Repeated stress has been reported to cause reversible impairment to the hippocampus. Glutamatergic and cholinergic systems were proposed to be involved in responses seen after exposure to stress and cholinesterase inhibitors. Effects of concurrent exposure to repeated stress and chlorpyrifos (CPF) on concentrations of excitatory amino acids, activities of cholinergic enzymes, and maximum binding density (Bmax) and equilibrium dissociation rate constant (Kd) of NMDA and total muscarinic receptors were studied in Long-Evans rats. The study was divided into 4 experiments. The first experiment was to find the dose of CPF to use for studies on the interaction of stress and CPF. From the results obtained, 60% of the maximum tolerated dose was chosen. An experiment to determine effects of repeated stress and CPF on cholinergic enzymes and glutamate included groups of rats (n=7-8) that were handled 5 days/week; restrained 1 hour/day for 5 days/week; swum 30 minutes for 1 day/week; or restrained 4 days/week and swum for 1 day/week, for 28 days. On day 24, each group was injected either with corn oil or CPF 160 mg/kg sc 4 hours after restraint. On day 28, blood samples were collected for acetylcholinesterase (AChE) activity. Brains were dissected into hippocampus (HP) and cerebral cortex (CC) to determine activities of acetylcholinesterase (AChE), carboxylesterase (Cbxy), and choline acetyltransferase (ChAT), and glutamate and aspartate concentrations. CPF inhibited AChE activity in blood, CC and HP, but stress did not affect AChE activity. Repeated restraint with swim reduced Cbxy and CPF inhibited Cbxy. Restraint with swim had a statistical trend to increase concentrations of glutamate in the HP more than swim alone (p = 0.064); but CPF had no effect on glutamate in the HP. CPF decreased concentrations of elevated aspartate in the HP of rats that were restrained and swum. The results suggested that restraint with swim indirectly elevated acetylcholine in the CC, and tended to increase glutamate in the HP. The experiment designed to study the effects of concurrent exposure to stress and CPF on NMDA and total muscarinic receptors was designed similar to the previous study, except that endpoints were Bmax and Kd of NMDA and total muscarinic receptors in the HP and CC, and NMDA receptors in the hypothalamus (HT). Restrained rats had higher Kd of NMDA receptors in the HP than control and restrained with swim rats; however, Bmax was similar. CPF deceased Bmax and Kd of total muscarinic receptors in the CC of swum rats (237.64 &#177; 17.36 fmol/mg protein, 0.216 &#177; 0.023 nM) and CPF also decreased Bmax of total muscarinic receptors in the CC of restrained rats (229.08 &#177; 17.36 fmol/mg protein). There were no effects of stress, CPF, or interactions of stress and CPF on NMDA receptors in the CC or on total muscarinic receptors in the HP. In summary, CPF was capable of modulation of total muscarinic receptors of swum and restrained rats, suggesting that cholinergic transmission in the CC for cognition, sensory and motor activity may be modified. Furthermore, we examined effects of stress and CPF on concentrations of monoamines. Swim stress and CPF individually decreased concentrations of norepinephrine in the HP, whereas swim and restraint with swim decreased concentrations of norepinephrine but increased concentrations of DOPAC in the HT. Swim stress increased concentrations of dopamine in the HT more than control or restraint. CPF did not alter concentrations of norepinephrine, dopamine, or DOPAC in the HT. The interactions of repeated stress and CPF on serotonin approached significance in the HP (p = 0.06) and HT (p = 0.08). CPF increased serotonin concentrations in rats that were handled and restrained but not swum. CPF reduced the elevated concentrations of serotonin in restrained rats and restrained with swim rats (p < 0.05). Swim and restraint with swim were potential stress models that altered noradrenergic, dopaminergic, and serotonergic responses in the HT. In summary, repeated stressors had effects on glutamatergic, cholinergic, and monoamine systems. CPF had effects on cholinergic and monoamine systems but the interactions between stress and CPF were few. / Ph. D.

In certain species, puberty is thought to be a period of susceptibility to various stressors, resulting in pathological behavioral and physiological changes subsequent to exposure during this period. However, juvenile male golden hamsters appear to be fairly resilient to pubertal stress, as compared to adult hamsters and many other species. In these experiments, repeatedly stressed juvenile male hamsters were found to be avoidant of aggressive adult male social stimuli, but did not display anxious behavior outside of a social context. In addition, several long-term changes in neural activity were associated with social stress during early puberty. The medial preoptic area and medial preoptic nucleus, and ventral tegmental area showed decreased neural activity in subjugated juveniles than in naïve individuals. Since these brain areas are involved in the expression of motivated behaviors, specifically sexual behavior, and reward pathways, we next investigated sexual behavior in virgin juveniles. When placed in a confined space with receptive females, consummatory behavior in subjugated juveniles was similar to those observed in naive juveniles. Appetitive aspects of sexual behavior were also tested in a Y-maze to allow subjects to choose whether to approach a social stimulus. When given a choice between a sexually receptive and non-receptive female social stimulus, socially stressed individuals showed anxiety related behaviors and did not show a preference. However, naïve hamsters preferred the non-receptive female. Interestingly, this effect was less significant in naïve animals tested during late puberty and early adulthood, and a preference for sexually receptive females was not observed. In addition, stressed hamsters tested with harnessed females at mid-puberty were slower to approach females, indicating altered motivation to approach adult conspecifics. This research is unique in that it is the first to suggest the disconnect between the development of consummatory and appetitive aspects of sexual behavior. Together, these data examine the effects of stress on the development of pubertal social behaviors. / text

Stress in adolescence is a putative risk factor for developing mental illnesses such as schizophrenia and mood disorders. Symptoms for these illnesses first emerge in late adolescence and early adulthood, with both incidence and severity being sexually dimorphic. Animal models can shed light on the neurobiological underpinnings of these disorders by allowing one to explore the relationship between a risk factor such as stress, and development of symptoms. In the current work the role of adolescent stress is explored in the development of biomarkers that are associated with adolescent-onset illnesses using Long Evans rats. Repeated exposure to predator odour was combined with social isolation during adolescence to create a novel stressor model. The specific objectives of this study were to determine (i) if repeated predator odour exposure altered measures related to sensorimotor gating (measured as prepulse inhibition, PPI), startle, and emotionality, and (ii) whether social support affected the outcome of predator odour stress. Predator odour elicited immediate avoidance, which did not habituate with repeated exposures, suggesting a strong behavioural stress response. In contrast to past work, few significant long-term effects were observed in animals exposed to predator odour compared with ones exposed to a non-threatening odour. Unexpectedly, animals exposed to a no odour (control) condition displayed altered PPI, startle response, anxiety-related behaviour, and memory, compared to rats exposed to a non-threatening, control odour or a predator odour. Moreover, the no odour animals showed altered expression of dopamine D2R receptor protein in the medial prefrontal cortex. The outcomes for this group were remarkably similar to those seen in animals raised in social isolation, suggesting an underlying similarity in the neurobiological mechanisms associated with these experiences that likely can be traced to being raised in environments lacking adequate social and physical complexity. Sex differences were noted in PPI, startle response, tests of anxiety- and depression-like behaviour, memory, and levels of dopamine D2R receptors, although the sex of the animal did not interact with stressor treatment to affect these measures. In conclusion, results of the current work provide further evidence for the importance of the social and physical environment to normal development during adolescence, as well as the importance of being male versus female.