The 5-HT(1A) receptor active compounds (R)-8-OH-DPAT and (S)-UH-301 modulate auditory evoked EEG responses in rats.

Schizophrenics commonly demonstrate abnormalities in central filtering capability following repetitive sensory stimuli. Such sensory inhibition deficits can be mirrored in rodents following administration of psycho-stimulatory drugs. In the present study, male Sprague-Dawley rats were implanted with brain surface electrodes to record auditory evoked EEG potentials in a paired-stimulus paradigm, using 87 dB clicks delivered 0.5 s apart. Amphetamine (1.83 mg/kg, i.p.) produced the expected loss of sensory inhibition, as defined by an increase in the ratio between test (T) and conditioning (C) amplitudes at N40, a mid-latency peak of the evoked potentials. Also, the 5-HT(1A) agonist (R)-8-OH-DPAT caused a significant increase in the TC ratio at the highest dose studied (0.5 mg/kg s.c.), while the 5-HT(1A) antagonist (S)-UH-301 did not significantly affect the TC ratio at any dose studied (0.1-5 mg/kg s.c.). When administered with amphetamine, a lower dose of 8-OH-DPAT (0.1 mg/kg) and the highest dose of UH-301 tested (5 mg/kg, s.c.) were able to reverse the amphetamine-induced increase in TC ratio. The findings suggest that 5-HT(1A) signaling is involved in sensory inhibition and support the evaluation of 5-HT(1A) receptor active compounds in conditions with central filtering deficits, such as schizophrenia.

Activity of alpha7-selective agonists at nicotinic and serotonin 5HT3 receptors expressed in Xenopus oocytes.

Nicotinic receptors containing alpha7 subunits are widely distributed in the central nervous system and are thought to be involved in a number of functions. However, it has been difficult to study alpha7-containing receptors in vivo because of a paucity of selective agonists. A new spirooxazolidinone compound, AR-R17779, was recently described as potent agonist at alpha7 receptors, but electrophysiological studies at other types of nicotinic receptors have not been carried out. We characterized the activity of AR-R17779 at alpha7, alpha4beta2, alpha3beta4, alpha3beta2, alpha3beta2alpha5 receptors expressed in Xenopus oocytes. In addition, since there is significant homology between nicotinic alpha7 and serotonin 5HT(3) receptors, the activity of AR-R17779 at expressed 5HT(3a) receptors was also examined. Finally, actions of tropisetron and ondansetron, two 5HT(3) antagonists, were explored. AR-R17779 was found to activate alpha7 receptors, but had no activity at other types of nicotinic receptors, and also had no activity at 5HT(3a) receptors. Tropisetron activated, while ondansetron acted as an antagonist, at alpha7 nicotinic receptors. The two 5HT(3) antagonists also acted as antagonists at alpha4beta2 and alpha3beta4 nicotinic receptors. Thus, AR-R17779 was confirmed to be a selective nicotinic alpha7 receptor agonist and to be without activity at 5HT(3) receptors. In contrast, the actions of tropisetron and ondansetron on nicotinic receptors were complex.

Age-related sex differences in spatial learning and basal forebrain cholinergic neurons in F344 rats.

Basal forebrain cholinergic neurons are important for spatial learning in rodents. Spatial learning ability is reportedly better in males than females, and declines with age. To examine the role of cholinergic function in sex- or age-related differences in spatial learning, we compared the size of basal forebrain cholinergic neurons (BFCN) of young and aged male and female Fischer 344 (F344) rats that had been trained in the Morris water maze. Young male and female rats were equally proficient in finding the platform during training trials, but probe tests revealed that young male rats had better knowledge of the platform's precise location. Impairments in spatial learning were observed in aged rats, and the advantage of males over females was lost. BFCN were significantly larger in young male than young female rats, and were correlated with spatial memory performance for both groups. BFCN were smaller in aged than young males; no change was seen between young and aged females. In the groups of aged rats the correlation between neuron size and spatial memory was lost. The present findings provide further evidence of a role for the basal forebrain cholinergic system in spatial learning, but reveal a complex interaction between sex, age and behavioral performance.

