Developmental changes of glutamate receptors in the rat cerebral cortex and hippocampus

 We studied the immunohistochemial localization of the glutamate receptors (GluR-1, -2, and -3,) in the developing rat cerebral cortex and hippocampus using antibodies to GluR1 and to an epitope common to GluR2 and GluR3 (GluR2/3) subunits. In the cerebral cortex, GluR1 immunoreactivity appeared in the neurons from postnatal day (PND) 0, increased with maturation, was highest at PND 10, decreased until PND 30, and thereafter remained at the same level as on PND 0. GluR2/3 immunoreactivity appeared earlier in scattered neurons on embryonal day (ED) 18, increased with maturation and reached a peak between PND 10 and PND 15, after which the immunoreactivity gradually decreased and reached a plateau at PND 30. For both GluR1 and GluR2/3, some of the pyramidal neurons showed intense staining. In the pyramidal layers of the hippocampus, GluR1 and GluR2/3 immunoreactivity was found in all the pyramidal neurons of the CA1–4 area from ED 20. In the dentate gyrus of the hippocampus, GluR1 and GluR2/3 immunoreactivity was found in the neurons of the granule cells after PND 0. Immunoreactivity in the neurons of the subiculum was found after PND 5 and that of the polymorphic cell layers was found after PND 15–20. Our results indicate that the development of glutamate receptor subunits in the rat cerebral cortex and hippocampus is expressed in different spatial patterns and distinct temporal patterns throughout development and is scheduled during the early postnatal period, when synaptic plasticity or synaptic connection occurs in these regions. Accepted: 13 June 1996     

Benzodiazepine receptors in rat cerebral cortex and hippocampus undergo rapid and reversible changes after acute stress

Rats were submitted to forced swimming and were killed 15 min after initiation of the stress and at 1 h, 1 day and 4 days thereafter. Immediately after the stress there was a decrease of 30% in the density of [³H]flunitrazepam binding sites in the cerebral cortex and of 27% in the hippocampal formation, with no changes in all the other brain areas studied. These changes in the number of benzodiazepine receptors were also corroborated by the binding of [³H]ethyl-β-carboline carboxylate. For both ligands there were no changes in affinity. These effects were selective for the benzodiazepine receptors and no changes in α₁, α₂ and β adrenoceptors and in dopaminergic receptors were observed. One hour after the stress, the number of benzodiazepine receptors had recovered in the cerebral cortex (8% above the control) and had increased greatly in the hippocampal formation ( + 53%). One day after the stress, the [³H]flunitrazepam binding in the cerebral cortex reached the normal level but it was still slightly elevated ( + 16%) in the hippocampus.These results are discussed in relation to some contradictory findings in the literature and to the fact that the hippocampal formation is related to neural mechanisms underlying behavior and neuroendocrine regulation.     

Upregulation of glutamate receptors in rat cerebral cortex with neuronal migration disorders

Neuronal migration disorders (NMDs) constitute the main pathologic substrate of medically intractable epilepsy in human. This study is designed to investigate the changes in expression of glutamate receptor subtypes on radiation-induced NMD in rats. The lesion was produced by intrauterine irradiation (240 cGy) on E17 rats, and then 10 weeks old rats were used for the study. The pathologic and immuno-histochemical findings for glutamate receptor subunit proteins on NMD cortex were correlated with development of behavioral seizures and EEG abnormality. Spontaneous seizures uncommonly occurred in NMD rats (5%); however, clinical stages of seizures were significantly increased in NMD rats by an administration of kainic acid. Brains taken from irradiated rats revealed gross and histopathologic features of NMD. Focal cortical dysplasia was identified by histopathology and immunohistochemistry with neurofilament protein (NF-M/H). Significantly strong NR1 and NR2A/B immunoreactivities were demonstrated in cytomegalic and heterotopic neurons of NMD rats. The results of the present study indicate that epileptogenesis of NMD might be caused by upregulation of glutamate receptor expression in dysplastic neurons of the rat cerebral cortex with NMDs.     

