71 0001-8244/00/0100-0071$18.00/0 © 2000 Plenum, Publishing Corporation Behavior Genetics, Vol. 30, No. 1, 2000 files in the Roman/Verh rats an inbreeding program was initiated in 1993, derived through brother/ sister mating from both lines (Driscoll et al., 1998; Escori- huela et al., 1999), and leading to the RHA-I/Verh and RLA-I/Verh strains. Behavioral and neuroen- docrine studies performed thus far have shown that RHA-I/Verh rats are also less inhibited (emotional/ anxious), display lower corticosterone and prolactin responses to stress, and are rather novelty seekers when compared to their RLA-I/Verh counterparts (e.g., Driscoll et al., 1998; Escorihuela et al., 1999; Steimer et al., 1997). Male RHA-I/Verh rats have also been reported to show a lower acoustic startle response, before or after foot shock sensitization, than RLA-I/Verh rats (Schwegler et al., 1997). The acoustic startle response is a widely used measure for emotional responses which is enhanced by fear conditioning, as well as by unconditioned footshocks and other stressful manip- ulations (Davis et al., 1993; Lang et al., 1990). The present study was aimed at investigating, in inbred RHA-I/Verh and RLA-I/Verh rats of both sexes, possible effects exerted upon the acoustic INTRODUCTION The Swiss sublines of Roman high-avoidance (RHA/Verh) and low-avoidance (RLA/Verh) rats, have been psychogenetically selected for good ver- sus extremely poor (respectively) two-way active avoidance in the shuttle box for more than 25 years (Driscoll and Bättig, 1982; Escorihuela et al., 1995). The two rat lines differ in many other behavioral and neuroendocrine/neurochemical characteristics, in the direction of a higher emotionality/stress reactivity in the RLA/Verh line (Driscoll et al., 1998), which conforms to evidence showing that the initial phases of shuttle box avoidance acquisition depend upon emotionality/anxiety levels (Escorihuela et al., 1995; Fernández-Teruel et al., 1991). With the aim of identifying the possible genetic bases responsible for the divergent emotionality pro- Differential Effects of Cohort Removal Stress on the Acoustic Startle Response of the Roman/Verh Rat Strains R. Aguilar, 1 L. Gil, 1 A. Tobeña, 1 R. M. Escorihuela, 1 and A. Fernández-Teruel 1,2 Received 26 June 1999—Final 13 Aug. 1999 Male and female rats of the inbred Roman/Verh strains, which have been psychogenetically selected and bred for good (RHA-I/Verh) and extremely poor (RLA-I/Verh) two-way avoid- ance acquisition, were evaluated in an acoustic startle response test. One half of the rats of each strain and sex were previously subjected to 30 min of isolation by removal of their cage partners (“cohort removal”). During the testing session each animal received 40 acoustic stim- uli at interstimulus intervals of 30 seconds. The hyperemotional RLA-I/Verh rats (especially the males) showed stronger acoustic startle responses than did their RHA-I/Verh counterparts. Startle amplitudes of the RLA-I/Verh males were further enhanced by the stress of 30 min isolation, whereas cohort removal did not significantly affect startle response amplitudes in RHA-I/Verh rats of either sex or in females of the RLA-I/Verh strain. KEY WORDS: Genetic selection; emotionality; anxiety; amygdala; acoustic startle responses; cohort removal; isolation stress. 1 Medical Psychology Unit, Department of Psychiatry and Forensic Medicine, School of Medicine, Autonomous University of Barcelona, 08193 Bellaterra, Barcelona, Spain. 2 To whom correspondence should be addressed. Fax: 34-93- 5811435.