Intraspinal release of substance P and calcitonin gene-related peptide during opiate dependence and withdrawal

Cardiovascular and behavioural effects induced by naloxone-precipitated morphine withdrawal in rat: characterization with tachykinin antagonists

This study examined the intracerebroventricular (i.c.v.) effects of three selective tachykinin receptor antagonists on the cardiovascular and behavioural responses induced by naloxone-precipitated morphine withdrawal in rats. I.c.v. injection of naloxone (10 microg) to morphine pre-treated rats (i.c.v. for 5 days) induced an immediate increase in blood pressure ( approximately 10 mmHg) and behavioural activity (sniffing > rearing > face washing approximately grooming approximately wet dog shake) without causing significant heart rate changes. The prior i.c.v. injection of the NK(1) receptor antagonist (6.5 nmol LY306740) reduced face washing and grooming during morphine withdrawal. NK(2) and NK(3) receptor antagonists (6.5 nmol SR48968 and R820) did not affect behavioural effects, yet the co-injection of the three tachykinin antagonists reduced all behavioural activity. The pressor response was not affected by the selective inhibition of NK(1) and NK(3) receptors while both blood pressure and heart rate were markedly enhanced by SR48968 during morphine withdrawal. The potentiating effect of SR48968 was prevented following simultaneous blockade of the three tachykinin receptors. In addition to confirming the involvement of central tachykinins in behavioural manifestations to morphine withdrawal, data suggest a modulatory function for tachykinins, especially the NK(2) receptor, in brain autonomic control of blood pressure and heart rate in supraspinal noloxone-precipitated withdrawal.

Release of immunoreactive brain-derived neurotrophic factor in the spinal cord of the rat following sciatic nerve transection

Using the antibody microprobe method, the sites of spinal release of immunoreactive brain-derived neurotrophic factor (BDNF) was studied in normal rats, and rats with prior sciatic nerve transection. In normal rats, a significant basal release of immunoreactive BDNF was found in the superficial dorsal horn. Following sciatic nerve transection (performed 14 days previously), release of BDNF was found throughout the whole of the dorsal horn, extending into deeper laminae. Electrical stimulation of the ipsilateral sciatic nerve at a strength adequate to excite either A fibres (20 Hz at 2x threshold voltage) or A and C fibres (2 Hz at 20x threshold voltage) did not alter the basal release of immunoreactive BDNF in normal or in nerve-injured rats. The results suggest that BDNF is released from the central terminals of primary afferent fibres, but such release is not solely dependent upon action potential invasion of these terminals. The increased extent of release following nerve transection is consistent with the hypothesis that BDNF plays a role in the central response to peripheral nerve injury.

Polyarthritis-associated changes in the opioid control of spinal CGRP release in the rat

As a model of chronic inflammatory pain, Freund's adjuvant-induced polyarthritis has been shown to be associated with marked alterations in the activity of opioid- and calcitonin gene-related peptide (CGRP)-containing neurons in the dorsal horn of the spinal cord in rats. Possible changes in the interactions between these two peptidergic systems in chronic inflammatory pain were investigated by comparing the effects of various opioid receptor ligands on the spinal outflow of CGRP-like material (CGRPLM) in polyarthritic and age-paired control rats. Intrathecal perfusion of an artificial cerebrospinal fluid in halothane-anaesthetized animals allowed the collection of CGRPLM released from the spinal cord and the application of opioid receptor ligands. The blockade of kappa-opioid receptors similarly increased CGRPLM release in both groups of rats as expected of a kappa-mediated tonic inhibitory control of CGRP-containing fibres in control, as well as in polyarthritic rats. In contrast, the higher increase in CGRPLM outflow due to the preferential blockade of mu opioid receptors by naloxone in polyarthritic rats as compared to non-suffering animals supports the idea of a reinforced mu opioid receptor-mediated tonic inhibitory control of CGRP-containing fibres in rats suffering from chronic pain. Even more strikingly, the differences observed in the effects of delta-opioid receptor ligands on CGRPLM outflow suggest that delta receptors are functionally shifted from a participation in a phasic excitatory control in non-suffering rats to a tonic inhibitory control in polyarthritic rats. These data indicate that agonists acting at the three types of opioid receptors all exert a tonic inhibitory influence on CGRP-containing nociceptive primary afferent fibres within the spinal cord of polyarthritic rats. Such a convergence probably explains why morphine and other opioids are especially potent to reduce pain in subjects suffering from chronic inflammatory diseases.

