Functional mapping of the neural circuitry of rat maternal motivation: effects of site-specific transient neural inactivation.

The present review focuses on recent studies from our laboratory examining the neural circuitry subserving rat maternal motivation across postpartum. We employed a site-specific neural inactivation method by infusion of bupivacaine to map the maternal motivation circuitry using two complementary behavioural approaches: unconditioned maternal responsiveness and choice of pup- over cocaine-conditioned incentives in a concurrent pup/cocaine choice conditioned place preference task. Our findings revealed that, during the early postpartum period, distinct brain structures, including the medial preoptic area, ventral tegmental area and medial prefrontal cortex infralimbic and anterior cingulate subregions, contribute a pup-specific bias to the motivational circuitry. As the postpartum period progresses and the pups grow older, it is further revealed that maternal responsiveness becomes progressively less dependent on the medial preoptic area and medial prefrontal cortex infralimbic activity, and more distributed in the maternal circuitry, such that additional network components, including the medial prefrontal cortex prelimbic subregion, are recruited with maternal experience, and contribute to the expression of late postpartum maternal behaviour. Collectively, our findings provide strong evidence that the remarkable ability of postpartum females to successfully care for their developing infants is subserved by a distributed neural network that carries out efficient and dynamic processing of complex, constantly changing incoming environmental and pup-related stimuli, ultimately allowing the progression of appropriate expression and waning of maternal responsiveness across the postpartum period.

The medial preoptic area is necessary for motivated choice of pup- over cocaine-associated environments by early postpartum rats.

Converging evidence suggests that the motivation to seek cocaine during the postpartum period is significantly impacted by the competing incentives of offspring, a stimulus unique to this life stage. In the present study, the functional role of the medial preoptic area (mPOA), a critical site involved in maternal responsiveness, on processing incentive value of pup-associated cues and influencing response allocation for pup- over cocaine-associated environments was investigated using a concurrent pup/cocaine choice conditioned place preference (CPP) paradigm. Early postpartum females with bilateral guide cannulae aimed into the mPOA or into anatomical control sites were conditioned, from postpartum days (PPD) 4 to 7, to associate different uniquely featured environments with pups or cocaine. CPP was tested on PPD8 following intra-mPOA infusions of either 2% bupivacaine or saline vehicle. In two additional experiments, the effects of intra-mPOA infusions of bupivacaine on expression of conditioned responding induced by environments associated with either pups or cocaine were examined separately. Transient inactivation of the mPOA selectively blocked the conditioned preferences for pup-associated environments, significantly contrasting the robust pup-CPP found in non-surgical and intra-mPOA vehicle-treated females. In contrast, mPOA inactivation failed to alter cocaine-CPP in postpartum females. When given a choice between environments associated with pups or cocaine, transient functional inactivation of the mPOA altered choice behavior, biasing the preference of females toward cocaine-associated environments, such that almost all preferred cocaine- and none the pup-associated option. The anatomical specificity was revealed when inactivation of adjacent regions to the mPOA did not affect CPP responses for pups. The findings support a critical role for the mPOA in mediating pup-seeking behavior, and further suggest that the competing properties of pups over alternative incentives, including drugs of abuse, rely on mPOA integrity to provide relevant pup-related information to the circuitry underlying the choice behavior between pups and alternative stimuli.

Juvenile rats show reduced c-fos activity in neural sites associated with aversion to pups and inhibition of maternal behavior.

Juvenile rats (18-23 days old) interact avidly with pups as novel stimuli and show maternal behavior after only 1-3 days of pup exposure; adults initially avoid pups and require 3-9 days of pup exposure. Upon exposure to pups as novel stimuli, adults had more c-Fos-immunoreactive neurons in the hypothalamus and amygdala--regions associated with aversion to pups--than adults exposed to familiar pup stimuli (maternal) or not exposed to pups (p < .05). In juvenile rats exposed to pups as novel stimuli, only the medial amygdala had a small significant increase of c-Fos neurons. In juveniles, this blunted engagement of c-Fos neurons may reflect the diminished activation of inhibitory neurons, facilitating the interaction of juveniles with pups as novel stimuli and onset of maternal behavior.

