Subrata Basu Ray

Mu opioid receptors in developing human spinal cord.

The distribution of mu opioid receptors was studied in human fetal spinal cords between 12–13 and 24–25 wk gestational ages. Autoradiographic localisation using [3H] DAMGO revealed the presence of mu receptors in the dorsal horn at all age groups with a higher density in the superficial laminae (I–II). A biphasic expression was noted. Receptor density increased in the dorsal horn, including the superficial laminae, between 12–13 and 16–17 wk. This could be associated with a spurt in neurogenesis. The density increased again at 24–25 wk in laminae I–II which resembled the adult pattern of distribution. A dramatic proliferation of cells was noted from the region of the ventricular zone between 16–17 and 24–25 wk. These were considered to be glial cells from their histological features. Mu receptor expression was noted over a large area of the spinal cord including the lateral funiculus at 24–25 wk. This may be due to receptor expression by glial cells. The study presents evidence of mu receptor expression by both neurons and glia during early development of human spinal cord.

Differential expression of L- and N-type voltage-sensitive calcium channels in the spinal cord of morphine+nimodipine treated rats.

We have earlier reported that nifedipine and nimodipine, both L-type voltage-sensitive calcium channel (L-VSCC) antagonists, attenuate the development of tolerance to chronic administration of morphine in the rat. In the present study, we have investigated the expression of L- and N-type VSCC using immunohisto-chemistry, in the cervical region of the spinal cords from animals treated chronically with morphine alone or in combination with nimodipine. The highest expression of both VSCCs within the spinal cord was detected within the superficial laminae of the dorsal horn, which indicates that these channels play an important role in the spinal processing of pain. After morphine tolerance, the expression of both the channels in the superficial laminae was significantly higher than control animals. However, morphine+nimodipine administration produced a differential effect, that is, the expression of L-VSCC decreased while that of N-VSCC increased. The study shows that the expression of these channels is plastic and subject to change depending upon the drug administered. This in turn can determine overall responsiveness to morphine.

Up-regulation of Μ-opioid receptors in the spinal cord of morphine-tolerant rats

Though morphine remains the most powerful drug for treating pain, its effectiveness is limited by the development of tolerance and dependence. The mechanism underlying development of tolerance to morphine is still poorly understood. One of the factors could be an alteration in the number of Μ-receptors within specific parts of the nervous system. However, reports on changes in the Μ-opioid receptor density in the spinal cord after chronic morphine administration are conflicting. Most of the studies have used subcutaneously implanted morphine pellets to produce tolerance. However, it does not simulate clinical conditions, where it is more common to administer morphine at intervals, either by injections or orally. In the present study, rats were made tolerant to morphine by injecting increasing doses of morphine (10-50 mg/kg, subcutaneously) for five days.In vitro tissue autoradiography for localization of Μ-receptor in the spinal cord was done using [3H]-DAMGO. As compared to the spinal cord of control rats, the spinal cord of tolerant rats showed an 18.8% increase or up-regulation in the density of Μ-receptors in the superficial layers of the dorsal horn. This up-regulation of Μ-receptors after morphine tolerance suggests that a fraction of the receptors have been rendered desensitized, which in turn could lead to tolerance

Antinociceptive effect of intrathecal loperamide: Role of mu-opioid receptor and calcium channels

