Activity in the ferret: oestradiol effects and circadian rhythms

Anim. Behav.,1985, 33, 150-154

Activity in the ferret: oestradiol effects and circadian rhythms E. R. S T O C K M A N * , H. E. A L B E R S t & M. J. B A U M * $ *Department of Nutrition and Food Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. t Worcester Foundation for Experimental Biology, 222 Maple Avenue, Shrewsbury, MA 01545, U.S.A.

Abstract. The present study was conducted to determine whether oestradiol increases activity in the European ferret (Mustelafuro), whether this effect is sexually dimorphic, and whether a 24-h rhythm is present in the ferret's daily activity. The activity of male and female adult, postpubertally gonadectomized ferrets was monitored while they were maintained singly on a 13: I I light-dark cycle, before and after implantation with oestradiol-17fi. Gonadectomized male and female ferrets exhibited equal levels of activity, and neither sex exhibited a significant change in activity following oestradiol implantation. None of the ferrets exhibited a strong circadian rhythm, although weak 24-h rhythms and shorter harmonic rhythms were present. Golden hamsters (Mesocricetus auratus), monitored, in an identical manner, exhibited strong circadian rhythms. It was concluded that oestradiol administration may not cause an increase in activity in the ferret, and that this species lacks a strong circadian activity rhythm.

In many mammalian species, perinatal exposure to androgen reduces an animal's ability to exhibit feminine receptivity in response to oestrogen in adulthood (Baum 1979). Female rats (Rattus norvegicus), for example, show higher levels of lordosis than males or perinatally androgenized females in response to the administration of oestradiol plus progesterone (Barraclough & Gorski 1962; Davidson 1969), or to oestradiol administration alone (Whalen et al. 1971). In the European ferret (Mustelafuro), however, oestradiol is equally effective in promoting feminine sexual receptivity in gonadectomized males and females (Baum 1976; Baum et al. 1982). For this reason, we have been interested in determining whether other oestrogenmediated behaviours are sexually dimorphic in the ferret. Activity seemed a reasonable behaviour to test in the ferret because oestradiol induces an increase in activity in the rat, as measured either in home-cage wheel-running (Gerall et al. 1973) or in the open field (Blizard et al. 1975). Oestradiol stimulates significantly more activity in neonatally unmanipulated female rats than in males or neonatally androgenized females (Blizard et al. 1975; Gentry & Wade 1976; Blizard 1983). No study has linked activity in the ferret with oestradiol, but Lockie (1966) has described increases in the territory size of wild female muste$ To whom reprint requests should be addressed.

lines during the breeding season (Lockie 1966). We reasoned that this territorial expansion might reflect an increase in activity due to increasing endogenous oestrogen levels at the onset of the breeding season. The present study was designed to establish whether oestradiol induces an increase in activity in the laboratory-reared ferret, and whether this effect, if present, is sexually dimorphic. A second aim of the present study was to determine whether the ferret exhibits a strong 24-h activity rhythm. Diurnal fluctuations in melatonin secretion by the ferret pineal have been observed (LynCh et al., personal communication), but no study to date has focused upon the activity rhythms of this species. If the ferret were found to exhibit a circadian rhythm, it is possible that the responsiveness of this rhythm to oestrogen might serve as another index of sexual differentiation in this species. In the golden hamster, Mesocricetus auratus (Zucker et al. 1980) and the rat (Albers 1981), the responsiveness of the circadian system to oestrogen is subject to sexual differentiation. Females, but not males or neonatally androgenized females, respond to oestrogen administration by shortening their free-running period. Other investigators of rest activity cycles have employed measurement techniques different from ours (Sterman et al. 1965; Hawking et al. 1971; Kavanau 1971). In order to establish that our methods would detect activity rhythms when pre150

Stockman et al.: Ferret activity sent, we tested six golden hamsters in the same manner as the ferrets. This rodent's activity cycle is known to be quite precise (Pittendrigh & Daan 1976). METHODS

