Enhancement of memory processing in an inhibitory avoidance and radial maze task by post-training infusion of bombesin into the nucleus tractus solitarius

Bombesin attenuated ischemia-induced spatial cognitive and synaptic plasticity impairment associated with oxidative damage

The dysfunction of spatial cognition is a character to various neurological disorders and therapeutic strategy. However, it is limited to known risk factors clinically so far. Gastrin releasing peptide (GRP) signaling is a neuropeptide system mediating emotional memory events. However, the effects of GRP agonist on spatial cognition and hippocampal synaptic plasticity are rarely investigated, especially in pathologic condition. This study was designed to investigate the long-term effects of GRPR agonist, bombesin, against cognitive impairment induced by chronic cerebral ischemia in rats and its possible mechanisms. Our results revealed that bombesin administration (30 μg/kg/day, for 14 continuous days) significantly protected the cognitive and synaptic plasticity impairments as assessed by the Morris water maze and long-term potentiation tests. The mechanism studies demonstrated that bombesin significantly alleviated the decreased activity of total superoxide dismutase (T-SOD), catalase (CAT) and altered the increased the content of malondialdehyde (MDA). Besides, the decreased expression of synapse plasticity-related proteins, calcium- calmodulin- dependent protein kinase II (CaMKII) and synaptophysin (SYP) in the hippocampus were increased with drug treatment. In conclusion, bombesin could protect the oxidative stress and expression of proteins, which were important for synaptic plasticity and cognitive function impairment induced by chronic cerebral ischemia. Our study is presented to provide novel insights into the effects of bombesin on spatial learning and memory, which should be further explored as a potential drug in disorders involving deficits in cognitive function.

Chemical stimulation or glutamate injections in the nucleus of solitary tract enhance conditioned taste aversion

Taste memory depends on motivational and post-ingestional consequences after a single taste-illness pairing. During conditioned taste aversion (CTA), the taste and visceral pathways reach the nucleus of the solitary tract (NTS), which is the first relay in the CNS and has a vital function in receiving vagal chemical stimuli and humoral signals from the area postrema that receives peripheral inputs also via vagal afferent fibers. The specific aim of the present set of experiments was to determine if the NTS is involved in the noradrenergic and glutamatergic activation of the basolateral amygdala (BLA) during CTA. Using in vivo microdialysis, we examined whether chemical NTS stimulation induces norepinephrine (NE) and/or glutamate changes in the BLA during visceral stimulation with intraperitoneal (i.p.) injections of low (0.08 M) and high (0.3 M) concentrations of lithium chloride (LiCl) during CTA training. The results showed that strength of CTA can be elicited by chemical NTS stimulation (Ringer's high potassium solution; 110 mM KCl) and by intra-NTS microinjections of glutamate, immediately after, but not before, low LiCl i.p. injections that only induce a week aversive memory. However visceral stimulation (with low or high i.p. LiCl) did not induce significantly more NE release in the amygdala compared with the NE increment induced by NTS potassium depolarization. In contrast, high i.p. concentrations of LiCl and chemical NTS stimulation induced a modest glutamate sustained release, that it is not observed with low LiCl i.p. injections. These results indicate that the NTS mainly mediates the visceral stimulus processing by sustained releasing glutamate in the BLA, but not by directly modulating NE release in the BLA during CTA acquisition, providing new evidence that the NTS has an important function in the transmission of signals from the periphery to brain systems that process aversive memory formation.

Gastrin-releasing peptide receptor signaling in the integration of stress and memory

Neuropeptides act as signaling molecules that regulate a range of aspects of brain function. Gastrin-releasing peptide (GRP) is a 27-amino acid mammalian neuropeptide, homolog of the amphibian peptide bombesin. GRP acts by binding to the GRP receptor (GRPR, also called BB2), a member of the G-protein coupled receptor (GPCR) superfamily. GRP produced by neurons in the central nervous system (CNS) plays a role in synaptic transmission by activating GRPRs located on postsynaptic membranes, influencing several aspects of brain function. Here we review the role of GRP/GRPR as a system mediating both stress responses and the formation and expression of memories for fearful events. GRPR signaling might integrate the processing of stress and fear with synaptic plasticity and memory, serving as an important component of the set of neurobiological systems underlying the enhancement of memory storage by aversive information.

