The SHR Exhibits Less “Anxiety” but Increased Sensitivity to the Anticonflict Effect of Clonidine Compared to Normotensive Controls

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Abstract
The behaviour of the adult (16 weeks) spontaneously hypertensive rat (SHR) was compared to that of age‐matched normotensive Wistar (WISTAR) and Wistar‐Kyoto (WKY) rats in two animal models of anxiety, Vogel's conflict test (VT) and Montgomery's conflict test (MT). With respect to “inborn anxiety”, the three strains ranked differently in the two tests, WISTAR>WKY = SHR in the VT, but WKY>WISTAR>SHR in the MT. However, the SHR was the least “anxious” strain in both tests. The SHR also appeared more active and reactive than both the other strains. In the SHR and in the WKY, but not in the WISTAR, the α2‐adrenoceptor agonist clonidine in a narrow low dose‐range produced anxiolytic‐like effects of similar magnitudes. The SHR, however, responded in a ten‐fold lower dose than did the WKY. The present findings, in conjunction with previous data, are not in all parts compatible with the locus ceruleus hypothesis of anxiety and may indicate that the anticonflict effect of clonidine is mediated through activation of postsynaptic α2‐adrenoceptors rather than of α2‐autoreceptors.