Effects of centrally administered corticotropin-releasing factor (CRF) and alpha-helical CRF on the vocalizations of isolated guinea pig pups

The CRF system and social behavior: a review

The corticotropin-releasing factor (CRF) system plays a key role in a diversity of behaviors accompanying stress, anxiety and depression. There is also substantial research on relationships between social behaviors and the CRF system in a variety of taxa including fish, birds, rodents, and primates. Some of these relationships are due to the broad role of CRF and urocortins in stress and anxiety, but these peptides also modulate social behavior specifically. For example, the social interaction (SI) test is often used to measure anxiety-like behavior. Many components of the CRF system including CRF, urocortin1, and the R1 receptor have been implicated in SI, via general effects on anxiety as well as specific effects depending on the brain region. The CRF system is also highly responsive to chronic social stressors such as social defeat and isolation. Animals exposed to these stressors display a number of anxiety- and stress-related behaviors, accompanied by changes in specific components the CRF system. Although the primary focus of CRF research on social behavior has been on the deleterious effects of social stress, there are also insights on a role for CRF and urocortins in prosocial and affiliative behaviors. The CRF system has been implicated in parental care, maternal defense, sexual behavior, and pair bonding. Species differences in the ligands and CRF receptors have been observed in vole and bird species differing in social behavior. Exogenous administration of CRF facilitates partner preference formation in monogamous male prairie voles, and these effects are dependent on both the CRF R1 and R2 receptors. These findings are particularly interesting as studies have also implicated the CRF and urocortins in social memory. With the rapid progress of social neuroscience and in understanding the complex structure of the CRF system, the next challenge is in parsing the exact contribution of individual components of this system to specific social behaviors.

Behavioral effects of peripheral corticotropin-releasing factor during maternal separation may be mediated by proinflammatory activity

When guinea pig pups are separated from their mothers in a novel environment, an initial period of active behavior (vocalizing, locomotor activity) wanes after an hour or so and is replaced by a second, passive stage characterized by a crouched stance, closed eyes, and extensive piloerection. If pups are given a peripheral injection of 7-14μg of corticotropin-releasing factor (CRF) prior to testing, the passive behaviors occur immediately upon separation. We found that intracerebroventricular infusion of 1-10μg of CRF did not increase passive behavior relative to vehicle infusion, but that peripheral injection of the anti-inflammatory cytokine, interleukin-10, attenuated the passive behavior induced by peripheral CRF injection. These results together with previous findings suggest that peripheral CRF administration affects behavior of separated guinea pig pups through a mechanism that involves peripheral proinflammatory activity. The possible role of endogenous peripheral CRF in the behavioral response of untreated pups during maternal separation is considered.

Alpha-melanocyte-stimulating hormone attenuates behavioral effects of corticotropin-releasing factor in isolated guinea pig pups

During a 3-hr period of social isolation in a novel environment, guinea pig pups exhibit an initial active phase of behavioral responsiveness, characterized primarily by vocalizing, which is then followed by a stage of passive responsiveness in which pups display a distinctive crouch, eye-closing, and extensive piloerection. Prior treatment of pups with alpha-melanocyte-stimulating hormone (alpha-MSH) reduces each of the passive behaviors. The onset of passive responding during separation can be accelerated with peripheral injection of corticotropin-releasing factor (CRF). To examine whether CRF produces its effects through a mechanism similar to that of prolonged separation, we examined the effect of administering alpha-MSH to pups injected with CRF. As expected, CRF markedly enhanced passive responding during a 60-min period of separation. alpha-MSH delivered by either intracerebroventricular infusion or intraperitoneal injection significantly reduced each of the passive behavioral responses without significantly affecting active behavior. These findings, together with previous results indicating that it is the anti-inflammatory property of alpha-MSH that is responsible for its behavioral effects during prolonged separation, suggest that peripheral CRF speeds the induction of passive responding through a mechanism involving enhanced proinflammatory activity.

