Reciprocal interactions between the amygdala and ventrolateral periaqueductal gray in mediating of Q/N(1-17)-induced analgesia in the rat

The opioid peptide, Orphanin FQ/nociceptin (OFQ/N(1-17))(,) its active fragments, and a related precursor peptide each produce analgesia following microinjection into the amygdala of rats. OFQ/N(1-17)-induced analgesia elicited from the amygdala is blocked by amygdala pretreatment of either general, mu, kappa, or delta-opioid antagonists even though OFQ/N(1-17) binds poorly to these receptor subtypes, and the antagonists bind poorly to the ORL-1/KOR-3 receptor. Agonists at mu and kappa opioid receptors as well as beta-endorphin each produce analgesia elicited from the amygdala that is blocked by opioid antagonist pretreatment in the ventrolateral periaqueductal gray (vlPAG) of rats. The present study examined whether pretreatment of general and selective opioid antagonists in the vlPAG blocked OFQ/N(1-17)-induced analgesia on the tail-flick test elicited from the amygdala, and whether pretreatment of general and selective opioid antagonists in the amygdala blocked OFQ/N(1-17)-induced analgesia elicited from the vlPAG of rats. OFQ/N(1-17)-induced analgesia elicited from the amygdala was significantly and markedly reduced following vlPAG pretreatment with a dose range of either naltrexone, beta-funaltrexamine (beta-FNA, mu), nor-binaltorphamine (NBNI, kappa) or naltrindole (NTI, delta). In contrast, opioid antagonists administered into misplaced mesencephalic control placements ventral and lateral to the vlPAG actually enhanced OFQ/N(1-17)-induced analgesia elicited from the amygdala. OFQ/N(1-17)-induced analgesia elicited from the vlPAG was significantly and markedly reduced following amygdala pretreatment with naltrexone and NBNI, to a lesser degree by NTI, and was unaffected by beta-FNA. Yet, opioid antagonists administered into misplaced amygdala control placements were generally ineffective in altering OFQ/N(1-17)-induced analgesia elicited from the vlPAG. Latencies were transiently increased by general, but not selective opioid antagonist treatment alone in the amygdala, but not the vlPAG. These data indicate reciprocal and regional interactions between the amygdala and vlPAG in the mediation of OFQ/N(1-17) by classic opioid receptor subtype antagonists in rats.     

Nociceptin/orphanin FQ into the rat periaqueductal gray decreases the withdrawal latency to heat and loading, an effect reversed by (Nphe)nociceptin(1–13)NH2

The present study investigated the effect of intraperiaqueductal grey injection of nociceptin/orphanin FQ (N/OFQ) and an antagonist (Nphe¹)nociceptin(1–13)NH₂ on the hindpaw withdrawal response to thermal and mechanical stimulation in rats. N/OFQ (5 nmol) significantly decreased the nociceptive thresholds in both tests and 1, 5 and 10 nmol of (Nphe¹)nociceptin(1–13)NH₂ significantly reversed this effect in a dose dependent way. Our results demonstrate, that N/OFQ has a nociceptive action, possibly through inhibition of PAG neurons. This effect is blocked by the antagonist (Nphe¹)nociceptin(1–13)NH₂ probably via ORL1 receptors in the periaqueductal grey.     

Nociceptin/orphanin FQ and [Pheψ(CH2-NH)Gly]nociceptin(1–13)NH2 modulates the activity of hypothalamic paraventricular nucleus neurons in vitro

Nociceptin, also known as orphanin FQ (N/OFQ), an endogenous ligand for the orphan opioid receptor-like₁ (ORL₁) receptor, is moderately expressed in the hypothalamic paraventricular nucleus (PVN) involved in the integrative control of the function of the endocrine and autonomic nervous systems. Our previous study demonstrated that intracerebroventricular administration of N/OFQ elicits an inhibitory action on the function of the cardiovascular and sympathetic nervous systems in conscious rats. However, the effects of N/OFQ on PVN neurons have not been examined. We investigated the effects of N/OFQ on PVN neurons using a whole-cell patch-clamp recording technique in rat brain slices. N/OFQ (30–1000 nM) hyperpolarized membrane potentials in type 1 and type 2 neurons of the PVN classified by the electrophysiological property. [Phe¹ψ(CH₂-NH)Gly²]nociceptin(1-13)NH₂ (Pheψ) (1–9 μM), a presumed competitive antagonist of the ORL₁ receptor, also hyperpolarized membrane potential in both types of neurons. In voltage clamp studies, N/OFQ (3–3000 nM) activated a K⁺ current concentration-dependently in 69.7% of PVN neurons with an EC₅₀ of 72.4±12 nM. Pheψ (100–9000 nM) also activated a K⁺ current with an EC₅₀ of 818±162 nM in PVN neurons, and significantly reduced the amplitude of the N/OFQ-stimulated current. The N/OFQ-induced current was not antagonized by the classical opioid receptor antagonist naloxone and putative antagonist nocistatin. These findings suggest that N/OFQ may have a functional role in the PVN.     

