Analgesic activity of substance P in rats: apparent mediation by met-enkephalin release

Ion Channels Involved in Substance P-Mediated Nociception and Antinociception

Substance P (SP), an 11-amino-acid neuropeptide, has long been considered an effector of pain. However, accumulating studies have proposed a paradoxical role of SP in anti-nociception. Here, we review studies of SP-mediated nociception and anti-nociception in terms of peptide features, SP-modulated ion channels, and differential effector systems underlying neurokinin 1 receptors (NK1Rs) in differential cell types to elucidate the effect of SP and further our understanding of SP in anti-nociception. Most importantly, understanding the anti-nociceptive SP-NK1R pathway would provide new insights for analgesic drug development.

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.

Study on the distribution sites and the molecular mechanism of analgesia after intracerebroventricular injection of rat/mouse hemokinin-1 in mice

Hemokinin-1 is a peptide encoded by Pptc, which belongs to the family of mammalian tachykinins. Our previous results showed that rat/mouse hemokinin-1 (r/m HK-1) produced striking analgesia after intracerebroventricular (i.c.v.) injection in mice, and the analgesia could be blocked by the NK1 receptor antagonist and the opioid receptor antagonist, respectively. However, the precise distribution sites and the molecular mechanism involved in the analgesic effect after i.c.v. administration of r/m HK-1 are needed to be further investigated deeply. Using the fluorescence labeling method, our present results directly showed that r/m HK-1 peptides were mainly distributed at the ventricular walls and several juxta-ventricular structures for the first time. Our results showed that the mRNA expressions of NK1 receptor, PPT-A, PPT-C, KOR, PDYN, DOR and PENK were not changed markedly, as well as the protein expression of NK1 receptor was hardly changed. However, both the transcripts and proteins of MOR and POMC were up-regulated significantly, indicating that the analgesic effect induced by i.c.v. administration of r/m HK-1 is related to the activation of NK1 receptor first, then it is related to the release of endogenous proopiomelanocortin, as well as the increased expression level of μ opioid receptor. These results should facilitate further the analysis of the analgesia of r/m HK-1 in the central nerval system in acute pain and may open novel pharmacological interventions.

Monoaminergic Antidepressants in the Relief of Pain: Potential Therapeutic Utility of Triple Reuptake Inhibitors (TRIs)

Over 75% of depressed patients suffer from painful symptoms predicting a greater severity and a less favorable outcome of depression. Imaging, anatomical and functional studies have demonstrated the existence of common brain structures, neuronal pathways and neurotransmitters in depression and pain. In particular, the ascending serotonergic and noradrenergic pathways originating from the raphe nuclei and the locus coeruleus; respectively, send projections to the limbic system. Such pathways control many of the psychological functions that are disturbed in depression and in the perception of pain. On the other hand, the descending pathways, from monoaminergic nuclei to the spinal cord, are specifically implicated in the inhibition of nociception providing rationale for the use of serotonin (5-HT) and/or norepinephrine (NE) reuptake inhibitors (SSRIs, NRIs, SNRIs), in the relief of pain. Compelling evidence suggests that dopamine (DA) is also involved in the pathophysiology and treatment of depression. Indeed, recent insights have demonstrated a central role for DA in analgesia through an action at both the spinal and suprasinal levels including brain regions such as the periaqueductal grey (PAG), the thalamus, the basal ganglia and the limbic system. In this context, dopaminergic antidepressants (i.e., containing dopaminergic activity), such as bupropion, nomifensine and more recently triple reuptake inhibitors (TRIs), might represent new promising therapeutic tools in the treatment of painful symptoms with depression. Nevertheless, whether the addition of the dopaminergic component produces more robust effects than single- or dual-acting agents, has yet to be demonstrated. This article reviews the main pathways regulating pain transmission in relation with the monoaminergic systems. It then focuses on the current knowledge regarding the in vivo pharmacological properties and mechanism of action of monoaminergic antidepressants including SSRIs, NRIs, SNRIs and TRIs. Finally, a synthesis of the preclinical studies supporting the efficacy of these antidepressants in analgesia is also addressed in order to highlight the relative contribution of 5-HT, NE and DA to nociception.

Effects of neurokinin-1 receptor agonism and antagonism in the rostral ventromedial medulla of rats with acute or persistent inflammatory nociception

The rostral ventromedial medulla (RVM), a central relay in the bulbospinal pathways that modulate nociception, contains high concentrations of substance P (Sub P) and neurokinin-1 (NK1) receptors. However, the function of Sub P in the RVM is poorly understood. This study characterized the actions of Sub P in the RVM in the absence of injury and then used two NK1 receptor antagonists, L-733,060 and L-703, 606, to probe the role of endogenously released Sub P in the development and maintenance of persistent inflammatory nociception of immune or neurogenic origin. In uninjured rats, microinjection of Sub P in the RVM produced a transient thermal antinociception that was attenuated by pretreatment with L-733,060 or L-703,606. It did not alter threshold to withdrawal from tactile stimulation with von Frey filaments. Microinjection of the antagonists alone did not alter paw withdrawal latency (PWL) or threshold suggesting that Sub P is not tonically released in the RVM in the absence of injury. However, microinjection of either antagonist in the RVM was sufficient to reverse heat hyperalgesia 4 h, 4 days or 2 weeks after intraplantar (ipl) injection of complete Freund's adjuvant (CFA). Antagonism of NK1 receptors in the RVM did not prevent or reverse tactile hypersensitivity induced by CFA, but did attenuate that produced by capsaicin. NK1 receptor antagonism did not prevent the development of thermal hyperalgesia, tactile hypersensitivity or spontaneous pain behaviors induced by mustard oil (MO). The results suggest that Sub P has bimodal actions in the RVM and that following inflammatory injury, it can play a critical role as a pronociceptive agent in the development and maintenance of hyperalgesia and tactile hypersensitivity. However, its actions are highly dependent on the stimulus modality and the type of injury, and this may be an additional basis for the poor efficacy of NK1 receptor antagonists in clinical trials.

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.

