Viljami Jokinen

Ketamine coadministration attenuates morphine tolerance and leads to increased brain concentrations of both drugs in the rat

The effects of ketamine in attenuating morphine tolerance have been suggested to result from a pharmacodynamic interaction. We studied whether ketamine might increase brain morphine concentrations in acute coadministration, in morphine tolerance and morphine withdrawal.

Pregabalin enhances the antinociceptive effect of oxycodone and morphine in thermal models of nociception in the rat without any pharmacokinetic interactions.

Oxycodone is increasingly being used in combination with pregabalin. Pregabalin use is prevalent in opioid‐dependent individuals. A high number of deaths caused by the co‐use of gabapentinoids and opioids occur. It is not known whether pregabalin affects concentrations of oxycodone or morphine in the central nervous system.

Differential Spinal and Supraspinal Activation of Glia in a Rat Model of Morphine Tolerance

Development of tolerance is a well known pharmacological characteristic of opioids and a major clinical problem. In addition to the known neuronal mechanisms of opioid tolerance, activation of glia has emerged as a potentially significant new mechanism. We studied activation of microglia and astrocytes in morphine tolerance and opioid-induced hyperalgesia in rats using immunohistochemistry, flow cytometry and RNA sequencing in spinal- and supraspinal regions. Chronic morphine treatment that induced tolerance and hyperalgesia also increased immunoreactivity of spinal microglia in the dorsal and ventral horns. Flow cytometry demonstrated that morphine treatment increased the proportion of M2-polarized spinal microglia, but failed to impact the number or the proportion of M1-polarized microglia. In the transcriptome of microglial cells isolated from the spinal cord (SC), morphine treatment increased transcripts related to cell activation and defense response. In the studied brain regions, no activation of microglia or astrocytes was detected by immunohistochemistry, except for a decrease in the number of microglial cells in the substantia nigra. In flow cytometry, morphine caused a decrease in the number of microglial cells in the medulla, but otherwise no change was detected for the count or the proportion of M1- and M2-polarized microglia in the medulla or sensory cortex. No evidence for the activation of glia in the brain was seen. Our results suggest that glial activation associated with opioid tolerance and opioid-induced hyperalgesia occurs mainly at the spinal level. The transcriptome data suggest that the microglial activation pattern after chronic morphine treatment has similarities with that of neuropathic pain.

Interactions of (2S,6S;2R,6R)‐Hydroxynorketamine, a Secondary Metabolite of (R,S)‐Ketamine, with Morphine

Ketamine and its primary metabolite norketamine attenuate morphine tolerance by antagonising N‐methyl‐d‐aspartate (NMDA) receptors. Ketamine is extensively metabolized to several other metabolites. The major secondary metabolite (2S,6S;2R,6R)‐hydroxynorketamine (6‐hydroxynorketamine) is not an NMDA antagonist. However, it may modulate nociception through negative allosteric modulation of α7 nicotinic acetylcholine receptors. We studied whether 6‐hydroxynorketamine could affect nociception or the effects of morphine in acute or chronic administration settings. Male Sprague Dawley rats received subcutaneous 6‐hydroxynorketamine or ketamine alone or in combination with morphine, as a cotreatment during induction of morphine tolerance, and after the development of tolerance induced by subcutaneous minipumps administering 9.6 mg morphine daily. Tail flick, hot plate, paw pressure and rotarod tests were used. Brain and serum drug concentrations were quantified with high‐performance liquid chromatography–tandem mass spectrometry. Ketamine (10 mg/kg), but not 6‐hydroxynorketamine (10 and 30 mg/kg), enhanced antinociception and decreased rotarod performance following acute administration either alone or combined with morphine. Ketamine efficiently attenuated morphine tolerance. Acutely administered 6‐hydroxynorketamine increased the brain concentration of morphine (by 60%), and brain and serum concentrations of 6‐hydroxynorketamine were doubled by morphine pre‐treatment. This pharmacokinetic interaction did not, however, lead to altered morphine tolerance. Co‐administration of 6‐hydroxynorketamine 20 mg/kg twice daily did not influence development of morphine tolerance. Even though morphine and 6‐hydroxynorketamine brain concentrations were increased after co‐administration, the pharmacokinetic interaction had no effect on acute morphine nociception or tolerance. These results indicate that 6‐hydroxynorketamine does not have antinociceptive properties or attenuate opioid tolerance in a similar way as ketamine.

A Novel Small Molecule GDNF Receptor RET Agonist, BT13, Promotes Neurite Growth from Sensory Neurons in Vitro and Attenuates Experimental Neuropathy in the Rat