Long-term effects of BIBN-99, a selective muscarinic M2 receptor antagonist, on improving spatial memory performance in aged cognitively impaired rats.

Aged Long-Evans rats were screened for spatial memory deficits using the Morris water maze task. Rats found to have impaired performance on the task (aged-impaired, AI) were then treated with a selective muscarinic M2 receptor antagonist, 5,11-dihydro-8-chloro-11-[[4-[3-[(2,2-dimethyl-1-oxopentyl)ethylamino]propyl]-1-piperidinyl]acetyl]-6H-pyrido[2,3-b][1,4]benzodiazepin-6-one (BIBN-99; 0.5 mg/kg, s.c.), for 3 successive days while receiving additional water maze training. BIBN-99 significantly improved performance in the task during the 3 days of drug treatment. Treatment was then ceased for the remainder of the study and rats were tested again in the water maze on days 10, 17, and 24. Compared to vehicle-treated rats, enhanced performance was observed in the AI rats that had previously been treated with BIBN-99. These results indicate that BIBN-99 enhances spatial learning in AI animals and that enhanced (or long-term) memory persists in the absence of the drug. In a second experiment, a 2-month delay was imposed in between the original water maze screening and the drug treatment regime. Again, BIBN-99 significantly improved performance in AI rats. This latter study suggests that reference memory does not decay, even in an AI animal that had displayed poor learning following original water maze screening. Together, these studies help provide further insight into possible mechanism(s) of reference memory and its potential clinical usefulness.

Effects of hippocampal lesions on patterned motor learning in the rat.

Motor skill learning in rats has been linked to cerebellar function as well as to cortical and striatal influences. The present study evaluated the contribution of the hippocampus to motor learning. Adult male rats received electrolytic lesions designed to selectively destroy the hippocampus; a sham-lesioned group of animals served as a control. The animals with hippocampal lesions acquired a patterned motor learning task as well as sham controls. In contrast, rats with hippocampal lesions were impaired in spatial, but not cued, learning in the Morris water maze. In addition, lesioned rats showed profound impairment in the novel object recognition memory task, when a 1-h delay was used between training and testing. Taken together, these results suggest that the hippocampus is not necessary during acquisition of the motor learning task.

Assessment of inhibition and epileptiform activity in the septal dentate gyrus of freely behaving rats during the first week after kainate treatment.

Mossy fiber reorganization has been hypothesized to restore inhibition months after kainate-induced status epilepticus. The time course of recovery of inhibition after kainate treatment, however, is not well established. We tested the hypothesis that if inhibition is decreased after kainate treatment, it is restored within the first week when little or no mossy fiber reorganization has occurred. Chronic in vivo recordings of the septal dentate gyrus were performed in rats before and 1, 4, and 7-8 d after kainate (multiple injections of 5 mg/kg, i.p.; n = 17) or saline (n = 11) treatment. Single and paired-pulse stimuli were used to assess synaptic inhibition. The first day after kainate treatment, only a fraction of rats showed multiple population spikes (35%), prolonged field postsynaptic potentials (76%), and loss of paired-pulse inhibition (29%) to perforant path stimulation. Thus, inhibition was reduced in only some of the kainate-treated rats. By 7-8 d after treatment, nearly all kainate-treated rats showed partial or full recovery in these response characteristics. Histological analysis indicated that kainate-treated rats had a significant decrease in the number of hilar neurons compared to controls, but Timm staining showed little to no mossy fiber reorganization. These results suggest that a decrease in synaptic inhibition in the septal dentate gyrus is not a prerequisite for epileptogenesis and that most of the recovery of inhibition occurs before robust Timm staining in the inner molecular layer.

Exposing rats to a predator impairs spatial working memory in the radial arm water maze.

