Chronic psychosocial stress affects corticotropin-releasing factor in the paraventricular nucleus and central extended amygdala as well as urocortin 1 in the non-preganglionic Edinger-Westphal nucleus of the tree shrew

The Lack of TRPA1 Ion Channel Does Not Affect the Chronic Stress-Induced Activation of the Locus Ceruleus

We have previously proven the involvement of transient receptor potential ankyrin 1 (TRPA1) in stress adaptation. A lack of TRPA1 affects both urocortin 1 (member of the corticotropin-releasing hormone (CRH) family) content of the Edinger-Westphal nucleus. The noradrenergic locus ceruleus (LC) is also an important player in mood control. We aimed at investigating whether the TRPA1 is expressed in the LC, and to test if the response to chronic variable mild stress (CVMS) is affected by a lack of TRPA1. The TRPA1 expression was examined via RNAscope in situ hybridization. We investigated TRPA1 knockout and wildtype mice using the CVMS model of depression. Tyrosine hydroxylase (TH) and FOSB double immunofluorescence were used to test the functional neuromorphological changes in the LC. No TRPA1 expression was detected in the LC. The TH content was not affected by CVMS exposure. The CVMS-induced FOSB immunosignal did not co-localize with the TH neurons. TRPA1 is not expressed in the LC. A lack of functional TRPA1 receptor neither directly nor indirectly affects the TH content of LC neurons under CVMS.

Anxiolytic reversal of classically conditioned / chronic stress-induced gene expression and learning in the Stress Alternatives Model

Social decision-making is critically influenced by neurocircuitries that regulate stress responsiveness. Adaptive choices, therefore, are altered by stress-related neuromodulatory peptide systems, such as corticotropin releasing factor (CRF). Experimental designs that take advantage of ecologically salient fear-inducing stimuli allow for revelation of neural mechanisms that regulate the balance between pro- and anti-stress responsiveness. To accomplish this, we developed a social stress and conditioning protocol, the Stress Alternatives Model (SAM), that utilizes a simple dichotomous choice, and produces distinctive behavioral phenotypes (Escape or Stay). The experiments involve repeated social aggression, a potent unconditioned stimulus (US), from a novel larger conspecific (a 3X larger Rainbow trout). Prior to the social interaction, the smaller test fish is presented with an auditory conditioning stimulus (water off = CS). During the social aggression, an escape route is available, but is only large enough for the smaller test animal. Surprisingly, although the new aggressor provides vigorous attacks each day, only 50% of the test fish choose Escape. Stay fish, treated with the CRF1 antagonist antalarmin, a potent anxiolytic drug, on day 4, promotes Escape behavior for the last 4 days of the SAM protocol. The results suggest that the decision to Escape, required a reduction in stress reactivity. The Stay fish that chose Escape following anxiolytic treatment, learned how to use the escape route prior to stress reduction, as the Escape latency in these fish was significantly faster than first time escapers. In Escape fish, the use of the escape route is learned over several days, reducing the Escape latency over time in the SAM. Fear conditioning (water off + aggression) resulted in elevated hippocampal (DL) Bdnf mRNA levels, with coincident reduction in the AMPA receptor subunit Glua1 expression, a result that is reversed following a one-time treatment (during SAM aggression on day 4) with the anxiolytic CRF1 receptor antagonist antalarmin.

Fluoxetine treatment supports predictive validity of the three hit model of depression in male PACAP heterozygous mice and underpins the impact of early life adversity on therapeutic efficacy

According to the three hit concept of depression, interaction of genetic predisposition altered epigenetic programming and environmental stress factors contribute to the disease. Earlier we demonstrated the construct and face validity of our three hit concept-based mouse model. In the present work, we aimed to examine the predictive validity of our model, the third willnerian criterion. Fluoxetine treatment was applied in chronic variable mild stress (CVMS)-exposed (environmental hit) CD1 mice carrying one mutated allele of pituitary adenylate cyclase-activating polypeptide gene (genetic hit) that were previously exposed to maternal deprivation (epigenetic hit) vs. controls. Fluoxetine reduced the anxiety level in CVMS-exposed mice in marble burying test, and decreased the depression level in tail suspension test if mice were not deprived maternally. History of maternal deprivation caused fundamental functional-morphological changes in response to CVMS and fluoxetine treatment in the corticotropin-releasing hormone-producing cells of the bed nucleus of the stria terminalis and central amygdala, in tyrosine-hydroxylase content of ventral tegmental area, in urocortin 1-expressing cells of the centrally projecting Edinger-Westphal nucleus, and serotonergic cells of the dorsal raphe nucleus. The epigenetic background of alterations was approved by altered acetylation of histone H3. Our findings further support the validity of both the three hit concept and that of our animal model. Reversal of behavioral and functional-morphological anomalies by fluoxetine treatment supports the predictive validity of the model. This study highlights that early life stress does not only interact with the genetic and environmental factors, but has strong influence also on therapeutic efficacy.

Neurodegeneration in the centrally-projecting Edinger-Westphal nucleus contributes to the non-motor symptoms of Parkinson's disease in the rat

BACKGROUND

The neuropathological background of major depression and anxiety as non-motor symptoms of Parkinson's disease is much less understood than classical motor symptoms. Although, neurodegeneration of the Edinger-Westphal nucleus in human Parkinson's disease is a known phenomenon, its possible significance in mood status has never been elucidated. In this work we aimed at investigating whether neuron loss and alpha-synuclein accumulation in the urocortin 1 containing (UCN1) cells of the centrally-projecting Edinger-Westphal (EWcp) nucleus is associated with anxiety and depression-like state in the rat.

METHODS

Systemic chronic rotenone administration as well as targeted leptin-saporin-induced lesions of EWcp/UCN1 neurons were conducted. Rotarod, open field and sucrose preference tests were performed to assess motor performance and mood status. Multiple immunofluorescence combined with RNAscope were used to reveal the functional-morphological changes. Two-sample Student's t test, Spearman's rank correlation analysis and Mann-Whitney U tests were used for statistics.

RESULTS

In the rotenone model, besides motor deficit, an anxious and depression-like phenotype was detected. Well-comparable neuron loss, cytoplasmic alpha-synuclein accumulation as well as astro- and microglial activation were observed both in the substantia nigra pars compacta and EWcp. Occasionally, UCN1-immunoreactive neuronal debris was observed in phagocytotic microglia. UCN1 peptide content of viable EWcp cells correlated with dopaminergic substantia nigra cell count. Importantly, other mood status-related dopaminergic (ventral tegmental area), serotonergic (dorsal and median raphe) and noradrenergic (locus ceruleus and A5 area) brainstem centers did not show remarkable morphological changes. Targeted partial selective EWcp/UCN1 neuron ablation induced similar mood status without motor symptoms.

