The effect of cholinergic, GABAergic, serotonergic, and glutamatergic receptor modulation on posttrial memory processing in the hippocampus.

Though the hippocampus is widely recognized as important in learning and memory, most of the evidence for this comes from animal lesion and human pathological studies. Due to the relatively small number of drugs that have been tested in the hippocampus for their ability to alter posttrial memory processing, there is a general impression that memory processing involves only a few neurotransmitters. We have evaluated the effects of cholinergic, GABAergic, serotonergic, and glutamatergic receptor agonists and antagonists for their ability to facilitate or impair retention. CD-1 mice received acute intrahippocampal drug infusion following footshock avoidance training in a T-maze. Retention was tested 1 week after training and drug administration. The results indicate that receptor agonists of acetylcholine and glutamate improved retention, while antagonists impaired retention. However, scopolamine did not impair retention, but M1 and M2 antagonists did. Receptor agonists of serotonin and GABA impaired retention, while antagonists improved retention. Drugs acting on 5-HT-1 and 5-HT-2 as well as GABA(A) and GABA(B) receptor subtypes did not differentially effect retention.

Modulation of memory processing in the cingulate cortex of mice.

To evaluate the possible role of the cingulate cortex in memory processing for training using a noxious stimulus, we trained mice on foot shock avoidance in a T-maze. Cholinergic, GABAergic, serotonergic, and glutamatergic agonists and antagonists were administered into the cingulate cortex immediately after training. Retention for the foot shock avoidance training was tested 1 week later. The results indicate that muscarinic and nicotinic agonists improved retention, while antagonists impaired it. GABA and serotonin agonists impaired retention, while antagonists improved it. Drugs acting on GABA(A) and GABA(B) receptors had similar effects on retention, as did drugs acting on serotonin 1 and 2 receptor subtypes. Glutamate improved retention, and AP5, an antagonist of the excitatory amino acid site of the NMDA receptor, impaired retention. The cingulate cortex, like other parts of the limbic system, is involved in memory processing that occurs shortly after training.

Beta-amyloid precursor polypeptide in SAMP8 mice affects learning and memory.

Senescence accelerated (SAMP8 [P8]) mice develop age-related deficits in memory and learning. We show that increased expression of amyloid precursor protein (APP) and its mRNA in the hippocampus are also age-related. Immunocytochemical data suggest that a critical amount of APP expression may be needed to generate amyloid (Abeta) protein plaques in the hippocampus. Deficits in acquisition and retention test performance were alleviated by administration of antibody to Abeta protein into the cerebral ventricles. This reversal of cognitive deficits provides a link between increased expression of both APP and Abeta protein and learning and memory loss in these mice.

Site-directed antisense oligonucleotide decreases the expression of amyloid precursor protein and reverses deficits in learning and memory in aged SAMP8 mice.

beta amyloid protein (Abeta) is a 40-43 amino acid peptide derived from amyloid precursor protein (APP). Abeta has been implicated as a cause of Alzheimer's disease (AD). Mice with spontaneous or transgenic overexpression of APP show the histologic hallmarks of AD and have impairments in learning and memory. We tested whether antisense phosphorothiolated oligonucleotides (AO) directed at the Abeta region of the APP gene given with or without antibody directed at Abeta could reverse the elevated protein levels of APP and the behavioral impairments seen in SAMP8 mice, a strain which spontaneously overexpresses APP. We found that intracerebroventricular (ICV) administration of antibody with either of two AOs directed at the midregion of Abeta improved acquisition and retention in a footshock avoidance paradigm, whereas two AOs directed more toward the C-terminal, a random AO, and vehicle were without effect. Three injections of the more potent AO given without antibody reduced APP protein levels by 43-68% in the amygdala, septum, and hippocampus. These results show that AO directed at the Abeta region of APP can reduce APP levels in the brain and reverse deficits in learning and memory.

Peripheral steroid sulfatase inhibition potentiates improvement of memory retention for hippocampally administered dehydroepiandrosterone sulfate but not pregnenolone sulfate.

