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

Bombesins: A New Frontier in Hybrid Compound Development

Recently, bombesin (BN) and its analogs have attracted much attention as excellent anticancer agents because they interact with specific receptors widely distributed on the surface of various cancer cells. However, their biological properties proceed far beyond this, given a broad spectrum of activity. Bombesin receptor ligands are effective drugs for the treatment of rheumatoid arthritis or gastrointestinal diseases. However, most diseases are complex, and the use of polytherapy may lead to pharmacokinetic and pharmacodynamic drug-drug interactions, resulting in side effects. Therefore, there is a need to develop effective compounds that also contain BN or its analogs, which are combined with other structural entities, thus generating a so-called hybrid drug. Hybrid drugs that contain bombesin pharmacophore(s) may be proposed as a solution to the problem of polytherapy or the lack of an effective cure. Such structures have now demonstrated the desired efficacy, though information on these aforementioned compounds is relatively scarce. Therefore, our paper aims to encourage researchers to focus on bombesins. Herein, we indicate that the hybrid approach should also be firmly applied to bombesins and the BN receptor family. This paper's structure is divided into two main sections demonstrating bombesins and their properties, as well as recent data on bombesin-based hybrid compounds and their potential usefulness in medicine. Overall, it refers to the discovery and synthesis of modified bombesin-based hybrid compounds.

Gastrin-releasing peptide regulates fear learning under stressed conditions via activation of the amygdalostriatal transition area

The amygdala, a critical brain region responsible for emotional behavior, is crucially involved in the regulation of the effects of stress on emotional behavior. In the mammalian forebrain, gastrin-releasing peptide (GRP), a 27-amino-acid mammalian neuropeptide, which is a homolog of the 14-amino-acid amidated amphibian peptide bombesin, is highly expressed in the amygdala. The levels of GRP are markedly increased in the amygdala after acute stress; therefore, it is known as a stress-activated modulator. To determine the role of GRP in emotional behavior under stress, we conducted some behavioral and biochemical experiments with GRP-knockout (KO) mice. GRP-KO mice exhibited a longer freezing response than wild-type (WT) littermates in both contextual and auditory fear (also known as threat) conditioning tests only when they were subjected to acute restraint stress 20 min before the conditioning. To identify the critical neural circuits associated with the regulation of emotional memory by GRP, we conducted Arc/Arg3.1-reporter mapping in the amygdala with an Arc-Venus reporter transgenic mouse line. In the amygdalostriatal transition area (AST) and the lateral side of the basal nuclei, fear conditioning after restraint stress increased neuronal activity significantly in WT mice, and GRP KO was found to negate this potentiation only in the AST. These results indicate that the GRP-activated neurons in the AST are likely to suppress excessive fear expression through the regulation of downstream circuits related to fear learning following acute stress.

Bombesin-like peptide recruits disinhibitory cortical circuits and enhances fear memories

Disinhibitory neurons throughout the mammalian cortex are powerful enhancers of circuit excitability and plasticity. The differential expression of neuropeptide receptors in disinhibitory, inhibitory, and excitatory neurons suggests that each circuit motif may be controlled by distinct neuropeptidergic systems. Here, we reveal that a bombesin-like neuropeptide, gastrin-releasing peptide (GRP), recruits disinhibitory cortical microcircuits through selective targeting and activation of vasoactive intestinal peptide (VIP)-expressing cells. Using a genetically encoded GRP sensor, optogenetic anterograde stimulation, and trans-synaptic tracing, we reveal that GRP regulates VIP cells most likely via extrasynaptic diffusion from several local and long-range sources. In vivo photometry and CRISPR-Cas9-mediated knockout of the GRP receptor (GRPR) in auditory cortex indicate that VIP cells are strongly recruited by novel sounds and aversive shocks, and GRP-GRPR signaling enhances auditory fear memories. Our data establish peptidergic recruitment of selective disinhibitory cortical microcircuits as a mechanism to regulate fear memories.

