Determinants of mother-young contact in Norway rats

Rat dams choose to care for their pups in a relatively warm area, apparently because they have an elevated thermal set point for the regulation of their body temperature. Since the duration of their contact bouts with their pups is limited by an acute hyperthermia that they experience while on the pups, their choice limits the time that they can spend with their pups. When dams could choose the temperature at which they cared for their young they spent only about half the time in contact with their offspring as did dams caring for their pups at the ambient temperature of the laboratory. We conclude that rat dams do not attempt to maximize the duration of their contact bouts with their offspring. Dams also do not attempt to minimize the amount of time that they spend away from their offspring. The duration of the intervals between contact bouts was influenced by the ability of the dams to dissipiate the heat gained during the contact bout with the pups. Again, dams did not seek out a cool area in which to spend their interbout intervals, preferring to remain in relatively warm areas. Their thermal preference facilitated the retention of their body heat and prolongation of the interbout intervals. Dams may return to the nest periodically for brief bouts during which the state of the pups is monitored. Pups were found to play both passive and active roles in modulating the overall pattern of maternal contact, for dams maintained contact with pups if the pups were cool, but failure to attach to the nipples soon after contact was established frequently led to rapid bout termination, particularly with warm pups. Contact is maintained if the pups are cool or if they attach to the nipples. Finally, it was found that pups could actively curtail interbout intervals by emitting ultrasonic vocalizations.     

The effects of lactation and ambient temperature on the body temperature of female Norway rats

Norway rat dams were placed in an ambient temperature of 4°C, 22°C, or 28°C. Body temperatures were recorded over a two hour period on three days; each female was observed on Day 4 and Day 10 of lactation and three days after lactation was stopped. Body temperatures were initially higher during lactation than after lactation had stopped. Dams were less able to maintain their body temperatures in the 28°C ambience on Day 4 of lactation than after lactation, and on Day 10 of lactation, were even less able to maintain their body temperatures than on Day 4. During lactation, dams were able to maintain and even increase their body temperatures in the 4°C ambience. The data support the hypothesis that the acute hyperthermia encountered by rat dams during contact with their litters may be due to a physical restriction on maternal heat loss.     

Thermal control of mother-young contact in rats

The factors involved in the control of nest bout duration by mother rats were analyzed. Disruption of prolactin or adrenocortical secretions halted the normal progressive decline in daily nesting time, but the behavioral effects of hormone disruption were not mediated by a suppression of milk delivery. Rather, these hormones stimulate a chronic elevation in the body temperature of mother rats which makes mothers vulnerable to the acute thermal consequences of huddling with their litter. The temperature of a mother rat rises further after her ability to dissipate heat decreases when she comes in contact with her young, a rise that eventually limits the duration of nest bouts. Decreasing the efficiency of heat dissipation during nest bouts by elevating room temperature, pup temperature, or by removing the tail (a major pathway for heat dissipation) decreased nest bout duration. Increasing the rate of maternal heat loss by decreasing room temperature, pup temperature, or by partially removing the insulating fur of dams increased nest bout duration. We conclude that nest bouts are normally limited by a rise in maternal temperature, and that the rate of temperature rise determines the duration of each bout.     

Thermal control of mother-young contact revisited: hyperthermic rats nurse normally

Morphine (MOR) is known to inhibit maternal behavior and induce hyperthermia; at appropriate doses, concurrent administration of naloxone (NAL) counteracts its disruption of maternal behavior but not the hyperthermia. We used these findings to evaluate the view that lactating rats terminate nursing due to intolerable hyperthermia. After a dam-litter separation of 4 h on Day 7 postpartum (PP), mother-litter interactions were observed continuously for 1 h. One hour before reunion, the dams received two injections (1 ml/kg ip each) of saline (SAL), MOR (20 mg/kg) and/or NAL (1 mg/kg) in the following combinations (n = 7 each): SAL + SAL, SAL + NAL, MOR + SAL or MOR + NAL. MOR profoundly disrupted maternal behavior, thereby preventing litter weight gains; these effects were completely counteracted by NAL, which alone had no discernible effects. In contrast, MOR-induced hyperthermia (approximately 0.7 degrees C increase in each hour, before and after reunion with pups) was not antagonized by NAL at the doses used. Thus, an additional 0.7-1.4 degrees C of body temperature (T) did not delay the onset or reduce the duration of nursing compared with SAL-treated controls. Further, there were no group differences in behaviors displayed both shortly before and after a nursing bout that included milk ejections or in the resumption of nursing. Together with earlier methodological and empirical criticisms of the thermal control theory, as well as knowledge about the somatosensory determinants of nursing, the present results suggest that nursing bouts in lactating rats are not limited by the mother's T.     