Dehydroepiandrosterone sulfate (DHEAS) improves memory retention when administered peripherally. Estrone-3-O-sulfamate (EMATE), a steroid sulfatase inhibitor, potentiates the effect of DHEAS on memory retention such that lower doses of DHEAS improve memory retention. It is not clear if this effect is mediated by both compounds entering the central nervous system. In the current studies, mice were trained to avoid footshock in a T-maze and memory retention was tested 1 week later. DHEAS, injected into the hippocampus after training, improved memory retention in a dose-dependent manner. In previous studies, pregnenolone sulfate (PREGS) improved memory retention when injected into the hippocampus. EMATE, administered peripherally, potentiated the effect of centrally administered DHEAS on memory retention. However, EMATE did not potentiate the effect of centrally administered PREGS. It was concluded that EMATE, acting peripherally, increased plasma levels of DHEAS which entered the brain and added to the effect of centrally administered DHEAS. The failure of EMATE to potentiate PREGS is discussed.

Septo-hippocampal drug interactions in post-trial memory processing.

To determine if serotonin and GABA regulate post-trial memory processing of the cholinergic projection from the septum to the hippocampus, mice were trained on footshock avoidance in a T-maze. Immediately after training, drugs were injected into the septum, hippocampus or both. Retention was tested 1 week after training and drug administration. Ketanserin, a serotonin type 2 receptor antagonist at a dose of 0.5 ng, had no measurable effect on retention, but it reduced the dose of bicuculline, in the septum, or arecoline in the hippocampus that was needed to improve retention. DOI, a serotonin type 2 receptor agonist at a dose of 2.5 ng, had the opposite effect of increasing the doses of bicuculline and arecoline needed to improve retention. Bicuculline, a GABA(A) receptor antagonist at a dose of 0.1 pg, did not affect retention when injected alone into the septum, but it reduced the dose of arecoline needed to improve retention in the hippocampus. Muscimol, a GABA(A) receptor agonist at a dose of 5 ng, injected into the septum, increased the dose of arecoline needed to improve retention. The results of this study are compatible with models that propose that serotonin innervation from the median raphe drives GABA interneurons in the medial septum that synapse on cholinergic neurons projecting to the hippocampus.

Changes in membrane fatty acids and delta-9 desaturase in senescence accelerated (SAMP8) mouse hippocampus with aging.

Senescence accelerated mice (SAMP8) exhibit age induced impairments such as loss of memory and learning disabilities by the age of 8-10 months. Analysis of hippocampus of SAMP8 mice revealed that delta 9-desaturase (delta9desaturase) activity reduced up to 44-50% with age. Correspondingly, levels of unsaturated fatty acids are also lowered in the aged animals approximately to the same levels. RNase protection assay showed that delta9specific message decreased similarly with age. As such a decrease is known to cause alterations in membrane fluidity and affect cellular signaling pathways, these results suggest that lowering of delta9gene expression may be partly involved in age induced impairments.

Effect of age on calcium-dependent proteins in hippocampus of senescence-accelerated mice.

The senescence-accelerated P8 mouse (SAMP8) is a well-characterized model for the age-related decline in acquisition and retention. Calcium-dependent protein kinase C (PKC) and calcium-calmodulin-dependent protein kinase (CAM K) have been implicated in these processes in the hippocampus. Therefore, the expression of hippocampal PKC and CAM K was determined in SAMP8 mice aged 4, 8, and 12 months. As measured by Western blotting, total hippocampal PKC-gamma protein declined linearly with age. In addition, the distribution of the PKC-gamma also changed with age. The amount of PKC in the particulate fraction declined linearly with age relative to the soluble PKC. The decline in total PKC and particulate PKC correlated with the previously reported decline in retention but not with the decline in acquisition. Western blotting revealed no consistent change in CAM KII protein levels. In addition to protein levels, Ca-dependent protein kinase activity may also be affected by changes in intracellular Ca concentration. Therefore, the levels of calbindin and the plasma membrane Ca pump, two proteins involved in maintaining low levels of intracellular Ca, were measured in the hippocampus. Calbindin protein declined progressively with age, but there was no significant change in total plasma membrane Ca pump expression. These studies demonstrate a decrease in the amount and distribution of hippocampal PKC-gamma in the SAMP8 between 4 and 12 months that is associated with decreased retention.