The putative OP4 antagonist, [Nphe]nociceptin(1-13)NH2, prevents the effects of nociceptin in neuropathic rats

Nociceptin or orphanin FQ (N/OFQ) is the natural ligand of the opioid receptor-like 1 receptor (ORL-1), which has been also classified as the fourth member of the opioid family of receptors and named OP₄. Elucidation of the biological role of N/OFQ has been hampered by the lack of compounds that selectively block the OP₄ receptor. Recently, a N/OFQ derivative, [Nphe¹]N/OFQ(1-13)NH₂, has been found to possess OP₄ antagonistic properties both in vitro and in vivo models. We investigated its spinal effect in the chronic constriction injury of the sciatic nerve in the rat, a model relevant to neuropathic pain in humans. Intrathecal (i.t.) administration of N/OFQ (0.2–20 nmoles) dose-dependently reversed mechanical allodynic-like behavior, while [Nphe¹]N/OFQ(1-13)NH₂ (20–120 nmoles, i.t.) was ineffective on its own. [Nphe¹]N/OFQ(1-13)NH₂ (60–120 nmoles, i.t.) antagonized N/OFQ (about 80% of reduction) but did not modify the activity of morphine (20 nmoles, i.t.). These results further support, for the first time in a chronic model of pain, the specific antagonistic profile of [Nphe¹]N/OFQ(1-13)NH₂vs the OP₄ receptor. This pseudopeptide is an interesting pharmacological tool to better clarify the role of N/OFQ in pathophysiology.     

Opioid antagonists in the periaqueductal gray inhibit morphine and β-endorphin analgesia elicited from the amygdala of rats

In addition to brainstem sites of action, analgesia can be elicited following amygdala microinjections of morphine and μ-selective opioid agonists. The present study examined whether opioid analgesia elicited by either morphine or β-endorphin in the amygdala could be altered by either the general opioid antagonist, naltrexone, the μ-selective antagonist, β-funaltrexamine (BFNA) or theδ₂ antagonist, naltrindole isothiocyanate (Ntii) in the periaqueductal gray (PAG). Both morphine (2.5–5 μg) and β-endorphin (2.5–5 jig) microinjected into either the baso-lateral or central nuclei of the amygdala significantly increased tail-flick latencies and jump thresholds in rats. The increases were far more pronounced on the jump test than on the tail-flick test. Placements dorsal and medial to the amygdala were ineffective. Naltrexone (1–5 μg) in the PAG significantly reduced both morphine (tail-flick: 70–75%; jump: 60–81%) and β-endorphin (tail-flick: 100%; jump: 93%) analgesia elicited from the amygdala, indicating that an opioid synapse in the PAG was integral for the full expression of analgesia elicited from the amygdala by both agonists. Both BFNA (68%) and Ntii (100%) in the PAG significantly reduced morphine, but not β-endorphin analgesia in the amygdala on the tail-flick test. Ntii in the PAG was more effective in reducing morphine (60%) and β-endorphin (79%) analgesia in the amygdala on the jump test than BFNA (15–24%). Opioid agonist-induced analgesia in the amygdala was unaffected by opioid antagonists administered into control misplacements in the lateral mesencephalon, and the small hyperalgesia elicited by opioid antagonists in the PAG could not account for the reductions in opioid agonist effects in the amygdala. These data indicate that PAGδ₂ and to a lesser degree, μ opioid receptors are necessary for the full expression of morphine and β-endorphin analgesia elicited from the amygdala.     

