Biomedical subjects

D L Hammond

Publications and source records attributed to D L Hammond.

At least 37 records · Page 2Linked to original sources

PubMed1997

Differential distribution of (GABA)A receptor subunits on bulbospinal serotonergic and nonserotonergic neurons of the ventromedial medulla of the rat.

Spinally projecting neurons of the ventromedial medulla (VMM) compose an important efferent pathway for the modulation of nociception. These neurons receive a substantial gamma-aminobutyric acid (GABA)-ergic input, but the GABA receptor that mediates this input is unknown. This study examined the distribution of GABA(A) receptor alpha1 and alpha3 subunits in serotonergic and nonserotonergic neurons of the VMM that project to the dorsal horn in the rat. A pledget of Gelfoam soaked in Fluoro-Gold was placed at the thoracolumbar junction of the spinal cord to label spinally projecting neurons. Alternate sections of the medulla were then incubated with a mixture of antisera to either serotonin and the alpha1 subunit, or to serotonin and the alpha3 subunit of the GABA(A) receptor. Nearly 30% of spinally projecting neurons in the VMM were immunoreactive for the alpha1 subunit. A similar percentage of spinally projecting neurons in the VMM were immunoreactive for the alpha3 subunit, although diffuse cellular labeling combined with intense staining of processes in the neuropil precluded a rigorous semi-quantitative estimation of this population. No alpha1-subunit-immunoreactive neurons colocalized serotonin. In contrast, serotonergic neurons were immunoreactive for the alpha3 subunit. However, these double-labeled neurons were a modest percentage of the serotonergic population. A small percentage of spinally projecting serotonergic neurons was immunoreactive for the alpha3 subunit. These results suggest that significant numbers of spinally projecting serotonergic and nonserotonergic neurons of the VMM possess GABA(A) receptors that differ in their respective subunit compositions and that both classes of neurons may mediate the antinociception produced by the microinjection of GABA(A) receptor antagonists in the VMM.

Animals↗

PubMed1997

Role of spinal gamma-aminobutyric acidA receptors in formalin-induced nociception in the rat.

This study investigated the role of gamma-aminobutyric acid (GABA) and GABA(A) receptors in the spinal cord in the expression of pain behaviors evoked by injection of formalin in concentrations ranging from 0.25 to 2.5% in the hindpaw of the rat. Two approaches were used. The first approach compared the effect of drug treatment to saline at each concentration of formalin. The second approach examined the effect of drug treatment on the concentration-response functions of formalin, i.e., its EC50. Intrathecal (i.t.) pretreatment with 0.03 to 0.3 microg of bicuculline, a GABA(A) receptor antagonist, dose-dependently increased the number of flinches and weighted pain scores in the interphase and phase 2, but did not alter responses in phase 1. In the interphase, the EC50 values of formalin for number of flinches or weighted pain score in bicuculline-pretreated rats were decreased to one-third or one-fourth, respectively, of their values in saline-pretreated rats. In phase 2, the EC50 values of formalin for number of flinches or weighted pain score in bicuculline-pretreated rats were similarly decreased to one-half of their value in saline-pretreated rats. These results suggest that formalin was a significantly more noxious stimulus in the presence of bicuculline. Pretreatment with the GABA(A) receptor agonists, muscimol (0.3 microg) or isoguvacine (10 or 30 microg i.t.), significantly decreased the number of flinches in phase 1 and phase 2, but produced only a marginal decrease in the weighted pain score at the highest doses. These findings suggest that there is little tonic activation of GABA(A) receptors by GABA in the spinal cord before or immediately after the injection of formalin. However, approximately 10 min after the induction of injury by formalin, there is a release of GABA and activation of GABA(A) receptors in the spinal cord that 1) contributes to the period of quiescence between phase 1 and phase 2 and 2) coincidentally diminishes the magnitude of pain behaviors in phase 2, possibly by limiting the development of central sensitization in the spinal cord.

Animals↗

PubMed1997

Differences in the antinociceptive effects of alpha-2 adrenoceptor agonists in two substrains of Sprague-Dawley rats.