Opioids modulate the calcitonin gene-related peptide8-37-mediated hindpaw withdrawal latency increase in thermally injured rats

The present study was performed to explore the modulatory potential of different endogenous opioid systems on transmission of presumed nociceptive information at the spinal cord level in thermally injured rats. Thermal injury was performed by dipping the left paw into water 60 degrees C for 20 s. This induced a significant bilateral decrease in hindpaw withdrawal latency HWL to pressure. Intrathecal administration of 10 nmol of CGRP8-37 induced a significant bilateral increase in HWL in the thermally injured group and in the intact controls. The effect of different opioid receptor antagonists on the increased latency to withdrawal response induced by intrathecal injection of 10 nmol of CGRP8-37 was explored in the thermally injured rats. The effect was reversed by intrathecal injection of 40 and 80 nmol of: b-funaltrexamine (mu opioid receptor antagonist) and naltrindole (delta opioid receptor antagonist), but not by norbinaltorphimine (kappa opioid receptor antagonist). The results of the present study show that intrathecal CGRP8-37 increases hindpaw withdrawal latency in thermally injured rats, an effect reduced by a mu as well as by a delta opioid receptor antagonist.

A novel technique for producing antibody-coated microprobes using a thiol-terminal silane and a heterobifunctional crosslinker

Antibody-coated microprobes are used to measure neuropeptide release in the central nervous system. Although they are not quantitative, they provide the most precise spatial resolution of the location of in vivo release of any currently available method. Previous methods of coating antibody microprobes are difficult and time-consuming. Moreover, using these methods we were unable to produce evenly coated antibody microprobes. This paper describes a novel method for the production of antibody microprobes using thiol-terminal silanes and the heterobifunctional crosslinker, 4-(4-N-maleimidophenyl)butyric acid hydrazide HCl 1/2 dioxane (MPBH). Following silation, glass micropipettes are incubated with antibody to substance P (SP) that has been conjugated to MPBH. This method results in a dense, even coating of antibody without decreasing the biological activity of the antibody. Additionally, this method takes considerably less time than previously described methods without sacrificing the use of antibody microprobes as micropipettes. The sensitivity of the microprobes for SP is in the picomolar range, and there is a linear correlation between the log of SP concentration (M) and B/B0 (r2 = 0.98). The microprobes are stable for up to 3 weeks when stored in 0.1 M sodium phosphate buffer with 50 mM NaCl (pH 7.4) at 5 degrees C. Finally, insertion into the exposed spinal cord of an anesthetized rat for 15 min produces no damage to the antibody coating.

Opioid antagonists naloxone, beta-funaltrexamine and naltrindole, but not nor-binaltorphimine, reverse the increased hindpaw withdrawal latency in rats induced by intrathecal administration of the calcitonin gene-related peptide antagonist CGRP8-37

We recently demonstrated that intrathecal administration of calcitonin gene-related peptide 8-37 (CGRP8-37), a selective antagonist of calcitonin gene-related peptide receptors, dose-dependently increased the latency to hindpaw withdrawal responses induced by both thermal and mechanical stimulation in intact rats, indicating a role for CGRP and its receptors in the transmission of presumed nociceptive information in the spinal cord. The present study was performed to explore the interaction between CGRP and opioids in the spinal cord of rats. The effects of naloxone, a non-selective opioid receptor antagonist, and three different selective opioid receptor antagonists on the increased latency to withdrawal response induced by intrathecal injection of CGRP8-37 were explored. Intrathecal administration of 10 nmol of CGRP8-37 induced a significant bilateral increase in hindpaw withdrawal latency to both thermal and mechanical stimulation. The effect was partly reversed by intrathecal injection of 4 or 8 micrograms of naloxone, 10 nmol of either the mu opioid receptor antagonist beta-funaltrexamine or the delta opioid receptor antagonist naltrindole, but not by 10 nmol of the kappa opioid receptor antagonist nor-binaltorphimine. These results indicate that mu and delta, but not kappa, opioid receptors are involved in the modulation of post-synaptic effects and/or release of CGRP and other neurotransmitters.