This series of studies investigated the effects of predator exposure on working memory in rats trained on the radial arm water maze (RAWM). The RAWM is a modified Morris water maze that contains four or six swim paths (arms) radiating out of an open central area, with a hidden platform located at the end of one of the arms. The hidden platform was located in the same arm on each trial within a day and was in a different arm across days. Each day rats learned the location of the hidden platform during acquisition trials, and then the rats were removed from the maze for a 30-min delay period. During the delay period, the rats were placed either in their home cage (nonstress condition) or in close proximity to a cat (stress condition). At the end of the delay period, the rats were run on a retention trial, which tested their ability to remember which arm contained the platform that day. The first experiment confirmed that the RAWM is a hippocampal-dependent task. Rats with hippocampal damage were impaired at learning the location of the hidden platform in the easiest RAWM under control (non-stress) conditions. The next three experiments showed that stress had no effect on memory in the easiest RAWM, but stress did impair memory in more difficult versions of the RAWM. These findings indicate that the capacity for stress to impair memory is influenced not only by the brain memory system involved in solving the task (hippocampal versus nonhippocampal), but also by the difficulty of the task. This work should help to resolve some of the confusion in the literature regarding the heterogeneous effects of stress on hippocampal-dependent learning and memory.

Differential activation of adenylyl cyclases by spatial and procedural learning.

Adenylyl cyclases (ACs) are involved in a variety of advanced CNS functions, including some types of learning and memory. At least nine AC isoforms are expressed in the brain, which are divisible into three broad classes based on the ability of Ca(2+) to modulate their activity. This study examined the hypothesis that different learning tasks would differentially activate ACs in selected brain regions. The ability of forskolin or Ca(2+) to enhance AC activity in the hippocampus, parietal cortex, striatum, and cerebellum was examined after mice had been trained in either a spatial or procedural learning task using a Morris water maze. Sensitivity of ACs to forskolin was enhanced to a greater degree in most brain regions after procedural learning, but Ca(2+)-sensitive ACs in the hippocampus were more sensitive to spatial learning. Because nonspecific behavioral elements, such as stress or motor activity, were similar in both experimental tasks, these results provide the first evidence that acquisition of different kinds of learning is associated with selective changes in particular AC species in a mammalian brain and support the idea that different biochemical processing, involving particular isoforms of ACs, subserves different memory systems.

Exposing rats to a predator blocks primed burst potentiation in the hippocampus in vitro.

This study evaluated the effects of acute psychological stress (cat exposure) in adult male rats on electrophysiological plasticity subsequently assessed in the hippocampus in vitro. Two physiological models of memory were studied in CA1 in each recording session: (1) primed burst potentiation (PBP), a low-threshold form of plasticity produced by a total of five physiologically patterned pulses; and (2) long-term potentiation (LTP), a suprathreshold form of plasticity produced by a train of 100 pulses. Three groups of rats were studied: (1) undisturbed rats in their home cage (home cage); (2) rats placed in a chamber for 75 min (chamber); and (3) rats placed in a chamber for 75 min in close proximity to a cat (chamber/stress). At the end of the chamber exposure period, blood samples were obtained, and the hippocampus was prepared for in vitro recordings. Only the chamber/stress group had elevated (stress) levels of corticosterone. The major finding was that PBP, but not LTP, was blocked in the chamber/stress group. Thus, the psychological stress experienced by the rats in response to cat exposure resulted in an inhibition of plasticity, which was localized to the intrinsic circuitry of the hippocampus. This work provides novel observations on the effects of an ethologically relevant stressor on PBP in vitro and of the relative insensitivity of LTP to being modulated by psychological stress. We discuss the relevance of these electrophysiological findings to our behavioral work showing that predator stress impairs spatial memory.

Acute application of NGF increases the firing rate of aged rat basal forebrain neurons.

Nerve growth factor (NGF) has been widely used in animal models to ameliorate age-related neurodegeneration, but it cannot cross the blood-brain barrier (BBB). NGF conjugated to an antibody against the transferrin receptor (OX-26) crosses the BBB and affects the biochemistry and morphology of NGF-deprived basal forebrain neurons. The rapid actions of NGF, including electrophysiological effects on these neurons, are not well understood. In the present study, two model systems in which basal forebrain neurons either respond dysfunctionally to NGF (aged rats) or do not have access to target-derived NGF (intraocular transplants of forebrain neurons) were tested. One group of transplanted and one group of aged animals received unconjugated OX-26 and NGF comixture as a control, while other groups received replacement NGF in the form of OX-26-NGF conjugate during the 3 months preceding the electrophysiological recording session. Neurons from animals in both the transplanted and aged control groups showed a significant increase in firing rate in response to acute NGF application, while none of the conjugate-treated groups or young intact rats showed any response. After the recordings, forebrain transplants and aged brains were immunocytochemically stained for the low-affinity NGF receptor. All conjugate treatment groups showed significantly greater staining intensity compared to controls. These data from both transplants and aged rats in situ indicate that NGF-deprived basal forebrain neurons respond to acute NGF with an increased firing rate. This novel finding may have importance even for long-term biological effects of this trophic factor in the basal forebrain.

