Infralimbic Cortex Biases Preference Decision Making for Offspring over Competing Cocaine-Associated Stimuli in New Mother Rats.

In the context of drug abuse, converging evidence suggests that cocaine use in new mothers is significantly reduced by the competing motivation related to child rearing. Given the demonstrated importance of the medial prefrontal cortex (mPFC) in decision-making processes, we investigated the contribution of distinct regions of the mPFC [anterior cingulate (Cg1), prelimbic (PrL), infralimbic (IL)] to decision making in new mother rats performing a concurrent pup/cocaine choice conditioned place preference (CPP) task. When given a choice, inactivation of IL cortex significantly biased decision making of mother rats toward cocaine-associated cues, highly contrasting the distribution of preferences by control groups. In contrast, inactivation of PrL cortex had the opposite effect, significantly increasing offspring bias in the decision making, such that none of the mothers chose the cocaine-associated alternative. Cg1 inactivation was without effect. Functional inactivation of these same mPFC subregions had no effect in a non-conflict CPP task in which context-induced cocaine or pup seeking were examined separately. Notably, inactivation of the IL cortex also interfered with maternal behavior. Taken together, we have identified a specific role of the IL cortex in the prioritization of offspring over drug competing alternatives, thus promoting resistance to drug use in new mothers.

Decreased mesolimbic dopaminergic signaling underlies the waning of maternal caregiving across the postpartum period in rats.

RATIONALE: Mesolimbic dopamine (DA) signaling is essential for the high maternal caregiving characteristic of the early postpartum period, but little is known about dopamine's role in the expression of maternal caregiving thereafter. OBJECTIVES: We tested the hypothesis that decreased mesolimbic dopaminergic signaling is particularly responsible for the natural decline in maternal caregiving that occurs as the postpartum period progresses. METHODS: Sprague-Dawley (SD) mother rats received intraperitoneal injections of either vehicle, the DA D1 receptor agonist SKF38393, the DA D2 receptor agonist quinpirole, or both agonists twice daily from postpartum days 9 to 15. In a separate experiment involving Long-Evans (LE) rats, we examined whether DA D1 and D2 receptor mRNAs in the nucleus accumbens (NA) shell and ventral tegmental area (VTA), along with DA turnover in the VTA, decline across the postpartum period in parallel with the decreasing maternal behavior. RESULTS: All drug treatments significantly maintained higher frequencies of active maternal behaviors (nesting, pup licking, retrieval) compared to vehicle. Furthermore, the majority of mothers treated with SKF38393 either alone or combined with quinpirole maintained full expression of maternal behavior during behavioral testing. D2 receptor mRNA levels were found to be lower in the late postpartum NA shell and VTA compared to early postpartum, but D1 receptor mRNA levels in the NA shell were higher in the late postpartum period. Furthermore, both late postpartum and recently parturient LE mothers had higher VTA DA turnover compared to nulliparae, suggesting changes in mesolimbic signal-to-noise ratio both at the end and beginning of motherhood. CONCLUSIONS: Collectively, our results suggest that alterations in mesolimbic DA is part of the neural substrate responsible for dynamic maternal caregiving across the entire postpartum period.

Using wheel availability to shape running behavior of the rat towards improved behavioral and neurobiological outcomes.

BACKGROUND: Though voluntary wheel running (VWR) has been used extensively to induce changes in both behavior and biology, little attention has been given to the way in which different variables influence VWR. This lack of understanding has led to an inability to utilize this behavior to its full potential, possibly blunting its effects on the endpoints of interest. NEW METHOD: We tested how running experience, sex, gonadal hormones, and wheel apparatus influence VWR in a range of wheel access "doses". RESULTS: VWR increases over several weeks, with females eventually running 1.5 times farther and faster than males. Limiting wheel access can be used as a tool to motivate subjects to run but restricts maximal running speeds attained by the rodents. Additionally, circulating gonadal hormones regulate wheel running behavior, but are not the sole basis of sex differences in running. COMPARISON WITH EXISTING METHOD(S): Limitations from previous studies include the predominate use of males, emphasis on distance run, variable amounts of wheel availability, variable light-dark cycles, and possible food and/or water deprivation. We designed a comprehensive set of experiments to address these inconsistencies, providing data regarding the "microfeatures" of running, including distance run, time spent running, running rate, bouting behavior, and daily running patterns. CONCLUSIONS: By systematically altering wheel access, VWR behavior can be finely tuned - a feature that we hypothesize is due to its positive incentive salience. We demonstrate how to maximize VWR, which will allow investigators to optimize exercise-induced changes in their behavioral and/or biological endpoints of interest.