Cellular basis for the effects of substance P in the periaqueductal gray and dorsal raphe nucleus

Substance P (SP) is known to act at supraspinal sites to influence pain sensitivity as well as to promote anxiety. The effects of SP could be mediated in part by actions in the periaqueductal gray (PAG) and the dorsal raphe nucleus (DRN), adjoining mesencephalic cell groups that are strategically positioned to influence both nociception and mood. Previous studies have indicated that SP regulates both enkephalin and serotonin neurotransmission in these brain regions. To determine the mechanism underlying the effects of SP in the PAG and DRN, the distribution of the principal receptor for SP, the neurokinin 1 (NK1) receptor, was examined with respect to other neurotransmitter markers. PAG neurons that had NK1 receptor immunolabeling were interdigitated with and received contacts from enkephalin-containing neurons. However, only a few (16/144; 11%) neurons with NK1 receptor also contained enkephalin immunoreactivity after colchicine treatment. In the DRN, dendrites containing NK1 receptor were selectively distributed in the dorsomedial subdivision. The majority (132/137; 96%) of these dendrites did not contain immunoreactivity for the serotonin-synthesizing enzyme tryptophan hydroxylase. In contrast, neuronal profiles with NK1 receptor in both the PAG and the DRN often contained immunolabeling for glutamate. Light and electron microscopic examination revealed that 48-65% of cell bodies and dendrites with NK1 receptor were dually immunolabeled for glutamate. These data suggest that SP directly acts primarily on glutamatergic neurons in the PAG and DRN. To a lesser extent, enkephalin-containing neurons may be targeted. Through these actions, it may subsequently influence activity of larger populations of neurons containing enkephalin as well as serotonin. This circuitry could contribute to, as well as coordinate, effects of SP on pain perception and mood.

The NK1 receptor is essential for the full expression of noxious inhibitory controls in the mouse

Behavioral analysis of the NK1 receptor gene knock-out (NK1-/-) mouse indicated that substance P was closely involved in orchestrating the physiological and behavioral response of the animal to major environmental stressors. In particular, endogenous pain control mechanisms, such as stress-induced analgesia were substantially impaired in mutant mice, suggesting a reduction in descending inhibitory controls to the spinal cord from the brainstem. To directly test the integrity of descending controls in NK1-/- mice, we have analyzed c-Fos expression in laminae I-II of the lumbar and cervical cord and in the rostral ventromedial medulla in an experimental paradigm known to require recruitment of descending inhibitory controls. Anesthetized mice were stimulated with water at 50 degrees C either on their forepaw, hindpaw, or on both the hindpaw plus forepaw concurrently. Wild-type mice, naive or treated with an NK1 antagonist (RP67580) or its inactive isomer (RP68651), were compared with NK1-/- mice. C-Fos expression at the lumbar laminae I-II level was significantly reduced, whereas it was significantly greater in the raphe magnus and pallidus nuclei in the double stimulation situation in wild-type compared with NK1-/- mice. Blocking the NK1 receptor pharmacologically reproduced, in an enantiomere-selective manner, the data from NK1-/- mice, with no evidence for recruitment of descending inhibition at the lumbar cord level after forepaw stimulation. The present study demonstrates that the NK1 receptor is essential for the full development of noxiously evoked descending inhibition.

Effect of cerebral ventricles perfusion with naloxone on trigemino-hypoglossal reflex in rats

The goal of this study was to determine whether opioid receptor antagonist naloxone abolishes the influence of periaqueductal central gray (PAG) on nociceptive evoked tongue jerks (ETJ) -- a trigemino-hypoglossal reflex induced by tooth pulp stimulation. In rats under chloralose anesthesia three series of experiments were performed. In the first two groups perfusions of lateral ventricles-cerebellomedullary cistern with McIlwain-Rodnight's solution and naloxone were carried out. In group 3 naloxone was infused through a catheter through the jugular vein. The amplitudes of tongue jerks induced by tooth pulp stimulation were recorded during subsequent 10 min perfusions. Mean amplitude of tongue movements induced by tooth pulp stimulation was regarded as the indicator of the magnitude of trigemino-hypoglossal reflex. We observed that perfusion of the cerebral ventricles with naloxone (100 nmol/ml) increased the trigemino-hypoglossal reflex up to 143%. The amplitude of ETJ was significantly reduced during PAG stimulation with a train of electrical impulses. After obtaining a significant -- 93% -- inhibition of ETJ (7% of the control), naloxone (100 nmol/ml) was added to the perfusion fluid. This led to a significant increase of the reflex up to 68%. Infusion of naloxone through the jugular vein did not affect the reflex. The above results suggest that the inhibition of ETJ due to PAG stimulation is partially reversed by naloxone and mediated via interactions with endogenous opioid systems involved in modulation of nociception.

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

We investigated and compared the effects of two amphibian tachykinins, the NK1 receptor agonist PG-SPI and the NK3 receptor agonist PG-KII, and the mammalian tachykinins substance P, neurokinin A and neurokinin B on the reaction time to a painful radiant heat stimulus (tail-flick test in rats) after intracerebroventricular injection. PG-SPI (1, 10 and 20 microg) and PG-KII (1, 5 and 10 microg) significantly increased the reaction time. Substance P (10 microg) injected intracerebroventricularly induced antinociception, whereas neurokinin A and neurokinin B did not. Like analgesia evoked by exogenous substance P, PG-SPI-evoked analgesia was blocked by pretreatment with naloxone. Naloxone left PG-KII antinociception unchanged, but the NK3 receptor selective antagonist markedly reduced it. These findings suggest NK1 and NK3 tachykinin receptor system involvement in supraspinal analgesia in rats.

Immunocytochemical localization of substance P receptor in rat periaqueductal gray matter: a light and electron microscopic study

The distribution of substance P receptor (SPR) protein in the rat periaqueductal gray matter (PAG) was investigated with a polyclonal antibody in the four subdivisions obtained by cytochrome-oxidase histochemistry (Co-hi). At light microscopic analysis, immunoreactivity appeared particularly dense in the dorsal subdivision of the PAG, was less intense in the other subdivisions, and formed several longitudinally organized columns. SPR-like immunoreactivity (SP(R-i)) was localized mostly to cell bodies and dendrites of small and medium-sized neurons, which constituted about 6% of the total neuronal population of the PAG. At the electron microscopic level, SP(R-i) could be observed on postsynaptic as well as on nonsynaptic regions of both cell bodies and dendrites. A small proportion of axons (4.2%) and axon terminals (5.3%) showed SP(R-i), the majority of labeled axon terminals, amounting to about 70% of synapsing elements, formed asymmetric synapses with dendrites. Rare astroglial processes displaying SP(R-i) were also observed scattered throughout the neuropil of all PAG subdivisions. Our observations suggest that 1) also in the PAG, SP may act in a diffuse, nonsynaptic manner, probably on targets that are distant from its sites of release; and 2) SP may modulate excitatory neurotransmission acting presynaptically on those labeled axons that form asymmetric synapses.

