Stressor predictability and rat brain noradrenaline metabolism.

This study examined the effects of stressor predictability on regional rat brain noradrenaline (NA) turnover, by measuring levels of a principal metabolite of NA (3-methoxy-4-hydroxyphenylethyleneglycol sulfate, MHPG-SO4). Male Wistar rats were exposed to one of three shock conditions for 19 hr: nonshock, signalled, and unsignalled shocks. Rats in the shock conditions received shock (1.2 mA intensity, 2 sec duration) on a 2.5 min variable time (VT) either preceded by a 12-sec, 10-W light signal (signal-shock interval of 10 sec) or not preceded by this signal. The tail electrodes for these rats were in series, so that the shock received by all rats was of exactly the same number and duration. After 19 hr in a VT-2.5 min shock session, the rats exposed to unsignalled shock (unpredictable group) showed significantly greater increases in MHPG-SO4 levels in the hypothalamus, amygdala, midbrain, cerebral cortex, thalamus and locus coeruleus, as well as in plasma corticosterone levels. Rats exposed to signalled shock (predictable group) showed significant increases in MHPG-SO4 levels in the first four of these regions, as compared to the nonshocked rats. Moreover, the unpredictably shocked rats exhibited greater elevations in MHPG-SO4 levels in the hypothalamus, amygdala, and thalamus, as well as in plasma corticosterone levels, when compared to the predictably shocked rats. These results are consistent with previous reports showing that unsignalled shock induced extensive somatic effects in comparison to signalled shock. The present study suggests that the presence of a signal attenuates the extent of NA release in some brain regions resulting from irregular inescapable shock stress.