CONCLUSIONS

Our findings collectively suggest that neurodegeneration of urocortinergic EWcp contributes to the mood-related non-motor symptoms in toxic models of Parkinson's disease in the rat.

Distribution of tyrosine-hydroxylase-immunoreactive neurons in the hypothalamus of tree shrews

The tree shrew (Tupaia belangeri chinensis) is the closest living relative of primates. Yet, little is known about the anatomical distribution of tyrosine hydroxylase (TH)-immunoreactive (ir) structures in the hypothalamus of the tree shrew. Here, we provide the first detailed description of the distribution of TH-ir neurons in the hypothalamus of tree shrews via immunohistochemical techniques. TH-ir neurons were widely distributed throughout the hypothalamus of tree shrew. The majority of hypothalamic TH-ir neurons were found in the paraventricular hypothalamic nucleus (PVN) and supraoptic nucleus (SON), as was also observed in the human hypothalamus. In contrast, rare TH-ir neurons were localized in the PVN and SON of rats. Vasopressin (AVP) colocalized with TH-ir neurons in the PVN and SON in a large number of neurons, but oxytocin and corticotropin-releasing hormone did not colocalize with TH. In addition, colocalization of TH with AVP was also observed in the other hypothalamic regions. Moreover, TH-ir neurons in the PVN and SON of tree shrews expressed other dopaminergic markers (aromatic l-amino acid decarboxylase and vesicular monoamine transporter, Type 2), further supporting that TH-ir neurons in the PVN and SON were catecholaminergic. These findings provide a detailed description of TH-ir neurons in the hypothalamus of tree shrews and demonstrate species differences in the distribution of this enzyme, providing a neurobiological basis for the participation of TH-ir neurons in the regulation of various hypothalamic functions.

Regulation of sympathetic vasomotor activity by the hypothalamic paraventricular nucleus in normotensive and hypertensive states

The hypothalamic paraventricular nucleus (PVN) is a unique and important brain region involved in the control of cardiovascular, neuroendocrine, and other physiological functions pertinent to homeostasis. The PVN is a major source of excitatory drive to the spinal sympathetic outflow via both direct and indirect projections. In this review, we discuss the role of the PVN in the regulation of sympathetic output in normal physiological conditions and in hypertension. In normal healthy animals, the PVN presympathetic neurons do not appear to have a major role in sustaining resting sympathetic vasomotor activity or in regulating sympathetic responses to short-term homeostatic challenges such as acute hypotension or hypoxia. Their role is, however, much more significant during longer-term challenges, such as sustained water deprivation, chronic intermittent hypoxia, and pregnancy. The PVN also appears to have a major role in generating the increased sympathetic vasomotor activity that is characteristic of multiple forms of hypertension. Recent studies in the spontaneously hypertensive rat model have shown that impaired inhibitory and enhanced excitatory synaptic inputs to PVN presympathetic neurons are the basis for the heightened sympathetic outflow in hypertension. We discuss the molecular mechanisms underlying the presynaptic and postsynaptic alterations in GABAergic and glutamatergic inputs to PVN presympathetic neurons in hypertension. In addition, we discuss the ability of exercise training to correct sympathetic hyperactivity by restoring blood-brain barrier integrity, reducing angiotensin II availability, and decreasing oxidative stress and inflammation in the PVN.

Both Basal and Acute Restraint Stress-Induced c-Fos Expression Is Influenced by Age in the Extended Amygdala and Brainstem Stress Centers in Male Rats

The hypothalamus-pituitary-adrenal axis (HPA) is the main regulator of the stress response. The key of the HPA is the parvocellular paraventricular nucleus of the hypothalamus (pPVN) controlled by higher-order limbic stress centers. The reactivity of the HPA axis is considered to be a function of age, but to date, little is known about the background of this age-dependency. Sporadic literature data suggest that the stress sensitivity as assessed by semi-quantitation of the neuronal activity marker c-Fos may also be influenced by age. Here, we aimed at investigating the HPA activity and c-Fos immunoreactivity 2 h after the beginning of a single 60 min acute restraint stress in eight age groups of male Wistar rats. We hypothesized that the function of the HPA axis (i.e., pPVN c-Fos and blood corticosterone (CORT) level), the neuronal activity of nine stress-related limbic areas (i.e., magnocellular PVN (mPVN), medial (MeA), central (CeA), basolateral nuclei of the amygdala, the oval (ovBNST), dorsolateral (dlBNST), dorsomedial (dmBNST), ventral and fusiform (fuBNST) divisions of the bed nucleus of the stria terminalis (BNST)), and two brainstem stress centers such as the centrally projecting Edinger-Westphal nucleus (cpEW) and dorsal raphe nucleus (DR) show age dependency in their c-Fos response. The somatosensory barrel cortex area (S1) was evaluated to test whether the age dependency is specific for stress-centers. Our results indicate that the stress-induced rise in blood CORT titer was lower in young age reflecting relatively low HPA activity. All 12 stress-related brain areas showed c-Fos response that peaked at 2 months of age. The magnitude of c-Fos immunoreactivity correlated negatively with age in seven regions (MeA, CeA, ovBNST, dlBNST, dmBNST, fuBNST and pPVN). Unexpectedly, the CeA, ovBNST and cpEW showed a considerable basal c-Fos expression in 1-month-old rats which decreased with age. The S1 showed a U-shaped age-related dynamics in contrast to the decline observed in stress centers. We conclude that the age- and brain area dependent dynamics in stress-induced neuronal activity pattern may contribute to the age dependance of the stress reactivity. Further studies are in progress to determine the neurochemical identity of neurons showing age-dependent basal and/or stress-induced c-Fos expression.

