Interactions between cholecystokinin and opioids in the isolated guinea-pig ileum

A1-adenosine acute withdrawal response and cholecystokinin-8 induced contractures are regulated by Ca(2+)- and ATP-activated K(+) channels

In isolated guinea-pig ileum (GPI), the A1-adenosine acute withdrawal response is under the control of several neuronal signalling systems, including the μ/κ-opioid and the cannabinoid CB1 systems. It is now well established that after the stimulation of the A1-adenosine system, the indirect activation of both μ/κ-opioid and CB1 systems is prevented by the peptide cholecystokinin-8 (CCk-8). In the present study, we have investigated the involvement of the Ca(2+)/ATP-activated K(+) channels in the regulation of both acute A1-withdrawal and CCk-8-induced contractures in the GPI preparation. Interestingly, we found that: (a) the A1-withdrawal contracture is inhibited by voltage dependent Ca(2+)-activated K(+) channels, Kv, while it is enhanced by the voltage independent Ca(2+)-activated K(+) channels, SKCa; (b) in the presence of CCk-8, the inhibitory effect of the A1 agonist, CPA, on the peptide induced contracture is significantly enhanced by the voltage independent Ca(2+)-activated K(+) channel, SKCa; and (c) the A1-withdrawal contracture precipitated in the presence of CCk-8 is controlled by the ATP-sensitive potassium channels, KATP. Our data suggest, for the first time, that both Ca(2+)- and ATP-activated K(+) channels are involved in the regulation of both A1-withdrawal precipitated and CCk-8 induced contractures.

The NOP receptor involvement in both withdrawal- and CCk-8-induced contracture responses of guinea pig isolated ileum after acute activation of κ-opioid receptor

In isolated guinea-pig ileum (GPI), the κ-opioid acute withdrawal response is under the control of several neuronal signaling systems, including the μ-opioid, the A(1)-adenosine and the CB(1) receptors, which are involved in the inhibitory control of the κ-withdrawal response. After κ-opioid system stimulation, indirect activation of μ-opioid, A(1)-adenosine and CB(1) systems is prevented by the peptide cholecystokinin-8 (CCk-8). In the present study, we have investigated whether the NOP system is also involved in the regulation of the acute κ-withdrawal response. Interestingly, we found that in GPI preparation, the NOP system is not indirectly activated by the κ-opioid receptor stimulation, but instead this system is able by itself to directly regulate the acute κ-withdrawal response. Specifically, our results clearly highlight first the existence of an endogenous tone of the NOP system in GPI, and second that it behaves as a functional anti-opioid system. We also found that, the NOP receptor system is involved in the regulation of the CCk-8-induced contracture intensity, only when in the presence of the κ-opioid receptor stimulation. This effect seems to be regulated by an activation threshold mechanism. In conclusion, the NOP system could act as neuromodulatory system, whose action is strictly related to the modulation of both excitatory and inhibitory neurotransmitters released in GPI enteric nervous system.

Biphasic regulation of the acute μ-withdrawal and CCk-8 contracture responses by the ORL-1 system in guinea pig ileum

The cloning of the opioid-receptor-like receptor (ORL-1) and the identification of the orphaninFQ/nociceptin (OFQ/N) as its endogenous agonist has revealed a new G-protein-coupled receptor signalling system. The structural and functional homology of ORL-1 to the opioid receptor systems has posed a number of challenges in the understanding the often competing physiological responses elicited by these G-protein-coupled receptors. We had previously shown that in guinea pig ileum (GPI), the acute μ-withdrawal response is under the inhibitory control of several systems. Specifically, we found that the exposure to a μ-opioid receptor agonist activates indirectly the κ-opioid, the A(1)-adenosine and the cannabinoid CB(1) systems, that in turn inhibit the withdrawal response. The indirect activation of these systems is prevented by the peptide cholecystokinin-8 (CCk-8). In the present study, we have investigated whether the ORL-1 system is also involved in the regulation of the acute μ-withdrawal response. Interestingly, we found that in GPI preparation, the ORL-1 system is not indirectly activated by the μ-opioid receptor stimulation, but instead the system is able by itself to directly regulate the acute μ-withdrawal response. Moreover, we have demonstrated that the ORL-1 system behaves both as anti-opioid or opioid-like system based on the level of activation. The same behaviour has also been observed in presence of CCk-8. Furthermore, in GPI, the existence of an endogenous tone of the ORL-1 system has been demonstrated. We concluded that the ORL-1 system acts as a neuromodulatory system, whose action is strictly related to the modulation of excitatory neurotrasmitters released in GPI enteric nervous system.

