Sex differences in neurochemical markers that correlate with behavior in aging mice.

Sex differences in neurochemical markers that correlate with behavior in aging mice NEUROBIOL AGING. We examined whether the enzymatic activities of choline acetyltransferase (ChAT) and glutamic acid decarboxylase (GAD) were altered similarly with age in male and female mice, and whether these changes were correlated with age-related alterations in memory and anxiety. ChAT and GAD activities were measured in neocortex, hippocampus, and striatum of behaviorally characterized male and female C57BL/6 mice (5, 17, and 25 months). Generally, ChAT activity was increased, and GAD activity decreased, with age. However, disparate changes were revealed between the sexes; activities of both enzymes were decreased in 17-month males, whereas alterations in females were not observed until 25-months. Furthermore, enzyme-behavior correlations differed between the sexes; in males, ChAT activity was related to one behavioral task, whereas in females, activities of both enzymes were correlated with multiple tasks. Significant enzyme-behavior correlations were most evident at 17 months of age, likely the result of behavioral and enzymatic sex differences at this age. These data represent the first comprehensive report illustrating differential alterations of ChAT and GAD activities in aging male and female mice.     

Reference memory, anxiety and estrous cyclicity in C57BL/6NIA mice are affected by age and sex

Age-related changes in learning and memory are common in rodents. However, direct comparisons of the effects of aging on learning and memory in both males and females are lacking. The present study examined whether memory deteriorates with increasing age in C57BL/6NIA mice, and whether age-related changes in learning and memory are similar in both sexes. Male and female mice (five, 17 and 25 months of age) were tested in a battery of behavioral tasks including the Morris water maze (spatial and non-spatial reference memory), simple odor discrimination (olfactory reference memory), plus maze (anxiety/exploration), locomotor activity, and basic reflexes. Five-month-old mice learned the water maze and odor discrimination tasks rapidly. Relative to five-month-old mice, 25-month-old mice exhibited impaired spatial and olfactory reference memory, but intact non-spatial reference memory. The spatial reference memory of 17-month-old mice was also impaired, but less so than 25-month mice. Seventeen-month-old mice exhibited intact non-spatial (visual and olfactory) reference memory. Five and 25-month-old mice had similar levels of plus maze exploration and locomotor activity, whereas 17-month-old mice were more active than both groups and were slightly less exploratory than five-month-old mice. Although sex differences were not observed in the five- and 25-month groups, 17-month-old females exhibited more impaired spatial reference memory and increased anxiety relative to 17-month-old males. Estrous cycling in females deteriorated significantly with increased age; all 25-month-old females had ceased cycling and 80% of 17-month-old females displayed either irregular or absent estrous cycling. This study is the first to directly compare age-related mnemonic decline in male and female mice. The results suggest that: (i) aged mice exhibit significant deficits in spatial and olfactory reference memory relative to young mice, whereas middle-aged mice exhibit only a moderate spatial memory deficit and; (ii) spatial reference memory decline begins at an earlier age in females than in males, a finding that may be related to the cessation of estrous cycling.     

Reference memory,anxiety and estrous cyclicity in C57BL/6NIA mice are affected by age and sex

Age-related changes in learning and memory are common in rodents. However, direct comparisons of the effects of aging on learning and memory in both males and females are lacking. The present study examined whether memory deteriorates with increasing age in C57BL/6NIA mice, and whether age-related changes in learning and memory are similar in both sexes. Male and female mice (five, 17 and 25 months of age) were tested in a battery of behavioral tasks including the Morris water maze (spatial and non-spatial reference memory), simple odor discrimination (olfactory reference memory), plus maze (anxiety/exploration), locomotor activity, and basic reflexes. Five-month-old mice learned the water maze and odor discrimination tasks rapidly. Relative to five-month-old mice, 25-month-old mice exhibited impaired spatial and olfactory reference memory, but intact non-spatial reference memory. The spatial reference memory of 17-month-old mice was also impaired, but less so than 25-month mice. Seventeen-month-old mice exhibited intact non-spatial (visual and olfactory) reference memory. Five and 25-month-old mice had similar levels of plus maze exploration and locomotor activity, whereas 17-month-old mice were more active than both groups and were slightly less exploratory than five-month-old mice. Although sex differences were not observed in the five- and 25-month groups, 17-month-old females exhibited more impaired spatial reference memory and increased anxiety relative to 17-month-old males. Estrous cycling in females deteriorated significantly with increased age; all 25-month-old females had ceased cycling and 80% of 17-month-old females displayed either irregular or absent estrous cycling.This study is the first to directly compare age-related mnemonic decline in male and female mice. The results suggest that: (i) aged mice exhibit significant deficits in spatial and olfactory reference memory relative to young mice, whereas middle-aged mice exhibit only a moderate spatial memory deficit and; (ii) spatial reference memory decline begins at an earlier age in females than in males, a finding that may be related to the cessation of estrous cycling.     