In this study, we examined whether Sprague-Dawley rats obtained from two different vendors, Harlan and Sasco, differ with respect to the types of alpha-2 adrenoceptors in the spinal cord that mediate antinociception. This hypothesis was tested using two alpha-2 adrenoceptor agonists, dexmedetomidine and ST-91, which are relatively selective for alpha-2A and alpha-2B adrenoceptors, respectively, and two different measures of nociception, the tail-flick and the 55 degrees C hot-plate test. Dexmedetomidine and ST-91 each increased tail-flick latency to a similar extent in both Harlan and Sasco rats, although dexmedetomidine was more efficacious than ST-91 in each substrain. However, the efficacy of these agonists was markedly different in Harlan and Sasco rats when the hot-plate test was used. For example, ST-91 was a full agonist in the hot-plate test in Harlan rats but a weak partial agonist in Sasco rats. Dexmedetomidine was a very weak partial agonist in Harlan rats and ineffective in the hot-plate test in Sasco rats. These findings suggest that (1) both spinal alpha-2A and alpha-2B receptors modulate nociceptive responses in the tail-flick test in both Harlan and Sasco rats; (2) hot-plate responses are mediated predominantly by alpha-2B adrenoceptors, with a minimal contribution by alpha-2A adrenoceptors in the Harlan rat and (3) hot-plate responses are not appreciably affected by either alpha-2A or alpha-2B adrenoceptors in the Sasco rat. These findings confirm previous reports that intrathecal administration of alpha-2 adrenoceptor agonists produces thermal antinociception in the rat. However, the magnitude of the antinociceptive effect is dependent on the receptor selectivity of the agonist used, cutaneous tissue stimulated to elicit nociceptive responses and substrain of rat.

Adrenergic alpha-2 Receptor Agonists↗

PubMed1997

Modulation of nociception by microinjection of delta-1 and delta-2 opioid receptor ligands in the ventromedial medulla of the rat.

In this study, we characterized the role of delta-1 and delta-2 opioid receptors in the ventromedial medulla (VMM) in the modulation of thermal nociception. Male Sprague-Dawley rats were prepared with an intracerebral guide cannula aimed at the nucleus raphe magnus or nucleus reticularis gigantocellularis pars alpha. Microinjection of the delta-1 opioid receptor agonist [D-Pen2,D-Pen5]enkephalin (DPDPE) or the delta-2 opioid receptor agonist [D-Ala2, Glu4]deltorphin (DELT) in the VMM increased response latency in the radiant heat tail-flick test with respective ED50 values (95% CL) of 0.66 (0.07-1.5) nmol and 0.1 (0.03-0.21) nmol. In the 55 degrees C hot-plate test, DELT produced a modest, transient increase in response latency and DPDPE was ineffective. The antinociception produced by DPDPE was antagonized by microinjection at the same site of 1.5 pmol of the delta-1 opioid receptor antagonist 7-benzylidenenaltrexone (BNTX) but not by 0.15 nmol of the delta-2 opioid receptor antagonist naltriben (NTB). Conversely, the antinociception produced by DELT was antagonized by microinjection at the same site of 0.15 nmol of NTB but not by 1.5 pmol of BNTX. These doses of BNTX or NTB alone did not alter either tail-flick or hot-plate latency when microinjected in the VMM. However, at 10-fold higher doses, BNTX lost its selectivity for the delta-1 opioid receptor, and NTB by itself increased tail-flick and hot-plate latencies. These results collectively implicate both delta-1 and delta-2 opioid receptors in the VMM in the modulation of nociception. They also indicate that the antinociceptive effects of DPDPE and DELT can be distinguished by BNTX and NTB, providing additional support for the existence of delta-1 and delta-2 opioid receptor subtypes at supraspinal loci. Finally, the failure of effective doses of either BNTX or NTB to alter nociceptive threshold suggests that neurons in the VMM do not receive a tonic, inhibitory enkephalinergic input mediated by delta-1 or delta-2 receptors.

Analgesics, Opioid↗

PubMed1996

Antinociception produced by systemic R(+)-baclofen hydrochloride is attenuated by CGP 35348 administered to the spinal cord or ventromedial medulla of rats.