Developmental profile of neuregulin receptor ErbB4 in postnatal rat cerebral cortex and hippocampus

We investigated the cellular and subcellular distributions of neuregulin tyrosine kinase receptor ErbB4 in the postnatal rat frontal cortex and hippocampus by light-, confocal- and electron-microscopic immunocytochemistry. At birth, ErbB4-immunoreactivity (ErbB4-IR) was prominent in the apical cytoplasm and dendrites of cortical plate neurons and hippocampal pyramidal cells. Throughout postnatal development and in adulthood, ErbB4-IR in both regions remained confined to the somatodendritic compartment of neurons, which increased in number to reach the adult pattern by the end of the first postnatal month (P30). At all ages examined, double-labeling experiments revealed that ErbB4-IR always co-localized with the neuronal marker neuronal nuclei (NeuN) and never with glial markers Nestin or glial fibrillary acidic protein (GFAP). Immunoperoxidase labeling at the ultrastructural level confirmed the exclusive localization of ErbB4-IR in somatodendrites, and notably in dendritic spines. Immunogold labeling showed preponderant ErbB4-IR in the cytoplasm, where it was associated with microtubules. Furthermore, ErbB4-IR was abundant in the nucleus of adult cortical and hippocampal neurons, suggesting a role for ErbB4 nuclear signaling in the brain beyond embryonic development. Taken together, these results show that ErbB4 is expressed by neuronal somatodendrites in cerebral cortex and hippocampus from birth to adulthood, and support a role for neuregulins in dendritic growth and plasticity.     

Developmental changes in release properties of the CA3-CA1 glutamate synapse in rat hippocampus

Developmental changes in release probability (Pr) and paired-pulse plasticity at CA3-CA1 glutamate synapses in hippocampal slices of neonatal rats were examined using field excitatory postsynaptic potential (EPSP) recordings. Paired-pulse facilitation (PPF) at these synapses was, on average, absent in the first postnatal week but emerged and became successively larger during the second postnatal week. This developmental increase in PPF was associated with a reduction in Pr, as indicated by the slower progressive block of the N-methyl-D-aspartate (NMDA) EPSP by the noncompetitive NMDA receptor antagonist MK-801. This developmental reduction in Pr was not homogenous among the synapses. As shown by the MK-801 analysis, the Pr heterogeneity observed among adult CA3-CA1 synapses is present already during the first postnatal week, and the developmental Pr reduction was found to be largely selective for synapses with higher Pr values, leaving Pr of the vast majority of the synapses essentially unaffected. A reduction in Pves, the release probability of the individual vesicle, possibly caused by reduction in Ca2+ influx, seems to explain the reduction in Pr. In vivo injection of tetanus toxin at the end of the first postnatal week did not prevent the increase in PPF, indicating that this developmental change in release is not critically dependent on normal neural activity during the second postnatal week.     

Arginine vasopressin receptors in pig cerebral microvessels, cerebral cortex and hippocampus

We tested for the presence of arginine vasopressin (AVP) receptors in pig cerebral microvessels, cerebral cortex and hippocampus by specific binding methods with [3H]AVP as the ligand. The specific binding of [3H]AVP to all preparations was saturable and Scatchard analysis indicated a single class of high affinity binding sites (dissociation constant of 1-2 nM). Maximal binding capacity in cerebral microvessels was about 60% that of the cerebral cortex; and there were no apparent differences in the maximal binding capacity between cerebral cortex and hippocampus. These findings suggest the existence of AVP receptor sites in cerebral microvessels and support the hypothesis that AVP has a role in the control of the brain microcirculation.     

Complex structure of quisqualate-sensitive glutamate receptors in rat cortex

Multiplicity of excitatory amino acid receptors has previously been demonstrated. The functional size of the quisqualate-type receptors was investigated by high energy radiation inactivation analysis of the binding of [3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid ([3H]AMPA) to rat cortical membranes. The AMPA binding site is coupled to a high molecular weight modulatory unit, which down-regulates the affinity of the binding site for quisqualate, glutamate and AMPA. The estimated molecular target size of the AMPA binding site is different from the reported molecular weight of the glutamate binding site isolated from bovine brain. Furthermore, enhanced [3H]AMPA binding induced by chaotropic ions seems not to be a consequence of removal of the modulatory unit.     

