diprenorphine and Neuralgia

The Federal Pain Research Strategy recommended development of nonopioid analgesics as a top priority in its strategic plan to address the significant public health crisis and individual burden of chronic pain faced by >100 million Americans. Motivated by this challenge, a natural product extracts library was screened and identified a plant extract that targets activity of voltage-gated calcium channels. This profile is of interest as a potential treatment for neuropathic pain. The active extract derived from the desert lavender plant native to southwestern United States, when subjected to bioassay-guided fractionation, afforded 3 compounds identified as pentacyclic triterpenoids, betulinic acid (BA), oleanolic acid, and ursolic acid. Betulinic acid inhibited depolarization-evoked calcium influx in dorsal root ganglion (DRG) neurons predominantly through targeting low-voltage-gated (Cav3 or T-type) and CaV2.2 (N-type) calcium channels. Voltage-clamp electrophysiology experiments revealed a reduction of Ca, but not Na, currents in sensory neurons after BA exposure. Betulinic acid inhibited spontaneous excitatory postsynaptic currents and depolarization-evoked release of calcitonin gene-related peptide from lumbar spinal cord slices. Notably, BA did not engage human mu, delta, or kappa opioid receptors. Intrathecal administration of BA reversed mechanical allodynia in rat models of chemotherapy-induced peripheral neuropathy and HIV-associated peripheral sensory neuropathy as well as a mouse model of partial sciatic nerve ligation without effects on locomotion. The broad-spectrum biological and medicinal properties reported, including anti-HIV and anticancer activities of BA and its derivatives, position this plant-derived small molecule natural product as a potential nonopioid therapy for management of chronic pain.

Topics: Animals; Anti-Inflammatory Agents, Non-Steroidal; Betulinic Acid; Calcium Channels, N-Type; Calcium Channels, T-Type; CHO Cells; Cricetulus; Diprenorphine; Disease Models, Animal; Female; Ganglia, Spinal; HIV Infections; Inhibitory Postsynaptic Potentials; Male; Mice; Mice, Inbred C57BL; Neuralgia; Neurons; Paclitaxel; Pentacyclic Triterpenes; Peripheral Nerve Injuries; Rats; Rats, Sprague-Dawley; Triterpenes; Tritium

This study used positron emission tomography (PET) and [11C]diprenorphine to compare the in vivo distribution abnormalities of brain opioid receptors (OR) in patients with peripheral (n=7) and central post-stroke pain (CPSP, n=8), matched for intensity and duration. Compared with age- and sex-matched controls, peripheral neuropathic pain (NP) patients showed bilateral and symmetrical OR binding decrease, while in CPSP binding decrease predominated in the hemisphere contralateral to pain. In CPSP patients, interhemispheric comparison demonstrated a significant decrease in opioid binding in posterior midbrain, medial thalamus and the insular, temporal and prefrontal cortices contralateral to the painful side. Peripheral NP patients did not show any lateralised decrease in opioid binding. Direct comparison between the central and peripheral groups confirmed a significant OR decrease in CPSP, contralateral to pain. While bilateral binding decrease in both NP groups may reflect endogenous opioid release secondary to chronic pain, the more important and lateralised decrease specific to CPSP suggests opioid receptor loss or inactivation in receptor-bearing neurons. Opioid binding decrease was much more extensive than brain anatomical lesions, and was not co-localised with them; metabolic depression (diaschisis) and/or degeneration of OR neurons-bearing secondary to central lesions appears therefore as a likely mechanism. Central and peripheral forms of NP may differ in distribution of brain opioid system changes and this in turn might underlie their different sensitivity to opiates.

Topics: Adult; Aged; Biological Availability; Brain; Carbon Radioisotopes; Central Nervous System Diseases; Diprenorphine; Female; Humans; Male; Middle Aged; Neuralgia; Peripheral Nervous System Diseases; Positron-Emission Tomography; Radiopharmaceuticals; Receptors, Opioid; Tissue Distribution