Spinal Nociceptive Responses Research Articles

Characterization of biphasic modulation of spinal nociceptive transmission by neurotensin in the rat rostral ventromedial medulla.

Modulation of spinal nociceptive transmission by neurotensin microinjected in the rostral ventromedial medulla (RVM) was examined in anesthetized, paralyzed rats. Forty-three spinal dorsal horn neurons in the L3-L5 spinal segments responding to mechanical and noxious thermal stimulation (50 degrees C) of the plantar surface of the ipsilateral hind foot were studied. Spinal units were classified as either wide dynamic range or nociceptive specific and were located in spinal laminae I-V. Microinjection of neurotensin (0.03 pmol/0.2 microl) into the RVM produced a significant facilitation (135% of control) of spinal unit responses to noxious thermal stimulation (50 degrees C) that lasted approximately 12 min. In contrast, injection of greater doses of neurotensin (300 or 3,000 pmol) produced an inhibition of spinal unit responses to noxious heat (51.7 and 10.6% of control, respectively) that had a longer duration (60-120 min). The effects of neurotensin on wide-dynamic-range and nociceptive-specific neuron responses to noxious heat were qualitatively and quantitatively similar. Spinal unit responses to graded heating of the skin (42-50 degrees C) were completely inhibited after microinjection of 3,000 pmol of neurotensin into the RVM. Injection of a lesser dose of neurotensin (300 pmol), however, resulted in a partial inhibition of spinal unit responses and significantly reduced the slope of the stimulus-response function to graded heating of the skin. Transection of either the ipsilateral or contralateral dorsolateral funiculus (DLF) significantly reduced the inhibition of spinal nociceptive transmission produced by neurotensin (3,000 pmol) in the RVM, whereas bilateral transection of the DLFs completely blocked the effect. In contrast, bilateral transection of the DLFs had no effect on facilitation of spinal nociception by neurotensin (0.03 pmol) in the RVM. The inhibition of spinal nociceptive transmission by neurotensin (3,000 pmol) in the RVM was completely blocked by injection of the nonpeptide neurotensin receptor antagonist SR48692 (30 fmol) into the RVM 10 min before neurotensin. To confirm a specific block of neurotensin-receptor-mediated effects by the antagonist, a subsequent injection of L-glutamate into the RVM was performed. L-Glutamate (100 nmol) was found to inhibit the nociceptive responses of those spinal units whose responses were no longer inhibited by neurotensin. In contrast, injection of SR48692 (30 fmol) into the RVM failed to block the facilitation of spinal unit responses to noxious heat produced by a subsequent injection of neurotensin (0.03 pmol) into the same site. The present series of experiments demonstrate a specific role for neurotensin in the RVM in the modulation of spinal nociceptive transmission, because the peptide was found to both facilitate and inhibit spinal neuron responses to noxious thermal stimulation. Additionally, the facilitatory and inhibitory effects of neurotensin appear to occur via interaction with multiple neurotensin receptors in the RVM that activate independent systems that descend in the ventrolateral funiculi and DLFs, respectively. The results from these experiments are consistent with prior studies demonstrating that the RVM both facilitates and inhibits spinal nociceptive transmission, and they complement previous work showing that neurotensin in the RVM modulates spinal nociceptive behavioral responses.

Thyrotropin-releasing hormone facilitates spinal nociceptive responses by potentiating NMDA receptor-mediated transmission

The interaction of thyrotropin-releasing hormone (TRH) with NMDA receptor-mediated responses has been investigated in α-chloralose-anaesthetized spinalized rats with respect to its relevance to spinal nociceptive transmission. The effects of TRH and of the uncompetitive NMDA antagonist ketamine were tested on responses of dorsal horn wide dynamic range neurones to noxious pinch, heat and electrical stimuli in parallel with those to iontophoretically applied N-methyl- d-aspartate (NMDA) and AMPA (α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid). Tests with NMDA blocking doses of ketamine (4 mg/kg i.v.) demonstrated a variable NMDA receptor-mediated component of all synaptic responses. TRH (0.5–1 mg/kg i.v.) enhanced the responses to NMDA (but not AMPA) in parallel with an increase of responses to all noxious stimuli and the ‘wind-up’ component of the responses to repeated electrical stimulation. This potentiation was completely reversed by a subsequent administration of ketamine (4 mg/kg i.v.). The results indicate that TRH facilitates nociceptive transmission in the spinal dorsal horn via a selective positive modulation of NMDA receptor-mediated transmission.

