Glutamate release in the ventromedial hypothalamus of the female rat during copulation: modulation by estradiol

Binding of glutamate or its ionotropic receptor agonists in the ventromedial hypothalamus (VMH) of female rats inhibits both appetitive and consummatory aspects of sexual behavior. Because vaginocervical stimulation activates glutamate neurons in the VMH, and administration of estradiol benzoate (EB) and progesterone (P) delays this effect, the present study examined the effects of hormonal priming on glutamate release within the VMH of female rats paired with sexually vigorous males. Ovariectomized, sexually experienced rats were implanted with guide cannula aimed at the ventrolateral VMH, through which microdialysis probes were inserted prior to testing. Females were assigned randomly to one of three hormone treatment conditions: EB+P, EB alone, or the oil vehicle. Testing was conducted over 5h, including a 120-min period of habituation to the testing chamber, a 60-min period of baseline sample collection, and a 120-min period during which a sexually vigorous male was introduced into the testing chamber. Dialysates were collected every 20min during the test and were analyzed for glutamate using HPLC. Females primed with oil had large and significant increases in glutamate release from baseline once the male was introduced to the chamber. Treatment with EB alone decreased glutamate release in response to male cues. Although treatment with EB+P did not differ significantly from EB alone, the degree of reduced glutamate release was less than with EB alone. These results indicate that priming with EB reduces glutamate transmission in the VMH in response to male cues. Taken together with our previous findings, estradiol blunts the activation of glutamate neurons in the VMH thus allowing female rats to copulate.

Cerebrospinal fluid tau cleaved by caspase-6 reflects brain levels and cognition in aging and Alzheimer disease

Caspase-6 (Casp6) activation in the brain is implicated early in the pathogenesis of Alzheimer disease (AD). In view of the need for early AD diagnosis, brain Casp6 activity was investigated by measuring Tau cleaved by Casp6 (TauΔCasp6) protein in postmortem cerebrospinal fluid (CSF) of 7 non-cognitively impaired; 5 mild cognitively impaired; and 12 mild, moderate, and severe AD patients. Levels of TauΔCasp6 in CSF accurately reflected the levels of active Casp6 and TauΔCasp6 detected using immunohistochemistry in hippocampal sections from the same individuals. Levels of CSF TauΔCasp6 significantly correlated with AD severity and with lower Global Cognitive Scores; Mini-Mental State Examination scores; and episodic, semantic, and working memory scores. Regression analyses suggested that the CSF TauΔCasp6 levels combined with TauΔCasp6 brain pathology predict cognitive performance. These results indicate that CSF TauΔCasp6 levels hold promise as a novel early biomarker of AD.

Caspase-6 activity predicts lower episodic memory ability in aged individuals

Caspase-6 (Casp6), a cysteinyl protease that induces axonal degeneration, is activated early in Alzheimer Disease (AD) brains. To determine whether Casp6 activation is responsible for early cognitive impairment, we investigated the abundance of Casp6 activity, paired helical filament-1 (PHF-1) phosphorylated Tau and amyloid beta peptide (Aβ) pathology by immunohistochemistry in the hippocampal formation of aged non-cognitively impaired (NCI) individuals. Casp6 activity was restricted to the entorhinal cortex (ERC) and CA1 regions of the hippocampus. Pathology scores were then correlated with cognitive scores obtained within 1 year of death. Regression analyses revealed that ERC and CA1 Casp6 activity were the main contributor to lower episodic memory performance, whereas ERC PHF-1 pathology predicted lower semantic and working memory performance. Aβ did not correlate with any of the cognitive tests. Because Casp6 activity and PHF-1 pathology are intimately associated with AD pathology and memory decline is an early event in AD, we conclude that Casp6 activity and PHF-1 immunoreactivity in ERC identifies aged individuals at risk for developing AD.

