Endogenous GABA concentration in cortical synaptosomes from young and aged rats

Insights from auditory cortex for GABA+ magnetic resonance spectroscopy studies of aging

Changes in levels of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) may underlie aging-related changes in brain function. GABA and co-edited macromolecules (GABA+) can be measured with MEGA-PRESS magnetic resonance spectroscopy (MRS). The current study investigated how changes in the aging brain impact the interpretation of GABA+ measures in bilateral auditory cortices of healthy young and older adults. Structural changes during aging appeared as decreasing proportion of grey matter in the MRS volume of interest and corresponding increase in cerebrospinal fluid. GABA+ referenced to H₂ O without tissue correction declined in aging. This decline persisted after correcting for tissue differences in MR-visible H₂ O and relaxation times but vanished after considering the different abundance of GABA+ in grey and white matter. However, GABA+ referenced to creatine and N-acetyl aspartate (NAA), which showed no dependence on tissue composition, decreased in aging. All GABA+ measures showed hemispheric asymmetry in young but not older adults. The study also considered aging-related effects on tissue segmentation and the impact of co-edited macromolecules. Tissue segmentation differed significantly between commonly used algorithms, but aging-related effects on tissue-corrected GABA+ were consistent across methods. Auditory cortex macromolecule concentration did not change with age, indicating that a decline in GABA caused the decrease in the compound GABA+ measure. Most likely, the macromolecule contribution to GABA+ leads to underestimating an aging-related decrease in GABA. Overall, considering multiple GABA+ measures using different reference signals strengthened the support for an aging-related decline in auditory cortex GABA levels.

Neuroanatomical changes associated with cognitive aging

The literature on the neuroanatomical changes that occur during normal, non-demented aging is reviewed here with an emphasis on the improved accuracy of studies that use stereological techniques. Loss of neural tissue involved in cognition occurs during aging of humans as well as the other mammals that have been examined. There is considerable regional specificity within the cerebral cortex and the hippocampus in both the degree and cellular basis for loss. The anatomy of the prefrontal cortex is especially vulnerable to the effects of aging while the major subfields of the hippocampus are not. A loss of neurons, dendrites and synapses has been documented, as well as changes in neurotransmitter systems, in some regions of the cortex and hippocampus but not others. Species differences are also apparent in the cortical white matter and the corpus callosum where there are indications of loss of myelin in humans, but most evidence favors preservation in rats. The examination of whether the course of neuroanatomical aging is altered by hormone replacement in females is just beginning. When hormone replacement is started close to the time of cycle cessation, there are indications in humans and rats that replacement can preserve neural tissue but there is some variability due to the type of hormones and regimen of administration.

Selective age-related changes in neuronal markers and smooth muscle reactivity in cerebrovascular beds of Fischer 344 rats

Choline accumulation, choline acetyltransferase (ChAT) and glutamic acid decarboxylase (GAD) activities were measured simultaneously in various cerebrovascular beds and brain areas from Fischer 344 rats aged 4.5 and 22 months. A slight (25%) but not significant decrease in choline accumulation was observed concomitantly with a significant increase (187%, p less than 0.05) in ChAT activity in the major cerebral arteries of the 22-month-old rats. In small cortical pial vessels and selected brain regions, cholinergic and GABAergic biochemical markers remain unaltered in aged rats. The vasomotor reactivity of the basilar artery was investigated in rats of 4.5, 12, 22 and 30 months of age. In the 22-month-old rats, maximal responses to 5-hydroxytryptamine (-25%, no significant) and prostaglandin F2 alpha (-30%, p less than 0.05 by ANOVA) were less intense as compared to other age-groups despite preserved contractile responses to dopamine, uridine triphosphate or a depolarizing concentration of K+. Relaxations induced by histamine, acetylcholine, noradrenaline, adenosine and somatostatin were strictly comparable among the different age-groups. The sensitivity of the basilar artery to all vasoactive agents failed to demonstrate any correlation with age. Our study suggests that cerebrovascular cholinergic and GABAergic markers undergo minor and selective changes with increasing age. Further, basilar artery vasomotor functions appeared relatively spared by the aging process despite age-related selective decreases in contractile responses to 5-hydroxytryptamine and prostaglandin F2 alpha.

