Early Life Determinants of Vulnerability to Pyridostigmine Bromide
Principal Investigator: Shelley A. Weaver, PhD
Background: After the first Gulf War, a large number of veterans registered nonspecific complaints of an unknown etiology, which has come to be known as Gulf War Illness. A unique condition affecting troops was the use of the acetylcholinesterase inhibitor pyridostigmine bromide (PB). PB can be considered an interoceptive stressor due to potential cholinergic-based side effects such as gastrointestinal distress. Although PB may be considered a "safe" medication for most military personnel, a minority of soldiers that exhibit enhancements of particular aspects of the stress response to interoceptive stressors may be vulnerable to persistent pathological effects of PB.
Method: We have identified an animal model of vulnerability to PB based on early environment. Bidirectional stress responses are obtained by separating mothers from pups for varying amounts of time with long maternal separations (MS) yielding exaggerated stress responses and short separations (handled; H) resulting in dampened responses. In a preliminary study MS rats given an acute dose of .5 mg/kg PB had a significant increase (p < .05) in acoustic startle response (ASR) both 1 and 15 days after PB treatment. PB did not affect ASRs in H rats. Thus the MS rat provides an animal model of vulnerability to PB.
Interoceptive stressors like PB are experienced in the context of a particular external environment. For example, sensitization is expected when novel stressors are presented in the context of chronic exposure to homotypic stress 2. By contrast, environmental factors such as social support can reduce stress responses3 and in animals, housing groups of rats in enriched environments (EE) can also dampen stress responses 4. Thus, the demonstration of vulnerability is not an end of itself; a particular environmental context could result in full-blown illness or counteract vulnerability. We hypothesize that exposure to chronic stress will exacerbate the vulnerability of MS rats to PB whereas EE will eliminate any such vulnerability. This will be evident in behavioral, peripheral and central indicators of stress and anxiety. By contrast, the H rat will not be affected by either environment with respect to persistent effects of PB. Thus, postnatally determined vulnerability to PB can be considered a "state" which can be manipulated by environmental cues whereas postnatally determined immunity to PB can be considered a "trait" which is not altered by environmental cues.
For each of two studies we will generate MS and H Long-Evans Hooded rats through repeated daily separation of litters from mothers for 180 or 15 minutes per day respectively for the first two weeks of life. In the first study adult rats will be subjected to repeated restraint stress, or nonstress control, for 4 days prior to being given 0.5 mg/kg PB i.p., or an equal volume of saline. Plasma ACTH and corticosterone responses to restraint and PB will be measured and a subset of rats will be sacrificed and neuronal activity in particular brain regions will be determined using Fos expression. In the second study rats will be group housed in complex cages containing a variety of novel objects (EE) or singly housed in standard shoe box cages from weaning until acute drug treatment at 90 days of age. Again, Fos expression in the brain will be determined. For both studies a subset of rats will also be tested for ASR 1 and 14 days after PB treatment.
Impact: These results of these studies will provide information on central, peripheral and behavioral responses to PB in an animal model of PB vulnerability and how further environmental manipulations can ameliorate or exaggerate this vulnerability.
Status: Project is ongoing.