Energetic limits on reproduction: Interaction of thermal and dietary factors

The time that food-restricted Norway rat dams spent in contact with their offspring was elevated only during that portion of the day in which their body temperatures were depressed. These data support a thermal model for the limitation of mother-young contact. The depression in maternal body temperature appeared to be due to a direct limitation on available fuel, rather than being mediated by a depression in circulating hormone levels.     

Thermal control of maternal contact bouts: the interbout interval

Cooling mother rats by means of sodium salicylate injections did not curtail the intervals between maternal contact bouts. To the contrary, the interbout intervals were prolonged, suggesting that the relief from hyperthermia is a necessary but not a sufficient condition for the reinitiation of pup contact by mother rats. The ability of sodium salicylate to suppress maternal temperature suggests that pyrogenic and lactational increases in temperature may have some common feature.     

Thermal control of mother-young contact in Norway rats: factors mediating the chronic elevation of maternal temperature

The chronically elevated heat production of lactating Norway rats makes them vulnerable to acute hyperthermia during pup contact and thereby limits the duration of such interactions. High lactational levels of progesterone and corticosterone may act in concert to increase maternal heat load. Specifically, progesterone appears to increase maternal thermal set point and corticosterone is necessary for the increase in maternal heat production. Thyroid hormones and brown adipose tissue do not seem to contribute to the chronic increase in maternal heat production. While mammary tissue does contribute to maternal heat load, it is no more hypermetabolic than other maternal tissues.     

The effect of constant light on maternal physiology and behavior

In contrast to previous findings in nonlactating rats, constant light does not disrupt maternal water or food intake. Mothers and pups in constant light, however, gained less weight than their counterparts in the light-dark cycle. While mothers did not differ in their total daily contact time or daily bout frequency, the distribution of the contact time was shifted in constant light. These data may be consistent with the idea that mothers maintain a free-running rhythm under such conditions.     

Diurnal and ultradian variations of plasma concentrations of eleven adrenal steroid hormones in human males

Summary The diurnal variations of the plasma concentrations of eleven steroid hormones and of corticotropin (ACTH) were studied in ten young healthy males. The plasma steroids progesterone, pregnenolone, deoxycorticosterone, 17-OH-progesterone, 17-OH-pregnenolone, deoxycortisol, 18-OH-deoxycorticosterone, corticosterone, aldosterone, cortisol and 18-OH-corticosterone, as well as plasma ACTH, were measured at 30-min intervals in the morning and in the evening and at 2-h intervals during the rest of the day.Steroids were extracted from 1 ml plasma, fractionated by high-pressure liquid chromatography (HPLC) and finally quantified by radioimmunoassay (RIA). Plasma concentrations of ACTH were radioimmunoassayed after extraction from 2 ml plasma.More or less pronounced circadian and episodic variations were apparent for plasma levels of all steroids studied, as well as of ACTH. According to related profiles of diurnal variations of plasma concentrations, three different categories of steroids were tentatively crystallized. Category 1 includes 17-OH-pregnenolone, deoxycortisol, corticosterone, 18-OH-deoxycorticosterone, deoxycorticosterone, cortisol and 18-OH-corticosterone, exhibiting a rhythm partly synchronous with that of the pituitary secretory activity of ACTH. Category 2, including progesterone, pregnenolone and 17-OH-progesterone, exhibited a time course of plasma concentrations assuming a regulation predominantly dictated by the testicular secretory activity. Lastly, aldosterone exerted a variation of plasma concentrations which was obviously regulated by the renin-angiotensin system under the present conditions.The paper was presented in part to the Joint Congress of the Scandinavian and German Societies of Clinical Chemistry, Hamburg 8. — 11.10.1980     

Excessive water intake in captive Norway rats with scar-markings

Wild Norway rats (Rattus norvegicus) with scar-markings, presumably an index of low social status, were exclusive in drinking excessive amounts of water (> 1cc/g body weight/day) after capture. Unscarred wild rats had intakes similar to albino rats after 50 days captivity. In a second experiment there was no reliable relation between adrenal weight of captives and scar-marking or time in captivity up to 1 yr.     

