Through V2, not V1 receptor relating to endogenous opiate peptides, arginine vasopressin in periaqueductal gray regulates antinociception in the rat

Our previous study has proven that central arginine vasopressin (AVP) plays an important role in antinociception, and pain stimulation raises AVP concentration in the periaqueductal gray (PAG). The nociceptive effect of AVP in PAG was investigated in the rat. The results showed that microinjection of AVP into PAG increased pain threshold, whereas microinjection of V2 receptor antagonist-d(CH2)5[d-Ile2, Ile4, Ala9-NH2]AVP into PAG decreased pain threshold in a dose-dependent manner, but local administration of V1 receptor antagonist-d(CH2)5Tyr(Me)AVP did not change pain threshold; Pain stimulation elevated AVP, Leucine-enkephalin (L-Ek), Methionine-enkephalin (M-Ek) and beta-endorphin (beta-Ep), not dynorphinA(1-13) (DynA(1-13)) concentrations in PAG perfuse liquid; PAG pre-treatment with naloxone, an opiate receptor antagonist or V2 receptor antagonist completely reversed AVP-induced increase in pain threshold, however, PAG pre-treatment with V1 receptor antagonist did not influence this effect of AVP administration. The data suggest that AVP in the PAG, through V2 rather than V1 receptor, regulates antinociception, which progress relates to enkephalin and endorphin.     

Tolerance Effects Induced by NSAID Microinjections into the Central Nucleus of the Amygdala in Rats

Recent investigations have shown that microinjections of non-opioid analgesics, nonsteroidal anti-inflammatory drugs, NSAIDs, into some brain areas, particularly, into the midbrain periaqueductal gray matter (PAG) and rostral ventro-medial medulla (RVM), cause antinociception with some effects of tolerance. Our preliminary findings have also shown the same effects of tolerance after intraperitoneal injections. The present study was designed to examine whether microinjections of metamizole (Analgin), ketorolac, and xefocam into the central nucleus of the amygdala (Ce) lead to the development of tolerance in rats, and to ascertain whether this nucleus is the pain-modulating pathway through PAG. Our investigation revealed that microinjections of NSAIDs into the Ce both unilaterally (the left side) and bilaterally produced antinociception, as indicated by a latency increase in tail-flick reflex (TF) compared to controls with saline, on the first experimental day for Analgin (P < 0.001), ketorolac (P < 0.001), and xefocam (P < 0.001). However, when these drug microinjections were repeated during subsequent days, the antinociceptive effects progressively diminished so that on the fifth experimental day the TF latency was similar to that in the rats that received repeated injections of only saline. These results show that, alongside with PAG and RVM, the Ce is an important site of the endogenous antinociceptive system, which triggers the descending pain control mechanism and thus inhibits nociceptive transmission. On the other hand, our data confirm the results of other authors that NSAIDs are closely related to endogenous opioids, and tolerance to these non-opioid drugs probably depends on opioid tolerance.     

Supraspinal circuitry mediating opioid antinociception: Antagonist and synergy studies in multiple sites

Supraspinal opioid antinociception is mediated by sensitive brain sites capable of supporting this response following microinjection of opioid agonists. These sites include the ventrolateral periaqueductal gray (vIPAG), the rostral ventromedial medulla (RVM), the locus coeruleus and the amygdala. Each of these sites comprise an interconnected anatomical and physiologically relevant system mediating antinociceptive responses through regional interactions. Such interactions have been identified using two pharmacological approaches: (1) the ability of selective antagonists delivered to one site to block antinociception elicited by opioid agonists in a second site, and (2) the presence of synergistic antinociceptive interactions following simultaneous administration of subthreshold doses of opioid agonists into pairs of sites. Thus, the RVM has essential serotonergic, opioid, cholinergic and NMDA synapses that are necessary for the full expression of morphine antinociception elicited from the vIPAG, and the vIPAG has essential opioid synapses that are necessary for the full expression of opioid antinociception elicited from the amygdala. Further, the vIPAG, RVM, locus coeruleus and amygdala interact with each other in synergistically supporting opioid antinociception.     