Preference for cocaine- versus pup-associated cues differentially activates neurons expressing either Fos or cocaine- and amphetamine-regulated transcript in lactating, maternal rodents.

We studied the neuronal basis of the motivational response to two powerful but radically different rewards-cocaine and maternal nurturing of pups in the postpartum rat (dam) which is in a unique motivational state. We used a place preference method designed to offer a choice between cues associated with a natural reinforcer (pups) and those associated with a pharmacologic reinforcer (cocaine). Using c-Fos or cocaine- and amphetamine-regulated transcript (CART) immunocytochemistry, we identified the neuronal groups that are activated when the dams expressed a preference for either cues-associated with pups or cues-associated with cocaine. Dams that preferred the cocaine-associated cues had more c-Fos positive neurons in medial prefrontal cortex, nucleus accumbens, and basolateral nucleus of amygdala than pup-associated cue preferring dams or control. Except for the accumbens, there was activation of neurons in these same regions with the pup-associated cue preference. In the nucleus accumbens only CART-immunoreactive (not c-Fos) neurons were activated with pup-cue preference. Notably, the medial preoptic area was the single area where greater activation of neurons was seen with a preference for pup-associated versus cocaine-associated cues. These responses were identified in the absence of the stimuli (cocaine or pups) and are proposed to be, in part, activation of these neurons related to motivational processing. Neither the distribution of neurons responding to pup-associated cue preference nor the demonstration that CART-expressing neurons are responsive to reward-associated cue preference has been previously reported. We hypothesize that the expression of preference for cocaine versus pup-associated cues is made possible by the concerted activity of these regionally distributed networks of neurons that are in part specific to the preference response.

The content of dopamine, serotonin, and their metabolites in the neural circuit that mediates maternal behavior in juvenile and adult rats.

Continuous exposure of non-parturient rats to pups can induce maternal behavior similar in most aspects to that found in the postpartum rat. Surprisingly, young juvenile rats (20-24 days of age) only require 1-3 days of exposure to pups, while adults require 4-8 days before maternal behavior emerges. Dopamine (DA) and possibly serotonin (5-HT) may mediate the expression of adult maternal behavior. We hypothesize that postnatal changes in DA and 5-HT within the neural circuit that supports maternal behavior including the medial preoptic area (MPOA), medial and cortical amygdala (MCA), and nucleus accumbens (NAC), may underlie these differences in responsiveness across juveniles and adults. We measured DA, 5-HT, and their metabolites in postmortem samples of these regions in maternal and non-maternal juvenile and adult females. The only difference found across behavioral groups was that the MPOA of adults induced into maternal behavior by pup exposure had more DA than did that of isolated adult females or maternal juveniles. However, when adults versus juveniles were compared, the content of DA and 3,4-dihydroxyphenylacetic (DOPAC) was higher in the adult than in the juvenile NAC and MCA; the content of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) in these structures did not vary across the age groups. In contrast, higher levels of 5-HT and 5-HIAA were found in the MPOA in juveniles compared to adults. We propose that these region-specific age differences in DA and 5HT may underlie differences in juvenile-adult responses to pups.

Comparison of two positive reinforcing stimuli: pups and cocaine throughout the postpartum period.

This set of experiments investigated the appetitive or motivational processes underlying the performance of maternal behavior. The place preference paradigm was adapted to simultaneously investigate the reinforcing properties of cocaine and pups for maternal, lactating dams. These modifications allowed the authors to assess which stimulus, either a 10 mg/kg s.c. injection of cocaine or 3 pups, had the strongest reinforcing value. At Postpartum Days 10 and 16, the dams preferred the cocaine cue-associated chamber, whereas the dams tested at Postpartum Day 8 preferred the pup cue-associated chamber. Overall, the data revealed an interaction between the postpartum period at testing and the exhibited preference for cocaine or pups. Further testing will investigate the neural circuitry underlying the appetitive processes of each stimulus.

