Fear conditioning and shock intensity: the choice between minimizing the stress induced and reducing the number of animals used.

Many fear conditioning studies use electric shock as the aversive stimulus. The intensity of shocks varies throughout the literature. In this study, shock intensities ranging from 0 to 1.5 mA were used, and the effects on the rats assessed by both behavioural and biochemical stress parameters. Results indicated a significant difference with respect to defaecation and freezing behaviour between controls and those animals that received a shock. Significant differences in corticosterone levels were also noted between controls and those groups that received a shock. No significant differences were found between the shock groups with regards to the stress parameters measured in our fear conditioning paradigm, indicating that the two shock groups were similarly stressed. Increased significance levels were noted for freezing behaviour as well as a lower standard error of means was found in the highest shock intensity group. We therefore recommend using the higher shock intensity (1.5 mA) as the rats in the higher shock intensity group were more homogeneously fear-conditioned and therefore the results should be more reproducible and robust than in the lower shock intensity group. This would allow for fewer rats to be used in order to gain an accurate impression of the conditioning paradigm employed.

Maternal separation in rats leads to anxiety-like behavior and a blunted ACTH response and altered neurotransmitter levels in response to a subsequent stressor.

Adverse early life experiences can have a negative impact on behavior later in life. We subjected rat pups to maternal separation and determined the effect's thereof on adult behavior. We removed rat pups from their mothers for 3 h daily from postnatal days 2 to 14. While controls were reared normally on day 60, the behaviors of the rats were tested using the elevated plus-maze. Some rats were subsequently subjected to restraint stress for a 10-min period. Trunk blood was collected for basal, as well as 15- and 60-min postrestraint stress ACTH determinations. Neurotransmitter levels (noradrenaline (NA), serotonin (5HT), and their metabolites, MHPG and 5HIAA, respectively) were also determined at basal, immediately and 15-min post-restraint stress in the hypothalamus, hippocampus, and frontal cortex in another group of animals. The amount of entries into the arms of the elevated plus-maze was significantly reduced in the separated animals, indicating decreased locomotion. They spent significantly more time in the closed arms of the maze. A significant increase in defecation frequency was noted. These observations suggested anxious behavior. Basal ACTH levels were significantly higher in separated animals. At 15-min post-restraint stress, the ACTH levels were significantly lower than controls, indicating a blunted stress response. A decrease in noradrenaline was noted first in limbic regions and an increase in 5HIAA levels was found in the frontal cortex and hippocampus. We conclude that maternal separation induced abnormal behaviors and stress responses that were associated with altered neurotransmitter levels.

Overcrowding induces anxiety and causes loss of serotonin 5HT-1a receptors in rats.

Unfavorable conditions under which children grow up may contribute significantly to the development of psychiatric abnormalities in adult life. We studied the effects of overcrowding at an early age and how it may result in anxiety later in life. Sprague-Dawley rats were housed 10 animals per cage, from birth until 4 weeks post-weaning. Rats housed 3 per cage served as controls. The Elevated Plus maze was used to determine their anxious behavior. Thereafter, the serotonergic system in the hippocampus was investigated. Overcrowded rats were significantly more anxious than controls. The number of 5HT-1a receptors in the hippocampus decreased significantly and their affinity for the ligand, OH-DPAT, increased significantly. There was no significant difference in the hippocampal levels of serotonin between overcrowded rats and controls. Our study shows that overcrowding during childhood can result in anxious behavior and that the serotonergic system appears to play a role in its manifestation.