Avoidance expression in rats as a function of signal-shock interval: strain and sex differences

Thomas R Minor,Pelin Avcu,Xilu Jiao,Kevin C H Pang,Kevin D Beck,Richard J Servatius,Nora Ko

doi:10.3389/fnbeh.2015.00168

Abstract

Inbred Wistar Kyoto (WKY) rats express inhibited temperament, increased sensitivity to stress, and exaggerated expressions of avoidance. A long-standing observation for lever press escape/avoidance learning in rats is the duration of the warning signal (WS) determines whether avoidance is expressed over escape. Outbred female Sprague-Dawley (SD) rats trained with a 10-s WS efficiently escaped, but failed to exhibit avoidance; avoidance was exhibited to a high degree with WSs longer than 20-s. We examined this longstanding WS duration function and extended it to male SD and male and female WKY rats. A cross-over design with two WS durations (10 or 60 s) was employed. Rats were trained (20 trials/session) in four phases: acquisition (10 sessions), extinction (10 sessions), re-acquisition (8 sessions) and re-extinction (8 sessions). Consistent with the literature, female and male SD rats failed to express avoidance to an appreciable degree with a 10-s WS. When these rats were switched to a 60-s WS, performance levels in the initial session of training resembled the peak performance of rats trained with a 60-s WS. Therefore, the avoidance relationship was acquired, but not expressed at 10-s WS. Further, poor avoidance at 10-s does not adversely affect expression at 60-s. Failure to express avoidance with a 10-s WS likely reflects contrasting reinforcement value of avoidance, not a reduction in the amount of time available to respond or competing responses. In contrast, WKY rats exhibited robust avoidance with a 10-s WS, which was most apparent in female WKY rats. Exaggerated expression of avoidances by WKY rats, especially female rats, further confirms this inbred strain as a model of anxiety vulnerability.

Highlights

Avoidance encompasses efforts, thoughts, and behaviors to forestall or eliminate a predicted aversive event or state
Acquisition was poor with a 10-s warning signal (WS) in male and female SD rats; each exhibited less than 20% avoidance by the end of 10 session of training
Male Wistar Kyoto (WKY) rats achieved a modest degree of avoidance (∼60%) by the end of 10th session, whereas acquisition by female WKY rats with a 10-s WS was similar to that expressed with a 60-s WS

Summary

Introduction

Thoughts, and behaviors to forestall or eliminate a predicted aversive event or state. One animal model is discrete trial lever press or bar press avoidance. A lever press is not among species specific defense reactions that confound the interpretation of avoidance learning and expression (Bolles, 1970). A lever press during the WS prevents foot shock and constitutes an avoidance response. In the absence of an avoidance response, intermittent foot shocks are delivered for a period of time. A lever press after the initiation of foot shock terminates shock and initiated a safety period, which may be signaled. An escape response is generally acquired early in training with avoidance emerging and reaching asymptotic performance over several sessions of training (Servatius et al, 2008; Jiao et al, 2011, 2014; Pang et al, 2011; Avcu et al, 2014; Beck et al, 2014). Avoidance is not expressed as quickly as other preparations (e.g., shuttle box), the slowness in acquisition is cited as a virtue (Bolles, 1970)

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Discussion

Conclusion