Original Research, Animal Studies, Basic Neurobiology, Pharmacology/Drug Development, Method

Developing potent non-opioid pain medications is an integral part of the battle to conquer both chronic pain and the current opioid crisis. Although most screening approaches use in vitro surrogate targets, in vivo screening of analgesic candidates is a necessary pre-clinical step in drug discovery. Here, we report the design of a new automated behavioral testing apparatus based on the principle of a thermal place preference test (TPPT). This new design can detect, quantify, and differentiate behavioral responses to cold stimuli between sham and chronic constriction injury (CCI) rodents with up to 12 animals tested simultaneously. At an optimized temperature pair of 12.5°C vs 30.0°C (± 0.5℃), the TPPT design has captured the antinociceptive effects of morphine and pregabalin on CCI rats in individual 10-min tests. Moreover, it can differentiate analgesic effects by morphine or pregabalin from anxiolytic effects by diazepam. The results, along with the relatively low cost to construct the apparatus and moderately high throughput, make our TPPT design applicable for behavioral studies of chronic pain in rodents and for high-throughput in vivo screening of the next generation of pain medications.

Both in vivo neuropharmacology and optogenetic stimulation can be used to decode neural circuitry, and can provide therapeutic strategies for brain disorders. However, current neuronal interfaces hinder long-term studies in awake and freely behaving animals, as they are limited in their ability to provide simultaneous and prolonged delivery of multiple drugs, are often bulky and lack multifunctionality, and employ custom control systems with insufficiently versatile selectivity for output mode, animal selection and target brain circuits. Here, we describe smartphone-controlled, minimally invasive, soft optofluidic probes with replaceable plug-like drug cartridges for chronic in vivo pharmacology and optogenetics with selective manipulation of brain circuits. We demonstrate the use of the probes for the control of the locomotor activity of mice for over four weeks via programmable wireless drug delivery and photostimulation. Owing to their ability to deliver both drugs and photopharmacology into the brain repeatedly over long time periods, the probes may contribute to uncovering the basis of neuropsychiatric diseases.

Optopharmacology is a very promising approach based on the use of light-deliverable drugs, which allows manipulating physiological processes with high spatiotemporal resolution. Light-dependent drugs (i.e. caged-compounds) targeting G protein-coupled receptors (GPCRs) have been developed to provide great pharmacological precision on the control of pain. Metabotropic glutamate type 5 (mGlu) receptors are widely expressed through the pain neuraxis and play a key role in pain transmission. In line with this, selective mGlu receptor negative allosteric modulators (NAMs) have consistently shown analgesic activity in experimental animal models of inflammatory pain. Accordingly, we synthesized a light-deliverable drug (i.e. caged compound) using the chemical structure of raseglurant, a mGlu receptor NAM, as a molecular scaffold. And thereafter, we evaluated the analgesic activity of the caged compound in formalin-injected (hind paw) mice upon light irradiation (405 nm). Of note, light was both delivered at the peripheral (i.e. hind paw) and central level (i.e. thalamus), by means of brain-implanted fiber-optics. The novel light-deliverable drug, JF-NP-26, showed antinociceptive activity upon violet light irradiation either of the hind paw or the thalamus, demonstrating the ability of precisely activating, in time and space, the caged compound. Here, we describe in detail the protocol used to perform a reliable and reproducible formalin nociception test in mice using an optopharmacology approach (i.e. light-deliverable compounds).