JD 82 Competition between

In female rats, food intake is reduced during the period in the estrous cycle when the rat is sexually receptive and ovulation is imminent (Wade and Gray, 1979). By contrast, food intake is increased during lactation in the postpartum period (Linden et al, 1990) and sexual behaviour and ovulation are inhibited (Sodersten et al, 1983). There are several examples demonstrating that the neuroendocrine control mechanisms of ingestive and sexual behaviour are linked to each other. For example, lesions in the ventromedial hypothalamus (VMH) disrupt sexual behaviour (Pfaff, 1980) and stimulate food intake (Shimizu et al, 1987). Administration of estrogen into the VMH stimulates sexual behaviour and decreases food intake in ovariectomized female rats (Wade and Gray, 1979). The ingestive behaviour of male rats is suppressed by the presence of sexually receptive females and the sexual behaviour of males is not affected by the presence of a bottle of sucrose when a sexually receptive female is present (Saito et al, 1999). This suggests that the presence of a sexual partner inhibits appetitive ingestive behaviour, i.e. the responses used by male rats to obtain food. Lactation is a state characterized by extensive neuronal and hormonal plasticity. Maternal behaviour is induced, there is a marked hyperphagia and significant increase in plasma levels of prolactin (PRL), and the normal estrus cycle is suppressed. It is well known that PRL also has numerous effects in mammalian brain functions including induction of maternal behaviour (Bridges et al, 1997), increased food intake (Sauve and Woodside, 1996), and inhibition of GnRH secretion (Bohnet et al, 1976). During lactation, rats become more aggressive to intruders, less emotionally responsive to novel situations and clearly demonstrate reduced anxiety (Neumann, 2003). Exposure to pups enhances the foraging ability of primiparous as well as nulliparous rats in foraging task tests (Lambert et al, 2005), and the aim of the present study was to initiScand. J. Lab. Anim. Sci. 2006 Vol. 33 No. 2