Male Ferrets Research Articles

Hearing in the ferret (Mustela putorius): effects of primary auditory cortical lesions on thresholds for pure tone detection.

1. Pure tone thresholds were determined for five adult male ferrets before and after bilateral ablation of primary auditory cortex. Complete audiograms ranging from 0.016 to 48 kHz were obtained for two animals. The remaining three animals were tested at five frequencies selected to assess hearing throughout the audible range (0.125, 0.5, 2.0, 8.0, and 32.0 kHz). 2. Shortly after surgery one animal had elevated thresholds across the entire frequency range with the most pronounced hearing loss above 12.0 kHz. Four other animals had no elevation of thresholds at low and midrange frequencies but suffered a hearing loss at very high frequencies (32 kHz). 3. Repeated testing over a period of several months revealed substantial recovery of sensitivity. There was complete recovery of sensitivity in the low- and middle-frequency range of the audiogram. Some hearing loss persisted at the extreme upper end of the audiogram (32 kHz), but in two cases there was evidence of recovery at this frequency as well. 4. Following determination of absolute thresholds all animals were assessed for their ability to localize sound in space. Minimum audible angles were obtained on midline as well as within both left and right hemifields, i.e., around 0, -60 and +60 degrees azimuth. All animals had severe and persistent deficits in their ability to localize brief sounds within the lateral fields, but were still capable of midline localization.

Differential effects of testosterone, 5 alpha-dihydrotestosterone and oestradiol-17 beta on plasma concentrations of LH in castrated ferrets.

The biological activity of testosterone often depends on the conversion of testosterone within the target cell to an androgenic or oestrogenic metabolite. The purpose of this study was to compare the relative ability of testosterone and two of its metabolites, dihydrotestosterone (DHT) and oestradiol, to suppress LH secretion in castrated male ferrets. Castrated ferrets were treated with five different doses of steroid by implanting various numbers of s.c. silicone elastomer capsules packed with either testosterone, DHT or oestradiol. The lowest dose of oestradiol (0.1 mm capsule length/100 g body weight, mean estimated total release rate of 25 ng/day) significantly suppressed plasma concentrations of LH in castrated ferrets. Higher amounts of DHT (2.5 mm capsule length/100 g body weight, mean estimated total release rate of 88 ng/day) were required for a significant reduction in plasma concentrations of LH. Concentrations of LH were also significantly lowered by testosterone when administered at a 2.5 mm capsule length/100 g body weight; however, estimated total release rate was 312 ng/day from these capsules. The fact that oestradiol was more effective than DHT, and that DHT was more effective than testosterone in inhibiting LH secretion in castrated ferrets, suggests that in gonadally intact ferrets, testosterone may be converted to DHT or oestradiol within target cells that mediate steroid negative feedback on LH secretion.

An olfactory recognition system in the ferret Mustela furo L. (Carnivora: Mustelidae)

This study demonstrates that ferrets can use variations in odours from anal sac secretions as a communication system. Odour preference tests showed that ferrets can discriminate between male and female ferret anal sac odours, between strange and familiar, familiar and their own, and fresh and 1-day-old odours. They did not discriminate between fresh and 2-h-old odours, nor did male ferrets discriminate between the odours of oestrous and anoestrous females. Ferrets were more attracted to the odours of the opposite sex than to those of their own sex. When faced by an opponent, male ferrets were more aggressive in the presence of their own rather than their opponent's odour, and less aggressive with their opponent's odour than with that of a known, dominant animal's odour. These results are consistent with both a sex attraction role and a territorial defence role for anal sac odours. A scent-matching mechanism for territorial defence is supported, although a neighbour-neighbour recognition/avoidance mechanism cannot be rejected. Gas chromatography revealed sexually and individually distinct profiles of volatile compounds in anal sac extracts, but no consistent seasonal trends. Females had high concentrations of 2,3-dimethylthietane and/or 3,4-dimethyl-1,2-dithiolane. Males usually had high concentrations of indole. 2-Propylthietane was an important constituent in most individuals. These differences in concentration were significant and could provide an olfactory recognition system of sex and individual identity.