Chronic fluoxetine treatment induces structural plasticity and selective changes in glutamate receptor subunits in the rat cerebral cortex

It has been postulated that chronic administration of antidepressant drugs induces delayed structural and molecular adaptations at glutamatergic forebrain synapses that might underlie mood improvement. To gain further insight into these changes in the cerebral cortex, rats were treated with fluoxetine (flx) for 4 weeks. These animals showed decreased anxiety and learned helplessness. N-methyl-d-aspartate (NMDA) and α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunit levels (NR1, NR2A, NR2B, GluR1 and GluR2) were analysed in the forebrain by both western blot of homogenates and immunohistochemistry. Both methods demonstrated an upregulation of NR2A, GluR1 and GluR2 that was especially significant in the retrosplenial granular b cortex (RSGb). However, when analysing subunit content in postsynaptic densities and synaptic membranes, we found increases of NR2A and GluR2 but not GluR1. Instead, GluR1 was augmented in a microsomal fraction containing intracellular membranes. NR1 and GluR2 were co-immunoprecipitated from postsynaptic densities and synaptic membranes. In the immunoprecipitates, NR2A was increased while GluR1 was decreased supporting a change in receptor stoichiometry. The changes of subunit levels were associated with an upregulation of dendritic spine density and of large, mushroom-type spines. These molecular and structural adaptations might be involved in neuronal network stabilization following long-term flx treatment.     

Time-dependent modulation of AMPA receptor phosphorylation and mRNA expression of NMDA receptors and glial glutamate transporters in the rat hippocampus and cerebral cortex in a pilocarpine model of epilepsy

The pilocarpine model in rodents reproduces the main features of mesial temporal lobe epilepsy related to hippocampus sclerosis (MTLE-HS) in humans. It has been demonstrated in this model that the phosphorylation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor GluR1 subunit is increased 1 h after pilocarpine treatment. Moreover, alterations in the levels of glutamate transporters have been associated with chronic epilepsy in humans. Despite these studies, the profile of these changes has not yet been addressed. We analyzed the protein content and phosphorylation profile of the AMPA receptor GluR1 subunit by western blotting. We also used quantitative real-time polymerase chain reaction to analyze the expression of glial glutamate transporters and the N-methyl-d-aspartate receptor NR1 subunit in the hippocampus (Hip) and cerebral cortex (Ctx) at different time points after pilocarpine-induced status epilepticus (Pilo-SE) in male adult Wistar rats. Biochemical analysis was performed in the Hip and Ctx at 1, 3, 12 h (acute period), 5 days (latent period), and 50 days (chronic period) after Pilo-SE. Key findings include an increase in the phosphorylation of GluR1-Ser⁸⁴⁵ in the Ctx and GluR1-Ser⁸³¹ in the Hip at different times during the acute period, and a decrease in the total content of the GluR1 subunit in the Ctx in the latent period. There was a down-regulation of the mRNA expression and protein levels of EAAT1 and EAAT2, and a decrease of the NR1 mRNA expression, in the Ctx during the latent period. Notably, during the chronic period, the EAAT2 mRNA expression and protein levels decreased while the NR1 mRNA levels increased in the Hip. Taken together, our findings suggest a time- and structure-dependent imbalance of glutamatergic transmission in response to Pilo-SE, which might be associated with either epileptogenesis or the seizure threshold in MTLE-HS.     

局灶性脑缺血大鼠脑组织皮质运动区和海马星形胶质细胞的动态变化

目的 观察大鼠局灶性脑缺血后皮质运动区和海马星形胶质细胞的变化。方法 雄性SD大鼠144只,按随机数字法分成正常组、假手术组和手术组,每组48只。手术组用大脑中动脉电凝术(MCAO)建立局灶性脑缺血模型;假手术组的操作步骤和手术组相同,只是不将暴露的大脑中动脉凝闭;正常组不做任何处理。应用免疫组化检测正常组、假手术组以及手术组术后1、2、3、5、7、14、28、56 d皮质运动区和海马星形胶质细胞神经胶质酸性蛋白(GFAP)表达的变化。结果与正常组和假手术组比较,手术组大鼠缺血侧和健侧MCAO术后皮质运动区GFAP阳性细胞计数术后第1天明显降低,第3天开始升高,缺血侧和健侧分别于术后第7天、第5天升至最高(85.6±3.3、75.3±2.9),随后逐渐下降并均在术后第14天出现第2个低谷,其后又开始逐渐上升接近正常。与正常组和假手术组比较,手术组海马CA1区GFAP阳性细胞计数缺血侧和健侧自术后第1天即开始升高,术后第7天升至最高( 106.5±3.6、108.4±3.0),其后进入平台期;CA2区、CA3区缺血侧和健侧术后第3天开始明显升高,CA2区缺血及健侧最高值则分别出现在术后第5天(106.9±4.4)、第7天(107.5±3.8);CA3区为第5天(130.9±3.7)、第5天(129.2±4.0),术后第14天开始下降,但仍然高于正常组和假手术组。结论 大鼠局灶性脑缺血后,不同脑区星形胶质细胞的表达随时间变化的模式不同,在皮质运动区存在两谷一峰的现象,在海马表达逐渐增强,并维持在一个较高的水平,这种差别可能与不同脑区神经的可塑性有关。     