Gastrin-releasing peptide receptors in the central nervous system: role in brain function and as a drug target

Neuropeptides acting on specific cell membrane receptors of the G protein-coupled receptor (GPCR) superfamily regulate a range of important aspects of nervous and neuroendocrine function. Gastrin-releasing peptide (GRP) is a mammalian neuropeptide that binds to the GRP receptor (GRPR, BB2). Increasing evidence indicates that GRPR-mediated signaling in the central nervous system (CNS) plays an important role in regulating brain function, including aspects related to emotional responses, social interaction, memory, and feeding behavior. In addition, some alterations in GRP or GRPR expression or function have been described in patients with neurodegenerative, neurodevelopmental, and psychiatric disorders, as well as in brain tumors. Findings from preclinical models are consistent with the view that the GRPR might play a role in brain disorders, and raise the possibility that GRPR agonists might ameliorate cognitive and social deficits associated with neurological diseases, while antagonists may reduce anxiety and inhibit the growth of some types of brain cancer. Further preclinical and translational studies evaluating the potential therapeutic effects of GRPR ligands are warranted.

Memory modulation

Our memories are not all created equally strong: Some experiences are well remembered while others are remembered poorly, if at all. Research on memory modulation investigates the neurobiological processes and systems that contribute to such differences in the strength of our memories. Extensive evidence from both animal and human research indicates that emotionally significant experiences activate hormonal and brain systems that regulate the consolidation of newly acquired memories. These effects are integrated through noradrenergic activation of the basolateral amygdala that regulates memory consolidation via interactions with many other brain regions involved in consolidating memories of recent experiences. Modulatory systems not only influence neurobiological processes underlying the consolidation of new information, but also affect other mnemonic processes, including memory extinction, memory recall, and working memory. In contrast to their enhancing effects on consolidation, adrenal stress hormones impair memory retrieval and working memory. Such effects, as with memory consolidation, require noradrenergic activation of the basolateral amygdala and interactions with other brain regions.

Effects of gastrin-releasing peptide agonist and antagonist administered to the basolateral nucleus of the amygdala on conditioned fear in the rat

RATIONALE

Bombesin (BB)-like peptides have been shown to affect neuroendocrine and neural functions related to the stress response and the modulation of conditioned fear. In line with this view, central administration of gastrin-releasing peptide (GRP; a mammalian analogue of BB) or its receptor antagonist (D-Tpi6, Leu13 psi[CH2NH]-Leu14) BB(6-14) (RC-3095) modulates conditioned fear.

OBJECTIVE

The present study examined the effects of bilateral infusions of GRP or its receptor antagonist (RC-3095) into the basolateral nucleus of the amygdala (BLA) on the conditioned emotional response.

METHODS

The effects of GRP (150, 300, and 600 ng/0.5 microl) and/or RC-3095 (50, 500, and 1,000 ng/0.5 microl) on contextual and cued fear conditioning were assessed following direct bilateral infusion of these compounds into the BLA.

RESULTS

Both GRP and RC-3095 (all doses) reduced freezing during the contextual testing period but did not influence responding in the cued test. Although both compounds reduced freezing in the contextual paradigms, at a moderate dose pretreatment with RC-3095 attenuated the GRP-elicited decrease in contextual freezing.

CONCLUSIONS

It appears that manipulation of GRP at the BLA may influence the expression of learned fear and that these effects preferentially influence contextual versus cue-dependent emotional responses.