Difluoromethylornithine (DFMO) reduces deficits in isolation-induced ultrasonic vocalizations and balance following neonatal ethanol exposure in rats

Neonatal ethanol (EtOH) exposure is associated with central nervous system dysfunction and neurotoxicity in rats. Increases in polyamine levels have been implicated as one underlying mechanism for some of EtOH's effects on the developing brain. In this study we addressed whether the inhibition of polyamine biosynthesis by alpha-difluoromethylornithine (DFMO) could reduce behavioral deficits induced by early EtOH exposure. Male and female rat pups received ethanol (6 g/kg/day EtOH i.g.), or isocaloric maltose (control) from postnatal days (PND) 1-8. On PND 8, animals were injected with either saline or DFMO (500 mg/kg, s.c.) immediately following the final neonatal treatment. Subjects were tested for isolation-induced ultrasonic vocalizations (USV) on PND 16; spontaneous activity in an open field apparatus on PND 20 and 21; and balance on PND 31. Animals exposed to EtOH neonatally displayed an increased latency to the first USV and reduced frequencies of USV, hyperactivity and preference for the center of the open field and poorer balance relative to controls. DFMO minimized these deficits in latency to the first USV and balance. These data provide further support that polyamines play a role in some of the functional deficits associated with EtOH exposure during early development and that reducing polyamine activity can improve outcome.

Alpha-melanocyte stimulating hormone reduces putative stress-induced sickness behaviors in isolated guinea pig pups

We have proposed that passive responses observed following maternal separation in guinea pig pups represent "stress-induced sickness behaviors" mediated by proinflammatory processes. In this study, the anti-inflammatory peptide, alpha-melanocyte stimulating hormone (alpha-MSH) administered intracerebroventricularly, but not intraperitoneally, reduced the passive responses of crouching, eye-closing, and extensive piloerection relative to levels following administration of vehicle. These findings support our hypothesis and are as would be expected if pro-inflammatory processes act centrally to promote the passive behaviors of separated guinea pig pups.

Centrally administered norepinephrine modifies the behavior induced by corticotropin-releasing factor in neonatal chicks

We previously reported that glucagon-like peptide-1 decreased corticotropin-releasing factor (CRF)-induced behaviors in neonatal chicks, and such an effect is hypothesized to act through norepinephrine (NE). Experiments were designed to explore the effect of the NE on CRF-induced behaviors. In experiment 1, the chicks were intracerebroventricularly (i.c.v.) administered saline, 0.1 microg of CRF, 50.0 microg of NE, or 0.1 microg of CRF with 50.0 microg of NE. Behavior was monitored for the 10 min immediately after i.c.v. injection, and plasma corticosterone was analyzed at the end of behavior tests. Compared with the control, chicks were excited by CRF as evidenced by increased spontaneous activity and distress vocalizations (DVs). NE decreased the spontaneous activity of chicks, and the differences diverged with time. DVs completely disappeared in the presence of NE, and sleep-like (sitting with eyes closed) behavior was observed in the same birds. NE-treated birds spent most of the time in a sleep-like posture irrespective of CRF treatment. CRF-treated chicks had increased plasma corticosterone, whereas NE injection caused a decrease in corticosterone. In experiment 2, the effect of NE was further studied using i.c.v. administration with either 0.1 microg of CRF alone or 0.1 microg of CRF plus 12.5, 25.0, or 50.0 microg of NE. NE dose dependently modified the CRF-induced locomotor activity in the open field, and DVs disappeared when chicks were given any dose of NE with CRF. With these findings taken together, it is suggested that central NE interacts the CRF-induced behaviors in neonatal chicks.

Enduring maternal influences in a precocial rodent

The guinea pig is highly developed at birth and requires little active maternal care. Yet the mother and other social figures markedly influence biobehavioral processes of the offspring. Here, responses of guinea pigs and nonhuman primates to maternal and other social separation procedures are compared, and influences of social partners on endocrine responses and behavior in periadolescent guinea pigs are described.

The pharmacology of CP-154,526, a non-peptide antagonist of the CRH1 receptor: a review