Immunocytochemical localization of hypothalamic luteinizing hormone-releasing hormone in male ferrets

LHRH-containing neurons within the hypothalamus were immunocytochemically identified in adult male ferrets that were either gonadally intact, castrated, or castrated and treated with testosterone. The distribution of LHRH-immunopositive neuron cell bodies was similar in the three treatment groups. The majority of these cells was located mediobasally in the retrochiasmatic area, including some within the ventrolateral aspects of the arcuate nucleus. These soma were associated with a dense basal LHRH fiber plexus which extended to the median eminence. A smaller number of cell bodies was found slightly more dorsal and lateral to the major concentration at the base and midline. Isolated LHRH perikarya were occasionally observed in dorsal areas of the hypothalamus. There were no differences in the mean total number of hypothalamic LHRH cell bodies identified in the three treatment groups. These results indicate that the documented negative feedback effects of testosterone on LH secretion in male ferrets are not the result of an alteration in the absolute number of neurons capable of synthesizing LHRH.

Effect of birth on plasma testosterone, brain aromatase activity, and hypothalamic estradiol in male and female ferrets.

The present studies examined the patterns of circulating testosterone (T) within 0-24 h after birth in male and female ferrets along with concomitant changes in neural aromatase activity and hypothalamic concentrations of estradiol (E2). Plasma and brain samples were obtained 0 and 2 h (cesarean delivery) or 0, 2, 12, and 24 h (natural delivery) after birth. Plasma T levels were significantly higher in male neonates 2 h after birth than at 0 h in both cesarean-delivered (9.48 +/- 1.25 vs. 3.37 +/- 0.60 ng/ml) and naturally delivered (19.28 +/- 2.94 vs. 5.13 +/- 1.93 ng/ml) ferrets, while female neonates showed no significant changes in T over these sampling times. T levels had returned to 0 h levels by 12 h in naturally delivered males. T was significantly lower in females than in males 0, 2, and 24 h after natural delivery, whereas T levels were equivalent in males and females immediately after cesarean delivery. Male kits kept on a heating pad for 2 h after natural delivery had lower plasma T levels than males that were left with their mothers over this same period. Brain aromatase activity in anterior hypothalamus-preoptic area, medial basal hypothalamus (MBH), temporal lobe, and cerebral cortex was equivalent in males and females at all postpartum ages, regardless of whether delivery occurred by cesarean section or naturally. However, in naturally delivered kits of both sexes significant elevations in aromatase activity occurred in MBH and temporal lobe 24 h postpartum. Finally, E2 concentrations in anterior hypothalamus-preoptic area and MBH were equivalent 0 and 2 h postpartum in males and females, regardless of whether they were delivered naturally or by cesarean section. The observed postnatal elevation in T may contribute to brain and behavioral sexual differentiation of male ferrets. It is unclear, however, whether such an effect of T depends on its neural aromatization to E2.

Evidence implicating aromatization of testosterone in the regulation of male ferret sexual behavior

We compared the effects of the aromatase inhibitor, 1,4,6-androstatriene-3,17-dione (ATD) and castration on the expression of mating behavior in adult male ferrets which were in breeding condition. Males implanted SC with Silastic capsules containing ATD displayed significantly less neck gripping, mounting and intromittive behavior than intact males which received empty capsules, although the ATD-induced reductions in behavior were not as large as those seen after castration. ATD had no effect on mating behavior in castrated males. As reported in another publication [13], brain aromatase activity was significantly reduced in both the intact and castrated males treated with ATD in the present study. Plasma estradiol (E 2) levels were uniformly low in intact and castrated males, regardless of whether they received ATD or no steroid. As expected, plasma testosterone (T) levels were significantly lower in castrated than in intact males, and ATD treatment did not affect these values. These results suggest that E 2 formed via the neural aromatization of T contributes to the activation of masculine sexual behavior in intact male ferrets in breeding condition.

