Concurrent measurement of substance P and serotonin in spinal superfusates: failure of capsaicin and p-chloroamphetamine to co-release

This study examined the efflux of endogenous serotonin and substance P-like immunoreactive material into in vivo superfusates of the rat spinal cord under quiescent conditions and during the addition of either capsaicin or p-chloroamphetamine to the superfusate. Using a method which permitted concurrent measurement of serotonin and substance P-like immunoreactive material in the same sample of superfusate, it was found that capsaicin increased the efflux of substance P-like immunoreactive material from the spinal cord but did not alter the efflux of serotonin. Conversely, the addition of p-chloroamphetamine caused the efflux of serotonin from the spinal cord to increase, but did not affect the efflux of substance P-like immunoreactive material.

Spinal cord monoamines modulate the antinociceptive effects of vaginal stimulation in rats

Perispinal administration (into the lumbar intrathecal space) of phentolamine (40 micrograms), an alpha-adrenergic receptor blocking agent, reduced the analgesic effect of vaginal stimulation by 39.7% (measured by tail flick latency) and 57.1% (measured by vocalization threshold) as compared to controls. Perispinal administration of methysergide (10 micrograms), a serotoninergic receptor blocking agent, reduced the analgesic effect of vaginal stimulation by 48.5% (measured by vocalization threshold), although it did not significantly affect the tail flick measure. In a separate experiment, vaginal stimulation activated the release of norepinephrine and serotonin into a superfusate of the spinal cord. During vaginal stimulation, levels of norepinephrine and serotonin increased about 2-fold above resting levels. These findings indicate that vaginal stimulation releases norepinephrine and serotonin into the spinal cord, thereby exerting an analgesic effect.

Studies on the release by somatic stimulation from rat and cat spinal cord of active materials which displace dihydromorphine in an opiate-binding assay

Using the spinal superfusion procedure, in anesthetized rats and cats, the presence of active factors which displace dihydromorphine in brain opiate binding studies, has been observed. Separation of this activity on a Sephadex G-10 column reveals the presence of two fractions which occur before (Fraction I) and after (Fraction II) the salt peak which account for over 70% of the observed dihydromorphine-displacing activity. The ratio of activity in Fraction II/Fraction I is 33 and 21, in the resting spinal perfusates of the rat and cat, respectively. High intensity, bilateral stimulation of the sciatic nerve in cats, results in a 30- and 5.4-fold increase in the levels of Fraction I and Fraction II, respectively, over pre-stimulation levels. In rat, bilateral stimulation of the hind paws, resulted in a frequency-dependent increase in the levels of Fraction I (1.9- and 3.2-fold at 5 and 50 Hz, respectively). Dynorphin 1-13 fragment elutes at least partly in Fraction I. With regard to Fraction II, the peak co-chromatographs with hexapeptide derivatives of enkephalin. Met- and Leu-enkephalin (Fraction III), elute off the column at a point where opiate receptor displacing activity is relatively small. Electrophoretic separation of Fraction I radioreceptor activity of alkaline and acid pH on agarose columns revealed two principle peaks which co-migrated with alpha-neoendorphin and dynorphin 1-13. Fraction II activity appeared primarily in a single peak which was isographic with enkephalin hexapeptides. Using radioimmunoassays, detectable levels of dynorphin and Met-enkephalin were observed and sciatic nerve stimulation resulted in significant increases. Neither column-coupled radioreceptor assays nor radioimmunoassays revealed the presence of beta-endorphin. The present experiments demonstrate the releasability by high intensity somatic stimulation of a variety of opioid peptides present in spinal terminals. Significantly, however, the majority of this activity appears to be found in fractions different from those of the pentapeptide enkephalins.

Drug-induced alterations in the efflux of 5-hydroxytryptamine and of 5-hydroxyindoleacetic acid into superfusates of the rat spinal cord

The effect of dl-p-chloroamphetamine, fluoxetine and probenecid on the efflux of endogenous 5-hydroxytryptamine (5HT) and 5-hydroxyindoleacetic acid (5HIAA) into superfusates of the spinal cord of anesthetized rats was examined. Mean basal efflux of 5HT and 5HIAA was 0.27 and 15.56 ng/ml superfusate, respectively. The addition of dl-p-chloroamphetamine to the superfusate produced a dose-dependent increase in the efflux of 5HT into the superfusate, but did not increase the efflux of 5HIAA. Probenecid (200 mg/kg i.p.) increased the basal efflux of both 5HT and 5HIAA as compared to control values. When administered systemically, fluoxetine (10 mg/kg i.p. or s.c.) decreased the basal efflux of 5HIAA, but did not alter the basal efflux of 5HT as compared to control values. In contrast, when administered in the superfusate, fluoxetine produced a dose-dependent increase in the basal efflux of 5HT, but did not alter the basal efflux of 5HIAA. The release of 5HT produced by the addition of 2.5 X 10(-4) M dl-p-chloroamphetamine to the superfusate was not prevented in rats pretreated systemically with fluoxetine, although when administered in the superfusate fluoxetine inhibited the dl-p-chloroamphetamine-induced release of 5HT.