Targeting the gastrin-releasing peptide receptor pathway to treat cognitive dysfunctionassociated with Alzheimer's Disease

Increasing evidence indicates that bombesin (BB)-like peptides (BLPs), such as the gastrin-releasing peptide (GRP) and its receptor (GRPR), might play a role in neurological and psychiatric disorders. The present study reviews findings from animal and human studies suggesting that the GRPR should be considered a target for the treatment of cognitive dysfunction in patients with Alzheimer’s disease (AD). Abnormalities in GRPR-triggered signaling have been described in both fibroblasts from patients with AD, and in transgenic mouse models of AD. Pharmacological and genetic preclinical studies have indicated that BLPs and the GRPR are importantly involved in regulating cognitive function. Moreover, drugs acting at the GRPR have been shown to enhance memory and ameliorate cognitive dysfunction in experimental models of amnesia associated with AD. Taken together, these findings support the view that the GRPR is a novel therapeutic target for the treatment of memory deficits associated with AD.

Basic Fibroblast Growth Factor Prevents the Memory Impairment Induced by Gastrin-Releasing Peptide Receptor Antagonism in Area CA1 of the Rat Hippocampus

Increasing evidence indicates that the gastrin-releasing peptide receptor (GRPR) is implicated in regulating synaptic plasticity and memory formation in the hippocampus and other brain areas. However, the molecular mechanisms underlying the memory-impairing effects of GRPR antagonism have remained unclear. Here we report that basic fibroblast growth factor (bFGF/FGF-2) rescues the memory impairment induced by GRPR antagonism in the rat dorsal hippocampus. The GRPR antagonist [D-Tpi(6), Leu(13) psi(CH(2)NH)-Leu(14)] bombesin (6-14) (RC-3095) at 1.0 microg impaired, whereas bFGF at 0.25 microg enhanced, 24 h retention of inhibitory avoidance (IA) when infused immediately after training into the CA1 hippocampal area in male rats. Coinfusion with an otherwise ineffective dose of bFGF blocked the memory-impairing effect of RC-3095. These findings suggest that the memory-impairing effects of GRPR antagonists might be partially mediated by an inhibition in the function and/or expression of neuronal bFGF or diminished activation of intracellular protein kinase pathways associated with bFGF signaling.

Opposite effects of low and high doses of the gastrin-releasing peptide receptor antagonist RC-3095 on memory consolidation in the hippocampus: possible involvement of the GABAergic system

Although the gastrin-releasing peptide receptor (GRPR) has recently emerged as a system importantly involved in regulating memory formation, the role of hippocampal GRPRs in memory remains controversial. The present study examined the effects of GRPR antagonism on memory consolidation in area CA1 of the hippocampus. Male Wistar rats received bilateral infusions of the GRPR antagonist [D-Tpi6, Leu13 psi(CH2NH)-Leu14] bombesin (6-14) (RC-3095; 1, 3, or 10 microg/side) into the dorsal hippocampus immediately after inhibitory avoidance (IA) training. RC-3095 at 1 microg impaired, whereas the dose of 10 microg enhanced, 24-h IA retention. A second experiment showed that the RC-3095-induced enhancement of memory consolidation was prevented by pretraining infusion of an otherwise ineffective dose of the gamma-aminobutyric acid type A (GABA(A)) receptor agonist muscimol. The results indicate that high doses of GRPR antagonists can induce enhancement of memory consolidation in the hippocampus. In addition, the memory-enhancing effect of GRPR antagonists might be mediated by inhibition of GABAergic transmission.