Serum Metabolites Differentiate Amnestic Mild Cognitive Impairment From Healthy Controls and Predict Early Alzheimer's Disease via Untargeted Lipidomics Analysis

Background and Aim: Alzheimer's disease (AD) is the most common type of dementia and presents with metabolic perturbations early in the disease process. In order to explore biomarkers useful in predicting early AD, we compared serum metabolites among patients suffering different stages of AD.

Methods: We recruited 107 participants including 23 healthy controls (HC), 21 amnestic mild cognitive impairment (aMCI), 24 non-amnestic mild cognitive impairment (naMCI) and 39 AD patients. Via liquid chromatography-mass spectrometry based serum untargeted lipidomics analysis, we compared differences in serum lipid metabolites among these patient groups and further elucidated biomarkers that differentiate aMCI from HC.

Results: There were significant differences of serum lipid metabolites among the groups, and 20 metabolites were obtained under negative ion mode from HC and aMCI comparison. Notably, 16:3 cholesteryl ester, ganglioside GM3 (d18:1/9z-18:1) and neuromedin B were associated with cognition and increased the predictive effect of aMCI to 0.98 as revealed by random forest classifier. The prediction model composed of MoCA score, 16:3 cholesteryl ester and ganglioside GM3 (d18:1/9z-18:1) had good predictive performance for aMCI. Glycerophospholipid metabolism was a pathway common among HC/aMCI and aMCI/AD groups.

Conclusion: This study provides preliminary evidence highlighting that 16:3 cholesteryl ester were useful for AD disease monitoring while ganglioside GM3 (d18:1/9z-18:1) and neuromedin B discriminated aMCI from HC, which can probably be applied in clinic for early predicting of AD.

Gastrin-releasing peptide attenuates fear memory reconsolidation

BACKGROUND

Gastrin Releasing Peptide (GRP) may play a role in fear learning. The GRP Receptor is expressed in the basolateral amygdala and hippocampus, and central administration of GRP mediates fear learning. The effects of GRP on reconsolidation, however, have been minimally explored. Reconsolidation, the process by which formed memories are rendered labile following recall, provides a window of opportunity for pharmacological intervention. Although evidence suggests the window of opportunity to alter reactivated consolidation memory can be as long as 6 h, shorter intervals have not been extensively investigated.

METHOD

Male Sprague-Dawley rats received six 1.0 mA continuous footshocks. 24 h later, were re-exposed to the context (shock chamber). Immediately following memory retrieval rats received i.p. injection of GRP (10 nmol/kg), Flumazenil (1 mg/kg), GRP + Flumazenil (10 nmol/kg GRP with 1 mg/kg Flumazenil), or Vehicle. Other groups received GRP or Vehicle at 0, 10, 30, or 60 min post-reactivation. 24 h and 5 days later rats were assessed for fear expression upon re-exposure to the fearful stimulus.

RESULTS

GRP significantly attenuated the reconsolidation of learned fear when administered immediately (but not 10 min or longer) following recall. Some of the variability in the impact of treatments aimed at disrupting fear memories may be governed, in part, by the time-frame of the reconsolidation window. Our results indicate that the effect of immediate administration persisted for at least 5 days. Co-administration of benzodiazepine-receptor antagonist Flumazenil blocked this effect, suggesting the effect is mediated via a GABAergic mechanism.

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.