Effects on nociceptive threshold and blood pressure of intrathecally administered morphine and alpha-adrenergic agonists

In the present experiments, the time course of the analgesic response (tail flick in rats, skin twitch in cat) and blood pressure effects of intrathecally administered morphine and the alpha-agonists, clonidine and ST-91 were examined in rats and cats. The clonidine analogue, ST-91 has approximately 1/50 the lipid solubility of clonidine. Intrathecally administered morphine and morphine followed by naloxone had no effect on resting blood pressure in unanaesthetized cats and in rats and cats anaesthetized with chloralose-urethane. Inactive when given systemically, ST-91 had a powerful antinociceptive effect when given intrathecally. At analgesic doses, this alpha-agonist produced only a transient hypertension. At doses 4-6 times that required to produce a significant antinociceptive effect, a pressor effect was observed. Intrathecal injection of clonidine in smaller doses produced a transient hypotension, with significant pressor effects present with larger doses.

Selective antagonism of the antinociceptive effect of intrathecally applied alpha adrenergic agonists by intrathecal prazosin and intrathecal yohimbine

We have attempted to define the alpha adrenoceptor subtype(s) on which intrathecally applied alpha adrenergic agonists act to produce their antinociceptive effect. The potencies of intrathecal (i.t.) prazosin or i.t. yohimbine to antagonize the elevations of thermal nociceptive threshold induced by i.t. 2-[2,6-diethylphenylamino]-2-imidazoline (ST-91), methoxamine or norepinephrine (NE) were determined in the rat. Tail-flick and hot plate tests were used to determine thermal nociceptive threshold. At the ID50 level, the alpha-2 selective antagonist yohimbine was significantly more potent than the alpha-1 selective antagonist prazosin at blocking the analgesia produced by the alpha-2 selective agonist ST-91, whereas prazosin was significantly more potent than yohimbine at antagonizing the analgesia produced by the alpha-1 selective agonist methoxamine or by the nonselective alpha agonist NE. Analgesic doses of methoxamine evoked a readily observable disturbance of motor and autonomic function, whereas such effects were not observed after analgesic doses of ST-91 or NE. Both i.t. methoxamine and i.t. ST-91 elevated thermal nociceptive threshold in rats depleted of lumbar spinal cord NE by pretreatment 7 days before with i.t. 6-hydroxydopamine. Our results suggest that stimulation of either one of two separate populations of postsynaptic spinal alpha adrenoceptors will inhibit spinal nociceptive transmission. One of these populations appears to be composed of alpha-2 adrenoceptors. The subtype classification of the alpha adrenoceptors composing the other population remains unclear.

Effect of intrathecal capsaicin analogues on the immunofluorescence of peptides and serotonin in the dorsal horn in rats

The intrathecal administration of capsaicin, a homovanillylamide derivative, has been demonstrated to cause analgesia in response to thermal stimuli. This analgesia has been correlated with a profound depletion of spinal substance P, a putative primary afferent transmitter. We studied the effects of capsaicin, a series of capsaicin analogues, piperine and kainic acid on the immunohistochemical staining of substance P, cholecystokinin, somatostatin, methionine-enkephalin and serotonin. Capsaicin and an analogue 1-nonenoyl-vanillylamide significantly elevated the tail flick latency and when the spinal cords of the rats were analyzed immunohistochemically, a profound depletion of substance P and cholecystokinin was observed. The spinal somatostatin-immunoreactivity of these rats was slightly reduced. Piperine also depleted substance P and reduced somatostatin staining but did not alter the staining intensity or density of cholecystokinin, methionine-enkephalin or serotonin. Kainate-depleted methionine-enkephalin but did not alter any other neuropeptides studied or serotonin. These results may indicate a link between capsaicin-induced analgesia and the concomitant depletion of cholecystokinin and substance P.

