From the Cover: PNAS Plus: Amygdala lesions disrupt modulation of functional MRI activity evoked by facial expression in the monkey inferior temporal cortex

Significance

Successful social interaction depends on the ability to recognize others, evaluate their mental states (e.g. intentions, desires, and beliefs), and “read” their emotional states. Here, we show that, in monkeys, damage to the amygdala, a brain structure that is central to the expression of emotion, significantly disrupts the processing of emotional facial expression in high-level visual cortical areas involved in face recognition. These findings suggest that the projections of the amygdala to visual cortical areas likely enhance the sensory processing of biologically important signals, including those related to potential environmental threats and social contexts.     

Influence of social rank on the development of long-term ethanol drinking trajectories in cynomolgus monkeys

Background

Chronic stress contribute to the development of alcohol use disorder (AUD). However, characterizing the role of chronic social stressors in the development of problematic drinking trajectories in humans is complicated by practical and ethical constraints. Group-housed nonhuman primates develop social dominance hierarchies that represent a continuum of social experiences from enrichment in higher-ranked (dominant) monkeys to chronic social stress in lower-ranked (subordinate) individuals. This framework provides a translationally relevant model of chronic social stress that can be used to characterize its effects on vulnerability to AUD.

Methods

Twelve male cynomolgus monkeys living in three social groups with established social dominance hierarchies were provided access to ethanol and water for 22 h/day, 4–5 days/week, for 1 year. Ethanol-free periods (2- or 3-day “weekends” or longer periods up to 10 days) were spent in social groups to maintain the stability of the social hierarchies. Observational studies conducted 6 months into the year of drinking assessed signs of ethanol withdrawal. After 1 year, monkeys were individually housed 24 h/day, 7 days/week for four consecutive weeks to examine the effect of eliminating the “weekends” spent socially housed.

Results

Subordinate monkeys had significantly higher mean daily ethanol intakes than dominant monkeys across 1 year of open access. Subordinates also had higher intakes on the first day back drinking following ethanol-free periods of 9–10 days. Moreover, during the last 4 weeks of open access, intakes on the first drinking day after an ethanol-free weekend increased significantly in subordinate monkeys. This effect diminished when all monkeys were individually housed for 4 weeks, indicating that the increased intake in subordinates was driven by the social environment.

Conclusions

These data demonstrate that social subordination, which is associated with chronic social stress, results in increased vulnerability to the development and maintenance of heavy drinking trajectories.     

Cardiac-Specific Overexpression of Oxytocin Receptor Leads to Cardiomyopathy in Mice

Background

Oxytocin (Oxt) and its receptor (Oxtr) gene system has been implicated in cardiomyogenesis and cardioprotection; however, effects of chronic activation of Oxtr are not known. We generated and investigated transgenic (TG) mice that overexpress Oxtr specifically in the heart.

Oxytocin in cardiac ontogeny

Previous studies demonstrated the presence of oxytocin (OT) and oxytocin receptors (OTRs) in the heart. The present work provides results supporting a potential role of OT in cardiomyogenesis. Here, we show a maximal OT and OTR protein level in the developing rat heart at day 21 of gestation and postnatal days 1-4, when cardiac myocytes are at a stage of intense hyperplasia. Between postnatal days 1 and 66, OT decreased linearly in all heart chambers (4.1- to 6.6-fold). Correspondingly, immunocytochemistry demonstrated that OTRs, which were eminent in postnatal cardiomyocytes, declined with age to low levels in adults. Interestingly, in coronary vasculature, OTRs developed in endothelial cells at postnatal days 12 and 22 and achieved a plateau in adult rats. These findings suggest that OT can be involved in developmental formation of the coronary vessels. In vivo, the OT/OTR system in the fetal heart was sensitive to the actions of retinoic acid (RA), recognized as a major cardiac morphogen. RA treatment produced a significant increase (2- to 3-fold) both in the OT concentration and in the OT mRNA levels. Ex vivo, an OT antagonist inhibited RA-mediated cardiomyocyte differentiation of P19 embryonic stem cells. The decline of cardiac OT expression from infancy to adulthood of the rat and changes in cell types expressing OTR indicate a dynamic regulation of the OT system in the heart rather than constitutive expression. The results support the hypothesis that RA induces cardiomyogenesis by activation of the cardiac OT system.     