Leptin and neuropeptide Y (NPY) modulate nitric oxide synthase: further evidence for a role of nitric oxide in feeding.

Recent studies have suggested a role for nitric oxide in the regulation of food intake. Neuropeptide Y (NPY) is one of the most potent orexigenic agents. Chronic administration of leptin decreases food intake. This study examined the effects of NPY and leptin on nitric oxide synthase (NOS) in the hypothalamus. Previously it has been demonstrated that obese (ob/ob) mice have elevated NOS levels in the hypothalamus. In this study we demonstrated that the administration of leptin (6 microg/day) subcutaneously (SC) for 3 days decreased body weight (P < 0.001) and food intake P < 0.001) in obese (ob/ob) mice as expected. In addition, leptin decreased NOS in the hypothalamus nu 37% (P < 0.01) and in brown adipose tissue by 69% (P < 0.01) but not in white adipose tissue. NPY was administered intracerebroventricularly to CD-1 mice at doses of 0.25 and 0.50 microg. Mice were sacrificed 15 min after injection and NOS was measured in their hypothalami. NPY at the lower dose increased NOS in the hypothalamus by 147%. These results, taken together, with previously published studies support the concept that nitric oxide may play a role as a mediator of the effects of NPY and leptin on food intake. The alterations of NOS in brown adipose tissue following leptin administration could result in changes in blood flow or metabolism in the brown fat.

Effects of wheatgerm agglutinin and aging on the regional brain uptake of HIV-1GP120.

HIV-1 is associated with infection and altered functions of the CNS, especially in the elderly. Most studies indicate that HIV-1 is not evenly distributed throughout the CNS but is concentrated in deep brain nuclei. This study examined whether regional or age-related differences in the permeability of the blood-brain barrier to gp120, the viral coat of HIV-1, exist. The initial concentration of gp120 in 10 brain regions correlated with vascular content in young and old mice. Susceptibility to wheatgerm agglutinin (WGA)-induced uptake of gp120, which relates to endothelial cell internalization, varied regionally, with no induction of uptake into the striatum or hypothalamus but with large increases in the cerebellum, cortex, and midbrain. Transport across the BBB, as measured by the unidirectional influx rate (Ki), also varied regionally with the hypothalamus, hippocampus, and pons-medulla showing the highest values for Ki and the striatum the lowest. These regional variations in the permeability of the BBB to gp120 could contribute to the inhomogeneous distribution of HIV-1 within the CNS whereas the failure to see differences with aging suggests other causes underlie the susceptibility of the elderly to the CNS manifestations of AIDS.

The pharmacology of post-trial memory processing in septum.

The septum is recognized as important in learning and memory, but relatively little is known about the role of specific neurotransmitter receptors in memory processing in the septum. We evaluated the role of the classical neurotransmitters in mice that were prepared for intraseptal microinfusion of drug solution after footshock avoidance training in T-maze. Retention for the footshock training was determined 1 week after training and drug administration. The results indicated that receptor agonists of dopamine, norepinephrine, glutamate and acetylcholine improved retention, while the antagonists impaired retention. Receptor agonists of serotonin, gamma-amino butyric acid (GABA) and opioids impaired retention, while antagonists improved retention.

Learning and memory in the SAMP8 mouse.

