Rigor and reproducibility in rodent behavioral research

Behavioral neuroscience research incorporates the identical high level of meticulous methodologies and exacting attention to detail as all other scientific disciplines. To achieve maximal rigor and reproducibility of findings, well-trained investigators employ a variety of established best practices. Here we explicate some of the requirements for rigorous experimental design and accurate data analysis in conducting mouse and rat behavioral tests. Novel object recognition is used as an example of a cognitive assay which has been conducted successfully with a range of methods, all based on common principles of appropriate procedures, controls, and statistics. Directors of Rodent Core facilities within Intellectual and Developmental Disabilities Research Centers contribute key aspects of their own novel object recognition protocols, offering insights into essential similarities and less-critical differences. Literature cited in this review article will lead the interested reader to source papers that provide step-by-step protocols which illustrate optimized methods for many standard rodent behavioral assays. Adhering to best practices in behavioral neuroscience will enhance the value of animal models for the multiple goals of understanding biological mechanisms, evaluating consequences of genetic mutations, and discovering efficacious therapeutics.

Noradrenergic Transmission at Alpha1-Adrenergic Receptors in the Ventral Periaqueductal Gray Modulates Arousal

BACKGROUND

Dysregulation of arousal is symptomatic of numerous psychiatric disorders. Previous research has shown that the activity of dopamine (DA) neurons in the ventral periaqueductal gray (vPAG) tracks with arousal state, and lesions of vPAGDA cells increase sleep. However, the circuitry controlling these wake-promoting DA neurons is unknown.

METHODS

This study combined designer receptors exclusively activated by designer drugs (DREADDs), behavioral pharmacology, electrophysiology, and immunoelectron microscopy in male and female mice to elucidate mechanisms in the vPAG that promote arousal.

RESULTS

Activation of locus coeruleus projections to the vPAG or vPAGDA neurons induced by DREADDs promoted arousal. Similarly, agonist stimulation of vPAG alpha1-adrenergic receptors (α1ARs) increased latency to fall asleep, whereas α1AR blockade had the opposite effect. α1AR stimulation drove vPAGDA activity in a glutamate-dependent, action potential-independent manner. Compared with other dopaminergic brain regions, α1ARs were enriched on astrocytes in the vPAG, and mimicking α1AR transmission specifically in vPAG astrocytes via Gq-DREADDS was sufficient to increase arousal. In general, the wake-promoting effects observed were not accompanied by hyperactivity.

CONCLUSIONS

These experiments revealed that vPAG α1ARs increase arousal, promote glutamatergic input onto vPAGDA neurons, and are abundantly expressed on astrocytes. Activation of locus coeruleus inputs, vPAG astrocytes, or vPAGDA neurons increase sleep latency but do not produce hyperactivity. Together, these results support an arousal circuit whereby noradrenergic transmission at astrocytic α1ARs activates wake-promoting vPAGDA neurons via glutamate transmission.

Increased expression of AT-1/SLC33A1 causes an autistic-like phenotype in mice by affecting dendritic branching and spine formation

The import of acetyl-CoA into the lumen of the endoplasmic reticulum (ER) by AT-1/SLC33A1 regulates Nε-lysine acetylation of ER-resident and -transiting proteins. Specifically, lysine acetylation within the ER appears to influence the efficiency of the secretory pathway by affecting ER-mediated quality control. Mutations or duplications in AT-1/SLC33A1 have been linked to diseases such as familial spastic paraplegia, developmental delay with premature death, and autism spectrum disorder with intellectual disability. In this study, we generated an AT-1 Tg mouse model that selectively overexpresses human AT-1 in neurons. These animals demonstrate cognitive deficits, autistic-like social behavior, aberrations in synaptic plasticity, an increased number of dendritic spines and branches, and widespread proteomic changes. We also found that AT-1 activity regulates acetyl-CoA flux, causing epigenetic modulation of the histone epitope H3K27 and mitochondrial adaptation. In conclusion, our results indicate that increased expression of AT-1 can cause an autistic-like phenotype by affecting key neuronal metabolic pathways.