Who, what, where, when (and maybe even why)? How the experience of sexual reward connects sexual desire, preference, and performance

Although sexual behavior is controlled by hormonal and neurochemical actions in the brain, sexual experience induces a degree of plasticity that allows animals to form instrumental and Pavlovian associations that predict sexual outcomes, thereby directing the strength of sexual responding. This review describes how experience with sexual reward strengthens the development of sexual behavior and induces sexually-conditioned place and partner preferences in rats. In both male and female rats, early sexual experience with partners scented with a neutral or even noxious odor induces a preference for scented partners in subsequent choice tests. Those preferences can also be induced by injections of morphine or oxytocin paired with a male rat's first exposure to scented females, indicating that pharmacological activation of opioid or oxytocin receptors can "stand in" for the sexual reward-related neurochemical processes normally activated by sexual stimulation. Conversely, conditioned place or partner preferences can be blocked by the opioid receptor antagonist naloxone. A somatosensory cue (a rodent jacket) paired with sexual reward comes to elicit sexual arousal in male rats, such that paired rats with the jacket off show dramatic copulatory deficits. We propose that endogenous opioid activation forms the basis of sexual reward, which also sensitizes hypothalamic and mesolimbic dopamine systems in the presence of cues that predict sexual reward. Those systems act to focus attention on, and activate goal-directed behavior toward, reward-related stimuli. Thus, a critical period exists during an individual's early sexual experience that creates a "love map" or Gestalt of features, movements, feelings, and interpersonal interactions associated with sexual reward.

Accumbal dopamine function in postpartum rats that were raised without their mothers

Postpartum rats that had been previously raised in an artificial rearing (AR) apparatus, without their mothers or siblings during the preweaning period, show altered maternal responses towards their own offspring in adulthood. In mother-reared (MR) rats, nucleus accumbens (NAC) dopamine (DA) responses to pups evoke a robust sustained rise during the postpartum period and following treatment with estrogen/progesterone parturient-like hormones (Afonso et al., 2009). These MR females had siblings that received AR rearing with varying amounts of preweaning tactile stimulation (ARmin; ARmax). The present study examined NACshell DA responses to pup and food stimuli in these AR rats, and statistically compared them to their MR siblings. Microdialysis samples were collected from adult (90 days postnatal) AR females in different parity states (cycling vs. postpartum, Exp. 1), or after ovariectomy with different hormone treatments (sham vs. hormone, Exp. 2. After basal sample collection, pup and then food stimuli were individually presented to the females in the dialysis chamber. As with their MR siblings, basal DA concentrations were lower and pup-evoked DA responses greater in hormonally-primed AR females than in non-primed AR controls. Compared to their postpartum MR sisters (Exp. 1), AR rats had increased basal DA levels, reduced pup related DA elevations, and disrupted maternal behavior. The postpartum AR impairment in pup-evoked DA was reversed by additional pre-weaning tactile stimulation. Exogenous hormones (Exp. 2) eliminated AR impairments on pup-evoked DA responses. Although MR and AR siblings had comparable DA responses to food stimuli, upon reanalyzing MR data it was found that only postpartum dams had DA responses to pups greater than to food. These data suggest that that the hormonally induced suppression of basal DA levels may reflect saliency of pups which was greater in MR than in AR dams. Preweaning tactile stimulation could partially reverse these effects only in naturally cycling or parturient animals.

Hormones that increase maternal responsiveness affect accumbal dopaminergic responses to pup- and food-stimuli in the female rat

The present study investigated hormonal mediation of maternal behavior and accumbal dopamine (DA) responses to pup-stimuli, as measured in microdialysis samples collected from the nucleus accumbens shell of female rats in non-homecage environment. In Experiment 1, samples were collected before and after continuous homecage pup experience from either intact postpartum or cycling females. In Experiment 2, samples were collected before and after responding maternally in homecage from ovariectomized females given either parturient-like hormone or sham treatments. After baseline sample collection in the dialysis chamber, pup and food stimuli were individually presented to females. Upon sampling completion, all animals were placed back into their homecage with donor pups for several days, and then the sample collection procedure was repeated. Prior to stimulus presentation, postpartum and hormone-treated females had decreased basal DA release compared to their controls. In response to pup stimuli, only postpartum and hormone-treated females had increased DA release compared to basal release (both sampling days). In response to food stimuli, all females had increased DA responses from basal; although there were group differences on the initial day of sampling. Findings suggest that hormones associated with inducing maternal behavior in the postpartum rat play a significant role in modifying accumbal dopaminergic responses on first exposure to pup stimuli in the rat. However, the postpartum experience provides further modifications to this brain region to promote DA responses to pup stimuli.