Docosahexaenoic Acid Helps to Lessen Extinction Memory in Rats

Abstract: Memory extinction is referred to as a learning process in which a conditioned response (CR) progressively reduces over time as an animal learns to uncouple a response from a stimulus. Extinction occurs when the rat is placed into a context without shock after training. Docosahexaenoic acid (DHA, C22:6, n-3) is implicated in memory formation in mammalian brains. In a two-way active shuttle-avoidance apparatus, we examined whether DHA affects the extinction memory and the expression of brain cognition-related proteins, including gastrin-releasing peptide receptor (GRPR), brain-derived neurotrophic factor receptor (BDNFR) tyrosine kinase receptor B (TrKB), and N-methyl-d-aspartate receptor (NMDAR) subunits NR2A and NR2B. Also, the protein levels of GRP, BDNF, postsynaptic density protein-95 (PSD-95), and vesicular acetylcholine transporter (VAChT), and the antioxidative potentials, in terms of lipid peroxide (LPO) and reactive oxygen species (ROS), were examined in the hippocampus. During the acquisition phase, the rats received a conditioned stimulus (CS-tone) paired with an unconditioned stimulus (UCS foot shock) for three consecutive days (Sessions S1, S2, and S3, each consisting of 30-trials) after 12 weeks of oral administration of DHA. After a three-day interval, the rats were re-subjected to two extinction sessions (S4, S5), each comprising 30 trials of CS alone. During the acquisition training in S1, the shock-related avoidance frequency (acquisition memory) was significantly higher in the DHA-administered rats compared with the control rats. The avoidance frequency, however, decreased with successive acquisition trainings in sessions S2 and S3. When the rats were subjected to the extinction sessions after a break for consolidation, the conditioned response (CR) was also significantly higher in the DHA-administered rats. Interestingly, the freezing responses (frequency and time) also significantly decreased in the DHA-administered rats, thus suggesting that a higher coping capacity was present during fear stress in the DHA-administered rats. DHA treatments increased the mRNA levels of GRPR, BDNF receptor TrKB, and NMDAR subunit NR2B. DHA also increased the protein levels of GRP, BDNF, PSD-95, and VAChT, and the antioxidative potentials in the hippocampus. These results suggest the usefulness of DHA for treating stress disorders.

Gastrin-releasing peptide facilitates glutamatergic transmission in the hippocampus and effectively prevents vascular dementia induced cognitive and synaptic plasticity deficits

Neuronal gastrin-releasing peptide (GRP) has been proved to be an important neuromodulator in the brain and involved in a variety of neurological diseases. Whether GRP could attenuate cognition impairment induced by vascular dementia (VD) in rats, and the mechanism of synaptic plasticity and GRP's action on synaptic efficiency are still poorly understood. In this study, we first investigated the effects of GRP on glutamatergic transmission with patch-clamp recording. We found that acute application of GRP enhanced the excitatory synaptic transmission in hippocampal CA1 neurons via GRPR in a presynaptic mechanism. Secondly, we examined whether exogenous GRP or its analogue neuromedin B (NMB) could prevent VD-induced cognitive deficits and the mechanism of synaptic plasticity. By using Morris water maze, long-term potentiation (LTP) recording, western blot assay and immunofluorescent staining, we verified for the first time that GRP or NMB substantially improved the spatial learning and memory abilities in VD rats, restored the impaired synaptic plasticity and was able to elevate the expression of synaptic proteins, synaptophysin (SYP) and CaMKII, which play pivotal roles in synaptic plasticity. These results suggest that the facilitatory effects of GRP on glutamate release may contribute to its long-term action on synaptic efficacy which is essential in cognitive function. Our findings present a new entry point for a better understanding of physiological function of GRP and raise the possibility that GRPR agonists might ameliorate cognitive deficits associated with neurological diseases.

Long-term behavioral effects of neonatal blockade of gastrin-releasing peptide receptors in rats: similarities to autism spectrum disorders

Gastrin releasing peptide, the mammalian counterpart of the amphibian peptide, bombesin, has been increasingly implicated in regulating normal brain function as well as in the pathogenesis of psychiatric and/or neurodevelopmental disorders. We have previously shown that the neonatal blockade of the gastrin-releasing peptide receptor (GRPr) in rats produces long-lasting consequences during central nervous system development that are commonly observed in neurodevelopmental disorders such as autism spectrum disorders. The present investigation assessed in further detail, long-term behavioral effects of neonatal GRPr blockade. During postnatal days 1-10, male Wistar rat pups (n=5-10/litter) were injected (subcutaneously) with the GRPr antagonist, RC-3095 (1 mg/kg), or a vehicle (control), twice daily. Following the drug treatment regimen, several behaviors were assessed (starting on postnatal day 14) including specific social behaviors (namely, group huddling characteristics, social interaction, and social approach), restrictive/repetitive and stereotyped behaviors (y-maze, repetitive novel object contact task, observation for stereotypies) and anxiety/fear-related responses (open field, elevated plus maze and contextual fear conditioning). Rats treated neonatally with RC-3095 showed reduced sociability, restrictive interests, motor stereotypies and enhanced learned fear response compared to the controls (vehicle-treated rats). These behavioral abnormalities are consistent with those observed in autism spectrum disorders and provide further evidence that neonatal blockade of GRPr could potentially serve as a useful model to gain a better understanding of the underlying neurodevelopmental disruptions contributing to the expression of autism-relevant phenotypes.

