Adult Male Marmosets Research Articles

Long-range projections of oxytocin neurons in the marmoset brain.

The neurohormone oxytocin (OT) has become a major target for the development of novel therapeutic strategies to treat psychiatric disorders such as autism spectrum disorder because of its integral role in governing many facets of mammalian social behavior. Whereas extensive work in rodents has produced much of our knowledge of OT, we lack basic information about its neurobiology in primates making it difficult to interpret the limited effects that OT manipulations have had in human patients. In fact, previous studies have revealed only limited OT fibers in primate brains. Here, we investigated the OT connectome in marmoset using immunohistochemistry, and mapped OT fibers throughout the brains of adult male and female marmoset monkeys. We found extensive OT projections reaching limbic and cortical areas that are involved in the regulation of social behaviors, such as the amygdala, the medial prefrontal cortex, and the basal ganglia. The pattern of OT fibers observed in marmosets is notably similar to the OT connectomes described in rodents. Our findings here contrast with previous results by demonstrating a broad distribution of OT throughout the marmoset brain. Given the prevalence of this neurohormone in the primate brain, methods developed in rodents to manipulate endogenous OT are likely to be applicable in marmosets.

Functional Cardiovascular Characterization of the Common Marmoset (Callithrix jacchus).

Appropriate cardiovascular animal models are urgently needed to investigate genetic, molecular, and therapeutic approaches, yet the translation of results from the currently used species is difficult due to their genetic distance as well as their anatomical or physiological differences. Animal species that are closer to the human situation might help to bridge this translational gap. The common marmoset (Callithrix jacchus) is an interesting candidate to investigate certain heart diseases and cardiovascular comorbidities, yet a basic functional characterization of its hemodynamic system is still missing. Therefore, cardiac functional analyses were performed by utilizing the invasive intracardiac pressure-volume loops (PV loop) system in seven animals, magnetic resonance imaging (MRI) in six animals, and echocardiography in five young adult male common marmosets. For a direct comparison between the three methods, only data from animals for which all three datasets could be acquired were selected. All three modalities were suitable for characterizing cardiac function, though with some systemic variations. In addition, vena cava occlusions were performed to investigate the load-independent parameters collected with the PV loop system, which allowed for a deeper analysis of the cardiac function and for a more sensitive detection of the alterations in a disease state, such as heart failure or certain cardiovascular comorbidities.

Atrophic astrocytes in aged marmosets present tau hyperphosphorylation, RNA oxidation, and DNA fragmentation

Astrocytes perform multiple essential functions in the brain showing morphological changes. Hypertrophic astrocytes are commonly observed in cognitively healthy aged animals, implying a functional defense mechanism without losing neuronal support. In neurodegenerative diseases, astrocytes show morphological alterations, such as decreased process length and reduced number of branch points, known as astroglial atrophy, with detrimental effects on neuronal cells. The common marmoset (Callithrix jacchus) is a non–human primate that, with age, develops several features that resemble neurodegeneration. In this study, we characterize the morphological alterations in astrocytes of adolescent (mean 1.75 y), adult (mean 5.33 y), old (mean 11.25 y), and aged (mean 16.83 y) male marmosets. We observed a significantly reduced arborization in astrocytes of aged marmosets compared to younger animals in the hippocampus and entorhinal cortex. These astrocytes also show oxidative damage to RNA and increased nuclear plaques in the cortex and tau hyperphosphorylation (AT100). Astrocytes lacking S100A10 protein show a more severe atrophy and DNA fragmentation. Our results demonstrate the presence of atrophic astrocytes in the brains of aged marmosets.