This study examined the sites in the central nervous system at which subcutaneously-administered R(+)-baclofen hydrochloride (baclofen), the most active isomer of this prototypic gamma-aminobutyric acid (GABA)B receptor agonist, acts to produce antinociception in the rat. To determine whether baclofen acts in the spinal cord, either saline or the GABAB receptor antagonist CGP 35348 was injected intrathecally in rats pretreated 24 min earlier with 1 or 3 mg/kg s.c. baclofen. Intrathecal (i.t.) injection of 3 or 10 micrograms of CGP 35348 antagonized the increase in tail-flick and hot-plate latency produced by either dose of baclofen. To determine whether baclofen acts at sites in the ventromedial medulla (VMM), either saline or CGP 35348 was microinjected in the nucleus raphe magnus or nucleus reticularis gigantocellularis pars alpha of rats pretreated 24 min earlier with 1 or 3 mg/kg s.c. baclofen. Microinjection of 0.5 or 3 micrograms of CGP 35348 at sites in the VMM produced at best only a very modest attenuation of the antinociceptive effects of baclofen. These data suggest that systemically-administered baclofen acts at sites in both the spinal cord and the VMM, but that its antinociceptive effects are likely to be mediated to a greater extent by a spinal, rather than medullary site of action. However, a definitive comparison of the relative contribution of GABAB receptors in these two regions is precluded by differences in the diffusion and concentrations of the antagonist in the spinal cord and brainstem. Finally, microinjection of 0.5 or 3.0 micrograms of CGP 35348 in the nucleus raphe magnus or nucleus reticularis gigantocellularis pars alpha of saline-pretreated rats did not alter tail-flick or hot-plate latency. This finding suggests that, unlike GABAA receptors, GABAB receptors do not mediate the tonic GABAergic input to neurons in these nuclei.

Analgesics, Non-Narcotic↗

PubMed1996

Releases of norepinephrine and dopamine in ventriculocisternal perfusions in hepatectomized and laparotomized rats.

The completely hepatectomized rat has frequently been used as a model to study changes in the economy of norepinephrine (NE) and dopamine (DA) in hepatic coma. Hypothermia characteristically develops in hepatectomized rats and also occurs in patients in hepatic coma and is associated with improved survival in both. The aims of the present study were to measure both release and uptake of NE and release of DA in brain in warm (37 degrees C) and cool (30-32 degrees C) rats at 3-5 h after laparotomy or hepatectomy. Ventriculocisternal perfusions of the brain were performed on rats under basal conditions and during releases evoked by 40 mM K+. Basal releases of NE and DA and evoked release of DA were greater in the warm hepatectomized rats than in all other groups. In some studies, 10(-5) M amitriptyline was added to the perfusates to assess whether neuronal uptake was changed after hepatectomy. Uptake of released NE was equally robust in cool hepatectomized as in cool laparotomized rats but could not be measured in warm hepatectomized rats because of amitriptyline toxicity in these rats. Decreases in NE and increases in DA content were found in most areas of the brain after perfusion. Increased releases of NE and DA may contribute to the pathogenesis of hepatic encephalopathy.

Animals↗

PubMed1996

Antagonism of the antinocifensive action of halothane by intrathecal administration of GABAA receptor antagonists.