PRTFDC1 is a genetic modifier of HPRT-deficiency in the mouse

Lesch-Nyhan disease (LND) is a severe X-linked neurological disorder caused by a deficiency of hypoxanthine phosphoribosyltransferase (HPRT). In contrast, HPRT-deficiency in the mouse does not result in the profound phenotypes such as self-injurious behavior observed in humans, and the genetic basis for this phenotypic disparity between HPRT-deficient humans and mice is unknown. To test the hypothesis that HPRT deficiency is modified by the presence/absence of phosphoribosyltransferase domain containing 1 (PRTFDC1), a paralog of HPRT that is a functional gene in humans but an inactivated pseudogene in mice, we created transgenic mice that express human PRTFDC1 in wild-type and HPRT-deficient backgrounds. Male mice expressing PRTFDC1 on either genetic background were viable and fertile. However, the presence of PRTFDC1 in the HPRT-deficient, but not wild-type mice, increased aggression as well as sensitivity to a specific amphetamine-induced stereotypy, both of which are reminiscent of the increased aggressive and self-injurious behavior exhibited by patients with LND. These results demonstrate that PRTFDC1 is a genetic modifier of HPRT-deficiency in the mouse and could therefore have important implications for unraveling the molecular etiology of LND.

Behavioral responses of dopamine beta-hydroxylase knockout mice to modafinil suggest a dual noradrenergic-dopaminergic mechanism of action

Modafinil is approved for use in the treatment of excessive daytime sleepiness. The precise mechanism of modafinil action has not been elucidated, although both dopamine (DA) and norepinephrine (NE) systems have been implicated. To explore the roles of DA and NE in the mechanism of modafinil-induced arousal, dopamine beta-hydroxylase knockout (Dbh -/-) mice were examined in behavioral paradigms of arousal (photobeam breaks and behavioral scoring of sleep latency). Dbh -/- mice completely lack NE but have hypersensitive DA signaling. It was hypothesized that Dbh -/- mice would be unresponsive to modafinil if the compound acts primarily via NE, but would be hypersensitive to modafinil if it acts primarily via DA. Dbh -/- mice had increased sensitivity to the locomotor-activating and wake-promoting effects of modafinil. Paradoxically, the alpha1-adrenergic receptor antagonist, prazosin, attenuated the effects of modafinil in control mice, but not in Dbh -/- mice. Blockade of DA receptors with flupenthixol decreased modafinil-induced locomotion and wake in both control and Dbh -/- mice. These results suggest that both NE and DA are involved in the behavioral effects of modafinil in control mice, but the requirement for NE can be bypassed by hypersensitive DA signaling.

The effects of norepinephrine transporter inactivation on locomotor activity in mice

BACKGROUND

Acute administration of different classes of antidepressants can enhance or reduce spontaneous locomotor activity in a novel environment, but the effects of chronic antidepressant treatment on spontaneous locomotor activity in novel and familiar environments are less well characterized. Because norepinephrine is an important regulator of spontaneous locomotor activity, we speculated that norepinephrine transporter blockade contributes to the effects of some antidepressants on spontaneous locomotor activity.

METHODS

Antidepressant drugs (reboxetine, desipramine, imipramine, venlafaxine, bupropion) were administered acutely (intraperitoneal) or chronically (via osmotic minipump) to control and norepinephrine transporter knockout mice, and spontaneous locomotor activity in novel or familiar environments was recorded.

RESULTS

Acute treatment with most norepinephrine transporter-blocking antidepressants decreased spontaneous locomotor activity in a novel environment, whereas chronic treatment decreased spontaneous locomotor activity in both novel and familiar environments. The exception was bupropion, a dual norepinephrine transporter/dopamine transporter blocker, which tended to increase spontaneous locomotor activity. Coadministration of reboxetine and the dopamine transporter blocker GBR 12909 also increased spontaneous locomotor activity. Norepinephrine transporter knockout mice had low basal spontaneous locomotor activity, which was increased by bupropion, whereas reboxetine had no effect in norepinephrine transporter knockout mice.

CONCLUSIONS

Acute or chronic inactivation of the norepinephrine transporter decreases spontaneous locomotor activity in novel and familiar environments unless coupled with dopamine transporter blockade.

The effects of chronic norepinephrine transporter inactivation on seizure susceptibility in mice

Epilepsy and depression are comorbid disorders, but the mechanisms underlying their relationship have not been identified. Traditionally, many antidepressants have been thought to increase seizure incidence, although this remains controversial, and it is unclear which medications should be used to treat individuals suffering from both epilepsy and depression. Since the neurotransmitter norepinephrine (NE) has both antidepressant and anticonvulsant properties, we speculated that NE transporter (NET) inhibitor antidepressants might be therapeutic candidates for comorbid individuals. To test this idea, we assessed the effects of chronic administration (via osmotic minipump) of the selective NET inhibitor reboxetine on flurothyl-induced seizures in mice. We found that reboxetine had both proconvulsant and anticonvulsant properties; it lowered both seizure threshold and maximal seizure severity. NET knockout (NET KO) mice essentially phenocopied the effects of reboxetine on flurothyl-induced seizures, and the trends were extended to pentylenetetrazole and maximal electroshock seizures (MES). Furthermore, reboxetine had no further effect in NET KO mice, demonstrating the specificity of reboxetine for the NET. We next tested the chronic and acute effects of other classes of antidepressants (desipramine, imipramine, sertraline, bupropion, and venlafaxine) on seizure susceptibility. Only venlafaxine was devoid of proconvulsant activity, and retained some anticonvulsant activity. These results suggest that chronic antidepressant drug treatment has both proconvulsant and anticonvulsant effects, and that venlafaxine is a good candidate for the treatment of epilepsy and depression comorbidity.