Long-term stability of neural signals from microwire arrays implanted in common marmoset motor cortex and striatum

Current neuroprosthetics rely on stable, high quality recordings from chronically implanted microelectrode arrays (MEAs) in neural tissue. While chronic electrophysiological recordings and electrode failure modes have been reported from rodent and larger non-human primate (NHP) models, chronic recordings from the marmoset model have not been previously described. The common marmoset is a New World primate that is easier to breed and handle compared to larger NHPs and has a similarly organized brain, making it a potentially useful smaller NHP model for neuroscience studies. This study reports recording stability and signal quality of MEAs chronically implanted in behaving marmosets. Six adult male marmosets, trained for reaching tasks, were implanted with either a 16-channel tungsten microwire array (five animals) or a Pt-Ir floating MEA (one animal) in the hand-arm region of the primary motor cortex (M1) and another MEA in the striatum targeting the nucleus accumbens (NAcc). Signal stability and quality was quantified as a function of array yield (active electrodes that recorded action potentials), neuronal yield (isolated single units during a recording session), and signal-to-noise ratio (SNR). Out of 11 implanted MEAs, nine provided functional recordings for at least three months, with two arrays functional for 10 months. In general, implants had high yield, which remained stable for up to several months. However, mechanical failure attributed to MEA connector was the most common failure mode. In the longest implants, signal degradation occurred, which was characterized by gradual decline in array yield, reduced number of isolated single units, and changes in waveform shape of action potentials. This work demonstrates the feasibility of long-term recordings from MEAs implanted in cortical and deep brain structures in the marmoset model. The ability to chronically record cortical signals for neural prosthetics applications in the common marmoset extends the potential of this model in neural interface research.

Behavioral phenotypes associated with MPTP induction of partial lesions in common marmosets (Callithrix jacchus)

Parkinson’s disease is a chronic neurodegenerative disorder with the core motor features of resting tremor, bradykinesia, rigidity, and postural instability. Non-motor symptoms also occur, and include cognitive dysfunction, mood disorders, anosmia (loss of smell), and REM sleep disturbances. As the development of medications and other therapies for treatment of non-motor symptoms is ongoing, it is essential to have animal models that aid in understanding the neural changes underlying non-motor PD symptoms and serve as a testing ground for potential therapeutics. We investigated several non-motor symptoms in 10 adult male marmosets using the MPTP model, with both the full (n=5) and partial (n=5) MPTP dosing regimens. Baseline data in numerous domains were collected prior to dosing; assessments in these same domains occurred post-dosing for 12 weeks. Marmosets given the partial MPTP dose (designed to mimic the early stages of the disease) differed significantly from marmosets given the full MPTP dose in several ways, including behavior, olfactory discrimination, cognitive performance, and social responses. Importantly, while spontaneous recovery of PD motor symptoms has been previously reported in studies of MPTP monkeys and cats, we did not observe recovery of any non-motor symptoms. This suggests that the neurochemical mechanisms behind the non-motor symptoms of PD, which appear years before the onset of symptoms, are independent of the striatal dopaminergic transmission. We demonstrate the value of assessing a broad range of behavioral change to detect non-motor impairment, anosmia, and differences in socially appropriate responses, in the marmoset MPTP model of early PD.

Hormonal stimulation and paternal experience influence responsiveness to infant distress vocalizations by adult male common marmosets, Callithrix jacchus

Parental experience and hormones play a large role in the common marmoset (Callithrix jacchus) father's care of their offspring. We tested the effect of exogenous estradiol or testosterone on the responsiveness of common marmosets to respond to infant distress vocalizations and whether males who haven't become fathers yet (paired males) would have increased responsiveness to infant distress calls with either steroid or whether parental experience is the most important component for the onset of paternal care. Sixteen male marmosets (8 fathers, 8 paired males) received a vehicle, low dose or high dose of estradiol and additional 16 males were tested with testosterone at three doses for their response either to a vocal control or a recording of an infant distress call for 10min. Without steroid stimulation fathers were significantly more likely to respond to the infant distress stimulus than paired males. Low dose estradiol stimulation resulted in a significant increase in fathers' behavioral response towards the infant distress stimulus but not in paired males. Fathers also showed a significant increase in infant responsiveness from the vehicle dose to the estradiol low dose treatment, but not to the estradiol high dose treatment. Testosterone treatment did not show significant differences between infant responsiveness at either dose and between fathers and paired males. We suggest that neither steroid is involved in the onset of paternal care behaviors in the marmoset but that estradiol may be involved in facilitating paternal motivation in experienced fathers.