The medial preoptic area, necessary for adult maternal behavior in rats, is only partially established as a component of the neural circuit that supports maternal behavior in juvenile rats.

To determine whether the neurons of the medial preoptic area (MPOA) are necessary for pup-induced maternal behavior (MB) in juvenile and adult rats, subjects received bilateral injections of the neurotoxin N-Methyl-D-Aspartic acid into the MPOA. Controls were intact or were sham treated by surgical placement of the syringe barrel. The rats were then induced into MB by constant pup exposure. Starting at 27 (juvenile) or 60 (adult) days of age, rats were tested for MB for 12 consecutive days. After histological analysis, rats were categorized as having either large or small lesions of the MPOA. In juveniles, large lesions of the MPOA blocked retrieval and impaired nest-building, but crouching behavior was unaffected; small lesions had no effect on MB. In contrast, in adults, large or small lesions severely impaired all components of MB. The results suggest that in juvenile rats, the role of the MPOA neurons in MB is only partially established, whereas by 60 days of age, the unsubstitutable role of the MPOA in the neural circuit that mediates MB is fully established.

Induction of c-fos-like and fosB-like immunoreactivity reveals forebrain neuronal populations involved differentially in pup-mediated maternal behavior in juvenile and adult rats.

Juvenile rats can exhibit maternal behavior after being exposed continuously to rat pups, a process called sensitization. Maternal behavior in juveniles is robust and is similar to adult maternal behavior (Mayer and Rosenblatt [1979] Dev. Psychobiol. 12:407-424; Gray and Chesley [684] J. Comp. Psychol. 98:91-99). In this study, immunocytochemical detection of the protein products of two immediate-early genes, c-fos and fosB, was used as a tool to identify forebrain neuronal populations involved in the maternal behavior of 27-day-old juvenile rats compared with 60-day-old adults. To sensitize them, rats were exposed continuously to foster pups. Once they were maternal, they were isolated from pups overnight, reexposed to pups for 2 hours, and then killed. Nonmaternal control animals also were isolated overnight and were either reexposed to pups for 2 hours or kept isolated from pups before killing. The lateral habenula (LH) was the only area in which both maternal juveniles and maternal adults had more c-Fos-immunoreactive (-Ir) neurons compared with controls. In maternal adults, the number of neurons that expressed c-Fos and FosB immunoreactivity increased in the medial preoptic area (MPO) and the ventral bed nucleus of the stria terminalis (BSTv), whereas the dorsal bed nucleus of the stria terminalis (BSTd) and the medial and cortical nuclei of the amygdala (MEA and COA, respectively) had increases only in the number of neurons that expressed c-Fos immunoreactivity. In contrast, juveniles, whether or not they were maternal, had the same number of c-Fos-IR and FosB-Ir neurons in all these areas. The adult-like increase in the number of c-Fos-Ir neurons found in maternal juveniles suggests that the juvenile LH participates in the neural circuit that supports maternal behavior in an adult-like manner. The lack of c-fos or fosB induction in the MPO, BSTv, BSTd, COA, or MEA of maternal juveniles compared with maternal adults may reflect the immaturity of these brain regions in juvenile rats. Exactly what this immaturity consists of and when the responses of these regions become adult-like remain to be determined.

Microinfusion of cocaine into the medial preoptic area or nucleus accumbens transiently impairs maternal behavior in the rat.