Opioid sensitivity of analgesia induced by microinjections of nonsteroidal anti-inflammatory drugs into the <Emphasis Type="Italic">nucleus raphe magnus</Emphasis>

Our recent investigations demonstrated that microinjections of three nonsteroidal anti-inflammatory drugs (NSAIDs), Analgin, ketorolac, or xefocam, into the central nucleus of the amygdala produce tolerance to these drugs and cross-tolerance to morphine. We observed the same phenomenon in the midbrain periaqueductal gray matter. In this report, we show that microinjections of NSAIDs into the nucleus raphe magnus (NRM) produces antinociception, as indicated by latency increases in both tail-flick (TF) and hot-plate (HP) reflexes compared to controls with saline microinjected into the same nucleus. Furthermore, microinjection of the μ-opioid antagonist naloxone into the NRM significantly decreased antinociceptive effects of NSAIDs characterized by the TF and HP latencies on the 1st experimental day. On the 2nd day, naloxone also provided some trend effects in both TF and HP tests. These results strongly support the suggestion that the endogenous opioid system is significantly involved in NSAID-induced antinociception and tolerance.     

Systemic morphine-induced release of serotonin in the rostroventral medulla is not mimicked by morphine microinjection into the periaqueductal gray

We used in vivo microdialysis in awake rats to test the hypothesis that intravenous morphine increases serotonin (5-HT) release within the rostral ventromedial medulla (RVM). We also injected morphine into various sites along the rostrocaudal extent of the periaqueductal gray (PAG), and examined the extent of its diffusion to the RVM. Intravenous morphine (3.0 mg/kg) produced thermal antinociception and increased RVM dialysate 5-HT, 5-hydroxyindole acetic acid (5-HIAA), and homovanillic acid (HVA) in a naloxone-reversible manner. As neither PAG microinjection of morphine (5 micro g/0.5 micro L) nor RVM administration of fentanyl or d-Ala(2),NMePhe(4),Gly-ol(5)]enkephalin (DAMGO) increased RVM 5-HT, we were unable to determine the precise site of action of morphine. Surprisingly, peak morphine levels in the RVM were higher after microinjection into the caudal PAG as compared to either intravenous injection or microinjection into more rostral sites within the PAG. Naloxone-precipitated withdrawal in morphine-tolerant rats not only increased extracellular 5-HT in the RVM, but also dopamine (DA) and HVA. We conclude that substantial amounts of morphine diffuse from the PAG to the RVM, and speculate that opioid receptor interactions at multiple brain sites mediate the analgesic effects of PAG morphine. Further studies will be required to elucidate the contribution of 5-HT and DA release in the RVM to opioid analgesia and opioid withdrawal.     

Oxytocin in the periaqueductal grey regulates nociception in the rat

Studies have demonstrated that oxytocin (OXT) plays important roles in pain modulation in the central nervous system, and there are OXT receptors in the periaqueductal grey (PAG). The experiment was designed to investigate the effect of OXT in the PAG on antinociception. The results showed that (1) intra-PAG injection of OXT increased the pain threshold, whereas the local administration of the high specific OXT receptor antagonist, desGly-NH(2), d(CH(2))(5)[D-Tyr(2), Thr-sup-4]OVT decreased the pain threshold in a dose-dependent manner; (2) Pain stimulation could elevate OXT concentration in the PAG perfusion liquid. The data suggested that OXT in the PAG was involved in the antinociceptive process through the OXT receptor.     