BACKGROUND: The hind brain and the spinal cord, regions that contain high concentrations of gamma-aminobutyric acid (GABA) and GABA receptors, have been implicated as sites of action of inhalational anesthetics. Previous studies have established that general anesthetics potentiate the effects of gamma-aminobutyric acid at the GABAA receptor. It was therefore hypothesized that the suppression of nocifensive movements during anesthesia is due to an enhancement of GABAA receptor-mediated transmission within the spinal cord. METHODS: Rats in which an intrathecal catheter had been implanted 1 week earlier were anesthetized with halothane. Core temperature was maintained at a steady level. After MAC determination, the concentration of halothane was adjusted to that at which the rats last moved in response to tail clamping. Saline, a GABAA, a GABAB, or glycine receptor antagonist was then injected intrathecally. The latency to move in response to application of the tail clamp was redetermined 5 min later, after which the halothane concentration was increased by 0.2%. Response latencies to application of the noxious stimulus were measured at 7-min intervals during the subsequent 35 min. To determine whether these antagonists altered baseline response latencies by themselves, another experiment was conducted in which the concentration of halothane was not increased after intrathecal administration of GABAA receptor antagonists. RESULTS: Intrathecal administration of the GABAA receptor antagonists bicuculline (0.3 micrograms) or picrotoxin (0.3, 1.0 micrograms) antagonized the suppression of nocifensive movement produced by the small increase in halothane concentration. In contrast, the antinocifensive effect of the increase in halothane concentration was not attenuated by the GABAB receptor antagonist CGP 35348 or the glycine receptor antagonist strychnine. By themselves, the GABAA receptor antagonists did not alter response latency in rats anesthetized with sub-MAC concentrations of halothane. CONCLUSIONS: Intrathecal administration of bicuculline or picrotoxin, at doses that do not change the latency to pinch-evoked movement when administered alone, antagonized the suppression of noxious-evoked movement produced by halothane concentrations equal to or greater than MAC. These results suggest that enhancement of GABAA receptor-mediated transmission within the spinal cord contributes to halothane's ability to suppress nocifensive movements.

Analgesics↗

PubMed1995

Microinjection of morphine into the rat medullary dorsal horn produces a dose-dependent increase in facial scratching.

It has been proposed that opioids act at the level of the medulla to produce facial pruritus. Supporting this hypothesis, microinjection of mu-opioid receptor agonists into the medullary dorsal horn (MDH; trigeminal subnucleus caudalis) of monkeys produces facial scratching behavior. The present study sought to establish a rodent model of opioid-induced facial pruritus. To this end, morphine (0.1, 0.3 or 1.0 micrograms/0.2 microliter) or saline (0.2 microliter) was unilaterally microinjected into the MDH of male Sprague-Dawley rats. Behavior for the 20 min preceding and the 80 min after this microinjection was videotaped. Morphine produced dose-dependent increases in facial scratching behavior ipsilateral to the microinjections with the peak effect at 30-40 min after microinjection. Facial scratching continued for the entire 80 min post-microinjection test period. Morphine also produced a lesser degree of facial scratching contralateral to the microinjections. Increases in facial scratching ipsilateral to the microinjection of 0.3 microgram morphine into the MDH were attenuated by 0.4 mg/kg s.c. naloxone. These findings support the hypothesis that the MDH is a critical site of action of opioid agonists in producing facial pruritus.

Animals↗

PubMed1995

Antinociception and delta-1 opioid receptors in the rat spinal cord: studies with intrathecal 7-benzylidenenaltrexone.

As part of the continuing investigation of the role of spinal delta opioid receptors in antinociception, this study characterized the ability of 7-benzylidenenaltrexone (BNTX), a selective delta-1 opioid receptor antagonist, to antagonize the antinociception produced in the rat by intrathecal (i.t.) administration of the respective delta-1 and delta-2 opioid receptor agonists, DPDPE and [D-Ala2, Glu4]deltorphin (DELT), or the mu receptor agonist DAMGO. In the tail flick test, 10-min pretreatment with 1 microgram of BNTX, increased the ED50 value of DPDPE from 27.5 micrograms (42.6 nmol) to 114.8 micrograms (177.8 nmol), but did not increase the ED50 values of either DELT or DAMGO. Increasing the dose of BNTX to 3 micrograms did not produce a significantly greater antagonism of the antinociceptive effects of DPDPE and did not antagonize the antinociceptive effects of DAMGO. However, it did enhance the antinociceptive effects of DELT decreasing its ED50 from 5.3 to 0.18 micrograms in the tail flick test. In the hot plate test, 10 min pretreatment with 1 microgram of BNTX selectively antagonized the antinociceptive effects of DPDPE, but did not antagonize the actions of DAMGO or DELT. Increasing the dose of BNTX to 3 micrograms also did not produce a significantly greater antagonism of the antinociceptive effects of DPDPE in the hot plate test, but did antagonize both the increase in hot plate latency and the modest decrement in motor function produced by 30 micrograms i.t. of DELT. However, the antagonism of these effects of DELT occurred much later in time than BNTX's antagonism of the antinociceptive effects of DPDPE.