Since CRH has been shown to mediate stress-induced physiological and behavioral changes, it has been hypothesized that CRH receptor antagonists may have therapeutic potential in disorders that involve excessive CRH activity. CP-154,526 and its close analog antalarmin are potent, brain-penetrable, selective nonpeptide CRH1 receptor antagonists that were discovered in an effort to develop compounds with efficacy in CNS disorders precipitated by stress. Since its discovery many investigators have used CP-154,526 as a tool to study the pharmacology of CRH and its receptors and to evaluate its therapeutic potential in a variety of CNS and peripheral disorders. Systemically-administered CP-154,526 has been demonstrated to antagonize CRH- and stress-induced neuroendocrine, neurochemical, electrophysiological, and behavioral effects. These findings support the hypothesis that CRH1 receptor antagonists may have therapeutic utility in a number of neuropsychiatric disorders. CP-154,526, as well as other CRH1 receptor antagonists that have since been discovered, have also shown activity in several preclinical models of anxiety, depression, and substance abuse, while having little effect on locomotor activity and motor function. Although these effects are on occasion inconsistent among different laboratories, clinical evaluation of CRH1 antagonists appears justified on the basis of these and clinical data implicating the involvement of CRH in several CNS disorders. The effects of CRH1 antagonists on cognition, neurodegeneration, inflammation, and the gastrointestinal system have not been as extensively characterized and additional studies will be necessary to evaluate their therapeutic potential in these areas.

Effects of a novel potential antidepressant on the behavior and cortisol levels of isolated guinea pig pups

A novel, potential antidepressant, E-6006 citrate (E-6039), dose-dependently reduced the vocalizations emitted by isolated guinea pig pups. The (+)-E-6006, but not the (-)-E-6006, enantiomer also reduced vocalizing. There were no reliable effects of E-6039 on locomotor activity, crouching, or other behavioral measures, but both E-6039 and the (+)-E-6006 enantiomer elevated plasma cortisol levels during isolation. The contrasting effects of E-6039 on vocalizations and plasma cortisol are discussed in terms of E-6039's putative ability to inhibit release of substance P. The reduction in the vocalizations of isolated guinea pig pups corroborates positive results with this drug in other antidepressant screens utilizing mice and rats, and provides further support for the potential of E-6039 as an antidepressant compound.

Short- and long-term consequences of corticotropin-releasing factor in early development

Corticotropin-releasing factor (CRF) mediates various stress-related responses in adult animals. Little is known about the effects of CRF during early development. Young mammals often vocalize when isolated in novel surroundings. Heightened levels of CRF inhibit vocalizing in isolated rat and guinea pig pups. Still lower levels of CRF may facilitate or permit vocalizing in rat pups. In guinea pigs, CRF appears to move pups from an initial active, to a subsequent passive, stage of behavioral responsiveness. CRF activity prior to birth can also affect the young. Exposing pregnant female rats to stressors during the last trimester of pregnancy alters the morphological and behavioral development of the offspring. Effects of gestational stress can be mimicked by injecting pregnant females with CRF during the last trimester. CRF appears to mediate both short- and long-term responses to stressors during developmental in rodents.

Is there a future for neuropeptide receptor ligands in the treatment of anxiety disorders?

This review provides an overview of preclinical and clinical evidence of a role for the neuroactive peptides cholecystokinin (CCK), corticotropin-releasing factor (CRF), neuropeptide Y (NPY), tachykinins (i.e., substance P, neurokinin [NK] A and B), and natriuretic peptides in anxiety and/or stress-related disorders. Results obtained with CCK receptor antagonists in animal studies have been highly variable, and clinical trials with several of these compounds in anxiety disorders have been unsuccessful so far. However, future investigations using CCK receptor antagonists with better pharmacokinetic characteristics and animal models other than those validated with the classical anxiolytics benzodiazepines may permit a more precise evaluation of the potential of these compounds as anti-anxiety agents. Results obtained with peptide CRF receptor antagonists in animal models of anxiety convincingly demonstrated that the blockade of central CRF receptors may yield anxiolytic-like activity. However, the discovery of nonpeptide and more lipophilic CRF receptor antagonists is essential for the development of these agents as anxiolytics. Similarly, there is clear preclinical evidence that the central infusion of NPY and NPY fragments selective for the Y1 receptor display anxiolytic-like effects in a variety of tests. However, synthetic nonpeptide NPY receptor agonists are still lacking, thereby hampering the development of NPY anxiolytics. Unlike selective NK1 receptor antagonists, which have variable effects in anxiety models, peripheral administration of selective NK2 receptor antagonists and central infusion of natriuretic peptides produce clear anxiolytic-like activity. Taken as a whole, these findings suggest that compounds targeting specific neuropeptide receptors may become an alternative to benzodiazepines for the treatment of anxiety disorders.