Role of cholinergic, opioidergic and GABAergic neurotransmission of the dorsal hippocampus in the modulation of nociception in guinea pigs

AIMS: Several physiological, pharmacological and behavioral lines of evidence suggest that the hippocampal formation is involved in nociception. The hippocampus is also believed to play an important role in the affective and motivational components of pain perception. Thus, our aim was to investigate the participation of cholinergic, opioidergic and GABAergic systems of the dorsal hippocampus (DH) in the modulation of nociception in guinea pigs. MAIN METHODS: The test used consisted of the application of a peripheral noxious stimulus (electric shock) that provokes the emission of a vocalization response by the animal. KEY FINDINGS: Our results showed that, in guinea pigs, microinjection of carbachol, morphine and bicuculline into the DH promoted antinociception, while muscimol promoted pronociception. These results were verified by a decrease and an increase, respectively, in the vocalization index in the vocalization test. This antinociceptive effect of carbachol (2.7 nmol) was blocked by previous administration of atropine (0.7 nmol) or naloxone (1.3 nmol) into the same site. In addition, the decrease in the vocalization index induced by the microinjection of morphine (2.2 nmol) into the DH was prevented by pretreatment with naloxone (1.3 nmol) or muscimol (0.5 nmol). At doses of 1.0 nmol, muscimol microinjection caused pronociception, while bicuculline promoted antinociception. SIGNIFICANCE: These results indicate the involvement of the cholinergic, opioidergic and GABAergic systems of the DH in the modulation of antinociception in guinea pigs. In addition, the present study suggests that cholinergic transmission may activate the release of endorphins/enkephalin from interneurons of the DH, which would inhibit GABAergic neurons, resulting in antinociception.     

Non-peptidergic small diameter primary afferents expressing VGluT2 project to lamina I of mouse spinal dorsal horn

The aim of this study was to investigate the expression of prostaglandin EP1 receptor within the ventrolateral periaqueductal grey (VL PAG). The role of VL PAG EP1 receptor in controlling thermonociception and rostral ventromedial medulla (RVM) activity in healthy and neuropathic rats was also examined. EP1 receptor was indeed found to be expressed within the VL PAG and co-localized with vesicular GABA transporter. Intra-VL PAG microinjection of ONO-DI-004, a selective EP1 receptor agonist, dose-dependently reduced tail flick latency as well as respectively increasing and decreasing the spontaneous activity of ON and OFF cells. Furthermore, it increased the ON cell burst and OFF cell pause. Intra-VL PAG prostaglandin E2 (PGE2) behaved similarly to ONO-DI-004. The effects of ONO-DI-004 and PGE2 were antagonized by intra-VL PAG L335677, a selective EP1 receptor antagonist. L335677 dose-dependently increased the tail flick latency and ongoing activity of the OFF cells, while reducing the ongoing ON cell activity. It also decreased the ON cell burst and OFF cell pause. In neuropathic rats using spare nerve injury (SNI) of the sciatic nerve model, EP1 receptor expression decreased in the VL PAG. However, ONO-DI-004 and L335677 were able to alter pain responses and ON and OFF cell activity, as they did in healthy animals. Collectively, these data show that within the VL PAG, EP1 receptor has a facilitatory effect on the nociceptive response and consistently affects RVM neuron activity. Thus, the blockade of EP1 receptor in the VL PAG leads to antinociception in neuropathic pain conditions, despite its down-regulation. The expression of EP1 receptor on GABAergic neurons is consistent with an EP1 receptor blockade-induced disinhibition of the antinociceptive descending pathway at VL PAG level.     

Endocannabinoid modulation of sympathetic and cardiovascular responses to acute stress in the periaqueductal gray of the rat

Activation of the sympathetic nervous system is fundamental to the coordinated response to stress or danger. The midbrain periaqueductal gray (PAG) contains the neural substrate required to recruit the sympathetic nervous system and organize the physiological and behavioral responses required to respond to imposed challenges. Endocannabinoids have been shown to influence associated behavioral responses. The defense response was used in this study as a working model to examine endocannabinoid modulation of the sympathetic response to acute stress in the anesthetized rat. Microinjection of the cannabinoid 1 (CB1) receptor agonist anandamide into the defense pathway of the dorsal PAG could elicit an increase in renal sympathetic nerve activity and blood pressure, twitching of the whiskers, and movement of the limbs. The response was attenuated by prior microinjection of the CB1 receptor antagonist AM-281 at the same site. Electrical stimulation of the hypothalamic defense area could evoke similar sympathoexcitatory and pressor responses, which were significantly attenuated by microinjection of AM-281 into the dorsal PAG. These data indicate that endocannabinoids can modulate the sympathetic and cardiovascular components of the acute stress response via CB1 receptors at the level of the PAG.     

