Open-field behavior and the peripheral sympathetic nervous system in the MR/N and MNR/N rat strains.

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The Maudsley Reactive (MR) and Maudsley Non-Reactive (MNR) strains of rats, respectively selected by P. L. Broadhurst for high and low open-field defecation (OFD), were acquired by the National Institutes of Health in 1963 at the 21st generation of inbreeding and have been inbred as the MR/N and MNR/N strains at that location for more than 40 generations. The present experiment shows that, despite the lack of deliberate selection for more than 40 generations, the strains still differ predictably in OFD. Strain differences in open-field activity, originally found to be inversely associated with those in OFD, have also been preserved. The existence of an association between the peripheral sympathetic system and OFD originally established in the Har derivation of the Maudsley strains was also confirmed in the MR/N and MNR/N strains: MR/N male rats, with the relatively higher levels of OFD, exhibited lower concentrations of norepinephrine (NE) in spleen and descending colon than MNR/N rats. The pattern of biochemical and morphological findings obtained in this study and previous data indicate that the MNR/N and the MNR/Har strains are both derived from the British stocks of this strain carrying the agouti allele (AA). A distinction should be made between these strains and the MNRA/Har strain, which carries the nonagouti allele (aa) and which has been genetically isolated from the Nonreactive strains bearing the agouti allele since early in the selection experiment.