Receptive field focus of visual area V4 neurons determines responses to illusory surfaces

Illusory figures demonstrate the visual system’s ability to infer surfaces under conditions of fragmented sensory input. To investigate the role of midlevel visual area V4 in visual surface completion, we used multielectrode arrays to measure spiking responses to two types of visual stimuli: Kanizsa patterns that induce the perception of an illusory surface and physically similar control stimuli that do not. Neurons in V4 exhibited stronger and sometimes rhythmic spiking responses for the illusion-promoting configurations compared with controls. Moreover, this elevated response depended on the precise alignment of the neuron’s peak visual field sensitivity (receptive field focus) with the illusory surface itself. Neurons whose receptive field focus was over adjacent inducing elements, less than 1.5° away, did not show response enhancement to the illusion. Neither receptive field sizes nor fixational eye movements could account for this effect, which was present in both single-unit signals and multiunit activity. These results suggest that the active perceptual completion of surfaces and shapes, which is a fundamental problem in natural visual experience, draws upon the selective enhancement of activity within a distinct subpopulation of neurons in cortical area V4.Visual illusions are valuable stimuli for studying the neural basis of visual processing because they reveal the brain’s internal mechanisms for interpreting sensory input. Illusory figures, for example, exploit the visual system’s capacity to construct contours, shapes, and surfaces despite the lack of a continuous physical border (1, 2). Illusory figures are perceived by a range of phylogenetically diverse species, including monkeys, cats, owls, and bees, pointing to perceptual completion as a fundamental aspect of natural vision (3).Neural correlates of illusory figures have been found in a wide range of brain areas. Recordings in monkeys revealed that illusory figures evoke spiking responses from neurons in visual areas as early as V1 and V2 and as late as the inferotemporal cortex (4–9). Neuroimaging studies in humans similarly found responses to illusory figures throughout visual cortex (10–13).Several theoretical models postulate mechanisms of illusory figure perception (14–19). A common feature of these models is spatial integration of the inducing elements combined with an active interpolation to complete the surface. These processes are frequently assigned to neurons in midlevel areas, whose receptive fields are large enough to cover separate elements yet sensitive enough to distinguish between local features such as orientation, curvature, and colinearity (20, 21). A range of evidence suggests that visual area V4 in particular may play an active role in surface completion. First, the receptive fields of V4 neurons are large by comparison with V1 and V2 receptive fields and are therefore able to integrate information across spatially separated stimulus components (22). Second, psychophysical studies demonstrate that the perception of certain similar illusory figures varies over visual space in a manner consistent with the retinotopy of V4 (23, 24). Third, both human (10–13) and nonhuman primate (25) functional imaging studies reveal responses to illusory contours and surfaces in area V4. Fourth, ablation of area V4 in the macaque selectively impairs performance on discrimination tasks that involve illusory contours (26).Here we investigate the neural representation of illusory surfaces in macaque area V4 using Kanizsa patterns known to give rise to the perception of illusory surfaces. Illusion-promoting patterns elicited electrophysiological responses that were often rhythmic and were significantly enhanced in their firing rate compared with physically similar control patterns that did not promote the illusion. This enhancement depended critically on the spatial alignment of the illusory surface with the point of peak V4 receptive field sensitivity, or “RF focus.” Only neurons with receptive fields focused on the illusory surface showed elevated responses to the illusory surface, whereas those with receptive fields focused on the inducing elements did not. This effect was observed for neurons whose receptive fields, as defined by conventional mapping techniques, were several degrees in size and overlapped with both the illusory surface and the inducer elements. These observations suggest that V4 neurons play an active role in the representation of illusory surfaces and are sensitive to stimulus details much finer than would be predicted based on receptive field size alone.     

