Effects of US devaluation on win-stay and win-shift radial maze performance in rats

Previous studies have shown double dissociations between win-stay and win-shift radial maze learning in terms of their underlying neural substrates. To examine the content of the associations formed in the two tasks, the authors devalued the food unconditioned stimulus (US) by taste aversion to differentiate stimulus-stimulus(CS-US) and stimulus-response (CS-CR) learning. US devaluation was performed in rats that were over- or undertrained on the win-stay task. Devaluation substantially reduced food consumption on the maze but failed to disrupt choice accuracy, regardless of the amount of training. Devaluation did not affect latency in overtrained rats but did increase latency in undertrained rats. In the win-shift task, devaluation caused rats to reject the reinforcer, yet they continued to accurately win-shift, but with significantly longer latencies (Experiment 3). The results suggest that an S-R association may mediate performance after extended win-stay training. In contrast, a US representation appears to be recalled during early win-stay and win-shift performance.

Effects of US devaluation on win-stay and win-shift radial maze performance in rats

Previous studies have shown double dissociations between win-stay and win-shift radial maze learning in terms of their underlying neural substrates. To examine the content of the associations formed in the two tasks, the authors devalued the food unconditioned stimulus (US) by taste aversion to differentiate stimulus-stimulus(CS-US) and stimulus-response (CS-CR) learning. US devaluation was performed in rats that were over- or undertrained on the win-stay task. Devaluation substantially reduced food consumption on the maze but failed to disrupt choice accuracy, regardless of the amount of training. Devaluation did not affect latency in overtrained rats but did increase latency in undertrained rats. In the win-shift task, devaluation caused rats to reject the reinforcer, yet they continued to accurately win-shift, but with significantly longer latencies (Experiment 3). The results suggest that an S-R association may mediate performance after extended win-stay training. In contrast, a US representation appears to be recalled during early win-stay and win-shift performance.

Scopolamine and Pavlovian fear conditioning in rats: dose-effect analysis

Muscarinic-cholinergic antagonism produces learning and memory deficits in a wide variety of hippocampal-dependent tasks. Hippocampal lesions produce both acquisition deficits and retrograde amnesia of contextual fear (fear of the place of conditioning), but do not impact fear conditioning to discrete cues (such as a tone). In order to examine the effects of muscarinic antagonism in this paradigm, rats were given 0.01 to 100 mg/kg of scopolamine (or methylscopolamine) either before or after a fear conditioning session in which tones were paired with aversive footshocks. Fear to the context and the tone were assessed by measuring freezing in separate tests. It was found that pretraining, but not post-training, scopolamine severely impaired fear conditioning; methylscopolamine was ineffective in disrupting conditioning. Although contextual fear conditioning was more sensitive to cholinergic disruption, high doses of scopolamine also disrupted tone conditioning. Scopolamine did not affect footshock reactivity, but did produce high levels of activity. However, hyperactivity was not directly responsible for deficits in conditioning. It was concluded that scopolamine disrupts CS-US association formation or CS processing, perhaps through an attenuation of hippocampal theta rhythm.

Attenuated sensitivity to neuroactive steroids in gamma-aminobutyrate type A receptor delta subunit knockout mice

gamma-Aminobutyric acid (GABA) type A receptors mediate fast inhibitory synaptic transmission and have been implicated in responses to sedative/hypnotic agents (including neuroactive steroids), anxiety, and learning and memory. Using gene targeting technology, we generated a strain of mice deficient in the delta subunit of the GABA type A receptors. In vivo testing of various behavioral responses revealed a strikingly selective attenuation of responses to neuroactive steroids, but not to other modulatory drugs. Electrophysiological recordings from hippocampal slices revealed a significantly faster miniature inhibitory postsynaptic current decay time in null mice, with no change in miniature inhibitory postsynaptic current amplitude or frequency. Learning and memory assessed with fear conditioning were normal. These results begin to illuminate the novel contributions of the delta subunit to GABA pharmacology and sedative/hypnotic responses and behavior and provide insights into the physiology of neurosteroids.

Survival of Escherichia coli O157:H7 after freezing and thawing in ground beef patties

Survival of Escherichia coli O157:H7 strains QA 326, and ATCC 43889, 43894, and 43895 after freezing (-20 degrees C, 24 h) and thawing (4 degrees C for 12 h, 23 degrees C for 3 h, or microwave heating of 700 W for 120 s) in ground beef patties was determined by reference most probable number (MPN), hydrophobic grid membrane filter SD-39 agar, and sorbitol MacConkey agar (SMA) spread-plating methods. Populations decreased from 0.62 to 2.52 log10 CFU/g, with the extent varying significantly by strain. Strain QA 326 populations almost always decreased the most, up to 1.87 log10 CFU/g more than the least sensitive strain. Microwave heating was the most lethal thawing treatment for strain QA 326, and 4 degrees C thawing was the most lethal treatment for strain ATCC 43894. Thawing treatments varied in relative lethality for the other two strains. For strain QA 326 (4 degrees C and microwave thaw treatments) and strain ATCC 43889 (4 and 23 degrees C thawing), the enumeration method significantly affected a population decrease. The SD-39 agar method best recovered strain QA 326 while the SD-39 agar method and the reference MPN method best recovered strain ATCC 43889 after 4 and 23 degrees C thawing, respectively. The greatest difference in population decrease measured by any two methods was 0.58 log10 CFU/g. Results showed (i) a wide range in freeze-thaw sensitivity among E. coli O157:H7 strains, (ii) no thawing method had consistently and significantly greater lethality, and (iii) the reference MPN, SD-39 agar, and SMA methods differed little in ability to enumerate E. coli O157:H7.

Evaluating survival of Escherichia coli O157:H7 in frozen and thawed apple cider: potential use of a hydrophobic grid membrane filter-SD-39 agar method

To determine the susceptibility of Escherichia coli O157:H7 to freezing and thawing in apple cider, methods that recover injured cells are needed for accurate enumeration. This study compared the ISO-GRID hydrophobic grid membrane filter (HGMF) SD-39 agar method to two other methods: a reference most probable number (MPN) method, and plating on sorbitol MacConkey agar (SMA). To determine numbers of injured cells, SMA spread plating was also compared to Trypticase soy agar (TSA) spread plating. Two strains of E. coli O157:H7 QA 326 and ATCC 43895, were inoculated into presterilized apple cider (10 ml) which was then frozen (-20 degrees C for 24 h). Samples were thawed at 4 degrees C for 4 h, or at 23 degrees C for 1.5 h, or in a microwave oven (700 W for 10 s). Substantial cell death (0.69- to 6.33 log10 CFU/ml decreases) and injury (0.70- to 2.38-log10 CFU/ml decreases) occurred during freezing and thawing. The extent of death and injury varied with strain and thawing method. The TSA spread plating method recovered the most cells while the HGMF method always recovered more viable cells than the reference MPN method and also either recovered significantly more (P < 0.05) cells or a not significantly different number of cells than SMA spread plating. Some injured cells of both strains were not counted by the HGMF method. Significant numbers of cells injured by freezing and thawing at 4 degrees C in apple cider were enumerated in the cider was diluted 1:2 Trypticase soy broth immediately before plating. Two epifluorescent microscopic methods showed that injury was not associated with loss of cell membrane integrity.