Cocaine was microinfused bilaterally (50 microg/0.5 microl/side) into the medial preoptic area (MPOA) or nucleus accumbens (NA), 2 regions within the rat brain neural circuit known to mediate maternal behavior (MB). Additionally, 2 sites not involved in this neural circuit, the dorsal striatum and dorsal medial hippocampus, were used as control sites. Microinfusion of cocaine into the MPOA or NA impaired MB, whereas infusion into the control sites did not. MB impairment was not temporally coincident with the increased locomotor activity, also documented after cocaine infusion into the MPOA or NA, arguing strongly that impaired MB is a direct, specific effect of cocaine in these areas, not a derivative of increased motor activity. This is the first demonstration that cocaine action on single central nervous system (CNS) sites can impair MB to the same extent as systemic injections. Thus, cocaine's simultaneous effect on multiple CNS sites is not required for MB impairment.

Estrogen implants in the lateral habenular nucleus do not stimulate the onset of maternal behavior in female rats.

The natural onset of maternal behavior in the rat is hormonally mediated. Estrogen, progesterone, and prolactin administered to ovariectomized females in amounts and sequences that produce circulating levels similar to those found during pregnancy stimulate the onset of maternal behavior. In fact, maternal behavior can be stimulated by estrogen alone, administered either peripherally or by implant in the central nervous system. The lateral habenula (Lhb), which is a necessary component in the neural circuit that supports maternal behavior, contains a subset of neurons with estrogen receptors. The present study investigated whether estradiol implants directly in the Lhb are sufficient to stimulate maternal behavior. Female rats, hysterectomized and ovariectomized on day 16 of pregnancy, received estrogen implants in the Lhb or, as a positive control, in the medial preoptic area (MPOA). An additional control group received cholesterol implants in the Lhb. All females were tested for pup retrieval, nest building, crouching behavior, locomotor activity, and carrying behavior. Estradiol implants into the Lhb did not stimulate the onset of maternal behavior. Females with estrogen implants in the Lhb scored significantly lower in pup retrieval and crouching behavior compared to females with implants in the MPOA and were not significantly different from females with cholesterol implants in the Lhb. There were also no significant differences in overall activity or carrying behavior among the groups.

Plasma cocaine levels and locomotor activity after systemic injection in virgin and in lactating maternal female rats.

We have determined the temporal pattern of plasma cocaine levels and increased activity that result from acute systemic injections of cocaine to female rats in two different endocrine and behavioral states, in nonmaternal virgins and in lactating maternal dams. Plasma levels of cocaine as well as ambulatory and rearing activity were determined every 30 min for a total of 300 min after subcutaneous injections of either 10, 20, or 40 mg/kg of cocaine. Virgin females had no prior drug history, whereas lactating, maternal dams had received two cocaine injections before activity testing. Within 30 min after an injection, cocaine in the plasma and activity were substantially elevated, and generally remained so for 270-300 min. Overall, plasma cocaine levels and activity were well correlated and followed a predictable dose-response pattern. The onset, peak, duration, and decline of activity corresponded generally to the onset, peak, duration, and decline of plasma cocaine. For virgins, mean ambulatory activity increased 2.5-4.0-fold over baseline, whereas in lactating females activity increased 5-11-fold over baseline. Stereotypy did not occur. Although the general responsivity of these females to cocaine was very similar to that reported for males, there are differences in the timing of peak activity and the return of activity to baseline when the virgins and the lactating dams are compared to each other and to reports by others on male rats. These data support the hypothesis that endocrine or behavioral state may influence the responsiveness of animals to cocaine.

Evidence for estrogen receptor in cell nuclei and axon terminals within the lateral habenula of the rat: regulation during pregnancy.