RP 67580, a selective antagonist of neurokinin-1 receptors, modifies some of the naloxone-precipitated morphine withdrawal signs in rats

In order to clarify the participation of substance P in the expression of opiate withdrawal, we have investigated the effects induced by the new selective neurokinin-1 antagonist RP 67580 on naloxone-induced morphine withdrawal syndrome in rats. Intracerebroventricular administration of RP 67580 elicited a decrease in 7 of the 13 withdrawal signs evaluated. Mastication, salivation and signs related to the motor component of withdrawal (jumping, rearing and locomotor activity) were particularly reduced. One sign, wet dog shakes, was increased, but it was also enhanced by the inactive enantiomer RP 68651. Our results indicate that blockade of NK1 receptors induces a decrease in the expression of naloxone-precipitated morphine abstinence in rats, and support the participation of substance P in the opiate withdrawal response.

Opioid control of the release of calcitonin gene-related peptide-like material from the rat spinal cord in vivo

The possible control by opioids of the spinal release of calcitonin gene-related peptide-like material (CGRPLM) was investigated in halothane-anaesthetized rats whose intrathecal space was perfused with an artificial cerebrospinal fluid. Morphine (20 mg/kg i.v.; or at 10-100 microM added to the perfusing fluid), the mu selective agonist DAGO (10 microM) and the kappa selective agonist U 50488 H (10 microM) did not affect the spontaneous outflow of the CGRPLM. In contrast, the selective delta agonist DTLET (10 microM) significantly increased CGRPLM release. The latter effect could be prevented by the selective delta antagonist naltrindole (10 microM) as expected from the involvement of this class of opioid receptors. However, the addition of naltrindole alone to the perfusing fluid did not modify CGRPLM outflow, indicating that endogenous opioids do not exert a tonic control of CGRP-containing fibers through the stimulation of delta receptors. In contrast, intrathecal perfusion with naloxone (10 microM) or nor-binaltorphimine (10 microM), a selective antagonist of kappa receptors, produced a marked increase in spinal CGRPLM release, suggesting that endogenous opioids acting at mu and kappa receptors, respectively, exert a tonic inhibitory control of CGRP-containing fibers. Indeed, a significant decrease in the spinal release of CGRPLM release could be evoked by the combined addition of U 50488 H (10 microM) plus DAGO (10 microM) to the perfusing medium, indicating that the simultaneous stimulation of both kappa and mu receptors is required for this negative control to occur. This could notably be achieved with morphine (10 microM) in the presence of naltrindole (10 microM) which also produced a significant reduction in the spinal release of CGRPLM. In conclusion, morphine per se did not change CGRPLM release because this drug triggers opposite positive (through the stimulation of delta receptors) and negative (through the concomitant stimulation of both kappa and mu receptors) control mechanisms within the rat spinal cord.

Morphine-induced modulation of calcitonin gene-related peptide levels

Calcitonin gene-related peptide (CGRP) is a novel calcium-modulatory product of the gene that encodes for calcitonin. Acute administration of morphine decreases levels of CGRP in rat corpus striatum. Tolerance to morphine did not alter the levels of CGRP in any brain region or in the spinal cord of the rat. CGRP did not alter the tolerance to the antinociceptive effects of morphine. Chronic naltrexone increased the levels of CGRP in the hypothalamus. Concurrent chronic administration of naltrexone plus morphine raised the levels of CGRP in the medulla, midbrain, and spinal cord. CGRP enhances naloxone-precipitated withdrawal jumping in mice. In rats, during withdrawal the levels of CGRP were tripled in the corpus striatum and significantly reduced in the hippocampus and hypothalamus. In the corpus striatum, CGRP enhances forskolin-stimulated cyclic adenosine monophosphate (cAMP) accumulation when such accumulation is suppressed (as with the chronic opiate administration), but conversely depresses forskolin-stimulated cAMP accumulation under normal conditions (as with chronic vehicle administration). These data are consistent with the hypothesis that CGRP acts as a modulatory peptide in opiate-sensitive systems and tonic opioid control of CGRP levels exists in brain.