Opioid activity released from cat spinal cord by sciatic nerve stimulation

Spinal superfusion was performed in anesthetized cats before and during sciatic nerve stimulation. The superfusates were fractionated on Sephadex G-10 columns and thereafter on electrophoresis and HPLC. The endorphin activity was monitored by radioreceptor and radioimmunoassays. In additional experiments, chromatographic fractions were subjected to enzymatic digestion prior to radioimmunoassay. Nerve stimulation caused a release of at least three different endorphins which separated on electrophoresis, one of which comigrated with [Met]enkephalin-Lys6. The identity of this peptide was further supported by HPLC analysis and radioimmunoassay. Furthermore, enzymatic degradation experiments provided evidence for the presence of enkephalin sequences in all three components released by stimulation. There were also increased dynorphin concentrations during stimulation. These findings suggest that at least two different endorphin systems (enkephalin and dynorphin) are functionally present in spinal cord and may be activated by somatic stimulation.

Differential association of spinal mu, delta and kappa opioid receptors with cutaneous thermal and visceral chemical nociceptive stimuli in the rat

The intrathecal administration of several mu (morphine), delta (d-ala2-d-leu5-enkephalin, dimeric leu-enkephalin) and mixed mu/delta (beta-endorphin) agonists produced a dose-dependent inhibition of all cutaneous thermal (Tail Flick/Hot Plate) nociceptive responses in the rat. The kappa agonist U50488H had no analgesic potency in thermal nociceptive tests. On a visceral chemical test (writhing) and agonists exerted a powerful suppression of the response. In contrast at doses 10 to 50 times the ED50 on cutaneous thermal tests, delta agonists had no effect on the writhing response. At higher intrathecal doses, delta ligands produced flaccidity. These observations suggest the existence of three discriminable populations of opioid receptors in the spinal cord whose activation has different effects on the animals response to noxious stimuli.

The analgetic effects of an intrathecally administered partial opiate agonist, nalbuphine hydrochloride

Nalbuphine (0.1-200 micrograms), unlike morphine (0.1-10 micrograms) administered intrathecally had no effect on the tail flick or hot plate response latencies. In contrast, both intrathecal nalbuphine and morphine inhibited, in a monotonic, dose dependent fashion, the writhing evoked by intraperitoneally administered acetic acid (ED50 = 38 nmol and 1.12 nmol, respectively.) The effects of intrathecal nalbuphine and morphine was antagonized by an equal dose of naloxone administered systemically. Co-intrathecal administration of morphine and nalbuphine revealed that a maximum inhibition of writhing could be obtained with low doses of either drug, while the effects of higher doses of either drug were attenuated as compared to the effects produced by the high dose of either drug alone. These data are consistent with the suggestion that nalbuphine exerts its agonistic effect through a mechanism that is pharmacologically distinct from that of morphine, and the likelihood of two populations of opioid receptors associated with the pain response evoked by thermal and visceral afferents, respectively is considered.

Opiate-mediated inhibition of the release of cholecystokinin and substance P, but not neurotensin from cat hypothalamic slices

The neuroactive peptides neurotensin (NT), substance P (SP) and cholecystokinin (CCK) have been shown to be distributed in the hypothalamus. These peptides may be part of hypothalamic mechanisms which regulate the release of pituitary hormones and feeding behavior. Numerous experiments have demonstrated opiate modulation of anterior pituitary hormone release. These effects have been reported to be mediated via a hypothalamic mechanism, which modulates the secretion of releasing, release inhibiting factors or other neuroactive peptides such as SP, CCK and NT. We have examined the effects of morphine on the potassium-stimulated, calcium-dependent release of SP, CCK and NT from cat hypothalamic slices. The potassium-stimulated release of SP and CCK was profoundly depressed by the addition of morphine (10(-5) M) in a naloxone-reversible manner. This morphine inhibition was shown to be stereospecific, levorphanol (10(-7) M) depressed the release, while dextrophan (10(-7) M) was inactive. Gel filtration chromatography of the potassium-stimulated release was determined to be isographic with authentic NT, SP and CCK-8, respectively. There was no indication of any gastrin-like activity. These data may suggest a regulatory mechanism through which opiates exert some of their neuroendocrine or feeding regulatory effects.