Stress and eating: a dual role for bombesin-like peptides

The current obesity “epidemic” in the developed world is a major health concern; over half of adult Canadians are now classified as overweight or obese. Although the reasons for high obesity rates remain unknown, an important factor appears to be the role stressors play in overconsumption of food and weight gain. In this context, increased stressor exposure and/or perceived stress may influence eating behavior and food choices. Stress-induced anorexia is often noted in rats exposed to chronic stress (e.g., repeated restraint) and access to standard Chow diet; associated reduced consumption and weight loss. However, if a similar stressor exposure takes place in the presence of palatable, calorie dense food, rats often consume an increase proportion of palatable food relative to Chow, leading to weight gain and obesity. In humans, a similar desire to eat palatable or “comfort” foods has been noted under stressful situations; it is thought that this response may potentially be attributable to stress-buffering properties and/or through activation of reward pathways. The complex interplay between stress-induced anorexia and stress-induced obesity is discussed in terms of the overlapping circuitry and neurochemicals that mediate feeding, stress and reward pathways. In particular, this paper draws attention to the bombesin family of peptides (BBs) initially shown to regulate food intake and subsequently shown to mediate stress response as well. Evidence is presented to support the hypothesis that BBs may be involved in stress-induced anorexia under certain conditions, but that the same peptides could also be involved in stress-induced obesity. This hypothesis is based on the unique distribution of BBs in key cortico-limbic brain regions involved in food regulation, reward, incentive salience and motivationally driven behavior.

Discovery of novel anxiolytic agents--the trials and tribulations of pre-clinical models of anxiety

Anxiety disorders are the most common class of mental disorders present in the general population with an estimated lifetime prevalence of any anxiety disorder being approximately 15%, while the 12-month prevalence is more than 10%. They are classified into simple phobias, social phobias, obsessive-compulsive disorder (OCD) and panic attacks. Anxiety disorders are more prevalent in females than males and respond to pharmacological and non-pharmacological (behavioral) treatments. Anxiety disorders are complex with genetic and environmental factors interacting to produce the final psychopathology. There are many tests used to detect behaviors that indicate heightened anxiety in rodents however there are few pathological models of anxiety in rodents. Most compound testing is performed on naive, non-pathologically anxious, male animals which is a potential limitation to current strategies since these animals do not reflect the anxious patient. This article briefly describes some of the most common anxiety tests used in rodent research and concludes with a short perspective on areas the field could concentrate on to improve the understanding and successful translation of novel targets into new therapies in the clinic.

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.

Activation of gastrin-releasing peptide receptors at the infralimbic cortex elicits gastrin-releasing peptide release at the basolateral amygdala: implications for conditioned fear

The basolateral amygdala (BLA) and infralimbic (IL) cortex share strong reciprocal interconnections and are key structures in conditioned fear circuitry. Gastrin-releasing peptide (GRP) or its receptor antagonists can modulate the conditioned fear response when exogenously administered at either of these sites, and increased release of GRP at the BLA occurs in response to conditioned fear recall. The present study sought to determine whether a functional pathway utilizing GRP exists between the IL cortex and BLA and whether this pathway is also influenced by amygdala corticotropin-releasing factor (CRF) release. To this end, we assessed the effects of intra-IL cortex injection of GRP or GRP co-administered with a receptor antagonist, RC-3095, on the downstream release of GRP and/or CRF at the BLA. Results showed that microinjection of GRP at the IL cortex increased the release of GRP, but not CRF, at the BLA, an effect blocked by co-administration of RC-3095. Administration of RC-3095 into the IL cortex on its own, however, also elicited the release of GRP (but not CRF) at the BLA. These findings suggest that a functional pathway utilizing GRP (among other factors) exists between the IL cortex and BLA that may be relevant to conditioned fear, but that GRP and CRF do not interact within this circuitry. Moreover, the finding that the release profile of GRP was similar following administration of either GRP or its receptor antagonist, lends support to the view that RC-3095 has partial agonist properties. Together these findings provide further evidence for the involvement of GRP in fear and anxiety-related disorders.