Take the monkey and run

BackgroundThe common marmoset (Callithrix jacchus) is a small, New World primate that is used extensively in biomedical and behavioral research. This short-lived primate, with its small body size, ease of handling, and docile temperament, has emerged as a valuable model for aging and neurodegenerative research.A growing body of research has indicated exercise, aerobic exercise especially, imparts beneficial effects to normal aging. Understanding the mechanisms underlying these positive effects of exercise, and the degree to which exercise has neurotherapeutic effects, is an important research focus. Thus, developing techniques to engage marmosets in aerobic exercise would have great advantages. New methodHere we describe the marmoset exercise ball (MEB) paradigm: a safe (for both experimenter and subjects), novel and effective means to engage marmosets in aerobic exercise. We trained young adult male marmosets to run on treadmills for 30min a day, 3 days a week. ResultsOur training procedures allowed us to engage male marmosets in this aerobic exercise within 4 weeks, and subjects maintained this frequency of exercise for 3 months. Comparison with existing methodsTo our knowledge, this is the first described method to engage marmosets in aerobic exercise. A major advantage of this exercise paradigm is that while it was technically forced exercise, it did not appear to induce stress in the marmosets. ConclusionsThese techniques should be useful to researchers wishing to address physiological responses of exercise in a marmoset model.

Alloparental Responsiveness to Newborns by Nonreproductive, Adult Male, Common Marmosets (Callithrix jacchus)

Parental care in mammals is influenced by sensory stimuli from infants, and by changes in the hormone levels of caretakers. To determine the responsiveness to infant cues in nonreproductive adult male common marmosets (Callithrix jacchus) with and without previous experience in caretaking, we exposed 12 males to newborn marmosets and assessed their cortisol plasma levels and behavioral response. Newborn marmosets housed in transparent enclosures were placed inside the cages of the adult male subjects. Males were exposed four times to two different experimental conditions: (a) newborn enclosures remained closed during the observation period and (b) newborn enclosures were opened during the observation period to allow direct social interaction by the adult males. Blood samples from adult males were collected after each behavioral observation trial to measure the levels of cortisol. The behavioral responses of adult males exposed to the closed and open newborn enclosures showed a significant difference only with respect to the frequency of displacements, where males moved among the quadrants of their own cages with greater frequency when the newborn enclosure was sealed. Experienced males approached newborn enclosures more frequently, spent more time in close proximity, and carried and recovered newborns more quickly than inexperienced males. The successive exposure to newborns increased the responsiveness in inexperienced males. The highest levels of plasma cortisol in adult males were recorded following periods of exposure to the sealed newborn enclosures. This suggests that successive exposure to newborns and previous alloparental caregiving experience while living in family groups influences the responsiveness of male marmosets to the sensory cues of newborns.

Oxytocin changes primate paternal tolerance to offspring in food transfer

Oxytocin facilitates social recognition in rats and mice, onset of maternal behavior in virgin mice and formation of pair bonds without copulation in prairie voles. However, the relationship between this peptide and paternal behavior in primates remains largely unknown. We investigated whether oxytocin affects paternal behavior in common marmosets. In these primates, fathers as well as mothers take care of their infants, and transferring food to the infants is one of their more obvious caretaking behaviors. We tested whether oxytocin and an oxytocin receptor antagonist affect the transfer of food to offspirng by fathers. After intracerebroventricular infusion of the vehicle, oxytocin, or the oxytocin receptor antagonist, the fathers' behavior, including picking up food, transferring food to the offspring, and refusing to transfer food to the offspring, was analyzed. Compared with the vehicle, oxytocin reduced the frequency of refusal. This was not caused by reduction of appetite. Although the oxytocin receptor antagonist did not change the frequency of refusal behavior of the fathers statistically significant manner, these observations suggest that the tolerance of the adult male marmoset toward its offspring as shown by the transfer of food is increased by oxytocin administered into the central nervous system.

Metabolic Fate of Phenothiazine in the Marmoset (Callithrix jacchus)

The fate of [35S]-phenothiazine, a veterinary anthelmintic, has been investigated in the adult male marmoset (Callithrix jacchus) following oral administration. A near complete recovery of radioactivity (c. 95%) was achieved in 0-3 days, with just over one-third of the dose (c. 37%) being present in the urine and the remainder (c. 58%) being accounted for in the faeces. The majority of the urinary radioactivity (c. 91%) was present as conjugates, tentatively identified as phenothiazine N-glucuronide and leucophenothiazone sulphate. Smaller amounts of phenothiazone, thionol, phenothiazine sulphoxide and unchanged phenothiazine were also identified. The only compound identified in the faeces was unchanged phenothiazine.