Atlas of the Striatum and Globus Pallidus in the Tree Shrew: Comparison with Rat and Mouse

The striatum and globus pallidus are principal nuclei of the basal ganglia. Nissl- and acetylcholinesterase-stained sections of the tree shrew brain showed the neuroanatomical features of the caudate nucleus (Cd), internal capsule (ic), putamen (Pu), accumbens, internal globus pallidus, and external globus pallidus. The ic separated the dorsal striatum into the Cd and Pu in the tree shrew, but not in rats and mice. In addition, computer-based 3D images allowed a better understanding of the position and orientation of these structures. These data provided a large-scale atlas of the striatum and globus pallidus in the coronal, sagittal, and horizontal planes, the first detailed distribution of parvalbumin-immunoreactive cells in the tree shrew, and the differences in morphological characteristics and density of parvalbumin-immunoreactive neurons between tree shrew and rat. Our findings support the tree shrew as a potential model for human striatal disorders.

Construct and face validity of a new model for the three-hit theory of depression using PACAP mutant mice on CD1 background

Major depression is a common cause of chronic disability. Despite decades of efforts, no equivocally accepted animal model is available for studying depression. We tested the validity of a new model based on the three-hit concept of vulnerability and resilience. Genetic predisposition (hit 1, mutation of pituitary adenylate cyclase-activating polypeptide, PACAP gene), early-life adversity (hit 2, 180-min maternal deprivation, MD180) and chronic variable mild stress (hit 3, CVMS) were combined. Physical, endocrinological, behavioral and functional morphological tools were used to validate the model. Body- and adrenal weight changes as well as corticosterone titers proved that CVMS was effective. Forced swim test indicated increased depression in CVMS PACAP heterozygous (Hz) mice with MD180 history, accompanied by elevated anxiety level in marble burying test. Corticotropin-releasing factor neurons in the oval division of the bed nucleus of the stria terminalis showed increased FosB expression, which was refractive to CVMS exposure in wild-type and Hz mice. Urocortin1 neurons became over-active in CMVS-exposed PACAP knock out (KO) mice with MD180 history, suggesting the contribution of centrally projecting Edinger-Westphal nucleus to the reduced depression and anxiety level of stressed KO mice. Serotoninergic neurons of the dorsal raphe nucleus lost their adaptation ability to CVMS in MD180 mice. In conclusion, the construct and face validity criteria suggest that MD180 PACAP HZ mice on CD1 background upon CVMS may be used as a reliable model for the three-hit theory.

Whole-brain mapping of afferent projections to the bed nucleus of the stria terminalis in tree shrews

The bed nucleus of the stria terminalis (BST) plays an important role in integrating and relaying input information to other brain regions in response to stress. The cytoarchitecture of the BST in tree shrews (Tupaia belangeri chinensis) has been comprehensively described in our previous publications. However, the inputs to the BST have not been described in previous reports. The aim of the present study was to investigate the sources of afferent projections to the BST throughout the brain of tree shrews using the retrograde tracer Fluoro-Gold (FG). The present results provide the first detailed whole-brain mapping of BST-projecting neurons in the tree shrew brain. The BST was densely innervated by the prefrontal cortex, entorhinal cortex, ventral subiculum, amygdala, ventral tegmental area, and parabrachial nucleus. Moreover, moderate projections to the BST originated from the medial preoptic area, supramammillary nucleus, paraventricular thalamic nucleus, pedunculopontine tegmental nucleus, dorsal raphe nucleus, locus coeruleus, and nucleus of the solitary tract. Afferent projections to the BST are identified in the ventral pallidum, nucleus of the diagonal band, ventral posteromedial thalamic nucleus, posterior complex of the thalamus, interfascicular nucleus, retrorubral field, rhabdoid nucleus, intermediate reticular nucleus, and parvicellular reticular nucleus. In addition, the different densities of BST-projecting neurons in various regions were analyzed in the tree shrew brains. In summary, whole-brain mapping of direct inputs to the BST is delineated in tree shrews. These brain circuits are implicated in the regulation of numerous physiological and behavioral processes including stress, reward, food intake, and arousal.

Effects of the Synthetic Neurosteroid: 3β-Methoxypregnenolone (MAP4343) on Behavioral and Physiological Alterations Provoked by Chronic Psychosocial Stress in Tree Shrews

BACKGROUND

Most currently available active antidepressant drugs are selective serotonin/noradrenaline reuptake inhibitors. However, as their clinical efficacy is not immediate, long-term administration is often accompanied by substantial side effects, and numerous patients remain non- or partial responders. We have recently found that the synthetic neurosteroid derivative 3β-methoxypregnenolone, which binds to the microtubule-associated protein-2, can provide a novel therapeutic approach in experimental model of depressive disorders in rats. To further validate the antidepressant-like efficacy of 3β-methoxypregnenolone, we investigated effects of a longer treatment (4-week oral administration; 50mg/kg/d) in a nonrodent species, the tree shrew, exposed to psychosocial stress that elicits close-to-human alterations observed in patients with depressive disorders.

METHODS

During the experimental period, physiological parameters were registered, including core body temperature and electroencephalogram, while animals were videotaped to analyze their avoidance behavior. Morning urine samples were collected for measurements of cortisol and noradrenaline levels.

RESULTS

We found that treatment with 3β-methoxypregnenolone abolished stress-triggered avoidance behavior and prevented hormone hypersecretion, hypothermia, and sleep disturbances, further suggesting its antidepressant-like efficacy. Comparative treatment with fluoxetine also prevented some of the physiological alterations, while the hypersecretion of cortisol and sleep disturbances were not or partially restored by fluoxetine, suggesting a better efficacy of 3β-methoxypregnenolone. Alpha-tubulin isoforms were measured in hippocampi: we found that 3β-methoxypregnenolone reversed the specific decrease in acetylation of α-tubulin induced by psychosocial stress, while it did not modify the psychosocial stress-elicited reduction of tyrosinated α-tubulin.

CONCLUSIONS

Taken together, these data strongly suggest a potent antidepressant-like effect of 3β-methoxypregnenolone on translational parameters.