Molecular mechanisms mediating gastrin-releasing peptide receptor modulation of memory consolidation in the hippocampus

Although the gastrin-releasing peptide-preferring bombesin receptor (GRPR) has been implicated in memory formation, the underlying molecular events are poorly understood. In the present study, we examined interactions between the GRPR and cellular signaling pathways in influencing memory consolidation in the hippocampus. Male Wistar rats received bilateral infusions of bombesin (BB) into the dorsal hippocampus immediately after inhibitory avoidance (IA) training. Intermediate doses of BB enhanced, whereas a higher dose impaired, 24-h IA memory retention. The BB-induced memory enhancement was prevented by pretraining infusions of a GRPR antagonist or inhibitors of protein kinase C (PKC), mitogen-activated protein kinase (MAPK) kinase and protein kinase A (PKA), but not by a neuromedin B receptor (NMBR) antagonist. We next further investigated the interactions between the GRPR and the PKA pathway. BB-induced enhancement of consolidation was potentiated by coinfusion of activators of the dopamine D1/D5 receptor (D1R)/cAMP/PKA pathway and prevented by a PKA inhibitor. We conclude that memory modulation by hippocampal GRPRs is mediated by the PKC, MAPK, and PKA pathways. Furthermore, pretraining infusion of BB prevented beta-amyloid peptide (25-35)-induced memory impairment, supporting the view that the GRPR is a target for the development of cognitive enhancers for dementia.

Non-associative learning and anxiety in rats treated with a single systemic administration of the gastrin-releasing peptide receptor antagonist RC-3095

The gastrin-releasing peptide receptor (GRPR) has been implicated in the modulation of emotionally-motivated memory. In the present study, we investigated the role of the GRPR on non-emotional, non-associative memory, and anxiety. Adult male Wistar rats were given a systemic injection of the GRPR antagonist [D-Tpi6, Leu(13) psi(CH2NH)-Leu14] bombesin (6-14) (RC-3095) (0.2, 1.0 or 5.0mg/kg) 30 min before exposure to an open field or an elevated plus maze. Habituation to the open field was tested in a retention trial carried out 24 h after the first exposure to the open field. Rats given RC-3095 at the doses of 1.0 or 5.0mg/kg showed impaired habituation. Animals treated with 5.0mg/kg of RC-3095 spent significantly more time in the closed arms of the elevated plus maze. No effects of RC-3095 on locomotion or exploratory behavior were observed. The results implicate the GRPR in the regulation of non-emotional, non-associative memory as well as in anxiety.

Immunohistochemical localization of gastrin-releasing peptide receptor in the mouse brain

Gastrin-releasing peptide (GRP) is a mammalian bombesin (BN)-like peptide that binds with high affinity to the GRP receptor (GRP-R). Previous behavioral studies using mice and rats showed that the GRP/GRP-R system mediates learning and memory by modulating neurotransmitter release in the local GABAergic network of the amygdala and the nucleus tractus solitarius (NTS). To date, the precise distribution of GRP-R in the brain has not been elucidated. We used a synthetic peptide derived from mouse GRP-R to generate affinity-purified antibodies to GRP-R and used immunohistochemistry to determine the distribution of GRP-R in the mouse brain. The specificity of anti-GRP-R antibody was confirmed in vitro using COS-7 cells transiently expressing GRP-R and in vivo using GRP-R-deficient and wild-type mouse brain sections. GRP-R immunoreactivity was widely distributed in the isocortex, hippocampal formation, piriform cortex, amygdala, hypothalamus, and brain stem. In particular, GRP-R immunoreactivity was observed in the lateral (LA), central, and basolateral amygdaloid (BLA) nuclei and NTS, which are important regions for memory performance. Double-labeling immunohistochemistry demonstrated that subpopulations of GRP-R are present in GABAergic neurons in the amygdala. Consequently, GRP-R immunoreactivity was observed in the GABAergic neurons of the limbic region. These anatomical results provide support for the idea that the GRP/GRP-R system mediates memory performance by modulating neurotransmitter release in the local GABAergic network.