Estrogen and the neural mediation of female-male mounting in the rat

Sexually receptive females mount sexually sluggish males to entice them to copulate, and estrogen and male olfactory cues mediate this female-male mounting (FMM) in the rat. This study examined whether brain regions that concentrate steroid hormones and receive olfactory projections were important for the mediation of FMM. Fos induction was observed within the medial amygdala, medial preoptic area, and ventromedial hypothalamus of ovariectomized, hormone-primed rats that displayed FMM compared with rats that did not. Excitotoxic lesions of those regions eliminated FMM, whereas implants of crystalline estradiol benzoate to the ventromedial hypothalamus, but not the medial preoptic area or medial amygdala, restored FMM. These data indicate that the ventromedial hypothalamus is a critical area of convergence of hormonal, olfactory, and somatosensory inputs for FMM.

Medial prefrontal cortex lesions in the female rat affect sexual and maternal behavior and their sequential organization

Temporal sequences of sexual and maternal behaviors in female rats and their correlation with each other and with performance on a sensory-motor gating response inhibition task assessed by prepulse inhibition (PPI) were investigated following medial prefrontal cortex (mPFC) lesions. Following excitotoxic mPFC (n = 10) or sham (n = 9) lesions, sexual behaviors across the ovarian cycle were scored. After mating and parturition, maternal interactions were scored until pups reached postnatal Day 10. After resumption of the ovarian cycle, the female rats were tested for PPI. Compared with sham lesions, mPFC lesions impaired proceptive behaviors and some maternal behaviors (e.g., pup retrieval, pup licking) but did not affect others (e.g., nest building, pup mouthing). Lesions disrupted temporal sequences of solicitations (number of male orientations followed, within 4 s, by a level change) and pup retrievals (number of pup retrievals followed, within 5 s, by another retrieval). These sequential behavior patterns were significantly correlated with each other and with PPI. However, when PPI effects were partialled out, group differences were less strong, but persisted. This study demonstrated that mPFC manipulations affect actions rich in sequential structure in response to biologically relevant stimuli.

Inhibition of platelet-activating factor receptors in hippocampal plasma membranes attenuates the inflammatory nociceptive response in rats

Evidence suggests that platelet-activating factor (PAF) is a mediator in inflammatory-based pain. Using the biphasic formalin model in rats, we recently demonstrated that PAF antagonists which were selective for either intracellular or plasma membrane PAF receptors decreased the late-phase of the nociceptive response. Inasmuch as both of the PAF antagonists previously used were administered systemically, and reportedly are able to cross the blood-brain barrier, the anatomic locations at which PAF affects pain processing remained to be elucidated. Since PAF is required for hippocampal-dependent memory consolidation, and since the hippocampus has been shown to mediate the late-phase of formalin-induced nociception, the present study investigated the effects on nociception of administration of PAF antagonists within the hippocampus, and of using agents specific for either plasma membrane (BN 52021) or intracellular (BN 50730) PAF binding sites. Intrahippocampal injections of BN 52021 decreased the late-phase of the nociceptive response in a concentration-dependent manner. In contrast, intrahippocampal administration of BN 50730 had no effect on inflammatory nociception. These findings suggest that hippocampal plasma membrane PAF receptors, but not intracellular PAF binding sites, mediate tonic inflammatory pain processing in rats.

Sensory mediation of female–male mounting in the rat: II. Role of tactile and conspecific cues

Although previous research has shown that olfactory cues mediate female-male mounting (FMM) in the rat, the role of other sensory modalities on FMM has not been investigated. The present study examined the display of female mounting of castrated male rats in bilevel chambers following different tactile or locomotor activity manipulations. Female rats (N = 40) were ovariectomized (OVX), primed with estrogen (E) and progesterone (P), and given either vaginocervical stimulation (VCS), flank/perineum stimulation (FPS), combined VCS and FPS, or general handling, immediately before each test with a castrated male rat for five trials. Compared to handling, the FPS females showed an increase in FMM behavior, whereas females given VCS, or combined VCS and FPS, showed a decrease in FMM behavior. A second experiment examined the effect of a 15-min delay between stimulation and testing using identical experimental and control conditions. There were no significant differences in the amount of FMM behavior between these groups. Finally, OVX rats primed with E and P were tested with castrated males that had been given injections of a ketamine/xylazine anesthetic mixture, saline, or amphetamine, to induce three levels of conspecific locomotor activity: none, moderate, or high, respectively. A positive linear relationship was found between male activity level and FMM. These data indicate that both tactile cues and cues associated with locomotor activity of the stimulus male modulate FMM.