The habenular complex is involved in several estrogen-dependent reproductive behaviors in female rats, namely, sexual behavior, maternal behavior, and postpartum sexual behavior. Although it is known that estrogen acts in other brain regions to mediate these behaviors, it is not known whether estrogen may also act directly on the habenular complex. To address this possibility, we examined this region for the presence of estrogen receptor (ER). This analysis was carried out in separate experiments by using in situ hybridization, immunocytochemistry at the light and electron microscopic levels, and steroid autoradiography. Neurons within the lateral habenula (LHb), but not the medial habenula, express ER mRNA, contain ER immunoreactivity (ER-ir) in their nuclei, and concentrate radiolabelled estradiol, providing strong evidence for the presence of functional ER in the lateral habenula. There were also ER-ir containing punctate fibers within the LHb, which, at the electron microscopic level, in part, proved to be axons and presynaptic axonal terminals. Both the level of ER-ir in cell nuclei and the density of ER-ir fibers within the LHb were regulated during the course of pregnancy and the postpartum period, suggesting that the sensitivity of the LHb to estrogen may be altered during this time. Taken together, these results demonstrate that the LHb is likely a more estrogen-sensitive region than was previously considered, and they suggest alternative mechanisms of action for ER. ER within the LHb may play a critical role in the involvement of the LHb in estrogen-dependent female reproductive behaviors.

Intact neurons of the lateral habenular nucleus are necessary for the nonhormonal, pup-mediated display of maternal behavior in sensitized virgin female rats.

Our research has demonstrated that the lateral habenular nucleus (Lhb) is necessary for the hormonal onset but not the postpartum maintenance of maternal behavior in the rat (K. P. Corodimas, J. S. Rosenblatt, & J. I. Morrell, 1992; K. P. Corodimas, J. S. Rosenblatt, M. E. Canfield, & J. I. Morell, 1993; T. Matthews-Felton, K. P. Corodimas, J. S. Rosenblatt, & J. I. Morell, 1995). To test the role of the Lhb in the nonhormonal onset of maternal behavior, we used the sensitization model in which the continual exposure of females to pups induces maternal behavior. Ovariectomized females received bilateral cytotoxic lesions of neurons of either the Lhb or the dorsal medial cingulate cortex-hippocampus, or they were unoperated. Maternal behavior, activity, and oromotor carrying capability were tested. Complete lesions of the neurons of the Lhb induced significant deficits in pup retrieval and nest building. Sniffing, licking, and crouching behaviors were unaltered. Activity and carrying ability were normal. These results indicate a role for the Lhb that extends to the nonhormonally dependent onset of maternal behavior, but they also indicate a more limited role than in the mediation of the hormonal onset of the behavior.

Atlas of the neurons that express mRNA for the long form of the prolactin receptor in the forebrain of the female rat.

Prolactin has a variety of important physiological effects on peripheral tissue and on the brain. The behavioral effects of prolactin include the induction of maternal behavior and increased food intake. Prolactin acts via its cognate receptors which have two forms, a short and a long form. The long form of the receptor is predominant in the preoptic area-hypothalamus and is positioned to support maternal behavior since this form is regulated across pregnancy and lactation (Nagano and Kelly [1994] J. Biol. Chem. 269:13337-13345; Sugiyama et al. [1994] J. Endocrinol. 141:325-333). By using in situ hybridization with [33P] labelled cRNA probe specific for the long form of the receptor mRNA(L-PRL mRNA) we have mapped, in brains from 2- and 21-day-old pregnant females, the neuroanatomical distribution of neurons expressing the long form of the receptor. Many neurons with high expression of L-PRL mRNA were located in the anteroventral periventricular nucleus, the medial preoptic area (MPO), specific subdivisions of the paraventricular and supraoptic nuclei, and in the arcuate and ventromedial nuclei. Labelled neurons were also found in limbic system structures such as the bed nucleus of stria terminalis (BST) and the medial nucleus of the amygdala, in a few thalamic nuclei, and in the central gray. All cells throughout the choroid plexus expressed high levels of L-PRL mRNA. The levels of L-PRL mRNA were higher in females on day 21 of pregnancy in the MPO and in the choroid plexus, than in females on day 2 of pregnancy; levels in the ventromedial nucleus of the hypothalamus (VMH) were unchanged across pregnancy. The neuroanatomical distribution of neurons expressing L-PRL mRNA may have special relevance for the mediation of maternal behavior, lactation, sexual behavior, and feeding.