Distribution of corticotropin-releasing factor in the tree shrew brain

Corticotropin-releasing factor (CRF) in the brain plays an important role in regulations of physiological and behavioral processes, yet CRF distribution in tree shrew brain has not been thoroughly and systematically reported. Here we examined the distribution of CRF immunoreactivity in the brain of tree shrews (Tupaia belangeri chinensis) using immunohistochemical techniques. CRF-immunoreactive (-ir) cells and fibers were present in the rhinencephalon, telencephalon, diencephalon, mesencephalon, metencephalon and myelencephalon of saline- and colchicine-treated tree shrews. Laminar distribution of CRF-ir cells was found in the main olfactory bulb and neocortex. Compared with saline-treated tree shrews, a larger number of CRF-ir cells in colchicine-treated tree shrews were found in the bed nucleus of the stria terminalis, paraventricular hypothalamic nucleus, medial preoptic area, dorsomedial hypothalamic nucleus, reuniens thalamic nucleus, inferior colliculus, Edinger-Westphal nucleus, median raphe nucleus, locus coeruleus, parabrachial nucleus, dorsal tegmental nucleus, lateral reticular nucleus, and inferior olive. CRF-ir fibers from the hypothalamic paraventricular nucleus projected toward and through the internal zone of the median eminence. In addition, density of CRF immunoreactivity is significantly different in the bed nucleus of the stria terminalis, central amygdaloid nucleus, suprachiasmatic nucleus, median raphe nucleus, Edinger-Westphal nucleus, locus coeruleus and inferior olive between tree shrews and rats after saline or colchicine treatment. Our findings provide, for the first time, the comprehensive description of CRF immunoreactivity and whole brain mapping of CRF in tree shrews, which is an anatomical basis for the participation of CRF system in the regulation of numerous behaviors.

Immunohistochemical mapping of neuropeptide Y in the tree shrew brain

Day-active tree shrews are promising animals as research models for a variety of human disorders. Neuropeptide Y (NPY) modulates many behaviors in vertebrates. Here we examined the distribution of NPY in the brain of tree shrews (Tupaia belangeri chinensis) using immunohistochemical techniques. The differential distribution of NPY-immunoreactive (-ir) cells and fibers were observed in the rhinencephalon, telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon of tree shrews. Most NPY-ir cells were multipolar or bipolar in shape with triangular, fusiform, and/or globular perikarya. The densest cluster of NPY-ir cells were found in the mitral cell layer of the main olfactory bulb (MOB), arcuate nucleus of the hypothalamus, and pretectal nucleus of the thalamus. The MOB presented a unique pattern of NPY immunoreactivity. Laminar distribution of NPY-ir cells was observed in the MOB, neocortex, and hippocampus. Compared to rats, the tree shrews exhibited a particularly robust and widespread distribution of NPY-ir cells in the MOB, bed nucleus of the stria terminalis, and amygdala as well as the ventral lateral geniculate nucleus and pretectal nucleus of the thalamus. By contrast, a low density of neurons were scattered in the striatum, neocortex, polymorph cell layer of the dentate gyrus, superior colliculus, inferior colliculus, and dorsal tegmental nucleus. These findings provide the first detailed mapping of NPY immunoreactivity in the tree shrew brain and demonstrate species differences in the distribution of this neuropeptide, providing an anatomical basis for the participation of the NPY system in the regulation of numerous physiological and behavioral processes.

Agomelatine in the tree shrew model of depression: effects on stress-induced nocturnal hyperthermia and hormonal status

The antidepressive drug agomelatine combines the properties of an agonist of melatonergic receptors 1 and 2 with an antagonist of the 5-HT2C receptor. We analyzed the effects of agomelatine in psychosocially stressed male tree shrews, an established preclinical model of depression. Tree shrews experienced daily social stress for a period of 5 weeks and were concomitantly treated with different drugs daily for 4 weeks. The effects of agomelatine (40 mg/kg/day) were compared with those of the agonist melatonin (40 mg/kg/day), the inverse 5-HT2C antagonist S32006 (10mg/kg/day), and the SSRI fluoxetine (15 mg/kg/day). Nocturnal core body temperature (CBT) was recorded by telemetry, and urinary norepinephrine and cortisol concentrations were measured. Chronic social stress induced nocturnal hyperthermia. Agomelatine normalized the CBT in the fourth week of the treatment (T4), whereas the other drugs did not significantly counteract the stress-induced hyperthermia. Agomelatine also reversed the stress-induced reduction in locomotor activity. Norepinephrine concentration was elevated by the stress indicating sympathetic hyperactivity, and was normalized in the stressed animals treated with agomelatine or fluoxetine but not in those treated with melatonin or S32006. Cortisol concentration was elevated by stress but returned to basal levels by T4 in all animals, irrespective of the treatment. These observations show that agomelatine has positive effects to counteract stress-induced physiological processes and to restore the normal rhythm of nocturnal CBT. The data underpin the antidepressant properties of agomelatine and are consistent with a distinctive profile compared to its constituent pharmacological components and other conventional agents.

Distribution of vasopressin, oxytocin and vasoactive intestinal polypeptide in the hypothalamus and extrahypothalamic regions of tree shrews

Vasopressin (VP), oxytocin (OXT) and vasoactive intestinal polypeptide (VIP) in the brain modulate physiological and behavioral processes in many vertebrates. Day-active tree shrews, the closest relatives of primates, live singly or in pairs in territories that they defend vigorously against intruding conspecifics. However, anatomy concerning peptidergic neuron distribution in the tree shrew brain is less clear. Here, we examined the distribution of VP, OXT and VIP immunoreactivity in the hypothalamus and extrahypothalamic regions of tree shrews (Tupaia belangeri chinensis) using the immunohistochemical techniques. Most of VP and OXT immunoreactive (-ir) neurons were found in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus. In addition, VP-ir or OXT-ir neurons were scattered in the preoptic area, anterior hypothalamic areas, dorsomedial hypothalamic nucleus, stria terminalis, bed nucleus of the stria terminalis and medial amygdala. Interestingly, a high density of VP-ir fibers within the ventral lateral septum was observed in males but not in females. Both VP-ir and VIP-ir neurons were found in different subdivisions of the suprachiasmatic nucleus (SCN) with partial overlap. VIP-ir cells and fibers were also scattered in the cerebral cortex, anterior olfactory nucleus, amygdala and dentate gyrus of the hippocampus. These findings provide a comprehensive description of VIP and a detailed mapping of VP and OXT in the hypothalamus and extrahypothalamic regions of tree shrews, which is an anatomical basis for the participation of these neuropeptides in the regulation of circadian behavior and social behavior.