Six male and six female adult, postpubertally gonadectomized ferrets were maintained singly in opaque wooden cages, measuring 45 by 60 by 60 cm. These cages were fitted with wire-mesh lids and placed in a light-proof, sound-proof cubicle. A white-noise generator was used to mask sounds. The animals were provided with Purina Cat Chow and water ad libitum, and maintained on a lighting regimen consisting of 13 h of light and 11 h of darkness per day. During the first half-hour of the light period, two 5-W bulbs were switched on at 15-rnin intervals to simulate dawn. These bulbs were placed so that only reflected light was visible to the animals. At the end of the first 30 min, the overhead fluorescent lights were switched on. This schedule was reversed during the last 30 min of the light period to simulate dusk. Activity was monitored by means of capacitative sensors (Columbus Instruments), equipped with electromechanical counters, placed under each cage. These devices were connected to a recording printer, set to print at 30-rain intervals. Six gonadally intact male golden hamsters were monitored in the same apparatus, under the same lighting regimen. They were housed singly and provided with Purina Laboratory Rodent Chow and water ad libitum. All ferrets and hamsters were allowed to habituate to the lighting regime for at least one week before activity monitoring began. Each ferret's activity was monitored for at least 14 days to establish baseline levels. Then each ferret was removed from the apparatus, anaesthetized with 40 mg ketamine and 0-4 mg acepromazine per kg of body weight, and implanted subcutaneously with an oestradiol-17/~ capsule measuring 65 mm per kg of body weight. The capsules were prepared by filling lengths of 1.47 • 1-96-mm silastic tubing with a 9:1 mixture of cholesterol and oestradiol, then sealing both ends with silastic elastomer. This dose has been found to induce serum oestradiol levels of 80 + 10 pg/ml in gonadectomized female ferrets (K. Ryan, unpublished data, N = 8), which is within the range reported for females in natural oestrus (Erskine & Baum, 1984). Each animal was

151

returned to the activity apparatus approximately 48 h after the operation, and activity was recorded for the next 20 days. Mean daily activity in the absence of hormone, and following oestradiol implantation, was calculated for each ferret. These data were subjected to a two-way analysis of variance, with sex as a main effect and hormone condition as a repeated measure. To evaluate periodic variations in activity, the complete activity records of all animals were subjected to time-series analyses (Rummel et al. 1974). RESULTS Within a week of oestradiol implantation, all of the female ferrets exhibited increases in vulval diameter indicative of oestrus. Activity levels following implantation, however, were not significantly higher than baseline levels in ferrets of either sex, and no sex difference was observed (Table I). The analysis of variance yielded no significant effects and no interaction. Daily variations in activity are shown in Fig. I. Table I. Effect of oestradiot on total daily activity in gonadectomized male and female ferrets Activity counts Subject sex

N

Pre-homaone

Oestradiol

Males Females

6 6

276.15-!-35-36 284-78_+41.94 252.824-34-80 261.87+ 37"10

Note: Data are expressed as mean _+s~ counts/day.

e~-41 Males 0 - - - 0 Females

40O O~

~ 500 ,b

I

cf

i

;~00

0

I i 6

12

18 D,4KF

24

50

36

Figure 1. Mean daily activity scores of six male and six female gonadectomized ferrets. Arrows indicate the day of oestradiol implantation.

Animal Behaviour, 33, 1

152 ACTIVITY O

4B FERRET

DAYS

I

I

HAMSTER

I

Figure 2. Double-plotted activity distributions of one

gonadectomized male ferret and one gonadally intact male hamster. Numerals at top indicate hours. Blacklines indicate half-hour periods in which activity exceeded the mean for the day. The horizontal bar indicates periods of darkness (black), light (white) and twilight (slashed).