Chronic methionine sulfoximine administration reduces synaptosomal aspartate and glutamate in rat striatum

Methionine sulfoxime (MS) is an inhibitor of glutamine synthetase, an astroglial enzyme believed to be involved in the maintenance of glutamine, a major precursor for neurotransmitter pools of the excitatory amino acids aspartate and glutamate in striatal afferent axon terminals. MS was infused for 7 days (24 micrograms/day) into the lateral cerebral ventricle of rats. On the side of MS infusion, there was a decrease of striatal synaptosomal aspartate (61%), glutamine (63%), and glutamate (48%), while taurine and gamma-aminobutyrate were unaltered when compared to vehicle-treated rats. The results indicate that chronic MS infusion is an effective means by which neurotransmitter aspartate and glutamate levels can be selectively reduced in the striatum.

Striatal preprotachykinin and preproenkephalin mRNA levels and the levels of nigral substance P and pallidal Met5-enkephalin depend on corticostriatal axons that use the excitatory amino acid neurotransmitters aspartate and glutamate: quantitative radioimmunocytochemical and in situ hybridization evidence

Because excitatory amino acid (EAA) neurotransmission has been implicated in long-term postsynaptic events, we conducted an initial study to determine whether or not the EAA-utilizing corticostriatal projection might influence peptide biosynthesis in neurons of the rat's basal ganglia. The content of EAAs in the caudatoputamen was reduced by frontal cortical ablation or by chronic intracerebroventricular infusion of methionine sulfoximine (MS). At 7 days following cortical ablation striatal Asp and Glu were reduced by 15% and 24%, respectively, while MS infusion (24 micrograms/day) for 7 days reduced synaptosomal levels of Asp by 61% and Glu by 48%. With either treatment, quantitative radioimmunocytochemistry revealed that substance P (SP) in the substantia nigra was increased by approximately 38%, while Met5-enkephalin (ME) in the globus pallidus was not changed. In situ hybridization with oligonucleotide probes revealed changes in the rostral striatum of preprotachykinin (PPT) and preproenkephalin (PPE) mRNA levels: cortical ablation reduced PPT mRNA by 17% and PPE mRNA by 20% dorsally, while it increased PPE mRNA (but not PPT mRNA) by 23% ventrally. Likewise, the infusion of MS decreased PPT (32%) and PPE mRNA (28%) dorsally, and increased PPE mRNA (50%) ventrally. In addition to the 7 day time point, the same measurements of EAAs, peptides and mRNAs were made at 14, 21 and 28 days after cortical excisions. At 14 days, the level of striatal Asp had returned to control value, but Glu remained depressed by 21%; nigral SP remained increased by 24%, and pallidal ME decreased by 15%. PPT and PPE mRNA remained depressed dorsally by 15% and 25%, respectively, while the increase in PPE mRNA noted ventrally at 7 days had returned to control values by 14 days. With the exception of Glu, which remained depressed by 18% at 21 and 28 days, all other values had returned to control levels by 21 days. The results indicate that a large reduction in EAA neurotransmission can influence differentially the steady-state levels of neuropeptides in striatal neurons and this change is brought about, at least in part, by an alteration in gene transcription.

GABA content and synthesis in the aging rat brain

The content and synthesis of GABA were measured in the cortex and striatum of young adult (4 months), mature (14 months), and old (24 months) male Wistar rats. GABA synthesis was determined from the GABA accumulation induced by aminooxyacetic acid (AOAA). Aging did not affect GABA content in the cortex (1.03 +/- 0.04 mumoles/g in young and 1.06 +/- 0.04 mumoles/g in old rats), or in the striatum (1.63 +/- 0.04 mumoles/g in young and 1.56 +/- 0.05 mumoles/g in old rats). Aging did not significantly change the AOAA-induced GABA accumulation in the striatum (+34% in young, +16% in mature, and +28% in old rats), but significantly reduced it in the cortex where this process takes place to a greater extent than in the striatum: +164% in young, +116% in mature, and +120% in old animals. It can be concluded that while in the striatum aging did not affect AOAA-induced GABA accumulation, in the cortex this was less in mature than young rats, with no further change in the old ones. GABA content was not affected by aging in either region.