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

Early Attachment Disruption, Inflammation, and Vulnerability for Depression in Rodent and Primate Models

Early experiments in nonhuman primates established the relation between disruption of filial attachment and depressive-like outcomes. Subsequent studies in rats and mice have been instrumental in linking depressive-like outcomes to disturbances in maternal behavior. Another aspect of attachment disruption, absence of the attachment object per se, may be studied more effectively in a different laboratory rodent-the guinea pig. Here, we discuss the rationale for using guinea pigs for this work. We then review guinea pig studies providing evidence for inflammatory mechanisms mediating both depressive-like behavior during separation as well as sensitization of stress responsiveness such as is thought to lead to increased vulnerability to depression at later ages. Finally, we discuss recent complementary work in adult monkeys that suggests cross-species generalizability of broad principles derived from the guinea pig experiments. Overall, the findings provide experimental support for human research implicating inflammatory mechanisms in the development of increased stress responsiveness and vulnerability to depression following attachment disruption and other forms of early-life stress. Specifically, the findings suggest inflammatory mechanisms may set in motion a cascade of underlying processes that mediate later increased stress responsiveness and, therefore, depression susceptibility.

Pharmacological and methodological aspects of the separation-induced vocalization test in guinea pig pups; a systematic review and meta-analysis

The separation-induced vocalization test in guinea pig pups is one of many that has been used to screen for anxiolytic-like properties of drugs. The test is based on the cross-species phenomenon that infants emit distress calls when placed in social isolation. Here we report a systematic review and meta-analysis of pharmacological intervention in the separation-induced vocalization test in guinea pig pups. Electronic databases were searched for original research articles, yielding 32 studies that met inclusion criteria. We extracted data on pharmacological intervention, animal and methodological characteristics, and study quality indicators. Meta-analysis showed that the different drug classes in clinical use for the treatment of anxiety disorders, have comparable effects on vocalization behaviour, irrespective of their mechanism of action. Of the experimental drugs, nociception (NOP) receptor agonists proved very effective in this test. Analysis further indicated that the commonly used read-outs total number and total duration of vocalizations are equally valid. With regard to methodological characteristics, repeated testing of pups as well as selecting pups with moderate or high levels of vocalization were associated with larger treatment effects. Finally, reporting of study methodology, randomization and blinding was poor and Egger's test for small study effects showed that publication bias likely occurred. This review illustrates the value of systematic reviews and meta-analyses in improving translational value and methodological aspects of animal models. It further shows the urgent need to implement existing publication guidelines to maximize the output and impact of experimental animal studies.

Sociality and sickness: have cytokines evolved to serve social functions beyond times of pathogen exposure?

During pathogen exposure or some forms of stress, proinflammatory processes induce an array of motivated and behavioral adjustments termed "sickness behaviors". Although withdrawal from social interactions is a commonly observed sickness behavior, the relation between social behavior and sickness is much more complex. Sickness can suppress or stimulate social behavior. Sickness can serve as a social cue. Stressors that are social in nature can induce sickness behaviors, and sickness behavior can be readily suppressed by meaningful social stimuli. The nature, context, and timing of these effects together suggest that cytokine-induced behavior may play a role in mediating social interactions in various non-pathological conditions.

Maternal inhibition of infant behavioral response following isolation in novel surroundings and inflammatory challenge

During isolation in a novel environment, guinea pig pups gradually begin to display passive behavior that appears to be mediated by proinflammatory activity, that is, "sickness behavior.". Administration of substances that increase proinflammatory activity [corticotropin-releasing factor (CRF), lipopolysaccharide (LPS)] prior to isolation induces passive behavior from the beginning of the isolation episode. Here, we show that reunion with the mother in the novel environment rapidly and potently suppresses the passive behavior of isolated pups (Experiment 1); inhibits the passive behavior of pups administered CRF (10 µg, subcutaneous; Experiment 2); and inhibits the passive behavior of male, though not female, pups administered LPS (250 µg/kg, intraperitoneal; Experiment 3). Together these findings suggest that the presence of the mother either recruits other processes that moderate the impact of proinflammatory processes on brain mechanisms mediating the passive response or initiates compensatory mechanisms that counter the effect of proinflammatory activity. Further, the results suggest that for physically ill animals of social species, the adaptive advantage that accrues from maintaining normal social interactions may sometimes outweigh the advantage gained by engaging in sickness behavior.

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.

Early attachment-figure separation and increased risk for later depression: potential mediation by proinflammatory processes

Early maternal separation and other disruptions of attachment relations are known to increase risk for the later onset of depressive illness in vulnerable individuals. It is suggested here that sensitization involving proinflammatory processes may contribute to this effect. This argument is based on: (1) current notions of the role of proinflammatory cytokines in depressive illness; (2) evidence that proinflammatory cytokines mediate depressive-like behavior during separation in a rodent model of infant attachment; and (3) comparisons of the effects of early proinflammatory activation versus maternal separation on later proinflammatory activity and biobehavioral processes related to depression. The possible interaction of proinflammatory processes and corticotropin-releasing factor in the sensitization process is discussed.

Anti-inflammatory agents attenuate the passive responses of guinea pig pups: evidence for stress-induced sickness behavior during maternal separation

A previous study found that intracerebroventricular (ICV) infusion of 25 microg of alpha-MSH reduced the passive responses (crouched stance, eye-closing, piloerection) of guinea pig pups during a 3-h isolation in a novel environment. Because alpha-MSH has broad anti-inflammatory properties, the results suggested that proinflammatory factors play a role in mediating the behavior of isolated infants. The present study further investigated this possibility. In Experiment 1, injection of lipopolysacchride (LPS) increased the number of 60-s intervals in which pups expressed the same three responses during a 1-h test, and ICV infusion of alpha-MSH significantly reduced the effect of LPS on crouching and piloerection. In Experiment 2, the prostaglandin synthesis inhibitor indomethacin (10 mg/kg) reduced the number of 60-s intervals in which pups exhibited both crouching and the full suite of passive responses during a 3-h isolation in a novel environment. Together these results provide further support for the hypothesis that the passive behaviors exhibited during prolonged isolation are "stress-induced sickness behaviors" mediated by proinflammatory factors.