The SAMP8 (P8) mouse strain develops deficits in learning and memory relatively early in its lifespan. This review provides an overview of the age-related changes that occur in P8 mice. Behavioral studies with P8 mice show impaired acquisition and retention as early as 4 months of age. Deficits in acquisition and retention occur with both aversive and appetitive training tasks. Anatomical studies have detected a number of age-related changes that occur in the central nervous system of P8 mice. The age-related increase in amyloid beta protein is well correlated with the age-related decline in learning and memory. Antibody to amyloid beta protein injected prior to training alleviated impaired acquisition and retention, whereas post-training injections alleviated retention deficits in older P8 mice. Biochemical studies have detected numerous age-related changes with reduced NMDA receptor activity most closely related to impaired learning and memory in P8 mice. Pharmacological studies have found age-related functional changes in the ability of drugs to improve memory processing in P8 mice in the septum and the hippocampus. The specific pattern of pharmacological changes and the inferred change in neurotransmitter activity suggest that age-related impairment in memory processing may be due to impaired septohippocampal interactions. The proposal that P8 mice may be a useful model for studying the early phases of age-related dementia of the Alzheimer type, while still requiring considerable study, seems reasonable.

Age-related changes in septal serotonergic, GABAergic and glutamatergic facilitation of retention in SAMP8 mice.

SAMP8/TaJf(P8) mouse strain has an inherited age-related impairment of learning and memory with its onset relatively early in its lifespan. Previously, it was reported that cholinergic and glutamatergic drugs injected into the hippocampus after behavioral training showed considerable shifts in the dose that improved retention in mice at 12 compared to 4 months of age. Cholinergic neurons in the septum supply most of the acetylcholine released in the hippocampus. In the present study, we determined if altered functional status of neurotransmission in the septum might account for the decrease in cholinergic and glutamatergic activity in the hippocampus of older SAMP8 mice. After training on footshock avoidance, P8 mice received a drug injection into the septum. Retention was tested 1 week later. The results indicate that bicuculline, GABA-A, and saclofen, GABA-B, receptor antagonist had to be injected at a higher dose in 12- than in 4-month-old mice to improve retention. The serotonergic antagonists, ketanserin and methiothepin, both showed dose response shifts such that less drug was needed to improve retention in 12- as compared to 4-month-old mice. It required four times more L-glutamate to improve retention in 12- than in 4-month-old mice. Agonists for acetylcholine, dopamine and norepinephrine receptors or an opiate antagonist required little or no change in the dose needed to improve retention in older P8 mice. SAMP8 mice may show an age-related impairment of septohippocampal functioning.

Effect of histamine H2 and H3 receptor modulation in the septum on post-training memory processing.

We compared the effects of modulating the postsynaptic histamine receptor subtype 2 (H2) and inhibitory presynaptic autoreceptor subtype 3 (H3) on memory processing in the septum. Mice were partially trained on footshock avoidance in a T-maze. Immediately after training, saline or a drug solution was infused into the septum. One week later, retention was tested by continuing training until the mice made five avoidance responses in six consecutive trials. The results indicate that dimaprit, an H2 agonist, facilitated retention (25 and 50 pg) with a U-shaped dose-response curve typical of drugs acting at postsynaptic receptors. Cimetidine, an H2 antagonist, impaired retention (15-50 ng). The H3 agonist. imetit, impaired retention (25-200 ng), while the H3 antagonist, thioperamide, facilitated retention (10-400 ng). An unusual feature of the dose-response curve for thioperamide was that it did not appeal to yield a U-shaped curve as occurs with drugs acting postsynaptically, but facilitated retention to approximately the same degree from 50 to 400 ng. As histamine neurons project to various limbic system structures involved in memory processing, it may play an important role in regulating the activity of structures such as the septum, hippocampus and amygdala.

Potentially predictive and manipulable blood serum correlates of aging in the healthy human male: progressive decreases in bioavailable testosterone, dehydroepiandrosterone sulfate, and the ratio of insulin-like growth factor 1 to growth hormone.