Interaction of contact velocity and cord compression in determining the severity of spinal cord injury.

Rate, depth, and duration of compression are the principal determinants of experimental spinal cord injury (SCI) severity. Since existing models do not allow independent control of these variables, the interaction of these factors has not been fully elucidated. The purpose of this study was to define the interactive relation of velocity (V) and compression (C) in SCI using a constrained stroke pneumatic impactor that allowed independent control of these variables. Computer simulation of previous weight-drop experiments using a lumped-mass model were compared with this series of impacts. After rigid stabilization of adjacent cervical vertebrae, the spinal cord (SC) in 48 male ferrets was compressed through the C6-C7 interspace. Maximum compression was 25%, 35%, 50%, or 65% of the SC diameter. Impacts were delivered at five velocities from 1.5 m/s to 6 m/s. Severity of injury was determined by pre and postinjury measurement of somatosensory evoked potentials (SEP) and blinded quantitation of histopathologic findings. Twenty-three of 48 animals (48%) had measurable SEP at 4 hs. No animal recovered SEP at 65% compression at any velocity, or at a velocity of 6 m/s for any level of compression. Recovery of SEP and histologic severity score correlated better with velocity-compression product, or maximum viscous response (VC) than with either V or C alone. The mean histologic severity score was higher in animals exhibiting no recovery of SEP. Modeling the viscoelastic elements of the SC using existing force-time and force-deflection data suggests that estimates of compression from current weight-drop techniques may be inaccurate. At low contact velocity, functional and anatomic damage is best predicted by maximum SC compression. However, as velocity increases, SCI severity becomes a function of the viscous response (VC), demonstrating the rate sensitivity of spinal cord tissue. Tolerance to SC compression decreases as the rate of deformation increases. This helps to explain apparent discrepancies between compression and severity of experimental SCI.

Role for prenatal estrogen in the development of masculine sexual behavior in the male ferret

Previous studies have shown that neonatal exposure to testosterone is essential for coital masculinization in male ferrets. In the present experiments, masculine sexual behavior was diminished in male ferrets by prenatal exposure to drugs which inhibited estrogenic stimulation of the brain. Similarly timed prenatal treatments with testosterone failed to masculinize the behavior of female offspring. We hypothesize that prenatal exposure of the male ferret to estrogen, derived from the neural aromatization of circulating androgen, may sensitize the developing brain to the subsequent masculinizing action of testosterone shortly after birth.

Sex difference in the effect of mating on the pulsatile secretion of luteinizing hormone in a reflex ovulator, the ferret.

Sex differences in the pulsatile secretion of LH were examined in male and female ferrets after mating. Female ferrets which were either gonadally intact and in estrus or gonadectomized and maintained on a pulsed regimen of daily estradiol (E2) injections exhibited a prolonged rise in plasma LH, characterized by an elevation in mean LH levels and an increase in the number of LH pulses after receiving an intromission from a stud male. By contrast, no such increase in LH secretion occurred in males which achieved an intromission with a female, regardless of whether they were gonadally intact and in breeding condition or gonadectomized and given pulsed estrogen. In fact, intact breeding males which achieved an intromission had significantly fewer LH pulses 1-5 h later than unmated males bled serially over the same time period. This decrease in LH pulse frequency was followed by a significant rise in mean plasma levels of androgen 5-12 h later. When a sexually dimorphic LH response to intromission was observed in gonadectomized E2-treated ferrets, we asked whether this could reflect a sex difference in pituitary responsiveness to the endogenous release of GnRH. Thus, plasma LH levels were measured in gonadectomized and gonadectomized E2-treated ferrets for 2 h after iv injection of GnRH. In the absence of gonadal steroids, ferrets exhibited a sex-specific difference in LH responsiveness to GnRH; however, no sex difference was apparent under the influence of E2. These findings demonstrate that ferrets' sexually dimorphic LH responses to intromission probably reflect a sex difference in the processing of somatosensory inputs from the genitalia or in the neural control of GnRH release into the pituitary portal vessels.

