Effects of intrathecal monoamine antagonists and naloxone on the descending inhibition of the spinal transmission of noxious input in rats: study with a new experimental model

ATP-sensitive potassium channels and endogenous adenosine are involved in spinal antinociception produced by locus coeruleus stimulation

The effects of locus coeruleus stimulation on nociceptive evoked discharges of thalamic parafascicular (PF) neurons were investigated in lightly urethane-anesthetized rats, aiming to study the mechanisms underlying these effects. Intrathecal (i.t.) administration of aminophylline (an adenosine antagonist), glibenclamide (an ATP-sensitive potassium [K+(ATP)] channels blocker), nicrorandil (Nico; an agonist of K+(ATP) channel and a K+(ATP) channel opener), and 5'-N-ethylcarboxamido-adenosine (NECA; an adenosine agonist) were used. The results showed that (1) locus coeruleus stimulation significantly inhibited the nociceptive evoked discharges of parafascicular neurons, (2) locus coeruleus stimulation-produced antinociception in PF neurons was blocked by both it. glibenclamide and i.t. aminophylline, (3) nociceptive discharges of PF neurons were also suppressed by both i.t. NECA and i.t. nicorandil, and (4) i.t. glibenclamide showed no effect on the suppression of nociceptive discharges induced by NECA, whereas aminophylline blocked the suppression of nociceptive discharges induced by nicorandil. These results suggest that (a) K+(ATP) channels and endogenous adenosine may be involved in the mediation of antinociception induced by norepinephrine, which is released in the dorsal horn by descending fibers originating from the locus coeruleus and (b) the opening of K+(ATP) channels may precede the release of endogenous adenosine in the process of suppressing nociceptive transmission at the spinal level.

Spinal antinociceptive effect of agmatine and tentative analysis of involved receptors: study in an electrophysiological model of rats

The present study was designed to evaluate the antinociceptive effect of agmatine and the receptors involved at the spinal level by using an experimental model in which agmatine was intrathecally (i.t.) administered while the changes of nociceptively-evoked discharges in thalamic parafascicular (PF) neurons were monitored in anesthetized Wistar rats. The results showed that: (1) i.t. agmatine dose-dependently suppressed the nociceptive discharges of PF neurons induced by the tail pinch; (2) i.t. yohimbine did not block the agmatine-induced suppressive effect of nociceptive discharges in these neurons; and (3) the agmatine-induced suppression could be blocked significantly by i.t. idazoxan. The results suggest that agmatine suppresses the transmission of nociceptive inputs at the spinal level mainly through the activation of imidazoline receptors other than alpha(2)-adrenoceptors.

Inhibition of spinal nociceptive responses after intramuscular injection of capsaicin involves activation of noradrenergic and opioid systems

Extracellular recordings of wide dynamic range neurones in the dorsal horn driven by electrical stimulation of the sciatic nerve were performed in intact urethane-anaesthetized Sprague-Dawley rats. The electrically evoked neuronal responses were defined as A- and C-fibres responses according to latencies, and the effect of a deep nociceptive conditioning stimulus induced by 200 microg capsaicin (8-methyl-N-vanillyl-6-noneamide) injected into the contralateral gastrocnemius-soleus muscle was studied for at least 30 min. Independent of the size and location of the receptive field of the neurone under study, a clear inhibition of the neuronal responses was observed. The electrically evoked C-fibre responses were inhibited to 53% of baseline 15-30 min after injection of capsaicin. This inhibition was only slightly attenuated by 125 nmol of the alpha-adrenoceptor antagonist phentolamine or 250 nmol of the opioid receptor antagonist naloxone applied directly onto the spinal cord when the two compounds were administered separately 5 min before capsaicin. In contrast, when a mixture of the two compounds was given 5 min before capsaicin, the effect of capsaicin was completely abolished. These results indicate that activation of the capsaicin-sensitive afferents in the gastrocnemius-soleus muscle inhibits the electrically evoked C-fibre responses in the dorsal horn by activating noradrenergic and opioidergic inhibitory systems. Moreover, our data indicate that the activation of these two systems following injection of capsaicin has a sub-additive inhibitory effect on the wide dynamic range neurones in the spinal cord. We conclude that only one of these systems is sufficient for the inhibition to occur.