Neisseria infection of rhesus macaques as a model to study colonization,transmission, persistence,and horizontal gene transfer

The strict tropism of many pathogens for man hampers the development of animal models that recapitulate important microbe–host interactions. We developed a rhesus macaque model for studying Neisseria–host interactions using Neisseria species indigenous to the animal. We report that Neisseria are common inhabitants of the rhesus macaque. Neisseria isolated from the rhesus macaque recolonize animals after laboratory passage, persist in the animals for at least 72 d, and are transmitted between animals. Neisseria are naturally competent and acquire genetic markers from each other in vivo, in the absence of selection, within 44 d after colonization. Neisseria macacae encodes orthologs of known or presumed virulence factors of human-adapted Neisseria, as well as current or candidate vaccine antigens. We conclude that the rhesus macaque model will allow studies of the molecular mechanisms of Neisseria colonization, transmission, persistence, and horizontal gene transfer. The model can potentially be developed further for preclinical testing of vaccine candidates.     

Neural responses to sanction threats in two-party economic exchange

Sanctions are used ubiquitously to enforce obedience to social norms. However, recent field studies and laboratory experiments have demonstrated that cooperation is sometimes reduced when incentives meant to promote prosocial decisions are added to the environment. Although various explanations for this effect have been suggested, the neural foundations of the effect have not been fully explored. Using a modified trust game, we found that trustees reciprocate relatively less when facing sanction threats, and that the presence of sanctions significantly reduces trustee''s brain activities involved in social reward valuation [in the ventromedial prefrontal cortex (VMPFC), lateral orbitofrontal cortex, and amygdala] while it simultaneously increases brain activities in the parietal cortex, which has been implicated in rational decision making. Moreover, we found that neural activity in a trustee''s VMPFC area predicts her future level of cooperation under both sanction and no-sanction conditions, and that this predictive activity can be dynamically modulated by the presence of a sanction threat.     

Cellular-resolution gene expression profiling in the neonatal marmoset brain reveals dynamic species- and region-specific differences

Precise spatiotemporal control of gene expression in the developing brain is critical for neural circuit formation, and comprehensive expression mapping in the developing primate brain is crucial to understand brain function in health and disease. Here, we developed an unbiased, automated, large-scale, cellular-resolution in situ hybridization (ISH)–based gene expression profiling system (GePS) and companion analysis to reveal gene expression patterns in the neonatal New World marmoset cortex, thalamus, and striatum that are distinct from those in mice. Gene-ontology analysis of marmoset-specific genes revealed associations with catalytic activity in the visual cortex and neuropsychiatric disorders in the thalamus. Cortically expressed genes with clear area boundaries were used in a three-dimensional cortical surface mapping algorithm to delineate higher-order cortical areas not evident in two-dimensional ISH data. GePS provides a powerful platform to elucidate the molecular mechanisms underlying primate neurobiology and developmental psychiatric and neurological disorders.

The mammalian brain contains many functionally distinct regions that are extensively interconnected, enabling rapid and accurate information processing. Each region contains diverse cell types that differ in their molecular, morphological, electrophysiological, and functional characteristics (1, 2). Advanced neuroscience technologies, which are generally optimized and extensively applied in mice, provide complicated approaches to analyze these characteristics and model the genetic basis of this regional- and cell-type diversity. However, clear cognitive differences arise from interspecies differences in these key characteristics, and most contemporary technologies are difficult to apply in species whose study involves extensive ethical, practical, and experimental impediments. The ability to compare gene expression patterns between primates and model species such as mice is crucial for understanding the human brain (3, 4).Recent developments in technologies, such as microarray, single-cell RNA-sequencing (RNA-seq), and Drop-seq, have allowed for high-resolution genome-wide expression analysis in specific regions of the brain or in thousands of individual cells simultaneously (5–10). However, these methods provide limited anatomical information and no morphological observations. Slide-seq solves these problems by transferring RNA from tissue sections onto a surface covered in DNA-barcoded beads, allowing the preservation of positional information (11). However, the unbiased nature of next-generation sequencing technologies may hinder the molecular identification of specific cell types or their direct analysis. In situ hybridization (ISH) databases of humans and nonhuman primates (NHPs) have revealed brain gene expression profiles at cellular resolution, while preserving key morphological and anatomical characteristics, ultimately providing important information about the genetic conservation of analogous brain regions (12). However, these studies are highly resource intensive, requiring substantial time and cost. Consequently, such efforts have been limited, focusing on a few brain regions in limited species (13). Thus, interspecific comparison of brain-cell heterogeneity has not been possible.Here, we describe the development and implementation of an unbiased gene expression profiling system (GePS) for use with the Marmoset Gene Atlas (MGA) (https://gene-atlas.brainminds.riken.jp/) (14), which is adaptable for use with other atlases. The system, developed as a part of “Brain Mapping by Integrated Neurotechnologies for Disease Studies (Brain/MINDS),” a brain mapping program in Japan (15, 16), enables systematic and automatic analysis of gene expression across brain regions. Brain/MINDS aims to develop knowledge-based tools to explore primate-specific brain structure, function, and connectivity in health and disease. Specifically, we analyzed the expression patterns of identified risk genes for psychiatric disorders in the neonatal marmoset brain. There are >2,000 gene expression profiles for the neonatal common-marmoset brain in the MGA. Although each ISH section is complemented by an adjacent neuroanatomically stained section (visualizing the Nissl substance), enabling users to compare gene expression against a brain atlas, mapping gene expression patterns over the entire brain is both time and labor intensive. A large mouse ISH database, the Allen Brain Atlas (ABA; http://developingmouse.brain-map.org/), demonstrated that an anatomic gene expression atlas (AGEA) can characterize local gene expression in the mouse brain without prior knowledge of classical anatomy (17). To enhance the MGA in this manner, we developed GePS to systematically and automatically identify gene expression patterns in specific regions of the neonatal marmoset brain.GePS, including the gene expression comparison system and cortical surface projection mapping function, is publicly available as an open resource on our website (https://gene-atlas.brainminds.riken.jp/). It provides primate-specific whole brain–gene expression patterns and will be a valuable platform in the field of primate neuroscience, helping reveal primate-specific brain functions and connections and providing insights into brain evolution and pathology.     