Cystic mucinous hyperplasia in the gallbladder of a ferret

An 8-year-old male ferret had icterus, hepatomegaly and an enlarged gallbladder. Microscopically, the gallbladder had numerous mucinous cysts and papillary hyperplasia of the mucosa, consistent with a diagnosis of cystic mucinous hyperplasia. No previous published reports of this condition in ferrets were found.

Evidence that a decrease in testosterone negative feedback mediates the pubertal increase in luteinizing hormone pulse frequency in male ferrets.

Neuroendocrine mechanisms regulating luteinizing hormone (LH) secretion during puberty were investigated in intact male ferrets and ferrets castrated at 8 wk of age that received s.c. implants of either empty or testosterone-filled Silastic capsules. To synchronize puberty onset among individuals, ferrets were exposed to short days between 8 and 16 wk of age, and then transferred to long days. Testis growth began in intact ferrets soon after photoperiod transition. Blood samples were obtained at 11, 15, 19, and 23 wk of age. LH pulse frequency was low in intact ferrets at 11 and 15 wk of age (less than or equal to 0.27 pulses/h), but rose to 0.94 pulses/h by 23 wk of age. No age-related increase in LH pulse frequency was observed in untreated castrated ferrets. LH pulses were rare in testosterone-treated castrated ferrets at 11 and 15 wk of age; but by 23 wk of age, frequency rose to 0.33 pulses/h. Thus, testis maturation in ferrets is accompanied by a dramatic increase in LH pulse frequency. No steroid-independent developmental increase in LH pulse frequency occurs in castrated ferrets. Furthermore, doses of testosterone that prevent LH secretion in young castrated ferrets do not as effectively suppress LH pulses in older ferrets. These data suggest that a decrease in the efficacy of testosterone negative feedback mediates the pubertal rise in LH pulse frequency in male ferrets.

Differential effects of reoxygenation on intracellular calcium and isometric tension.

We used the bioluminescent Ca2+ indicator, aequorin to record intracellular calcium transients during reoxygenation of hypoxic ferret ventricular muscle in order to determine whether alterations in the amplitude and time course of isometric contraction are mediated by changes in [Ca2+]i. Papillary muscles less than or equal to 1 mm in diameter were removed from the hearts of male ferrets and perfused with a bicarbonate-buffered physiologic salt solution at 30 degrees C. Muscles were stimulated to contract isometrically at 0.33 Hz and were loaded with aequorin by a chemical procedure. Hypoxia was induced by changing the gas mixture bubbling the perfusate to 95% N2, 5% CO2; reoxygenation was accomplished by switching the gas mixture to 95% O2, 5% CO2. Hypoxia produced a decrease in peak Ca2+ and tension that was reversed by reoxygenation. However, the effects on tension of changes in oxygenation were greater than expected from the degree of change in [Ca2+]i. The time courses of the Ca2+ transient and isometric twitch moved in opposite directions and were respectively prolonged/abbreviated by hypoxia and abbreviated/prolonged by reoxygenation. These results indicate that changes in the amplitude and time course of the isometric twitch induced by hypoxia and reoxygenation cannot be attributed alone to changes in intracellular Ca2+ availability and are caused in part by a significant decrease in the calcium sensitivity of the contractile apparatus.

Food intake and struvite crystalluria in ferrets.

Four adult, castrated, male ferrets were studied in two similar trials for effects of food intake on variables hypothesized to promote struvite (ammonium, magnesium, phosphate hexahydrate) crystal formation in urine. Struvite crystalluria occurred in three of the four ferrets. Urine pH (UpH) averaged 6.6 for these ferrets. UpH in the ferret without crystalluria was 6.0. By simple linear regression analysis, no relationship was found between the amount of food ingested and the urinary concentration and excretion of magnesium and phosphorous. However, urine osmolality and excretion of both protein and ammonium were correlated to food intake (P less than .05). Ways in which these effects could promote struvite crystal formation are discussed.

