Angel I. Melo

Mothering begets mothering: The transmission of behavior and its neurobiology across generations

Early experiences exert their effects on adult parental behavior in part by altering the development of neurobiological mechanisms that initiate or support the initiation and sustenance of adult parental behavior. The effects of parental behavior on sensory, perceptual and emotional mechanisms in offspring constitute an experientially based mechanism by which neurobiological factors regulating behavior can be transferred from generation to generation somewhat independently of genetic endowment.

Maternal behavior in New Zealand white rabbits: Quantification of somatic events, motor patterns, and steroid plasma levels

Several parameters associated with maternal behavior were quantified under laboratory conditions in New Zealand white rabbits. Digging behavior appeared earliest (8-6 days prepartum), its decline preceding the onset of straw carrying (3-1 days prepartum). Hair pulling consummated the construction of the maternal nest. Food intake significantly decreased on days 2 and 1 prepartum. On parturition day, all females spent 300-500 s with the litter while, for the rest of lactation, nursing bouts lasted 199 +/- 7 s. Milk yield increased linearly up to lactation day 19, declining thereafter. Pup weight increased linearly throughout lactation despite the decline in milk yield. Plasma estradiol (E) levels did not significantly vary across pregnancy: 60 +/- 2 pg/ml (days 10-25) and 75 +/- 6 pg/ml (day 30). The testosterone (T) levels at these times were: 200 +/- 10 and 308 +/- 0.03 pg/ml, respectively. Testosterone significantly declined from pregnancy day 30 to lactation day 1 (202 +/- 0.02 pg/ml). Progesterone (P) levels significantly declined from pregnancy day 20 (9 +/- 1 ng/ml) onwards. Progesterone levels were negligible across lactation. Thus, mother rabbits display a sequence of motor patterns and somatic events correlated with changes in plasma levels of T and P against a background of E.

Estradiol, progesterone, and Prolactin Regulate Maternal Nest-Building in Rabbits

Maternal nest‐building in rabbits, expressed across the last third of pregnancy, consists of: digging a burrow, collecting straw and shaping it into a nest inside the burrow, plucking body hair and lining the straw nest with it. The sequential expression of these activities is correlated with specific changes in the plasma concentration of estradiol, progesterone (P), and prolactin (PRL). To further substantiate the participation of these hormones in the control of maternal nest‐building we explored in ovariectomized (ovx) New Zealand white rabbits the capacity of several combinations of such hormones to stimulate digging, straw‐carrying, and hair‐pulling. Does given estradiol benzoate (EB; 5 μg/day from days 3 to 21) plus P (2 or 10 mg/day from days 4 to 16) dug into a substrate from the fourth day of the P treatment until withdrawal of this hormone. The intensity of this effect was greater in the group treated with the high dose of P. Straw‐carrying and hair‐pulling occurred after P withdrawal in a dose‐response way. Food intake, which declines in pregnant females shortly before parturition, decreased to the same extent in both groups of ovx EB‐treated does after P withdrawal. A significant increase in PRL plasma levels was observed on day 9 in does given EB plus 2 mg P/day and at two days following P withdrawal in does given EB plus 10 mg P/day. When such ovx EB/P‐treated does were given bromocriptine to block PRL release (1 or 3 mg/Kg/day, from days 11 to 21) the expression of digging was unmodified. By contrast, bromocriptine abolished the display of straw‐carrying and hair‐pulling, and also prevented the decline in food intake normally following P withdrawal. The addition of ovine PRL to ovx EB/P‐treated does given bromocriptine reduced the expression of digging, did not restore straw‐carrying or hair‐pulling, and provoked a sharp decline in food intake. The possible mechanisms of interaction between PRL and steroid hormones for the regulation of specific aspects of the pregnant does physiology and behavior are discussed.