Glucose improvement of memory: a review

The memory-improving action of glucose has now been studied for almost 20 years and the study of this phenomenon has led to a number of important developments in the understanding of memory, brain physiology and pathological consequences of impaired glucose tolerance. Glucose improvement of memory appears to involve two optimal doses in animals (100 mg/kg and 2 g/kg) that may correspond to two physiological mechanisms underlying glucose effects on memory. In humans, there have been few dose-response studies so the existence of more than one effective dose in humans is uncertain. Many tasks are facilitated by glucose in humans but tasks that are difficult to master or involve divided attention are improved more readily that easier tasks. There are a number of hypotheses about the physiological bases of the memory-improving action of glucose. Peripheral glucose injections could alleviate localized deficits in extracellular glucose in the hippocampus. These localized deficits may be due to changes in glucose transporters in that structure. Because certain neurotransmitters such as acetylcholine are directly dependent on the glucose supply for their synthesis, glucose is thought to facilitate neurotransmitter synthesis under certain circumstances. However, these hypotheses cannot account for the specificity of the dose-response effect of glucose. A number of peripheral mechanisms have been proposed, including the possibility that glucose-sensitive neurons in the brain or in the periphery may serve as glucose sensors and eventually produce neural changes that would facilitate memory processing. These latter results could be of importance because the mechanisms they suggest appear to be dose-dependent, a crucial characteristic to explain the dose-dependent effects of glucose. There may be an advantage to develop hypotheses that include both peripheral and central actions of glucose. There is evidence that impaired glucose regulation is associated with impaired cognition, particularly episodic memory. This impairment is minimal in young people but increases in older people (65 years and over) where it may compound other aging processes leading to reduced brain function. A small number of studies showed that glucose improvement of memory is associated with poor glucose regulation although this may not be the case for diabetic patients. Results of a few studies also suggest that drug treatments that improve glucose regulation also produce cognitive improvement in diabetic patients.

Bombesin/gastrin-releasing peptide receptors in the basolateral amygdala regulate memory consolidation

Several receptor and intracellular signalling systems in the basolateral amygdala (BLA) regulate memory formation. In the present study, we show that bombesin/gastrin-releasing peptide (GRP) receptors in the BLA are involved in the consolidation of affectively motivated memory. Adult male rats were trained in a single-trial step-down inhibitory avoidance task and tested for retention 24 h later. Post-training systemic injection of the bombesin/GRP receptor antagonist (D-Tpi6, Leu13 psi[CH2NH]-Leu14) bombesin (6-14) (RC-3095) impaired memory retention. In rats implanted under thionembutal anaesthesia with guide cannulae aimed at the BLA, post-training bilateral infusion of RC-3095 into the BLA dose-dependently impaired retention. Pre-training unilateral muscimol inactivation of the BLA blocked the memory-impairing effect of post-training systemic administration of RC-3095. The results suggest that bombesin/GRP receptors in the BLA are involved in the consolidation of aversive memory, and the BLA mediates the memory-impairing effect of systemic bombesin/GRP receptor blockade.

Distributions of two chicken bombesin receptors, bombesin receptor subtype-3.5 (chBRS-3.5) and gastrin-releasing peptide receptor (chGRP-R) mRNAS in the chicken telencephalon

