CIRCADIAN ACTIVITY, NOCICEPTIVE THRESHOLDS, NIGROSTRIATAL AND MESOLIMBIC DOPAMINERGIC ACTIVITY IN THE NAPLES HIGH-EXCITABILITY AND LOW-EXCITABILITY RAT LINES

These experiments were designed to further characterize the differenti al phenotypic constellation of the Naples High- (NHE) and Naples Low-E xcitability (NLE) lines. In order to determine possible differences be tween NHE and NLE rats in activity and circadian rhythms, besides reac tivity to novelty (selection trait), adult male rats of both strains w ere tested during two 10-min exposures to a Lat-maze. They were then k ept in activity cages continuously for 3 days. Moreover, nociceptive t hresholds were measured with the hot-plate and the tail-flick test, to probe the possibility that these rats could be differentially sensiti ve to nociceptive stimuli. Further, the integrity of the nigro-striata l and mesolimbic system was investigated by measuring tyrosine-hydroxy lase activity in the striatum and 3,4-dihydroxyphenylacetic acid (DOPA C) levels in the striatum as well as in the nucleus accumbens. In addi tion, TH activity was measured in the adrenals to probe the sympatheti c section of the neurovegetative system. The results indicate that NHE and NLE rats differ by a factor of two in their phasic activity in a Lat-maze. In contrast, no differences in 24-h activity during the dark or light phase could be observed in the activity cages. However, NHE rats anticipated the light-on stimulus in the morning by reducing thei r activity 1 h earlier than NLE rats. Further, no difference could be found with the hot-plate and the tail-flick test. Finally, biochemical analyses revealed no difference in the NHE and NLE rats in the main t erminal zone of mesolimbic system (n. accumbens) nor of nigrostriatal system (striatum) nor in the adrenal glands. In conclusion, since the only consistent difference between NHE and NLE rats appears to be reac tivity to spatial novelty, an hippocampus-dependent behavioral trait ( selection trait), independent of altered activity in the sympathetic s ystem or dopaminergic activity in the major dopaminergic brain systems , the usefulness of these strains as genetic model to test current hyp otheses of spatial processor device(s) in the mammalian brain is suppo rted.