Medullary μ and δ opioid receptors modulate mesencephalic morphine analgesia in rats

Supraspinal opioid analgesia is mediated in part by connections between the midbrain periaqueductal gray (PAG) and rostral ventral medulla (RVM) which includes the nuclei raphe magnus and reticularis gigantocellularis. Serotonergic 5HT₂ and 5HT₃ receptor subtypes appear to participate in this pathway since general and selective serotonergic antagonists microinjected into the RVM significantly reduced morphine analgesia elicited from the PAG. Since both an enkephalinergic pathway between the PAG and RVM and intrinsic enkephalinergic cells in the RVM exist, the present study evaluated the abilities of general (naltrexone), μ-selective (β-funaltrexamine: B-FNA) andδ₂-selective (naltrindole) opioid receptor subtype antagonists microinjected into the RVM to alter morphine (2.5 μg) analgesia elicited from the PAG as measured by the tail-flick and jump tests. Mesencephalic morphine analgesia was significantly reduced after pretreatment in the RVM with naltrexone (1–10 μg), B-FNA (0.5–5 μg) or naltrindole (0.5–5 μg). Naltrexone in the RVM failed to alter basal nociceptive thresholds and none of the opioid antagonists were effective in reducing mesencephalic morphine analgesia when they were microinjected into placements lateral or dorsal to the RVM. These data indicate that μ andδ₂ opioid receptors in the RVM modulate the transmission of opioid pain-inhibitory signals from the PAG.     

Opioid supraspinal analgesic synergy between the amygdala and periaqueductal gray in rats

Analgesia can be elicited following microinjections of morphine, μ-selective agonists and β-endorphin into the amygdala. These analgesic responses are mediated by opioid synapses in the periaqueductal gray (PAG) since general (naltrexone), μ (β-funaltrexamine) and δ₂ (naltrindole isothiocyanate) opioid antagonists administered into the PAG significantly reduce both morphine and β-endorphin analgesia elicited from the amygdala. Supraspinal multiplicative opiate analgesic interactions have been observed between the PAG and rostroventromedial medulla (RVM), the PAG and locus coeruleus (LC), and the RVM and LC. The present study further examined the relationship between the amygdala and PAG in analgesic responsiveness by determining whether multiplicative analgesic interactions occur following paired administration of subthreshold doses of morphine into both structures, β-endorphin into both structures, morphine into one structure and β-endorphin into the other structure, or morphine and β-endorphin into one structure. Co-administration of subthreshold doses of morphine into both the amygdala and PAG results in a profound synergistic interaction on the jump test, but not the tail-flick test. Co-administration of subthreshold doses of β-endorphin into both structures also results in a profound test-specific synergistic interaction. In both cases, the magnitude of the interaction was similar regardless of the site receiving the fixed dose of the opioid, and the site receiving the variable dose of the opioid. Co-administration of β-endorphin (1 μg) into the amygdala and morphine (1 μg) into the PAG produced a potent interaction, but co-administration of morphine (1 μg) into the amygdala and β-endorphin (1 μg) into the PAG failed to produce interactive effects. Finally, co-administration of morphine (1 μg) and β-endorphin (1 μg) into either the amygdala alone or the PAG alone failed to produce an interaction, indicating the importance of regional opioid activation. These data are discussed in terms of the test-specificity of nociceptive processing in the amygdala, in terms of the multiple modulatory mechanisms mediating β-endorphin analgesia in the PAG, and in terms of whether the interactions are either mediated by anatomical connections between the amygdala and PAG or by mechanisms initiated by these two sites converging at another site or sites.     

Anti-opioid efficacy of Neuropeptide FF in morphine-tolerant mice

The modulatory effects of 1DMe ( -Tyr-Leu-(NMe)Phe-Gln-Pro-Gln-Arg-Phe-NH₂), an agonist of Neuropeptide FF (NPFF) receptors, on opioid antinociceptive activity have been compared in naive and tolerant mice in the tail-flick and the hot-plate tests. In naive mice, 1DMe alone had no effect on pain threshold but decreased dose-dependently (3–22 nmol) the analgesic activity of morphine in both tests. In tolerant mice, injections of 60-fold lower doses of 1DMe (0.05–0.5 nmol) reverse morphine-induced analgesia in the tail-flick test but this anti-opioid effect was no longer observed with the highest doses of 1DMe tested (3–22 nmol). In the hot-plate test, the anti-opioid action of 1DMe was not detected, whatever doses tested. Neither the NPFF-like immunoreactivity content of spinal cord and of olfactory bulbs, nor the density of NPFF receptors in olfactory bulbs, were altered. These results indicate that a chronic morphine treatment modifies the pharmacological properties of NPFF but the type of pain test is crucial in determining NPFF effects.     