The medial prefrontal cortex and nucleus accumbens mediate the motivation for voluntary wheel running in the rat.

Voluntary wheel running in rats provides a preclinical model of exercise motivation in humans. We hypothesized that rats run because this activity has positive incentive salience in both the acquisition and habitual stages of wheel running and that gender differences might be present. Additionally, we sought to determine which forebrain regions are essential for the motivational processes underlying wheel running in rats. The motivation for voluntary wheel running in male and female Sprague-Dawley rats was investigated during the acquisition (Days 1-7) and habitual phases (after Day 21) of running using conditioned place preference (CPP) and the reinstatement (rebound) response after forced abstinence, respectively. Both genders displayed a strong CPP for the acquisition phase and a strong rebound response to wheel deprivation during the habitual phase, suggesting that both phases of wheel running are rewarding for both sexes. Female rats showed a 1.5 times greater rebound response than males to wheel deprivation in the habitual phase of running, while during the acquisition phase, no gender differences in CPP were found. We transiently inactivated the medial prefrontal cortex (mPFC) or the nucleus accumbens (NA), hypothesizing that because these regions are involved in the acquisition and reinstatement of self-administration of both natural and pharmacological stimuli, they might also serve a role in the motivation to wheel run. Inactivation of either structure decreased the rebound response in the habitual phase of running, demonstrating that these structures are involved in the motivation for this behavior.

Flexibility and adaptation of the neural substrate that supports maternal behavior in mammals.

Maternal behavior is species-specific and expressed under different physiological conditions, and contexts. It is the result of neural processes that support different forms (e.g. postpartum, cycling sensitized and spontaneous maternal behavior) and modalities of mother-offspring interaction (e.g. maternal interaction with altricial/precocious young; selective/non-selective bond). To understand how the brain adapts to and regulates maternal behavior in different species, and physiological and social conditions we propose new neural models to explain different forms of maternal expression (e.g. sensitized and spontaneous maternal behavior) and the behavioral changes that occur across the postpartum period. We emphasize the changing role of the medial preoptic area in the neural circuitry that supports maternal behavior and the cortical regulation and adjustment of ongoing behavioral performance. Finally, we discuss how our accumulated knowledge about the psychobiology of mothering in animal models supports the validity of animal studies to guide our understanding of human mothering and to improve human welfare and health.

New theoretical and experimental approaches on maternal motivation in mammals.

Maternal behavior is expressed in different modalities, physiological conditions, and contexts. It is the result of a highly motivated brain, that allows the female to flexibily adapt her caring activities to different situations and social demands. To understand how mothers coordinate maternal and other motivated behaviors we discuss the limitations of current theoretical approaches to study maternal motivation (e.g. distinction between appetitive and consummatory behaviors), and propose a different approach (i.e. motorically active vs. passive motivations) and a distinction between maternal motivated state and maternal motivated behaviors. We review the evidence supporting dopamine mediation of maternal motivation and describe how different phases of the dopaminergic response - basal, tonic, and phasic release in the nucleus accumbens - relate to increased salience, invigorating behavior, and behavioral switching. The existing and new experimental paradigms to investigate maternal motivation, and its coexpression and coordination with other social or non-social motivations are also analyzed. An example of how specificity of motivational systems (e.g. maternal and sexual behavior at postpartum estrus) could be processed at the neural level is also provided. This revision offers new theoretical and experimental approaches to address the fundamental question of how mothers flexibly adapt and coordinate the different components of maternal behavior with other motivated behaviors, also critical for the survival of the species.

Behavioral differences between late preweanling and adult female Sprague-Dawley rat exploration of animate and inanimate stimuli and food.