Bombesin (BN)-like peptide receptors are known to be essential to the regulation of not only homeostasis, including feeding behavior, but also of emotional systems in mammal. Recently, two novel BN receptors, chicken BN-like peptide receptor subtype-3.5 (chBRS-3.5) and gastrin-releasing peptide receptor (chGRP-R), have been identified. Here, we report the localizations of these receptors' mRNAs in the chick brain through development using in situ hybridization. First, chBRS-3.5 mRNA signals were found in the dorsal ventricular ridge at embryonic day (ED) 9. Strong signals were observed in the hyperpallium accessorium, nidopallium and nucleus basorostralis pallii, and moderate signals were found in the hippocampus, cortex piriformis, hyperpallium intercalatum, area temporo-parieto-occipitalis, nucleus striae terminalis lateralis, nucleus olfactorius anterior and organum septi lateralis at ED16. This wide expression in the pallium persisted during posthatch periods. Abundant expressions in the hyperpallium, nidopallium, considered to be similar to the mammalian cortex, as well as in the hippocampus, indicate participation of these molecules in the processing of sensory information, motor function, learning and memory. Telencephalic areas devoid of chBRS-3.5 signals were the entopallium, arcopallium anterius, globus pallidus, nucleus intrapeduncularis, tuberculum olfactorius, nucleus septalis lateralis, hypothalamic and thalamic areas. In contrast to chBRS-3.5, chGRP-R mRNA signals were found in the pallidum at ED5 and 9. At ED16, chGRP-R mRNA signals were localized in the medial striatum and hypothalamus. GRP-R expression in the hypothalamic region was phylogenically conserved. Thus, chBRS-3.5 mRNA signals were distributed in a broader region and were more intense than chGRP-R mRNA. Taken together, chGRP-R and chBRS-3.5 mRNA occurred in similar regions of mammals that express GRP-R. BN/GRP-immunoreactive neurons and varicosities were found mainly in the pallium, especially in the hyperpallium accessorium and nidopallium, and this distribution coincided with that of chBRS-3.5 mRNA. This result suggests that the endogenous ligands for chBRS-3.5 were likely BN-like peptides produced in the pallium.

Intrahippocampal infusion of the bombesin/gastrin-releasing peptide antagonist RC-3095 impairs inhibitory avoidance retention

Bombesin (BN)-like peptides regulate cell proliferation and cancer growth as well as neuroendocrine and neural functions. We evaluated the effects of the BN/gastrin-releasing peptide (GRP) antagonist RC-3095 on memory formation. Male Wistar rats were given a bilateral infusion of saline or RC-3095 (0.2, 1.0 or 5.0 microg) into the dorsal hippocampus either immediately or 2 h after training in an inhibitory avoidance (IA) task. Retention test trials were carried out 1.5 h (short-term memory) and 24 h (long-term memory) after training. RC-3095 impaired both short- and long-term retention only when given immediately after training. The results suggest that the hippocampal BN/GRP receptor system modulates IA memory formation.

Blockade of bombesin-like peptide receptors impairs inhibitory avoidance learning in mice

Several studies reported that peripheral administration of bombesin (BN) and gastrin-releasing peptide (GRP) improved some forms of memory performance. In the present study, we examined the role of endogenous BN-like peptide(s) for the acquisition of inhibitory avoidance learning in mice using BN-like peptide receptor antagonists. An administration of [Leu(13)-(psi-CH(2)NH)-Leu(14)]BN (antagonizes GRP-R>neuromedin B receptor (NMB-R)) impaired the performance of inhibitory avoidance learning in all doses (16, 32, 64 nmol/kg). While the effect was somewhat lesser than [Leu(13)-(psi-CH(2)NH)-Leu(14)]BN, BIM23127 (antagonizes NMB-R>GRP-R) also impaired performance in a moderate dose (32 nmol/kg). These results showed that endogenous BN-like peptides have some role(s) for the modulation of learning and memory, and suggest that NMB/NMB-R pathway may also be involved in the memory acquisition and modulation as well as GRP/GRP-R pathway.

Bombesin and its family of peptides: prospects for the treatment of obesity

Bombesin, its family of bombesin-like peptides, and many other peptides/hormones modulate biological and behavioral functions in animals. Among the wide variety of functions influenced by bombesin/bombesin-like peptides, the most prominent may be their role in feeding-related behavior. Over many years, intensive psychopharmacological studies have addressed the mechanisms by which these peptides induce feeding suppression, and the results suggest the applicability of bombesin/bombesin-like peptides for the treatment of eating disorders and/or obesity in humans. Recent studies using gene-knockout mice also shed new light on the relationship between bombesin/bombesin-like peptides and feeding behavior. In addition, genetic analyses of the possible links between bombesin/bombesin-like peptides/receptors and human obesity have also been undertaken. Here, we briefly review the literature pertaining to the relationship between bombesin/bombesin-like peptides and feeding behavior-with particular attention to human subjects-and discuss the pharmacotherapeutic potential of bombesin/bombesin-like peptides with regard to obesity.