Gastrin-releasing peptide signaling plays a limited and subtle role in amygdala physiology and aversive memory

Links between synaptic plasticity in the lateral amygdala (LA) and Pavlovian fear learning are well established. Neuropeptides including gastrin-releasing peptide (GRP) can modulate LA function. GRP increases inhibition in the LA and mice lacking the GRP receptor (GRPR KO) show more pronounced and persistent fear after single-trial associative learning. Here, we confirmed these initial findings and examined whether they extrapolate to more aspects of amygdala physiology and to other forms of aversive associative learning. GRP application in brain slices from wildtype but not GRPR KO mice increased spontaneous inhibitory activity in LA pyramidal neurons. In amygdala slices from GRPR KO mice, GRP did not increase inhibitory activity. In comparison to wildtype, short- but not long-term plasticity was increased in the cortico-lateral amygdala (LA) pathway of GRPR KO amygdala slices, whereas no changes were detected in the thalamo-LA pathway. In addition, GRPR KO mice showed enhanced fear evoked by single-trial conditioning and reduced spontaneous firing of neurons in the central nucleus of the amygdala (CeA). Altogether, these results are consistent with a potentially important modulatory role of GRP/GRPR signaling in the amygdala. However, administration of GRP or the GRPR antagonist (D-Phe(6), Leu-NHEt(13), des-Met(14))-Bombesin (6-14) did not affect amygdala LTP in brain slices, nor did they affect the expression of conditioned fear following intra-amygdala administration. GRPR KO mice also failed to show differences in fear expression and extinction after multiple-trial fear conditioning, and there were no differences in conditioned taste aversion or gustatory neophobia. Collectively, our data indicate that GRP/GRPR signaling modulates amygdala physiology in a paradigm-specific fashion that likely is insufficient to generate therapeutic effects across amygdala-dependent disorders.

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.

Biology of mammalian bombesin-like peptides and their receptors

PURPOSE OF REVIEW

This review will highlight recent advances in the understanding of mammalian bombesin receptor-related pathophysiological roles in disease states and new insights into bombesin receptor pharmacology.

RECENT FINDINGS

Studies regarding bombesin-like peptides and mammalian bombesin receptor functions have demonstrated significant biological impact on a broad array of physiological and pathophysiological conditions. Pharmacological experiments in vitro and in vivo as well as utilization of genetic rodent models of the gastrin-releasing peptide receptor (GRP-R/BB2) and neuromedin B receptor (NMB-R/BB1) further delineated roles in memory and fear behavior, inhibition of tumor cell growth, mediating signals for pruritus and male reproductive behavior. All three mammalian bombesin receptors were shown to possess some role in the regulation of energy balance. Novel synthesis of selective high affinity agonists and antagonists of the orphan bombesin receptor subtype-3 (BRS-3/BB3) has been accomplished and will facilitate further studies using animal model systems.

SUMMARY

Mammalian bombesin receptors participate in the regulation of energy homeostasis and may represent an attractive target for pharmacological treatment of obesity and certain eating disorders. Novel pharmacological insights of bombesin-like peptides and the interaction with their respective receptors have been elucidated to aid future treatment and imaging of epithelial cell-derived tumors.

Haploinsufficiency in peptidylglycine alpha-amidating monooxygenase leads to altered synaptic transmission in the amygdala and impaired emotional responses

