Sexual experience increases oxytocin, but not vasopressin, receptor densities in the medial preoptic area, ventromedial hypothalamus, and central amygdala of male rats.

Oxytocin (OT) and vasopressin (VP) are considered to be principal neurochemical substrates of bonding in monogamous species. We have reported previously that conditioning of a sexual partner preference in male rats resulted in conditioned activation of OT and VP neurons in hypothalamic paraventricular and supraoptc nuclei. Here we asked whether such conditioning would also alter OT or VP receptor densities. Sexually naïve male rats were assigned to one of three groups (n = 15/group). The Paired group received 9 copulatory training trials with sexually receptive females scented with a neutral almond odor. The Unpaired group received 9 copulatory training trials with unscented sexually receptive females. The Naïve group were not given sexual experience. Paired and Unpaired males were given a final test in an open field with two receptive females, one scented and the other unscented, to assess the development of conditioned ejaculatory preference (CEP), which was expressed significantly in the Paired group. Brains from rats in the three groups were then assessed for OT receptor (OTR) or VP1a receptor (VPR) densities within cortical, limbic and hypothalamic structures using autoradiography with selective 125I-labeled receptor ligands. Sexual experience alone increased OTR significantly in the medial preoptic area (mPOA), ventromedial hypothalamus (VMH), and central nucleus of the amygdala (CeA) in both Paired- and Unpaired-trained males compared to sexually Naïve males. No differences were found for experience on VPR densities in any region. These data add to a growing body of evidence that sexual experience alters brain function and processing of sex-related cues, and suggest that enhanced activation of OTRs in the mPOA, VMH, and CeA by conditioned OT release in those regions may underlie CEP in the male rat.

Appetitive olfactory conditioning in the neonatal male rat facilitates subsequent sexual partner preference.

Pairing a neutral odor with a male rat's initial sexual experiences to ejaculation produces a subsequent conditioned ejaculatory preference (CEP) in which males ejaculate preferentially with receptive females that bear the odor relative to unscented receptive females. In 1986, Fillion and Blass reported that neonatal male rats exposed to a neutral lemon odor (citral) painted on their mother's ventrum while nursing ejaculated faster as adults with sexually receptive, citral-scented females compared to unscented receptive females. The present study examined whether the same odor paired with tactile reward in neonatal male rats would alter the subsequent expression of a CEP. Newborn Long-Evans male rats were separated from their mothers each day beginning on Postnatal Day 1 and placed into a Plexiglas cage that contained either unscented or citral-scented bedding (N = 8/group). During each trial, rats were stroked from head to toe with a soft, narrow paintbrush, after which they were returned to their mothers. Males were weaned at 21 days of age and housed in same-treatment pairs for an intervening 50 days. Following habituation to a large open field, males were presented with two sexually receptive Long-Evans females, one scented with citral, and the other unscented, for a 30-min test of copulation. Males in the Paired group copulated and ejaculated preferentially with the scented female whereas males in the Unpaired group showed no preference. Pairing a neutral odor with a reward state in infancy generates a preference in male rats to ejaculate with sexually receptive females bearing the same odor in adulthood.

Differential role of oxytocin and vasopressin in the conditioned ejaculatory preference of the male rat.

The neuropeptides oxytocin (OT) and vasopressin (AVP) are critical in the formation of pair bonding in the vole, and potentially in other species. The finding that normally promiscuous male rats display a conditioned ejaculatory preference (CEP) for females that bear an odor paired with the sexual reward state induced by ejaculation prompted us to examine whether similar mechanisms may mediate this rudiment of pair bonding. Sexually naïve Long-Evans male rats were given 9 multi-ejaculatory copulation trials at 4-day intervals with either almond-scented (paired) or unscented (unpaired) sexually receptive females. CEP was examined in an open field, in which each male had unrestricted access to two receptive females, one scented with almond and the other unscented. Males in both groups were given two reconditioning trials and presented with the almond odor on gauze for 1 h prior to sacrifice. Neuronal activation was assessed by immunohistochemical detection of Fos protein within OT or AVP neurons. Exposure to the odor induced significantly greater activation of OT neurons in parvocellular neurons of the paraventricular nucleus (PVN) and of AVP in magnocellular neurons of the supraoptic nucleus (SON) in the paired group compared to the unpaired group. The second experiment examined whether oxytocin or vasopressin could enhance the acquisition of a CEP. Sexually naïve male Long-Evans rats received a subcutaneous injection of OT, AVP, or the saline vehicle, prior to their first sexual experience with an almond-scented female. CEP was examined 4 days later in the open field. Males injected with OT, but not AVP or saline, displayed significant CEP. The selective activation of OT neurons by the conditioned odor in the paired group, and the ability of OT injections to enhance the association of the odor and sexual reward, indicates that enhanced OT transmission is critical in the formation of CEP in male rats.

Conditioned ejaculatory preference in male rats paired with haloperidol-treated females.

We have shown that males trained to copulate with the same almond-scented female in environments that restrict access to the female during copulation develop a conditioned ejaculation preference (CEP) for their familiar female over a novel one. These findings suggest that copulation in environments in which the female spends more time away from the male and in which the male anticipates female copulatory contact facilitates the development of CEP. However, conditioned place and partner preference studies have shown that males prefer easy over obstructed access to the female partners during copulation. The objective of this study was to attempt to understand the discrepancy between the findings by examining the importance of female proceptive behavior and genital stimulation in the development of CEP while keeping the environment constant. Experiment 1 investigated the effect of administering a range of doses of haloperidol, a dopamine D2-receptor antagonist, to females, on female and male sexual behavior. Haloperidol treatment significantly reduced female proceptive behaviors in a dose-dependent manner. Experiment 2 examined the importance of female proceptive behaviors in the development of CEP by inhibiting those behaviors with haloperidol. Following alternating copulatory trials with scented or unscented haloperidol- or saline-treated females, males were given a partner preference test. Males displayed a preference to copulate with females bearing cues predictive of prior copulation with haloperidol-treated females, suggesting that males do not require female proceptive behaviors for the development of CEP. We conclude that copulation to ejaculation at a male's preferred rate is essential for the development of CEP, as it is for the development of a sexually-conditioned place preference.

Neurochemical basis of conditioned partner preference in the female rat: I. Disruption by naloxone.

The effects of the opioid antagonist naloxone were examined on the development of conditioned partner preference induced by paced copulation in female rats. In Experiment 1, ovariectomized, hormone-primed rats were conditioned to associate scented and unscented male rats with paced and nonpaced copulation, respectively. Female rats in Experiment 2 associated albino or pigmented male rats with paced or nonpaced copulation. Naloxone or saline was administered before each conditioning trial. During a final drug-free preference test, female rats could choose to copulate with either a pacing related or unrelated male. Saline-trained female rats in the paired group copulated preferentially with the pacing-related male rat, whereas naloxone-trained female rats did not show a preference. The authors concluded that opioids mediated the conditioned partner preference induced by paced copulation.

Cecum location in rats and the implications for intraperitoneal injections.

Intraperitoneal injection is a common route for parenteral administration of drugs in rodents. A serious consequence associated with this technique, however, is the puncture of vital organs such as the cecum, which causes pain and occasionally peritonitis. Reports have described the cecum as located on either side of the lower abdominal cavity, contributing to the idea that intraperitoneal injections can be performed in either side. The authors investigated the location of the cecum in adult male and female albino and pigmented rat strains, and evaluated the consequences of intraperitoneal injections in the right and left portion of the lower abdomen. Of the rats they investigated, 71.8% had ceca on the left side of the abdomen. The authors also found that injections on the left side were more likely to result in punctured ceca.