The late preweanling rat has potential as a preclinical model for disorders initially manifested in early childhood that are characterized by dysfunctional interactions with specific stimuli (e.g., obsessive-compulsive disorder and autism). No reports, however, of specific-stimulus exploration in the late preweanling rat are found in the literature. We examined the behavioral responses of normal late preweanling (PND 18-19) and adult rats when presented with exemplars of categorically-varied stimuli, including inanimate objects systematically varied in size and interactive properties, biological stimuli, and food. Preweanlings were faster to initiate specific stimulus exploration and were more interactive with most specific stimuli than adults; the magnitude of these preweanling-adult quantitative differences ranged from fairly small to very large depending upon the stimulus. In contrast, preweanlings were adult-like in their interaction with food and prey. Preweanling response to some stimuli, for example to live pups, was qualitatively different from that of adults; the preweanling behavioral repertoire was characterized by pup-seeking while the adult response was characterized by pup-avoidance. The specific stimulus interactions of preweanlings were less impacted than those of adults by the time of day of testing and placement of a stimulus in an anxiety-provoking location. The impact of novelty was stimulus dependent. The differences in interactions of preweanlings versus adults with specific stimuli suggests that CNS systems underlying these behavior patterns are at different stages of immaturity at PND 18 such that there may be an array of developmental trajectories for various categories of specific stimuli. These data provide a basis for the use of the preweanling as a preclinical model for understanding and medicating human disorders during development that are characterized by dysfunctional interactions with specific stimuli.

Effect of the adenosine A2A receptor antagonist MSX-3 on motivational disruptions of maternal behavior induced by dopamine antagonism in the early postpartum rat.

RATIONALE: Mesolimbic dopamine (DA), particularly in the nucleus accumbens, importantly regulates activational aspects of maternal responsiveness. DA antagonism and accumbens DA depletions interfere with early postpartum maternal motivation by selectively affecting most forms of active maternal behaviors, while leaving nursing behavior relatively intact. Considerable evidence indicates that there is a functional interaction between DA D2 and adenosine A(2A) receptors in striatal areas, including the nucleus accumbens. OBJECTIVE: This study was conducted to determine if adenosine A(2A) receptor antagonism could reverse the effects of DA receptor antagonism on early postpartum maternal behavior. METHODS: The adenosine A(2A) receptor antagonist MSX-3 (0.25-2.0 mg/kg, IP) was investigated for its ability to reverse the effects of the DA D2 receptor antagonist haloperidol (0.1 mg/kg, IP) on the maternal behavior of early postpartum female rats. RESULTS: Haloperidol severely impaired the expression of active maternal components, including retrieval and grouping the pups at the nest site, pup licking, and nest building. Co-administration of MSX-3 (0.25-2.0 mg/kg, IP) with haloperidol produced a dose-related attenuation of the haloperidol-induced behavioral deficits in early postpartum females. Doses of MSX-3 that effectively reversed the effects of haloperidol (0.5, 1.0 mg/kg), when administered in the absence of haloperidol, did not affect maternal responding or locomotor activity. CONCLUSIONS: Adenosine and DA systems interact to regulate early postpartum maternal responsiveness. This research may potentially contribute to the development of strategies for treatments of psychiatric disorders during the postpartum period, with particular emphasis in maintaining or restoring the mother-infant relationship.

Transient inactivation of the ventral tegmental area selectively disrupts the expression of conditioned place preference for pup- but not cocaine-paired contexts.

The ventral tegmental area (VTA) plays a critical role in motivated behavior. However, it remains unclear whether intact VTA function is necessary for motivated behavior to seek contexts repeatedly paired with natural stimuli and/or pharmacological stimuli. In the present study, conditioned place preference (CPP) was induced with highly salient natural or drug stimuli attributed with strong incentive-motivational value in each of 2 female models: Postpartum females were conditioned to associate one unique context in the CPP apparatus with young offspring (pups) and a second context with a neutral stimulus, and virgin females were conditioned to associate unique contexts with cocaine (5 mg/kg ip) and saline injections. Immediately before CPP testing, each female received a microinfusion of bupivacaine bilaterally into the VTA to transiently inactivate the region; subjects were also tested after saline microinfusion into the VTA. Postpartum females' preference for the pup-paired context was abolished by VTA inactivation but was restored to high control levels after saline microinfusion. In separate tests, VTA inactivation also reduced motivated pup licking and pup retrieval in postpartum females, suggesting that intact VTA function is required for the expression of both pup CPP and motivated pup-directed behaviors. Cocaine CPP remained unaffected by VTA inactivation. Locomotion was not affected by VTA microinfusions but was severely impaired by bupivacaine microinfusions into the substantia nigra. We concluded that the VTA is differentially involved in the expression of conditioned preference for contexts paired with pups, a salient natural stimulus, and contexts paired with cocaine.