Developmental changes in odor-evoked activity in rat piriform cortex

In adult rats, odor-evoked Fos protein expression is found in rostrocaudally-oriented bands of cells in anterior piriform cortex (APC), likely indicating functionally distinct subregions, while activated cells in posterior piriform cortex (PPC) lack apparent spatial organization. To determine whether these patterns are present during early postnatal life, and whether they change during development, Fos expression was assessed following acute exposure to single aliphatic acid odors in developing rats beginning at postnatal day 3 (P3). In the olfactory bulb, Fos-immunoreactive cells were present in the granule cell, mitral cell and glomerular layers at the earliest ages examined. Cells immunopositive for Fos were clustered in areas previously reported as active in response to these odors. In piriform cortex, activation in layers II/III shared some features with that seen in the adult; in APC, rostro-caudally oriented bands of Fos-positive cells alternated with bands relatively free of label, while labeled cells were found dispersed throughout PPC. However, in P3-P7 animals, Fos-positive cells in APC were found in a central rostro-caudally oriented band that was flanked by two bands relatively free of Fos-positive cells. This contrasted with the adult pattern, a central cell-poor band flanked by cell-rich bands, which was observed beginning at P10. These results suggest that subregions of APC visualized by odor-evoked Fos expression are active and functionally distinct shortly after birth. Changes in activity within these subregions during early postnatal development coincide with a shift toward adult-like olfactory learning behavior in the second postnatal week, and may play a role in this behavioral shift.     

急性脑缺血损伤大鼠海马神经元谷氨酸转运体的表达

目的：研究急性脑缺血损伤大鼠海马神经元谷氨酸转运体（EAAC1）的表达变化。 方法： 采用EAAC1反义寡核苷酸脑内注射，用插线法建立大鼠局灶性脑缺血模型(MCAO)。运用Western blot法和TTC染色观察缺血区EAAC1表达和梗塞体积；采用RT-PCR 和Western blot法，测定海马EAAC1 mRNA和蛋白在缺血1 h、6 h、24 h的变化。结果： 注射EAAC1反义寡核苷酸组大鼠梗塞体积［(105.67±8.70) mm3］显著小于正义组。缺血1 h大鼠海马EAAC1 mRNA表达（0.963±0.117）与假手术组（0.907±0.113）无明显差异，缺血6h、24h持续高于缺血1 h（分别为1.116±0.104和1.428±0.078）。而海马EAAC1蛋白表达（0.640±0.027）在缺血24 h高于假手术组，缺血1 h和6h EAAC1表达与假手术组比较无显著差异（分别为0.330±0.018、0.330±0.015）。结论： EAAC1可促进缺血脑损伤,在急性脑缺血病理过程中表达增加。     

Developmental changes in cerebral autoregulatory capacity in the fetal sheep parietal cortex

We validated laser Doppler flowmetry (LDF) for long-term monitoring and detection of acute changes of local cerebral blood flow (lCBF) in chronically instrumented fetal sheep. Using LDF, we estimated developmental changes of cerebral autoregulation. Single fibre laser probes (0.4 mm in diameter) were implanted in and surface probes were placed on the parietal cerebral cortex at 105 ± 2 (   n = 7  ) and 120 ± 2 days gestational age (dGA, n = 7). Basal lCBF was monitored over 5 days followed by a hypercapnic challenge (fetal arterial partial pressure of CO₂,   P _a,CO2  : 83 ± 3 mmHg) during which lCBF changes obtained by LDF were compared to those obtained with coloured microspheres (CMSs). Mean arterial blood pressure (MABP) was increased and decreased using phenylephrine and sodium nitroprusside at 110 ± 2 and 128 ± 2 dGA. Intracortical and cortical surface laser probes gave stable measurements over 5 days. The lCBF increase during hypercapnia obtained by LDF correlated well with flows obtained using CMS ( r = 0.89, P < 0.01). The signals of intracortical and surface laser probes also correlated well ( r = 0.91, P < 0.01). Gliosis of 0.35 ± 0.06 mm around the tip of intracortical probes did not affect the measurements. The range of MABP over which cerebral autoregulation was observed increased from 20–48 mmHg at 110 dGA to 35 to > 95 mmHg at 128 dGA ( P < 0.05). Since MABP increased from 33 to 54 mmHg over this period ( P < 0.01), the range between the lower limit of cerebral autoregulation and the MABP increased from 13 mmHg at 110 dGA to 19 mmHg at 128 dGA ( P < 0.01). LDF is a reliable tool to assess dynamic changes in cerebral perfusion continuously in fetal sheep.     