The mammalian amygdala expresses various neuropeptides whose signaling has been implicated in emotionality. Many neuropeptides require amidation for full activation by peptidylglycine α-amidating monooxygenase (PAM), a transmembrane vesicular cuproenzyme and regulator of the secretory pathway. Mice heterozygous for the Pam gene (PAM(+/-)) exhibit physiological and behavioral abnormalities related to specific peptidergic pathways. In the present study, we evaluated emotionality and examined molecular and cellular responses that characterize neurophysiological differences in the PAM(+/-) amygdala. PAM(+/-) mice presented with anxiety-like behaviors in the zero maze that were alleviated by diazepam. PAM(+/-) animals were deficient in short- and long-term contextual and cued fear conditioning and required higher shock intensities to establish fear-potentiated startle than their wild-type littermates. Immunohistochemical analysis of the amygdala revealed PAM expression in pyramidal neurons and local interneurons that synthesize GABA. We performed whole-cell recordings of pyramidal neurons in the PAM(+/-) amygdala to elucidate neurophysiological correlates of the fear behavioral phenotypes. Consistent with these observations, thalamic afferent synapses in the PAM(+/-) lateral nucleus were deficient in long-term potentiation. This deficit was apparent in the absence and presence of the GABA(A) receptor antagonist picrotoxin and was abolished when both GABA(A) and GABA(B) receptors were blocked. Both evoked and spontaneous excitatory signals were enhanced in the PAM(+/-) lateral nucleus. Phasic GABAergic signaling was also augmented in the PAM(+/-) amygdala, and this difference comprised activity-independent and -dependent components. These physiological findings represent perturbations in the PAM(+/-) amygdala that may underlie the aberrant emotional responses in the intact animal.

In vivo levels of corticotropin-releasing hormone and gastrin-releasing peptide at the basolateral amygdala and medial prefrontal cortex in response to conditioned fear in the rat

Given the modulatory effect of exogenously administered corticotropin-releasing hormone (CRH) and gastrin-releasing peptide (GRP) on conditioned fear, the present study sought to measure the fear-induced endogenous release of CRH and GRP at the medial prefrontal cortex (mPFC) and basolateral amygdala (BLA) using in vivo microdialysis. Rats were divided into 2 training conditions; tone only (cue), or tone paired with shock. The day after conditioning, animals were tested for fear by scoring freezing behavior in response to the tone alone in cages different from the cages they were previously conditioned in. Freezing was scored for 10 min. Dialysates were collected over 20 min intervals from 2h prior to testing (to establish baseline values) through to 3h post-testing continually uninterrupted. Analyses of dialysates revealed that at the BLA, the release of both CRH and GRP was increased over time and that peptide release was significantly higher in animals that had previously received shock relative to rats that had not. Further, the release of CRH and GRP was significantly correlated with freezing levels (an indication of fear in the rat) such that animals that had higher levels of freezing also had higher interstitial peptide levels. These effects appeared site-specific, as they were not apparent at the mPFC. It appears that at the BLA, the release of CRH and GRP is related to fear.

Neonatal gastrin-releasing peptide receptor blockade reduces maternal odor preference in rats

Alterations in attachment behavior might play a role in the dysfunction in social behavior displayed by autistic infants. Here we show that neonatal gastrin-releasing peptide receptor (GRPR) blockade induces a reduction in maternal odor preference, a task involving attachment behavior, in infant rats. These findings provide the first evidence that the GRPR regulates odor preference, supporting the view that the GRPR is involved in attachment and social behaviors.

Phosphoinositide 3-kinase is required for bombesin-induced enhancement of fear memory consolidation in the hippocampus

Increasing evidence indicates that the neuronal gastrin-releasing peptide-preferring bombesin receptor (GRPR) is a key molecular regulator of fear memory formation. However, the downstream signaling events remain poorly understood. The protooncogene product phosphoinositide 3-kinase (PI3K) has been implicated in regulating memory formation, as well as in mediating cellular responses to GRPR activation in glioma and neuroblastoma cells. We show here that GRPR modulation of fear memory consolidation in the rat hippocampus requires PI3K activation. Male Wistar rats received bilateral infusions of the GRPR agonist bombesin (BB) or the PI3K inhibitor LY294002 into the CA1 region of the dorsal hippocampus immediately after inhibitory avoidance (IA) conditioning. BB enhanced, whereas LY294002 impaired, IA memory retention. The BB-induced memory enhancement was blocked by coinfusion of either a GRPR antagonist or LY294002. These findings provide the first evidence suggesting that PI3K signaling is required for GRPR regulation of CNS function.