Cholecystokinin receptor mechanism(s) and morphine tolerance in mice

Different lipid profiles as constituencies of liquid formula diets do not influence pain perception and the efficacy of opioids in a human model of acute pain and hyperalgesia

Nutritional support and pain control by medication are often used concomitantly, but interactions are hardly investigated. A randomised, double-blind, cross-over study in ten right-handed volunteers was performed evaluating the influence of cholecystokinin (CCK)-excretion on the perception of pain in a standardised model. CCK-excretion was induced by a liquid formula diet with either long- or medium-chain triglycerides (LCT, MCT). Plasma samples were drawn over a 60 min period in 15-min intervals and CCK and somatostatin (SMS) were measured by radioimmunoassay (RIA). Gastric emptying was evaluated by C-13-breath testing. Transcutaneous electrical stimulation at a high current density (5 Hz, 70.1+/-5.8 mA) was used to provoke acute pain and stable areas of secondary mechanical hyperalgesia and pinprick allodynia for 2 h. Ongoing pain ratings as well as extension of pinprick-hyperalgesia and allodynia were compared between both liquid formula diets. In a second series of experiments, alfentanil (4.1+/-0.5 mg) was administered for 90 min using target-controlled infusions and measurements were performed as stated above. Oral administration of LCT as well as MCT may lead to different CCK blood levels, but we found no evidence for CCK-induced effects on pain sensation, touch-evoked allodynia, secondary hyperalgesia or morphine-induced anti-nociception in humans. In our studies, liquid formula diets did not influence acute pain perception or the efficacy of opioids in a human model of pain.

Expression of calcitonin gene-related peptide, substance P and protein kinase C in cultured dorsal root ganglion neurons following chronic exposure to mu, delta and kappa opiates

The mechanisms involved in morphine tolerance are poorly understood. It was reported by our group that calcitonin gene-related peptide (CGRP)-like immunoreactivity (IR) was increased in the spinal dorsal horn during morphine tolerance [Ménard et al. (1996) J. Neurosci. 16, 2342-2351]. More recently, we observed that it was possible to mimic these results in cultured dorsal root ganglion (DRG) neurons allowing for more detailed mechanistic studies [Ma et al. (2000) Neuroscience 99, 529-539]. The aim of the present series of experiments was to further validate the DRG cell culture model by establishing which subtypes of opioid receptors are involved in the induction of CGRP in cultured rat DRG neurons, and to examine the signaling pathway possibly involved in the induction of CGRP-like IR following repeated opiate treatments. Other neuropeptides known to be expressed in DRG neurons, such as substance P (SP), neuropeptide Y (NPY) and galanin, were investigated to assess specificity. Following treatment with any of the three opioid agonists (mu, DAMGO; delta, DPDPE; kappa, U50488H), the number of CGRP- and SP-IR cultured DRG neurons increased significantly, and in a concentration-dependent manner, with the effects of kappa agonist being less pronounced. NPY and galanin were not affected.Double-immunofluorescence staining showed that the three opioid receptors were co-localized with both CGRP- and SP-like IR.Protein kinase C (PKC)-like IR was found to be significantly increased following a repetitive treatment with DAMGO. Double-immunofluorescence staining showed the co-localization of PKCalpha with CGRP- and SP-IR in cultured DRG neurons. Moreover, a combined treatment with DAMGO and a PKC inhibitor (chelerythrine chloride or Gö 6976) was able to block the effects of the opioid on increased CGRP-like IR. These data suggest that the three opioid receptors may be involved in the induction of CGRP and SP observed following chronic exposure to opiates, and that PKC probably plays a role in the signaling pathway leading to the up-regulation of these neuropeptides. These findings further validate the DRG cell culture as a suitable model to study intracellular pathways that govern changes seen following repeated opioid treatments possibly leading to opioid tolerance.

A phase 1 study of the cholecystokinin (CCK) B antagonist L-365,260 in human subjects taking morphine for intractable non-cancer pain

To investigate the safety and tolerability of L-365,260 in human subjects taking morphine for intractable pain. An open label study of nine adult subjects. Two doses of L-365,260 were administered to all subjects separated by a 4 h interval (three received 10 mg, three 30 mg and three 60 mg). Haemodynamic and respiratory variables were recorded from immediately prior to first drug administration to T + 600 min. In addition, continuous electrocardiogram (ECG) monitoring and serial 12 lead ECGs were recorded along with pain and side effect measurements. No major side effects were observed. L-365,260 was well tolerated. No abnormalities in blood pressure, heart rate, respiratory rate or ECG measurements were recorded. Minor side effects were observed. L-365,260 can be safely administered at the doses investigated to human subjects receiving morphine for intractable pain.