Optical imaging in galagos reveals parietal-frontal circuits underlying motor behavior

The posterior parietal cortex (PPC) of monkeys and prosimian galagos contains a number of subregions where complex, behaviorally meaningful movements, such as reaching, grasping, and body defense, can be evoked by electrical stimulation with long trains of electrical pulses through microelectrodes. Shorter trains of pulses evoke no or simple movements. One possibility for the difference in effectiveness of intracortical microstimulation is that long trains activate much larger regions of the brain. Here, we show that long-train stimulation of PPC does not activate widespread regions of frontal motor and premotor cortex but instead, produces focal, somatotopically appropriate activations of frontal motor and premotor cortex. Shorter stimulation trains activate the same frontal foci but less strongly, showing that longer stimulus trains do not produce less specification. Because the activated sites in frontal cortex correspond to the locations of direct parietal-frontal anatomical connections from the stimulated PPC subregions, the results show the usefulness of optical imaging in conjunction with electrical stimulation in showing functional pathways between nodes in behavior-specific cortical networks. Thus, long-train stimulation is effective in evoking ethologically relevant sequences of movements by activating nodes in a cortical network for a behaviorally relevant period rather than spreading activation in a nonspecific manner.     

Cellular responses to Loa loa experimental infection in mandrills (Mandrillus sphinx) vaccinated with irradiated infective larvae

In order to shed light on the mechanisms of antifilarial protective immunity, we investigated the course of experimental loaiosis after vaccination in a nonhuman primate host, Mandrillus sphinx. Six vaccinated (V) mandrills received 50 irradiated L3 while six nonvaccinated (NV) received saline solution on days -60, -30 and -15. All animals were challenged with 100 intact L3 (day 0). Parasitological and immunological status were followed for 9 months. Vaccination delayed the appearance and mean peak of microfilaraemia. Five mandrills (Mf-) were never microfilaraemic (one V mandrill) or microfilaraemic on only one occasion (2 V and 2 NV), the other seven having stable microfilaraemia (Mf+). The cytokine response of peripheral blood mononuclear cells to L3 (L3 Ag) was Th2 dominated, while microfilariae (Mf Ag) elicited a Th0-like response. During vaccination, Th2 cytokine production significantly increased in V mandrills against L3 Ag, as well as Mf Ag, whereas Th1 cytokines decreased. On day 60 postinoculation, cellular proliferation was higher in V mandrills in response to L3 and Mf Ags and PHA-L mitogen. At the end of prepatency (on day 130), mandrills with delayed appearance of microfilaraemia exhibited a high, transient IL-2 and IL-4 secretion in response to L3 Ag. Finally, high anti-Mf Th2 cytokine levels characterized Mf-mandrills not only during prepatency, but also (for IL-5) before immunization. However, the presence of a balanced Th1 anti-L3 response during prepatency in the amicrofilaraemic mandrill suggests its importance in protective immunity. Taken together, our data suggest that Th2 cells and also Th1 components of the antifilarial response, especially to larval antigen, may contribute to parasite elimination.     