Morphine is a gold standard analgesic commonly used to alleviate pain. However, its use is associated with unavoidable side effects including the risk for addiction. Peripherally administered loperamide lacks effect on the central nervous system as it is a substrate for the permeability glycoprotein (P-gp) efflux pump which blocks its entry into brain. However, when administered intrathecally, loperamide has been reported to produce analgesia. The present study investigates the mechanism of the central analgesic effect of loperamide. Adult male Sprague-Dawley rats were subjected to surgery for catheter placement. Following baseline testing, different groups of rats were administered fixed intrathecal doses (1 μg, 3 μg, 10 μg and 30 μg) of loperamide and morphine. Analgesia was compared employing Hargreaves paw withdrawal apparatus at 15 min, 30 min, 60 min, 90 min and 120 min. Additionally, CTOP, a specific mu-opioid receptor antagonist was co-administered with loperamide to examine the mu-opioid receptor mediated loperamide analgesia. Furthermore, nefiracetam, a calcium channel opener, was co-administered with loperamide or morphine to evaluate the involvement of Ca(2+) channels in Loperamide showed an analgesic effect which was comparable to morphine. However, loperamide produced longer analgesia and the analgesic effect was significantly better at 42 h and 49 h compared to morphine. CTOP completely reversed loperamide analgesia. Though nefiracetam significantly reversed loperamide analgesia, it did not have any effect on morphine induced analgesia. Our findings suggest that loperamide administered intrathecally produces analgesia which is mediated through mu-opioid receptor and subsequent blockade of downstream calcium channels.

The enigma of morphine tolerance: recent insights.

Pain has been described as a necessary evil. Necessary because it is a protective reflex and evil since lives have been made miserable due to it. Morphine and its related alkaloids are still the most effective analgesics in a physician’s armoury for treating pain. However, many physicians hesitate to use opioids because of side effects – the primary reason being the development of tolerance and dependence on opioids. Other, equally detrimental effects are inhibition of gastrointestinal motility and respiratory depression. However, a general consensus among physicians has emerged that opioids – particularly long-acting opioids – should form the mainstay of treatment of chronic pain of both cancerous and noncancerous etiology (McCarberg and Barkin 2001).

Enhanced analgesic effect of morphine-nimodipine combination after intraspinal administration as compared to systemic administration in mice

Calcium plays an important role in the pathophysiology of pain. A number of studies have investigated the effect of L-type calcium channel blockers on the analgesic response of morphine. However, the results are conflicting. In the present study, the antinociceptive effect of morphine (2–5 Μg) and nimodipine (1 Μg) co-administered intraspinally in mice was observed using the tail flick test. It was compared to the analgesic effect of these drugs (morphine — 250 Μg subcutaneously; nimodipine — 100 Μg intraperitoneally) after systemic administration. Nimodipine is highly lipophilic and readily crosses the blood brain barrier. Addition of nimodipine to morphine potentiated the analgesic response of the latter when administered through the intraspinal route but not when administered through systemic route. It may be due to direct inhibitory effect of morphine and nimodipine on neurons of superficial laminae of the spinal cord after binding to Μ-opioid receptors and L-type calcium channels respectively.

Role of Somatostatin and Somatostatin Receptor type 2 in Postincisional Nociception in Rats

Somatostatin (SST) and the somatostatin receptor type 2 (sstr2) are expressed in the superficial part (Laminae I-III) of the dorsal horn of the spinal cord. Since the neurons in these laminae also receive nociceptive sensation from the periphery, it was hypothesized that both SST and sstr2 could be involved in the modulation of nociceptive transmission. To the best of knowledge, there are no studies on the involvement of SST and sstr2 in hind paw incision model in rats, which mimics postoperative pain in humans. Sprague-Dawley rats were subjected to hind paw incision under isoflurane anaesthesia and the resulting mechanical allodynia and thermal hyperalgesia were evaluated for 5 days. In another set of animals, the spinal cord was isolated at specified time intervals after incision and examined for SST and sstr2 expression using immunohistochemistry and immunoblotting procedures. Finally, nociceptive parameters were again evaluated in incised rats, which had received SST (400 µg/kg i.p. three times per day). Blood glucose level and locomotor activity were determined after SST treatment. Both allodynia and hyperalgesia were highest immediately after incision. Spinal SST expression increased at 2 h. A further increase was noted on day 3. Expression of sstr2 increased initially but decreased at day 1. These changes could be due to exocytosis of SST and internalization of the ligand-receptor complex. SST injection significantly attenuated mechanical allodynia but not thermal hyperalgesia. Significant change in blood glucose level or locomotor activity was absent. SST appears to contribute to postincisional pain. This finding could be of clinical relevance.