The influence of social cues on circadian activity rhythm resynchronisation to the light–dark cycle in common marmosets Callithrix jacchus

The influence of a previously synchronised conspecific of the same sex on the circadian activity rhythm resynchronisation rate to the LD cycle was assessed in five adult male marmosets during a 6 h LD cycle delay, followed by a 6 h advance, in the absence and presence of a donor. After phase delay with donor, activity onset and offset were resynchronised in fewer days. During the transients, the occurrence of a mid-activity delay, an activity increase in the L-phase and a decrease in the D-phase indicates a faster resynchronisation. After phase advance, activity onset resynchronised promptly and the number of transients for the offset and mid-activity during the transients did not differ. Daily activity increased at phase shifts. Social cues may influence photic synchronisation in marmosets, probably due to direct effects of socially-induced arousal on the circadian pacemaker or by indirect effects, thus increasing photic input or masking.

Non-invasive measurement of small peptides in the common marmoset ( Callithrix jacchus): A radiolabeled clearance study and endogenous excretion under varying social conditions

A non-invasive assay for measurement of oxytocin (OT) and vasopressin (AVP) in primates would enable researchers to study the relationship between the endocrine system and behavior without disturbing potentially endangered animals in their natural habitats. In order to test whether or not OT specifically would be measurable in the urine of a primate, 10 μCi of tritium-labeled OT were injected into the peripheral blood supply of four adult male common marmosets ( Callithrix jacchus), with continuous urinary collection over 48 h. When urine was processed by HPLC separation and beta counting for radioactive clearance, the label was present in all samples in the fraction where OT elutes. Large amounts of OT were also seen in a fraction other than that containing the OT standard, indicating that OT is measurable but that it also undergoes substantial metabolic breakdown. In a second experiment, we isolated six common marmosets for 48 h and then exposed them to social contact to evaluate the effect of changing social stimuli on endogenous urinary measurement of both OT and AVP. Both were measured after HPLC separation to isolate the intact molecule and also to control for cross-reactivity with metabolites in subsequent RIA. Cortisol was also measured to objectively evaluate the stress response. A priori assumptions were that urinary OT and AVP would be lower during a period of isolation and higher during periods of social contact. These assumptions were met, leading us to conclude that peripheral OT and AVP are measurable via urinary assay and that such an assay is a valid means of evaluating social condition in this species.

Bifunctional Gonadotropin-Releasing Hormone Antagonist-Progesterone Analogs with Increased Efficacy and Duration of Action

GnRH peptide analogs are widely used to treat diverse clinical conditions. However, they have poor oral activity and exhibit rapid metabolic clearance, thus requiring injection and depot formulation. Because steroid hormones are bound to plasma proteins, we explored the possibility of conjugating hydroxylated progesterones to GnRH analogs to reduce metabolic clearance of the peptides. Conjugation of [D-Lys6]GnRH agonist to the alpha11-hydroxyl of alpha11-hydroxyl progesterone via a hemi-succinate bridge increased the plasma half-life after iv injection in rabbits by 3.6-fold while retaining high binding affinity, thus providing proof of concept. Five GnRH antagonists were then synthesized with 21-hydroxyprogesterone conjugated via C21-hydroxyl to positions six (conjugates A and B) and position seven (conjugates C and D) of GnRH antagonists. In the fifth compound the NH2 terminus of a GnRH antagonist lacking the first two amino acids was conjugated via the C21-hydroxyl to 21-hydroxyprogesterone (conjugate E). All five analogs bound to guinea pig progesterone binding globulin with relatively high affinities (264-1020 nM). Moreover, all five conjugates retained high progestogenic activity in stimulating a progesterone-response-element-driven chloramphenicol acetyltransferase reporter gene in the T47D breast cancer cell line. Conjugation via the epsilon-amino function of D-Lys6 (conjugates A and B) produced compounds with high binding affinity for the human GnRH receptor (15 and 7 nM) comparable to that of the unconjugated GnRH antagonists (4 and 26 nM). Conjugation via the epsilon-amino function of Lys7 (conjugates C and D) or the NH2 terminus of an N-terminally truncated antagonist (conjugate E) produced compounds of low binding affinity. Conjugates A and B also exhibited high functional antagonism of GnRH stimulation of inositol phosphate production in COS-7 cells expressing the human GnRH receptor (2.6 and 16 nM) compared with the unconjugated antagonists (1.3 and 122 nM). In accordance with their poor receptor binding affinity, conjugates C, D, and E had poor functional antagonism. Preliminary dose-finding studies in female marmosets showed transitory progesterone inhibition by 0.25 mg and prolonged suppression of 12 and 17 d by 0.5- and 1.0-mg doses. Injection of conjugate A in adult male marmosets (0.5 mg sc) rapidly suppressed plasma testosterone levels, which remained suppressed for at least 3 d. In contrast, the unconjugated parent antagonist alone or with progesterone suppressed testosterone for only 8 h to 1 d. The findings demonstrate that conjugation of progesterone to GnRH antagonists conveys plasma binding and progestogenic properties and increases their efficacy and duration of action in vivo. These new GnRH antagonists show promise as therapeutic agents for hormone-dependent diseases and as contraceptives.