Failed remyelination of the nonhuman primate optic nerve leads to axon degeneration,retinal damages,and visual dysfunction

White matter disorders of the central nervous system (CNS), such as multiple sclerosis (MS), lead to failure of nerve conduction and long-lasting neurological disabilities affecting a variety of sensory and motor systems, including vision. While most disease-modifying therapies target the immune and inflammatory response, the promotion of remyelination has become a new therapeutic avenue to prevent neuronal degeneration and promote recovery. Most of these strategies have been developed in short-lived rodent models of demyelination, which spontaneously repair and do not reflect the size, organization, and biology of the human CNS. Thus, well-defined nonhuman primate models are required to efficiently advance therapeutic approaches for patients. Here, we followed the consequence of long-term toxin-induced demyelination of the macaque optic nerve on remyelination and axon preservation, as well as its impact on visual functions. Findings from oculomotor behavior, ophthalmic examination, electrophysiology, and retinal imaging indicate visual impairment involving the optic nerve and retina. These visual dysfunctions fully correlated at the anatomical level, with sustained optic nerve demyelination, axonal degeneration, and alterations of the inner retinal layers. This nonhuman primate model of chronic optic nerve demyelination associated with axonal degeneration and visual dysfunction, recapitulates several key features of MS lesions and should be instrumental in providing the missing link to translate emerging repair promyelinating/neuroprotective therapies to the clinic for myelin disorders, such as MS.

White matter disorders are a large group of neurological diseases of various origins. Those affecting the central nervous system (CNS), such as multiple sclerosis (MS), lead to failure of nerve conduction, axon degeneration, and result in long-lasting neurological disabilities and tissue atrophy (1). The loss of myelin and healthy axons are believed to be responsible for irreversible damages, which affect a variety of sensory and motor systems, including vision. In MS, 70% of patients are affected with optic neuritis. It can manifest in an acute episode with decreased vision that can recover over several weeks in the majority of patients, while permanent visual symptoms persist in 40 to 60% of patients (2, 3). Chronic optic neuritis can lead to significant optic nerve atrophy and retinal alterations, affecting mainly the retinal inner layers, including the retinal nerve fiber and ganglion cell layers (4). Several visual assays, including visual fields (VF) (5), pupillary responses to luminance and color (pupillary light reflex, PLR) (6), electroretinograms (ERG) (7), optical coherence tomography (OCT) (4, 8), and visual evoked potential (VEP) (9–11) are routinely performed to assess noninvasively the anatomical and electrophysiological perturbations of visual functions in MS. While functional recovery was reported in some patients (9), the lack of anatomical–electrophysiological correlation has prevented to attribute directly these improvements to remyelination or other regenerative processes.Animal models of demyelination induced by toxins, such as lyso-phosphatidylcholine (LPC), are suitable for studying the mechanisms of demyelination/remyelination and developing approaches aimed at promoting CNS remyelination, as they show little inflammation and, therefore, provide means to assay directly the effect of a therapy on remyelination. However, most of these models are developed in short-lived rodents and spontaneously repair, thus lacking the long-lasting progressive degenerative disease context of MS. Besides, these models do not reflect the size or complex organization of the human primate CNS (12). They do not inform on the biology of primate cells, which differs from rodents (13, 14), nor on the security, toxicity, and long-term efficacy of cell- or compound-based promyelinating/neuroprotective therapies. Thus, experiments in long-lived nonhuman primates appear an essential step toward clinical trials.While promoting remyelination may prevent axon degeneration, only a few promyelination strategies have been translated to the clinic (15,16). One of the roadblocks is the absence of studies addressing the clinical benefit of promyelination approaches that could be applied to the clinic (17). A positive correlation between changes in VEP parameters and the degree of demyelination/remyelination was established in rodents (18–21), cats (22), and dogs (23), and exploited successfully to follow promyelination therapies in rodents (24, 25). OCT has been used to identify loss of optic nerve and retinal damages in animal models of myelin disorders as well (23, 26). While used seldomly in nonhuman primates (27), none of these clinical assays were exploited to monitor the impact of optic nerve demyelination in nonhuman primates.We previously demonstrated that LPC injection in the macaque optic nerve induced demyelination with fair axon preservation but little remyelination up to 2 mo post demyelination (28). Taking advantage of the fact that nonhuman primates are long-lived and are able to perform several tasks awake, as do humans, we questioned whether this model could be used to follow the consequence of long-term demyelination on axon preservation, and whether multimodal noninvasive assays, such as VF, VEP, OCT, and PLR could be instrumental to follow/predict the functional and anatomical outcome of optic nerve demyelination. Using multidisciplinary approaches, we provide compelling evidence that LPC-induced demyelination of the macaque optic nerve leads to modified VF, VEP, PLR, and altered inner retinal layers, but preserved photoreceptors based on OCT and ERG. These clinical and functional anomalies were correlated at the histological level with failed remyelination and progressive optic nerve axon loss, followed by neuronal and fiber loss of the inner retinal layers. The postmortem validation of OCT, VEP, and PLR as pertinent markers of optic nerve demyelination/degeneration could further help the translation of therapeutic strategies toward the clinic for myelin diseases associated with long-term demyelination of the optic nerve.     