Tachykinin NK-1 and NK-3 selective agonists induce analgesia in the formalin test for tonic pain following intra-VTA or intra-accumbens microinfusions

Experiments were designed to examine the analgesic effects induced by selective tachykinin receptor agonists microinfused into either the ventral tegmental area (VTA) or nucleus accumbens septi (NAS). Rats were tested in the formalin test for tonic pain following an injection of 0.05 ml of 2.5% formalin into one hind paw immediately after bilateral intra-VTA infusions of either the NK-1 agonist, GR-73632 (0.005, 0.05 or 0.5 nmol/side), the NK-3 agonist, senktide (0.005, 0.5 or 1.5 nmol/side), or saline. Two weeks later, the saline-treated rats were assessed in the tail-flick test for phasic pain after infusions of the tachykinin agonists. Tail-flick latencies were recorded following immersion of the tail in 55 degrees C hot water at 10 min intervals for 1 h immediately after intra-VTA infusions of either GR-73632 (0.5 nmol/side), senktide (1.5 nmol/side) or saline. In a second group of rats, the same effects were studied after infusions into the nucleus accumbens (NAS) of GR-73632 (0.005, 0.5 or 1.5 nmol/side), senktide (0.005, 0.5 or 1.5 nmol/side), or saline. In both the VTA and NAS, the NK-1 and the NK-3 agonists caused significant analgesia in the formalin test, although the NK-1 agonist appeared to be more effective. Naltrexone (2.0 mg/kg) pretreatment failed to reverse the analgesic effects in the formalin test induced by intra-VTA infusions of the substance P (SP) analog, DiMe-C7 (3.0 microg/side), GR-73632 (0.5 nmol/side), or senktide (1.5 nmol/side). Neither compound given at either site was effective in the tail-flick test. These findings suggest that SP-dopamine (DA) interactions within the mesolimbic DA system play an important role in the inhibition of tonic pain. Furthermore, they support our earlier ideas that activation of midbrain DA systems by SP might play a role in stress- and/or pain-induced analgesia.

Opiate receptors modulate estrogen-induced cholecystokinin and tachykinin but not enkephalin messenger RNA levels in the limbic system and hypothalamus

Cholecystokinin, substance P and methionine enkephalin all regulate the display of reproductive behaviour. Their expression is exquisitely regulated by estrogen in the limbic-hypothalamic circuit, a circuit that regulates the display of estrogen-sensitive female reproductive behavior. Relatively little is known, however, about the interaction of endogenous opioid peptides with cholecystokinin and substance P in the limbic-hypothalamic circuit. Opiates antagonize the release of cholecystokinin and substance P in the hypothalamus and periaqueductal gray and stimulate cholecystokinin messenger RNA levels in the amygdala. To determine the effect of endogenous opioid input on estrogen-induced cholecystokinin, enkephalin and substance P expression, in situ hybridization histochemistry was used to examine estrogen-induced messenger RNA levels of these neuropeptides in specific nuclei of the limbic system and hypothalamus in the presence of opiate receptor antagonists. Estrogen treatment of ovariectomized rats significantly elevated cholecystokinin messenger RNA levels in the central portion of the medial preoptic nucleus, the encapsulated portion of the bed nucleus of the stria terminalis and the posterodorsal medial amygdala, as well as increased preproenkephalin and preprotachykinin messenger RNA levels in the ventromedial hypothalamic nucleus and the posterodorsal medial amygdala. The universal opiate receptor antagonist naltrexone and the delta-opiate receptor antagonist naltrindole each potentiated the estrogen-induced increase and elevated cholecystokinin messenger RNA levels an additional 1.9- to 2.8-fold depending on the nucleus examined, but had no effect on the estrogen-induced expression of either preproenkephalin or preprotachykinin messenger RNA. beta-Funaltrexamine, a mu-opiate receptor antagonist, had no effect on the medial preoptic or medial amygdaloid cholecystokinin messenger RNA levels or on the estrogen-induced expression of preproenkephalin messenger RNA but did cause a decrease in estrogen-induced cholecystokinin messenger RNA levels in the bed nucleus of the stria terminalis and a decrease in the preprotachykinin messenger RNA levels in the ventromedial hypothalamic nucleus. These results indicate that endogenous opioids, acting on the delta-opiate receptor within nuclei of the limbic-hypothalamic circuit, restrain the estrogen-induced increase of cholecystokinin messenger RNA expression. Activation of the mu-opiate receptor, however, may facilitate cholecystokinin messenger RNA expression in the bed nucleus of the stria terminalis and preprotachykinin messenger RNA expression in the ventromedial hypothalamic nucleus. Thus, endogenous opioid peptides may act in a site- and receptor-specific manner to modulate estrogen-induced neuropeptide levels in the limbic system and hypothalamus.

Antinociceptive involvement of substance P in the spinal cord of mice: dose effects of substance P on the behavior elicited by intrathecally administered NMDA

The functional interaction between substance P (SP) and N-methyl-D-aspartate (NMDA) was studied to clarify the diversity of the roles of SP in nociceptive processes at the spinal level in mice. Behavioral responses elicited by intrathecal co-administration of NMDA (0.25 nmol) with various doses of SP (0.3-12 pmol) were observed for 1 min. The high dose of SP (12 pmol) potentiated NMDA-induced responses, which consisted of caudally directed licking and biting, while the low dose of SP (1 pmol) significantly reduced the responses by 40% compared to control mice administered NMDA alone. The antinociceptive effect of the low dose of SP was negated by co-administration of the opioid receptor antagonist naloxone. Furthermore, the antinociception produced by SP was present in mice pre-treated with systemic administration of capsaicin during the neonatal period. These results suggest that one of the roles SP plays at the spinal level is an involvement in antinociception. The activities of excitatory dorsal horn neurons are considered to be inhibited by endogenous opioid peptides released from inhibitory dorsal horn neurons directly stimulated by SP.

Age-related changes in nociception, behavior, and monoamine levels in rats

1. Pain threshold, behavioral parameters, and monoamine levels were compared in two groups of rats: adult (12 months old) and old rats (25 months old). 2. No differences in nociception were found between the two groups using the tail-shock test. 3. Behavioral experiments with the holeboard test showed that locomotor activity and exploration activity were lower in aged animals, whereas no significant differences were found in emotivity. 4. Using high-performance liquid chromatography (HPLC) techniques, we found that serotonin and dopamine showed lower levels in the old group.