Endogenous nociceptin/orphanin‐fq in the dorsal hippocampus facilitates despair‐related behavior

Nociceptin/orphanin‐FQ (N/OFQ) peptide and its receptor (NOP: N/OFQ opioid peptide receptor) are highly expressed in the hippocampus, but their functional role remains poorly understood. We recently showed that hippocampal N/OFQ inhibits learning and memory abilities in mice. Here, we investigated whether the endogenous peptide also regulated emotional responses at the level of the hippocampus. Bilateral infusions of the selective NOP receptor antagonist, UFP‐101 (1–3 nmol/side), into the dorsal hippocampus produced antidepressant‐like effects in the mouse forced swim and tail suspension tests comparable with those obtained with the prototypical antidepressant, fluoxetine (10–30 mg/kg, intraperitoneal). In the light‐dark test, neither UFP‐101 (1–3 nmol/side) nor N/OFQ peptide (1–3 nmol/side) modified anxiety measures when injected at behaviorally active doses in the dorsal hippocampus. These findings show a clear dissociation in the involvement of hippocampal N/OFQ system in anxiety‐ and despair‐related behaviors. We conclude that the dorsal hippocampus is a brain region in which there is an important N/OFQ modulation of mnemonic processes and adaptive emotional responses associated to despair states. © 2010 Wiley‐Liss, Inc.     

Pharmacological characterization of buprenorphine, a mixed agonist–antagonist with κ3 analgesia

Buprenorphine is a mixed opioid agonist/antagonist analgesic. This study was designed to determine the role of opioid receptor subtypes, especially κ₃, in buprenorphine-induced analgesia in mice. Buprenorphine, when injected systemically, revealed a potent analgesic effect by tailflick assay, with a biphasic dose–response curve, which was reversed by naloxone. The presence of analgesic cross-tolerance between buprenorphine and naloxone benzoylhydrazone (NalBzoH) and morphine indicated a role for κ₃ and μ receptor subtype in buprenorphine analgesia. Additional studies with selective opioid antagonists indicated κ₁ mechanisms of action. We did not detect any involvement of the δ receptor subtype. Low doses of buprenorphine antagonized morphine analgesia, while high doses of buprenorphine coadministered with morphine elicited increasing analgesia in a dose-dependent manner. These findings suggest that buprenorphine elicits analgesia through an interaction with κ₃ receptors and to a lesser extent with κ₁ as well as its activity as partial μ receptor agonist.     

Met-Enkephalin-Arg-Phe-like immunoreactive substances mediate electroacupuncture analgesia in the periaqueductal gray of the rabbit

The present study was undertaken to investigate whether the C-terminal extended Met-enkephalin heptapeptide (Met-enkephalin-Arg⁶-Phe⁷, MEAP) played a role in mediating the analgesic effect of electroacupuncture in rabbits. MEAP and its degrading enzyme inhibitor captopril as well as antiserum against MEAP were injected into the periaqueductal gray (PAG) via a previously implanted cannula. Their effects on nociception were tested by the escape response latency (ERL) elicited by radiant heat applied on the skin of the snout. (1) Microinjection of MEAP (30–240 nmol) into PAG produced a dose-dependent analgesic effect which was 2.5 times more potent than Met-enkephalin (MEK) and 3 times less potent than morphine. The complete reversal of the analgesia elicited by 240 nmol of MEAP by a small dose of naloxone (0.1 mg/kg, i.v.) indicates that the effect of MEAP is mediated by naloxone sensitive opioid receptors. (2) In rabbits, a dose-dependent analgesia was elicited by an intra-PAG injection of captopril (60–240 nmol). A single dose of 240 nmol captopril increased ERL by more than 100%. This effect could be reversed by 30 nmol of naloxone injected into the same site, or by antiserum recognizing MEAP (1 μL, titer 1:1500) but not by antiserum recognizing MEK (1 μl, 1:8000) suggesting that captopril was able to protect MEAP from degradation. (3) Intra-PAG injection of 60 nmol of captopril significantly potentiated the after effect of electroacupuncture (EA) induced analgesia. This effect could be blocked either by 30 nmol (but not 7.5 nmol) of naloxone, or by 1 μl (but not 0.1 μl) of MEAP antiserum. These results suggest that MEAP may cause analgesia by acting in PAG, and could be operative in EA analgesia.     