Analgesics↗

PubMed1995

Microinjection of baclofen in the ventromedial medulla of rats: antinociception at low doses and hyperalgesia at high doses.

Neurons of the nucleus raphe magnus (NRM) and adjacent nucleus reticularis gigantocellularis pars alpha (NGCp alpha) receive a tonic inhibitory input from gamma-aminobutyric acid (GABA)-ergic neurons that is mediated by GABAA receptors. However, comparatively little is known about the role of GABAB receptors in these nuclei. The present study examined the effects on nociceptive threshold of microinjection of a wide dose range (0.1-150 ng) of the GABAB receptor agonist, baclofen hydrochloride (BAC), in the NRM, NGCp alpha or the nucleus reticularis giganto-cellularis (NGC). Microinjection of low doses of R(+)-BAC (0.1-1.0 ng) in the NRM or the NGCp alpha, but not the NGC increased response latencies in the tail flick test. As the dose of BAC was increased to 30.0 to 50.0 ng, response latencies in the tail flick test diminished to control values. Microinjection of the highest dose, 150 ng, in either the NRM, NGCp alpha or NGC significantly decreased response latencies in the tail flick test. Response latencies in the hot plate test were not increased by microinjection of 0.1 to 5.0 ng of R(+)BAC in the NRM, NGCp alpha or NGC. Although hot plate latency was increased after microinjection of 30.0 to 50.0 ng of R(+)-BAC in these nuclei, the effect was confounded by the occurrence of motor dysfunction. The effects of BAC were stereospecific as nociceptive threshold and motor function were not altered by microinjection of either 0.5 ng or 150 ng of the less active stereoisomer, S(-)-BAC. The biphasic effect of R(+)-BAC suggests the existence of multiple mechanisms by which GABAB receptors modulate the activity of neurons in the ventromedial medulla. It is proposed that the antinociception produced by low doses of R(+)-BAC results from disinhibition (activation) of neurons in the NRM and NGCp alpha as a consequence of the presynaptic inhibition of inhibitory GABAergic and/or noradrenergic inputs. The decrement in antinociceptive effect and hyperalgesia produced by higher doses of R(+)-BAC may result from a presynaptic inhibition of excitatory inputs to the NRM and NGCp alpha, postsynaptic hyperpolarization of neurons in these nuclei or the unmasking of a descending facilitatory pathway originating from the NRM, NGCp alpha and NGC.

Animals↗

PubMed1994

Enkephalin analogs as systemically active antinociceptive agents: O- and N-alkylated derivatives of the dipeptide amide L-2,6-dimethyltyrosyl-N-(3-phenylpropyl)-D-alaninamide.

A number of O- and N-alkylated derivatives of the antinociceptive, orally active, mu-opioid-selective truncated enkephalin analog L-2,6-dimethyltyrosyl-N-(3-phenylpropyl)-D-alaninamide (2, SC-39566) were synthesized to explore the structure-activity relationships of the series. The parent molecule is quite forgiving of substitution on the tyrosyl phenolic moiety and on the alanyl nitrogen. The tyrosyl and (phenylpropyl)amide NH sites, however, appear to be critical to interactions with the receptor, for even modest changes at these sites cause great loss of binding potency.

Alkylation↗

PubMed1994

Delta antagonist and kappa agonist activity of Naltriben: evidence for differential kappa interaction with the delta 1 and delta 2 opioid receptor subtypes.