Importance of mother/young contact at parturition and across lactation for the expression of maternal behavior in rabbits

We prevented mother/pup contact at parturition or across early or midlactation to investigate the importance of such interaction for maintaining material behavior in rabbits. When pup contact was prevented across lactation Days 1-7 or 11-17 (by anesthetizing multiparous mothers during the oxytocin-induced milk letdown; Experiment 1), nursing incidence was reduced to 40% and 83%, respectively, on the day following anesthesia withdrawal. Both groups also showed a decreased milk output, long latencies to initiate nursing, and several entrances into the nest box not associated with nursing. In Experiment 2 we prevented mother/litter contact at parturition to determine the specific role of pup contact at this time. We found a reduction in the incidence of nursing on postpartum Day 1 from 80% (in control primiparous mothers) to 33%. By contrast, 100% of both deprived and control multiparous mothers displayed nursing on Day 1. These mothers also showed the unusual behaviors found in Experiment 1 and an extemporaneous nest-building. We conclude that: (a) mother/young contact at parturition is crucial for establishing maternal responsiveness in primiparous does, (b) the experience acquired by raising a previous litter allows the retention of maternal responsiveness despite a lack of pup contact at parturition, (c) maternal responsiveness is maintained across early lactation by daily interaction with pups, and (d) interaction with pups across midlactation allows the finely tuned display of maternal behavior.

Pharmacological evidence that prolactin acts from late gestation to promote maternal behaviour in rabbits.

We investigated the role of prolactin and suckling stimulation in the expression of maternal behaviour of primiparous rabbits. Bromocriptine (1 mg/kg/day), given to intact mothers across postpartum days 1–5, decreased serum concentrations of prolactin to undetectable levels, reduced crouching, and increased time inside the nest. Failure of maternal nest‐building, provoked by bromocriptine injections from pregnancy day 26 to parturition or to postpartum day 5, correlated with a stronger reduction in crouching and an increased time inside the nest, measures of disturbed maternal behaviour, on postpartum days 3 and 5. Preventing suckling by thelectomy did not prevent prolactin release but reduced crouching incidence and increased the time spent inside the nest on postpartum days 3 and 5. Bromocriptine, injected in thelectomized mothers across postpartum days 1–5, further reduced the incidence of crouching and increased the time spent inside the nest on postpartum days 3 and 5. We conclude that prolactin acting prepartum facilitates maternal behaviour initiation in rabbits and, together with pup stimulation, maintains this behaviour across lactation.

Sex steroid regulation of chin-marking behavior in male New Zealand rabbits

Chin-marking behavior (chinning) was evaluated daily in nine intact adult male rabbits. All subjects (Ss) displayed chinning (mean of means +/- SE = 61 +/- 7 marks/10 min) but the frequency of this behavior varied largely across them (range of mean chinning frequency = 19-84 marks/10 min). Chinning frequency showed abrupt variations at intervals of 2-3 days, but periodogram analysis did not reveal the existence of an endogenous rhythm in this behavior. Castration significantly decreased (mean of means +/- SE = 29 +/- 9 marks/10 min; p < 0.01). but did not suppress chinning. Testosterone propionate (TP; 1 mg/day for 16 days) restored chinning in castrated Ss to slightly below precastration levels (mean +/- S.E. V 53 +/- 13 marks/10 min). The daily administration of 1 microgram estradiol benzoate (EB) plus 1 mg dihydrotestosterone propionate (DHTP) stimulated chinning within 2 days (mean increase = 147%; p < 0.005). DHTP (1 mg/day) given alone stimulated chinning only after 11 days of treatment (mean increase = 475%; p < 0.01). At higher doses, both DHTP (10 mg/day) and EB (10 or 50 micrograms/day) stimulated chinning by 450%, 80%, and 100%, respectively, over baseline values. Results indicate that chinning largely depends on testicular steroids. Androgen receptor occupation by T or DHT, which is enhanced by E, optimally activates chinning.

Progesterone, but Not LHRH or Prostaglandin E2, Induces Sequential Inhibition of Lordosis to Various Lordogenic Agents

In experiment I we studied the capacity of progesterone (P) and two nonsteroidal agents that activate lordosis, but do not bind to the progestin receptor (PR), i.e. luteinizing hormone-releasing hormone (LHRH) and prostaglandin E2 (PGE2) to induce sequential inhibition (SI) in ovariectomized estradiol-primed rats. The administration of 1 mg P, 5 micrograms LHRH or 100 micrograms PGE2 induced significant lordosis within 4 h. An injection of 1 mg P, 24 h after the administration of the above lordogenic agents, induced significant lordosis in rats pretreated with LHRH or PGE2, but not in those pretreated with P. Thus, only P induced SI (p < 0.025). In experiment II we investigated if progestin-induced SI results in a reduced capacity of the subjects to respond only to P or to other lordogenic agents. The synthetic progestin norgestrel (400 micrograms administered 24 h earlier) significantly reduced the responsiveness to P (p < 0.01), LHRH (p < 0.01), PGE2 (p < 0.025) and dibutyryl cyclic AMP (db cAMP p < 0.01). Results suggest that SI is triggered only by agents that bind to the PR (experiment I) and that it decreases the responsiveness of rats not only to P but also to other lordogenic agents (experiment II).