Pulsatile release of luteinizing hormone and testosterone in male ferrets.

The temporal organization of LH and testosterone secretion was examined in male European ferrets. Hormone levels were measured in frequent blood samples taken via an indwelling jugular cannula from sexually mature and castrated ferrets. Intact ferrets discharge LH and testosterone in discrete pulses, but the frequency and amplitude of these pulses vary within and between individual males. The average frequency of LH pulses was 1.14 +/- 0.25 pulses/h, with an amplitude of 1.59 +/- 0.23 ng/ml in 11 ferrets. Testosterone pulse frequency and amplitude were 0.62 +/- 0.04 pulses/h and 16.96 +/- 2.5 ng/ml, respectively. The frequency, amplitude, and duration of hormone pulses were similar during the light and dark phases of the light-dark cycle. LH and testosterone peaks were temporally coupled with LH pulses preceding testosterone pulses by 10-20 min. However, not all LH pulses evoked a rise in testosterone. Frequently, trains of 2 or more LH pulses gave rise to a single testosterone pulse. Castration provoked a rapid increase in the frequency of LH pulses, and the interpulse interval became strikingly uniform within hours after orchidectomy. The amplitude of LH pulses, in contrast, increased gradually over the first 6 postcastration days and then plateaued at about 4.5 ng/ml. These findings demonstrate that LH pulses constitute functionally important signals to the testis, as evidenced by temporally related increments in testosterone secretion. Moreover, distinct differences in the development of the postcastration rise in the frequency and amplitude of LH pulses suggest that testosterone operates via multiple mechanisms to regulate LH release in the adult male. Finally, this study emphasizes the utility of the ferret as an animal model to study neural determinants of LH release in the male.

Oestrogen modulates dopamine-controlled behaviours in the male ferret

Oestradiol (100 μg/kg) was administered to male ferrets on three successive days. Two and three days after the last oestradiol injection the effects of apomorphine (0.5 mg/kg) on stereotyped, predatory and scent marking behaviour were intensified. Thus, three days of oestradiol administration produced a supersensitive dopaminergic system two days after the last oestradiol injection. However, no initial antidopaminergic effect of oestradiol could be demonstrated one day after the last oestradiol injection.

Effect of Arginine-Free Diet on Ammonia Metabolism in Young and Adult Ferrets

Two-month-old, male ferrets were fasted for 16 h and fed a synthetic, arginine-free diet. Within 2–3 h after ingesting the diet, they developed hyperammonemia and encephalopathy. Ammonia levels in the serum, brain and cerebrospinal fluid were greatly elevated compared to those of ferrets fed the synthetic diet supplemented with arginine. Orotic acid and glucose levels in serum were also significantly elevated. Urinary orotic acid was significantly increased but citrate and creatinine level were unaltered. Adult (18-mo-old) ferrets did not develop hyperammonemia and encephalopathy after eating the arginine-free diet. Serum and urinary orotic acid levels were significantly elevated in the adult ferrets fed arginine-free diet. Hyperammonemia and encephalopathy were prevented in young ferrets by supplying dietary arginine and abbreviated by ornithine injections given during encephalopathy. These results suggest that young ferrets are unable to meet their ornithine needs from precursors other than arginine, whereas adult ferrets appear to be able to synthesize ornithine from sources other than dietary arginine. Intraperitoneal injection of sodium benzoate to young ferrets fed arginine-free diet failed to decrease serum ammonia levels.