The selective delta 2 receptor antagonist Naltriben (NTB) has played an important role in the identification of subtypes of the delta opioid receptor, termed delta 1 and delta 2, and their role in antinociception. However, the majority of these studies have been conducted in the mouse. The present study determined the opioid receptor selectivity of subcutaneously (s.c.) administered NTB in the rat. Five minute pretreatment with 1 mg/kg s.c. NTB antagonized the increase in TFL produced by i.t. administration of equieffective doses of the delta 2 receptor agonist [D-Ala2,Glu4]deltorphin (DELT) or the delta 1 receptor agonist [D-Pen2, D-Pen5]enkephalin (DPDPE), but did not antagonize the mu receptor agonist [D-Ala2, MePhe4, Gly-ol5]enkephalin (DAMGO). These data confirm previous reports that NTB is a selective delta opioid receptor antagonist. However, this dose of NTB antagonized DELT and DPDPE to an equivalent extent, suggesting that its selectivity for the delta 2 receptor is not maintained after s.c. administration in the rat. A lower dose of NTB (0.56 mg/kg s.c.) was ineffective. When the dose of NTB was increased to 3 mg/kg s.c. the antagonism of DELT and of DPDPE was unexpectedly lost. Pretreatment with the kappa receptor antagonist norbinaltorphimine (nor-BNI) partially restored the antagonism of DELT, but not DPDPE by this dose of NTB and did not modify the antagonism of DAMGO by NTB. These data suggest that high doses of NTB have kappa receptor agonist-like activity and support the proposal that kappa opioid agonists diminish the actions of delta receptor antagonists. They also suggest that nor-BNI-sensitive kappa opioid receptors interact with delta 2, but not delta 1 opioid receptors in the spinal cord.

Animals↗

PubMed1994

Inhibitory actions of delta 1-, delta 2-, and mu-opioid receptor agonists on excitatory transmission in lamina II neurons of adult rat spinal cord.

This study examined the electrophysiological consequences of selective activation of delta 1-, delta 2-, or mu-opioid receptors using whole-cell recordings made from visually identified lamina II neurons in thin transverse slices of young adult rat lumbar spinal cord. Excitatory postsynaptic currents (EPSCs) or potentials (EPSPs) were evoked electrically at the ipsilateral dorsal root entry zone after blocking inhibitory inputs with bicuculline and strychnine, and NMDA receptors with D-2-amino-5-phosphonopentanoic acid. Bath application of the mu receptor agonist [D-Ala2, N-MePhe4, Gly5-ol]enkephalin (DAMGO) or the delta 1 receptor agonist [D-Pen2, D-Pen5]enkephalin (DPDPE) produced a long-linear, concentration-dependent reduction in the amplitude of the evoked EPSP/EPSC. By comparison, the delta 2 receptor agonist [D-Ala2,Glu4]deltorphin (DELT) was unable to reduce the evoked EPSP/EPSC by more than 50% at 100 microM, the highest concentration tested. At concentrations that reduced evoked EPSP/EPSCs by 40-60%, neither DAMGO, DPDPE, nor DELT decreased the amplitude of the postsynaptic current produced by brief pressure ejection of (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid, suggesting a presynaptic site of action of these opioid receptor agonists. Bath application of 200 nM naltriben (NTB), a delta 2 receptor antagonist, competitively increased the EC75 of DELT by 15.3-fold, but did not antagonize either DPDPE or DAMGO. The EC75 of DELT was further increased by 169.7-fold in the presence of 1 microM NTB. However, this high concentration of NTB also increased the EC50 of DPDPE by about threefold in a noncompetitive manner and antagonized DAMGO in a noncompetitive manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1994

Antinociception produced by oral, subcutaneous or intrathecal administration of SC-39566, an opioid dipeptide arylalkylamide, in the rodent.

This study characterized the prototypic "minimum structure" enkephalin SC-39566 [2,6-dimethyl-L-tyrosinyl-D-alanine-(3-phenyl-1-propyl)-amide hydrochloride]. SC-39566 bound with highest affinity to mu opioid receptors (Ki, 0.13 nM), as well as to delta (Ki, 4.0 nM) opioid receptors in the rat brain, and with much lower affinity to kappa opioid receptors (Ki, 83.8 nM) in the guinea pig brain. In the mouse, SC-39566 inhibited phenylbenzoquinone-induced writhing and increased tail-flick and hot-plate latencies in a dose-dependent manner after either s.c. or p.o. (i.g.; intragastrical) administration. This antinociception was antagonized by the opioid antagonist naloxone, but not by alpha adrenergic, serotonergic, histaminergic, muscarinic cholinergic or dopaminergic receptor antagonists. In the rat, SC-39566 dose-dependently inhibited acetic-acid-induced writhing after s.c. or i.g. administration and increased response latencies in the tail-flick and hot-plate test after s.c. or intrathecal (i.t.) administration. The increase in tail-flick latency produced by s.c. SC-39566 in the rat was antagonized by s.c. naloxone with an apparent pA2 value of 7.9. Pretreatment with naltrindole, a delta opioid receptor antagonist, increased the ED50 of SC-39566 by only 1.7-fold. In addition, the increase in tail-flick latency produced by i.t. SC-39566 was not antagonized by i.t. administration of naltrindole or nor-binaltorphimine, a kappa receptor antagonist. These data suggest that the antinociceptive activity of SC-39566 is mediated predominantly by mu opioid receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Administration, Oral↗

