Effects of supraspinal administration of PG-SPI and PG-KII, two amphibian tachykinin peptides, on nociception in the rat

Neurokinin-1 receptor-based bivalent drugs in pain management: The journey to nowhere?

Hybrid compounds (also known as chimeras, designed multiple ligands, bivalent compounds) are chemical units where two active components, usually possessing affinity and selectivity for distinct molecular targets, are combined as a single chemical entity. The rationale for using a chimeric approach is well documented as such novel drugs are characterized by their enhanced enzymatic stability and biological activity. This allows their use at lower concentrations, increasing their safety profile, particularly when considering undesirable side effects. In the group of synthetic bivalent compounds, drugs combining pharmacophores having affinities toward opioid and neurokinin-1 receptors have been extensively studied as potential analgesic drugs. Indeed, substance P is known as a major endogenous modulator of nociception both in the peripheral and central nervous systems. Hence, synthetic peptide fragments showing either agonism or antagonism at neurokinin 1 receptor were both assigned with analgesic properties. However, even though preclinical studies designated neurokinin-1 receptor antagonists as promising analgesics, early clinical studies revealed a lack of efficacy in human. Nevertheless, their molecular combination with enkephalin/endomorphin fragments has been considered as a valuable approach to design putatively promising ligands for the treatment of pain. This paper is aimed at summarizing a 20-year journey to the development of potent analgesic hybrid compounds involving an opioid pharmacophore and devoid of unwanted side effects. Additionally, the legitimacy of considering neurokinin-1 receptor ligands in the design of chimeric drugs is discussed.

Supraspinal injection of Substance P attenuates allodynia and hyperalgesia in a rat model of inflammatory pain

The neuropeptide Substance P (SP), that has a high affinity for the neurokinin 1 (NK1) receptor, is involved in modulation of pain transmission. Although SP is thought to have excitatory actions and promote nociception in the spinal cord, the peptide induces analgesia at the supraspinal level. The aim of this study was to evaluate the role of supraspinal SP and the NK1 receptor in inflammatory pain induced by injection of carrageenan in the hind paw of the rat. There are two nociceptive behavioral responses associated with this pain state: mechanical allodynia and heat hyperalgesia. Because the NK1 receptor colocalizes with the MOP receptor in supraspinal sites involved in pain modulation, we also decided to study the possible involvement of the opioid system on SP-induced analgesia. We found that treatment with SP, at doses of 3.5, 5 and 7 μg/5 μl/rat i.c.v., clearly showed inhibition of allodynia and hyperalgesia. Pretreatment with the selective NK1 antagonist L-733,060 (10mg/kg i.p.) blocked the SP-induced analgesia, suggesting the involvement of the NK1 receptor. This SP-induced analgesia was significantly reduced by administration of the opioid antagonist naloxone (3mg/kg s.c.). This reduction occurred when SP was administered either before or after the carrageenan injection. These results suggest a significant antinociceptive role for SP and the NK1 receptor in inflammatory pain at the supraspinal level, possibly through the release of endogenous opioids.

Substance P drives endocannabinoid-mediated disinhibition in a midbrain descending analgesic pathway

Substance P is thought to play an essential role in several forms of supraspinally mediated analgesia. The actions of substance P on synaptic transmission within descending analgesic pathways, however, are largely unknown. Here, we used whole-cell recordings from rat midbrain slices to examine the effects of substance P on GABAergic and glutamatergic transmission within the periaqueductal gray (PAG), a key component of a descending analgesic pathway that projects via the rostral ventromedial medulla (RVM) to the spinal cord dorsal horn. We found that substance P reversibly decreased the amplitude and increased the paired-pulse ratio of evoked IPSCs recorded from identified PAG-RVM projection neurons and from unidentified PAG neurons. Substance P had no effect on miniature IPSCs, implying an indirect mode of action. The effects of substance P were abolished by metabotropic glutamate type 5 and cannabinoid CB1 receptor antagonists, but unaltered by NMDA, GABA(B), mu,delta-opioid, adenosine A(1), and 5HT(1A) receptor antagonists. Consistent with a role for endogenous glutamate in this process, substance P increased the frequency of action potential-dependent spontaneous EPSCs. Moreover, the effect of substance P on evoked IPSCs was mimicked and occluded by a glutamate transport inhibitor. Finally, these effects were dependent on postsynaptic G-protein activation and diacylglycerol lipase activity, suggesting the requirement for retrograde signaling by the endocannabinoid 2-arachidonoylglycerol. Thus, substance P may facilitate descending analgesia in part by enhancing glutamate-mediated excitation and endocannabinoid-mediated disinhibition of PAG-RVM projection neurons.