Characteristics of midbrain control of spinal nociceptive neurons and nonsomatosensory parameters in the pentobarbital-anesthetized rat

1. GABAergic mechanisms in the midbrain periaqueductal gray (PAG) have been proposed to control the activity of descending antinociceptive systems and defensive behavior. Here, the effect of neuronal disinhibition by gamma-aminobutyric acid (GABAA) receptor blockade at midbrain sites on spinal neuronal responses to noxious and innocuous skin stimulation was quantitatively characterized. It was compared with the effect of direct neuronal excitation by glutamate microinjections or electrical stimulation at identical sites. Changes in mean arterial blood pressure and other nonsomatosensory responses were also assessed. 2. Responses of 101 multireceptive lumbar spinal dorsal horn neurons to noxious radiant skin heating (50 degrees C, 10 s), innocuous skin brushing, and electrical stimulation of primary afferent A- and C-fibers were recorded in deeply pentobarbital-anesthetized rats. The mean blood pressure was continuously monitored in one carotid artery, and other nonsomatosensory parameters, such as frequency and depth of spontaneous respiration and contractions of abdominal and facial muscles, were classified according to their relative intensity into five groups. 3. A fine, multibarrel glass pipette was constructed for monopolar electrical stimulation and microinjection of the GABAA receptor antagonist bicuculline (40, 200, or 400 pmol), or glutamate (10-50 nmol), or Fast Green dye in 50 or 100 nl at identical sites in the midbrain. 4. Bicuculline microinjections into discrete regions of the PAG selectively abolished spinal neuronal responses to noxious skin stimulation but did not affect brush-evoked responses or responses to electrical A-fiber stimuli. This antinociception was often, albeit not necessarily, accompanied by tachypnoea and abdominal and facial muscle contractions and changes--mostly increases--in mean arterial blood pressure. Injections into other areas of the PAG and adjoining ventral tegmentum (VT) were less effective. The vast majority of injection sites in the lateral tegmentum (LT) were ineffective. 5. Glutamate microinjections at midbrain sites to detect areas of origin of descending antinociceptive neurons were characterized by a high incidence (greater than 50%) of false-negative results, as bicuculline was shown to be effective at numerous glutamate-insensitive sites. Glutamate microinjections into some sites of the PAG and adjoining VT reduced, but did not abolish, spinal neuronal responses to noxious skin heating. Injections into the LT were ineffective. 6. The efficacy of electrical stimulation at midbrain sites on spinal nociceptive responses had no predictive value for the effect of glutamate or bicuculline.(ABSTRACT TRUNCATED AT 400 WORDS)

Opioid receptor ligands in the neonatal rat spinal cord: binding and in vitro depression of the nociceptive responses

1. Opioid receptors in the neonatal rat spinal cord have been characterized by measurements of ligand binding to crude membrane fractions and by functional tests on the nociceptive spinal response in a spinal cord-tail preparation in vitro. 2. There were high affinity binding sites for [3H]-[D-Ala2, MePhe4, Gly(ol)5]enkephalin (DAGOL), [3H]-U69593, and [3H]-ethylketocyclazocine (EKC) on spinal cord membranes from neonatal rats. Hill slopes for binding of [3H]-DAGOL and [3H]-U69593 were close to unity. The Hill slope for binding of [3H]-EKC was less than unity, even after its interactions at mu-receptors had been blocked with 100 nM unlabelled DAGOL. Binding sites for [3H]-[D-Pen2, D-Pen5]enkephalin (DPDPE) could not be detected. 3. In competition assays U50488 was as potent as PD117302 and U69593 in competition for either [3H]-U69593 or [3H]-EKC binding sites. Hill slopes for a range of competing ligands at [3H]-DAGOL or [3H]-U69593 sites were close to unity. Hill slopes for competition at [3H]-EKC sites were less than one. 4. In the spinal cord-tail preparation from neonatal rats, opioid receptor agonists depressed spinal nociceptive responses evoked by application of capsaicin or heat to the tail. The order of potency was DAGOL greater than U69593 = PD117302 greater than morphine greater than U50488 = [D-Pen2, L-Pen5]enkephalin (DPLPE). 5. The antagonist naloxone was about equally potent against DAGOL, morphine and DPLPE, and about ten times less potent against U69593 and PD117302. The effects of U50488 were much less sensitive to blockade by naloxone than the effects of PD11703 or U69593. The Kappa antagonist, nor-binaltorphimine was equipotent against all three Kappa agonists. 6. The absence of delta-binding sites, and the low potency and relatively high sensitivity to naloxone suggest that DPLPE could be working at mu-receptors in the neonatal rat spinal cord. 7. The binding assays show that U50488 has the same affinity as PD1 17302 and U69593 for Kappa-receptors, yet it was less effective in the depression of nociceptive responses. This may be because U50488 has a relatively low efficacy at Kappa-receptors. It is possible that at high concentrations U50488 activates receptors not affected by other Kappa-ligands. These additional receptors may be non-opioid receptors (hence the insensitivity to naloxone), or they could be a subtype of Kappa-opioid receptor.

