Central bombesin activates the hypothalamic-pituitary-adrenal axis. Effects on regional levels and release of corticotropin-releasing hormone and arginine-vasopressin

Environmental enrichment as an intervention for adverse health outcomes of prenatal stress

Prenatal stress (PS) has complex neurological, behavioural and physiological consequences for the developing offspring. The phenotype linked to PS usually lasts into adulthood and may even propagate to subsequent generations. The often uncontrollable exposure to maternal stress and the lasting consequences emphasize the urgent need for treatment strategies that effectively reverse stress programming. Exposure to complex beneficial experiences, such as environmental enrichment (EE), is one of the most powerful therapies to promote neuroplasticity and behavioural performance at any time in life. A small number of studies have previously used EE to postnatally treat consequences of PS in the attempt to reverse deficits that were primarily induced in utero . This review discusses the available data on postnatal EE exposure in prenatally stressed individuals. The goal is to determine if EE is a suitable treatment option that reverses adverse consequences of stress programming and enhances stress resiliency. Moreover, this review discusses data with respect to relevant hypotheses including the cumulative stress and the mismatch hypotheses. The articles included in this review emphasize that EE reverses most behavioural, physiological and neural deficits associated with PS. Differing responses may be dependent on the timing and variability of stress and EE, exercise, and potentially vulnerable and resilient phenotypes of PS. Results from this study suggest that enrichment may provide an effective therapy for clinical populations suffering from the effects of PS or early life trauma.

Peripheral injection of bombesin induces c-Fos in NUCB2/nesfatin-1 neurons

As anorexigenic hormones bombesin and nucleobindin2 (NUCB2)/nesfatin-1 decrease food intake in rodents. Both hormones have been described in brain nuclei that play a role in the modulation of hunger and satiety, like the paraventricular nucleus of the hypothalamus (PVN) and the nucleus of the solitary tract (NTS). However, the direct interaction of the two hormones is unknown so far. The aim of study was to elucidate whether bombesin directly interacts with NUCB2/nesfatin-1 neurons in the PVN and NTS. Therefore, we injected bombesin intraperitoneally (ip) at two doses (26 and 32nmol/kg body weight) and assessed c-Fos activation in the PVN, arcuate nucleus (ARC) and NTS compared to vehicle treated rats (0.15M NaCl). We also performed co-localization studies with oxytocin or tyrosine hydroxylase. Bombesin at both doses increased the number of c-Fos positive neurons in the PVN (p<0.05) and NTS (p<0.05) compared to vehicle, while in the ARC no modulation was observed (p>0.05). In the PVN and NTS the number of c-Fos positive neurons colocalized with NUCB2/nesfatin-1 increased after bombesin injection compared to vehicle treatment (p<0.05). Moreover, an increase of activated NUCB2/nesfatin-1 immunoreactive neurons that co-expressed oxytocin in the PVN (p<0.05) or tyrosine hydroxylase in the NTS (p<0.05) was observed compared to vehicle. Our results show that peripherally injected bombesin activates NUCB2/nesfatin-1 neurons in the PVN and NTS giving rise to a possible interaction between bombesin and NUCB2/nesfatin-1 in the modulation of food intake.

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.

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.

Neuromodulation by oxytocin and vasopressin

Oxytocin (OT) and vasopressin (VP) are two closely related neuropeptides, widely known for their peripheral hormonal effects. Specific receptors have also been found in the brain, where their neuromodulatory actions have meanwhile been described in a large number of regions. Recently, it has become possible to study their endogenous neuropeptide release with the help of OT/VP promoter-driven expression of fluorescent proteins and light-activated ion channels. In this review, I summarize the neuromodulatory effects of OT and VP in different brain regions by grouping these into different behavioral systems, highlighting their concerted, and at times opposite, effects on different aspects of behavior.

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.

Effect of environmental enrichment on fearful behavior and gastrin-releasing peptide receptor expression in the amygdala of prenatal stressed rats

Prenatal stressed offspring exhibit more fearful behavior in behavioral tests, which can be reversed by environmental enrichment (EE). However, the physiological basis of these phenomena remains unclear. Previous studies revealed that abnormal fearful behavior of prenatally stressed offspring may be a consequence of increased activities of CRFergic systems (corticotropin-releasing factor and its receptors) in the amygdala. Gastrin-releasing peptide receptors (GRPR) also have an important role in regulating amygdala-dependent, fear-related learning. The aim of this study was to examine weather prenatal stress and EE can affect the expression of GRPR in the amygdala. We reported here that prenatal chronic stress (subjected to immobilization and bright light stress for 45 min three times per day) caused increased fearfulness in defensive withdrawal test but had no effect on the expression of GRPR in the amygdala. However, enriched environment housing treatment on postnatal days 21-60 can dramatically increase the expression of GRPR in amygdala and reduce fearfulness in the defensive withdrawal test. Our results demonstrate for the first time that EE can modify the expression of GRPR in the amygdala, which might contribute to our understanding of the physiological effects of environmental enrichment.

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.