Chronic morphine exposure increases the phosphorylation of MAP kinases and the transcription factor CREB in dorsal root ganglion neurons: an in vitro and in vivo study

Tolerance to opiates reduces their effectiveness in the treatment of severe pain. Although the mechanisms are unclear, overactivity of pro-nociceptive systems has been proposed to contribute to this phenomenon. We have reported that the development of morphine tolerance significantly increased calcitonin-gene-related-peptide-like immunoreactivity (CGRP-IR) in primary sensory afferents of the spinal dorsal horn, suggesting that changes in pain-related neuropeptides in the dorsal root ganglion (DRG) neurons may be involved (Menard et al., 1996, J. Neurosci., 16, 2342-2351). Recently, we have shown that repeated morphine treatments induced increases in CGRP- and substance P (SP)-IR in cultured DRG, mimicking the in vivo effects (Ma et al., 2000, Neuroscience, 99, 529-539). In this study, we investigated the intracellular signal transduction pathways possibly involved in morphine-induced increases in CGRP- and SP-IR in DRG neurons. Repeated morphine exposure (10-20 microm) for 6 days increased the number of neurons expressing phosphorylated (p) mitogen-activated protein (MAP) kinases, including the extracellular signal-regulated kinase (pERK), c-jun N-terminal kinase (pJNK) and P38 (pP38 MAPK). The number of neurons expressing phosphorylated cAMP responsive element binding protein (pCREB) was also markedly increased in morphine-exposed cultured DRG neurons. pERK-, pP38-, pJNK- and pCREB-IR were colocalized with CGRP-IR in cultured DRG neurons. Naloxone effectively blocked these actions of morphine, whereas a selective MEK1 inhibitor, PD98059, inhibited the morphine-induced increase in the phosphorylation of ERK and CREB, and the expression of CGRP and SP. Moreover, in morphine-tolerant rats, the number of pCREB-, CGRP- and SP-IR neurons in the lumbar DRG was also significantly increased. These in vitro and in vivo data suggest that the phosphorylation of MAP kinases and CREB plays a role in the morphine-induced increase in spinal CGRP and SP levels in primary sensory afferents, contributing to the development of tolerance to opioid-induced analgesia.

Morphine treatment induced calcitonin gene-related peptide and substance P increases in cultured dorsal root ganglion neurons

The mechanism of spinal tolerance to the analgesic effects of opiates is unclear at present. We have reported previously that calcitonin gene-related peptide-like immunoreactivity was significantly increased in primary afferents of the spinal dorsal horn during the development of morphine tolerance, suggesting that changes in the level of pain-related neuropeptides in dorsal root ganglion neurons may be involved [Menard D. P. et al. (1996) J. Neurosci. 16, 2342-2351]. In this study, we investigated if in vitro treatment with morphine can mimic the in vivo findings and induce increases in calcitonin gene-related peptide-like immunostaining in cultured dorsal root ganglion neurons from young (three-month-old) and middle-aged (10-month-old) adult rats. Following a repetitive exposure to morphine sulfate (1, 5, 10 microM) for six days, the number of calcitonin gene-related peptide- and substance P-immunoreactive neurons in cultured dorsal root ganglia from three- and 10-month-old rats was significantly increased. A lower concentration (0.5 microM) of morphine induced these increases only in dorsal root ganglion neurons from middle-aged rats. Morphine treatment was also found to increase the number of calcitonin gene-related peptide-immunoreactive neurons possessing multiple, long branches (i.e. with at least one branch >0.5mm). This apparent increase in the number of calcitonin gene-related peptide- and substance P-immunoreactive neurons observed following morphine treatment was blocked by naloxone, an opiate antagonist, indicating the involvement of genuine opioid receptors. No significant change in the number of neuropeptide Y- or galanin-immunoreactive neurons in cultured dorsal root ganglia was detected following any of these treatments. These data suggest that repeated exposure to morphine rather selectively increases calcitonin gene-related peptide- and substance P-like immunoreactivity in cultured dorsal root ganglion neurons. Moreover, the sensitivity to morphine-induced changes is greater in cultured dorsal root ganglion neurons from 10- compared to three-month-old rats. Hence, cultured dorsal root ganglion neurons can provide a model to investigate the cellular and molecular mechanisms underlying alterations in neuropeptide levels following repeated exposure to opiates and their relevance to the development of opioid tolerance.