The changing role of the medial preoptic area in the regulation of maternal behavior across the postpartum period: facilitation followed by inhibition.

Maternal behavior in rats undergoes considerable plasticity in parallel to the developmental stage of the pups, resulting in distinct patterns of maternal behavior and care at different postpartum time points. The medial preoptic area (mPOA) of the hypothalamus is one critical neural substrate underlying the onset and early expression of maternal behavior in rats but little is known about its specific functional role in the evolving expression of maternal behavior across the postpartum period. The present study uses a reversible local neural inactivation method to examine the role of the mPOA in the regulation of maternal behavior throughout the postpartum period, particularly extending into the late postpartum, a little examined period. This approach avoids the compensatory plasticity in CNS that occurs after permanent lesions, and allows the repeated testing of same individuals. Early (PPD7-8) and late (PPD13-14) postpartum maternal behavior was evaluated in female rats following infusions of bupivacaine or vehicle into the mPOA or into control areas. As expected, mPOA inactivation severely but transiently disrupted early postpartum maternal behavior whereas infusion of vehicle or inactivation of adjacent control sites did not. Later in the postpartum period, however, transient mPOA inactivation facilitated the expression of maternal behaviors, highly contrasting the behavioral expression levels characteristic of late postpartum. Results strongly demonstrate that the mPOA is differentially engaged throughout postpartum in orchestrating appropriate maternal responses with the developmental stage of the pups.

Exposure to pups influences the strength of maternal motivation in virgin female rats.

Following repeated exposure to foster pups, virgin female rats acquire and eventually express a full spectrum of maternal caretaking behaviors directed toward pups. Though these behaviors are vigorous, these females are reportedly less motivated to seek out and interact with pups (i.e. maternally motivated) than parturient females during early postpartum. The present study systematically assesses how the length of pup-exposure and nature of interactions between the female-pup dyad affect maternal motivation in the virgin female rat. Virgin females were exposed to young pups consistently (24 h/day) across a prolonged period (21 days), briefly (1 h/day) across a relatively brief period (7 days), or distally (pups inaccessible in mesh bag). During final pup-exposure days, females were conditioned and tested for their preference for a pup-associated chamber (e.g. maternal motivation) using conditioned place preference. Early postpartum females provided a comparison group. Fully maternal behavior only emerged in females given prolonged pup-exposure; this behavior improved significantly over time and was maximally expressed for a duration equivalent to early postpartum. Females given brief pup-exposure expressed only emergent maternal behaviors initiated by pups; distal pup-exposure evoked pup-avoidance. Virgin females given prolonged or brief pup-exposure expressed substantial pup-associated chamber preference, with more females preferring the pup-associated chamber following longer pup-exposures in a subtle stepwise relationship. Maternal motivation was strikingly similar in prolonged pup-exposure virgin and early postpartum females. Females given distal pup-exposure completely lacked maternal motivation. Maternal behavior did not predict chamber preference. Results suggest that pup-exposure, regardless of length, is sufficient to support strong maternal motivation, whereas parity is not required.

Incentive salience of cocaine across the postpartum period of the female rat.