Oxytocin therapy in hypopituitarism: Challenges and opportunities

Patients with hypopituitarism display impaired quality of life and excess morbidity and mortality, despite apparently optimal pituitary hormone replacement. Oxytocin is a neuropeptide synthesized in the anterior hypothalamus which plays an important role in controlling social and emotional behaviour, body weight and metabolism. Recent studies have suggested that a deficiency of oxytocin may be evident in patients with hypopituitarism and craniopharyngioma, and that this may be associated with deficits in cognitive empathy. Preliminary data hint at potential benefits of oxytocin therapy in improving these deficits and the accompanying metabolic disturbances that are common in these conditions. However, several challenges remain, including an incomplete understanding of the regulation and mechanisms of action of oxytocin, difficulties in accurately measuring oxytocin levels and in establishing a diagnosis of oxytocin deficiency, and a need to determine both the optimal mode of administration for oxytocin therapy and an acceptable safety profile with long‐term use. This review considers the data linking oxytocin to the neuropsychological and metabolic disturbances evident in patients with craniopharyngioma and hypopituitarism, and describes the challenges that need to be overcome before replacement therapy can be considered as a therapeutic option in clinical practice.     

Hypothalamic and Neurohypophysial Vasopressin and Oxytocin in Melatonin-Treated Pinealectomized Male Rats

The effect of melatonin on hypothalamic and neurohypophysial vasopressin and oxytocin was investigated in normal and pinealectomized rats. Pinealectomy was followed by a decrease of both vasopressin and oxytocin content in the hypothalamus and neurohypophysis. In unpinealectomized rats, melatonin decreased vasopressin and oxytocin storage in the hypothalamo-neurohypophysial system. Following pineal removal, melatonin did not augment the pinealectomy-induced decrease of vasopressin and oxytocin in the neurohypophysis; the hypothalamic storage of both neurohormones was even higher when compared with vehicle-treated animals.     

Investigation of Oxytocin Secretion in Anorexia Nervosa and Bulimia Nervosa: Relationships to Temperament Personality Dimensions

Published studies suggested an implication of oxytocin in some temperament characteristics of personality. Therefore, we measured oxytocin secretion in 23 women with anorexia nervosa (AN), 27 with bulimia nervosa (BN) and 19 healthy controls and explored the relationships between circulating oxytocin and patients' personality traits. Plasma oxytocin levels were significantly reduced in AN women but not in BN ones. In healthy women, the attachment subscale scores of the reward dependence temperament and the harm avoidance (HA) scores explained 82% of the variability in circulating oxytocin. In BN patients, plasma oxytocin resulted to be negatively correlated with HA, whereas no significant correlations emerged in AN patients. These findings confirm a dysregulation of oxytocin production in AN but not in BN and show, for the first time, a disruption of the associations between hormone levels and patients' temperament traits, which may have a role in certain deranged behaviours of eating disorder patients. Copyright © 2015 John Wiley & Sons, Ltd and Eating Disorders Association.     