Orphanin FQ/nociceptin analgesia in the rat

The heptadecapeptide orphanin FQ or nociceptin (OFQ/N), the endogenous ligand for the orphan opioid receptor, has a complex pharmacology in mice, eliciting either an anti-opioid/hyperalgesic action or analgesia depending upon the dose and testing paradigm. Unlike mice, orphanin FQ/nociceptin fails to elicit hyperalgesia in the rat following intracerebroventricular injection. Both OFQ/N and a truncated version, OFQ/N(1-11), produce a robust analgesic response. OFQ/N analgesia is readily antagonized by the opioid antagonists naloxone or diprenorphine, despite their very poor affinity for the cloned orphan opioid receptor. Antisense studies revealed that probes targeting the second and third coding exon of the orphan clone significantly attenuate OFQ/N analgesia, while the exon 1 probe was inactive. These results indicate that OFQ/N elicits a naloxone-sensitive analgesia in rats similar to that previously reported in mice.     

Effect of intrathecal nocistatin on nociceptin/orphanin FQ analgesia in chronic constriction injury rat

Nocistatin and nociceptin/orphanin FQ (N/OFQ) are two neuropeptides derived from the same precursor protein, prepronociceptin (ppOFQ), and exhibit different effects on spinal neurotransmission. Nocistatin does not bind to nociceptin/orphanin FQ peptide receptor (NOP), but intrathecal (i.t.) nocistatin has been reported to block the analgesic effect of i.t. N/OFQ. In this study, we investigated the effect of i.t. nocistatin on N/OFQ analgesia to radiant thermal stimuli in chronic constriction injury (CCI) rat. Firstly, to investigate the analgesic effect of N/OFQ, different doses of N/OFQ (3, 10, 30 microg) were intrathecally injected and foot withdrawal latency (FWL) to radiant heat was recorded. It is observed that 3 microg N/OFQ had no effect on FWL, 10 and 30 microg N/OFQ significantly increased FWL of CCI rat. Then, 10 microg N/OFQ, 10 microg nocistatin and a drug cocktail including 10 microg N/OFQ and 10 microg nocistatin were intrathecally injected. The results showed that FWL significantly decreased after using N/OFQ and nocistatin compared with using only N/OFQ, and 10 microg nocistatin had no effect on FWL versus control, suggesting that this dose of nocistatin per se had no effect on the pain threshold of CCI rat, but could block the analgesic effect of N/OFQ. These results indicated that i.t. N/OFQ dose-relatedly depressed thermal hyperalgesia produced by CCI and nocistatin could block N/OFQ analgesia at spinal level in CCI rat.     

The role of spinal cord 5-HT1A and 5-HT1B receptors in the modulation of a spinal nociceptive reflex

The role of the 5-hydroxytryptamine (5-HT) receptor subtypes in the spinal cord in the regulation of nociception is unknown. This study examined whether administration of different 5-HT₁ receptor agonists into the spinal subarachnoid space of mice modulates the nociceptive tail-flick reflex, and whether effects on the tail-flick reflex involve changes in tail skin temperature. The tail-flick latencies (the time needed to evoke the tail-flick reflex by noxious radiant heat) were significantly increased after intrathecal (i. th.) injection of 5-HT (10–20 μg), the 5-HT_1A/5-HT_1B receptor agonist5-methoxy-N,N-dimethyltryptamine (5-MeODMT, 10–20 μg), the selective 5-HT_1A receptor agonist8-hydroxy-2-di-n-propylamino)tetralin (8-OH-DPAT, 20 μg), and after i.th. injection of1(m-chlorophenyl)piperazine (mCPP, 5–20 μg) and5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole (RU 24969, 5–20 μg) which have high affinity for the 5-HT_1B receptors. None of the 5-HT₁ receptor agonists had the ability to change the tail skin temperature. The results show that in the mouse i.th. injection of both 5-HT_1A and 5-HT_1B receptor agonists has the ability to inhibit the tail-flick reflex without interfering with the tail skin temperature.     