Arginine vasopressin induces periaqueductal gray release of enkephalin and endorphin relating to pain modulation in the rat

Previous study has proven that microinjection of arginine vasopressin (AVP) into periaqueductal gray (PAG) raises the pain threshold, in which the antinociceptive effect of AVP can be reversed by PAG pretreatment with V2 rather than V1 or opiate receptor antagonist. The present work investigated the AVP effect on endogenous opiate peptides, oxytocin (OXT) and classical neurotransmitters in the rat PAG. The results showed that AVP elevated the concentrations of leucine-enkephalin (L-Ek), methionine-enkephalin (M-Ek) and beta-endorphin (beta-Ep), but did not change the concentrations of dynorphinA(1-13) (DynA(1-13)), OXT, classical neurotransmitters including achetylcholine (Ach), choline (Ch), serotonin (5-HT), gamma-aminobutyric acid (GABA), glutamate (Glu), dopamine (DA), norepinephrine (NE) and epinephrine (E), and their metabolic products in PAG perfusion liquid. Pain stimulation increased the concentrations of AVP, L-EK, M-Ek, beta-Ep, 5-HT and 5-HIAA (5-HT metabolic product), but did not influence the concentrations of DynA(1-13), OXT, the other classical neurotransmitters and their metabolic products. PAG pretreatment with naloxone - an opiate receptor antagonist completely attenuated the pain threshold increase induced by PAG administration of AVP, but local pretreatment of OXT or classical neurotransmitter receptor antagonist did not influence the pain threshold increase induced by PAG administration of AVP. The data suggested that AVP in PAG could induce the local release of enkephalin and endorphin rather than dynophin, OXT and classical neurotransmitters to participate in pain modulation.     

Antinociceptive and behavioral activation responses elicited by d-Pro(2)-endomorphin-2 in the ventrolateral periaqueductal gray are sensitive to sex and gonadectomy differences in rats

Sex differences have been observed in antinociception after morphine administered into either the lateral ventricles, rostral ventromedial medulla, or ventrolateral periaqueductal gray such that male rats exhibit significantly greater antinociception than female rats. Adult gonadectomy produced small, but significant changes in morphine antinociception relative to same-sex sham-operated controls. The present study examined whether sex and adult gonadectomy differences were observed in antinociceptive responses after D-Pro(2)-Endomorphin-2 (1-50 microg) elicited from the ventrolateral periaqueductal gray (vlPAG) on the tail-flick and jump tests in rats, and compared these effects with morphine antinociception. D-Pro(2)-Endomorphin-2 antinociception in the vlPAG was significantly greater in estrous-phase, sham-operated and ovariectomized female rats relative to sham-operated and castrated male rats on the tail-flick, but not jump test that differed markedly from the greater magnitude of morphine antinociception noted for male rats on both tests. In testing whether D-Pro(2)-Endomorphin-2's antinociceptive sex differences were secondary to alterations in activity, similar decreases in the pattern of total activity were observed after D-Pro(2)-Endomorphin-2 in the vlPAG in male and female rats. In evaluating whether male and female rats differed in their behavioral activation responses after D-Pro(2)-Endomorphin-2 in the vlPAG, significantly more excessive grooming, seizures, barrel rolls and explosive running behaviors were observed after D-Pro(2)-Endomorphin-2 in male, but not female rats during the precise periods of time when they were failing to display robust antinociceptive responses on the tail-flick test. Thus, the different patterns of sex differences after D-Pro(2)-Endomorphin-2 in the vlPAG appear to be attributable to sex-dependent alterations in behavioral activation rather than nociceptive processing per se.     