Effect of Polidan on ultrastructural changes in the cerebral cortex and hippocampus of rats

Electron microscopy and morphometry of neurons in layer V of the somatosensory area in the neocortex and CA3 field in the dorsal hippocampus showed that single and 5-fold intraperitoneal injection of Polidan was followed by ultrastructural and metabolic changes in neurons reflecting activation of protein synthesis. The number of free ribosomes decreased; the number of polysomes and count of ribosomes in polysomes and tubules of the granular endoplasmic reticulum increased. Study of mitochondria and neuropil showed that Polidan activates synthetic processes in the neocortex and hippocampus. It should be emphasized that single treatment with Polidan led to functional activation of synthetic processes, while 5-fold injection of Polidan was followed by hyperactivation of synthetic processes and depletion of ultrastructures in the neocortex and hippocampus. __________ Translated from Byulleten’ Eksperimental’noi Biologii i Meditsiny, Vol. 142, No. 8, pp. 221–226, August, 2006     

Region-specific modulation of electrically induced synchronous oscillations in the rat hippocampus and cerebral cortex

Strong tetanization induces synchronous membrane potential oscillations (seizure-like afterdischarge) in mature pyramidal cells of the hippocampal CA1 region. To investigate whether local networks in other brain regions can generate such an afterdischarge independently, we studied the inducibility of afterdischarge in individual 'isolated slices' of the rat hippocampal CA1 and CA3 regions, dentate gyrus (DG), entorhinal cortex (EC), and temporal cortex (TC) using intracellular and extracellular recordings. The strong tetanization constantly induced afterdischarges in the CA1 and CA3 pyramidal cells as well as in the EC and TC superficial principal cells. However, parameters of the afterdischarges, such as the frequency and duration of afterdischarges, varied among the regions. A mixture of N-methyl-D-aspartate (NMDA) and non-NMDA receptor antagonists or a GABA(A) receptor antagonist completely blocked the afterdischarges. Local GABA application during the afterdischarge elicited depolarization, rather than hyperpolarization. Moreover, reversal potentials of the afterdischarge were around -40 mV. In contrast, the tetanization resulted in occasional afterdischarge-like activities in DG slices, which were blocked by the non-NMDA or GABA(A) receptor antagonist. These findings suggest that the afterdischarges mediated through the excitatory GABAergic and glutamatergic transmissions might be common to, but be modulated differently by individual local networks in the hippocampus and cortex.     

Activity-dependent changes in synaptophysin immunoreactivity in hippocampus,piriform cortex,and entorhinal cortex of the rat

Synaptophysin, an integral membrane glycoprotein of synaptic vesicles, has been widely used to investigate synaptogenesis in both animal models and human patients. Kindling is an experimental model of complex partial seizures with secondary generalization, and a useful model for studying activation-induced neural growth in adult systems. Many studies using Timm staining have shown that kindling promotes sprouting in the mossy fiber pathway of the dentate gyrus. In the present study, we used synaptophysin immunohistochemistry to demonstrate activation-induced neural sprouting in non-mossy fiber cortical pathways in the adult rat. We found a significant kindling-induced increase in synaptophysin immunoreactivity in the stratum radiatum of CA1 and stratum lucidum/radiatum of CA3, the hilus, the inner molecular layer of the dentate gyrus, and layer II/III of the piriform cortex, but no significant change in layer II/III of the entorhinal cortex, 4 weeks after the last kindling stimulation. We also found that synaptophysin immunoreactivity was lowest in CA3 near the hilus and increased with increasing distance from the hilus, a reverse pattern to that seen with Timm stains in stratum oriens following kindling. Furthermore, synaptophysin immunoreactivity was lowest in dorsal and greatest in ventral sections of both CA3 and dentate gyrus in both kindled and non-kindled animals. This demonstrates that different populations of sprouting axons are labeled by these two techniques, and suggests that activation-induced sprouting extends well beyond the hippocampal mossy fiber system.     