Role of neuropeptides in anxiety, stress, and depression: from animals to humans

Major depression, with its strikingly high prevalence, is the most common cause of disability in communities of Western type, according to data of the World Health Organization. Stress-related mood disorders, besides their deleterious effects on the patient itself, also challenge the healthcare systems with their great social and economic impact. Our knowledge on the neurobiology of these conditions is less than sufficient as exemplified by the high proportion of patients who do not respond to currently available medications targeting monoaminergic systems. The search for new therapeutical strategies became therefore a "hot topic" in neuroscience, and there is a large body of evidence suggesting that brain neuropeptides not only participate is stress physiology, but they may also have clinical relevance. Based on data obtained in animal studies, neuropeptides and their receptors might be targeted by new candidate neuropharmacons with the hope that they will become important and effective tools in the management of stress related mood disorders. In this review, we attempt to summarize the latest evidence obtained using animal models for mood disorders, genetically modified rodent models for anxiety and depression, and we will pay some attention to previously published clinical data on corticotropin releasing factor, urocortin 1, urocortin 2, urocortin 3, arginine-vasopressin, neuropeptide Y, pituitary adenylate-cyclase activating polypeptide, neuropeptide S, oxytocin, substance P and galanin fields of stress research.

Central corticotropin releasing factor and social stress

Social interactions are a main source of stress in vertebrates. Social stressors, as well as other stressors, activate the hypothalamic–pituitary–adrenal (HPA) axis resulting in glucocorticoid release. One of the main components of the HPA axis is corticotropin releasing factor (CRF). The neuropeptide CRF is part of a peptide family including CRF, urocortin 1–3, urotensin 1–3, and sauvagine. The actions of the CRF family are mediated by at least two different receptors with different anatomical distribution and affinities for the peptides. The CRF peptides affect several behavioral and physiological responses to stress including aggression, feeding, and locomotor activity. This review will summarize recent research in vertebrates concerning how social stress interacts with components of the CRF system. Consideration will be taken to the different models used for social stress ranging from social isolation, dyadic interactions, to group dominance hierarchies. Further, the temporal effect of social stressor from acute, intermittent, to chronic will be considered. Finally, strains selected for specific behavior or physiology linked to social stress will also be discussed.

Reflections on the interaction of psychogenic stress systems in humans: the stress coherence/compensation model

Although stress simultaneously affects and causes changes in central nervous system systems together with the sympathetic nervous system and the hypothalamus-pituitary-adrenal axis, this interaction and its behavioral consequences are rarely assessed. The current paper first describes the different systems involved in the perception and processing of stressful stimuli on an anatomical and functional level, and the available measures to assess changes in these systems. It then explores, based on theoretical and empirical grounds, the interaction of the systems. This is followed by a review of previous stress models, and how these attempted to integrate the interaction of these systems. Then, it complements previous models by suggesting a complementary regulation of the stress systems, and discusses potential behavioral consequences. Finally, based on the three-system approach to assess stress it is argued that psychological measures, together with physiological and endocrine measures are indispensable. However, the lack of consensus on how to best assess the central and sympathetic nervous system components of stress make it more difficult to include measures of all systems routinely in future stress studies. Thus, the paper closes by giving some recommendations on how to include a minimum of feasible stress measures for all systems involved in stress processing and regulation.

Cannabinoid modulation of midbrain urocortin 1 neurones during acute and chronic stress

Neurones in the centrally projecting Edinger-Westphal nucleus (EWcp) are the main site of urocortin 1 (Ucn1) synthesis in the mammalian brain, and are assumed to play a role in the stress response of the animal. Because endocannabinoid signalling has also been strongly implicated in stress, we hypothesised that endocannabinoids may modulate the functioning of the urocortinergic EWcp. First, using in situ hybridisation, we demonstrated cannabinoid receptor 1 (CB1R) mRNA expression in mouse EWcp-neurones that were Ucn1-negative. Dual- and triple-label immunocytochemistry revealed the presence of CB1R in several GABA-immunopositive fibres juxtaposed to EWcp-Ucn1 neurones. To test functional aspects of such an anatomical constellation, we compared acute (1 h of restraint) and chronic (14 days of chronic mild stress) stress-induced changes in wild-type (WT) and CB1R knockout (CB1R-KO) mice. Acute and especially chronic stress resulted in an increase in Ucn1 content of the EWcp, which was attenuated in CB1R-KO mice. CB1R-KO mice had higher basal and chronic stress-induced adrenocorticotrophin and corticosterone levels and were more anxious on the elevated plus-maze versus WT. Collectively, our results show for the first time EWcp-Ucn1 neurones are putatively innervated by endocannabinoid sensitive, inhibitory, GABAergic afferents. In addition, we provide novel evidence that the absence of the CB1 receptor alters the Ucn1 mRNA and peptide levels in EWcp neurones, concomitant with an augmented stress response and increased anxiety-like behaviour.

Recognition memory in tree shrew (Tupaia belangeri) after repeated familiarization sessions

Recognition memories are formed during perceptual experience and allow subsequent recognition of previously encountered objects as well as their distinction from novel objects. As a consequence, novel objects are generally explored longer than familiar objects by many species. This novelty preference has been documented in rodents using the novel object recognition (NOR) test, as well is in primates including humans using preferential looking time paradigms. Here, we examine novelty preference using the NOR task in tree shrew, a small animal species that is considered to be an intermediary between rodents and primates. Our paradigm consisted of three phases: arena familiarization, object familiarization sessions with two identical objects in the arena and finally a test session following a 24-h retention period with a familiar and a novel object in the arena. We employed two different object familiarization durations: one and three sessions on consecutive days. After three object familiarization sessions, tree shrews exhibited robust preference for novel objects on the test day. This was accompanied by significant reduction in familiar object exploration time, occurring largely between the first and second day of object familiarization. By contrast, tree shrews did not show a significant preference for the novel object after a one-session object familiarization. Nonetheless, they spent significantly less time exploring the familiar object on the test day compared to the object familiarization day, indicating that they did maintain a memory trace for the familiar object. Our study revealed different time courses for familiar object habituation and emergence of novelty preference, suggesting that novelty preference is dependent on well-consolidated memory of the competing familiar object. Taken together, our results demonstrate robust novelty preference of tree shrews, in general similarity to previous findings in rodents and primates.

Elevated tph2 mRNA expression in a rat model of chronic anxiety

BACKGROUND

Allelic variations in TPH2, the gene encoding tryptophan hydroxylase 2, the rate-limiting enzyme for brain serotonin (5-HT) biosynthesis, may be genetic predictors of panic disorder and panic responses to panicogenic challenges in healthy volunteers. To test the hypothesis that tph2 mRNA is altered in chronic anxiety states, we measured tph2 expression in an established rat model of panic disorder.