Neuropathic pain caused by nerve damage is a common and severe class of chronic pain. Disease-modifying clinical therapies are needed as current treatments typically provide only symptomatic relief; show varying clinical efficacy; and most have significant adverse effects. One approach is targeting either neurotrophic factors or their receptors that normalize sensory neuron function and stimulate regeneration after nerve damage. Two candidate targets are glial cell line-derived neurotrophic factor (GDNF) and artemin (ARTN), as these GDNF family ligands (GFLs) show efficacy in animal models of neuropathic pain (Boucher et al., 2000; Gardell et al., 2003; Wang et al., 2008, 2014). As these protein ligands have poor drug-like properties and are expensive to produce for clinical use, we screened 18,400 drug-like compounds to develop small molecules that act similarly to GFLs (GDNF mimetics). This screening identified BT13 as a compound that selectively targeted GFL receptor RET to activate downstream signaling cascades. BT13 was similar to NGF and ARTN in selectively promoting neurite outgrowth from the peptidergic class of adult sensory neurons in culture, but was opposite to ARTN in causing neurite elongation without affecting initiation. When administered after spinal nerve ligation in a rat model of neuropathic pain, 20 and 25 mg/kg of BT13 decreased mechanical hypersensitivity and normalized expression of sensory neuron markers in dorsal root ganglia. In control rats, BT13 had no effect on baseline mechanical or thermal sensitivity, motor coordination, or weight gain. Thus, small molecule BT13 selectively activates RET and offers opportunities for developing novel disease-modifying medications to treat neuropathic pain.

Do Diuretics have Antinociceptive Actions: Studies of Spironolactone, Eplerenone, Furosemide and Chlorothiazide, Individually and with Oxycodone and Morphine

Spironolactone, eplerenone, chlorothiazide and furosemide are diuretics that have been suggested to have antinociceptive properties, for example via mineralocorticoid receptor antagonism. In co‐administration, diuretics might enhance the antinociceptive effect of opioids via pharmacodynamic and pharmacokinetic mechanisms. Effects of spironolactone (100 mg/kg, i.p.), eplerenone (100 mg/kg, i.p.), chlorothiazide (50 mg/kg, i.p.) and furosemide (100 mg/kg, i.p.) were studied on acute oxycodone (0.75 mg/kg, s.c.)‐ and morphine (3 mg/kg, s.c.)‐induced antinociception using tail‐flick and hot plate tests in male Sprague Dawley rats. The diuretics were administered 30 min. before the opioids, and behavioural tests were performed 30 and 90 min. after the opioids. Concentrations of oxycodone, morphine and their major metabolites in plasma and brain were quantified by mass spectrometry. In the hot plate test at 30 and 90 min., spironolactone significantly enhanced the antinociceptive effect (% of maximum possible effect) of oxycodone from 10% to 78% and from 0% to 50%, respectively, and that of morphine from 12% to 73% and from 4% to 83%, respectively. The brain oxycodone and morphine concentrations were significantly increased at 30 min. (oxycodone, 46%) and at 90 min. (morphine, 190%). We did not detect any independent antinociceptive effects with the diuretics. Eplerenone and chlorothiazide did not enhance the antinociceptive effect of either opioid. The results suggest that spironolactone enhances the antinociceptive effect of both oxycodone and morphine by increasing their concentrations in the central nervous system.

Novel agonist of GDNF family ligand receptor RET for the treatment of experimental neuropathy

Neuropathic pain is a chronic pain condition caused by lesion or disease affecting the somatosensory system. The glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) alleviate symptoms of NP and stimulate regeneration of sensory neurons in vivo. Here we report the development of the compound BT18 that selectively activates GFLreceptors, alleviates pain and restores damaged dorsal root ganglion (DRG) neurons in rat models of NP. Significance statement Neuropathic pain (NP) is a chronic syndrome caused by different diseases and lesions affecting nervous system. Earlier studies demonstrated that neurotrophic factors - the glial cell line-derived neurotrophic factor (GDNF) and artemin - could reverse the damage done by lesions in animal models of NP. We demonstrate for the first time that a small molecule can activate receptor of GDNF and artemin, it alleviates pain symptoms in vivo in two animal models of NP and restores to normal the molecular markers expressed in sensory neurons. This compound, termed BT18, can pave way for creating novel disease modifying therapies for NP.

The mineralocorticoid receptor antagonist spironolactone enhances morphine antinociception

Abstract Aims Spironolactone, an antimineralocorticoid, has been reported to potentiate the cataleptic effect of morphine in the rat. Since no previous research exists on the matter and the interaction might be clinically significant, the effects of spironolactone on morphine antinociception and pharmacokinetics in the rat were investigated. Methods Male SD rats were used to assess the effects of spironolactone on acute morphine-induced antinociception, development of morphine tolerance, and established morphine tolerance in the tail-flick and hot plate tests. Spironolactone was also administered with loperamide to assess whether spironolactone enhances the brain distribution of the acknowledged P-glycoprotein substrate across the blood-brain barrier. Results Spironolactone had no antinociceptive effects of its own but when co-administrated with morphine the antinociceptive effect of morphine was greatly enhanced. Morphine concentrations in the brain were increased fourfold in the spironolactone co-administrated group. Spironolactone did not inhibit the formation of pro-nociceptive morphine-3-glucuronide, nor did inhibit the development of tolerance. The peripherally restricted opioid, loperamide, had no antinociceptive effects by itself, but co-administration with spironolactone produced a clear change in the hot plate test. Conclusions Although mineralocorticoids have been proposed to take part in pain signaling, in our setting spironolactone did not have antinociceptive properties of its own. The increased antinociceptive effect of morphine is apparently caused by the increased morphine brain concentrations. We suggest this to be due to P-glycoprotein inhibition, as indicated by the loperamide assay. The clinical relevance of P-glycoprotein inhibition by spironolactone should be studied.