I

i LIGHTING REGIMEN

6oo! - 5004

FEMALES

,,;o 2,'|

~.~o

o3;0

o6;o

oggo

,~oo ,~[,o

TIME OF DAY

Figure 3. Mean baseline activity levelsof six mate and six

female gonadectomized ferrets during each half-hour of the day. Horizontal bar as in Fig, 2.

All the hamsters exhibited the strong nocturnal activity pattern typical of that species, but the ferrets did not exhibit a prominent 24-h rhythm. Example activity distributions are shown in Fig. 2. Time-series analyses revealed strong 24-h periodicity in the hamster activity records. The ferret activity records however, contained only weak 24-h rhythms, along with shorter harmonic (8- or 12-h) rhythms. Mean baseline activity levels of male and female ferrets (Fig. 3) tended to drop during, and just after, the transition from darkness to light. Activity levels were highest immediately before and after this period. A similar pattern occurred during oestradiol administration (data not shown). DISCUSSION Our data clearly do not support the notion that oestrogen exerts an activity-promoting effect in the ferret like that observed in the rat. Gerall et al. (1973), Gentry & Wade (1976) and Blizard (1983) observed significant increases in the running-wheel activity of rats within 2 weeks of the initiation of oestradiol treatment. In female rats, a more than threefold increase was observed within this time. By contrast, our data for male and female ferrets show no such increase following more than 3 weeks of oestradiol treatment. In the female rat, oestradiol induces a decline in food intake and in body weight, as well as an increase in energy expenditure through activity and heat loss (Gentry & Wade 1976; Wade 1976). The metabolic effects of oestrogen in the ferret may be somewhat different from its effects in the rat (Wade 1976). Consistent with this notion is our observation that the body weights of ferrets of either sex were not significantly altered following 21 days of oestradiol treatment (unpublished data). Although no sex difference in activity was observed either prior to or during oestradiol treatment, a number of oestrogen-mediated behaviours have been shown to be sexually dimorphic in the ferret. For example, higher levels of masculine coital behaviour were exhibited by male than by female ferrets in response to either oestradiol or testosterone administration in adulthood, following gonadectomy (Baum 1976). Recently, we have found that in response to oestradiol administration, gonadectomized female, but not male, ferrets approach stimulus males with shorter latencies then they do in the absence of this hormone.

Stockman et al.: Ferret activity Oestradiol also induces a preference for male, as opposed to female, stimulus animals in female ferrets, but not in males. Our finding that the circadian organization of ferret activity is rather weak is consistent with reports for three other carnivore species. Like the ferret, the bobcat, Felix rufus (Kavanau 1971) and the domestic cat and dog (Sterman et al. 1965; Hawking et al. 1971) are intermittently active during both day and night. Some carnivores, however, including the red fox (Vulpes vulpes), the tayra (Eira barbara) and the grison (Galictis vittatus), exhibit clearly nocturnal or diurnal activity patterns (Kavanau 1971). The least weasel (Mustela rixosa), a congeneric species, is primarily nocturnal in the laboratory, but only if the lighting regimen includes artificial twilights (Kavanau 1969). When the transitions between darkness and light are abrupt, this species is also arhythmic. The inhibitory effect of artificial 'dawn' upon the ferrets' activity may be due to a phenomenon known as 'masking' (Ashoff 1960), which is a modification in the expression of a rhythm by the lighting regimen independent of its effect on the circadian system. The daily transitions of light and dark have been previously observed to have potent inhibitory and facilitatory effects on the expression of a variety of behaviours in several species (Borbely & Huston 1974; Pohl 1976; Albers et al. 1982).

ACKNOWLEDGMENTS This research was supported by U.S. Public Health Service Grant H D 11722. This work was performed while E.R.S. was supported by N I M H N R S A 5 F32 MH08579-02. M.J.B. was the recipient of Research Scientist Development Award K O 2 - M H 00392 from the U.S. Public Health Service, and H.E.A. was supported by N I H G r a n t GM-31199.

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(Received 13 December 1983; revised 1 March 1984; MS. number: A4210)