Cognition enhancement strategies by ion channel modulation of neurotransmission.

Relatively few effective therapies exist for the multitude of disorders that comprise dementia, a clinical syndrome manifested by impairments in cognition, language and memory. Treatment of Alzheimer s disease (AD), the most common cause of dementia, is a primary goal of research in cognitive enhancement. However, despite intense research, effective pharmacological interventions remain to be developed. The preponderance of pharmacological strategies which are being pursued in AD research attempt to relieve cognitive and memory deficits which are attributed to cholinergic dysfunction. This paper briefly reviews the status of other efforts that have in common the potential to enhance the use-dependent activity of multiple neurotransmitters system through the modulation of gated ion channels. Discussed are recent advances in the areas of: 1) g-aminobutyric acid subtype A receptor/benzodiazepine (GABAA/BZ) inverse agonists; 2) nicotinic acetylcholine receptor (nAChR) agonists; 3) serotonin subtype 3 receptor (5-HT3R) antagonists; and 4) potassium (K+) M-channel inhibitors.

Multiple single units and population responses during inhibitory gating of hippocampal auditory response in freely-moving rats.

Paired clicks were presented to awake, freely-moving rats to examine neuronal activity associated with inhibitory gating of responses to repeated auditory stimuli. The rats had bundles of eight microwires implanted into each of four different brain areas: CA3 region of the hippocampus, medial septal nucleus, brainstem reticular nucleus, and the auditory cortex. Single-unit recordings from each wire were made while the local auditory-evoked potential was also recorded. The response to a conditioning stimulus was compared to the response to a test stimulus delivered 500 ms later: the ratio of the test response to the conditioning response provided a measure of inhibitory gating. Auditory-evoked potentials were recorded at all sites. Overall, brainstem reticular nucleus neurons showed the greatest gating of local auditory-evoked potentials, while the auditory cortex showed the least. However, except for the auditory cortex, both gating and non-gating of the evoked response were recorded at various times in all brain regions. Gating of the hippocampal response was significantly correlated with gating in the medial septal nucleus and brainstem reticular nucleus, but not the auditory cortex. Single-unit neuron firing in response to the clicks was most pronounced in the brainstem reticular nucleus and the medial septal nucleus, while relatively few neurons responded in the CA3 region of the hippocampus and the auditory cortex. Taken together, these data support the hypothesis that inhibitory gating of the auditory-evoked response originates in the non-lemniscal pathway and not in cortical areas of the rat brain.

The enhancement of hippocampal primed burst potentiation by dehydroepiandrosterone sulfate (DHEAS) is blocked by psychological stress.

This series of studies investigated the effects of psychological stress and the neurosteroid dehydroepiandrosterone sulfate (DHEAS) on hippocampal primed burst (PB) and long-term (LTP) potentiation, two electrophysiological models of memory. The DHEAS and stress manipulations were performed on awake rats, and then PB and LTP were recorded while the rats were anesthetized. DHEAS enhanced PB potentiation when administered to rats under non-stress conditions, but had no effect when given to stressed rats. Further study showed that DHEAS enhanced PB potentiation only when it was administered before, but not after, the rats were stressed. The DHEAS and stress manipulations had no effect on LTP. This study provides three major findings regarding stress, neurosteroids and hippocampal plasticity. First, DHEAS enhanced a threshold form of plasticity (PB potentiation), but had no effect on a supra-threshold form of plasticity (LTP). Second, stress blocked the DHEAS-induced enhancement of PB potentiation. Third, stress and DHEAS effects on the hippocampus were so durable they could be performed on awake animals and then be studied while the animals were anesthetized. That DHEAS enhanced a subset of forms of hippocampal plasticity under restricted behavioral conditions may help to resolve conflicting observations of DHEAS effects on cognition and mood in people.