Participation of mu-opioid, GABA(B), and NK1 receptors of major pain control medullary areas in pathways targeting the rat spinal cord: implications for descending modulation of nociceptive transmission

Several brain areas modulate pain transmission through direct projections to the spinal cord. The descending modulation is exerted by neurotransmitters acting both at spinally projecting neurons and at interneurons that target the projection neurons. We analyzed the expression of mu-opioid (MOR), gamma-aminobutyric acid GABA(B), and NK1 receptors in spinally projecting neurons of major medullary pain control areas of the rat: rostroventromedial medulla (RVM), dorsal reticular nucleus (DRt), nucleus of the solitary tract, ventral reticular nucleus, and lateralmost part of the caudal ventrolateral medulla. The retrograde tracer cholera toxin subunit B (CTb) was injected into the spinal dorsal horn, and medullary sections were processed by double immunocytochemistry for CTb and each receptor. The RVM contained the majority of double-labeled neurons followed by the DRt. In general, high percentages of MOR- and NK1-expressing neurons were retrogradely labeled, whereas GABA(B) receptors were mainly expressed in neurons that were not labeled from the cord. The results suggest that MOR and NK1 receptors play an important role in direct and indirect control of descending modulation. The co-localization of MOR and GABA(B) in DRt neurons also demonstrated by the present study suggests that the pronociceptive effects of this nucleus may be controlled by local opoidergic and GABAergic inhibition of the pronociception increased during chronic pain.

Dorsal horn NK1-expressing neurons control windup of downstream trigeminal nociceptive neurons

Windup is a progressive, frequency-dependent increase in the excitability of trigeminal and spinal dorsal horn wide dynamic range (WDR) nociceptive neurons to repetitive stimulation of primary afferent nociceptive C-fibers. Superficial dorsal horn neurokinin 1 receptor (NK1R)-expressing neurons were recently shown to regulate sensitization of WDR nociceptive neurons through activation of a defined spino-bulbo-spinal loop. However, the windup of WDR nociceptive neurons was not regulated through this loop. In the present study, we sought to identify the alternative circuit activated by dorsal horn NK1Rs that mediates WDR neuron windup. As a model we used the rat spinal trigeminal nucleus, in which the subnucleus oralis (Sp5O) contains a pool of WDR neurons that receive their nociceptive C-input indirectly via interneurons located in the medullary dorsal horn (MDH). First, we found that intravenous injection of NK1R antagonists (SR140333 and RP67580) produced a reversible inhibition of Sp5O WDR neuron windup. Second, we anatomically identified in the MDH lamina III a subpopulation of NK1R-expressing local interneurons that relay nociceptive information from the MDH to downstream Sp5O neurons. Third, using microinjections of NK1R antagonists during in vivo electrophysiological recordings from Sp5O WDR neurons, we showed that WDR neuron windup depends on activation of NK1Rs located in the MDH laminae I-III. We conclude that, in contrast to central sensitization that is controlled by a spino-bulbo-spinal loop, Sp5O WDR neuron windup is regulated through a local circuit activated by MDH lamina III NK1Rs.

Rat/mouse hemokinin-1, a mammalian tachykinin peptide, markedly potentiates the antinociceptive effects of morphine administered at the peripheral and supraspinal level

Rat/mouse hemokinin 1 (r/m HK-1) is a mammalian tachykinin peptide whose biological functions are not fully understood. Our recent report showed that i.c.v. administration of r/m HK-1 could produce dose- and time-related antinociceptive effect at nanomole concentration, and naloxone significantly antagonized this effect. Thus, we provide indirect evidence favoring a role of NK1 supraspinal receptors in the inhibitory control of descending pain pathways, a role that seems to partially involve the activation of the endogenous opioid systems. Based on this report, the present study was conducted to further investigate the direct functional interaction between supraspinal tachykinin (r/m HK-1) and opioid systems. The results demonstrate that i.c.v. administration of r/m HK-1 (5 nmol/kg) could significantly potentiate the antinociceptive effects of morphine which was injected at peripheral and supraspinal level. These antinociceptive effects were blocked by prior treatment with the classical opioid receptors antagonist naloxone, indicating that the potentiated analgesic response is mediated by opioid-responsive neurons. Consistent with previous biochemical data, a likely mechanism underlying the peptide-mediated enhancement of opioid analgesia may center on the ability of r/m HK-1 to release endogenous opioid peptides. We suggest that there may be a cascade amplification mechanism in pain modulation when the two agents were co-administrated. The synergistic analgesic relationship of morphine and r/m HK-1 established here supports the hypothesis that supraspinal tachykinin and peripheral and central opioid systems have a direct functional interaction in the modulation of local nociceptive responses.