Male marmoset monkeys express an adrenal fetal zone at birth, but not a zona reticularis in adulthood.

Neonatal human males produce high levels of dehydroepiandrosterone (DHEA) and its sulfo-conjugated form (DS) that decline within a few months of birth, due to regression of the adrenal fetal zone (FZ). Adult male humans and rhesus monkeys produce C19 steroids in abundance from the adrenal zona reticularis (ZR). Male marmoset monkeys produce DS at birth, but unlike humans and rhesus monkeys, do not produce comparable amounts of DHEA and DS in adulthood. To determine whether male marmosets express a functional ZR in adulthood, we examined adult and neonatal male marmosets for the presence of a ZR and FZ, respectively. Exogenous ACTH failed to stimulate DHEA or DS in adults, and dexamethasone treatment failed to suppress DHEA and DS, although cortisol levels changed as expected. In steroidogenic tissues, the key proteins necessary to synthesize C19 steroids from pregnenolone are P450c17, 3beta-hydroxysteroid dehydrogenase (3beta-HSD), nicotinamide adenine dinucleotide phosphate (reduced) oxido-reductase cytochrome P450 (reductase), and cytochromeb5 (cytb5). Adult adrenal cross sections showed P450c17 and reductase protein expression throughout the cortex but showed no expected decrease in 3beta-HSD and increase in cytb5 in the innermost region. Western analysis confirmed these data, demonstrating comparable P450c17 expression to rhesus monkeys, but not cytb5. HPLC analysis revealed similar 17alpha-hydroxylase action on pregnenolone for adult marmoset and rhesus adrenal microsomes but greatly diminished 17,20-lyase activity in marmosets. Neonatal marmoset adrenals exhibited staining indicative of a putative FZ (with P450c17, reduced 3beta-HSD and increased cytb5). We conclude that neonatal marmosets exhibit a C19 steroid-secreting FZ similar to humans, but adult males fail to acquire a functional ZR.

263.Effects of animal pairing on marmoset sperm collected by penile vibratory stimulation

The marmoset monkey (Callithrix jacchus) is an important model species for the development of reproductive technologies in humans and endangered primates. Obtaining sufficient sperm of high quality is a limiting factor in implementing marmoset IVF. A method for the collection of marmoset semen, using penile vibratory stimulation (PVS), has recently been described (1). Due to the high rate of copulation in marmosets, the pairing of males with females may affect semen collection. The aim of the present study was to determine whether the quantity and motility of sperm collected by PVS is affected by animal pairing. A total of 10 adult male marmosets were used, of which 3 were paired with another adult or juvenile male, and 7 were paired with an adult female. Semen was collected from each male on up to 5 separate occassions in sterile glass tubes. PVS involved the application of successive sequences of increasing vibration to the penis using a FertiCare personal vibrator. Immediately following collection, pre-warmed Hepes-buffered TALP medium (200 μl) was added to the ejaculate. Sperm suspensions were evaluated for total sperm count and sperm motility. Ejaculates were obtained from male-paired males on every attempt (12 of 12), whereas 5 of 23 attempts failed to yield an ejaculate from female-paired males. The number of sequences of stimulation needed to obtain an ejaculate differed between males but was unaffected by animal pairing. The ejaculates collected from female-paired males had lower total numbers of sperm (3.9���1.4�×�106 v. 10.1���2.2�×�106; P�<�0.05) with a lower percentage motile (35���9% v. 85 � 13%; P<0.01), compared with those from male-paired males. We conclude that housing males separately from females increases the quantity and motility of sperm collected by PVS. (1) Schneiders, A., Sonksen, J., Hodges, J. K. (2004) J. Med. Primatol. 33, 98–104.