Sexual dimorphism in the preoptic/anterior hypothalamic area of ferrets: Effects of adult exposure to sex steroids

The organization of neuronal cell bodies in the caudal preoptic area (POA) and rostral anterior hypothalamic area (AH) was studied in Nissl-stained brain sections from adult male and female ferrets. Computer-assisted image-analysis procedures were developed to help estimate the areas of cellular density and the sizes of individual perikarya. At the junction of the POA and AH, a bilateral dorsal-medial group of neurons was apparent only in male ferrets (dorsal nucleus). At the same coronal level, a ventral-medial group of neurons was apparent bilaterally in both males and females (ventral nucleus). The mean somal area of cells in the dorsal nucleus of males was significantly greater than the mean somal area of cells in the corresponding dorsal region of females or in the ventral nucleus of both sexes. The dorsal nucleus was clearly discernible in adult males regardless of their hormonal status, although cells in the dorsal nucleus were larger in intact breeding males or gonadectomized males given testosterone, estradiol or dihydrotestosterone than in gonadectomized males given no gonadal hormones or given progesterone. Neither the grouping of large cells nor the steroid-induced increase in cell size, characteristic of the male dorsal nucleus, was seen in the comparable dorsal region of females. The sex difference in cellular organization observed in the ferret at the junction of the POA and AH is the first difference of this type to be seen in the POA/AH of a non-rodent mammalian species. Its identification will, hopefully, aid in the analysis of the neural mechanisms that control various sex-specific behaviors in this species.

Hearing in the ferret (Mustela putorius): thresholds for pure tone detection.

Thresholds for pure tone detection were determined for two male ferrets (Mustela putorius) using standard behavioral testing procedures. Both animals had a broad range of hearing comparable to that seen in other Carnivores. The lower limit of hearing at 60 dB SPL was estimated from the average audiogram to be 36 Hz, and the upper limit of hearing was 44 kHz. The lowest thresholds were found in the region of 8-12 kHz with both animals exhibiting sharp tuning at best frequency.

Neonatal exposure of female ferrets to testosterone alters sociosexual preferences in adulthood

Sociosexual preferences in adult female ferrets, as measured in a choice arena, were altered by neonatal exposure to exogenous testosterone. Adult female ferrets showed a preference for males which did not depend on the presence of gonadal steroids, because gonadectomized and gonadectomized estrogen-treated females showed identical preferences for males. Adult castrated males showed no preference for females unless these males were treated with testosterone. A similar no-preference pattern was found in adult females that had received testosterone neonatally. Females exposed neonatally to dihydrotestosterone or estradiol exhibited the normal females' male-oriented preference. These results indicate that testosterone secreted by the testes in the developing male may interrupt the phenotypic female development pattern and hence prevent the emergence of a homosexual preference in adulthood in the male ferret.

Effect of prenatal exposure to aromatase inhibitor, testosterone, or antiandrogen on the development of feminine sexual behavior in ferrets of both sexes

Experiments were conducted to assess the role of prenatal exposure of the brain to estrogen in controlling the development of proceptive and receptive feminine sexual behavior in ferrets of both sexes. Female ferrets, deprived prenatally of estrogen via maternal ovariectomy on gestational day 30 (42-day gestation) plus SC implantation of an aromatase inhibitor, 1,4,6-androstatriene-3,17-dione (ATD), into their mothers, were significantly less receptive than control females when tested in adulthood after ovariectomy and administration of low (5 and 10 μg/kg) dosages of estradiol benzoate (EB). However, they were no less receptive than control females in response to a high (15 μg/kg) dosage of EB. Estrogen-deprived females showed EB-induced reductions in approach latencies to a stud male (proceptive responses) which were equivalent to those of control females. These results suggest that estrogenic stimulation of the female brain during fetal development enhances later receptive responsiveness to estradiol. However, prenatal estrogen appears not to be an absolute requirement for the development of receptive or proceptive coital capacity in females. In a previous study castrated male ferrets failed to show EB-induced reductions in the latency to approach a sexually active male, indicating that they were proceptively defeminized. In the present study females exposed prenatally to exogenous testosterone also failed to display significant EB-induced reductions in approach latencies to a sexually active male. Males deprived prenatally of estrogen ran faster to a stud male after adult administration of increasing dosages of EB. By contrast, control males as well as males exposed prenatally to androgen receptor antagonist, flutamide, displayed no EB-induced reduction in approach latencies. This suggests that in male ferrets prenatal neural aromatization of androgen to estrogen normally contributes to the process of proceptive defeminization.