Inhibitory Control of the Acute Mu-Withdrawal Response by Indirectly Activated Adenosine A1 and Kappa-Opioid Systems in the Guinea-Pig Ileum; Reversal by Cholecystokinin

In the isolated guinea-pig ileum (GPI), the acute mu-opioid withdrawal response is inhibited by the kappa-opioid system, indirectly activated by the opioid agonist; yet, other inhibitory mechanisms are probably operating. On the other hand, cholecystokinin (CCK-8) strongly enhances the withdrawal response. In this study, we have shown that the adenosine A1 antagonist 8-cyclopenthyl-1,3-dimethylxantine (CPT) increased the withdrawal response in dermorphin/naloxone (NLX) tests but lacked any effect if the withdrawal tests were carried out in presence of CCK-8. In tissue preparations coming from a same animal both CPT and the kappa-opioid antagonist, nor-binaltorphimine (BNI), increased the intensity of the withdrawal responses; the effects of the two antagonists were additive. The intensity of withdrawal contractile responses in presence of CCK-8 was similar to those obtained in presence of the two antagonists. Tissue preparations tested with dermorphin/CCK-8/NLX and then washed out yielded contractile responses when subsequently challenged with CPT, BNI or BNI+CPT, with a percentage markedly higher than the percentage of the response to NLX challenge. BNI+CPT also increased the intensity of the response to NLX challenge. These data suggest that acute exposure of GPI to dermorphin induces the activation of both the adenosine A1 and kappa-opioid systems, which in turns inhibit the mu-withdrawal response. CCK-8 antagonises the inhibitory effect of the indirectly activated systems.

Involvement of the Cannabinoid CB1 Receptor in the Opioid Inhibition of the Response to Cholecystokinin and Acute Withdrawal Response

Numerous recent studies have reported major functional interactions between cannabinoid and opioid systems. These interactions can be studied in the myenteric plexus-longitudinal muscle isolated preparations. We had previously shown that in the guinea-pig ileum (GPI), the opioid acute withdrawal response is under the inhibitory control of several systems; mu-opioid agonist exposure indirectly activates the kappa-opioid system; conversely, exposure to a kappa-opioid agonist indirectly activates the mu-system; the indirectly activated opioid system inhibits the withdrawal response. The adenosine A1 system is also indirectly activated by opioids and it inhibits the withdrawal response. We had also shown that indirect activation is prevented or antagonized by cholecystokinin (CCK-8). In GPI preparations briefly exposed to the mu-agonist, dermorphine (DERM) and then challenged with naloxone (NL), the cannabinoid CB1 antagonist, SR141716 (SR), increased the withdrawal responses to NL, but only did so in presence of a kappa-opioid and an adenosine A(1) antagonist. Under similar experimental conditions, SR also enhances the kappa-opioid withdrawal response. In opioid agonist/CCK-8/NL tests, SR antagonized the inhibition of the tissue response to CCK-8 induced by the mu- or kappa-opioid agonist and increased the kappa-withdrawal response, but not the mu-withdrawal response. However, the dose-response curve against dermorphine inhibition of the response to CCK-8 was bell-shaped and the highest SR concentration also significantly decreased the mu-withdrawal response. In preparations exposed to dermorphine or to the kappa-agonist, U-50,488H, the cannabinoid agonist WIN 55,212-2 increased the opioid-induced inhibition of the tissue response to CCK-8 and decreased the NL-induced responses. These results show that opioid exposure may also activate the cannabinoid CB1 system, which leads to an inhibition of the opioid acute withdrawal response. This phenomenon and the antagonistic effect of SR on the opioid-induced inhibition of the response to CCK-8 suggest that reciprocal interaction between opioid and cannabinoid systems are operating in the enteric nervous system.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.