The role of gastrin-releasing peptide on conditioned fear: differential cortical and amygdaloid responses in the rat

RATIONALE

Bombesin (BB), an amphibian peptide, was shown to affect the expression of the stress response. However, the physiological role of the mammalian counterparts of BB in mediating anxiety and fear responses remain to be characterized.

OBJECTIVE

This study examined the effects of gastrin-releasing peptide (GRP), a mammalian analogue of BB, and its receptor antagonist, BW2258U89, on conditioned emotional response (CER), using fear conditioning.

MATERIALS AND METHODS

The effects of these compounds on contextual and cued fear conditioning were assessed after direct bilateral infusions into the prelimbic (PrL) cortex, infralimbic (IL) cortex or central nucleus of the amygdala (CeA).

RESULTS

GRP (300 ng) microinjected into each of the three target nuclei significantly reduced freezing to contextual cues. Similarly, in the cued portion of CER, GRP administered to the IL cortex significantly reduced freezing. Administration of BW2258U89 resulted in dose-dependent and site-specific effects. At the IL cortex, the 50 ng dose decreased freezing to both contextual and cued fear conditioning. At the CeA, the 300 ng dose also decreased freezing, but at the 50 ng dose, it increased contextual freezing. At the PrL cortex, BW2258U89 did not affect freezing.

CONCLUSIONS

These results illustrate that (1) GRP system(s) can significantly affect the expression of learned fear, (2) some of the relevant brain sites mediating these effects include the PrL, IL and the CeA, and (3) such effects may be dependent upon whether responses were evoked by environmental contextual fear cues or by specific auditory cues that were explicitly paired with an aversive stimulus.

Bombesin receptors as a novel anti-anxiety therapeutic target: BB1 receptor actions on anxiety through alterations of serotonin activity

The effects of PD 176252 [3-(1H-indol-3-yl)-N-[1-(5-methoxy-pyridin-2-yl)-cyclohexylmethyl]-2-methyl-2-[3-(nitro-phenyl)ureido]propionamide], a nonpeptide bombesin (BB) BB1/BB2 receptor antagonist, were assessed in rats using several ethologically relevant tests of anxiety. Consistent with a role for the bombesin family of peptides in subserving anxiety behaviors, the antagonist increased social interaction (3.75 and 7.5 mg/kg, i.p.), dose-dependently attenuated the number of vocalizations emitted by guinea pig pups separated from their mother (1-30 mg/kg, i.p.), reduced latency to approach a palatable snack in an anxiogenic (unfamiliar) environment, and reduced the fear-potentiated startle response (5 and 10 mg/kg, i.p., and 100-200 ng per rat, i.c.v.). When administered directly to the dorsal raphé nucleus (DRN), PD 176252 (20-500 ng) increased social interaction under aversive conditions, as did the 5-HT1A receptor agonist 8-hydroxy-2(di-n-propylamino)tetralin (50 ng). Furthermore, intra-DRN microinfusion of the peptide antagonist (PD 176252) suppressed, whereas its agonist [neuromedin B (NMB)-30] promoted, the in vivo release of 5-HT in the ventral hippocampus. In parallel, the suppressed social interaction elicited by intra-DRN administration of NMB was attenuated by a systemically administered 5-HT2C (but not 5-HT1A) receptor antagonist. Together, these findings suggest that endogenous BB-like peptides at the DRN evoke the release of 5-HT from the limbic nerve terminals originating from the raphé, specifically at the ventral hippocampus, resulting in anxiogenesis. The finding that this action was attenuated by BB receptor (BB1 and/or BB2) antagonists suggests that these compounds may represent a novel class of anxiolytic agents.

Bombesin, but not amylin, blocks the orexigenic effect of peripheral ghrelin

The interaction between ghrelin and bombesin or amylin administered intraperitoneally on food intake and brain neuronal activity was assessed by Fos-like immunoreactivity (FLI) in nonfasted rats. Ghrelin (13 microg/kg ip) increased food intake compared with the vehicle group when measured at 30 min (g/kg: 3.66 +/- 0.80 vs. 1.68 +/- 0.42, P < 0.0087). Bombesin (8 microg/kg) injected intraperitoneally with ghrelin (13 microg/kg) blocked the orexigenic effect of ghrelin (1.18 +/- 0.41 g/kg, P < 0.0002). Bombesin alone (4 and 8 microg/kg ip) exerted a dose-related nonsignificant reduction of food intake (g/kg: 1.08 +/- 0.44, P > 0.45 and 0.55 +/- 0.34, P > 0.16, respectively). By contrast, ghrelin-induced stimulation of food intake (g/kg: 3.96 +/- 0.56 g/kg vs. vehicle 0.82 +/- 0.59, P < 0.004) was not altered by amylin (1 and 5 microg/kg ip) (g/kg: 4.37 +/- 1.12, P > 0.69, and 3.01 +/- 0.78, respectively, P > 0.37). Ghrelin increased the number of FLI-positive neurons/section in the arcuate nucleus (ARC) compared with vehicle (median: 42 vs. 19, P < 0.008). Bombesin alone (4 and 8 microg/kg ip) did not induce FLI neurons in the paraventricular nucleus of the hypothalamus (PVN) and coadministered with ghrelin did not alter ghrelin-induced FLI in the ARC. However, bombesin (8 microg/kg) with ghrelin significantly increased neuronal activity in the PVN approximately threefold compared with vehicle and approximately 1.5-fold compared with the ghrelin group. Bombesin (8 microg/kg) with ghrelin injected intraperitoneally induced Fos expression in 22.4 +/- 0.8% of CRF-immunoreactive neurons in the PVN. These results suggest that peripheral bombesin, unlike amylin, inhibits peripheral ghrelin induced food intake and enhances activation of CRF neurons in the PVN.