Age- and sex-related disturbance in a battery of sensorimotor and cognitive tasks in Kunming mice

A battery of tasks, i.e. beam walking, open field, tightrope, radial six-arm water maze (RAWM), novel-object recognition and olfactory discrimination, was used to determine whether there was age- and sex-related memory deterioration in Kunming (KM) mice, and whether these tasks are independent or correlated with each other. Two age groups of KM mice were used: a younger group (7-8 months old, 12 males and 11 females) and an older group (17-18 months old, 12 males and 12 females). The results showed that the spatial learning ability and memory in the RAWM were lower in older female KM mice relative to younger female mice and older male mice. Consistent with this, in the novel-object recognition task, a non-spatial cognitive task, older female mice but not older male mice had impairment of short-term memory. In olfactory discrimination, another non-spatial task, the older mice retained this ability. Interestingly, female mice performed better than males, especially in the younger group. The older females exhibited sensorimotor impairment in the tightrope task and low locomotor activity in the open-field task. Moreover, older mice spent a longer time in the peripheral squares of the open-field than younger ones. The non-spatial cognitive performance in the novel-object recognition and olfactory discrimination tasks was related to performance in the open-field, whereas the spatial cognitive performance in the RAWM was not related to performance in any of the three sensorimotor tasks. These results suggest that disturbance of spatial learning and memory, as well as selective impairment of non-spatial learning and memory, existed in older female KM mice.     

Age and sex-dependent decreases in ChAT in basal forebrain nuclei

Microdissection techniques were utilized to measure the activity of choline acetyltransferase (ChAT) (enzyme responsible for synthesis of acetylcholine) in individual basal forebrain nuclei of aged (24 month) and young (4 month) male and female rats. Small but consistent decreases in the activity of ChAT in aged rats were found, and the location of the changes was dependent on the sex of the rat. Aged female rats showed approximately 30% lower ChAT and 40% lower acetylcholinesterase (AChE) activity in the ventral globus pallidus (vGP). Aged males did not show decreased ChAT in the vGP but activity in the medial aspect of the horizontal diagonal band nucleus was 50% lower than in the young males. ChAT activity in four other closely aligned basal forebrain nuclei was not different between the young and aged rats. Analysis of cell number, density and area in the vGP by AChE histochemistry showed no significant differences between aged and young females. In addition, age and sex-dependent changes were measured in pituitary glucose-6-phosphate dehydrogenase activity. The relationship of the changes to age-dependent decrements in memory, the possible influence of gonadal hormones on aging, and the mechanisms responsible for age-related declines in ChAT activity are discussed.     

Sex differences in behavioral and neurochemical profiles after chronic stress: role of housing conditions

This experiment sought to identify the extent housing conditions can differentially enhance or dampen the effects chronic restraint stress has on exploration and object memory in male and female rats. Subjects were either pair- or singly housed during stress (21 days of 6-h restraint) and maintained under those conditions during poststress behavior testing (7 days). Neurochemical analysis of neural tissue was accomplished by HPLC with electrochemical detection. Interactions between stress and housing conditions were found across both sexes. Stress was associated with less activity in the center of the forced open-field in both sexes. Stress also decreased the latency for males to enter the free open-field to female levels. Object recognition was greatly impaired in double-housed males but unaffected by stress or housing in females. Object location memory was impaired in stressed males if they were singly housed, and females performed as well as control males only if they were stressed. Both sexes generally showed increased in hippocampal (CA3) norepinephrine levels in their respective stress groups. Singly housed subjects had higher CA1 serotonin levels compared to double-housed subjects, whereas in the prefrontal cortex, a general sex difference was found with females having higher levels of serotonin and dopamine metabolites. These results show that stress affects limbic neurochemistry across sex, although only males exhibit stress-dependent decrements in object memory. Housing condition also has a profound effect on neurochemistry and male performance on object recognition. Thus, housing condition is a critical variable for male models of stress that can influence the extent the stress manipulation affects behavior. The differences observed across sex are further discussed in the context of behavioral inhibition.     