Neuroanatomical distribution of aromatase MRNA in the rat brain: indications of regional regulation.

Aromatase, the enzyme responsible for the conversion of testosterone to estradiol, is found in the rat brain and is present in regions of the preoptic area, hypothalamus, and limbic system. Gonadal steroid hormones regulate aromatase activity levels in many brain regions, but not all. Using in situ hybridization, we examined the distribution of aromatase mRNA in the adult male forebrain, as well as the levels of aromatase mRNA in the brains of males and females, and the regulation by gonadal steroid hormones. In the adult male, many heavily labelled cells were found in the encapsulated bed nucleus of the stria terminalis (BNST), the medial preoptic nucleus (MPN), the ventromedial nucleus (VMN), the medial amygdala (mAMY) and the cortical amygdala (CoAMY). The regional distribution of aromatase mRNA was similar in males and females, but males tended to have a greater number of aromatase mRNA-expressing cells in each region compared to females. Aromatase mRNA levels in the BNST, MPN, VMN and mAMY tended to be lower in castrated males than in intact males, whereas aromatase mRNA levels were unaltered by castration in the CoAMY. Further analysis of individual cells expressing aromatase mRNA suggests that aromatase mRNA may be regulated by steroid hormones differentially in specific populations of cells in regions where enzyme activity levels are steroid-hormone-dependent.

Focal necrotizing panniculitis and vascular necrosis in rats given subcutaneous injections of cocaine hydrochloride.

Subcutaneous injections of cocaine hydrochloride in sterile physiological saline were administered to rats at a dosage of 20 or 40 mg.kg-1. Resultant skin lesions included focal areas of alopecia (within 1 to 2 days) which progressed to necrosis (within 2 to 7 days). Histologically, the skin lesions were characterized by necrotizing panniculitis and vascular necrosis, with only small numbers of inflammatory cells. The lesions may be ischemic in nature, and associated with cytotoxic properties of cocaine.

Levels of estrogen receptor immunoreactivity are altered in behaviorally-relevant brain regions in female rats during pregnancy.

Pregnancy and parturition are accompanied by unique behavioral changes. Only some of the neural mechanisms behind the dramatic changes in behavior are understood. Estrogen's action within the medial preoptic nucleus (MPN) is necessary for the induction of maternal behavior around the time of parturition, and estrogen acts within the ventromedial nucleus (VMN) to trigger postpartum sexual receptivity shortly after parturition. We have hypothesized that the sensitivity of various brain regions to estrogen may be altered by pregnancy to support these unique behavioral patterns. Using immunocytochemistry, this study examined whether the levels of estrogen receptor (ER) protein, within behaviorally relevant brain regions, differ among females on day 8, day 16, and day 22 of pregnancy, or on postpartum day 1. On day 16 and day 22 of pregnancy, the MPN contained a significantly greater number of cells expressing high levels of ER-ir compared to day 8 or postpartum day 1. In the VMN, the mean amount of ER-ir per cell was significantly higher on day 22 of pregnancy than on day 16 or postpartum day 1. In the bed nucleus of the stria terminalis, ER-ir levels were significantly increased on postpartum day 1 compared to day 22 of pregnancy. There were no significant changes in ER-ir in the medial amygdala. These results demonstrate regionally and temporally specific regulation of ER protein in the brain during pregnancy. Alterations in the levels of ER at critical times in regions such as the MPN and VMN may underlie the unique expression of maternal and sexual behavior that occur during pregnancy and at the time of parturition.

Distribution and steroid hormone regulation of aromatase mRNA expression in the forebrain of adult male and female rats: a cellular-level analysis using in situ hybridization.