Behavioral and analgesic effects induced by administration of nifedipine and nimodipine

Evidence exists that calcium antagonists can have effects on neural function. The aim of this work is to analyze the effect of two dihydropyridines, nifedipine and nimodipine, administered for 11 days on the behavior and pain sensitivity of rats. Nociception was tested using the tail electric stimulation test, and behavior parameters using a holeboard. Our results show that chronic administration of nifedipine or nimodipine induces analgesia that can be evaluated by tail withdrawal. However, neither the vocalization nor the vocalization after discharge were modified, so the analgesia may be mediated by spinal mechanisms. Rats treated with nifedipine or nimodipine exhibited a dose-dependent tendency to avoid the center of the field without modification of other parameters, suggesting an increased emotivity in the rats. This conclusion is supported by the fact that anxiogenic or anxiolytic drugs modify the pattern of locomotion without significant changes in other parameters related with the motility. The results from this study suggest the view of a complex mechanism of action underlying nifedipine- and nimodipine-mediated behavioral effects.

Repeated spinal cord stimulation decreases the extracellular level of gamma-aminobutyric acid in the periaqueductal gray matter of freely moving rats

Most of the previous experimental studies on the antinociceptive effects of electrical spinal cord stimulation (SCS) have focused on short-lasting effects mainly depending on spinal mechanisms. However, patients treated with SCS for chronic pain often report pain relief exceeding the period of stimulation for several hours. The long lasting effect of SCS might not only involve spinal, but also supraspinal mechanisms. A supraspinal region of major importance for the coordination of descending pain inhibition is the periaqueductal grey matter (PAG). The aim of the present microdialysis study, performed in awake freely moving rats, was to investigate if repeated SCS (two 30 min periods separated by a 90 min resting period) alters the extracellular neurotransmitter concentrations in the ventrolateral PAG. In a first series of experiments significantly decreased (-30%; P < 0.05; n = 7) gamma-aminobutyric acid (GABA) levels were detected immediately after the second SCS session. Neither the concentration of serotonin nor that of substance P-like immunoreactivity (SP-LI) was affected by SCS. The decrease of GABA after two SCS sessions was confirmed in a second series of experiments (-30%; P < 0.05; n = 7). No spontaneous decline of GABA was observed in sham-stimulated animals (n = 6). The glutamate concentration was also determined in this latter series of experiments and a significant decrease (-23%; P < 0.05; n = 5) was observed after the second SCS session. As GABA-neurons in the PAG exert a tonic depressive effect on the activity in descending pain inhibitory pathways, a decreased extracellular GABA level in this region, as detected following repeated SCS, might indicate an increased pain inhibition.

Intra-VTA infusions of the substance P analogue, DiMe-C7, and intra-accumbens infusions of amphetamine induce analgesia in the formalin test for tonic pain

Experiments were designed to examine the analgesic effects of SP injected into the ventral tegmental area (VTA). Rats received bilateral intra-VTA infusions of 3.0 micrograms/0.5 microliter/side of the SP analogue, DiMe-C7, or the vehicle, either immediately prior to or 25 min following an injection of 0.05 ml of 2.5% formalin into one hind paw. Formalin-induced pain responses were continuously recorded for 75 min. DiMe-C7 attenuated pain responses for approximately 30 min; the analgesia was more potent and longer-lasting when DiMe-C7 was infused after, rather than prior to, the early pain phase. In another set of experiments, rats were tested in the formalin test immediately following bilateral infusions of amphetamine (1.5 or 2.5 micrograms/0.05 microliter/side) into either the medial prefrontal cortex (mPFC) or the nucleus accumbens septi (NAS). Amphetamine failed to alter pain responses when infused into the mPFC, but both doses attenuated pain responses during 25 min when infused into the NAS. There was no evidence for pain inhibition in the tail-flick test for phasic pain following either intra-VTA DiMe-C7 or intra-NAS amphetamine. The finding that intra-VTA DiMe-C7 and intra-NAS amphetamine produces analgesia in the formalin, but not the tail-flick test, suggests that activation of mesolimbic dopamine (DA) neurons contributes to suppression of tonic pain. Because stressors attenuate tonic pain responses, and are known to cause SP release in the VTA, we speculate that SP-induced activation of midbrain DA systems may mediate a form of pain- or stress-induced pain inhibitory system.

A developmental study on stress-induced antinociception measured by the tail electric stimulation test

The possible influence of weaning on the development of different neural mechanisms involved in stress-induced antinociception (SIA) was studied. Male Wistar albino rats were used for studies on adult and pre- and postweanling rats of 20 and 25 days of age, respectively. Animals were stressed by warm-water (20C) swimming for 3-min periods. Antinociception was assessed by the tail electric stimulation test. The thresholds for the motor response (tail withdrawal) (TW), vocalization during stimulus (V), and vocalization after discharge (VAD) were recorded. These responses are considered to be integrated at spinal, medulla oblongata, and diencephalon-rhinencephalon levels, respectively. In 20-day-old neonates, swimming stress only induced significant increases in the VAD thresholds that were not significantly reversed by naloxone (NAL) (1 mg/kg). Twenty-five-day-old rats showed increased threshold for the three nociceptive responses after stress, the effects on TW and V being antagonized by NAL. Adult rats subjected to stress showed increased threshold for the three responses, an effect that was antagonized by NAL in all cases. These results suggest that the weaning period might be critical for the development of the mechanisms mediating SIA. Besides, a different involvement of opioid systems throughout development, particularly in relation to the affective/emotional component of pain, is also suggested.

Enkephalins, brain and immunity: modulation of immune responses by methionine-enkephalin injected into the cerebral cavity

There is a large number of interactions at molecular and cellular levels between the nervous system and the immune system. It has been demonstrated that the opioid neuropentapeptide methionine-enkephalin (Met-Enk) is involved in humoral and cell-mediated immune reactions. Met-Enk injected peripherally produces a dual and dose-dependent immunomodulatory effect: high doses suppress, whereas low doses potentiate the immune reactivity. The present mini-review concerns the immunological activity of Met-Enk after its administration into the lateral ventricles of the rat brain, and describes the extraordinary capacity of centrally applied Met-Enk to regulate/modulate the immune function. This survey is composed of sections dealing with (a) the role of opioid peptides in the central nervous system (CNS); (b) the activity of opioid peptides in the immune system; (c) the application of Met-Enk into the cerebral cavity; (d) the influence of centrally administered Met-Enk on nonspecific local inflammatory reaction; (e) the effect of Met-Enk injected intracerebroventricularly (i.c.v.) on specific delayed hypersensitivity skin reaction, experimental allergic encephalomyelitis, anaphylactic shock, plaque-forming cell response, and hemagglutinin production; (f) the central antagonizing action of quaternary naltrexone, an opioid antagonist that does not cross the brain-blood barrier, on Met-Enk-induced immunomodulation; (g) the alteration of immune responsiveness by i.c.v. injection of enkephalinase-degrading enzymes; (h) the participation of the brain-blood/blood-brain barrier in the CNS-immune system interaction; and (i) the role of opioid receptors in immunological activity of Met-Enk. A hypothesis has been advanced for the reaction of Met-Enk and opioid receptor sitting on the cell membrane. This concept suggests that the constellation of chemical residues of enkephalin and receptor in the microenvironment determines the binding between the opioid partners. The plurality of conformational structures of enkephalins and receptors makes possible their involvement in a variety of processes which occur in different physiological systems, including the nervous system and the immune system, and intercommunications between the two systems.

