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

Abstract

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.

Introduction

Maternal behavior in the rat consists of maternal nest building, and pup directed behaviors including retrieving, nursing, grooming, and protection from intruders. Parturient females are immediately maternally responsive to their pups. Less generally known is that fact that female, male, or juvenile rats can be induced to express maternal behavior by continuous exposure to new born pups provided from parturient female foster mothers. The expression of pup-induced maternal behavior has been studied extensively; it is virtually indistinguishable from hormonally-induced maternal behavior, except that parturient females nurse their pups. 47., 58.. Adult rats (>60 days of age) require 4–8 days of exposure to pups before maternal behavior emerges, while surprisingly young juvenile rats (20–26 days of age) only require 1–3 days of exposure to pups. This required exposure period increases as the juveniles approach puberty 6., 7., 43., 44., 45., 65.. These data suggest an intriguing difference in behavioral responsiveness across young juveniles and adults.

Key components of the neural circuit that mediate maternal behavior include the medial preoptic area (MPOA), the medial and cortical amygdala (MCA), and the nucleus accumbens (NAC), along with other limbic and hypothalamic structures [48]. Most of our knowledge of the neural circuit that mediates maternal behavior has been gathered from studies using adult animals; much less has been done using the juvenile. The MPOA is necessary for both the expression of and motivation to perform maternal behavior [48]. In adults both large and very small lesions in the MPOA disrupt maternal behavior while in the juvenile or pubertal rat, only large lesions disrupt the behavior, small lesions do not 36., 50., 51.. Moreover, increased expression of immunoreactive c-fos in the MPOA and other components of this neural circuit occurs in adults during pup-induced maternal behavior, however, in juveniles c-fos induction depends upon the age of the juveniles and perhaps on the extent of pup exposure 24., 37., 51., suggesting that this is a maturing circuit component.

In the non-maternal adult female, neuronal activity in the MCA inhibits maternal behavior by a mechanism that involves processing of olfactory cues from the initial exposure to pups 20., 47., 48.. The hormones of the peripartum period are thought to alter or release this basal inhibition so that the expression of maternal behavior can occur. Prolonged pup exposure also reduces this basal inhibition allowing the onset of pup-induced maternal behavior. Lesions in the medial amygdala had similar effects in both older juveniles and adults, shortening the time of pup exposure needed to induce maternal behavior [51]. The NAC is necessary for retrieval components of maternal behavior, and for processing of pup related stimuli likely to be related to mechanisms of motivation in the adult, not yet studied in the juvenile [48].

The scope of our knowledge concerning the neurotransmitters that mediate maternal behavior is limited to a few transmitters and to the adult model [48]. The literature provides the clearest case for a role for dopamine (DA) in the mediation of maternal behavior 5., 21., 22., 25., 26., 28., 49., 66.. Extensive lesions of the dopaminergic system 25., 26., 27. disrupt maternal behavior, and dopamine receptor antagonists infused locally into the NAC inhibited retrieval and licking components of maternal behavior [39]. These studies suggest that expression of the behavior requires dopamine. Increases in DA and DA metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in the extracellular space in the ventral striatum after separated mother rats and pups were reunited [28] suggest a role in the motivational components of the behavioral response.

The exploration of a role for other neurotransmitters in the mediation of maternal behavior is very limited; however, there is modest evidence that serotonin (5-HT) may be involved. Barofsky et al. [2] found that serotonergic lesions in the median raphe nucleus caused short-term disruption in maternal behavior but not the prolactin response with suckling, however, the study does not establish functional specificity since locomotor impairment was not ruled out. Ferreira et al. [17] found that buspirone (a partial 5-HT_1A agonist) inhibited maternal behavior; however, they did find reduced motor activity, limiting the specificity of the interpretation. Microdialysis also showed an increase in the concentration of the metabolite for 5-HT, 5-hydroxyindoleacetic (5-HIAA), in the ventral striatum when dams were reunited with pups [28]; perhaps motivational components of maternal behavior are supported by this system as well. Infusions of cocaine into the NAC or MPOA disrupt adult maternal behavior [74]; suggesting the exact level of DA and possibly 5-HT neurotransmitters is important. Thus, an excess of these transmitters could be as disruptive as their reduction or absence.

Our working hypothesis is that the brain regions mediating maternal behavior in the adult, also mediate maternal behavior in the juvenile, and that postnatal neurochemical changes within these regions may underlie the maturing time course of pup-induced maternal behavior. While it is formally possible that maternal behavior in the juvenile might be supported by different neural structures or transmitters than those that support maternal behavior in the adult, we know of no data to support this alternative hypothesis.

It has been hypothesized that adult rats, without benefit of the hormones of parturition, require longer exposure to pups to become maternal because their initial response to pups is a typical adult neophobic response to novel stimuli, while juveniles are generally less neophobic and affiliate readily with pups 18., 19., 43., 45., 55., 63.. We hypothesize that postnatal changes within the neural circuit may add a level of inhibition to social stimuli, and that this is the fundamental difference between the juvenile and adult state. The comparison of juveniles (20–27 days old) to adults (>60 days) offers the intriguing view before and after these hypothesized developmental changes in the inhibitory components of the neural circuit occur, and to consider that these two neurotransmitters might be part of the mechanistic basis for this additional behavioral inhibition.

Our goal was to examine the content of dopamine and serotonin and their metabolites in juveniles and adults within the neural circuit that mediates the expression of maternal behavior. We choose postmortem sampling of tissue punches as our approach so that we could examine multiple brain areas in juveniles and adults. An alternative, microdialysis would allow measurements in only one area at a time. First, the behavioral state of each animal was assessed. Then postmortem tissue analysis was used to determine, in each individual animal, the amount of these neurotransmitters and their metabolites in three different brain regions, the NAC, MPOA, and MCA.

Two specific questions were addressed. Does the amount of these neurotransmitters and their metabolites vary with the maternal state of the rat? Is the amount of these substances in these specific regions different in adults compared with juveniles?