Many of the effects of gonadal steroid hormones in the male brain are due to the actions of the testosterone metabolite estradiol, which is synthesized by the actions of the P450 enzyme aromatase. Aromatase activity is present in regions of the preoptic area, hypothalamus, and limbic system. Levels of aromatase activity in the brain are highly dependent on gonadal steroid hormones in many brain regions, but not all. We examined the distribution of aromatase mRNA in adult male and female rat brains as well as the regulation of the levels of aromatase mRNA in the brains of males by gonadal steroid hormones using in situ hybridization. This method was performed using a 35S-labelled cRNA probe, transcribed in vitro from the rat ovarian aromatase cDNA. In the adult male, many heavily labelled cells were found in the encapsulated bed nucleus of the stria terminalis (BNST), the medial preoptic nucleus (MPN), the ventromedial nucleus (VMN), the medial amygdala (mAMY), and the cortical amygdala (CoAMY). The regional distribution of aromatase mRNA was similar in females, but females tended to have a lower number of aromatase mRNA expressing cells in each region compared to males. Aromatase mRNA levels in the BNST, MPN, VMN, and mAMY tended to be lower in castrated males than in intact males, whereas aromatase mRNA levels were unaltered by castration in the CoAMY. The degree of reduction in mean levels of aromatase mRNA following castration does not simply account for the large changes measured in activity following castration. Examination of the entire population of individual cells expressing aromatase mRNA in castrated males suggests that aromatase mRNA may be regulated by steroid hormones differentially in specific populations of neurons within regions where activity is known to decrease following castration.

Cocaine transiently impairs maternal behavior in the rat.

This paradigm distinguished between two hypotheses not previously directly addressed. Do repeated exposures to cocaine at critical times during pregnancy, when the neural mechanisms that support maternal behavior are being read, alter some fundamental neural underpinning of maternal behavior in rats? Alternatively, does cocaine alter maternal behavior only when circulating? During the 4 hr after cocaine injection (20 or 40 mg/kg), there were significant deficits in maternal behavior. In contrast, 16 hr after cocaine injection, drug-injected females, in which plasma cocaine had fallen to nondetectable levels, showed the normal maternal behavior of saline-injected controls. This pattern of impaired maternal behavior after cocaine injection, followed by normal behavior as blood levels returned to zero, was replicated over 8 days. It was concluded that cocaine impairs maternal behavior only when circulating and does not have a residual effect in the transiently drug-free, chronically drug-treated dam.

Quantitative autoradiographic analysis of D1 and D2 dopamine receptors in rat brain in early and late pregnancy.

This study examined the levels of D1 and D2 dopamine receptors in the rat brain during pregnancy, a physiologically unique and important naturally occurring state. We are particularly interested in changes in the dopamine receptor complement of the brain during pregnancy because these receptors might support some components of the immediate postpartum onset of normal maternal behavior. Quantitative in vitro receptor autoradiography was applied particularly focusing on brain areas that control maternal behavior. The D1 dopamine receptor selective antagonist -3H-SCH23390 and the D2 dopamine receptor selective antagonist [3H]spiperone were used as the ligands. We examined the levels of binding to D1 and D2 dopamine receptors in brains in females on day 2 (early pregnancy) and day 21 (late, but prepartum pregnancy) of pregnancy. In addition, brains from females on diestrus-1 and from males provided reference points to the existing literature. Late in pregnancy females had significantly 18-27% lower levels of binding to D1 dopamine receptors in the lateral striatum, the medial striatum, and the nucleus accumbens when compared to all other groups. Late in pregnancy, females had also significantly 11-25% lower levels of binding to D2 dopamine receptors in the lateral striatum, the anterior striatum, the nucleus accumbens and the olfactory tubercle compared to all other experimental groups. We examined all of the brain regions already established to be important for maternal behavior, and found that dopamine receptor binding changed across pregnancy only in one such region, the nucleus accumbens. Thus pregnancy, perhaps the hormones of pregnancy, reduces the levels of D1 and D2 dopamine receptors in the striatum, and the nucleus accumbens, but not in other brain regions.