UNLABELLED: RATIONALE-OBJECTIVES: Our prior conditioned place preference (CPP) work demonstrates that late (day16) postpartum female rats consistently prefer cocaine- over pup-associated chambers, whereas far fewer early postpartum (day8) females prefer the cocaine-associated chamber. The present study examines early and late postpartum females' preference for a cocaine-associated chamber when contrasted with a chamber associated with saline (rather than pups). MATERIALS AND METHODS: Postpartum females were tested for conditioned preference for chambers associated with cocaine (10 mg/kg subcutaneous (SC) or 0.5, 5, 10, or 20 mg/kg intraperitoneal (IP) injections) versus saline; preferences of virgin female and male rats for select cocaine stimuli (10mg/kg SC or IP) were also tested. Locomotion was recorded during CPP conditioning and testing. RESULTS: Early and late postpartum females expressed strikingly similar preference for the cocaine-associated chamber across all administration routes and doses. IP cocaine produced an orderly, inverted U-shaped dose-preference curve, with preference peaking at the 5 mg/kg dose (83% of females). While many postpartum females preferred 10mg/kg cocaine administered either SC or IP, both virgin females and males expressed strong aversion to SC cocaine and, while virgin females strongly preferred IP cocaine, males remained relatively indifferent. Across 10mg/kg IP cocaine-conditioning sessions, locomotor sensitization occurred exclusively in cocaine- but not saline-preferring postpartum females. Locomotor rate was lower in preferred versus nonpreferred chambers at CPP test. CONCLUSIONS: Early and late postpartum females may be equally and uniquely susceptible to sampling and/or abuse of modestly salient doses of cocaine (10mg/kg SC; 5mg/kg IP) compared to virgin females and/or males.

Characterization of maternal motivation in the lactating rat: Contrasts between early and late postpartum responses.

We previously assessed the motivational properties of pups relative to those of cocaine in parturient female rats (dams) across the postpartum period and demonstrated that the larger subset of dams in early postpartum (PPD8) preferred the pup-associated chamber, whereas the majority of dams tested in late postpartum (PPD16) preferred the cocaine-associated chamber [Mattson, B.J., Williams, S., Rosenblatt, J.S., Morrell, J.I. 2001. Comparison of two positive reinforcing stimuli: pups and cocaine throughout the postpartum period. Behav. Neurosci., 115, 683-694; Seip, K.M., Morrell, J.I. 2007. Increasing the incentive salience of cocaine challenges preference for pup- over cocaine-associated stimuli during early postpartum: place preference and locomotor analyses in the lactating female rat. Psychopharmacology 194, 309-319]. The present study uses a dual-choice conditioned place preference to ask how the progression of the postpartum period, including natural pup development, influences maternal motivation for pups. Preferences for cued chambers associated with pups that were age-matched to the postpartum stage of the dam in contrast to a stimulus with little incentive salience were higher during the early than the late postpartum, suggesting that the incentive salience of pups diminishes as the postpartum period progresses. Preferences of the early postpartum dams deprived of pups for 15 min, 2, 6, 12 or 22 hrs prior to conditioning and testing did not differ statistically but there was a trend of more pup preference after 22 hr deprivation; pup age was not an important factor in early postpartum. In marked contrast, late postpartum dams only exhibited robust pup-associated place preference when they were conditioned with young (4-7 day-old) pups or after a 22 hr period of deprivation from contemporaneous pups. Together these results suggest that both forces are at work in the mother-pup dyad, changes in the pups as they develop and changes in the physiological and endocrine state of the female as she progresses through the postpartum period.

Behavioral responses during the initial exposures to a low dose of cocaine in late preweanling and adult rats.

Human drug experimentation begins during late childhood and early adolescence, a critical time in physical and CNS development, when the immature CNS is vulnerable to the long-term effects of psychoactive drugs. Few preclinical animal studies have investigated responses to such drugs in a developmental stage equivalent to late childhood of humans. We used a rodent model to examine behavioral responses of female Sprague-Dawley late preweanling and adult rats during acute and repeated exposures to a low dose of cocaine. Results show that after cocaine injection, preweanling rats (18-21 days old) have locomotor responses that differ from adults, but after postnatal day 22, the responses are indistinguishable from adults even though rats are still not weaned. Before day 22, locomotor effects of cocaine differ from those in adults in three ways: preweanlings are active for a longer time after cocaine injection at day 18; preweanling activity peaks more rapidly after subcutaneous administration; and after only three injections of cocaine, a tolerance-like pattern is seen in preweanlings whereas an emerging pattern of sensitization to cocaine is seen in adults. The behavioral patterns of this age group offer a preclinical model of the early effects of drugs of abuse.

Increasing the incentive salience of cocaine challenges preference for pup- over cocaine-associated stimuli during early postpartum: place preference and locomotor analyses in the lactating female rat.