Oxytocin responses to stress in lactating and hyperprolactinaemic rats

The plasma oxytocin (OT) response to acute stress was compared between virgin, lactating, and hyperprolactinaemic female rats. In virgin rats, brief immobilization was associated with a significant elevation of plasma OT to 24.7 +/- 3.7 pmol/l compared with 7.7 +/- 1.1 pmol/l in controls. In contrast, the stress response was absent in lactating (6 days post-partum) animals: control OT 9.4 +/- 2.2, immobilized OT 9.0 +/- 1.1 pmol/l. Hyperprolactinaemia produced by treatment with either dopamine antagonists (domperidone or haloperidol) or ovine prolactin was also associated with an impairment of the OT stress response in intact females, whereas domperidone treatment failed to modify the response in ovariectomized (OVX) rats. Following ovarian steroid replacement with oestradiol and progesterone, the inhibitory effect of domperidone was observed in OVX rats: control OT 11.1 +/- 2.5, immobilized OT 16.0 +/- 3.7 pmol/l. Treatment of OVX rats with oestradiol and progesterone, either separately or combined, did not modify the OT stress response. Plasma levels of vasopressin were not significantly modified in either control or immobilized rats of any experimental groups. The results indicate that hyperprolactinaemia may be a causative factor in the impairment of OT stress responses observed in lactating rats.     

Memory loss in a nonnavigational spatial task after hippocampal inactivation in monkeys

The hippocampus has a well-documented role for spatial navigation across species, but its role for spatial memory in nonnavigational tasks is uncertain. In particular, when monkeys are tested in tasks that do not require navigation, spatial memory seems unaffected by lesions of the hippocampus. However, the interpretation of these results is compromised by long-term compensatory adaptation occurring in the days and weeks after lesions. To test the hypothesis that hippocampus is necessary for nonnavigational spatial memory, we selected a technique that avoids long-term compensatory adaptation. We transiently disrupted hippocampal function acutely at the time of testing by microinfusion of the glutamate receptor antagonist kynurenate. Animals were tested on a self-ordered spatial memory task, the Hamilton Search Task. In the task, animals are presented with an array of eight boxes, each containing a food reinforcer; one box may be opened per trial, with trials separated by a delay. Only the spatial location of the boxes serves as a cue to solve the task. The optimal strategy is to open each box once without returning to previously visited locations. Transient inactivation of hippocampus reduced performance to chance levels in a delay-dependent manner. In contrast, no deficits were seen when boxes were marked with nonspatial cues (color). These results clearly document a role for hippocampus in nonnavigational spatial memory in macaques and demonstrate the efficacy of pharmacological inactivation of this structure in this species. Our data bring the role of the hippocampus in monkeys into alignment with the broader framework of hippocampal function.Although a role for the hippocampus in navigational spatial memory has been well documented in many species, including humans (1–4), rodents (5, 6), and monkeys (7–9), a role for hippocampus in nonnavigational spatial memory is less certain. Because nonhuman primates have a close neuroanatomical and behavioral homology to humans, they offer a unique opportunity to assess the neural substrates of spatial memory using nonnavigational tasks. However, studies using nonnavigational spatial tasks with nonhuman primates have found little or no effect of hippocampal lesions (10–15).The ability of hippocampal-lesioned monkeys to perform these tasks without impairment has been explained by the reliance on a strategy that is not dependent on hippocampus. Whereas the use of allocentric (world-centered) cues depends on the hippocampus, it is thought that the use of egocentric (body-centered) cues does not. Indeed, it has been suggested that the use of egocentric cues is supported by extrahippocampal substrates including striatum and parietal cortex (15–18).Following a hippocampal lesion, subjects may learn to resolve a given task by developing a strategy distinct from that used in the presence of an intact hippocampus. Several factors may support this compensatory strategy, including (i) postlesion training or retraining and (ii) postlesion network reorganization (19–21). Thus, although lesion studies are useful in documenting altered function after injury, they cannot directly address the role of the hippocampus in the uninjured brain.A technique frequently used in rodents to avoid the adaptations described above is the rapid and reversible inactivation of a brain region by focal intracerebral drug infusions (e.g., with drugs that block glutamatergic neurotransmission or enhance GABAergic neurotransmission). An important advantage of this approach is the ability to use each animal as its own control in a repeated-measures design. In monkeys, we (22–26) and others (27–29) have used this technique to probe functions of several brain regions, but it has yet to be applied to the hippocampus.In the present experiments, we infused the glutamate receptor antagonist kynurenate [KYNA, 100 mM; 1–3 µL per infusion (30, 31)] bilaterally into the hippocampus of four macaques. These animals were pretrained to a stable level of performance on a self-ordered sequencing spatial memory task, the Hamilton Search Task (32–35), in which the monkey was in a fixed orientation relative to the stimuli within a Wisconsin General Testing Apparatus. Self-ordered tasks require subjects to monitor self-generated choices from a set of stimuli until each stimulus has been chosen (e.g., refs. 36 and 37). Here, the stimuli were an array of eight boxes arranged in a horizontal row; the boxes were identical, except for their spatial position within the row (Fig. 1 and Methods). As a contrast, we also tested the animals on a version of the task, in which each box was uniquely colored (color-cued version), thereby eliminating reliance on spatial position. At the start of a run, each box was baited with a single food reward; when the monkey collected all eight rewards the run was ended. The monkey was allowed to open one box at a time (boxes snapped shut after release). After each opening, an opaque screen in front of the monkey was lowered and then raised again after a delay (either 1 s or 30 s); each access to the box array constituted one trial. The optimal strategy, opening each box only once, allows all eight rewards to be retrieved within eight trials. Returning to a previously opened box constitutes an error.Open in a separate windowFig. 1.(A) Spatial version of the task. (B) Intended infusion sites in the anterior hippocampus. (C and D) Three- to four-millimeter region of diffusion of MR contrast agent (Methods) shown as hypersignal within hippocampus. (E and F) Postmortem MRI and (G and H) histological confirmation of infusion sites. Arrows indicate cannula tracks in E–H.After training to criterion (Methods), cranial platforms were implanted to allow drug to be infused bilaterally into the hippocampus in awake animals (as described in refs. 22–26). Measures used to assess performance were number of trials needed to complete a run, number of correct openings in the first eight trials, and a repetition index (see Methods for calculations). These values were compared within subjects across treatments and task conditions.     

