An Investigation of the Effects of Maternal Separation and Novelty on Central Mechanisms Mediating Pituitary-Adrenal Activity in Infant Guinea Pigs (Cavia porcellus)

An Investigation of the Effects of Maternal Separation and Novelty on Central Mechanisms Mediating Pituitary-Adrenal Activity in Infant Guinea Pigs (Cavia porcellus)

Behavioral Neuroscience, 2010, Vol. 124, No. 6, 800-809

Chapter 8 discusses how drugs and hormones influence the brain and behavior. A stressor is defined as a stimulus that challenges the body’s homeostasis and triggers arousal. In the body, the hypothalamus produces neurohormones which proceed to stimulate the pituitary gland. The pituitary gland then directs the endocrine glands to produce and secrete hormones to targets throughout the body.

The article that I reviewed basically observed the physiological effects of stress inflicted on guinea pigs, which are highly filial creatures, when they are separated from their mother. Previous research revealed that a brief separation of infant guinea pigs from their mothers resulted in raised cortisol and ACTH levels in the blood. Environment also plays a role. Placed in a novel cage without their mothers, the guinea pigs’ hormone levels are greatly raised. Placed in a novel cage with their mothers or left in the home environment and separated, the hormone levels are not elevated.

The researchers in this study wanted to attain a better understanding of the central mechanisms regulating pituitary-adrenal activity during conditions of separation for the guinea pig. Research with rats seems to indicate a complicated pathway—the medial amygdale appears to activate the paraventricular nucleus to increase the release of CRF in the median eminence. As a result, the ACTH and cortisol is indirectly released.

The researchers used infant male guinea pigs from their research colony. Only one male from any particular litter was used. The guinea pigs were kept in opaque plastic cages with their siblings until about 16 days (+/- 1) after they were born. This is a week earlier than they would naturally be weaned.

The first experiment was designed to see if 15 to 240 minutes of separation in a novel environment would result in elevated levels of CRF or mRNA expression in the paraventricular nucleus. The conditions of the experiment were that pups either remained in their home condition or were separated into another clear plastic cage in an adjoining room for either 15, 30, 60, 120, or 240 minutes. At the end of each test, the infants were decapitated and their brains were removed in under 5 minutes. A significant main effect of condition was found between the 120/separation/novel condition and the undisturbed home condition, F (5, 17) = 3.44, p < .05. 

The second experiment examined the independent and combined effects of separation and exposure to novelty on CRF mRNA expression at 120 minutes. Again, the undisturbed home cage condition was the control condition. There were four experimental conditions in a 2 (mother, separation) x 2 (home, novel) factorial design. Results indicated that separation created an increase in cortisol levels in the novel environment but not in the home environment. Also, cortisol levels were greater in the separation/novel condition than in the home cage control condition. Separation also increased ACTH when in a novel environment. Likewise, CRF mRNA was increased in the novel/separation condition.

In general, this research indicates that the hormonal stress responses are likely to occur in a novel environment rather than at home, and that in the guinea pig, like the rat, the medial amygdale sends inhibitory projections to the BNST,  and those neurons are activated by novelty.