Preclinical Models of Attention Deficit Hyperactivity Disorder: Neurobiology, Drug Discovery, and Beyond

OBJECTIVE

We offer an overview of ADHD research using mouse models of nicotine exposure.

METHOD

Nicotine exposure of C57BL/6 or Swiss Webster mice occurred during prenatal period only or during the prenatal and the pre-weaning periods. Behavioral, neuroanatomical and neurotransmitter assays were used to investigate neurobiological mechanisms of ADHD and discover candidate ADHD medications.

RESULTS

Our studies show that norbinaltorphimine, a selective kappa opioid receptor antagonist is a candidate novel non-stimulant ADHD treatment and that a combination of methylphenidate and naltrexone has abuse deterrent potential with therapeutic benefits for ADHD. Other studies showed transgenerational transmission of ADHD-associated behavioral traits and demonstrated that interactions between untreated ADHD and repeated mild traumatic brain injury produced behavioral traits not associated with either condition alone.

CONCLUSION

Preclinical models contribute to novel insights into ADHD neurobiology and are valuable tools for drug discovery and translation to benefit humans with ADHD.

Difelikefalin, a peripherally restricted KOR (kappa opioid receptor) agonist, produces diuresis through a central KOR pathway

Difelikefalin is a peripherally restricted kappa opioid receptor (KOR) agonist that was recently approved by the FDA to treat pruritis in dialysis patients. Here, we investigated the cardiovascular and renal responses to difelikefalin, and using the KOR antagonist norbinaltorphimine (norBNI), examined whether any difelikefalin-induced changes in the renal excretion of water and/or electrolytes were mediated through a central or peripheral KOR pathway. The effects of norBNI pretreatment on nalfurafine, a KOR agonist that crosses the blood-brain barrier, were also examined. We hypothesized that difelikefalin would alter urine output differently than nalfurafine, given that KOR agonists produce diuresis via activating central KORs to inhibit vasopressin release. Following catheterization, conscious Sprague-Dawley rats were infused i.v. with isotonic saline and pretreated with norBNI centrally via an intracerebroventricular (ICV) cannula or peripherally via an intravenous catheter. After stabilization, difelikefalin or nalfurafine was administered i.v. and urine output, heart rate and mean arterial pressure (MAP) were recorded for 90 min. Difelikefalin produced a significant increase in urine output, and significant decrease in urinary sodium and potassium excretion, urine osmolality, and MAP. ICV norBNI pretreatment markedly attenuated the increase in urine output caused by difelikefalin and nalfurafine but did not inhibit the electrolyte effects. However, IV norBNI pretreatment prevented all responses to difelikefalin and nalfurafine. Together, these findings demonstrate that difelikefalin and nalfurafine utilize central KOR pathways to elicit diuresis and a decrease in MAP but enhance renal tubular electrolyte reabsorption through a peripheral KOR pathway, providing important insight into two clinically useful KOR agonists.

14-3-3γ mediates the long-term inhibition of peripheral kappa opioid receptor antinociceptive signaling by norbinaltorphimine

Long-term inhibition of kappa opioid receptor (KOR) signaling in peripheral pain-sensing neurons is a potential obstacle for development of peripherally-restricted KOR agonists that produce analgesia. Such a long-term inhibitory mechanism is invoked from activation of c-Jun N-terminal kinase (JNK) that follows a single injection of the KOR antagonist norbinaltorphimine (norBNI). This effect requires protein synthesis of an unknown mediator in peripheral pain-sensing neurons. Using 2D difference gel electrophoresis with tandem mass spectrometry, we have identified that the scaffolding protein 14-3-3γ is upregulated in peripheral sensory neurons following activation of JNK with norBNI. Knockdown of 14-3-3γ by siRNA eliminates the long-term reduction in KOR-mediated cAMP signaling by norBNI in peripheral sensory neurons in culture. Similarly, knockdown of 14-3-3γ in the rat hind paw abolished the norBNI-mediated long-term reduction in peripheral KOR-mediated antinociception. Further, overexpression of 14-3-3γ in KOR expressing CHO cells prevented KOR-mediated inhibition of cAMP signaling. These long-term effects are selective for KOR as heterologous regulation of other receptor systems was not observed. These data suggest that 14-3-3γ is both necessary and sufficient for the long-term inhibition of KOR by norBNI in peripheral sensory neurons.

Kappa Opioid Receptor Expression and Function in Cells of the Immune System

The kappa opioid receptor (KOR) is expressed on a number of hematopoietic cell populations, based on both protein binding analysis and the detection of kappa opioid receptor gene (Oprk1) transcripts. There are prominent Oprk1 splice variants that are expressed in the mouse and human brain cells and leukocytes. The activation of KOR results in reduced antibody production, an inhibition of phagocytic cell activity, an inhibition of T cell development, alterations in the production of various pro-inflammatory cytokines, chemokines, and the receptors for these mediators. Finally, the activation of KOR also leads to the regulation of receptor functional activity of chemokine receptors through the process of heterologous desensitization. The functional activity of KOR is important for the regulation of inflammatory responses and may provide opportunities for the development of therapeutics for the treatment of inflammatory disease states.

Effects of repeated treatment with monoamine-transporter-inhibitor antidepressants on pain-related depression of intracranial self-stimulation in rats

RATIONALE

Synaptic neurotransmission with dopamine (DA), norepinephrine (NE), and serotonin (5-HT) is terminated primarily by reuptake into presynaptic terminals via the DA, NE, and 5-HT transporters (DAT/NET/SERT, respectively). Monoamine transporter inhibitors constitute one class of drugs used to treat both depression and pain, and therapeutic effects by these compounds often require repeated treatment for days or weeks.

OBJECTIVES

The present study compared antinociceptive effects produced by repeated treatment with monoamine transporter inhibitors in a preclinical assay of pain-related depression of positively reinforced operant responding.

METHODS

Adult Sprague-Dawley rats equipped with microelectrodes targeting a brain-reward area responded for pulses of electrical brain stimulation in an intracranial self-stimulation (ICSS) procedure. Intraperitoneal injection of dilute lactic acid served as a noxious stimulus that repeatedly depressed ICSS and also produced weight loss during 7 days of repeated acid administration.

RESULTS

Acid-induced depression of both ICSS and body weight were completely blocked by repeated pretreatment with the nonsteroidal anti-inflammatory drug ketorolac. The DAT-selective inhibitor bupropion also fully blocked acid-induced ICSS depression and weight loss throughout all 7 days of treatment. The NET-selective inhibitor nortriptyline and the SERT-selective inhibitor citalopram were generally less effective, but both drugs blocked acid-induced ICSS depression by the end of the 7-day treatment. Acid-induced depression of ICSS and body weight were not blocked by the kappa opioid receptor (KOR) agonist U69593 or the KOR antagonist norbinaltorphimine.

CONCLUSIONS

These results support effectiveness of bupropion to alleviate signs of pain-related behavioral depression in rats and further suggest that nortriptyline and citalopram produce significant but less reliable effects.