PubMed1994

Activation of spinal delta-1 or delta-2 opioid receptors reduces carrageenan-induced hyperalgesia in the rat.

The role of spinal delta opioid receptors in mediating antinociception was studied by using the carrageenan-induced model of thermal hyperalgesia. Intrathecal administration of [D-Ala2, Glu4]-deltorphin (DELT), a delta-2 receptor agonist, or DPDPE, a delta-1 receptor agonist, produced a dose-dependent increase in paw-flick latency (PFL) with an ED50 of 14.0 micrograms for DELT and 30.4 micrograms for DPDPE. DAMGO, a mu receptor agonist, also increased the PFL in a dose-dependent manner when administered intrathecally with an ED50 of 0.02 microgram. Each opioid agonist increased the PFL to values that exceeded base-line latencies determined before the injection of carrageenan. However, DELT and DPDPE increased the PFL to a greater extent than did DAMGO. Coadministration of 30 micrograms of naltrindole shifted the dose-effect line of DELT to the right by 3.5-fold and that of DPDPE to the right by 2.5-fold, consistent with its characterization as a mixed delta-1/delta-2 receptor antagonist. Coadministration of 3 micrograms of naltriben (NTB) shifted the dose-effect line of DELT to the right by 3.2-fold, whereas 10 micrograms of NTB shifted the dose-effect line of DELT at least 15-fold to the right. Neither dose of NTB antagonized the effects of DPDPE. These data are consistent with characterization of NTB as a selective delta-2 receptor antagonist. The antinociception produced by DAMGO was noncompetitively antagonized by 30 micrograms of naltrindole and it was competitively antagonized by 10 micrograms of NTB. Thus, although NTB distinguishes between delta-1 and delta-2 opioid receptors, high doses may not effectively distinguish between delta and mu receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Antinociception produced by microinjection of L-glutamate into the ventromedial medulla of the rat: mediation by spinal GABAA receptors.

This study examined whether the antinociception produced by glutamatergic stimulation of neurons in the nucleus raphe magnus (NRM) or nucleus reticularis gigantocellularis pars alpha (NGCp alpha) is mediated by activation of GABAA receptors in the spinal cord. Two approaches were used. The first approach determined the ability of intrathecally (i.t.) administered bicuculline, a competitive GABAA receptor antagonist, to attenuate the antinociception produced by microinjection of L-glutamate in the NRM or NGCp alpha. Bicuculline was selected on the basis of an initial study that determined that 0.3 micrograms i.t. bicuculline caused a 3.4-fold rightward shift in the dose-effect relationship of the i.t.-administered GABAA agonist, isoguvacine, without producing allodynia or alterations in nociceptive threshold. The GABAA antagonist SR 95531 was judged unsuitable because it caused only a modest 1.7-fold rightward shift in the dose-effect relationship of isoguvacine at doses that did not produce allodynia. The second approach determined the ability of i.t. administered diazepam, a benzodiazepine receptor agonist, to enhance the antinociception produced by microinjection of L-glutamate in these nuclei. Diazepam was selected because of its ability to enhance the actions of GABA at the GABAA receptor. Rats were pretreated with i.t. administration of 0.3 micrograms bicuculline, 40 micrograms diazepam, or vehicle, after which 30 nmol L-glutamate was microinjected into the NRM or NGCp alpha. Microinjection of L-glutamate into the NRM or NGCp alpha in vehicle-pretreated rats significantly increased tail flick latency. The antinociception produced by microinjection of L-glutamate in the NGCp alpha was antagonized by bicuculline and enhanced by diazepam. In contrast, the antinociception evoked from sites in the NRM was only partially attenuated by bicuculline and was not enhanced by diazepam. In an ancillary experiment, i.t. administration of 0.3 micrograms bicuculline or 40 micrograms diazepam did not alter tail skin temperature or nociceptive threshold, suggesting that their effects on glutamate-induced antinociception were not secondary to alterations in tail skin temperature. Taken together, these results support the hypothesis that the antinociception produced by activation of neurons in the NGCp alpha, but not the NRM, is mediated in part by an action of GABA at GABAA receptors in the spinal cord.