RATIONALE/OBJECTIVES: Prior studies using a dual-choice conditioned place preference (CPP) procedure revealed that postpartum female rats (dams) strongly prefer chambers associated with pups over those associated with subcutaneously administered cocaine almost exclusively during early but not late postpartum (Mattson et al. 2001). The present study examines whether early postpartum dams retain strong pup-associated chamber preference when contrasted with a cocaine stimulus of greater incentive salience (intraperitoneal [IP] injections with brief conditioning sessions). Locomotor rate was measured during conditioning (stimuli-present) and test (stimulus-absent) sessions. MATERIALS AND METHODS: A three-chambered CPP apparatus was used to compare preferences for chambers associated with IP cocaine vs age-matched pups. Unconditioned stimuli were systematically assigned to the least-preferred chamber of separate groups of dams before conditioning. Control dams verified that unconditioned stimuli were necessary for CPP and stimulus-associated locomotion. RESULTS: Compared with most late postpartum dams (60%), only 31% of early postpartum dams preferred the cocaine-associated chamber (P < 0.05). Substantially more dams preferred the pup-associated chamber during early postpartum (27%) than late postpartum (5%; P < 0.05). Locomotor sensitization emerged across cocaine-conditioning sessions in cocaine-preferring but not pup-preferring dams (P < 0.05). Locomotor rates were consistently lower in preferred vs nonpreferred chambers during test. CONCLUSIONS: After increasing cocaine's incentive salience, more early postpartum dams prefer the cocaine-associated chamber than previously reported (Mattson et al. 2001). However, pup-associated chamber preference was still higher in early vs late postpartum. Pup- and cocaine-preferring dams expressed differences in the induction phase of locomotor sensitization across cocaine conditioning but expressed similar motoric patterns in their preferred chambers at test.

Comparison of infant and adult rats in exploratory activity, diurnal patterns, and responses to novel and anxiety-provoking environments.

Infant rats emerge from the maternal nest at Postnatal Day 17-18 to have their first critical environmental experiences; they may be particularly sensitive to experiences or experimental interventions that can affect their adult capacity. The authors address open questions on 2 components of normative environmental exploration, locomotor activity and response to anxiety-provoking locations, in Postnatal Day 18 infant and Postnatal Day 60 adult rats. The authors compare diurnal patterns of locomotor activity, wheel running, novel and familiar open-field activity, and 2 measures of anxiety. Infants have an equivalent capacity to adults for locomotor activity and wheel running and a fundamentally adult-like diurnal rhythm, except that they do not anticipate light-dark transitions, are more perturbable at their most somnolent, and are more or less active during specific limited phases than adults. Infants initially have a lower rate of locomotor activity in novel environments and have a greater willingness to be active in anxiety-provoking locations. Such differences may allow enhanced gathering of environmental information by the infant and are important to consider in the design of experiments using infants.

Plasma cocaine levels, metabolites, and locomotor activity after subcutaneous cocaine injection are stable across the postpartum period in rats.

Plasma levels of cocaine (COC) and two of its principle metabolites, benzoylecgonine (BE) and ecgonine methyl ester (EME) were determined by liquid chromatography-tandem mass spectrometry (LC/MS/MS) in samples collected up to 3 h after a subcutaneous injection of cocaine (10 mg/kg) on six different days between days 4 and 24 postpartum, a period of dramatic change in the endocrine state of the female rat. Locomotor activity was measured in the same animals during this period using automated animal activity monitors. Additional measures in males provide a link to existing literature. We found that plasma levels of cocaine and its metabolites, as well as their respective time courses, are remarkably uniform across the postpartum period in female rats, as are the effects of cocaine on locomotor activity. Data from males show accord with prior published values. COC and BE, but not EME levels, were higher in males, and the time courses of COC and BE levels after injection varied somewhat between postpartum females and males; however, neither baseline nor cocaine-induced locomotor activity differed between postpartum females and males. We conclude that in the postpartum rat, there are no significant differences in the peripheral processing or general accessibility of cocaine to the brain to activate motor systems across the postpartum period. These data are critical to our understanding of differences in the reward salience of cocaine across the postpartum period and in other adult rat models [Mattson BJ, Williams S, Rosenblatt JS, Morrell JI. Comparison of two positive reinforcing stimuli: pups and cocaine throughout the postpartum period. Behav Neurosci 2001;115:683-94, Mattson BJ, Williams SE, Rosenblatt JS, Morrell JI. Preferences for cocaine- or pup-associated chambers differentiate otherwise behaviorally identical postpartum maternal rats. Psychopharmacology (Berl) 2003;167:1-8].