A cross-sectional survey was made in 56 exceptionally healthy males, ranging in age from 20 to 84 years. Measurements were made of selected steroidal components and peptidic hormones in blood serum, and cognitive and physical tests were performed. Of those blood serum variables that gave highly significant negative correlations with age (r > -0.6), bioavailable testosterone (BT), dehydroepiandrosterone sulfate (DHEAS), and the ratio of insulin-like growth factor 1 (IGF-1) to growth hormone (GH) showed a stepwise pattern of age-related changes most closely resembling those of the age steps themselves. Of these, BT correlated best with significantly age-correlated cognitive and physical measures. Because DHEAS correlated well with BT and considerably less well than BT with the cognitive and physical measures, it seems likely that BT and/or substances to which BT gives rise in tissues play a more direct role in whatever processes are rate-limiting in the functions measured and that DHEAS relates more indirectly to these functions. The high correlation of IGF-1/GH with age, its relatively low correlation with BT, and the patterns of correlations of IGF-1/GH and BT with significantly age-correlated cognitive and physical measures suggest that the GH-IGF-1 axis and BT play independent roles in affecting these functions. Serial determinations made after oral ingestion of pregnenolone and data from the literature suggest there is interdependence of steroid metabolic systems with those operational in control of interrelations in the GH-IGF-1 axis. Longitudinal concurrent measurements of serum levels of BT, DHEAS, and IGF-1/GH together with detailed studies of their correlations with age-correlated functional measures may be useful in detecting early age-related dysregulations and may be helpful in devising ameliorative approaches.

Amylin and food intake in mice: effects on motivation to eat and mechanism of action.

Amylin is a hormone produced by the pancreatic islets of Langerhans. Amylin decreased food pellet consumption. Amylin also decreased lever pressing for milk solution whether or not the mice were prefed. Amylin did not produce a conditioned taste aversion in a two bottle test, whereas lithium chloride did. In addition, L-arginine, a precursor for nitric oxide synthesis, was demonstrated to inhibit the ability of amylin to decrease food intake. Amylin did not alter nitric oxide synthase activity in the fundus of the stomach. These studies demonstrated that amylin inhibits food intake at a higher range of doses than is typical of anorectic agents such as cholecystokinin. Amylin does not appear to decrease food intake by reducing the release of nitric oxide but may affect appetite by modulating serum glucose levels when co-released with insulin.

The cortical neuropeptide, cortistatin-14, impairs post-training memory processing.

Cortistatin-14, a neuropeptide, is present primarily in the cortex and hippocampus. In the hippocampus, cortistatin-14 inhibits pyramidal cell firing and co-exists with GABA. To determine if cortistatin-14 would impair retention, saline or cortistatin-14 were injected intracerebroventricularly after footshock avoidance training in CD-1 mice. After 1 week, training was resumed to determine the effect of cortistatin-14 on retention. Cortistatin-14 was found to impair retention relative to the control group at doses of 0.5-5.0 micrograms.

Inhibition of feeding by a nitric oxide synthase inhibitor: effects of aging.

Nitric oxide has been demonstrated to play a role in the modulation of food intake. With advancing age, there is a physiological decrease in food intake. The effect of the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME) on food intake in C57BL/6Nnia mice aged 3, 12 and 24 months was studied. L-NAME was more effective at decreasing food intake in 12- and 24-month-old mice than in the 3-month-old mice. NO synthase levels in the hypothalamus were increased in 16- and 25-month-old mice compared to 6-month-old mice (P < 0.01). NO synthase mRNA increased in 16- compared to 6-month-old mice, but decreased in 25-month-old mice. Overall, these studies may suggest that nitric oxide may play an increasingly important role in the feeding drive with advancing age.

Enhancement of hippocampal acetylcholine release by the neurosteroid dehydroepiandrosterone sulfate: an in vivo microdialysis study.

The effect of dehydroepiandrosterone sulfate (DHEAS) administered i.p. on the release of acetylcholine (ACh) from the hippocampus of anesthetized rats was examined using in vivo microdialysis. DHEAS significantly increased ACh release above the pre-treatment levels for all doses tested. The administration of 100 mg/kg significantly enhanced ACh release greater than 4-fold when compared to the saline-treated group 80 min following drug administration. This study is the first to demonstrate that the neurosteroid DHEAS, a negative allosteric modulator of the GABAA receptor, can enhance the release of ACh from neurons in the hippocampus.