Role of corticotropin-releasing factor in forced swimming test

Several aspects of the role of corticotropin-releasing factor (CRF) in the forced swimming test were investigated in this study by using two different administration schedules. I.c.v. microinjection of CRF produced a dose-dependent increase in swimming activity when the administration schedule originally reported for this test to screen antidepressant drugs was followed. The most effective doses were 1 and 3 microg of CRF. A lower dose of CRF (0.5 microg) was also effective when repetitive experimental stress was present. CRF receptor antagonist, alpha-helical CRF-(9-41) (alpha-helical CRF-(9-41)), was able to block CRF-induced increases in swimming in all sessions of the forced swimming test. However, the effects of CRF and CRF receptor antagonist depended on the administration schedule. A decrease in swimming in the forced swimming test was observed when CRF and CRF receptor antagonist were given together, using a different administration schedule. I.c.v. CRF was ineffective and CRF receptor antagonist alone produced an increase in swimming when administered according to this schedule. These behavioural responses were maintained after twelve days without any treatment. The results of the current study suggest that endogenous CRF seems to play a determinant role in behavioural responses in the forced swimming test. The involvement of the level of activation and memory processes in these behavioural responses is discussed.

Evidence that endogenous corticotropin-releasing factor suppresses behavioral responses of guinea pig pups to brief isolation in novel surroundings

Guinea pig pups were injected subcutaneously with a corticotropin-releasing factor antagonist (CRF12-41) or saline vehicle and then placed into a novel cage for 30 or 60 min. Isolated 20- to 26-day-old pups vocalized more and exhibited more locomotor activity when given 15 to 150 micrograms of CRF12-41 than when given saline. The presence of the mother in the test cage prevented the antagonist from affecting behavior. The influence of the antagonist during isolation was not limited to guinea pigs near weaning age: CRF12-41 increased levels of vocalizing in isolated, 4- to 6-day-old pups, though no changes were seen in locomotor activity. Results support the hypothesis that endogenous corticotropin-releasing factor, perhaps acting at a peripheral binding site, suppresses the active behavioral response characteristic of pups during the early phase of isolation in novel surroundings.

Peripheral administration of a corticotropin-releasing factor antagonist increases the vocalizing and locomotor activity of isolated guinea pig pups

Guinea pig pups vocalized more and were more active during a 60-min period of isolation in a novel environment when injected SC with 50 micrograms of the corticotropin-releasing factor (CRF) antagonist, [D-Phe]CRF12-41, than when injected with saline vehicle only. More pups exhibited eye closing and extensive piloerection when injected with saline than when injected with the antagonist. Plasma levels of cortisol following testing were not affected by the injections. These results support the hypothesis that endogenous CRF contributes to the normal decline in vocalizing and locomotor activity that guinea pig pups show with continued isolation. The results also suggest that endogenous CRF plays a role in the eye closing and piloerection of isolated pups. The finding that the dose of the antagonist used altered behavior, yet was not sufficient to lower plasma cortisol levels, supports earlier evidence that the observed effects of CRF are not due to the actions of ACTH or glucocorticoids.

Further characterization of the behavioral effects of peripherally administered corticotropin-releasing factor in guinea pigs

Guinea pig pups were either not injected (NI) or given SC injection of either saline vehicle (SAL) or 14 micrograms of corticotropin-releasing factor (CRF). In an isolation test, mean number of vocalizations and several measures of locomotor activity were markedly lower for CRF pups than for NI or SAL controls. CRF pups defecated less than did SAL pups. No differences were found among conditions for self-grooming. Behavioral freezing was shown by only two pups in the entire study. Significantly more CRF pups displayed piloerection, eye-closing, and a characteristic crouched stance than did controls. In a defensive withdrawal test, no differences among conditions were found for the proportion of pups entering a darkened chamber or for the latency to enter the chamber; however, CRF pups entered the chamber significantly fewer times during the 60-min test than did controls. There were no differences among conditions in the distance swum or number of turns made in a forced-swim test. These results replicate our earlier findings that peripheral injection of CRF suppresses vocalizing and a measure of locomotor activity in isolated guinea pig pups and identifies a number of additional behavioral effects. Of central interest here, the results indicate that the suppression of vocalizing and locomotion during isolation is not due to an increase in competing stress-related behavior or to diminished motor capacity.