SUSCEPTIBILITY AND RESISTANCE OF RATS TO STRESS-INDUCED DECREASES IN SWIM-TEST ACTIVITY - A SELECTIVE BREEDING STUDY

In this study, selective breeding was used to generate two populations of rats that differed in their susceptibility to showing decreased st ruggling activity in a swim test after being exposed to uncontrollable electric tail-shock. After five generations of selective breeding, we obtained a population that displayed large decreases in swim-test str uggling after shock (swim-test susceptible) and a population that disp layed no decrease in struggling after shock (swim-test resistant). Mal es of this fifth generation from the two selectively-bred populations were then compared for differences in non-swim behavioral measures (ho me-cage 24-h spontaneous ambulatory activity and food/water intake) an d several aspects of brain catecholaminergic activity, including elect rophysiological activity of locus coeruleus (LC) neurons, catecholamin e/metabolite concentrations in various brain regions, and in vivo tyro sine hydroxylase activity. Interestingly, swim-test resistant rats dis played larger decreases in home-cage ambulatory activity and water int ake after exposure to shock than did swim-test susceptible animals. Ma rked differences were also seen in measures of brain noradrenergic act ivity. Compared to the susceptible rats, resistant rats showed higher levels of evoked activity of LC neurons, larger shock-induced depletio ns of norepinephrine (NE) and 3-methoxy-4-hydroxyphenylglycol (MHPG) i n the LC, lower in vivo tyrosine hydroxylase (TH) activity in ventral bundle projection areas such as the hypothalamus, and larger amounts o f NE in dorsal bundle projection areas. Finally, swim-test resistant r ats had much higher concentrations of dopamine (DA) and dihydroxypheny lacetic acid (DOPAC) in striatum and nucleus accumbens than susceptibl e rats. These results appear to be explainable on the basis that diffe rences in swim-test struggling behavior for which the two populations were selectively bred were a consequence of differences in forebrain D A whereas stress-induced differences in other behavioral measures (i.e . spontaneous ambulation and intake) occurred because swim-test resist ant animals showed greater disturbance of the LC-NE system after uncon trollable shock.