In two species, females exhibit superior working memory and inferior reference memory on the water radial-arm maze

Male and female mice and rats were tested on a water escape version of the radial-arm maze designed to measure working and reference memory. In both species, females exhibited superior working memory during acquisition, and were better able to handle a higher memory load. However, male mice and rats exhibited better reference memory than females during the asymptotic portion of testing. Our data suggest that females may be better at working memory when both working and reference memory information must be learned simultaneously, and males better at reference memory when it has been differentiated from working memory.     

Motor and cognitive deficits in mice bred to have low or high blood pressure

Deviations from normal blood pressure can lead to a number of physiological and behavioral complications. We tested the hypothesis that hyper- or hypotension is associated with significant differences in motor activity and coordination, anxiety levels, and spatial learning and memory in male and female mice. Compared to normotensive control mice, hypertensive mice were hyperactive and their performance was significantly worse on the rotarod (males only), cued learning (males only), spatial learning/re-learning, and spatial memory. Hypotensive mice of both genders swam more slowly and performed even worse than hypertensive mice on the rotarod, cued learning, spatial learning/re-learning, and spatial memory tasks. Across all phenotypes, females were generally more active than males in the open field and exhibited more anxiety-like behaviors in the elevated zero maze. Alterations in hemodynamics and/or neurovascular unit function may account for the observed behavioral changes in the hypo- and hypertensive mice.     

Gender dimorphism in neutrophil priming and activation following trauma-hemorrhagic shock

Although gender influences T-cell, macrophage and organ functions following trauma-hemorrhage and resuscitation (T-H), it remains unknown whether it also influences polymorphonuclear cell (PMN) activity under such conditions. To study this, proestrus female and male Sprague-Dawley rats underwent trauma-hemorrhage followed by fluid resuscitation. Circulating PMNs were assessed for superoxide (O2-) and elastase production and tissues were analyzed for myeloperoxidase (MPO) activity and TBARS (thiobarbituric acid reactive substances) as a marker of oxidative injury, at 2 and 24 h after resuscitation. PMA stimulated O2- production was not influenced by T-H or gender. In contrast, fMLP-stimulated O2- and LPS-stimulated elastase release by PMNs from male T-H rats was greater than that of females. A significant MPO activity and TBARS in tissues of both male and female rats was induced; however, MPO activity and TBARS levels were higher in males following T-H. Levels of the chemokine CINC-1 were elevated in the lungs of male, but not of proestrus females after T-H. Thus, decreased PMN priming and activation in proestrus females, compared to males, occurs following T-H resulting in decreased cellular injury and organ damage that is likely to contribute to improved outcome under those conditions.     

Characterization of 50-kHz ultrasonic vocalizations in male and female rats

Male and female rats emit ultrasonic vocalizations in reproductive encounters. While estrous bedding has been used to elicit vocalizations of males, the number of responses is variable. We report a reliable method to assess vocalizations using exposure to a stimulus animal. The stimulus rat is placed behind a wire barrier for 5 min, then removed. Vocalizations are then recorded for 5 min. Experiment 1 validated this method and it was used for subsequent experiments. In Experiment 2, male rats were castrated and tested for the restoration of vocalizations. In one group, males were allowed to copulate freely; in the other, females had vaginal masks to prevent ejaculation, but not mounting. Vocalizations were restored only in males allowed to ejaculate. In Experiment 3, we measured vocalizations in sexually nai;ve and sexually experienced males following exposure to either castrated (CAS) males, testosterone (T)-treated males, ovariectomized (OVX) females, or OVX females receiving estrogen plus progesterone (E+P). Males vocalized most after exposure to E+P females, whether they were sexually experienced or naive. However, the rate of vocalizations was significantly higher after exposure to E+P females when the males were sexually experienced. In Experiment 4, we measured vocalizations in females following exposure to CAS males, T-treated males, OVX females, or E+P females. Females vocalized most after exposure to T-treated males. Our results show that (1) sexual experience facilitates vocalizations in male rats, (2) vocalizations are highest after exposure to hormonally receptive conspecifics, and (3) ultrasonic signaling is a sensitive index for assessing the hormonal disposition of conspecifics.     