Gender differences in the antinociceptive effect of tramadol,alone or in combination with gabapentin,in mice

Gender difference in the antinociceptive effect of tramadol and gabapentin (alone or in combination) were investigated in mice. For investigation of acute antinociceptive effect, tramadol and gabapentin were administered to mice by intraperitoneal injection and per os, respectively, and antinociceptive activity was measured by the tail-flick test 30 min after drug administration. For investigation of the development of antinociceptive tolerance to analgesics, mice were injected with tramadol (60 mg/kg), alone or in combination with gabapentin (75 mg/kg), twice daily for seven consecutive days and the tail-flicks were tested on experimental days 1, 3, 5 and 7. Results showed there was a lower ED₅₀ value of tramadol antinociception in males than in females, indicating that females were less sensitive to the drug. Gabapentin produces a limited antinociception in both males and females. The combination of gabapentin and tramadol produced synergistic effect without gender difference. Repeated administration of tramadol produced antinociceptive tolerance in both genders. Gabapentin produced synergistic effect in tramadol-tolerant mice and repeated administration of gabapentin did not alter the synergistic effect in tramadol-tolerant mice. Because females show a higher overall prevalence of pain and less sensitivity to opioids, our finding may suggest a clinical significance of combined use of the two drugs.     

Substance P and antinociception

Substance P (SP)-induced antinociception is still a topic of controversy. Some investigators have failed to see an antinociceptive effect of SP, particularly following intraperitoneal administration. In the present experiments SP induced significant hot plate antinociception in male mice, following intraperitoneal administration. SP exhibited a bell-shaped dose response curve, and the antinociceptive effect was dependent on the pH of the vehicle. The antinociceptive effect of SP lasted for at least 1 hr and was naloxone-reversible. The antinociceptive effect of SP could be prevented by housing subjects collectively rather than individually during the experiment. In conclusion, the bell-shaped dose response curve, the solution pH and different testing procedures all influence the effects of SP on nociception. Given this complexity, it is not surprising that some experiments fail to demonstrate antinociception following SP administration.     

Functionally Selective Signaling for Morphine and Fentanyl Antinociception and Tolerance Mediated by the Rat Periaqueductal Gray

Functionally selective signaling appears to contribute to the variability in mechanisms that underlie tolerance to the antinociceptive effects of opioids. The present study tested this hypothesis by examining the contribution of G protein-coupled receptor kinase (GRK)/Protein kinase C (PKC) and C-Jun N-terminal kinase (JNK) activation on both the expression and development of tolerance to morphine and fentanyl microinjected into the ventrolateral periaqueductal gray of the rat. Microinjection of morphine or fentanyl into the periaqueductal gray produced a dose-dependent increase in hot plate latency. Microinjection of the non-specific GRK/PKC inhibitor Ro 32-0432 into the periaqueductal gray to block mu-opioid receptor phosphorylation enhanced the antinociceptive effect of morphine but had no effect on fentanyl antinociception. Microinjection of the JNK inhibitor SP600125 had no effect on morphine or fentanyl antinociception, but blocked the expression of tolerance to repeated morphine microinjections. In contrast, a microinjection of Ro 32-0432 blocked the expression of fentanyl, but not morphine tolerance. Repeated microinjections of Ro 32-0432 blocked the development of morphine tolerance and inhibited fentanyl antinociception whether rats were tolerant or not. Repeated microinjections of SP600125 into the periaqueductal gray blocked the development of tolerance to both morphine and fentanyl microinjections. These data demonstrate that the signaling molecules that contribute to tolerance vary depending on the opioid and methodology used to assess tolerance (expression vs. development of tolerance). This signaling difference is especially clear for the expression of tolerance in which JNK contributes to morphine tolerance and GRK/PKC contributes to fentanyl tolerance.     

Chimeric peptide of Met-enkephalin and FMRFa induces antinociception and attenuates development of tolerance to morphine antinociception.