Animals↗

PubMed1993

Antagonism of L-baclofen-induced antinociception by CGP 35348 in the spinal cord of the rat.

This study examined the potency and selectivity of the GABAB receptor antagonist CGP 35348 (3-amino-propyl(diethoxy-methyl)phosphinic acid) in the spinal cord of the rat. Intrathecal (i.t.) administration of 3-30 micrograms CGP 35348 produced a dose-dependent antagonism of the antinociception produced by i.t. administered L-baclofen. Increasing doses of CGP 35348 produced progressive, rightward parallel shifts in the dose-effect relationship of L-baclofen in both the tail flick and hot plate tests. However, in the tail flick test, the magnitude of the shift was not proportional to the dose of CGP 35348 such that doses greater than 10 micrograms i.t. produced no further antagonism. Schild analysis using all three doses of CGP 35348 yielded a slope of -0.45. However, if the Schild analysis was confined to the two lowest doses of CGP 35348 at which progressive shifts were obtained, a slope of -0.91 and an apparent pA2 value of 9.3 were obtained. An apparent pA2 value of 9.0 was also obtained in the hot plate test using the two lowest doses of CGP 35348. These data suggest that CGP 35348 is a competitive GABAB receptor antagonist at low concentrations. However, the failure to observe greater antagonism at higher doses of CGP 35348 suggests that this drug may exhibit additional properties at higher concentrations that can mask or prevent the occurrence of further antagonism. In contrast, the antinociception produced by i.t. administration of the GABAA receptor agonist isoguvacine was not antagonized by 30 micrograms i.t. CGP 35348, a dose that shifted the dose-effect relationship of L-baclofen 10-fold to the right.(ABSTRACT TRUNCATED AT 250 WORDS)

Analgesics↗

PubMed1993

Intrathecal GABAB antagonists attenuate the antinociception produced by microinjection of L-glutamate into the ventromedial medulla of the rat.

This study examined whether the antinociception produced by glutaminergic stimulation of neurons in the nucleus raphe magnus (NRM) and nucleus reticularis gigantocellularis pars alpha (NGCp alpha) is mediated by an action of GABA at GABAB receptors in the spinal cord. Rats were pretreated with intrathecal (i.t.) administration of the selective GABAB receptor antagonists phaclofen (100 micrograms) or CGP 35348 (30 micrograms), the serotonin receptor antagonist methysergide (30 micrograms), or vehicle. Fifteen min later, 30 nmol L-glutamate was microinjected into the NRM, NGCp alpha, or sites in the medulla outside these two regions. Microinjection of L-glutamate into the NRM or NGCp alpha in vehicle-pretreated rats significantly increased tail flick latency. This increase was antagonized, but not abolished, by i.t. pretreatment with 30 micrograms CGP 35348 or 100 micrograms phaclofen. Pretreatment with 30 micrograms methysergide completely antagonized the antinociception produced by L-glutamate. Microinjection of L-glutamate at medullary sites outside the NMR or NGCp alpha did not produce antinociception. In an ancillary experiment, the possibility that the ability of methysergide, phaclofen or CGP 35348 to antagonize glutamate-induced antinociception was related to non-specific increases in tail skin temperature was explored. Although phaclofen or methysergide increased tail skin temperature, the magnitude and time course of this increase were not consistent with the antagonism of glutamate-induced antinociception. Moreover, administration of CGP 35348 resulted in a modest decrease in tail skin temperature. Thus, antagonism of glutamate-induced antinociception does not appear to result from non-specific alterations in tail skin temperature.(ABSTRACT TRUNCATED AT 250 WORDS)

Analgesics↗