Estrogen blocks the protective action of melatonin in a behavioral model of ethanol-induced hangover in mice

Melatonin has antioxidant and neuroprotective properties in human beings and experimental models, as well as 'anti-estrogenic' effects. Ethanol (EtOH) affects various behavioral parameters during a period known as ethanol-induced hangover. Our study evaluated the neuroprotective effect of melatonin on motor performance during ethanol hangover in male and female Swiss mice. The females were subjected to specific hormonal states: ovariectomized (OVX) and OVX estrogenized (OVX-E(2)). Mice received melatonin (25μg/ml) or vehicle in their drinking water for seven days and were given intraperitoneal (i.p.) injections of EtOH (3.8g/kg) or saline on the morning of the eighth day. Motor performance was evaluated by the tightrope test 6h after EtOH exposure (hangover onset). During ethanol hangover, males exhibited lower motor performance than controls (p<0.01) but pretreatment with melatonin significantly improved performance during hangover (p<0.05). In females, melatonin treatment before ethanol-induced hangover led to a better motor performance in OVX compared with intact females (p<0.01) and a lower performance in OVX-E(2) compared with not-estrogenized OVX (p<0.05). Consequently, estrogen reversed the motor performance enhancement afforded by melatonin. We conclude that estrogen interferes with the protective action of melatonin on motor performance during ethanol hangover.     

Estrogen replacement improves spatial reference memory and increases hippocampal synaptophysin in aged female mice

Estrogen deficiency during menopause is often associated with memory dysfunction. However, inconsistencies regarding the ability of estrogen to improve memory in menopausal women highlight the need to evaluate, in a controlled animal model, the potential for estrogen to alleviate age-related mnemonic decline. The current study tested whether estrogen could ameliorate spatial reference memory decline in aged female mice. At the conclusion of testing, levels of the presynaptic protein synaptophysin, and activities of the synthetic enzymes for acetylcholine and GABA, were measured in the hippocampus and neocortex. Aged (27-28-month-old) female C57BL/6 mice were given daily subcutaneous injections of 1 microg or 5 microg of beta-estradiol-3-benzoate dissolved in sesame oil. Control mice received daily injections of sesame oil or no injections. Estradiol treatment began 5 days prior to behavioral testing and continued throughout testing. Spatial and non-spatial memory were assessed in the Morris water maze. The 5 microg dose of estradiol significantly improved spatial learning and memory in aged females. The performance of 5 microg females improved significantly more rapidly than that of control females; estradiol-treated females performed at asymptotic levels by session 2. Furthermore, 5 microg females exhibited a more robust spatial bias than controls during probe trials. In contrast, 1 microg of estradiol did not improve spatial task performance. Neither dose affected performance of the non-spatial task. In the hippocampus, synaptophysin was increased in 5 microg females relative to controls. Estrogen did not affect enzyme activities in either brain region.This study is the first to examine the effects of estrogen replacement on spatial reference memory and synaptophysin expression in aged post-estropausal female rodents. The results suggest that: (1) estrogen can profoundly improve spatial reference memory in aged females, and (2) this improvement may be related to increased hippocampal synaptic plasticity, but not modulation of the synthetic enzymes for acetylcholine and GABA.     

Sexually dimorphic regulation of NK-1 receptor-mediated electrophysiological responses in vagal primary afferent neurons

Neurons can display sexual dimorphism in receptor expression, neurotransmitter release, and synaptic plasticity. We have detected sexual dimorphism in functional tachykinin receptors in vagal afferents (nodose ganglion neurons, NGNs) by studying the effects of hormonal variation on the depolarizing actions of substance P (SP) in female guinea pig NGNs. Using conventional "sharp" microelectrode recording plus measurement of serum 17beta-estradiol values, we examined SP responses in NGNs isolated from 1) ovariectomized females (OVX), 2) OVX females treated with 17beta-estradiol (OVX + E2), 3) pregnant females, and 4) males. Depending on various manipulations, 19-41% female NGNs were depolarized (16 +/- 1.1 mV, mean +/- SE) by 100 nM SP acting through NK-1 receptors. The NGNs of OVX + E2 females (41%, 15/37; 17 +/- 2.1 mV) and pregnant females (41%, 32/79; 16 +/- 1.7 mV) were more likely to respond to SP than those of control males (P < 0.001). The percentage of SP-sensitive NGNs from OVX females (19%, 21/109; 15 +/- 1.9 mV) was not significantly different (P = 0.361) from that of control males (13%, 11/83; 13 +/- 2.0 mV). The serum 17beta-estradiol values for OVX + E2, pregnant, and OVX females were 23.9 +/- 3.3 pg/ml (n = 8), 16.0 +/- 2.4 pg/ml (n = 4), and 3.9 +/- 0.3 pg/ml (n = 8), respectively. These data indicate that there is a gender difference in NK-1 receptor expression in guinea pig nodose neurons, and they suggest that estrogen may modulate SP responsiveness in these neurons.     

