Behavioral effects of neurotoxic lesions of the ascending monoamine pathways in the rat brain.

Intracranial microinjections of 6-hydroxydopamine or 5,6-dihydroxytryptamine into 6 ascending monoamine pathways produced the expected patterns of depletion of telencephalic serotonin, dopamine (DA), and norepinephrine (NE). Serotonin level was specifically lowered after dorsal or median raphe lesions but not after mesolimbic or nigrostriatal system lesions which lowered both NE and DA. Lesions in the locus coeruleus (LC) or ventral noradrenergic bundle lowered only NE, and LC lesions elevated serotonin level. Behavior was examined in an open field, 1-way active avoidance, and 2 passive avoidance (PA) tasks, and measures were taken of water consumption and body weight. Dorsal raphe lesions had no effect on any of the measures; the other 5 lesion groups exhibited deficient acquisition of the 1-way active avoidance task. In the appetitive PA task, only the substantia nigra lesion group exhibited a deficiency. In the step-through PA task, both the substantia nigra and the median raphe groups exhibited a deficit, with the median raphe group exhibiting hyperactivity in the start box during testing. Water consumption was decreased by lesions in the ventral noradrenergic bundle during the 1st postoperative week and was increased in the median raphe group by the 4th postoperative week. Lastly, lesions in the LC dramatically decreased activity in the open field. Results are discussed in regard to the search for specificity of behavioral functions of the distinct ascending monoamine pathways. (27 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Deficient passive and one-way active avoidance acquisition following medial forebrain bundle lesions in rats.

In 3 experiments with 93 Long-Evans male hooded rats, cathodal electrolytic lesions of the medial forebrain bundle (MFB) at posterior hypothalamic levels produced a mild, transient hypodipsia and lowered jump thresholds to footshock. Lesions produced marked deficits in passive avoidance performance in a paradigm that paired discrete, linearly incrementing footshock intensities with contact of a water spout following 48 hrs of water deprivation. Injections of levo-dextro-5-hydroxytryptophan (75 mg g, ip), the immediate metabolic precursor of serotonin, had no effect on the passive avoidance performance of either experimental or operated control Ss. Lesions of the MFB also resulted in deficient acquisition in a 1-trial step-through passive avoidance paradigm not using motivation to drink and caused a severe acquisition deficit in a 1-way active avoidance task. Lesions of the septal nuclei produced lowered jump thresholds but did not affect acquisition in the 1st passive avoidance task. Results are interpreted as indicating a lesion-induced deficiency in fear learning, independent of the serotonergic functions of the MFB. (45 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Conditioning decreases in plasma corticosterone level in rats by pairing stimuli with daily feedings.

Examined the nature of a stimulus-induced decrease in the plasma corticosterone levels of male hooded Long-Evans rats in 3 experiments. Ss maintained on a 23-hr food and water deprivation regimen were fed each morning immediately upon entrance of the experimenter into the otherwise isolated animal quarters. After only 14 feeding trials, Ss showed a marked decrease in corticosterone levels within 10 min of the experimenter's entrance whether fed and watered or not. The acquisition of this decrease was then examined over training trials and by comparing the conditioned animals (Group CD) with a group designed to control for pseudoconditioning, sensitization, and habituation (Group PC) during the use of the more distinctive conditioning stimulus of placing the rat's cage in a sound-attenuating chamber. During training, Group CD was fed .5–6 min after placement in the chamber, and Group PC was fed 1.5–2.5 hrs before or after placement. After 1 training trial, the CD and PC groups showed an increase in corticosterone levels in response to chamber placement when not fed or watered. However, after 14 training trials the CD group exhibited a significant decline, whereas the PC group exhibited a rise in corticosterone levels. Results indicate that external stimuli can cause a decrease in corticosterone level by virtue of prior association with feeding and drinking. This strongly suggests an acquired inhibition of the pituitary-adrenal system. (24 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)