Diurnal patterns of food intake and plasma corticosterone levels in lactating rats

The diurnal patterns of food intake and plasma corticosterone levels were determined in lactating Wistar-Imamichi rats maintained in a light (14 L: 10 D, lights on 0500 hr) and temperature (24±2°C) controlled animal room. Lactating mothers with four or more than eight pups showed a characteristic diurnal pattern of food intake with high rates of intake around the time of lights off. The diurnal rhythmicity of food intake seemed to disappear in late lactation in mothers with more than eight pups. The plasma corticosterone concentration on day 9 of lactation displayed a diurnal rhythm with high values at 1700 hr and 2100 hr. The rhythmicity of the corticosterone levels was abolished on day 16 of lactation and a high level was maintained throughout a 24 hr period. Since the diurnal rhythm of the corticosterone level was shifted in animals fed during the restricted period (0900–1700 hr) from days 2 to 9 of lactation, the appearance of diurnal rhythm of corticoid secretion seemed to depend on the diurnal feeding rhythm in lactation as well as in the estrous cycle.     

Effect of ambient temperature on the 24-hour sleep-wake cycle in normal and capsaicin-treated rats

The 24-hour sleep-wake cycle of untreated, normal rats and of capsaicin-treated rats was continuously recorded by telemetry. Recordings were made on two baseline days at 22 degrees C, two days at 29 degrees C, and two final days at 22 degrees C. In untreated animals the daily amount of waking was reduced by the elevated ambient temperature and nonREM sleep was enhanced. This effect was mainly due to the frequent interruption of the dark-time waking episodes by sleep. In capsaicin-treated animals, raising the ambient temperature did not significantly enhance sleep. However, in both groups of rats the slow wave sleep (SWS) fraction of nonREM sleep was increased after elevating the temperature to 29 degrees C. REM sleep showed a minor increase which was significant only for the capsaicin-treated group. The results suggest that a moderate increase of ambient temperature has two effects: (1) It causes an enhancement of sleep by a reduction in the duration of waking episodes, an effect that may represent a heat-defense response. The attenuation of this response in capsaicin-treated rats may be a consequence of the impairment of warm-receptors. (2) It favors the occurrence of SWS and REM sleep.     

Sexual preference in female rats during estrous cycle, pregnancy and lactation.

Choice of a male or female social contact was studied in intact female rats in a runway-choice apparatus during estrous cycle, pregnancy, and lactation. The male was chosen significantly more often during proestrus/estrus than during the diestrous days of the cycle. During pregnancy this preference in choice declined only to reappear gradually during the lactation period. The shifts in the level of motivation to seek out the male support previous studies and point to the significance of estrogen in producing the preference for the male.     

Ethanol selection in wild-trapped agouti and laboratory albino Norway rats (Rattus norvegicus)

Wild-caught agouti Norway rats and Wistar albino Norway rats were habituated to the laboratory setting for 2 months and then given an ad lib 2-bottle choice of water and ethanol solution at concentrations of 1–17%, increasing 1% on consecutive days. Wistar rats displayed their typical preference for low concentrations and strong avoidance of high concentrations of ethanol, but the wild rats showed only a weak aversion for the highest ethanol concentrations. The results are discussed with reference to the higher metabolism, larger adrenal glands, and relative inflexibility of wild rats observed in previous comparisons with laboratory rats.     

Ovarian influences on food intake and body weight regulation in lactating rats

In the following experiments, an attempt was made to determine the role of the ovary in the control of food intake and body weight regulation during lactation. In the first study, it was found that concentrations of estradiol benzoate effective in suppressing food intake and body weight in nonlactating animals were not effective during lactation. In the second experiment, ovariectomy during lactation was shown not to produce the usual increases in food intake and body weight or change in meal patterns known to occur after ovariectomy in the nonlactating rat. These results suggested that lactating animals behave the as though functionally ovariectomized and that the removal of the ovaries is of no additional consequence. The further observation that animals nursing small litters gained weight considerably more rapidly than animals nursing large litters led to the prediction that these animals would also be more responsive to the suppressive effects of EB. In the third study, EB in concentrations which are not effective in suppressing body weight in animals nursing large litters was found to suppress body weight in mothers with small litters. However, since these animals also showed a decline in milk yield, a number of alternative interpretations of these results were considered. These results, together with data concerning levels of ovarian hormones during gestation and lactation led to the hypothesis that pregnant and lactating animals undergo an elevation in body weight set-point, similar in magnitude and quality to elevations following ovariectomy in the nonlactating animal.     