Dorsal column stimulation induces release of serotonin and substance P in the cat dorsal horn

The neurohumoral mechanisms behind the pain-suppressing effect of dorsal column stimulation (DCS) still remain obscure. Experimental observations have indicated an inhibitory role for serotonin and, under certain conditions, also for substance P (SP), on nociceptive transmission in the spinal cord. Furthermore, some observations suggest that these substances might be involved in the effect of DCS. The present series of experiments was undertaken to investigate whether serotonin and SP are released in the dorsal horn by DCS. Twenty-one adult cats, in some experiments anesthetized, in others decerebrated at the midcollicular level, were used. Microdialysis probes were implanted bilaterally in lumbar dorsal horns (L5-L7) and perfused with Krebs' solution. Dialysates were analyzed for serotonin by high-performance liquid chromatography or for SP by radioimmunoassay. DCS was applied at the thoracolumbar junction with current parameters similar to those used clinically in humans. DCS induced a significant release of serotonin in the dorsal horn of decerebrated animals (173 +/- 83% increase; mean +/- standard error; n = 4; P less than 0.01), whereas the levels of the metabolite 5-hydroxyindoleacetic acid were not significantly influenced. In contrast, no release of SP could be recorded in response to DCS in the decerebrated preparation, although peripheral nociceptive stimulation (pinch) and noxious electric dorsal root stimulation induced an elevation of the SP levels. However, in intact animals DCS provoked a marked SP release in the dorsal horn (190 +/- 92% increase; n = 7; P less than 0.01). The release of serotonin and SP after DCS may indicate that these substances participate in the mediation of the pain alleviating effect of DCS.(ABSTRACT TRUNCATED AT 250 WORDS)

Naloxone blocks conditioned place preference induced by substance P and [pGlu6]-SP(6-11)

The effect of prior treatment with the opioid receptor (opioceptor) antagonist naloxone on conditioned place preference produced by the neurotachykinin substance P (SP) and its C-terminal hexapeptide analog [pGlu6]-SP(6-11) (SPC) was investigated in rats. Place conditioning was assessed using a circular open field partitioned into four quadrants that were equally preferred by the rats prior to drug treatment. On three successive days, rats received an intraperitoneal (i.p.) injection of naloxone-HCl (1 mg/kg) or vehicle 15 min before an i.p. injection of either 37 nmol/kg SP, equimolar dosed SPC or corresponding diluent vehicle. After injection the rats were placed into their assigned treatment corral for 15 min. During the test for conditioned corral preference (CCP), when provided a choice between the four quadrants, rats injected with SP or SPC spent more time in the treatment corral compared to vehicle controls, indicative of a positive reinforcing action of these peptides. The pre-treatment with naloxone blocked the positive reinforcing effects of both SP and SPC; when injected alone, naloxone did not influence the preference behavior. Gross locomotor activity was affected by neither treatment. Thus, the positive reinforcing effects of SP and SPC may be mediated via interactions with the endogenous opioid system(s).

Repeated electroconvulsive shock increases tachykinin and cholecystokinin mRNA expression in ventral periaqueductal gray

The effect of repeated electroconvulsive shock (five shocks during 10 days) on preprocholecystokinin and preprotachykinin-A messenger RNA expression was studied in the mesencephalic periaqueductal gray and adjacent areas of rat using in situ hybridization histochemistry with specific oligonucleotide probes. An increased number of preprocholecystokinin and preprotachykinin-A messenger RNA hybridization positive neurons (+30% and +47%, respectively) in the Edinger-Westphal nucleus was observed following repeated electroconvulsive shock. In addition, both preprocholecystokinin and preprotachykinin-A messenger RNA expression, measured as grain density over single neurons, was significantly increased (+37% and +45%, respectively). The results indicate that cholecystokinin- and substance P-containing neurons in the Edinger-Westphal nucleus are activated by repeated electroconvulsive shock, which may be related to the antidepressant and analgesic effects of electroconvulsive shock treatment.

Ultrastructure of substance P immunoreactive elements in the periaqueductal gray matter of the rat

Substance P (SP) is a non-opioid peptide that generates a potent analgesia when injected into the periaqueductal gray matter (PAG). The aim of this study was to investigate the fine neuronal structures and synaptic circuits involved in SP action in rats by means of electron microscopy, using immunocytochemical (ICC) pre-embedding methods. A conventional ultrastructural study, carried out to interpret the ICC data correctly, shows small sized nerve cell bodies with a high nucleus-cytoplasmic ratio; absence of an extensive granular endoplasmic reticulum; and few axo-somatic contacts having symmetrical and asymmetrical junctions in equal proportions. The large neuropil is characterized by numerous thin unmyelinated axons and axo-dendritic synapses mainly showing pleomorphic vesicles and asymmetrical junctions. The ICC analysis showed moderately labeled nerve cell bodies with the same structural, synaptic, and dimensional features as the negative cells. In the neuropil SP immunoreactivity is shown by dendrites, synapses, and thin elements which are unidentifiable structurally. No SP terminals synapsing on SP nerve cell bodies were found and only occasional SP light labeled terminals synapsing on negative perikarya were seen. The SP boutons generally have pleomorphic vesicles and asymmetrical junctions. On the basis of these data a possible excitatory activity of PAG SP synapses could be hypothesized. This activity would take place on postsynaptic neurons generally at a dendritic level. Our ultrastructural findings give support to an excitatory role carried out by SP neurons of the PAG, as suggested by the role of PAG circuitry on spinal nociception.