Behavioral effects of neurotrophic factor supplementation in aging.

Neurotrophic factors are now recognized to play important roles in the normal function of the mature central nervous system. This knowledge has motivated experiments to evaluate the potential benefits of administering neurotrophic factors to the aged brain. This article provides a review of studies to date that have determined the behavioral effects of such treatments. Nerve growth factor (NGF) administration appears to reliably enhance learning and memory in aged rats, while glial-derived neurotrophic factor (GDNF) causes some improvement in motor function. Problems associated with neurotrophic factor administration to humans are discussed.

Parallel nature of hippocampal synaptic plasticity.

This study demonstrates that the mechanisms involved in the production of long-term potentiation (LTP) in the hippocampus appear to be independent of those which generate shorter-lasting plasticity, but that both processes are activated concurrently following an LTP-inducing stimulus. Adult male Sprague-Dawley rats were anesthetized using either pentobarbital or secobarbital to record extracellular field potentials from the hippocampal CA1 pyramidal cell layer in response to stimulation of commissural afferents. Plasticity was generated by the delivery of a five-pulse patterned stimulus train, consisting of one priming pulse followed 170 milliseconds later by a burst of four pulses at 200 Hz. While similar LTP was observed in both groups, short-term plasticity was absent in the secobarbital-anesthetized animals. This result suggests that different plasticity mechanisms in the hippocampus are activated in parallel by the triggering stimulus.

A non-invasive system for delivering neural growth factors across the blood-brain barrier: a review.

Intraventricular administration of nerve growth factor (NGF) in rats has been shown to reduce age-related atrophy of central cholinergic neurons and the accompanying memory impairment, as well as protect these neurons against a variety of perturbations. Since neurotrophins do not pass the blood-brain barrier (BBB) in significant amounts, a non-invasive delivery system for this group of therapeutic molecules needs to be developed. We have utilized a carrier system, consisting of NGF covalently linked to an anti-transferrin receptor antibody (OX-26), to transport biologically active NGF across the BBB. The biological activity of this carrier system was tested using in vitro bioassays and intraocular transplants; we were able to demonstrate that cholinergic markers in both developing and aged intraocular septal grafts were enhanced by intravenous delivery of the OX-26-NGF conjugate. In subsequent experiments, aged (24 months old) Fischer 344 rats received intravenous injections of the OX-26-NGF conjugate for 6 weeks, resulting in a significant improvement in spatial learning in previously impaired rats, but disrupting the learning ability of previously unimpaired rats. Neuroanatomical analyses showed that OX-26-NGF conjugate treatment resulted in a significant increase in cholinergic cell size as well as an upregulation of both low and high affinity NGF receptors in the medial septal region of rats initially impaired in spatial learning. Finally, OX-26-NGF was able to protect striatal cholinergic neurons against excitotoxicity and basal forebrain cholinergic neurons from degeneration associated with chemically-induced loss of target neurons. These results indicate the potential utility of the transferrin receptor antibody delivery system for treatment of neurodegenerative disorders with neurotrophic substances.

Brief paradoxical sleep deprivation impairs reference, but not working, memory in the radial arm maze task.

Selective deprivation of paradoxical sleep after learning results in memory deficits in a variety of tasks. The present experiment was designed to examine the effects of paradoxical sleep deprivation (PSD) upon spatial working and reference memory. Adult male Sprague-Dawley rats were trained for 10 days in an eight-arm radial maze. Food rewards were available in four of the arms, while the other four arms were never baited. After each daily training session, different groups of rats were given 4 h of PSD, beginning either immediately, 4 h, or 8 h after the training experience. An additional group received PSD during the period 13-24 h following daily training. The group that received PSD for 4 h immediately following daily training showed significant impairment compared to the other groups, but the deficit was limited to the reference component of the task. This result suggests that PSD causes deficits only in long-term forms of spatial memory.

5-Hydroxytryptamine2 receptors modulate auditory filtering in the rat.