Effects of peptides, with emphasis on feeding, pain, and behavior A 5-year (1999-2003) review of publications in Peptides

Novel effects of naturally occurring peptides are continuing to be discovered, and their mechanisms of actions as well as interactions with other substances, organs, and systems have been elucidated. Synthetic analogs may have actions similar or antagonistic to the endogenous peptides, and both the native peptides and analogs have potential as drugs or drug targets. The journal Peptides publishes many leading articles on the structure-activity relationship of peptides as well as outstanding reviews on some families of peptides. Complementary to the reviews, here we extract information from the original papers published during the past five years in Peptides (1999-2003) to summarize the effects of different classes of peptides, their modulation by other chemicals and various pathophysiological states, and the mechanisms by which the effects are exerted. Special attention is given to peptides related to feeding, pain, and other behaviors. By presenting in condensed form the effects of peptides which are essential for systems biology, we hope that this summary of existing knowledge will encourage additional novel research to be presented in Peptides.

Substance P microinjected into the periaqueductal gray matter induces antinociception and is released following morphine administration

The aims of the present study were to investigate, in rats, the behavioral effects of substance P (SP) microinjected into the ventrolateral periaqueductal gray (PAG) and the effects of the neurokinin 1 (NK-1) receptor antagonist [d-Arg1, d-Trp7, 9, Leu11]-substance P (Spantide). The effect of morphine administration on the release of SP in the ventrolateral PAG was also investigated using microdialysis in awake rats. SP microinjected into the ventrolateral part of the PAG induced significant increases in the hindpaw withdrawal latencies (HWLs) to thermal and mechanical stimulation as an antinociceptive response. The NK-1 receptor antagonist blocked these effects but exhibited no antinociceptive effect alone. Subcutaneous administration of morphine increased basal SP-like immunoreactivity (SP-LI) release in the microdialysate obtained from the ventrolateral PAG of freely moving rats. Our results demonstrate that SP injected into the ventrolateral PAG induces an antinociceptive effect via activation of NK-1 receptors. Morphine administered systemically induces the release of SP in the ventrolateral PAG. We suggest that an increased release of SP in the PAG may contribute to opioid antinociception.

Effects of the peripherally acting NK receptor antagonist, SB-235375, on intestinal and somatic nociceptive responses and on intestinal motility in anaesthetized rats

We investigated the effects of the selective NK(3) tachykinin receptor antagonist, SB-235375, on noxious signalling from gut and skin and on intestinal motility in anaesthetized rats. We also measured penetrance into brain and spinal cord. Nociceptive responses in reaction to colorectal distension and skin pinch were assessed by recording the electromyogram (EMG) from the external oblique muscle (a visceromotor response). Motility was measured by recording intraluminal pressure waves during changes in baseline pressure in the jejunum. Colorectal compliance was assessed by measuring luminal pressure change during isovolumic distension. SB-235375 (20 mg kg(-1), by i.v. bolus) reduced the EMG response to colorectal distension by over 90%. The reduction was slow at onset, peaked at about 60 min, and lasted for over 2 h. Responses to noxious skin pinch were unchanged. Amplitudes of propulsive waves in the jejunum were slightly reduced, but their frequency of occurrence was unchanged. SB-235375 decreased colorectal compliance by 5-10%. There was undetectable penetration of i.v. SB-235375 into brain or spinal cord. We conclude that SB-235375 acts peripherally to substantially reduce nociceptive signalling from colorectum without affecting noxious signalling from skin and with little effect on intestinal motility.

Noxious-evoked c-fos expression in brainstem neurons immunoreactive for GABAB, mu-opioid and NK-1 receptors

Modulation of nociceptive transmission at the brainstem involves several neurochemical systems. The precise location and specific characteristics of nociceptive neurons activated in each system was never reported. In this study, the presence of GABA(B), mu-opioid, and neurokinin-1 (NK-1) receptors in brainstem nociceptive neurons was investigated by double-immunocytochemical detection of each receptor and noxious-evoked induction of the c-fos proto-oncogene. Noxious cutaneous mechanical stimulation significantly increased the proportions of neurons double-labelled for Fos and GABA(B) receptors in several brainstem regions, namely, the reticular formation of the caudal ventrolateral medulla (VLMlat and VLMrf), lateral reticular nucleus, spinal trigeminal nucleus, pars caudalis (Sp5C), nucleus of the solitary tract, dorsal reticular nucleus, ventral reticular nucleus, raphe obscurus nucleus and dorsal parabrachial nucleus (DPB). For mu-opioid receptors, the proportions of double-labelled neurons in noxious-stimulated animals were higher than in controls only in the VLMlat, VLMrf, Sp5C, DPB and A5 noradrenergic cell group. As for the NK-1 receptor, no significant differences were found between control and stimulated animals. According to these results, neurons expressing GABA(B), mu-opioid and NK-1 receptors at several pain control centres of the brainstem are differentially involved in processing nociceptive mechanical input. The data provide the definition of new supraspinal targets for selective modulation of nociceptive neurons in order to define better strategies of pain control.