The distribution of mRNA for the short form of the prolactin receptor in the forebrain of the female rat.

Prolactin exerts its diverse effects on peripheral tissue and on the brain via receptors that have two forms, a short form and a long form. The distribution of the mRNA for both forms of the receptor has been examined in brain and peripheral tissue regions using methods based on regional dissection. Although the cell-specific distribution of the long form of the prolactin receptor has been examined using in situ hybridization in the rat brain, the cell-specific distribution of the short form has not been described. In this study we mapped the distribution of neurons and other cells expressing the short from of the receptor transcript in the forebrain, ovary, and uterus of the female rat by using in situ hybridization with a 33P-labeled cRNA probe specific for the short form of the prolactin receptor mRNA (PRL-SR mRNA). Neurons expressing the PRL-SR mRNA were located predominantly in the preoptic area and hypothalamus as well as in certain limbic structures. Specific nuclei included the anteroventral periventricular nucleus, paraventricular and supraoptic nucleus, medial preoptic area, suprachiasmatic nucleus, and ventromedial and arcuate nuclei of the hypothalamus, as well as the bed nucleus of stria terminalis and the medial amygdala. Scattered neurons expressing PRL-SR mRNA were also found in the cortex, habenula, zona incerta, and thalamus. Cells in the choroid plexus expressed high levels of PRL-SR mRNA, as did the luteal cells of the corpus luteum and the epithelial cells of the uterine glands. These data confirm previous reports and extend our knowledge of the distribution of the short form of the receptor to the cellular level. The neuroanatomic distribution of neurons expressing PRL-SR mRNA suggests that they may influence the mediation and coordination of prolactin-regulated endocrine and behavioral events.

Immunocytochemical detection of estrogen receptor-alpha in the female rabbit forebrain: topography and regulation by estradiol.

Two antibodies (H222 and Zymed) directed towards different sites of the estrogen receptor-alpha (ERalpha) were used for the following objectives: (1). to map the ERalpha in the forebrain of ovariectomized (ovx) rabbits by immunocytochemistry and (2). to determine the effect of endogenous (intact non-pregnant animals) and exogenous (ovx, estrogen-treated animals) estradiol (E2) on the population of ERalpha in the forebrain. Similar results were obtained with both antibodies used: dense aggregations of ERalpha-immunoreactive (IR) neurons were found in the infundibular nucleus (IN), the medial preoptic area (POA), the bed nucleus of the stria terminalis (BNST), and some nuclei of the amygdala. By contrast, no ERalpha-IR neurons were present in the ventromedial hypothalamic nucleus (VMN), but a dense aggregation of ERalpha-IR neurons occurred lateral to it in nucleus X. Numerous ERalpha-IR neurons were observed in the paraventricular hypothalamic nucleus, but not in the supraoptic or suprachiasmatic nuclei. The hippocampus proper lacked ERalpha-IR neurons, but the ventral subiculum in the hippocampal formation had a dense group of such cells. Intact non-pregnant rabbits showed less ERalpha-IR neurons in all regions tested than ovx animals. This difference was particularly clear in the medial POA, amygdala and BNST, while the IN showed only a marginal decrease. The dorsal, but much less the ventral, part of nucleus X also showed a decrease in the number of ERalpha-IR neurons compared with ovx animals. E2 benzoate (5 microg/day for 5 days) reduced even further the number of ERalpha-IR neurons in all regions except in a circumscribed area of the IN and the ventral part of nucleus X. These results show the existence of both sensitive and insensitive neurons to the down-regulatory effect of E2 on the presence of ERalpha. Sensitive neurons are located in the telencephalon, POA and several hypothalamic nuclei (PVN), while insensitive neurons are mainly restricted to the IN and the ventral part of nucleus X in the basal hypothalamus.