A synthetic chimeric peptide of Met-enkephalin and FMRFamide (YGGFMKKKFMRFa), based on MERF was synthesized. This peptide was tested for possible antinociceptive effects using the tail flick test in mice. The effect of the chimeric peptide on morphine antinociception and development of tolerance to the antinociceptive action of morphine was also investigated. The chimeric peptide produced significant, dose-dependent antinociception (40, 60 and 90 mg/kg) in the tail flick test. Pretreatment with naloxone (5 mg/kg, IP) significantly attenuated the antinociceptive effect induced by the chimeric peptide (90 mg/kg, IP), indicating involvement of an opioidergic mechanism. In combination experiments with morphine, the antinociceptive dose of the chimeric peptide (60 mg/kg, IP) potentiated morphine (7 mg/kg, IP) antinociception. A low dose of the chimeric peptide (10 mg/kg, IP), that did not produce significant antinociception on its own, also potentiated morphine antinociception. In the tolerance studies, male albino mice received twice daily injections of morphine (20 mg/kg, IP) followed by either saline (0.1 ml) or chimeric peptide (80 mg/kg, IP) for a period of 4 days. A control group received twice daily injections of saline (0.1 ml) for the same period. When tested on Day 5, tolerance to antinociceptive action of morphine (15 mg/kg, IP) was evidenced by decreased response in chronic morphine plus saline treated mice compared to control group. Concurrent administration of chimeric peptide (80 mg/kg, IP) with morphine significantly attenuated the development of tolerance to the antinociceptive action of morphine. The preliminary results of this study demonstrate that peripherally administered chimeric peptide can produce dose dependent, naloxone reversible, antinociception; potentiate morphine antinociception and attenuate morphine tolerance, indicating a possible role of these type of amphiactive sequences in antinociception and its modulation. These chimeric peptides may also prove to be useful tools for further ascertaining the role of FMRFa family of peptides in mechanisms leading to opiate tolerance and dependence.     

Nimodipine is more effective than nifedipine in attenuating morphine tolerance on chronic co-administration in the rat tail-flick test

Opioids, when co-administered with L-type calcium channel blockers (L-CCBs) show morphine like higher antinociceptive effect. This antinociceptive effect has been further investigated using a different experimental paradigm. The effect of two different L-CCBs (nifedipine and nimodipine) on morphine-induced antinociception was studied by the tail-flick test (40 min after morphine administration) in adult Wistar rats. A fixed-dose of nimodipine or nifedipine (2 mg/kg, once daily) was combined with a fixed dose of morphine (10 mg/kg, twice daily) for 10 days. Co-administration of L-CCBs significantly increased the antinociceptive effect of morphine, even 12 hr after administration. Also, nimodipine was more effective than nifedipine. Nimodipine was further studied using a higher and escalating doses of morphine (20-30 mg/kg twice daily for 14 days). Nimodipine increased the antinociceptive effect of morphine in the latter part of the study (days nine to fourteen) though significant difference was observed on 11th evening and 12th morning. No obvious adverse effects were observed in the present study. The results show for the first time that nimodipine is more effective than nifedipine and that these L-CCBs continue to be effective, even 12 hr after administration in the tail-flick test.     

Corticosterone microinjected into nucleus pontis oralis increases tonic immobility in rats

Tonic immobility (TI) is also known as “immobility response”, “immobility reflex”, “animal hypnosis”, etc. It is an innate antipredatory behavior characterized by an absence of movement, varying degrees of muscular activity, and a relative unresponsiveness to external stimuli. Experimentally, TI is commonly produced by manually forcing an animal into an inverted position and restraining it in that position until the animal becomes immobile. Part of the neural mechanism(s) of TI involves the medullo-pontine reticular formation, with influence from other components of the brain, notably the limbic system. It has been observed that TI is more prolonged in stressed animals, and systemic injection of corticosterone (CORT) also potentiates this behavior. At present, the anatomical brain regions involved in the CORT modulation of TI are unknown. Thus, our study was made to determine if some pontine areas could be targets for the modulation of TI by CORT. A unilateral nucleus pontis oralis (PnO) microinjection of 1 μL of CORT (0.05 μg/1 μL) in rats resulted in clear behavioral responses. The animals had an increased duration of TI caused by clamping the neck (in this induction, besides of body inversion and restraint, there is also clamping the neck), with an enhancement in open-field motor activity, which were prevented by pretreatment injection into PnO with 1 μL of the mineralocorticoid-receptor antagonist spironolactone (0.5 μg/1 μL) or 1 μL of the glucocorticoid-receptor antagonist mifepristone (0.5 μg/1 μL). In contrast, these behavioral changes were not seen when CORT (0.05 μg/1 μL) was microinjected into medial lemniscus area or paramedian raphe. Our data support the idea that, in stressful situations, glucocorticoids released from adrenals of the prey reach the PnO to produce a hyper arousal state, which in turn can prolong the duration of TI.     