Voluntary Sodium Ingestion in Wild-Type and Oxytocin Knockout Mice

Oxytocin knockout (OT KO) mice acutely consume inappropriate amounts of sodium following overnight water deprivation suggesting that oxytocinergic neurons inhibit excessive sodium ingestion (Amico JA, Morris M, Vollmer RR. Mice deficient in oxytocin manifest increased saline consumption following overnight fluid deprivation. Am J Physiol – Regul Integr Comp Physiol 2001; 281:R1368–R1373). This study sought to determine whether oxytocin (OT) provides long-term regulation of voluntary sodium ingestion. Wild-type (WT) and oxytocin knockout male mice were provided choices between diets or drinking solutions that differed in their sodium content. Mice were given access for 1 week to two diets, one containing low sodium (0.01% sodium chloride [NaCl]) content and a second containing a normal sodium (1.0% NaCl) content. During the second week, the animals were given a choice between a low sodium diet and a high sodium (8.0% NaCl) diet. In the second week, mice consumed 4 times more sodium; however, there were no differences between WT and OT KO mice. In a second experiment, mice had access to a two-bottle choice of tap water and a 0.5 M NaCl solution made palatable by the addition of a 4.1% Intralipid emulsion. Both genotypes consumed large, but equivalent, volumes of the Intralipid/sodium solution. The ingestion of this sodium-rich solution stimulated thirst and enhanced the intake of water. Thus, the availability of palatable sodium-rich food or solutions can lead to excessive voluntary sodium ingestion. Compared with oxytocin knockout mice, enhanced voluntary ingestion of sodium-rich solid and liquid diets proceeded unimpeded in WT mice. Therefore, OT pathways may not be essential for regulating solute intake in this setting.