Variation in circulating and excreted estradiol associated with testicular activity in male marmosets.

Concentrations of estradiol (E2) are high in the urine of male marmosets, and links between E2 and paternal behavior have been proposed in black tufted-ear marmosets, Callithrix kuhlii. However, it is not clear whether urinary E2 in male marmosets: 1) represents production of E2 associated with testicular activity, 2) is associated with adrenal steroid production, or 3) merely reflects peripheral conversion of T to E2 prior to excretion. We tested the hypothesis that urinary E2 in male marmosets represents estrogen production-associated activity in the hypothalamus-pituitary-gonad (HPG) axis. We treated adult male marmosets with gonadotropin-releasing hormone (GnRH), and used saline-treated males as controls. We collected blood and urine samples from males before and after treatment, and assayed them for testosterone (T), estradiol (E2), and cortisol (CORT). Treatment with GnRH increased circulating T and E2, and prevented decreases in levels of urinary T and E2. Moreover, changes in plasma and urinary E2 after treatment were positively correlated with post-treatment changes in T. Thus, our data are consistent with both plasma and urinary E2 in male marmosets increasing as a result of testicular stimulation. However, treatment with GnRH did not affect plasma or urinary CORT concentrations of males, suggesting that the E2 excreted by males is not of adrenal origin. We also compared urinary T, E2, and CORT levels between intact and castrated male common marmosets (Callithrix jacchus). Urinary concentrations of T and E2, but not CORT, were significantly lower in castrated than in intact males, further suggesting that E2 in male marmosets varies with testicular activity.

Effects of dietary oltipraz and ethoxyquin on aflatoxin B1 biotransformation in non-human primates.

Following aflatoxin B1 (AFB) exposure, rats readily develop liver tumors. However, treatment of rats with a variety of compounds, including the synthetic dithiolthione oltipraz and the antioxidant ethoxyquin, protects these rodents from AFB-induced hepatocarcinogenesis. Several epidemiological studies strongly suggest that AFB is also a causative agent of liver cancer in humans. However, relatively little is known about the efficacy of cancer chemoprevention in human and non-human primates. To this end, we examined the effects of chemopreventive agents on AFB metabolism in non-human primates. Hepatic aflatoxin B1 metabolism profiles of macaque (Macaca nemestrina) and marmoset (Callithrix jacchus) monkeys were determined and compared to humans. Quantitatively, the oxidative metabolism of this mycotoxin was similar in the three primate species. In contrast to macaques, both humans and marmosets lacked AFB-glutathione conjugating activity. It was concluded that marmosets resembled human AFB metabolism more closely than the macaques, and therefore, marmoset monkeys were chosen for this study. Eleven adult male marmosets were randomly assigned to three groups. Animals received the synthetic dithiolthione oltipraz, the antioxidant ethoxyquin, or vehicle only. In addition, two single doses of AFB were also administered orally before and after animals were treated with aforementioned compounds. Both oltipraz and ethoxyquin induced aflatoxin B1-glutathione conjugating activity in the livers of some but not all marmosets. In addition, 10 microM oltipraz inhibited cytochrome P450-mediated activation of AFB to the ultimate carcinogenic metabolite, aflatoxin B1-8,9-epoxide, in vitro, up to 51%. Furthermore, animals treated in vivo with oltipraz, but not ethoxyquin, exhibited a significant reduction (53% average) in AFB-DNA adduct formation relative to the control animals (p < 0.05). Together, our data suggest that chemoprevention is also effective in primates; however, most likely to a lesser degree than in rodents.

