17.6 n&v 707 MH

iour’ to variation in the expression patterns of the receptors for two related signalling molecules, oxytocin and arginine vasopressin (vasopressin for short). These molecules are released in the brain after sex, and are also involved in other reproductionrelated behaviours and in unrelated functions such as water retention and stress. Notably, the vasopressin 1a receptor (V1aR) is expressed in greater numbers in the ventral pallidum region of the forebrain in male prairie voles than in male meadow voles. Although the exact relationships of the ventral pallidum to other brain regions and to behaviour are still unclear, the neighbouring nucleus accumbens is part of the brain’s ‘reward system’, which signals that a particular behaviour is worth doing again. In prairie voles, manipulation either of signals mediated by the neurotransmitter dopamine in the nucleus accumbens, or of vasopressin signalling within (at least in part) the ventral pallidum, can substitute for or block the effects of sexual activity on preferential association — showing the importance of both regions in forming attachments. The expression of dopamine receptors shows little natural variation between species, however, hinting that the key to the differing behaviour of prairie and meadow voles might lie in the variation in vasopressin receptors. Lim et al. now take us beyond correlations in the study of these receptors, by experimentally manipulating the expression of V1aR. The authors wanted to find out whether adult male meadow voles that have prairie-vole-like expression of news and views