Sex-dependent effects of neonatally administered morphiceptin and D-ala-D-leu-enkephalin on maze learning in rats

Newborn rats were injected (ip) with morphiceptin [72.7 micrograms/kg (a mu-type opioid receptor agonist)], D-alanine2-D-leucine5-enkephalin [79.4 micrograms/kg (a delta-receptor agonist)], or saline for 7 days after birth. Testing on a complex maze on Day 25 revealed a significant sex-dependent facilitation of performance by the opioid peptides. Peptide-treated females performed better than the males on the first day of training as measured by errors. Opioid treatment increased mortality, as three times as many peptide-treated animals died in comparison to the saline control group. Peptide treatment did not affect locomotor activity measured in an open field. Weight at Day 24 was also affected by the peptide treatment, females and males injected with the opioids being lighter and heavier, respectively, than the control group.     

Self-control in male and female rats

Eight male and 8 female Wistar rats were exposed to a discrete-trial procedure in which they chose between the presentation of a small (one pellet) or a large (three pellets) reinforcer. The delay to the small and large reinforcer was 6.0 s in the first condition of Experiment 1. Subjects consistently chose the large reinforcer. When the delay to the small reinforcer was decreased to 0.1 s in the next experimental condition, all subjects continued to choose the large 6.0-s delayed reinforcer. When the contingencies correlated with the two levers were reversed in the next experimental condition, the majority of subjects (5 males and 6 females) still chose the large delayed reinforcer over the small immediately presented reinforcer. The delay to the small reinforcer was maintained at 6.0 s, but the delay to the large reinforcer was varied among 9.0, 15.0, 24.0, and 36.0 s in Experiment 2, in which 4 males and 4 females participated. Most subjects consistently chose the large increasingly delayed reinforcer, although choice for the small 6.0-s delayed reinforcer developed in some females when the large reinforcer was delayed for 24.0 or 36.0 s. These choice patterns were not predicted from a literal application of a model that says choice should favor the alternative correlated with the higher (amount/delay) ratio.     

ACTH 4-9 analog can retard spatial alternation learning in brain damaged and normal rats

Thirty adult male hooded rats (Long-Evans strain) were assigned randomly to one of three lesion groups (n = 10) and prepared with medial frontal, posterior parietal, or sham neocortical injuries. Following a recovery interval of 10-12 days, access to water was limited to 30 min per day and the rats were shaped to traverse a T-maze for a reward of sweetened water. After a pretraining criterion was attained, osmotic minipumps (Alzet 2002) were installed subcutaneously. The minipumps delivered chronically for the next 14 to 15 days either 0 or 1.2 micrograms of ACTH 4-9 dissolved in bacteriostatic saline per day while the rats were trained on a reinforced spatial alternation task. Analysis of the number of errors made to a criterion of at least 80% correct alternations in two consecutive training sessions, or a ceiling of 62 errors (attained by two rats with parietal lesions), revealed that learning was impaired in the rats with parietal injuries. Contrary to our hypothesis, animals receiving ACTH 4-9 committed more errors than their counterparts receiving only saline.     

Intelligence and regional brain volumes in normal controls

The relationship between brain size and intelligence was examined in a large sample of healthy normal volunteers (N = 90). In addition to confirmation of the small but significant relationship between IQ and whole brain volume (r = 0.25, p < .05) using magnetic resonance imaging, regional differences were examined using a completely automated procedure to generate lobar volumes to address questions related to regional specificity and its relationship to intelligence. A significant positive relationship was found between measures of full scale IQ and frontal (r = 0.25, p < .05) and temporal (r = 0.28, p < .01) regions, although when correlations across different brain regions were tested against each other, there was no evidence of regional specificity. In addition, no laterality effects were seen. Correlations between WAIS-R subtest scores and brain regions indicated few correlations between verbal subtest scores and brain size, but several significant correlations between the performance subtest scores and frontal, temporal, parietal, and cerebellar brain regions. These results support the notion of a modest relationship between brain size and measures of global intelligence and suggest diffuse brain involvement on performance tasks which require integration and use of multiple cognitive domains.     

Estrogen abolishes latent inhibition in ovariectomized female rats

Estrogen is frequently prescribed as a method of birth control and as hormone replacement therapy for post-menopausal women with varied effects on cognition. Here the effects of estrogen on attention were examined using the latent inhibition (LI) behavioral paradigm. Ovariectomized (OVX) female rats were given either estrogen benzoate (EB, 10 or 100 microg/ml/kg; SC) or sesame oil vehicle. Males and OVX females receiving vehicle displayed normal LI. In contrast, LI was abolished in OVX females receiving EB. The lack of LI in OVX females receiving EB was a result of low suppression ratios, reflecting strong conditioning between the tone and the shock in these subjects even if they were pre-exposed to the tone. Thus, estrogen impaired the ability of OVX females to ignore irrelevant stimuli. Since different cognitive tasks vary in their required ability to ignore irrelevant stimuli, these results may account for some of the variations in the current literature on estrogen and cognition.