Sensory mediation of female–male mounting in the rat: I. Role of olfactory cues

Previous research has shown that olfactory cues mediate the mounting of female rats by male or other female rats. The present study examined whether olfactory cues might mediate the mounting of castrated, sexually inactive male rats by sexually receptive female rats (female-male mounting, or FMM). The effects of olfactory impairment, created by either olfactory bulbectomy (OBx) or olfactory occlusion (OOc), on FMM were investigated. Ovariectomized, hormone-primed female rats were given either OBx (OBx+) or sham (OBx-) surgeries. OBx+ females did not engage in any FMM after surgery, whereas sham-operated females continued to mount at baseline levels. This effect was replicated using OOc, a reversible form of olfactory impairment that involves the cannulation of the nasal cavity with a flexible tube. Females were either given the OOc surgery (OOc+), the OOc surgery with the tube removed immediately after placement (OOc-), or sham surgery in which the animal was only anesthetisized. OOc+ females, like OBx+ females, did not display FMM, whereas both control groups continued to mount at baseline levels. The effect of prior experience with FMM was also examined. Females were given either 0 or 5 encounters with castrated males prior to OBx+, OOc+, or OOc- surgeries. OBx+ and OOc+ females did not mount, regardless of prior mounting experience. These data indicate that the olfactory sense is a prime mediator of FMM, and that prior mounting experience does not offset the disruption of FMM caused by the elimination of olfactory cues.

Hormonal and experiential control of female-male mounting in the female rat

Mounting behavior in the female rat has been studied extensively in same-sex interactions, but not in the heterosexual dyad. The present study examined the display of female mounting of castrated noncopulating male rats (FMM). Ovariectomized (OVX) sexually naive female rats (N = 80) were given either estrogen (E) + progesterone (P), E + oil (O), P + O, or O + O treatment for five tests with castrated male rats. FMMs were observed in both the E + P and E + O females. The influence of the ovarian cycle on FMM was also investigated. Vaginal smears from sexually naive females (N = 16) were taken daily for 12 days immediately after testing with castrated males. FMM frequency was greatest during Proestrus. Finally, OVX females (N = 30) treated with E + P were given either 0, 1, 10 multi-ejaculatory heterosexual experiences with intact, sexually experienced males, prior to tests with intact, copulating or castrated, noncopulating males for five tests. Sexually naive females displayed a greater number of mounts relative to the sexually experienced females when tested with castrated, noncopulating males. In contrast, very few FMMs were observed in females of any group tested with intact, copulating males. These data suggest that FMM occurs naturally in rats as a "super-solicitational" behavior that is modified by hormone treatment and prior heterosexual experience.

Relationship between wheel running, feeding, drinking, and body weight in male rats

The amount of wheel running varies widely between rats. Wheel introduction and running also have profound effects on the animal's energy balance. We explored the effects of ad lib wheel access and running levels on feeding, drinking, and body weight in 30 young adult male rats with wheel access and in 30 rats without wheel access. The initial running period (Days 1-8) and a time of stable running [Days 29-32 (DEnd)] were analyzed using both between- and within-group approaches. Initially, wheel access suppressed feeding (by about 25% over the 8 days) but not drinking. There were no significant correlations between the amount of wheel running and the other behaviors. The degree of feeding suppression was also not correlated to the amount of running. When running had stabilized (animal ran from 841 to 13,124 wheel turns per day), food intake was increased by about 0.75 g per 1000 wheel turns. Running at this time correlated positively with feeding and drinking and negatively with body weight and weight gain. In animals without wheel access, feeding and drinking were positively correlated with weight and weight gain, but in animals with wheel access, these correlations were close to zero. Only early running predicted later levels of running but accounted for only 23% of the variance in running. Wheel access has profound but very different immediate and long-term effects on the rats' energy balance.