In vivo evidence for multiple opiate receptors mediating analgesia in the rat spinal cord

In rats implanted with chronic catheters in the spinal subarachnoid space, intrathecal injections of SKF 10047 and dynorphin did not produce any elevation of the nociceptive threshold as defined by hot-plate and tail-flick tests. In contrast, intrathecal ethylketocyclazocine (EKC) and (D-Ala2,D-Leu5)-enkephalin (DADL) administration resulted in a dose-dependent antinociceptive effect which was reversible with intraperitoneal naloxone. Calculation of the Schild dose-ratio plots for the data derived from systemically administered naloxone reveals a slope of--1 and a calculated pA2 value of 6.8 for EKC and 6.2 for DADL. Also, animals made tolerant to systemic morphine showed a diminished analgesic response to intrathecal morphine and EKC when compared to naive animals. There was, however, no significant change in the dose response curve of intrathecal DADL. Thus, these experiments suggest that in addition to mu receptors a separate subpopulation of delta but not kappa or sigma receptors are involved with spinally mediated analgesia.

Agonist regulation of muscarinic acetylcholine receptors in rat spinal cord

In vitro studies with cultured cells originating from nervous tissue have shown that chronic exposure to muscarinic agonists results in a loss of muscarinic receptors. To determine whether this type of regulation of muscarinic receptor number also occurs in vivo, we infused carbachol into the spinal cords of rats. A single carbachol injection into the lumbar spinal cord caused a significant increase in the nociceptive threshold. This effect of carbachol diminished to control levels after 12 h of repeated agonist injections every 4 h and was blocked by atropine. The desensitization to the antinociceptive effects of carbachol was associated with a loss of muscarinic receptors as determined by the binding of the muscarinic antagonist [3H]quinuclidinyl benzilate. After a 24-h exposure to carbachol given every 4h, there was about a 60% loss of binding sites. The loss of muscarinic receptors was also blocked by atropine and was reversible. These results represent direct evidence that a muscarinic agonist can regulate receptor number in the central nervous system and suggest that this loss of receptors is associated with a desensitization to the antinociceptive effects of carbachol injected into the spinal cord.

Histamine H1 receptors in the brain and spinal cord of the cat

The regional distribution of histamine H1 receptors in the feline brain and spinal cord was determined in vitro using the radioactively labeled histamine H1 antagonist, [3H]pyrilamine. This distribution of H1 receptors, which was different from that reported for other species, was highest in the hypothalamus and mammillary bodies. Intermediate levels of binding were observed in the cerebral cortical and limbic regions, corpus striatum, colliculi, cerebellum, and medulla. The lowest binding was found in the pons and spinal cord. Binding in the spinal cord was concentrated in the gray matter, but the number of binding sites detected in the dorsal and ventral horns did not differ. The equilibrium dissociation constants for [3H]pyrilamine were similar for the various regions and were in the range of 2--3 nM and the pharmacological characteristics of the feline brain and spinal H1 receptors were similar to those found in the brains of other mammalian species with the exception of the guinea pig.

Origin and distribution of cholecystokinin-containing nerve terminals in the lumbar dorsal horn and nucleus caudalis of the cat

Immunohistochemical bridge methods were used to localize cholecystokinin (CCK)-like immunoreactivity in the lumbar dorsal horn (DH) and nucleus caudalis (NC) of the cat. The CCK-positive structures were either dot- or fiber-like. The distribution of CCK-like immunoreactivity in the DH and NC was narrower than substance P (SP)-like immunoreactivity in adjacent sections. CCK- and SP-like immunoreactivity in the DH and NC was severely depleted 7--10 days following rhizotomy of either the dorsal roots or the 5th, 9th and 10th cranial nerves, respectively; enkephalin-like immunoreactivity in adjacent sections was unaffected. Cervical hemisection had no effect on either CCK- or SP-like immunoreactivity in the DH.

Studies on the location and release of cholecystokinin and vasoactive intestinal peptide in rat and cat spinal cord