METHODS

We implanted 16 adult, male rats with bilateral guide cannulae and then primed them with daily injections of the corticotropin-releasing factor (CRF) receptor agonist, urocortin 1 (UCN1, 6 fmoles/100 nl per side, n = 8) or vehicle (n = 8) into the basolateral amygdaloid complex (BL) for 5 consecutive days. Anxiety-like behavior was assessed, 24 hr prior to and 48 hr following priming, in the social interaction (SI) test. A third group (n = 7) served as undisturbed home cage controls. All rats were killed 3 days after the last intra-BL injection to analyze tph2 and slc6a4 (gene encoding the serotonin transporter, SERT) mRNA expression in the dorsal raphe nucleus (DR), the main source of serotonergic projections to anxiety-related brain regions, using in situ hybridization histochemistry.

RESULTS

UCN1 priming increased anxiety-related behavior in the SI test compared to vehicle-injected controls and elevated tph2, but not slc6a4, mRNA expression in DR subregions, including the ventrolateral DR/ventrolateral periaqueductal gray (DRVL/VLPAG), a subregion previously implicated in control of panic-related physiologic responses. Tph2 mRNA expression in the DRVL/VLPAG was correlated with increased anxiety-related behavior.

CONCLUSION

Our data support the hypothesis that chronic anxiety states are associated with dysregulated tph2 expression.

[Effects of some extenders and monoamines on sperm cryopreservation in tree shrews (Tupaia belangeri)]

The tree shrew may be an important experimental animal for disease models in humans. The effects of some extenders and momamines on sperm cryopreservation will provide helpful data for experimentation of strains and conservation of genetic resources in tree shrews. Epididymal sperm were surgically harvested from male tree shrews captured around Kunming, China and sperm motility, acrosome integrity and fertility were assessed during cryopreservation. In Experiment 1 eight extenders (TTE, TCG, TCF, TTG, BWW, BTS, DM, and SR) supplemented with 0.4 mol/L DMSO were used to dilute the sperm: only TTE, DM and SR showed no differences in motility and acrosome integrity compared to fresh controls after equilibration. After freezing and thawing, sperm in any extender showed lower motility than fresh control and sperm in DM showed higher motility than other groups. However, BWW produced the lowest motility. For acrosome integrity, TTE and DM showed higher than BWW, BTS and SR after equilibration. The parameter in DM was higher than other groups (except TTE) after thawing. In Experiment 2 four penetrating cryoprotectant agents (CPA) [dimethyl-formamide (DF), formamide (F), dimethylacetamide (DA), and acetamide (A)] at 0.2 mol/L, 0.4 mol/L, 0.8 mol/L, and 1.2 mol/L, respectively were added to the DM extender. Motility showed no difference among CPA groups and non-CPA group (control) after equilibration, but all thawed sperm showed lower values in motility and acrosome integrity than pre-freezing groups. However, sperm in 0.8 mol/L DF and 0.4 mol/L DMSO showed higher values in both parameters than that in other CPA groups (P>0.05). In Experiment 3 the fertilization rate of oocytes inseminated with 0.4mol/L DMSO (50%) were higher than that with 0.8mol/L DF (16%). In conclusion, non-ion extenders supplemented with egg yolk may be better for sperm cryopreservation in tree shrews and cryoprotectant effects of monoamines agents should be further studied in this species.

[Morphological characteristics and cryodamage of Chinese tree shrew (Tupaia belangeri chinensis) sperm]

The tree shrew (Tupaia belangeri chinensis) is a small non-rodent mammal, which is a relatively new experimental animal in medicine due to its close evolutionary relationship to primates and its rapid propagation. Sperm characteristics and cryopreservation in the tree shrew were the main contents of our spermatological research. Epididymal sperm were surgically harvested from male tree shrews captured from the Kunming area. The rate of testis weight to body weight was (1.05±0.07)%, volume of both testis was (1.12 ± 0.10) mL, total sperm from epididymis and vas deferens were 2.2-8.8×10(7), and sperm motility and acrosome integrity were (68.8 ± 3.9)% and (90.0 ± 2.1)%, respectively. Sperm ultrastructure of the tree shrew was examined by scanning electron microscopy and transmission electron microscopy. Tree shrew sperm had a round or oval shaped head of approximately 6.65×5.82 μm, and midpiece, principal piece, tail, and total sperm lengths were 13.39, 52.35, 65.74, and 73.05 μm, respectively. The mitochondria in the midpiece consisted of approximately 48 gyres and had a 9+9+2 axonemal pattern. After freezing and thawing, sperm showed partly intact acrosomes and plasma membrane defects, and sperm breakages, twists, and swellings were found. The tree shrew had similar ultrastructure with other mammalians except for the mitochondria number and the sperm size. Ultrastructural alteration is still the main cause resulting in poor sperm after cryopreservation.

Characterization of Genetic Differences within the Centrally Projecting Edinger-Westphal Nucleus of C57BL/6J and DBA/2J Mice by Expression Profiling

Detailed examination of the midbrain Edinger–Westphal (EW) nucleus revealed the existence of two distinct nuclei. One population of EW preganglionic (EWpg) neurons was found to control oculomotor functions, and a separate population of EW centrally projecting (EWcp) neurons was found to contain stress- and feeding-related neuropeptides. Although it has been shown that EWcp neurons are highly responsive to drugs of abuse and behavioral stress, a genetic characterization of the EWcp was needed. To identify genetic differences in the EWcp of inbred mouse strains that differ in behaviors relevant to EWcp function, we used publicly available tools from the Allen Brain Atlas to identify 68 transcripts that were selectively expressed in the EWcp, and examined their expression within tissue punch microdissection samples containing the EWcp of adult male C57BL/6J (B6) and DBA/2J (D2) mice. Using 96-well quantitative real-time PCR (qPCR) arrays that included the EWcp-specific genes, several other genes of interest, and five housekeeping genes, we identified strain differences in expression of 11 EWcp-specific genes (BC023892, Btg3, Bves, Cart, Cck, Ghsr, Neto1, Postn, Ptprn, Rcn1, and Ucn), two immediate early genes (Egr1 and Fos), and one dopamine-related gene (Drd5). All significant expression differences were greater in B6 vs. D2 mice, and several of these were verified either at the protein level using immunohistochemistry (IHC) or in silico using microarray data sets from whole brain and other brain areas. These results demonstrate a significant advance in our understanding of the EWcp on three levels. First, we generated a list of EWcp-specific genes (most of which had not yet been reported within the EWcp in the literature) that will be informative for future studies of EWcp function. Second, due to similarity in results from qPCR and IHC, we revealed that strain differences in basal EWcp neuropeptide content are accounted for by differential transcription and number of peptidergic neurons, rather than by differential rates of peptide release. And third, our identification of differentially expressed EWcp-specific genes between B6 and D2 mice may hold powerful insight into the neurogenetic contributions of the EWcp to stress- and addiction-related behaviors.