Activated platelets induce Weibel-Palade-body secretion and leukocyte rolling in vivo: role of P-selectin

The presence of activated platelets and platelet-leukocyte aggregates in the circulation accompanies major surgical procedures and occurs in several chronic diseases. Recent findings that activated platelets contribute to the inflammatory disease atherosclerosis made us address the question whether activated platelets stimulate normal healthy endothelium. Infusion of activated platelets into young mice led to the formation of transient platelet-leukocyte aggregates and resulted in a several-fold systemic increase in leukocyte rolling 2 to 4 hours after infusion. Rolling returned to baseline levels 7 hours after infusion. Infusion of activated P-selectin-/- platelets did not induce leukocyte rolling, indicating that platelet P-selectin was involved in the endothelial activation. The endothelial activation did not require platelet CD40L. Leukocyte rolling was mediated solely by the interaction of endothelial P-selectin and leukocyte P-selectin glycoprotein ligand 1 (PSGL-1). Endothelial P-selectin is stored with von Willebrand factor (VWF) in Weibel-Palade bodies. The release of Weibel-Palade bodies on infusion of activated platelets was indicated by both elevation of plasma VWF levels and by an increase in the in vivo staining of endothelial P-selectin. We conclude that the presence of activated platelets in circulation promotes acute inflammation by stimulating secretion of Weibel-Palade bodies and P-selectin-mediated leukocyte rolling.

Elevated soluble ICAM-1 levels induce immune deficiency and increase adiposity in mice

Elevated soluble intercellular adhesion molecule-1 (sICAM-1) levels have been found in many pathological conditions, including obesity. To determine the effects of elevated sICAM-1 on immune responses and metabolism, we generated a transgenic mouse model overexpressing the extracellular domain of mouse ICAM-1 in the liver. The mice, showing 10-fold higher sICAM-1 levels than wild-type mice, presented elevated neutrophil count. Despite this, after intraperitoneal injection of thioglycollate, neutrophil recruitment into the peritoneal cavity was reduced, and the delayed macrophage recruitment was also affected in the transgenic mice compared with wild-type mice. Inhibition of contact hypersensitivity response in the sICAM-1 transgenic mice was comparable to ICAM-1-deficient mice and characterized by significantly less ear swelling and inflammatory cell infiltration than in wild-type mice. sICAM-1transgenic mice were more susceptible to weight gain on a Western-type diet than wild-type mice, and older animals showed excessive fat accumulation, again reminiscent of ICAM-1-deficient mice. Together, these data indicate that sICAM-1 interferes with ICAM-1-mediated cell-cell interactions, which could produce immune-suppressant effects and alteration of metabolism in persons with high levels of this soluble adhesion receptor.

Antithrombotic activity of TNF-alpha

Basic and clinical observations suggest that thrombosis and inflammation are closely related. Here we addressed the role played by TNF-alpha in thrombus formation and growth in an in vivo mouse model. Using intravital microscopy, we show that systemic administration of TNF-alpha at doses found in sepsis transiently inhibits thrombus formation and delays arterial occlusion upon vascular injury. These results were reflected in a prolonged bleeding time. Platelets isolated from the TNF-alpha-treated mice showed a marked decrease in fibrinogen binding and P-selectin expression as well as reduced platelet aggregation in response to various agonists. In contrast, in vitro treatment of platelets with TNF-alpha did not affect their function. TNF receptor 1- and 2-deficient mice exhibited normal thrombogenesis in the presence of TNF-alpha. Additionally, the inhibitory effect of TNF-alpha was lost either after treatment with NG-monomethyl-l-arginine, an inhibitor of NO production, or in mice deficient for iNOS. These results indicate that under inflammatory conditions, when leukocytes need free passage to transmigrate into tissues, TNF-alpha decreases platelet activation and inhibits thrombi formation. This effect is not exerted directly on platelets but mediated through the rapid generation of NO in the vessel wall.