By radioimmunoassay vasoactive intestinal peptide (VIP) and cholecystokinin (CCK) are found in the cat lumbar spinal ganglion and spinal cord with levels in dorsal greater than ventral horn. Unilateral rhizotomy, but not cervical hemisection produced a significant but incomplete depletion of CCK and VIP immunoreactivity in dorsal, but not ventral horn. Intrathecal capsaicin (0.5 mg) had no effect on the levels of spinal VIP or CCK. Intrathecal colchicine (0.5 mg)produced a significant increase in the levels of VIP in the dorsal and ventral horn but had no effect on the levels of CCK. The present experiments, using a preparation which permits in situ superfusion of the spinal cord, demonstrated in the chloralose-urethanized cat and rat the presence of measurable levels of VIP and CCK. In rats, the addition of potassium (40 mM in excess) resulted in a 138% and 46% increase in the levels of CCK and VIP, respectively above resting levels (3.7 +/- 1.2 fmol/ml/10 min and 1.7 +/- 0.5 fmol/ml/10 min, respectively). The deletion of calcium and substitution of cobalt (2 mM) resulted in a significant reduction in the potassium-evoked release. Intrathecal picrotoxin doubled the levels of CCK, but had no effect on the levels of VIP in the spinal superfusates. Capsaicin (3 X 10(-4) M) had no effect on the levels of either peptide in rat spinal superfusate. In cats, bilateral electrical stimulation of the sciatic nerve at high, but not low intensity, resulted in a 218% and 132% increase above prestimulation baseline in the levels of CCK and VIP, respectively. Separation of immunoreactivity on a Sephadex G-50 superfine column of the spinal superfusates and the extracted material from cat spinal cord, revealed that the immunoreactive CCK species in tissue co-migrated with the 8 and 33 amino acid peptide fragments. In the release samples, however, all the radioimmunoassayable activity migrated with the peak corresponding with CCK. No other peaks were detected. Column separation of spinal cord and the superfusate obtained during basal and evoked release, revealed that all activity in both the tissue and perfusate samples, travelled in a single peak which co-migrated with authentic VIP.

Studies on the intrathecal effect of beta-endorphin in primate

The intrathecal administration of beta-endorphin in the primate through an indwelling spinal catheter, produced a significant elevation in the nociceptive threshold as measured by the discrete trial shock titration task. The time of onset, duration of effect and magnitude of effect were all dose-dependent over a range of 150-750 micrograms. The effects were antagonized in a dose-dependent fashion by the systemic administration of naloxone. Aside from the elevations in the shock titration threshold produced by intrathecal beta-endorphin, no untoward effects on the animal's motor function or behavioral reactivity was noted. Significantly, unlike morphine, intrathecal beta-endorphin failed to produce any signs of scratching behavior at the doses used in these experiments. Once daily administration of intrathecal beta-endorphin (500 micrograms) showed a significant progressive decline in the antinociceptive effect over an 8-day period. Animals made tolerant to beta-endorphin in this fashion showed a significantly reduced response to an otherwise active dose of intrathecal morphine, indicating evidence for cross tolerance.

Effects of thiorphan on the antinociceptive actions of intrathecal [D-Ala2,Met5] enkephalin

The intrathecal administration of thiorphan (dl-3-mercapto-2-benzylpropranoyl-glycine) had no effect in doses up to 70 microgram on the rat hot plate and tail flick. Administrations of these doses of thiorphan concurrently or up to 1 h prior to the intrathecal administration of [D-Ala2, Met5] enkephalin resulted in a dose dependent, leftward shift in the peptide dose response curve. The potentiation was maximal with 35 micrograms, higher doses producing no greater potentiation. The potentiated effects of [D-Ala2,Met5] enkephalin were totally antagonized by systemically administered naloxone. Similarly, in the primate, concurrent administration of thiorphan alone in doses up to 70 microgram, had little effect on the hot plate or tail flick in the rat. 200 microgram resulted in a significant increase in hot plate, but not tail flick response latency. The increase observed on the hot plate, was partially antagonized by naloxone. In the primate, doses of intrathecal thiorphan (400-800 microgram) had no effect on the shock titration threshold. Higher doses resulted in an increase in the titration threshold, which was not antagonized by naloxone. These experiments appear to indicate the relevance of the thiorphan-sensitive inactivation system in the spinal cord to the relative potency of exogenously administered opioid peptides.

The antinociceptive effects of epidural opiates in the cat: studies of the pharmacology and the effects of lipophilicity in spinal analgesia

In cats implanted chronically with catheters in the lumbar epidural space, the pharmacology of the analgesia of 4 opiates, which varied widely in their physicochemical properties, was studied. Results revealed the following order of epidural analgesic potency as indicated by suppression of the spinally mediate skin twitch reflex: lofentanyl greater than morphine greater than L-methadone greater than meperidine greater than D-methadone. With the doses used in these experiments the duration of action was: morphine greater than lofentanyl greater than L-methadone = meperidine. In addition, antagonism of these effects by systemic naloxone and the development of tolerance after daily epidural administration of morphine were demonstrated. These data, jointly, indicate a specific receptor mechanism of action. We suggest that this animal model may be useful for the evaluation of new opiates and non-opiates to be given via the epidural route.