A novel proline rich bombesin-related peptide (PR-bombesin) from toad Bombina maxima

A novel bombesin-related peptide was isolated from skin secretions of Chinese red belly toad Bombina maxima. Its primary structure was established as pGlu-Lys-Lys-Pro-Pro-Arg-Pro-Pro-Gln-Trp-Ala-Val-Gly-His-Phe-Met-NH(2.) The amino-terminal (N-terminal) 8-residue segment comprising four prolines and three basic residues is extensively different from bombesins from other Bombina species. The peptide was thus named proline rich bombesin (PR-bombesin). PR-bombesin was found to elicit concentration-dependent contractile effects in the rat stomach strip, with both increased potency and intrinsic activity as compared with those of [Leu(13)]bombesin. Analysis of different bombesin cDNA structures revealed that an 8 to 14- nucleotide fragment replacement in the peptide coding region (TGGGGAAT in the cDNAs of multiple bombesin forms from Bombina orientalis and CACCCCGGCCACCC in the cDNA of PR-bombesin) resulted in an unusual Pro-Pro-Arg-Pro-Pro motif in the N-terminal part of PR-bombesin.

Posttraining administration of gastrin-releasing peptide improves memory loss in scopolamine- and hypoxia-induced amnesic mice

We examined memory improvement with respect to the effects of gastrin-releasing peptide (GRP) in male C57BL/6J mice under conditions of experimentally induced amnesia. GRP was administered following training in a one-trial passive avoidance test. In Experiment 1, the drug scopolamine (1 or 2 mg/kg, ip) was used to induce amnesia prior to training, and GRP (32 nmol/kg, ip) or saline (control) was administered immediately after training. Results indicate that GRP at this dose improved memory only when the dosage of scopolamine was relatively low (1 mg/kg). In Experiment 2, CO2-induced amnesia was employed. Mice were placed in a chamber filled with CO2 or air (control) immediately after acquisition training. Subsequently, they were administered either saline or GRP (32 nmol/kg, ip). Significantly longer light-dark latency was observed in all mice that received GRP (both experimental and control groups). In total, our results indicate that the effect of GRP at this dose on the improvement of impaired memory is dependent on the degree of impairment. Furthermore, because CO2-induced hypoxia is known to decrease acetylcholine release in the brain, our results also suggest that GRP and its receptor may interact with the cholinergic system in the central nervous system.

Adrenergic activation of the nucleus tractus solitarius potentiates amygdala norepinephrine release and enhances retention performance in emotionally arousing and spatial memory tasks

It is well documented that noradrenergic systems in the amygdala modulate memory formation, however, less research has examined how sources of limbic norepinephrine contribute to this process. The amygdala receives a dense supply of norepinephrine from neurons in the nucleus of the solitary tract (NTS). The present experiments examined whether adrenergic activation of these NTS neurons affects memory in learning tasks that are sensitive to amygdala norepinephrine release. Separate groups of male Sprague-Dawley rats were trained in either an emotionally arousing or spatial memory task. They then received vehicle or the adrenergic agonist epinephrine (50, 125, or 250 ng/0.5 microl) into the NTS. Rats given the 125 ng dose had significantly longer retention latencies on a 48 h inhibitory avoidance retention test and made a significantly higher percentage of correct responses on an 18 h delayed radial maze retention test. A third experiment using in vivo microdialysis and high performance liquid chromatography (HPLC) demonstrated that intra-NTS infusion of a memory-enhancing dose of epinephrine potentiated amygdala norepinephrine release. Collectively, these results suggest that stimulation of the NTS contributes to memory processing by influencing noradrenergic systems in the amygdala.