Sensory processing deficits are a hallmark of schizophrenia and can be demonstrated by recording auditory evoked potentials (AEPs) elicited in response to closely paired click stimuli. In nonschizophrenic humans, as well as in rats, the amplitude of the response to the second click is reduced (filtered) compared with the first. In contrast, schizophrenics, or rats treated with amphetamine, generate AEPs that have smaller amplitudes and show little or no reduction in the response to the second click. We sought to evaluate the role of 5-hydroxytryptamine2 5-HT2 receptors in auditory filtering. Male Sprague-Dawley rats were implanted with a skull screw electrode to permit chronic recording of AEPs from a point approximating human vertex. During subsequent recording sessions, pairs of clicks (a conditioning click followed by a test click) were presented 500 msec apart. Parameters of N40, a dominant midlatency component of the AEP, were examined to evaluate the effects of a 5-HT2 receptor agonist, (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI), and a 5-HT2 receptor antagonist, ketanserin. Systemic administration of ketanserin reduced sensory filtering in a dose-dependent manner. Conversely, DOI significantly improved filtering. In addition, DOI dose-dependently antagonized the disruption of filtering induced by administration of amphetamine (1.83 mg/kg i.p.). Taken together, these results indicate an important role for 5-HT2 receptors in the modulation of auditory filtering.

Kainic acid lesions in adult rats as a model of schizophrenia: changes in auditory information processing.

Previous studies have suggested that intracerebroventricular kainic acid injections alter brain anatomy and neurochemistry in a manner similar to what is observed in schizophrenic patients. Disturbances in sensory information processing are one of the major symptoms of schizophrenia. Thus, the present experiments were designed to evaluate the hypothesis that hippocampal damage, induced by administration of kainic acid, would alter the processing of auditory stimuli in a paired-click paradigm. Adult male Sprague-Dawley rats were implanted for surface recording of auditory evoked potentials. At the time of electrode implantation, the rats also received bilateral injections of either kainic acid or the vehicle solution. In vehicle-treated rats, the midlatency N40 component of the auditory evoked potential was diminished in amplitude by approximately 60% in response to the second of a pair of clicks delivered 0.5 s apart. By contrast, no reduction of the N40 wave evoked by the second click was observed in kainate-treated rats. Further, administration of haloperidol, a prototypical neuroleptic agent, did not improve this auditory processing dysfunction in kainate-treated animals. Loss of auditory filtering in the paired-click paradigm and a lack of response to haloperidol in this test are typically observed in schizophrenic humans. Thus, the present results demonstrate that kainate-lesioned rats possess a functional schizophrenia-like abnormality, further reinforcing the utility of this model system for studying the basic neurobiology of schizophrenia-induced sensory processing deficits.

Carrier mediated delivery of NGF: alterations in basal forebrain neurons in aged rats revealed using antibodies against low and high affinity NGF receptors.

The distribution of low and high affinity nerve growth factor (NGF) receptors was investigated in the basal forebrain during aging and NGF treatment. A peripheral administration model for NGF was utilized. NGF was conjugated to a transferrin receptor antibody (OX-26-NGF), and this conjugate was injected into the tail vein of aged Fischer 344 male rats (24 months) twice weekly for 5 weeks (equivalent to 50 microg of NGF/injection). Controls were injected with a non-conjugated mixture of OX-26 and NGF. The aged rats treated with conjugate showed a significant increase in cell size of p75- and trkA-immunoreactive neurons in the medial septal nucleus and vertical limb of the diagonal band as compared to controls. A significant increase in cell size of trkA-immunoreactive neurons was also observed in the horizontal limb of the diagonal band in rats treated with conjugate. Rats treated with conjugate also showed a significant increase in overall staining density for p75 and trkA antibodies in the medial septal nucleus as compared to controls. A significant increase in staining density of p75-immunoreactive structures was also observed in the vertical and horizontal limbs of the diagonal band. Therefore, treatment with OX-26-NGF conjugate has regional effects on both the low and high affinity NGF receptors in terms of cell body size and staining density in the basal forebrain of aged rats. The current findings support the idea that this delivery system might be useful in therapeutic approaches involving the delivery of neurotrophic factors and other large molecules into the brain.