The effects of corticotropin-releasing factor and the urocortins on hypothalamic gamma-amino butyric acid release--the impacts on the hypothalamic-pituitary-adrenal axis

Corticotropin-releasing factor (CRF) and the urocortins (UCNs) are structurally and pharmacologically related neuropeptides which regulate the endocrine, autonomic, emotional and behavioral responses to stress. CRF and UCN1 activate both CRF receptors (CRFR1 and CRFR2) with CRF binding preferentially to CRFR1 and UCN1 binding equipotently to both receptors. UCN2 and UCN3 activate selectively CRFR2. Previously an in vitro study demonstrated that superfusion of both CRF and UCN1 elevated the GABA release elicited by electrical stimulation from rat amygdala, through activation of CRF1 receptors. In the present experiments, the same in vitro settings were used to study the actions of CRF and the urocortins on hypothalamic GABA release. CRF and UCN1 administered in equimolar doses increased significantly the GABA release induced by electrical stimulation from rat hypothalamus. The increasing effects of CRF and UCN1 were inhibited considerably by the selective CRFR1 antagonist antalarmin, but were not influenced by the selective CRFR2 antagonist astressin 2B. UCN2 and UCN3 were ineffective. We conclude that CRF1 receptor agonists induce the release of GABA in the hypothalamus as well as previously the amygdala. We speculate that CRF-induced GABA release may act as a double-edged sword: amygdalar GABA may disinhibit the hypothalamic CRF release, leading to activation of the hypothalamic-pituitary-adrenal axis, whereas hypothalamic GABA may inhibit the hypothalamic CRF release, terminating this activation.

Chronic stress induces sex-specific alterations in methylation and expression of corticotropin-releasing factor gene in the rat

BACKGROUND

Although the higher prevalence of depression in women than in men is well known, the neuronal basis of this sex difference is largely elusive.

METHODS

Male and female rats were exposed to chronic variable mild stress (CVMS) after which immediate early gene products, corticotropin-releasing factor (CRF) mRNA and peptide, various epigenetic-associated enzymes and DNA methylation of the Crf gene were determined in the hypothalamic paraventricular nucleus (PVN), oval (BSTov) and fusiform (BSTfu) parts of the bed nucleus of the stria terminalis, and central amygdala (CeA).

RESULTS

CVMS induced site-specific changes in Crf gene methylation in all brain centers studied in female rats and in the male BST and CeA, whereas the histone acetyltransferase, CREB-binding protein was increased in the female BST and the histone-deacetylase-5 decreased in the male CeA. These changes were accompanied by an increased amount of c-Fos in the PVN, BSTfu and CeA in males, and of FosB in the PVN of both sexes and in the male BSTov and BSTfu. In the PVN, CVMS increased CRF mRNA in males and CRF peptide decreased in females.

CONCLUSIONS

The data confirm our hypothesis that chronic stress affects gene expression and CRF transcriptional, translational and secretory activities in the PVN, BSTov, BSTfu and CeA, in a brain center-specific and sex-specific manner. Brain region-specific and sex-specific changes in epigenetic activity and neuronal activation may play, too, an important role in the sex specificity of the stress response and the susceptibility to depression.

The Edinger-Westphal nucleus: a historical, structural, and functional perspective on a dichotomous terminology

The eponymous term nucleus of Edinger-Westphal (EW) has come to be used to describe two juxtaposed and somewhat intermingled cell groups of the midbrain that differ dramatically in their connectivity and neurochemistry. On one hand, the classically defined EW is the part of the oculomotor complex that is the source of the parasympathetic preganglionic motoneuron input to the ciliary ganglion (CG), through which it controls pupil constriction and lens accommodation. On the other hand, EW is applied to a population of centrally projecting neurons involved in sympathetic, consumptive, and stress-related functions. This terminology problem arose because the name EW has historically been applied to the most prominent cell collection above or between the somatic oculomotor nuclei (III), an assumption based on the known location of the preganglionic motoneurons in monkeys. However, in many mammals, the nucleus designated as EW is not made up of cholinergic, preganglionic motoneurons supplying the CG and instead contains neurons using peptides, such as urocortin 1, with diverse central projections. As a result, the literature has become increasingly confusing. To resolve this problem, we suggest that the term EW be supplemented with terminology based on connectivity. Specifically, we recommend that 1) the cholinergic, preganglionic neurons supplying the CG be termed the Edinger-Westphal preganglionic (EWpg) population and 2) the centrally projecting, peptidergic neurons be termed the Edinger-Westphal centrally projecting (EWcp) population. The history of this nomenclature problem and the rationale for our solutions are discussed in this review.

Differential gene expression in mutant mice overexpressing or deficient in the serotonin transporter: a focus on urocortin 1

Transcriptome analyses were performed in the anterior raphe area of mutant mice deficient in the serotonin transporter (5-HTT KO) or overexpressing this protein (5-HTT TG), which exhibit opposite changes in anxiety-related behavior. Among genes with altered expression, the gene encoding the neuropeptide urocortin 1 was down-regulated in 5-HTT KO and up-regulated in 5-HTT TG mice. Expression of the gene encoding cocaine-and-amphetamine-related-peptide, which colocalizes with urocortin 1, was also increased in 5-HTT TG mutants. Real-time RT-PCR confirmed these data and immunoautoradiographic labeling showed that parallel changes in neuropeptide levels were confined to the non-preganglionic Edinger-Westphal nucleus. Thus, 5-HTT expression correlates with that of urocortin 1, suggesting that this peptide can be involved in the behavioral changes observed in 5-HTT mutant mice.