Intracerebroventricular galanin and N-terminal galanin fragment enhance the morphine-induced analgesia in the rat

Summary Behavioral effect of galanin and its fragments, galanin_1–15 and galanin_16–29 (200 ng, 1 and 5 g), after intracerebroventricular (i.c.v.) administration was studied in rats. The number of crossings and pippings and the time of locomotion (an open field test) showed a similar sedative action of galanin and galanin_16–29, with no significant effect of galanin_1–15. Galanin and its fragments, injected in doses of 200 ng, 1 and 5 g, did not affect nociception, as measured by a tail-flick and paw pressure test. Galanin and galanin_1–15, but not galanin_16–29 (5 g i.c.v.), injected together with morphine (2.5 g i.c.v.), significantly potentiated the analgetic effect of morphine assessed by a paw pressure test; a similar tendency was also observed in a tail-flick test. Galanin and its two fragments injected in doses of 200 ng, 1 and 5 g, did not change the effect of morphine given in a dose of 1 g. These data suggest that galanin, having no effect when given alone, potentiate the analgetic effect of morphine. The fact that the N-terminal fragment of galanin acts like a natural peptide suggests a receptor mediated action.In conclusion, the analgesic effect of morphine was potentiated by galanin and its N-terminal fragment galanin_1–15. On the other hand, behavioral study showed a similar sedative action of galanin and C-terminal fragment galanin_16–29. This suggests that the N- and C-terminal fragments of galanin are differentially involved in behavioral effects of the peptide.     

The presence of delta and mu-, but not kappa or ORL1 receptors in bovine pinealocytes

Physicians have noted since antiquity that their patients complained of less pain and required fewer analgesics at night-time. In humans, the circulating levels of melatonin, a pineal substance with analgesic and hypnotic properties, exhibit a pronounced circadian rhythm with serum levels being high at night and low during day-time. Moreover, pinealectomy abolishes the analgesic effects of melatonin, and naloxone disrupts the day–night rhythm of nociception. In this study, we have attempted to identify and characterize the nature and types of opioid receptor in bovine pinealocyte membranes, using a radioligand binding technique with the selective radioligands [³H]DAMGO, [³H]DPDPE, [³H]U69593 and [³H]orphanin-FQ (OFQ) for identifying mu (μ)-, delta (δ)-, kappa (κ)- and opioid receptor-like (ORL₁) receptors, respectively. The saturation experiments on bovine pinealocyte membranes for [³H]DPDPE binding provided B_max and K_d values of 553±24 fmol/mg protein and 1.3±0.6 nM; and for [³H]DAMGO binding provided B_max and K_d values of 6.3±1.3 fmol/mg protein and 1.2±0.4 nM, respectively. On the other hand, the specific radioligands ([³H]U69593 and [³H]OFQ) binding of κ and ORL₁ receptors were undetectable in bovine pinealocyte membranes. Furthermore, competitive experiments with opioid agonist and antagonist and related compounds confirmed the presence of μ- and δ-opioid binding sites in bovine pinealocyte membranes. These results indicate that neither κ nor ORL₁ receptors are present on the pinealocytes, and the majority of opioid receptors found in the bovine pineal gland are δ (possibly, both δ₁ and δ₂) types, with a minority being μ type, and that both are primarily located on the bovine pinealocyte membranes. These opioid receptors, by stimulating the activity of N-acetyltransferase, enhance the synthesis of melatonin.     

Biphasic alteration in the trigeminal nociceptive neuronal responses after intracerebroventricular injection of prostaglandin E2 in rats

To investigate the role of prostaglandin E₂ (PGE₂) in the brain in nociception electrophysiologically, we injected PGE₂ (0.1 fmol–1 nmol) into the lateral cerebroventricle (LCV) of anesthetized rats and observed the changes of the responses of the wide dynamic range (WDR) neurons in the trigeminal nucleus caudalis to noxious pinching of facial skin. The LCV injection of PGE₂ at 1 fmol and 10 fmol enhanced the responses of the majority of WDR neurons to noxious stimuli, whereas that of PGE₂ at 100 pmol and 1 nmol suppressed them. The enhancement and suppression of the nociceptive responses of WDR neurons were observed 15–25 min and 5–15 min after injection of PGE₂ at 10 fmol (3.53 pg) and 1 nmol (353 ng), respectively. On the other hand, the LCV injection of PGE₂ at both 10 fmol and 1 nmol had no effect on the responses of the low threshold mechanoreceptive neurons to skin brushing. These results provide electrophysiological evidence that brain-derived PGE₂ has biphasic effects on nociception, i.e., it induces mechanical hyperalgesia at lower doses and hypoalgesia at higher doses in rats.     