Sex hormones differentially influence voluntary running activity, food intake and body weight in aging female and male rats

The aim of this study was to examine the longer-term effects of reduced gonadal hormones on food intake, food efficiency, voluntary running activity and body weight in mature male and female rats, compared to age-matched controls. We hypothesized that hormonal effects would differ for rats that were not rapidly growing and our results are consistent with this hypothesis. 6-8?month male and female rats were divided into four groups: Female and male control groups and a female and male experimental group. Control groups were intact for 46?weeks. Experimental groups were intact during Phase I (16?weeks), ovariectomized or orchidectomized during Phase II (20?weeks), and received estrogen or testosterone hormone replacement therapy (HRT) during the final Phase III (10?weeks). Food intake and running distance were monitored daily and body weight was recorded weekly for 46?weeks. Contrary to findings for young and growing animals, we did not observe a (1) stabilization of food intake in female rats following OVX, (2) loss of body weight with ORX in males, or (3) complete restoration of running activity in ORX males given testosterone, compared to females given estrogen. Feeding efficiency was not affected by aging in females or males. Loss of estrogen increased energy intake whereas reduced testosterone in males resulted in a negative energy balance. Findings suggest variable hormonal effects for aging male/female rats.     

Sex-differences in age-related cognitive decline in C57BL/6J mice associated with increased brain microtubule-associated protein 2 and synaptophysin immunoreactivity

Understanding cognitive aging is becoming more important as the elderly population grows. Here, the effects of age and sex on learning and memory performance were compared in female and male young (3-4 months old) middle-aged (10-12 months old) and old (18-20 months old) wild-type C57BL/6J mice. Old males and females performed worse than young or middle-aged mice in novel location, but not novel object recognition tasks. Old mice, of both sexes, also showed impaired spatial water maze performance during training compared with young or middle-aged mice, however only old females failed to show robust spatial bias during probe trials. While there was no age-difference in passive avoidance performance for males, females showed an age-related decline. There was no difference in cognitive performance between young and middle-age mice of either sex on any task. Cognitive performance was associated with alterations in immunoreactivity of microtubule-associated protein 2-positive dendrites and synaptophysin-positive pre-synaptic terminals in hippocampal CA1, CA3, and dentate, entorhinal cortex, and central nucleus of amygdala. Overall, microtubule-associated protein 2 immunoreactivity was increased in old females compared with both young and middle-age females with no significant difference in males. In contrast, synaptophysin immunoreactivity increased from young to middle-age in females, and from middle-age to old in males; females had higher levels of synaptophysin immunoreactivity than males in middle-age only. Elevated levels of microtubule-associated protein 2 and synaptophysin may constitute a compensatory response to age-related functional decline in mice.     

Developmental effects of neonatal sex hormones on spatial and activity skills in the white rat.

The purpose of this experiment is to extend these experiments by using varying neonatal hormonal differentiation processes and to examine the effects of these different methods of neonatal feminization on changes in spatial and activity skills. The experiment involved three neonatally feminized groups of male rats: 'castration alone', 'estrogen alone', and 'estrogen injected castrates' together with one female sample, 'testosterone injected castrates', while treated and untreated male and female controls were also used. The data provide partial support for the major hypotheses that neonatal gonadectomy and opposite sex hormones administered to male and female castrated rats (together and independently for males) would reverse the normal sex-associated abilities of the white rate (higher male spatial learning and higher female activity). However, the feminization effect for the male 'estrogen alone' and 'castration alone' experimental groups was much greater than for the male estrogen injected castrates. The masculinized females, testosterone injected castrates, also had higher spatial learning and lower activity levels, while the feminized male's spatial and activity skills were also reversed. This confirmed in part the extent to which neonatal gonadal sex hormones are effective at critical periods of development in programming the brain in terms of sex-associated spatial and activity skills. Adult hormonal replacement therapy was also administered at 12 months and supported the hypothesis that sex hormones in adults would be mainly activational and have less marked effects than the significant directional changes obtained by these neonatal sex hormones and castration techniques.     