About a snail, a toad, and rodents: animal models for adaptation research

Neural adaptation mechanisms have many similarities throughout the animal kingdom, enabling to study fundamentals of human adaptation in selected animal models with experimental approaches that are impossible to apply in man. This will be illustrated by reviewing research on three of such animal models, viz. (1) the egg-laying behavior of a snail, Lymnaea stagnalis: how one neuron type controls behavior, (2) adaptation to the ambient light condition by a toad, Xenopus laevis: how a neuroendocrine cell integrates complex external and neural inputs, and (3) stress, feeding, and depression in rodents: how a neuronal network co-ordinates different but related complex behaviors. Special attention is being paid to the actions of neurochemical messengers, such as neuropeptide Y, urocortin 1, and brain-derived neurotrophic factor. While awaiting new technological developments to study the living human brain at the cellular and molecular levels, continuing progress in the insight in the functioning of human adaptation mechanisms may be expected from neuroendocrine research using invertebrate and vertebrate animal models.

Individual differences in reactivity to social stress predict susceptibility and resilience to a depressive phenotype: role of corticotropin-releasing factor

Previous social stress exposure is a common risk factor for affective disorders. However, factors that determine vulnerability or resiliency to social stress-induced psychopathologies remain unclear. Using a rodent model of social stress, the present study was designed to identify putative neurobiological substrates that contribute to social stress-induced psychopathology and factors that influence or predict vulnerability. The resident-intruder model of defeat was used as a social stressor in adult male Sprague Dawley rats. The average latency to assume a subordinate posture (signaling defeat) over seven daily defeat exposures was calculated and examined with respect to endpoints of hypothalamic-pituitary-adrenal activity, components of the corticotropin-releasing factor (CRF) system, and behaviors that are relevant to human depression. In the present studies, a bimodal distribution emerged in an otherwise homogeneous population of Sprague Dawley rats such that 42% of rats exhibited short defeat latencies (<300 sec), whereas 58% of rats resisted defeat and exhibited longer latencies (>300 sec). These two phenotypes were associated with distinct endocrine and behavioral profiles as well as differences in components of the CRF system. Notably, the short-latency subpopulation exhibited hypothalamic-pituitary-adrenal dysregulation and behavior similar to that observed in melancholic depression. Examination of components of the CRF system suggested that proactive behavior in resisting defeat exhibited by long-latency rats was associated with decreased efficacy of CRF. Together, these data suggest that inherent differences in stress reactivity, perhaps as a result of differences in CRF regulation, may predict long-term consequences of social stress and vulnerability to depressive-like symptoms.

Sex-specific expression of BDNF and CART in the midbrain non-preganglionic Edinger-Westphal nucleus in the rat

In mammals, females generally appear more vulnerable to stressors than males. The non-preganglionic Edinger-Westphal nucleus (npEW) has been implicated in regulation of the stress response. Brain-derived neurotrophic factor (BDNF) and cocaine- and amphetamine-related transcript peptide (CART) are sex-specifically involved in the stress response too, and are present in the human and rat npEW. We hypothesized that male and female rats would differ in the expression of BDNF and CART in the npEW. Using immunocytochemistry and in situ hybridization we found that BDNF, CART and the estrogen receptor beta (ERbeta) are colocalized in the npEW. Q-RT-PCR showed no differences in CART and BDNF coding mRNAs between males and females, but quantitative immunocytochemistry revealed a 16% lower number of BDNF-immunoreactive neurons, and 19% lower CART-immunoreactivity in females compared to males. Considering the fact that Ucn1, CART and BDNF are co-expressed in the npEW with ERbeta and their protein expression differs between males and females, we propose that the functioning of the npEW may contribute to the sex differences that exist in stress sensitivity.

Differential sensitivity of the perioculomotor urocortin-containing neurons to ethanol, psychostimulants and stress in mice and rats

The perioculomotor urocortin-containing population of neurons (pIIIu: otherwise known as the non-preganglionic Edinger-Westphal nucleus) is sensitive to alcohol and is involved in the regulation of alcohol intake. A recent study indicated that this brain region is also sensitive to psychostimulants. Since pIIIu has been shown to respond to stress, we investigated how psychostimulant-induced pIIIu activation compares to stress- and ethanol-induced activation, and whether it is independent from a generalized stress response. Several experiments were performed to test how the pIIIu responds to psychostimulants by quantifying the number of Fos immunoreactive nuclei after acute i.p. injections of saline, 10-30 mg/kg cocaine, 5 mg/kg methamphetamine, 5 mg/kg amphetamine, 2.5 g/kg ethanol, 2 h of restraint stress, 10 min of swim stress, or six applications of mild foot shock in male C57BL/6 J mice. We also compared Fos immunoreactivity in pIIIu after acute (20 mg/kg cocaine) and repeated cocaine exposure (7 days of 20 mg/kg cocaine) injections in male C57BL/6 J mice in order to investigate the potential habituation of this response. Finally, we quantified the number of Fos immunoreactive nuclei in pIIIu after administration of saline, 2.5 g/kg ethanol, 20 mg/kg cocaine, or 2 h of restraint stress in male Sprague-Dawley rats. We found that exposure to psychostimulants and ethanol induced significantly higher Fos levels in pIIIu compared to stress in mice. Furthermore, repeated cocaine injections did not decrease Fos immunoreactivity as would be expected if this response were due to stress. In rats, exposure to ethanol, psychostimulant and restraint stress all induced pIIIu Fos immunoreactivity compared to saline-injected controls. In both mice and rats, ethanol- and cocaine-induced Fos immunoreactivity occurred exclusively in urocortin 1-positive, but not in tyrosine hydroxylase-positive, cells. These results provide evidence that the pIIIu Fos-response to psychostimulants is independent of a generalized stress in mice, but not rats. They additionally show that the pIIIu response to stress differs significantly between species.

Housekeeping genes revisited: different expressions depending on gender, brain area and stressor

Housekeeping gene (HKG) mRNAs are used to normalize expression data of genes of interest in quantitative reverse transcriptase polymerase chain reaction studies. Such normalization assumes constant HKG gene expression under all circumstances. Although sporadic evidence suggests that HKG expression may not always fulfill this requirement and, therefore, such normalization may lead readily to erroneous results, this fact is generally not sufficiently appreciated by investigators. Here, we have systematically analyzed the expression of three common HKGs, glyceraldehyde-3-phosphate dehydrogenase, ribosomal subunit 18S and beta-actin, in two different stress paradigms, in various brain areas, in male and in female rats. HKG expressions differed considerably with respect to brain area, type of stressor and gender, in an HKG-specific manner. Therefore, we conclude that before final experimentation, pilot expression studies are necessary to select an HKG which expression is unaffected by the experimental factor(s), allowing reliable interpretation of expression data of genes of interest.