Effects of plus-maze experience and chlordiazepoxide on anxiety-like behavior and serotonin neural activity in the dorsal raphe nucleus in rats.

The extent to which rats express anxiety-like behavior on the elevated plus-maze (EPM) depends on their previous maze experience. Open-arm avoidance develops in maze-experienced rats, and is often accompanied by a diminished anxiolytic response to benzodiazepines. Regions of the dorsal raphe nucleus (DRN) were examined in male Sprague-Dawley rats using c-Fos and serotonin immunohistochemistry following a single exposure, a second exposure or no exposure to the EPM. We then examined the effect of the benzodiazepine anxiolytic chlordiazepoxide (CDP, 5 mg/kg) on EPM behavior and DRN neural activity. Enhanced open-arm avoidance was evident on the second EPM trial in both experiments. The observed pattern of c-Fos expression suggests that the first exposure to the maze activates serotonin cells in the rostral and dorsal regions of the DRN and that only the dorsal subregion is activated by a second exposure. CDP increased open-arm exploration during the first trial, which corresponded to decreased 5-hydroxytryptamine (5-HT) activity in the rostral and ventral subregions of the DRN. However, 5-HT activity in the DRN was reduced in rats on the second maze trial compared with the first trial, when CDP had no effect on open-arm exploration. These results suggest that open-arm avoidance in maze-experienced rats can be characterized as a coping response that is mediated by specific populations of 5-HT neurons in the DRN.

Measuring salivary cortisol in the behavioral neuroscience laboratory.

It is often difficult for instructors teaching laboratory courses in behavioral neuroscience to find appropriate experiments that can ethically examine biological parameters in human participants. In most instances, the default experiments that allow students to act as both experimenter and subject tend to be electrophysiological in nature (e.g., EEG, GSR, etc.). We report here the use of an experiment module that utilizes an easily-obtained enzyme immunoassay (EIA) kit to measure human salivary cortisol. Cortisol is a hormone of the adrenal cortex that can be used as a peripheral indicator of hypothalamic neural activity. Plasma (and salivary) cortisol levels rise due to circadian influences as well as perturbations in the organism's environment (i.e., stressors). The involvement of the hypothalamic-pituitary-adrenal (HPA) axis in the pathophysiology of depression makes this an appealing module to students in behavioral neuroscience laboratories. Measurement of salivary cortisol takes advantage of a simple, painless, non-invasive sampling procedure. The assay can be performed successfully by anyone with access to a plate reader, a shaker or rotary mixer, and a few commonly used pipettors. A single plate assay can be completed in two to three hours. Students in our behavioral neuroscience laboratory class have utilized this kit successfully to examine the circadian cortisol rhythm as well as the effect of stress/relaxation on cortisol levels.

Electrolytic lesions and pharmacological inhibition of the dorsal raphe nucleus prevent stressor potentiation of morphine conditioned place preference in rats.

RATIONALE: Exposure to a single session of uncontrollable inescapable shock (IS), but not to identical controllable escapable shock, produces a potentiation of morphine's rewarding properties that is unusual in that the stressor can be given a number of days before the drug administration in an environment quite different from the drug context. Many other behavioral outcomes of stressors that depend on the uncontrollability of the stressor are mediated by alterations in serotonergic (5-HT) neurons within the dorsal raphe nucleus (DRN). OBJECTIVES: The present experiments examined the role of the DRN and 5-HT in mediating the effect of IS on the rewarding properties of morphine as assessed by conditioned place preference (CPP). METHODS: In experiment 1, subjects received small electrolytic lesions of the DRN and were tested for morphine (3.0 mg/kg, SC) CPP after IS or control treatment. In experiment 2, subjects received an intra-DRN microinjection of the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 1.0 microg/0.5 microl) either before IS or before morphine (3.0 mg/kg, SC) injections during CPP testing. RESULTS: IS potentiated morphine CPP in controls, but both DRN lesion and intra-DRN 8-OH-DPAT, either before IS or before morphine administration, completely blocked this effect. CONCLUSIONS: These data implicate alterations in DRN 5-HT neurons in the potentiation of morphine reward produced by uncontrollable stress.

Serum cholesterol levels and stressor controllability in rats.

Whether an organism can control a stressful event is often an important variable determining the impact of the event on physiology and behavior. Numerous behavioral and physiological variables are more adversely affected by uncontrollable stress. The present experiment with rat subjects compared the effect of controllable stress (escape conditioning) or uncontrollable stress (yoked control group) vs. home cage controls on total cholesterol, as well as high-density lipoprotein (HDL) and low/very-low density lipoprotein (LDL/VLDL) serum cholesterol. Results indicated that both stressed groups had higher total and LDL/VLDL cholesterol levels than home cage controls. No group differences were observed with HDL cholesterol. The escape and yoked control subjects did not differ from each other in any dependent measure. Results are discussed in terms of the probable mediators of stress-induced cholesterol increases, and the fact that these mediators may be insensitive to stressor controllability.

Blockade of alpha1 adrenoreceptors in the dorsal raphe nucleus prevents enhanced conditioned fear and impaired escape performance following uncontrollable stressor exposure in rats.

Previous research has shown that the effect of exposure to uncontrollable stressors on conditioned fear responding and escape behavior in rats is dependent on serotonergic neural activity in the dorsal raphe nucleus (DRN). The role that norepinephrine released in the DRN plays in producing the behavioral consequences of exposure to inescapable tail shock in rats was investigated in the present study. The selective alpha1 adrenoreceptor antagonist benoxathian was injected into the DRN before exposure to inescapable tail shock or before behavioral testing conducted 24 h later. Benoxathian prevented the impairment of escape responding produced by inescapable shock, but did not reverse this effect when given before testing. The enhancement of conditioned fear produced by prior inescapable shock was attenuated by benoxathian administered before inescapable shock or before behavioral testing. These results support the view that noradrenergic input to the DRN is necessary to produce the behavioral effects of inescapable tail shock.