Chronic oral estrogen affects memory and neurochemistry in middle-aged female mice

This study tested whether chronic oral estrogen could improve memory and alter neural plasticity in the hippocampus and neocortex of middle-aged female mice. Ovariectomized C57BL/6 mice were administered 1,000, 1,500, or 2,500 nM 17beta-estradiol in drinking water for 5 weeks prior to and during spatial and object memory testing. Synaptophysin, nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF) levels were then measured in hippocampus and neocortex. The medium dose impaired spatial reference memory in the radial-arm maze, whereas all doses improved object recognition. The high dose increased hippocampal synaptophysin and NGF levels, whereas the medium dose decreased these neocortical levels. The high dose decreased neocortical BDNF levels. These data suggest that chronic oral estrogen selectively affects memory and neural function in middle-aged female mice.     

Effects of sex and ovarian hormones on the initial renal sympathetic nerve activity response to myocardial infarction

The physiological mechanisms contributing to sex differences following myocardial infarction (MI) are poorly understood. Given the strong relationship between sympathetic nerve activity (SNA) and outcome, we hypothesized there may be a sex difference in SNA responses to MI. In anaesthetized, open-chest male, female and ovariectomized (OVX) female Wistar rats, mean arterial pressure, heart rate and renal SNA were recorded in response to ligation of the left coronary artery. In males, renal SNA increased by 30 ± 6% in the first minute of coronary occlusion (P < 0.05) and remained elevated at 18 ± 7% above baseline (P < 0.05) at 2 h following MI. In response to MI, ovary-intact females displayed no change in renal SNA, whereas OVX females displayed a significant increase, similar to that seen in the males (increases of 43 ± 11% at 1 min and 21 ± 7% at 2 h post-MI, P < 0.05 versus intact females). Arterial baroreflex control of renal SNA had a smaller range in females (ovary intact and OVX) than males; no changes in arterial baroreflex responses were observed 1 h post-MI in males or females. Denervating the arterial baroreceptors abolished the renal SNA response to MI in the males, whereas in ovary-intact females and OVX females the response was unaltered. These findings suggest that ovarian hormones are able to blunt the initial sympathetic activation post-MI in females and that the importance of the arterial baroreflex in mediating initial sympathetic activation post-MI is different between the sexes.     

Sex, age, and training modulate spatial memory in the rhesus monkey (Macaca mulatta)

The authors tested 90 rhesus monkeys (Macaca mulatta) on a task of spatial memory, the spatial Delayed Recognition Span Test. The results showed that performance declined significantly with age, males had greater scores than females, and the rate of apparent decline with age was greater in males than in females. Both working and reference memory declined with age, but only working memory showed sex differences. The authors compared these data with that of 22 monkeys who were trained on a simpler version of the task before formal testing. Training had no effect on males but dramatically improved working memory in young females. The results confirm a male advantage in spatial working memory at a young age and confirm a greater decline with age in males than in females. It is important to note that prior training completely reverses the deficits of young females.     

Age-related changes in rostral basal forebrain cholinergic and GABAergic projection neurons: relationship with spatial impairment

Both cholinergic and GABAergic projections from the rostral basal forebrain contribute to hippocampal function and mnemonic abilities. While dysfunction of cholinergic neurons has been heavily implicated in age-related memory decline, significantly less is known regarding how age-related changes in codistributed GABAergic projection neurons contribute to a decline in hippocampal-dependent spatial learning. In the current study, confocal stereology was used to quantify cholinergic (choline acetyltransferase [ChAT] immunopositive) neurons, GABAergic projection (glutamic decarboxylase 67 [GAD67] immunopositive) neurons, and total (neuronal nuclei [NeuN] immunopositive) neurons in the rostral basal forebrain of young and aged rats that were first characterized on a spatial learning task. ChAT immunopositive neurons were significantly but modestly reduced in aged rats. Although ChAT immunopositive neuron number was strongly correlated with spatial learning abilities among young rats, the reduction of ChAT immunopositive neurons was not associated with impaired spatial learning in aged rats. In contrast, the number of GAD67 immunopositive neurons was robustly and selectively elevated in aged rats that exhibited impaired spatial learning. Interestingly, the total number of rostral basal forebrain neurons was comparable in young and aged rats, regardless of their cognitive status. These data demonstrate differential effects of age on phenotypically distinct rostral basal forebrain projection neurons, and implicate dysregulated cholinergic and GABAergic septohippocampal circuitry in age-related mnemonic decline.