 days followed by 12 days of testing on the place learning-set task (8 trials/day with each task). Mice were tested at 3, 14, and 25 months of age. C57 mice, 25 months of age, were significantly impaired in both the Morris and place learning-set task probe trial performance compared to mice 3 months of age (p < 0.05). These aged C57 mice also demonstrated a significant reduction in membrane-bound hippocampal protein kinase C (PKC) activity (p < 0.05) with no significant change in cytosolic PKC activity. F1 mice, however, showed no effect of age on probe trial performance on the spatial learning tasks. In addition, in a comparison of C57 and F1 mice within each age group, F1 mice demonstrated superior learning performance which was accompanied by a significant elevation in PKC activity (p < 0.05). Spatial learning performance of both strains significantly correlated with membrane-bound PKC activity (p < 0.01). These data provide additional support for our previous hypothesis of an involvement of hippocampal PKC activity in spatial learning and suggest that the amount of membrane-bound PKC activity may be a determinant of age-related decline in spatial learning.

BDNF and trkB mRNA expression in the hippocampal formation of aging rats

Quantitative in situ hybridization and northern blot analysis techniques were used to determine the topographical distribution and levels of mRNA coding for brain-derived neurotrophic factor (BDNF) and the tyrosine receptor kinase (trkB) mRNA in the hippocampal formation of two strains of male rat during aging. Age did not change the prevalence or regional distribution patterns of BDNF or trkB mRNA in the hippocampal formation throughout the lifespan of male Sprague-Dawley rats. There also were no significant differences in the prevalence or topographical distribution patterns of trkB mRNA transcripts during aging. Northern blot analysis of hippocampal RNA from male Fischer 344 confirmed that neither BDNF mRNA nor trkB mRNA levels changed with age. These findings suggest that age-related neurodegenerative changes, including the atrophy of the cholinergic septo-hippocampal pathway, are not the result of changes in hippocampal BDNF or trkB mRNA expression. Moreover, the prevalence and distribution of synaptosomal-associated protein 25 (SNAP-25), a neuron-specific protein located in synaptic terminals and a putative marker of synaptic integrity, did not change with age. These findings indicate that altered hippocampal synaptic plasticity which occurs in the aged rat brain is not a reflection of changes in the expression of BDNF or trkB receptor mRNA.

The neuropharmacological and neurochemical basis of place learning in the Morris water maze

The Morris water maze (MWM) offers several advantages over other methods of studying the neurochemical basis of learning and memory, particularly with respect to its ability to dissociate deficits in memory formation from deficits in sensory, motor, motivational and retrieval processes. The contributions of nearly all of the major neurotransmitter systems have been investigated and consistent patterns have emerged. Normal function in glutamatergic and cholinergic systems is necessary for spatial learning, as blockade of NMDA receptors and cholinergic hypofunction prevents spatial learning but does not impair recall. Peptides such as adrenal and sex hormones and somatostatin may also be necessary for spatial learning. In contrast, activity in either GABAergic or opioidergic systems impairs spatial learning, though by quite different means. GABAergic activity prevents memory function, whereas opioidergic activity reduces motivation. Normal monoaminergic activity is necessary for normal performance in the MWM, but not for spatial learning per se. However, noradrenergic and serotonergic systems may enhance cholinergic-mediated mnemonic processes. Further research into the relative contributions of different receptor subtypes as well as interactions between neurochemical systems should provide significant advances in our understanding of the neural basis of learning and memory in mammals.

Behavior-related cortical activity and swim-to-platform performance in the aged rat

Aged rats (26 months) usually retained normal acetylcholine-dependent and serotonin-dependent forms of neocortical low-voltage fast activity and serotonin-dependent hippocampal rhythmical slow-wave activity. In a simple swim-to-platform test, aged rats (23 and 26 months) performed normally in acquisition and in retention over a 7-day period. The results are discussed in relation to the common assumption that aged rats provide a valid model of human senile dementia.

Age-dependent modulation of in vivo cortical acetylcholine release by benzodiazepine receptor ligands

In vivo microdialysis was utilized to determine the effects of benzodiazepine receptor (BZR) ligands on cortical acetylcholine (ACh) release in awake young and aged rats. There were no significant differences in baseline cortical ACh release as a function of age. While administration of the BZR selective inverse agonist ZK 93 426 increased ACh release in both groups of animals, the aged rats exhibited a greater stimulation. Unexpectedly, under the present testing conditions, the BZR agonist chlordiazepoxide (CDP) had no systematic effect on ACh release in either group. The presence or absence of these drug effects or drug-age interactions was not secondary to the impact of these compounds on behavioral activity. Cortical ACh release could also be stimulated by turning off the lights in the observation room or by the systemic administration of scopolamine. Aged rats were at least as able as their younger counterparts to respond to these manipulations with increased release. These results suggest that basal and stimulated release of cortical ACh is not impaired at the ages studied. Moreover, selective inverse BZR agonists may be a potent way of trans-synaptically stimulating cortical cholinergic transmission.

Progressive decline in spatial learning and integrity of forebrain cholinergic neurons in rats during aging

Rats distributed over five different age groups, 3, 12, 18, 24 and 30 months of age, were screened for their spatial learning and memory ability in the Morris water maze, and the degree of place navigational impairments was correlated with morphological changes in the four major forebrain cholinergic cell groups (medial septum, MS; vertical limb of the diagonal band of Broca, VDB; nucleus basalis magnocellularis, NBM; and striatum) using choline acetyltransferase (ChAT) and nerve growth factor receptor (NGFr) histochemistry. Impaired place navigation developed progressively with age, such that 8% of the 12-month-old rats, 45% of the 18-month-old, 53% of the 24-month-old, and over 90% of the 30-month-old rats were behaviorally impaired. Significant reductions in the number of ChAT/NGFr-positive cell bodies, amounting to between 19 and 45%, were observed in all four cell groups, and the remaining cells were reduced in size (6-24% reduction in cross-sectional area in the oldest age groups). Although the morphological changes were less severe and tended to develop later than the behavioral impairments, there was overall a significant correlation between water maze performance and ChAT/NGFr-positive cell counts, and to a lesser degree also cell size in all four cell groups. These changes were also highly correlated with age. The highest correlations were seen in MS, VDB and NBM, which are known to play a role in spatial memory performance in young rats. The results indicate that degenerative and/or atrophic changes in the forebrain cholinergic system and decline in spatial learning ability are parallel processes during aging. Although the magnitude of the morphological changes does not appear to be substantial enough, by itself, to explain the severe spatial learning impairments that develop in the oldest animals, the present data are consistent with the view that impaired function in the forebrain cholinergic system can contribute to age-dependent cognitive decline in rodents.