Coaxing Delta Opioid Receptors into Action for Pain Relief
Daniel J. Shiwarski, Alycia Tipton, Melissa D. Giraldo, Brigitte F. Schmidt, Michael S. Gold, et al.
(see pages 3741–3752)
Opioid drugs and our brain's endogenous opioids work primarily at mu-type opioid receptors (μR) to dampen pain signals. But because the μR is expressed throughout the brain and body, μR agonists can cause side effects ranging from constipation to addiction. The related delta-type opioid receptor (δR), in contrast, has more limited expression, but it is found in nociceptive sensory neurons, making it an attractive target to produce pain relief with lower abuse potential. Researchers have been stymied in this effort, however, because effective pain relief requires extremely high doses of δR agonists for reasons that remain mysterious. Past studies have hinted that the receptor may be located inside sensory neurons with little availability at the surface.
Shiwarski et al. delved into how receptor trafficking to the cell surface might be controlled in sensory neurons by focusing on the phosphatidylinositol 3 kinase (PI3K) and phosphatase and tensin (PTEN) pathways, which figure prominently in neuronal differentiation and trafficking. They hypothesized that the balance of activity between the two pathways might control δR surface availability, with PI3K promoting membrane delivery and PTEN favoring intracellular retention. In heterologous PC12 cells expressing functional fluorescent δR, receptors are found on the cell surface, but treatment with nerve growth factor led to intracellular accumulation of δR. PTEN blockers prevented this accumulation and increased surface δR. Similarly, PTEN inhibition in neurons cultured from mouse trigeminal ganglia caused redistribution of δR to the cell surface. The Gi-coupled receptors were functional according to an assay that measured their inhibition of cAMP.
To test whether PTEN increased availability of functional endogenous δRs, the team measured calcium current inhibition in response to a δR agonist. While the magnitude of current inhibition remained steady, more trigeminal cells responded to the agonist following pretreatment with PTEN. In vivo, PTEN inhibition also increased trafficking of endogenous δR to the cell surface in trigeminal neurons. Finally, in a mouse model of chronic inflammatory pain, PTEN inhibition reversed mechanical pain hypersensitivity that was prevented by a δR-selective antagonist. The authors conclude that increasing δR availability at the sensory neuron surface may be a viable strategy to target δRs for pain relief.
Febrile Seizure Defects Rely on Neuronal Silencing Factor
Katelin P. Patterson, Jeremy M. Barry, Megan M. Curran, Akanksha Singh-Taylor, Gary Brennan, et al.
(see pages 3799–3812)
Febrile seizures, the most common childhood seizure, can cause lifelong memory and learning impairments, but how remains unknown. Patterson, Barry et al. used experimental febrile status epilepticus (eFSE) in rats to probe the neural underpinnings of seizure-related structural and functional changes. Neuron restrictive silencing factor (NRSF), a transcription factor, suppresses expression of certain genes during neuronal development and in mature neurons. Previous work has shown that NRSF underlies functional changes in hippocampal networks following eFSE.
NRSF binds the hcn1 gene that encodes the hyperpolarization-activated cyclic nucleotide-gated channel 1 (HCN1), which is expressed in the hippocampal formation and is reduced after eFSE. The researchers designed decoy oligodeoxynucleotides (ODN) containing the NRSF-hcn1 binding site sequence to sequester NRSF and render it inactive. ODNs with a scrambled sequence served as a control. NRSF binding to hcn1 chromatin three hours after eFSE was increased significantly in rats that received the scrambled ODN compared to those that got the NRSF-sequestering ODN, and the decoy ODN prevented eFSE-induced suppression of hcn1.
To measure cognitive deficits, rats underwent a spatial memory task that placed them in a rotating arena, one quadrant of which delivered a shock. One and two days after eFSE, rats that received the scrambled ODN spent more time in the shock quadrant, received more shocks, and spent less time in the quadrant opposite the shock compartment compared to rats that did not receive eFSE; the learning deficit persisted a month later. The result indicates that rats failed to learn which quadrant delivered a shock. The rats that received the NRSF-blocking ODN, however, were indistinguishable from rats that did not undergo eFSE, suggesting that NRSF activity underlies the memory defect.
To measure neuronal activity, the researchers recorded oscillations from the hippocampal CA1 region that are associated with spatial learning using implanted electrode arrays. Theta rhythms and slow and fast gamma oscillation kinetics were disrupted in rats that underwent eFSE, but not those receiving the decoy ODN.
Examination of post-mortem tissue revealed that eFSE caused aberrant synaptogenesis of excitatory synapses at granule cells. The results depict disturbances in brain development caused by eFSE that depend on NRSF gene suppression and suggest that interfering with NRSF activity following seizure could provide protection against lasting cognitive deficits.
This Week in The Journal was written by Stephani Sutherland, Ph.D.