5-HT 3 receptors modulate spinal nociceptive reflexes

The selective 5-HT 3 receptor agonist 2-methyl-serotonin (2-Me-5-HT) mimicked the antinociceptive activity of 5-HT when intrathecally administered to rats. Two hundred μg (i.t.) doses of these agonists produced similar increases in tail flick latency. However, equal doses of 2-Me-5-HT and 5-HT doubled and tripled, respectively, the mean response latency as measured by the hot plate test. The potent and selective 5-HT 3 receptor antagonists ICS 205-930 (3-tropanyl-indole-3-car☐ylate) and MDL 72222 (3-tropanyl-3, 5-dichlorobenzoate) antagonized the antinociceptive effects of both 5-HT and 2-Me-5-HT. However, there were differences in the efficacy of these antagonists. Thus, intrathecal pretreatment with ICS 205-930 (0.05 μg) or MDL 72222 (0.1 μg) blocked the antinociceptive effects of 5-HT (200 μg, i.t.) as measured by the tail flick test, however, higher doses (0.1 and 1.0 μg, respectively) were required in the hot plate test. Pretreatment with ICS 205-930 (0.1 μg) or MDL 72222 (0.1 μg) blocked the effects of 2-Me-5-HT (200 μg, i.t.) in both analgesiometric tests. It is concluded that 5-HT 3 receptors are intimately involved in the modulation of spinal nociceptive responses.

Reversal of the antinociceptive effects of intrathecally administered serotonin in the rat by a selective 5-HT 3 receptor antagonist

The ability of the highly selective 5-HT 3 receptor antagonist ICS 205–930 (3α-tropanyl-1H-indole-3-carboxylic acid ester) to block the increase in tail flick (TFL) and hot plate latencies (HPL) produced by intrathecally (i.t.) administered serotonin (5-HT) was examined in pargyline pretreated rats. ICS 205–930 (0.1 μg, i.t.) blocked the ability of 5-HT (200 μg) to increase TFL and HPL. Significant hyperalgesia, as measured by a decrease in TFL and HPL compared to saline controls, also resulted from either the coadministration of ICS 205–930 (10 μg) and 5-HT (200 μg) or from ICS 205–930 (100 μg) alone. These data suggest an important role for 5-HT 3 receptors in modulating spinal nociceptive responses.

Inhibition in spinal cord of nociceptive information by electrical stimulation and morphine microinjection at identical sites in midbrain of the cat : J. Neurophysiol., 51 (1984) 75–89