Interrelations of opioids with monoamines in descending inhibition of nociceptive transmission at the spinal level: an immunocytochemical study

This study was designed to reexamine a previous proposal of whether the opioid-like substances (OLS) being acting mainly as an intrinsic spinal mediator in the descending inhibition of nociception of the bulbospinally projecting NE-ergic, and/or 5-HT-ergic terminals in the dorsal horn by using an immunocytochemical method. The effects of intrathecal (i.t.) phentolamine (Ph), cyproheptadine (Cyp), and naloxone (Nal), administered separately or coadministered by two of them, on the expression of Fos-like-immunoreactive (FLI) neurons were observed on both sides of the lumbar dorsal horn of rats, in which equal volumes of formalin were injected into two hindpaws and the ipsilateral dorsolateral funiculus (DLF) was transected at the thoracic level antecedently. The results showed: (1) when rats were pretreated with i.t. saline, the number of nociceptive FLI neurons was significantly lowered 44% (p<0.01) on the side of the lumbar dorsal horn with intact DLF compared to the opposite side with sectioned DLF; (2) when rats were separately pretreated with i.t. Ph, Cyp and Nal, the reduction of FLI neurons on the DLF-intact side were decreased by 27% (p<0.01), 21% (p<0.01), and 25% (p<0.01), respectively; (3) when rats were pretreated with combined i.t. Ph+Cyp, the reduction on the intact side was eliminated almost completely (4%); (4) when rats were pretreated with combined i.t. Ph+Nal, the reduction on the intact side was 21% (p<0.01); and (5) when rats were pretreated with i.t. Cyp+Nal, the reduction on the intact side was 9.1%. These results suggest that: (1) nearly all the suppressive action exerted by the DLF-descending fibers are produced by the release of either NE or 5-HT as neurotransmitters at the spinal level; (2) most of the opioid-like substances act as an intrinsic spinal mediator mainly for the descending NE-ergic, but in a lesser extent for the 5-HT-ergic terminals in the dorsal horn circuitry; and (3) some OLS-ergic interneurons may only be activated by local nociceptive input.

Adenosine mediates spinal norepinephrine-produced antinociception as revealed by nociceptive discharges in parafascicular neurons in rats

The effects of intrathecal pretreatment with aminophylline on intrathecal norepinephrine-produced or serotonin-produced suppression of noxiously evoked discharges in thalamic parafascicular neurons were investigated in 35 urethane-anesthetized rats. The results showed that: (1) both intrathecal norepinephrine (15 nmol) or serotonin (20 nmol) produced significant suppression of noxiously evoked discharges in parafascicular neurons; (2) intrathecal aminophylline (120 nmol) blocked the norepinephrine-produced suppression of noxiously evoked discharges, while the same dose of aminophylline exhibited no significant effect on the serotonin-produced suppression of these discharges in parafascicular neurons. The results suggest that spinal norepinephrine-produced, but not serotonin-produced, antinociceptive effects may be mediated by adenosine as one of successive chemical links in the spinal dorsal horn circuitry.

Effects of intrathecal monoamine antagonists on the nociceptive c-Fos expression in a lesioned rat spinal cord

Effects of intrathecal (i.t.) administration of monoamine antagonists on formalin-induced neuronal c-Fos expression in two sides of the lumbar dorsal horn were observed in rats with unilateral transection of the dorsolateral funiculus at T11-12 level. The results showed that: 1) pretreated with i.t. normal saline (control) and then an equal volume of formalin was injected into the two hindpaws, the number of Fos-like immunoreactive neurons were 44% lower on the side of lumbar dorsal horn with intact dorsolateral funiculus (57 +/- 3.1 vs. 103 +/- 3.8). 2) Pretreatment with i.t. phentolamine (a non-selective alpha-adrenoceptor antagonist) caused an increase of Fos-like immunoreactive neurons on the intact side so showing only a reduction rate of 23% to the lesioned side (p < .01); 3) pretreatment with i.t. cyproheptadine (a 5-HT-receptor antagonist) caused a similar reduction rate of 21% (p < .01) of Fos-like immunoreactive neurons on the intact side; and 4) combined i.t. pretreatment with phentolamine and cyproheptadine caused a reduction of Fos-like immunoreactive neurons of only 4% on the intact side, namely, the differences in the number of Fos-like immunoreactive neurons on two sides of the lumbar spinal cord owing to the unilateral dorsolateral funiculus lesion were nearly abolished by i.t. coinjection of phentolamine and cyproheptadine. The results indicate that 1) peripheral noxious inputs can provoke a spinally-descending inhibitory effect on the spinal nociceptive transmission via the dorsolateral funiculus and 2) the descending fibers in dorsolateral funiculus exert their action mainly through the release of either norepinephrine or 5-HT at the spinal level.

Locus coeruleus stimulation modulates the nociceptive response in parafascicular neurons: an analysis of descending and ascending pathways