Correlation of substance P-induced desensitization with substance P amino terminal metabolites in the mouse spinal cord

Intrathecal injection of mice with substance P (SP) or its C-terminal fragments results in a behavioral syndrome characterized by reciprocal caudally directed biting and scratching. Repeated injection of SP, but not SP C-terminal fragments, results in a decrease in the intensity of, or desensitization to, these SP-induced behaviors. Peptidase inhibitors, phosphoramidon (PH), bacitracin (BAC), diprotin A (DPA) and angiotensin converting enzyme inhibitor (ACEI OR SQ20881), together with [3H]SP, were used to investigate the possible accumulation of tritiated N-terminal metabolites in the mouse spinal cord in vivo during the development of desensitization to SP. SP N-terminal metabolites in the spinal cord were quantified by reverse-phase HPLC. The magnitude of SP-induced desensitization correlated well (r = .95) with total SP N-terminal metabolites recovered from the spinal cords of the same mice studied in vivo. The magnitude of SP-induced desensitization was also found to be negatively correlated (r = .95) with total recovered intact [3H]SP. The rank order of potency of the peptidase inhibitors in decreasing the magnitude of SP-induced desensitization was BAC = PH much greater than ACEI greater than DPA. The order of potency for in vitro inhibition of SP metabolism using synaptic membrane-derived peptidases was BAC greater than PH much greater than ACEI. These results support the hypothesis that desensitization to SP-induced behaviors depends, at least in part, on the concentration of SP N-terminal metabolites in the spinal cord.

Involvement of spinal monoaminergic pathways in antinociception produced by substance P and neurotensin in rodents

The antinociceptive effects of substance P and of neurotensin have been determined in rodents after depletion of serotonin (5-HT) or noradrenaline (NA) in the spinal cord. The antinociceptive effect of substance P, given intraventricularly, in rats and mice was blocked after depletion of 5-HT in the spinal cord with the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) or with the inhibitor of the synthesis of 5-HT, p-chlorophenylalanine (PCPA), but not after depletion of NA in the spinal cord with the neurotoxin 6-hydroxydopamine (6-OHDA). Conversely, the antinociceptive effect of neurotensin in mice was blocked after lesion of spinal NA pathways with 6-OHDA. When 5-HT spinal pathways of mice were lesioned with 5,7-DHT, neurotensin-induced antinociception was blocked 7 but not 15 days after the lesion. p-Chlorophenylalanine failed to prevent this effect of neurotensin. The results suggest that the antinociceptive effect of substance P depends on the integrity of spinal 5-HT neurones, whereas that of neurotensin depends on spinal NA neurones and, only to a limited extent, on 5-HT neurones. It seems that different descending systems are involved in the antinociception elicited by these two neuropeptides.

Intrathecal administration of 5,6-DHT or 5,7-DHT reduces morphine and substance P-antinociceptive activity in the rat

The administration of SP (15 and 50 ug) and morphine sulphate (10 ug) either into the IV ventricle or intrathecally produces an analgesic effect. This effect was attenuated when the rats received previously an intrathecal dose of 5,6-DHT or 5,7-DHT. The consequences of neurotoxin administration upon monoamine descending systems were evaluated by measuring 14C-5HT and 3H-NA synaptosomal uptake in different structures of the CNS. SP levels were also determined in the animals injected with the neurotoxins. Our results confirm the existence of a relationship injected with the neurotoxins. Our results confirm the existence of a relationship between the 5HT and SP descending systems, which control the nociceptive information at the level of the dorsal horn of the spinal cord.

Interactions of substance P antagonists with serotonin in the mouse spinal cord

Administered intrathecally (IT) to mice, the putative substance P antagonist [D-Pro2,D-Trp7,9-substance P (DPDT) blocked substance P- and serotonin-induced reciprocal hindlimb scratching with ID50 values of 4.6 (2.9-6.9) and 3.0 (1.9-4.8) micrograms, respectively. The duration of this antagonistic effect was 90-120 min. In contrast, DPDT did not block bombesin-, somatostatin-, glycine- or glutamate-induced scratching. These data indicate that DPDT is an effective antagonist of serotonin-induced behaviors in the mouse spinal cord. Phenoxybenzamine (IT) also blocked substance P- and serotonin-induced scratching. Its onset of action was more rapid for serotonin than for DPDT implying differences in agonist-induced receptor activation. Methysergide (IT) blocked serotonin-induced scratching [ID50 = 0.7 (0.3-1.5) micrograms], but not substance P-induced scratching. Similar to DPDT, [D-Arg1,D-Trp7,9,Leu11]-substance P, [des-Arg1,D-Pro2, D-Trp7,9]-substance P(2-11) and [D-Pro4,D-Trp7,9]-substance P(4-11) blocked substance P and serotonin-induced scratching. In contrast, [D-Pro2,D-Phe7,D-Trp9]-substance P and [D-Pro4,D-Trp7,9,10]-substance P(4-11) blocked only substance P-induced scratching. Thus, some, but not all putative substance P antagonists may also be behavioral antagonists of serotonin in the mouse spinal cord.

Analgesic and cardiovascular effects of centrally administered substance P

Substance P (SP) injected intracerebroventricularly (ICV) into rabbits caused dose-related thermal analgesia with the maximum effect after 2 micrograms. The analgesia was measured by timing the withdrawal of the rabbit's ear from an infrared beam. Equimolar amounts of the related peptides physalaemin and eledoisin-related peptide also caused analgesia, but the SP N-terminal fragment (1-9) was inactive. This suggests that the analgesic message of SP resides within the C-terminal fragment. The analgesia caused by each peptide developed more rapidly but did not last as long as that after central injection of beta-endorphin. In separate experiments, 2 micrograms SP injected ICV increased blood pressure and decreased heart rate. The analgesic, bradycardic and pressor responses to central administration of SP were opposite to effects of peripherally administered SP, described previously. These results indicate that the effect induced by SP depends upon its specific neuroanatomical site of action.

Substance P, nicotinic acetylcholine receptors and antinociception in the rat

Intracerebroventricular injections of both nicotine and substance P (SP) induced antinociception in the tail-flick test in rats. The antinociceptive effect was blocked in both cases by mecamylamine and by naloxone, suggesting that central nicotinic acetylcholine receptors and endogenous opioids are implicated in the action of these drugs. A link between substance P neurones and central cholinergic systems, involving nicotinic receptors, was also suggested by the quickly developed cross-tolerance between the antinociceptive effect of substance P and nicotine. A smaller, subeffective dose of substance P was able to block, on acute administration, the antinociceptive action of nicotine, an effect not shared by the two other mammalian tachykinins, neurokinin A or neurokinin B. The results obtained in the present study appear to indicate a dual action of substance P on central nicotinic cholinoceptors.

Antinociceptive and Met-enkephalin releasing effects of tachykinins and substance P fragments

Substance P (SP), physalaemin, SP4-11, SP5-11 and the SP5-11 analog DiMe-C7 induce an antinociceptive effect in rats after intraventricular administration. Other tachykinins and the N-terminal fragments of SP are inactive. All antinociceptive peptides increase the Met-enkephalin efflux from slices of rat periaqueductal gray matter and their antinociceptive potency is correlated with their capacity to release Met-enkephalin. The results, discussed in the light of current theories on different tachykinin receptors, suggest that the SP-P receptor subtype may be involved in the control of noxious stimulation elicited by SP at supraspinal levels.

