Yes, I am ready now: differential effects of paced versus unpaced mating on anxiety and central oxytocin release in female rats

Abstract

Sexual activity and partner intimacy results in several positive consequences in the context of stress-coping, both in males and females, such as reduced state anxiety in male rats after successful mating. However, in female rats, mating is a rewarding experience only when the estrous female is able to control sexual interactions, i.e., under paced-mating conditions. Here, we demonstrate that sex-steroid priming required for female mating is anxiolytic; subsequent sexual activity under paced mating conditions did not disrupt this anxiolytic priming effect, whereas mating under unpaced conditions increased anxiety-related behavior. In primed females, the release of the neuropeptide oxytocin (OT) within the hypothalamic paraventricular nucleus was found to be elevated and to further increase during paced, but not unpaced mating. Central administration of an OT receptor antagonist partly prevented priming/mating-induced anxiolysis indicating the involvement of brain OT in the anxiolysis triggered by priming and/or sexual activity.These findings reveal that the positive consequences of mating in females are dependent on her ability to control sexual interactions, and that brain OT release is at least in part the underlying neurobiological correlate.

Figure 1. Paced mating (PM) arena.

PM was performed in an open-topped PM arena (34×31×53 cm) divided by a vertical barrier with an interspaces of 3.5 cm between the barrier and the cage's walls. The PM arena allows size-dependent withdrawal of the female to the other side of the cage and control of mating frequency and performance of microdialysis during ongoing PM. (A) anogenital investigation of the female by the male, (B) escape of the female to the other side of the barrier, (C) separation of male and female rats.

Figure 2. Effects of priming, unpaced (UPM) and paced mating (PM) on anxiety-related behavior.

Female, ovariectomized Wistar rats which were either steroid-primed or non-primed were tested on the elevated plusmaze (A, C), or in the black-white box (B, D) 30 min after a 30-min period of single-housing (SH), UPM (A, B) or PM (C, D). Priming-induced anxiolysis remained after PM, but not after UPM, indicated by longer and more frequent exploration of the open and unprotected arms of the plusmaze or the white compartment of the black-white box. Locomotor activity was reflected by the number of closed arm entries on the plusmaze (A, C). Data represent mean + S.E.M. Group size between 9 and 16. *** P<0.001, ** P<0.01, * P<0.05 versus non-primed SH; (*) P = 0.07 versus non-primed SH. # P<0.01 versus primed SH.

Figure 3. OT release within the PVN during UPM and PM in non-primed and primed female rats.

(A) PM triggers OT release into the extracellular fluid of the PVN of primed female Wistar rats as indicated by elevated OT content in microdialysates sampled during exposure to a sexually experienced male. Two 15-min microdialysates were sampled during single-housing (dialysate 1, 2), physical contact with an unknown female (social contact), UPM or PM (dialysate 3, 4), and after removal of the female or male (dialysate 5, 6). (B) Mean OT content in dialysates sampled from all non-primed or primed females during single-housing (mean samples 1 and 2). (C) Schematic drawing at the level of the hypothalamic paraventricular nucleus (PVN; bregma −1.88 mm), and microphotograph of Nissl-stained coronal brain section after removal of the microdialysis probe located inside the PVN. Scale bar, 1.0 mm. Data represent means + S.E.M. ** P<0.01 versus mating/contact all other groups, + P<0.05, ++ P<0.01 versus sample 3, primed PM group; and # P = 0.06 versus mean basal of all non-primed SH females.

Figure 4. Effects of an oxytocin receptor antagonist (OTA) on anxiety-related behavior in female rats.

Female rats were classified into three groups: (i) non-primed single-housed (NP-SH) (ii) primed single-housed (P-SH) and (iii) primed paced-mated (P-PM). NP and P rats were infused with either a selective OTA (0.75 µg/5 µl, intracerebroventricular, hatched bars) or vehicle (plane bars) into the lateral ventricle immediately after 30 min of PM or SH, and 30 min before testing on the plusmaze. A significant effect of OTA on anxiety-related behavior indicated by the percent time spent on the open arms of the maze was found after combining behavioral data from both primed groups (right, black columns). Locomotion reflected by the number of closed arm entries was not altered by any treatment. Data represent means + S.E.M. * P<0.05 versus vehicle-treated non-primed SH females, + P<0.05 versus vehicle-treated groups (combined primed single-housed and paced-mated groups treated with either vehicle or OTA).