The nociceptive responses in parafascicular neurons (PF) were recorded and studied following electrical stimulation of locus coeruleus (LC) combined with intrathecal (IT) or intracerebroventricular (ICV) administration of phentolamine (Ph), an alpha-adrenoceptor antagonist. The results revealed the following. (1) Three different PF neuronal populations were observed according to their response pattern following noxious stimulation: nociceptive-on, nociceptive-off, and nonresponsive units. Only the nociceptive-on units were studied further. (2) The nociceptive discharges in majority of PF neurons (66/87) were inhibited by electrical stimulation of the LC. (3) The inhibitory effect of LC stimulation was prevented and even reversed by pretreatment of IT Ph (40 nmol) in 22 units, or by dorsolateral funiculi transection in 24 units tested. (4) The inhibitory effect of LC stimulation was strengthened by preadministration of ICV Ph (40 nmol) in 17 units tested. (5) ICV administration of norepinephrine (NE 30 nmol) resulted in PF neurons a biphasic response to nociceptive stimulation: an early brief inhibition and a late long-lasting facilitation. (6) Pretreatment of ICV Ph (40 nmol) prior to NE injection prevented the NE-induced biphasic response. The results suggest that stimulation of LC modulates the nociceptive response of PF neurons through both ascending and descending routes. These two diverse routes exert two different effects: a predominantly inhibitory role on the nociceptive transmission at the spinal cord level by descending NE-ergic fibers, and a facilitatory role on the responsiveness of PF to noxious inputs by ascending fibers.

Adenosine and opiate-like substances mediates antinociception at the spinal cord

The effects of intrathecally administered naloxone or aminophylline on the antinociception produced by intrathecal NE, 5-HT, morphine or adenosine receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA) were observed in rats using the tail-flick test. The results show that: (1) the antinociception produced by NE with doses of 0.5 or 1.0 nmol could be completely blocked by both naloxone (240 nmol) and aminophylline (120 nmol); (2) neither naloxone (240 nmol) nor aminophylline (120 nmol) could alter the antinociception produced by 5-HT with doses of 60 or 120 nmol; and (3) the antinociception produced by morphine (0.5 nmol) could be blocked by both naloxone (240 nmol) and aminophylline (120 nmol), while the antinociception by NECA (0.5 nmol) could be blocked only by aminophylline (120 nmol), but not by naloxone (240 nmol). The results suggest that opiate-like substances (OLS) and adenosine are involved in the mediation of the NE-produced antinociception, but not in 5-HT-produced antinociception. Results also suggest that NE, OLS and adenosine may act in a sequential order in the performance of NE-induced antinociception at the spinal level.

Effects of intrathecal naloxone and atropine on the nociceptive suppression induced by norepinephrine and serotonin at the spinal level in rats

The interrelations among norepinephrine (NE), serotonin (5-HT), opiate-like substances (OLS), and acetylcholine (ACh) were investigated by using electrophysiological method combining with intrathecal (i.t.) injection. The results show that: (1) pretreatment with i.t. naloxone (Nal) completely reversed the NE-induced suppression of nociceptive discharges in parafascicular (PF) neurons, but partially reversed that of induced by i.t. 5-HT; (2) pretreatment with i.t. atropine (Atr) completely reversed the suppression induced by either NE or 5-HT. The results suggest that OLS may act as a necessary mediator for NE-induced suppression on the spinal transmission of nociceptive inputs, while it is only partially involved in the 5-HT-induced suppression, and moreover, that endogenous ACh is necessary for the performance of nociceptive suppression induced by either spinal NE or 5-HT administration.

Norepinephrine and serotonin-induced antinociception are blocked by naloxone with different dosages

The effects of intrathecally (IT) administered naloxone (Nal) on the antinociception produced by IT norepinephrine (NE), serotonin (5-HT), or morphine (Mor) were observed and compared in rats using the tail-flick (TF) assay. The results show that: a) NE, 5-HT, and Mor in doses of 1 nmol, 240 nmol, and 0.5 nmol, respectively, produce similar increases in amplitude and time in TF latency (TFL); b) Nal treatment of 240 and 360 nmol has no effects on TFL; c) the antinociception produced by NE (1 nmol) can be blocked by Nal (240 nmol); d) antinociception produced by Mor (0.5 nmol) can also be blocked by Nal (240 nmol); e) 240 nmol of Nal does not affect the 5-HT (120 nmol)-produced antinociception, while 360 nmol of Nal show a delayed blockade to the 5-HT (120 nmol)-produced antinociception. The results suggest that endogenous opiate-like substances may be involved in both NE- or 5-HT-produced antinociception at the spinal level, and these effects may be mediated through different types of opiate receptors.

Morphological evidence for the activation of descending modulatory control by nociceptive afferent pathways: an immunocytochemical study

Immunocytochemical technique was used to compare the content of substance P (SP), Met-enkephalin (Met-Enk) and neurotensin (NT) on two sides of the lumbar dorsal horn of rats in which the unilateral dorsolateral funiculus was transected while formalin (0.2 ml, 5%) was injected equally into two hindpaws. The results showed that the SP-like immunoreactivity (SP-LI) and Met-Enk-LI were significantly higher and the NT-LI was significantly lower in the superficial laminae of dorsal horn on the side ipsilateral to the intact DLF than that on the opposite side, implying that peripheral noxious inputs can activate the supraspinal descending inhibitory systems which in turn modulate the transmission of noxious message at the spinal level by changing the activities of related peptidergic neurons.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.