Simultaneous changes of the perivascular contact area and HIMOT activity in the pineal organ after bilateral adrenalectomy in the rat

Summary In the pineal organ of the female rat, proportional changes in the area of contact of pinealocyte processes and glial processes with the perivascular space were found after bilateral adrenalectomy. The contact area was evaluated by measuring the length of pinealocyte and glial cell membranes directly abutting on the basal lamina. In the normal female rat, 40% of the contact area is occupied by pinealocyte and 60% by glial cell processes. Fourteen days after bilateral adrenalectomy, this proportion is reversed. In addition, cell counts demonstrate that more pinealocytes gain access to the pericapillary space due to the experimental conditions.In order to prove whether or not these results indicate an increase of pineal endocrine activity, the melatonin-forming enzyme hydroxyindole-O-methyltransferase (HIOMT, E.C. 2.1.1.4) was assayed. After bilateral adrenalectomy the HIOMT activity was found to significantly increase.The authors wish to dedicate this paper to Professor Wolfgang Bargmann on the occasion of his 70th birthdayThis paper includes results of the Doctor's Thesis of Marlies Schmitter at the Medical Faculty of BonnSupported by grants from the Landesamt für Forschung, Nordrhein-Westfalen (BRD) and the Deutsche Forschungsgemeinschaft     

Temperature preferences and the effect of changes in serotonin in maturing mice

Behavior of maturing mice in a 20 cm long thermal gradient is described. Mean preferred temperatures (T_pref) under standard test conditions ranged from about 29° to 31° in untreated mice aged 3, 7, 10, and 14 days postpartum, which is similar to the temperature maintained in the nest by parent mice. Increased serotonin (5-HT) after injection of the precursor 5-hydroxytryptophan (30 mg/kg) was generally associated with increasted T_pref (7 and 10 days) as compared with vehicle injected littermate controls in a double-blind study. Depletion of 5-HT following p-chlorophenylalanine (400 mg/kg) was associated with decreased T_pref (3 and 7 days). Relationships with body temperature and thermoregulation are discussed and handling cautions are presented.     

Quantitative histoenzymologic characteristics of the submaxillary salivary glands during the ovarian cycle in albino rats

To study the connection between the salivary and endocrine glands and also the degree of participation of individual components of the salivary glands with different functions in endocrine regulation, the ovarian cycle was used. The concentrations of proteins, mucopolysaccharides, DNA, and RNA, and activity of NAD- and NADP-diaphorases, alkaline phosphatase, malate and isocitrate dehydrogenases, and -leucine-aminopeptidase in the submaxillary salivary glands were investigated. It was shown by cytospectrophotometric analysis that, depending on the phases of the ovarian cycle, synchronous changes in the activity of the enzymes studied took place in all parts of the salivary glands. Of the four consecutive phases of the sex cycle the greatest activity of the enzymes and concentration of proteins and mucopolysaccharides were observed in proestrus and metestrus. The metabolic processes in the salivary tubules were rather different from those in other parts of the glands, and this may be attributable to differences in the pattern of secretion production and also, possibly, hormone formation.Department of Pathological Anatomy, Academician I. P. Pavlov First Leningrad Medical Institute. (Presented by Academician of the Academy of Medical Sciences of the USSR A. V. Smol'yannikov.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 85, No. 6, pp. 736–739, June, 1978.     

Stimulus animal characteristics do not modulate the expression of partner preference by female rats

The stimulus animals used in tests of partner preference in female rats vary. To test the hypothesis that the alternative stimulus animal modulates the preference for an intact male, gonadectomized (GDX) female rats received estradiol benzoate plus progesterone or the oil vehicle and were tested for partner preference with either an intact male paired with a GDX hormone-primed female or an intact male paired with a GDX male. Rats were tested under conditions that limited physical contact (No Contact) or allowed sexual interaction (Contact). Stimulus animal condition was not a primary determinant of partner preference. In contrast, contact condition and hormone treatment modulated preference, as well as activity levels and the display of proceptive behaviors. Our findings demonstrate that the characteristics of the alternative stimulus animals tested here do not play a significant role in modulating partner preference in female rats.