Administered peptides inhibit the degradation of endogenous peptides. The dilemma of distinguishing direct from indirect effects

Virtually all peptides are biologically active following central administration as a consequence of both direct and indirect cellular actions. Direct effects are mainly interactions with specific membrane receptors but may include unions with other components of the receptor/effector complex. Significant indirect biological effects of exogenous peptides, including apparent secretagogue effects on endogenous peptides largely overlooked in practice, result from extensive competition with endogenous peptides for degradative enzymes (peptidases). A consequence of this competition is enhancement of tonic or intermittent activity of endogenous peptides. The pharmacological profile of any peptide reflects or includes, therefore, the spectrum of endogenous peptides that is protected from peptidase action. It is likely that certain pharmacologically active peptides, including a large number of di-, tri- and oligo-peptides, elicit responses mainly or exclusively by competing for peptidases. Therefore, reliable estimates of the relative contributions of direct and indirect actions of exogenous peptides may be difficult, if not impossible, to obtain.

The antinociceptive effects of histidyl-proline diketopiperazine and thyrotropin-releasing hormone in the mouse

Intracerebroventricular (i.c.v.) injection of histidyl-proline diketopiperazine [cyclo (His-Pro)], an active metabolite of thyrotropin-releasing hormone (TRH) in mice produced an antinociceptive effect in a dose-dependent manner as measured in four antinociceptive tests; tail-pressure, tail-flick, hot-plate and acetic acid writhing. The antinociceptive effect of cyclo (His-Pro) reached a maximum at 5 min and lasted for 30 min. The ED50 values for the tests were 760.0 (598.4-965.2), 540.0 (442.6-658.8), 595.0 (487.7-725.9) and 370.0 (286.8-477.3) nmol/mouse and the slope functions were 1.61, 1.56, 1.57 and 1.66, respectively. Pretreatment with naloxone (0.5, 2 and 8 mg/kg), an opioid antagonist, administered subcutaneously antagonized the antinociceptive effect of cyclo (His-Pro). TRH injected i.c.v. to mice also exerted a dose-dependent antinociceptive action as measured in three antinociceptive tests; tail-pressure, hot-plate and acetic acid writhing. The antinociceptive effect of TRH was only seen at 5 min. The ED50 values for each test were 112.0 (47.5-264.3), 19.2 (10.4-35.5) and 0.2 (0.1-0.4) nmol/mouse and the slope functions were 8.89, 4.14 and 3.94, respectively. TRH was without effect in the tail-flick test. In contrast to cyclo (His-Pro), TRH-induced antinociception was not antagonized by pretreatment with naloxone (0.5, 2 and 8 mg/kg). The data suggest that the two peptides have a different mechanism of antinociceptive action in relation to the involvement of the opioid system in the central nervous system.

Substance P levels in various regions of the rat central nervous system after acute and chronic morphine treatment

Substance P levels were measured in various CNS regions from rats treated acutely and chronically with morphine. There was no observable effect in the group treated with an acute dose of morphine (10 mg/kg) and sacrificed after 2 h. After 35 days chronic treatment with increasing doses of the drug, the rats were divided into three groups and sacrificed 2 h, 24 h and 7 days after the last injection. The substance P level was increased in the corpus striatum 2 h and 24 h and in the medulla oblongata and dorsal part of the spinal cord 2 h after withdrawal. Seven days after the last injection the levels had returned to normal in these areas. No effects were observed in the cerebral cortex, the hypothalamus or the ventral spinal cord at any time of measurement. Earlier studies have demonstrated that morphine inhibits release of substance P. The observed increase in tissue levels after long-term treatment is therefore interpreted as an accumulation of substance P in the neurons.

Pain threshold changes in rats following central injection of beta-endorphin, met-enkephalin, vasopressin or oxytocin antisera

Both opioid peptides such as beta-endorphin and met-enkephalin and nonopioid peptides such as vasopressin and oxytocin increase pain thresholds in rodents. Antisera raised against each of these peptides have been developed for use in immunocytochemical and radioimmunoassay procedures. The present study assessed whether central administration of antisera raised against beta-endorphin (ABE), met-enkephalin (AME), arginine, vasopressin (AAVP) or oxytocin (AOT) altered tail-flick latencies elicited by three different levels of radiant heat, jump thresholds, core body temperatures and locomotor activity. ABE induced a transient hyperalgesia on the tail-flick test at thermal levels at which beta-endorphin administration would elicit an analgesic effect. While met-enkephalin increases tail-flick latencies elicited by high thermal levels, AME failed to alter latencies at this level, but rather induced a short-acting hyperalgesia at a low thermal level. While vasopressin increased tail-flick latencies at high thermal levels, AAVP produced reciprocal decreases. Yet AAVP inexplicably induced analgesic effects at moderate and low thermal levels. Finally, while oxytocin increased latencies at high thermal levels, AOT failed to alter latencies. Rather, it decreased latencies at the moderate thermal level and increased latencies at the low thermal level. Neither jump thresholds nor core body temperatures were affected by any antiserum pretreatment. While activity levels were unaffected by either ABE, AME or AAVP pretreatment, AOT decreased activity in a fashion complementry to oxytocin-induced hyperactivity and seizures. There data are discussed in terms of tonic versus phasic influences of these peptides upon pain perception.

Antagonism of diazepam-induced feeding in rats by antisera to opioid peptides

Diazepam-induced feeding in rats is antagonized not only by the opiate antagonist naloxone but also intraventricular administration of specific antisera to the endogenous opioid peptides met-enkephalin or beta-endorphin. Pituitary beta-endorphin is probably not implicated in the diazepam effect since blockade with the glucocorticoid dexamethasone of the release of beta-endorphin from the anterior pituitary does not modify the diazepam-induced feeding, which is however prevented by TRH, a suggested physiological antagonist of some of the effects of opioid peptides. The possible central participation of both beta-endorphin and met-enkephalin in the ingestive behavior induced by diazepam gives further support to the postulated physiological role of endogenous opioids in appetite regulation.