The descending inhibition of noxious heat-evoked spinal neuronal excitation produced by morphine (MOR) and electrical brain stimulation (EBS) given at identical sites in the midbrain was quantitatively studied in the anesthetized cat. Fifty-two dorsal horn units driven by electrical stimulation of the posterior tibial and/or superficial peroneal nerves at A- and C-fiber strength and responding to noxious radiant heating (50 degrees C) of the skin of the foot-or toepads were studied. All units also responded to mechanical skin stimuli and were located primarily in laminae IV-VI of the dorsal horn. MOR (10-20 micrograms) was administered at 23 different sites in the midbrain. MOR attenuated the heat-evoked responses of 14 of 18 dorsal horn units studied to a mean 43% of the control heat-evoked response when administered at 18 sites in and immediately surrounding the periaqueductal gray (PAG). MOR administered at four sites in the PAG failed to significantly attenuate spinal nociceptive responses. The 14 sites where MOR was efficacious were distributed throughout the PAG. The efficacy of the MOR-produced inhibition was not correlated significantly to the distance from the cerebral aqueduct for the 18 PAG sites examined nor was there any difference in the spinal inhibitory effects of MOR, whether administered dorsally or ventrally in the PAG. MOR also failed to affect spinal neuronal heat-evoked responses (n = 5) when administered at five sites in the reticular formation ipsilateral to the PAG. EBS at the same 23 sites where MOR was given and at 34 additional sites in the midbrain attenuated the heat-evoked responses of all but one dorsal horn unit studied (mean maximal inhibition to 35% of control). In the PAG, stimulation ventrally inhibited a significantly greater proportion (13/21, 62%) of heat-evoked spinal neuronal responses to less than or equal to 25% of the control heat-evoked response than did stimulation in the dorsal PAG (5/15, 33%). The efficacy of the stimulation-produced inhibition was not, however, correlated significantly to the distance from the cerebral aqueduct for the total 57 midbrain sites examined, the 23 sites at which MOR was also tested, or only those EBS sites in and surrounding the PAG (n = 44).(ABSTRACT TRUNCATED AT 400 WORDS)

22. Comparison of μ, κ and σ opioids on spinal nociceptive responses

Whereas μ receptor activation reduces spinal responses to most peripheral nociceptive stimuli, κ and σ receptor activation can have differential effects between various types of nociceptive stimulus. This has led us to start a comparative study of μ (fentanyl), κ (U-50,488H) and σ (ketamine) preferring agonists on spinal nociceptive and other responses.

Comparison of mu, kappa and sigma preferring agonists for effects on spinal nociceptive and other responses in rats

The effects of the mu agonist fentanyl, the kappa agonist U-50,488H and the sigma agonist ketamine have been examined on the responses of single spinal neurones of anaesthetised, spinalised rats to a variety of peripheral somatic stimuli. On motoneurones all three compounds reduced nociceptive responses at doses which were low compared with those reported in behavioural tests. In the dorsal horn higher doses were required to achieve similar reductions.

Inhibition in spinal cord of nociceptive information by electrical stimulation and morphine microinjection at identical sites in midbrain of the cat.

The descending inhibition of noxious heat-evoked spinal neuronal excitation produced by morphine (MOR) and electrical brain stimulation (EBS) given at identical sites in the midbrain was quantitatively studied in the anesthetized cat. Fifty-two dorsal horn units driven by electrical stimulation of the posterior tibial and/or superficial peroneal nerves at A- and C-fiber strength and responding to noxious radiant heating (50 degrees C) of the skin of the foot-or toepads were studied. All units also responded to mechanical skin stimuli and were located primarily in laminae IV-VI of the dorsal horn. MOR (10-20 micrograms) was administered at 23 different sites in the midbrain. MOR attenuated the heat-evoked responses of 14 of 18 dorsal horn units studied to a mean 43% of the control heat-evoked response when administered at 18 sites in and immediately surrounding the periaqueductal gray (PAG). MOR administered at four sites in the PAG failed to significantly attenuate spinal nociceptive responses. The 14 sites where MOR was efficacious were distributed throughout the PAG. The efficacy of the MOR-produced inhibition was not correlated significantly to the distance from the cerebral aqueduct for the 18 PAG sites examined nor was there any difference in the spinal inhibitory effects of MOR, whether administered dorsally or ventrally in the PAG. MOR also failed to affect spinal neuronal heat-evoked responses (n = 5) when administered at five sites in the reticular formation ipsilateral to the PAG. EBS at the same 23 sites where MOR was given and at 34 additional sites in the midbrain attenuated the heat-evoked responses of all but one dorsal horn unit studied (mean maximal inhibition to 35% of control). In the PAG, stimulation ventrally inhibited a significantly greater proportion (13/21, 62%) of heat-evoked spinal neuronal responses to less than or equal to 25% of the control heat-evoked response than did stimulation in the dorsal PAG (5/15, 33%). The efficacy of the stimulation-produced inhibition was not, however, correlated significantly to the distance from the cerebral aqueduct for the total 57 midbrain sites examined, the 23 sites at which MOR was also tested, or only those EBS sites in and surrounding the PAG (n = 44).(ABSTRACT TRUNCATED AT 400 WORDS)