Circling behaviour produced by unilateral lesions in the region of the locus coeruleus in rats.

Rats with unilateral lesions in the region of the locus coeruleus circled tightly to the opposite side when given apomorphine or amphetamine. This turning behaviour was transient and disappeared within some 30 days after surgery. It was seen most obviously in animals with severe unilateral destruction of the locus coeruleus, which caused on average a 55% reduction in the level of noradrenaline in the ipsilateral cerebral cortex. It was not marked in animals with partial unilateral lesions of the locus coeruleus, which caused only an average fall in cortical noradrenaline of 22%. It was not seen in sham operated animals or animals in which lesions were placed into adjacent structures such as the cerebellum above, superior cerebellar peduncle laterally, and brain stem ventrally. A clue to the mechanism of this phenomenon may lie in the observation that dopamine in the ipsilateral striatum was increased 5 days after operation, when circling occurred, but had returned to normal by 30 days when circling had ceased. It is suggested that the lesion causes a reduction in impulse traffic in the ipsilateral nigrostriatal pathway, and that circling is due to preferential stimulation of the ipsilateral striatal dopamine receptors by both drugs; apomorphine directly, amphetamine by release of endogenous dopamine.