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The central administration of oxytocin and oxytocin analogs to steroid-primed female rats : an investigation of the effects of brain site, dose, receptor type, and time parameters on the generation of lordosis behavior Schulze, H. Georg


Oxytocin plays an important role in the orchestration of many behaviors, including reproductive behaviors, in the female rat. Although it is known to influence sexual receptivity, relatively little is known about the central sites of action of oxytocin and how these contribute to the generation of lordosis. Furthermore, it is not clear what the mechanisms of action of this peptide at these sites may be, and how temporal factors influence oxytocinergic activity. The purpose of this thesis is to investigate the role of oxytocin in the generation of lordosis behavior in the female rat from a variety of perspectives in order to gain a broader understanding of this behavior. The administration of low doses of oxytocin to the lateral ventricle of female rats treated with 5 µg estradiol benzoate and 150 µg progesterone inhibited lordosis behavior while higher doses had no effect. This contrasted with the administration of similar oxytocin doses to the medial preoptic area where low doses had little effect and higher doses tended to elevate primarily lordosis quotients. Higher oxytocin doses administered to the ventromedial hypothalamus extended lordosis durations but had a smaller effect on lordosis quotients. In the central nucleus of the amygdala, the same high doses also extended lordosis durations but not lordosis quotients. These results imply that different brain sites may contribute to different aspects of the lordosis behavior: the medial preoptic area appears to be responsible for lordosis frequency (as measured by the lordosis quotient), the amygdala for lordosis duration, and the hypothalamus perhaps for lordosis initiation. The infusion of an oxytocin agonist and antagonist into the lateral ventricles of steroid-primed female rats reduced lordosis quotients. The same agents infused into the medial preoptic area and the ventromedial hypothalamus did not produce any significant effects. These results are difficult to interpret but appear to be congruent with the notion that both the vasopressinergic system and the oxytocinergic system exercise control over lordosis behavior. The former system is inhibitory while the latter system appears to facilitate sexual behavior only when a certain threshold of neural activity has been reached. It furthermore appears likely that both peptides can act on the receptors of both systems. When infused into the left lateral ventricles of steroid-treated female rats, the oxytocin agonist employed in these studies facilitated lordosis behavior 32 h after estradiol administration but not 48 h later when most reported investigations have been carried out. This implies that sufficient' functional oxytocin receptors are induced within 32 h after estrogen treatment. It also raises the possibility that receptor induction and decay at different central sites may follow different time courses. Taken together, these studies indicate that several brain sites contribute to the generation of lordosis and that both the vasopressinergic and oxytocinergic systems may be involved. Furthermore, these systems may be differentiated by temporal patterns of activation.

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