Locally mediated analgesic effect of bradykinin type 2 receptor antagonist HOE 140 during acute inflammatory pain in rats.

Opioids like morphine form the mainstay of treatment for moderate to severe burn pain. However, lack of dedicated burn care service and potentially serious side effects of opioids often compromise effective treatment. Newer drugs as well as newer routes of administration of analgesic drugs are long-felt needs in the management of burn pain. Bradykinin is a potent inflammatory mediator present at sites of tissue damage. The present study investigated the analgesic effect of bradykinin type 2 receptor antagonist HOE 140 after direct intrawound administration in rats. Also, whether the analgesic effect was locally mediated was further evaluated. Tissue damage was produced by a surgical incision involving skin, fascia, and muscle. It has been reported that there are minor differences in inflammatory mediators underlying incision-related and burn injury–related pain. HOE 140 (1, 3, or 10 &mgr;g/10 &mgr;l physiological saline) was administered into the wound by a sterile micropipette. After an interval of 30 seconds, the wound was closed. HOE 140-induced analgesic effect was compared to other experimental groups of rats which did not receive any drug or those which were treated with either saline (vehicle) or water. Postincisional pain was determined by monitoring behavior, allodynia, and thermal hyperalgesia. Analgesic effect was also determined after drug administration in contralateral paw. HOE 140 (1, 3, 10 &mgr;g) significantly relieved mechanical allodynia and guarding in comparison with vehicle-treated group. The analgesic effect of HOE 140 was locally mediated. Healing of the wound was normal. In conclusion, the results suggest that bradykinin type 2 receptor antagonists such as HOE 140 could be useful in the treatment of acute inflammatory pain.

Role of neurokinin type 1 receptor in nociception at the periphery and the spinal level in the rat

Objectives:Noxious stimuli activate small to medium-sized dorsal root ganglion (DRG) neurons. Intense noxious stimuli result in the release of substance P (SP) from the central terminals of these neurons. It binds to the neurokinin type 1 receptor (NK1r) and sensitises the dorsal horn neurons. SP is also released from the peripheral terminals leading to neurogenic inflammation. However, their individual contribution at spinal and peripheral levels to postincisional nociception has not been delineated as yet.Methods:Sprague–Dawley rats were administered different doses (3–100 μg) of an NK1r antagonist (L760735) by intrathecal (i.t.) route before hind paw incision. On the basis of its antinociceptive effect on guarding behaviour, the 30 μg dose was selected for further study. In different sets of animals, this was administered i.t. (postemptive) and intrawound (i.w.). Finally, in another group, drug (30 μg) was administered through both i.t and i.w. routes. The antinociceptive effect was assessed and compared. Expression of SP was examined in the spinal cord. Intrawound concentration of SP and inflammatory mediators was also evaluated.Results:Postemptive i.t. administration significantly attenuated guarding and allodynia. Guarding was alone decreased after i.w. drug treatment. Combined drug administration further attenuated all nociceptive parameters, more so after postemptive treatment. Expression of SP in the spinal cord decreased post incision but increased in the paw tissue. Inflammatory mediators like the nerve growth factor also increased after incision.Conclusion:In conclusion, SP acting through the NK1r appears to be an important mediator of nociception, more so at the spinal level. These findings could have clinical relevance.

Nimodipine potentiates the analgesic effect of morphine in the rat hot-plate test: Implications in the treatment of pain.

abscess caused by Proteus penneri in a neutropenic host. J Clin Microbiol 1990;28:1645-6. 6. Latuszynski DK, Schoch P, Qadir MT, Cunha BA. Proteus penneri urosepsis in a patient with diabetes. Heart Lung 1998;27:146-8. 7. Liassine N, Labia R, Metral C, Ninet B, Madec S, Peron M, et al. Two Isolates of Proteus penneri, one ceftriaxone-susceptible and one ceftriaxone-resistant, isolated from a patient treated by ceftriaxone, produce the same beta-lactamase but with different levels of production. Abstr Intersci Conf Antimicrob Agents Chemother 2000;40:100.