Artificial feeding synchronizes behavioral, hormonal, metabolic and neural parameters in mother-deprived neonatal rabbit pups.

Nursing in the rabbit is under circadian control, and pups have a daily anticipatory behavioral arousal synchronized to this unique event, but it is not known which signal is the main entraining cue. In the present study, we hypothesized that food is the main entraining signal. Using mother‐deprived pups, we tested the effects of artificial feeding on the synchronization of locomotor behavior, plasma glucose, corticosterone, c‐Fos (FOS) and PERIOD1 (PER1) rhythms in suprachiasmatic, supraoptic, paraventricular and tuberomammillary nuclei. At postnatal day 1, an intragastric tube was placed by gastrostomy. The next day and for the rest of the experiment, pups were fed with a milk formula through the cannula at either 02:00 h or 10:00 h [feeding time = zeitgeber time (ZT)0]. At postnatal days 5–7, pups exhibited behavioral arousal, with a significant increase in locomotor behavior 60 min before feeding. Glucose levels increased after feeding, peaking at ZT4–ZT12 and then declining. Corticosterone levels were highest around the time of feeding, and then decreased to trough concentrations at ZT12–ZT16, increasing again in anticipation of the next feeding bout. In the brain, the suprachiasmatic nucleus had a rhythm of FOS and PER1 that was not significantly affected by the feeding schedule. Conversely, the supraoptic, paraventricular and tuberomammillary nuclei had rhythms of both FOS and PER1 induced by the time of scheduled feeding. We conclude that the nursing rabbit pup is a natural model of food entrainment, as food, in this case milk formula, is a strong synchronizing signal for behavioral, hormonal, metabolic and neural parameters.

Effect of forebrain implants of testosterone or estradiol on scent-marking and sexual behavior in male and female rabbits

Chinning consists of rubbing the chin against an object, thereby depositing secretions from the submandibular glands. As mating, chinning is stimulated in male and female rabbits by testosterone and estradiol, respectively. To investigate the brain sites where steroids act to stimulate chinning and mating we implanted into the ventromedial hypothalamus (VMH) or the medial preoptic area (MPOA) of gonadectomized male and female rabbits testosterone propionate (TP; males) or estradiol benzoate (EB; females) and quantified chinning and sexual behavior. EB implants into the VMH or MPOA reliably stimulated chinning in females. Most of those implanted into the VMH and around half of the ones receiving EB into MPOA or diagonal band of Broca (DBB) showed lordosis. Chinning, but not sexual behavior, was stimulated in males by TP implants into the MPOA or DBB. Neither chinning nor mounting were reliably displayed by males following TP implants into the VMH. Results indicate that, in females, the VMH is an estrogen-sensitive brain area that stimulates both chinning and lordosis while the MPOA seems to contain subpopulations of neurons involved in either behavior. In males, androgen-sensitive neurons of the MPOA, but not the VMH, are involved in chinning stimulation but it is unclear if these areas also participate in the regulation of copulatory behavior.

Chin-marking behavior in male and female New Zealand rabbits: Onset, development, and activation by steroids

Chin marking (chinning) frequency was determined daily in 25 male and 24 female New Zealand rabbits aged 31-150 days. Chinning appeared earlier in females (mean +/- SD = 41 +/- 16 days) than in males (47 +/- 13 days). Between days 30-50, females displayed chinning more frequently than males. Thereafter, chinning increased steadily in both sexes up to day 100. Chinning curves levelled on days 100 and 140 in males and females, respectively. Profile analysis of male and female chinning curves showed significant differences in their slopes and in their population means (p < 0.001). At 7 months of age (days 210-224), both sexes displayed adult chinning frequencies: mean of means +/- SD = 103 +/- 18 and 79 +/- 14 marks/10 min in males (n = 8) and females (n = 8), respectively. The administration of testosterone propionate (TP, 1 mg/day) or estradiol benzoate (EB, 1 microgram/day) to males and females, respectively, from days 31-50, stimulated higher chinning frequencies than those displayed by untreated animals. Results suggest that chinning frequency increases throughout development largely as a consequence of a concomitant increase in sex steroid secretion.