Visuotopic organisation of striate cortex in the marmoset monkey (Callithrix jacchus).

The visuotopic organisation of the primary visual cortex (V1) was studied by extracellular recordings in adult male marmosets (Callithrix jacchus) that were anaesthetised with sufentanil/nitrous oxide and paralysed with pancuronium bromide. Extensive sampling of the occipital region in four individuals and partial coverage of V1 in five others allowed not only the establishment of the normal visuotopy but also the study of interindividual variability. As in other primates, there was a single, continuous map of the contralateral hemifield in V1, with the upper visual quadrant represented ventrally and the lower quadrant represented dorsally. The surface area of V1, which was measured in two-dimensional reconstructions of the cortical surface, varied from 192 to 217 mm2. There was a marked emphasis on the representation of the foveal and parafoveal visual fields: the representation of the central 5 degrees of the visual field occupied 36-39% of the surface area of V1, whereas the central 10 degrees occupied 57-59%. No asymmetry between the representations of the upper and lower quadrants was apparent. The visual topography of V1 was highly consistent between individuals, relative to both sulcal landmarks and stereotaxic coordinates. The entire contralateral hemifield was represented in V1; in addition, neurones with receptive fields whose borders invaded the ipsilateral hemifield were observed within V1, less than 800 microns from the V1/V2 boundary. The total invasion of the ipsilateral hemifield was less than 0.5 degree at the centre of the fovea but reached 8 degrees at the periphery of the vertical meridian. Our results demonstrate that the organisation of V1 is similar in diurnal New and Old World simians, despite major variations in size, ecological niche, and timing of postnatal development across species.

The effect of stress on semen reduction in the marmoset monkey (Callithrix jacchus).

This study compared a number of semen parameters of two separate groups of the common marmoset monkey (Callithrix jacchus) in order to determine the effect of a continuous potentially stressful situation on these parameters, and thus on the monkey's reproductive ability. The semen from 16 adult male marmoset monkeys was collected and analysed to compare semen parameters between a 'normal' (control) group (n = 9) and a 'blood withdrawn' ('stress') group (n = 7). The semen parameter values observed in the control were: pH 7.51 +/- 0.22, volume 40.2 +/- 27.2 microliters, concentration 27.3 +/- 14.8 x 10(4)/microliters, motility 47.4 +/- 15.9%, grade of velocity 3.5 +/- 1.2, and normal morphological forms 51.8 +/- 13.7%. The 'blood withdrawn' group of marmoset monkeys showed significantly lower semen volume and sperm concentration than the 'normal' group. In addition, total count of spermatozoa, normal spermatozoa, motile spermatozoa and normal motile spermatozoa per ejaculate per monkey was significantly reduced in the 'blood withdrawn' group. The semen of these monkeys also revealed a significantly higher percentage of abnormally-shaped sperm heads than the normal group, and cases of impotence and sham ejaculation were recorded. Our study revealed that the continuous withdrawal of a small volume of blood from a group of marmoset monkeys appeared to be stressful to these monkeys and as a result, influenced their semen parameters, possibly making them less fertile. In addition, electroejaculation was found to be possibly harmful to the monkey's reproductive ability.

Sexual and aggressive behaviour of adult male marmosets ( Callithrix jacchus) castrated neonatally, prepubertally, or in adulthood

During early postnatal development, males of some primate species exhibit higher levels of plasma testosterone than females. The possible significance of this for behavioural development was examined in male marmosets castrated neonatally (NC), prepubertally (PC), or in adulthood (AC) and reared in their natal groups. Age-matched intact (AI) adult males served as controls. All males were removed from their natal groups as adults for pair testing with unfamiliar conspecific males and females. NC males did not mount, but were frequently aggressive during tests with females. NC males also elicited aggression from females but rarely from intact males. PC and AC males attempted copulation during the majority of tests with females; less aggression occurred but more so in tests with PC males than between females and AC males. Aggression between PC or AC males and intact males was much more frequent than when NC males met intacts in paired encounters. Results show that secretion of testicular hormones during infancy in male marmosets has important effects upon the development of sexual and aggressive behaviour. Castration in infancy has measurably different effects upon behavioural development than castration performed later in life (prepubertally or in adulthood).