Role of spinal α1-adrenergic mechanisms in the control of lower urinary tract in the rat

The role of spinal α₁-adrenergic mechanisms in the control of urinary bladder function was examined in urethane (1.2 g/kg s.c.) anesthetized and decerebrate unanesthetized female Sprague–Dawley rats (250–320 g). Bladder activity was recorded via a transurethral catheter during continuous infusion (0.21 ml/min) cystometrograms or under isovolumetric conditions. All drugs were administered intrathecally at the L₆-S₁ segmental level of spinal cord. During cystometrograms, 3 or 30 nmol of phenylephrine (α₁-adrenergic agonist) did not alter bladder activity; whereas 300 nmol increased the intercontraction interval by 98% and pressure threshold for inducing micturition by 115%, but did not change bladder contraction amplitude. A large dose of phenylephrine (3000 nmol) completely blocked reflex voiding and induced overflow incontinence at a high baseline pressure (mean: 33 cmH₂O; range: 28–42 cmH₂O). Under isovolumetric conditions, 3–30 nmol of phenylephrine abolished bladder activity for 22–45 min; whereas smaller doses (0.003–0.3 nmol) were inactive. Doxazosin (50 nmol), an α₁-adrenergic antagonist, decreased intercontraction intervals but did not change bladder contraction amplitude during cystometrograms. Under isovolumetric conditions this dose of doxazosin increased bladder contraction frequency and decreased bladder contraction amplitude. Smaller doses (5 or 25 nmol) of doxazosin did not alter bladder activity. These studies suggest that two types of spinal α₁-adrenergic mechanisms are involved in reflex bladder activity: (1) inhibitory control of the frequency of voiding reflexes presumably by regulating afferent processing in the spinal cord and (2) facilitatory modulation of the descending limb of the micturition reflex pathway.     

Inhibitory influences of FMRFamide and PLG on stress-induced opioid analgesia and activity

The effects of i.c.v. administrations of the peptide FMRFamide (Phe-Met-Arg-Phe-NH₂), as well as i.p. injections of PLG (Pro-Leu-Gly-NH₂) and the opiate antagonist, naloxone, on immobilization-induced analgesia and locomotor activity were examined in CF-1 and C57BL strains of mice. Both naloxone (1.0 mg/kg) and FMRFamide (0.10–1.0 μg) blocked the experimentally induced analgesia and activity, whereas PLG (0.10–10 mg/kg) suppressed only analgesia. These results indicate that FMRFamide (or FMRFamide-like neuropeptides) and PLG may function as differential antagonists of the behavioral and physiological consequences of endogenous opioid activation.     

Biphasic modulation in the trigeminal nociceptive neuronal responses by the intracerebroventricular prostaglandin E2 may be mediated through different EP receptors subtypes in rats

To determine which prostaglandin E₂ (PGE₂) receptor subtypes are involved in the brain-derived PGE₂-induced changes in nociception, we injected synthetic EP₁, EP₂ and EP₃ receptor agonists (0.01 fmol to 10 nmol) into the lateral cerebroventricle (LCV) of urethane-anesthetized rats and observed the changes in the responses of the wide dynamic range (WDR) neurons in the trigeminal nucleus caudalis to noxious pinching of facial skin. The enhancement and suppression of the nociceptive responses of the WDR neurons were observed after the LCV injection of MB28767 (an EP₃ receptor agonist) at a low dose range (1–100 fmol) and 17-phenyl-ω-trinor PGE₂ (an EP₁ receptor agonist) at high doses (1–10 nmol), respectively. Furthermore, the suppression of nociceptive neuronal responses after the LCV injection of PGE₂ (1 nmol) was completely blocked by SC19220 (an EP₁ receptor antagonist, 300 nmol). On the other hand, butaprost (an EP₂ receptor agonist) at any doses tested (0.1 fmol to 1 nmol) had no effect on the nociceptive responses. The LCV injection of MB28767 (10 fmol) and 17-phenyl-ω-trinor PGE₂ (1 nmol), which respectively enhanced and suppressed the nociceptive neuronal responses, did not affect the responses of the low threshold mechanoreceptive neurons to innocuous tactile stimuli. These results provide electrophysiological evidence that brain-derived PGE₂ induces mechanical hyperalgesia and hypoalgesia through EP₃ and EP₁ receptors, respectively, in the rat.