Basic and regulatory mechanisms of in vitro release of Met-enkephalin from the dorsal zone of the rat spinal cord

Under control conditions, superfused slices of the dorsal half of the lumbar enlargement from adult rats released Met-enkephalin-like material (MELM) that behaved as authentic Met-enkephalin under two different chromatographic procedures (Bio-gel filtration, HPLC). MELM release increased markedly on exposure of slices to batrachotoxin (0.5 microM) or to an excess of K+ (28 and 56 mM instead of 5.6 mM). The K+-evoked release was totally dependent on the presence of Ca2+ in the superfusing fluid whereas the spontaneous efflux of MELM was only partially Ca2+-dependent. Further experiments performed with tissues of polyarthritic rats indicated that the increase in their MELM levels was associated with a lower fractional rate constant of MELM release, therefore suggesting that spinal Met-enkephalin turnover might be reduced in chronically suffering animals. Examination of the possible modulation of MELM release by various neuroactive compounds present within the dorsal horn revealed that cholecystokinin (10 microM), but not its desulphated derivative, substance P-sulphoxide (10 microM), and to a lesser extent substance P, enhanced the K+-evoked MELM release. In contrast, gamma-aminobutyric acid (10 microM) and (-)-baclofen (1 microM) partially prevented the stimulatory effect of K+ on MELM release. Other compounds such as serotonin, somatostatin, and neurotensin altered neither the spontaneous nor the K+-evoked release of MELM.

Analgesic activity of substance P in rats: tolerance and cross-tolerance with enkephalin

In rats, tolerance to the analgesic effect of intraventricular substance P (SP) develops quickly. In rats made tolerant to [D-Ala2, D-Leu5]enkephalin the analgesic efficacy of SP is reduced significantly. The latter result suggests some overlap in the sites of action of SP and [D-Ala2,D-Leu5] enkephalin. Since SP seems to lack any direct effect on opiate receptors of the brain, the present data suggest that the analgesic effect of SP is mediated by enkephalin release at supraspinal levels which are related to pain control.

Prevention of stress-induced analgesia by substance P

It has been shown that a variety of stressful procedures, such as immobilization and footshock, can induce a significant degree of analgesia in mice. In addition, it has been shown that for some, but not all, stressful treatments, the analgesic effect is mediated via endogenous opioids. This report describes the effects of substance P, administered systemically, on both opioid-mediated immobilization-induced analgesia and non-opioid footshock-induced analgesia. Substance P completely blocked the opioid-mediated form of stress-induced analgesia while having no effect on the non-opioid form. Exogenous substance P appears to interact with endogenous opioid pain-suppressing systems.

Substance P-induced release of Met5-enkephalin from striatal and periaqueductal gray slices

Substance P(SP), the heptapeptide SP and the stable analogue (p-Glu5-MePhe8-MeGly9) SP (DiMe-C7) induce a Ca2+-dependent release of Met5-enkephalin (MET) from slices of periaqueductal gray matter (PAG) and striatum of rats. The MET release from striatal slices is greater than that from PAG slices because of the higher MET content of striatum. Intraventricular injection of SP and of the two related peptides induce analgesia in the rat, and their analgesic potency is in line with their capacity to release MET. Other neuropeptides which possess antinociceptive activity such as bombesin, neurotensin, vasopressin and somatostatin fail to release MET from PAG slices.

Antinociceptive effect of centrally administered cyclo(N-methyl-L-Tyr-L-Arg) in the rat

The tail-flick test was used to test cyclo(N-methyl-L-Tyr-L-Arg) (C.NMTA), kyotorphin (L-Tyr-L-Arg) and morphine for antinociceptive effects following injection into the cerebroventricles (lateral ventricle (VL), third ventricle (V3) and fourth ventricle (V4)) and into the spinal subarachnoid space in the rat. When injected into the VL, V3 and V4, but not into the spinal subarachnoid space, C.NMTA produced a dose-dependent inhibition of the tail-flick response to thermal stimulation. The ED50 values for each site were 61.0 (48.0-77.5), 40.0 (27.4-58.4) and 163.0 (86.7-306.4) nmol/rat, respectively. Behavioral sedation was seen when C.NMTA was injected into the VL, V3 and V4, but not into the spinal subarachnoid space. Kyotorphin was without antinociceptive effect when given by all routes. However, weak sedation was seen after injection into the cerebroventricles. Naloxone (2, 4 and 20 mg/kg), an opiate antagonist, injected intraperitoneally (i.p.) 20 min before C.NMTA injection, did not significantly alter the C.NMTA-induced antinociceptive effect. Additionally, naloxone (2, 4 and 20 mg/kg i.p.) did not abolish the sedative effect of this peptide. It is suggested that C.NMTA produced a naloxone-resistant antinociceptive effect mainly on the upper brain stem.

Substance P and behavior: opposite effects of N-terminal and C-terminal fragments

Most of the biological actions of substance P (SP) have been thought to be mediated by the carboxy-terminal portion of the peptide. Some of the behavioral effects produced by exogenous SP exhibit a strikingly different structure-activity relationship. The N-terminal heptapeptide fragment of SP, SP(1-7), inhibits nociceptive, aggressive and grooming behaviors and stimulates investigative motor behavior, but the C-terminal hexapeptide fragment analog pyroglutamyl-SP(7-11) exerts opposite effects. While the C-terminal fragment mimics the effects of administered intact SP on motor behaviors, the N-terminal fragment mimics the effects of intact SP on aggressive and nociceptive behaviors. The significant behavioral effects of SP(1-7) and the consistently opposite behavioral effects of N- and C-terminal fragments are important new findings.

Blockade by met-enkephalin antiserum of analgesia induced by substance P in mice

In the tail-flick test in mice, the intraventricular administration of Substance P (10-5,000 ng/mouse) produced a naloxone-reversible analgesic effect of rapid onset and long duration. The dose-response curve was bell-shaped, the analgesic effect being smaller after the largest doses. The analgesia was blocked by concomitant intraventricular administration of the antibody against met-enkephalin but not by the antibody against beta-endorphin. In the hot plate assay, Substance P produced analgesia in mice with high sensitivity to pain, and hyperalgesia in mice with lower sensitivity to pain than normal. The analgesia was blocked by the antibody against met-enkephalin but the hyperalgesia or the scratching response were not modified by the antiserum. The results appear to indicate a dual effect, analgesic or hyperalgesic, of Substance P in mice, the former probably being mediated by release of met-enkephalin.

Differential effects of D-Ala2 analogues of enkephalins on substance P-induced analgesia in rodents

The systemic administration of subanalgesic doses of [D-Ala2, D-Leu5]enkephalin significantly potentiated the analgesia elicited in rats or mice by intraventricular injection of substance P. On the contrary, systemic administration of low doses of [D-Ala2,Met5]enkephalinamide antagonized the substance P-induced analgesia. The results support the notion of different physiological functions for the enkephalins and suggest an integrated role for enkephalins and substance P in the control of pain at supraspinal levels.