Developmental changes in enzyme activities and in morphology of rat cortex mitochondria

Development of mitochondria in rat brain cortex was investigated in terms of mitochondrial respiratory enzyme activities, and structural and numerical developments of mitochondria. Measurements of succinate-O2 and NADH-O2 oxidoreductase activities of mitochondria resulted in simultaneous changes of activities in postnatal rat. Both oxidoreductase activities were still low at 0-5 days old, increased until 15 days, decreased slightly at 21 days and drastically in adult mitochondria. In morphological study, the cross-sectional area of mitochondrion per cell increased gradually until 21 days old, but decreased drastically in adult. The area of a mitochondrion at 5 days increased about 1.5-fold in comparison with that at 0 days, and maintained at 15 and 21 days. However, the values of area of one mitochondrion from 10 days and adult are about half of a maximum value (21 days). Numbers of mitochondrion per cell were still low at 0-5 days, and high constantly (about twice) at 10-21 days. These findings suggest that the organelle division of mitochondria may be carried out at 5-10 days postnatal. The number of adult rat mitochondria decreased slightly. The small and undeveloped mitochondria were observed at 0 day postnatal by use of transmission electron microscopy (TEM). However, during development from 5 days postnatal, larger and elongated mitochondria were observed, and the maximal complexity of structure of cristae is observed at 15 days and 21 days by TEM. In adult cortex, the small mitochondria were also observed with compact and dense cristae. Our results indicate that the changes of activities of mitochondrial respiratory enzymes in rat cortex is good correlated with the structural maturation of mitochondria.     

Tonic activation of NMDA receptors by ambient glutamate of non-synaptic origin in the rat hippocampus

In several neuronal types of the CNS, glutamate and GABA receptors mediate a persistent current which reflects the presence of a low concentration of transmitters in the extracellular space. Here, we further characterize the tonic current mediated by ambient glutamate in rat hippocampal slices. A tonic current of small amplitude (53.99 ± 6.48 pA at +40 mV) with the voltage dependency and the pharmacology of NMDA receptors (NMDARs) was detected in virtually all pyramidal cells of the CA1 and subiculum areas. Manipulations aiming at increasing d -serine or glycine extracellular concentrations failed to modify this current indicating that the glycine binding sites of the NMDARs mediating the tonic current were saturated. In contrast, non-transportable inhibitors of glutamate transporters increased the amplitude of this tonic current, indicating that the extracellular concentration of glutamate primarily regulates its magnitude. Neither AMPA/kainate receptors nor metabotropic glutamate receptors contributed significantly to this tonic excitation of pyramidal neurons. In the presence of glutamate transporter inhibitors, however, a significant proportion of the tonic conductance was mediated by AMPA receptors. The tonic current was unaffected when inhibiting vesicular release of transmitters from neurons but was increased upon inhibition of the enzyme converting glutamate in glutamine in glial cells. These observations indicate that ambient glutamate is mainly of glial origin. Finally, experiments with the use-dependent antagonist MK801 indicated that NMDARs mediating the tonic conductance are probably extra-synaptic NMDARs.     

Age-related changes in glutamate release in the CA3 and dentate gyrus of the rat hippocampus

The present studies employed a novel microelectrode array recording technology to study glutamate release and uptake in the dentate gyrus, CA3 and CA1 hippocampal subregions in anesthetized young, late-middle aged and aged male Fischer 344 rats. The mossy fiber terminals in CA3 showed a significantly decreased amount of KCl-evoked glutamate release in aged rats compared to both young and late-middle-aged rats. Significantly more KCl-evoked glutamate release was seen from perforant path terminals in the DG of late-middle-aged rats compared young and aged rats. The DG of aged rats developed an increased glutamate uptake rate compared to the DG of young animals, indicating a possible age-related change in glutamate regulation to deal with increased glutamate release that occurred in late-middle age. No age-related changes in resting levels of glutamate were observed in the DG, CA3 and CA1. Taken together, these data support dynamic changes to glutamate regulation during aging in subregions of the mammalian hippocampus that are critical for learning and memory.