Glucocorticoid receptor activation in the rat nucleus of the solitary tract facilitates memory consolidation: involvement of the basolateral amygdala

These experiments examined the involvement of glucocorticoid receptors (GRs or type II) located in the A2-noradrenergic cell group of the rat nucleus of the solitary tract (NTS) in modulating memory storage. Bilateral intra-NTS infusions (0.5 microL) of the specific GR agonist RU 28362 (11beta, 17beta-dihydroxy-6, 21-dimethyl-17alpha-pregna-4,6-trien-20yn-3-one), in doses ranging from 0.01 to 10.0 ng, immediately after inhibitory avoidance training produced a dose-dependent enhancement of 48 h retention performance. Infusions of 0.1 or 1.0 ng of the agonist enhanced retention, whereas lower or higher doses were ineffective. Post-training infusions of the GR antagonist RU 38486 [17beta-hydroxy-11beta-(4-dimethylaminophenyl)-17alpha-(1-pr opynyl)-o estra-4,9-dien-3-one, 0.01-10.0 ng] into the NTS did not significantly affect retention performance, but shifted the dose-response effects of post-training systemic injections of the synthetic glucocorticoid dexamethasone to the right. These results indicate that activation of GRs in the NTS can influence memory formation for inhibitory avoidance training, and suggest that the effects of circulating glucocorticoids on memory are mediated, in part, by an activation of GRs in the NTS. Additionally, pretraining infusions of the beta1-adrenergic antagonist atenolol (0.5 microg in 0.2 microL) into the basolateral nucleus of the amygdala (BLA), a brain structure which receives noradrenergic projections from the NTS and is implicated in memory storage modulation, blocked the memory-enhancing effects of the GR agonist (1.0 ng) infused into the NTS. These findings provide evidence that memory storage is modulated by glucocorticoid binding to GRs in noradrenergic cell bodies in the NTS and suggest that these modulatory effects are conveyed by ascending projections to the BLA.

Sustained bombesin exposure results in receptor down-regulation and tolerance to the chronic but not acute effects of bombesin on ingestion

Acute intracerebroventricular (i.c.v.) administration of bombesin (BN) reduces meal intake in fasted rats. The overall objective of the present study was to determine the behavioral and other ingestive effects of prolonged central administration of BN. In the first experiment, we characterized the effects of 8-day sustained central administration of BN (0, 0.01, 0.05, 0.1 and 5 micrograms/0.5 microliter/h) or its antagonist (BIM-26226; 0, 0.005, 0.05, 0.5 and 5.0 micrograms/0.5 microliter/h), in free feeding rats. Each dose was delivered over 48 h, in an ascending sequence. At the higher doses, the BN-exposed rats consumed significantly less food whereas those exposed to the BN antagonist ingested significantly more food than the controls (saline exposed), during the dark phase. Due to the limited 48-h exposure to these higher doses, we further investigated the effects of more sustained BN exposure. Thus BN was infused over a 7-day period, at a rate of 0.25 microgram BN/0.5 microliter/h. In this latter study, we determined the effects of sustained BN exposure on a) daily spontaneous ingestive pattern, b) the rat's ingestive and other behavioral responses to a subsequent acute BN (i.c.v.) challenge and, c) BN receptor binding profile in various brain regions. Over the initial 2 days of chronic infusion, BN significantly suppressed spontaneous ingestion, and this effect dissipated by 72 h. Upon acute challenge with bolus injection of BN (0.25 microgram; i.c.v.), both chronically BN-treated and control rats responded by decreased feeding and enhanced grooming behaviors. In terms of effects at the receptor level, chronic BN exposure resulted in significant down-regulation (reduced BN/GRP receptor density) at the PVN and the hippocampal dentate gyrus. These findings represent the first demonstration of concomitant behavioral and receptor based changes consequent to sustained exposure to BN. These data suggest that tolerance to feeding suppressant effects of BN develops gradually, which in part may be mediated by down-regulation of BN/GRP receptors at specific brain loci. Our results also suggest that despite the development of tolerance to the chronic or sustained effects of BN exposure, animals still respond robustly to acute fluctuations in peptide levels.