Role of giant cell nuclei of the reticular formation in mechanisms of analgesia during auricular electroacupuncture and action of morphine

The activity of antinociceptive mechanisms induced by auricular electroacupuncture (AEA) and intraperitoneal injection of morphine (M) was studied on rats subjected to bilateral lesion of gigantocellular nuclei (GCN). It was shown that under AEA hot-plate (HP) and tail-flick (TF) latencies (L) significantly increased as compared to the baseline level. A comparison of L shows that HP and TF in the experimental group were significantly shorter than in the control group. After M injection the rats of both the experimental and control groups showed a significant increase of HP L and TF L as compared to the baseline level, but there was no difference in L between the groups. It is concluded that neurochemical systems of GCN are involved in the mechanism of antinociception elicited in AEA while the mechanisms of antinociceptive effect of M do not involve GCN.     

Antinociceptive activity of impentamine, a histamine congener, after CNS administration

The brain neuromodulator histamine induces antinociception when administered directly into the rodent CNS. However, several compounds derived from H2 and H3 antagonists also produce antinociception after central administration. Pharmacological studies have shown that a prototype of these agents, improgan, induces analgesia that is not mediated by actions on known histamine receptors. Presently, the antinociceptive properties of a compound that chemically resembles both improgan and histamine were investigated in rats. Intraventricular (i.v.t.) administration of impentamine (4-imidazolylpentylamine) induced reversible, near-maximal antinociception on the hot plate and tail flick tests (15 microg, 98 nmol). The dose-response function was extremely steep, however, since other doses showed either no effect or behavioral toxicity. On the tail flick test, impentamine antinociception was resistant to antagonism by blockers of H1, H2, or H3 receptors, similar to characteristics previously found for improgan. In contrast, histamine antinociception was highly attenuated by H1 and H2 antagonists. These findings suggest that: 1) the histamine congener impentamine may induce antinociception by a mechanism similar to that produced by improgan, and 2) additional histamine receptors may be discovered that are linked to pain-attenuating processes.     

Synthesis and antinociceptive effects of endomorphin-1 analogs with C-terminal linked by oligoarginine

Endomorphins (EMs) cannot be delivered into the central nervous system (CNS) in sufficient quantity to elicit antinociception when given systemically because they are severely restricted by the blood-brain barrier (BBB). In the present study, we investigated herein a series of EM-1 analogs with C-terminal linked by oligoarginine in order to improve the brain delivery and antinociception after systemic administration. Indeed, all these analogs decreased the opioid receptor affinity and in vitro pharmacological activity. Moreover, analogs 4, 7-9 produced a less potent antinociceptive activity after intracerebroventricular (i.c.v.) administration, with the ED₅₀ values about 11- to 13-fold lower potencies than that of EM-1. Nevertheless, our results revealed that EM-1 failed to induce any significant antinociception at a dose of 50 μmol/kg after subcutaneous (s.c.) administration, whereas equimolar dose of these four analogs produced a little low but significant antinociceptive effects. Naloxone (10 nmol/kg, i.c.v.) significantly blocked the antinociceptive effects, indicating an opioid and central mechanism. These results demonstrated that C-terminal of EM-1 linked to oligoarginine improved the brain delivery, eliciting potent antinociception following peripheral administration.