Corticotropin-releasing hormone, arginine vasopressin, gastrin-releasing peptide, and neuromedin B alterations in stress-relevant brain regions of suicides and control subjects

BACKGROUND

Postmortem levels of several stress- and depression-relevant neuropeptides were assessed in brain regions of depressed suicides relative to control subjects that had died of other causes.

METHODS

Brains of suicides and those that died from other causes were collected soon after death (typically <6 hours). Immunoreactivity levels (ir) of corticotropin-releasing hormone (CRH-ir) and arginine vasopressin (AVP-ir), and the bombesin analogs, gastrin-releasing peptide (GRP-ir), and neuromedin B (NMB-ir), were assessed.

RESULTS

Levels of CRH-ir among suicides were elevated in the locus coeruleus (LC), frontopolar, dorsolateral prefrontal (DMPFC) and ventromedial prefrontal cortices, but were reduced at the dorsovagal complex (DVC). The concentration of AVP-ir was elevated at the paraventricluar hypothalamic nucleus, LC, and DMPFC, and reduced at the DVC. Finally, GRP and NMB variations, which might influence anxiety states, were limited, although GRP-ir within the LC of suicides was higher than in control subjects, while NMB-ir was reduced at the DVC of suicides.

CONCLUSIONS

The data show several neuropeptide changes in relation to suicide, although it is premature to ascribe these outcomes specifically to the suicide act versus depression. Likewise, it is uncertain whether the neuropeptide alterations were etiologically related to suicide/depression or secondary to the depressive state.

Demonstration of low flow push-pull perfusion

Methods to follow in vivo chemical composition provide information regarding the processes of intercellular communication. There is a need for methods that provide chemical information from small volumes of the central nervous system (CNS) without sacrificing neurochemical recovery. One method that offers potential for providing such information is push-pull perfusion. In this study a low flow push-pull perfusion system is introduced that provides high (70-80%) in vitro recoveries. A concentric probe design is used with a 27-gauge stainless steel outer cannula for saline infusion and an inner fused silica capillary for fluid withdrawal. Flow rates of 10-50 nl/min were reliably generated and were well matched in vitro. Sampling was performed in the striatum of an anesthetized rat generating a 0.5 microl sample every 12 min. Capillary electrophoresis was used to determine glutamate levels in each sample; the basal level was found to be 1.97+/-0.70 microM. The method described was also demonstrated to deliver L-trans-pyrrolidine-2,4-dicarboxylic acid through the perfusion solution while sampling. Post-sampling histological analysis demonstrates little tissue disturbance to the sampled region. These data provide evidence that low flow push-pull method is a viable alternative for studying neurochemical signaling in the CNS.

Gastrin-releasing peptide mediated regulation of 5-HT neuronal activity in the hypothalamic paraventricular nucleus under basal and restraint stress conditions

The purpose of this study was to examine the gastrin-releasing peptide (GRP) mediated regulation of 5-HT neuronal activity in the paraventricular nucleus of the hypothalamus under basal and restraint stress conditions. Intracerebroventricular (icv) administration of GRP (1, 10, 100 ng/rat) increased 5-HIAA concentrations in the paraventricular nucleus (PVN) of the hypothalamus, but was without effect in the accumbens, suprachiasmatic and arcuate nuclei. Administration of (Leu(13)-psi-CH(2)NH-Leu(14)) Bombesin (10, 100 and 1000 ng/rat; icv), a GRP antagonist, had no effect by itself on PVN serotonergic activity; however, a dose of 1 microg/rat of this compound, completely blocked the increase of 5-HIAA concentrations induced by GRP (10 ng). Restraint stress increased serotonergic activity -as shown by an elevation of 5-HIAA in the PVN- as well as plasma ACTH and corticosterone. This stress-induced activation of both the serotonergic neurons and the hypothalamus-pituitary-adrenal axis was blocked by CRF and GRP antagonists. Interestingly, when the activation of hypothalamic 5-HT neurons was induced by GRP administration, alpha-helical (9-41) CRF was ineffective. These data suggest that GRP, by acting on GRP receptors but not via CRF receptors, increases 5-HT neuronal activity in the PVN. In turn, it appears that endogenous GRP and CRF receptor ligands are both simultaneously involved in the regulation of the increase in 5-HT neuronal activity, ACTH and corticosterone secretion, under stress conditions.