Studies on the vascular permeability induced by intrathecal substance P and bradykinin in the rat

Modulation of peripheral inflammation by the substance P N-terminal metabolite substance P1-7

The N-terminal metabolite of the undecapeptide substance P (SP), substance P1-7 (SP1-7), is known to modulate nociception in the central nervous system (CNS) and often has opposite effects from SP. This study investigated the ability of SP(1-7) to modulate the vasodilatation response to SP in anaesthetized rats under different injury conditions using a blister model of inflammation on the hind footpad. The results indicated that SP1-7 inhibited the vascular response to SP in a dose-dependent manner. The putative antagonists naloxone and D-Pro2-D-Phe7-SP1-7 (D-SP1-7) reversed the effect of SP1-7. D-SP1-7 improved the responsiveness to SP under chronic nerve injury, which suggests a role for endogenous SP1-7 in this model. SP1-7 did not inhibit the response to electrical stimulation of the sciatic nerve, which indicates that the heptapeptide interacts at a post-terminal binding site. The current results suggest that SP1-7 may have inhibitory properties in inflammation, analogous to its antinociceptive role in the central nervous system.

The role of spinal neurokinin-1 and glutamate receptors in hyperalgesia and allodynia induced by prostaglandin E(2) or zymosan in the rat

Recent research has focused on prostaglandins in the central nervous system and their contribution to hyperalgesia and allodynia. This study sought to establish whether neurokinin-1 (NK-1) receptors and glutamate receptors are involved in the hyperalgesic and allodynic effects of spinally administered prostaglandin E2 (PGE2) in rats, and also to determine if the same receptors are involved the hyperalgesia induced by intraplantar administration of zymosan, an inflammatory agent which is known to evoke spinal PGE2 release. Spinal application of antagonists of the NK-1 receptor, the -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate glutamate or metabotropic glutamate receptor significantly attenuated the decrease in mechanical paw withdrawal response thresholds produced by either spinal administration of PGE2 or intraplantar administration of zymosan. The decrease in thermal paw withdrawal response latencies induced by PGE2, but not by zymosan, was significantly attenuated by spinal administration of an N-methyl--aspartate (NMDA) receptor antagonist, an AMPA/kainate receptor antagonist, or a metabotropic glutamate receptor antagonist. Allodynia induced by PGE2 was significantly alleviated by antagonists of NMDA or AMPA/kainate receptors. These results suggest that both PGE2-induced and zymosan-induced mechanical hyperalgesia are mediated in part through activation of NK-1, AMPA/kainate and metabotropic glutamate receptors. PGE2-induced, but not zymosan-induced, thermal hyperalgesia is mediated in part by activation of NMDA, AMPA/kainate and metabotropic glutamate receptors. Activation of both NMDA and AMPA/kainate receptors contribute to PGE2-induced allodynia.

ReN 1869, a novel tricyclic antihistamine, is active against neurogenic pain and inflammation

The tricyclic compound (R)-1-(3-(10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-ylidene)-1-propyl)-3-piperidine carboxylic acid (ReN 1869) is a novel, selective histamine H(1) receptor antagonist. It is orally available, well tolerated, easily enters the central nervous system (CNS) but no adverse effects are seen in mice at 300 mg/kg. ReN 1869 at 0.01-10 mg/kg is antinociceptive in tests of chemical nociception in rodents (formalin, capsaicin, phenyl quinone writhing) but not in thermal tests (hot plate and tail flick). ReN 1869 amplifies the analgesic action of morphine but does not show tolerance after chronic dosing. Moreover, the compound is effective against inflammation of neurogenic origin (antidromic nerve stimulation, histamine-evoked edema) but not in carrageenan-induced inflammation. We suggest that ReN 1869, via H(1) blockade, counteracts the effect of histamine liberated from activated mast cells and inhibits pain transmission in the dorsal spinal cord. ReN 1869 represents a new class of antihistamines with pain-relieving properties that probably is mediated centrally through histamine H(1) receptors but alternative mechanisms of action cannot be excluded.

Pathophysiology of the kallikrein-kinin system in mammalian nervous tissue

The nervous system and peripheral tissues in mammals contain a large number of biologically active peptides and proteases that function as neurotransmitters or neuromodulators in the nervous system, as hormones or cellular mediators in peripheral tissue, and play a role in human neurological diseases. The existence and possible functional relevance of bradykinin and kallidin (the peptides), kallikreins (the proteolytic enzymes), and kininases (the peptidases) in neurophysiology and neuropathological states are discussed in this review. Tissue kallikrein, the major cellular kinin-generating enzyme, has been localised in various areas of the mammalian brain. Functionally, it may assist also in the normal turnover of brain proteins and the processing of peptide-hormones, neurotransmitters, and some of the nerve growth factors that are essential for normal neuronal function and synaptic transmission. A specific class of kininases, peptidases responsible for the rapid degradation of kinins, is considered to be identical to enkephalinase A. Additionally, kinins are known to mediate inflammation, a cardinal feature of which is pain, and the clearest evidence for a primary neuronal role exists so far in the activation by kinins of peripherally located nociceptive receptors on C-fibre terminals that transmit and modulate pain perception. Kinins are also important in vascular homeostasis, the release of excitatory amino acid neurotransmitters, and the modulation of cerebral cellular immunity. The two kinin receptors, B2 and B1, that modulate the cellular actions of kinins have been demonstrated in animal neural tissue, neural cells in culture, and various areas of the human brain. Their localisation in glial tissue and neural centres, important in the regulation of cardiovascular homeostasis and nociception, suggests that the kinin system may play a functional role in the nervous system.

Increased pulmonary vascular permeability and oedema induced by intrathecally injected endothelins in rat

The intrathecal (i.t.) injection of endothelins to conscious rats was found to cause respiratory arrest. To gain some insights into this central phenomenon, peripheral vascular permeability and lung oedema were measured after i.t. and i.v. injections of these peptides. When injected at T-8 spinal cord level, endothelin-1 (65 and 650 pmol) and endothelin-3 (650 pmol) enhanced vascular permeability in the lungs by 22-fold and 7-fold, respectively, and caused sudden death at the highest dose. Less prominent increases (between 1.4- and 2.2-fold) of vascular permeability were observed in other tissues (trachea, kidney, ears, skin of hind paws and back skin) with endothelin-1. Endothelin-1 (650 pmol) caused a similar increase (27-fold) in lung vascular permeability when injected at T-2, although the response was significantly less (P < 0.05) if injected at the L-4 (15-fold) spinal cord level. Only endothelin-1 produced lung oedema when injected at the T-2 or T-8 level. In contrast, intravenous injection of endothelins-1 and -3 (650 pmol) did not produce lung oedema and the lung vascular permeability was increased by only 1.4-1.6-fold and all rats survived. The prior i.t. injection of 6.5 nmol BQ-123 (cyclo[D-Trp, D-Asp, L-Pro, D-Val, L-Leu]), a selective endothelin ET(A) receptor antagonist, prevented the increases of lung vascular permeability and oedema and the mortality induced by i.t. endothelin-1 (650 pmol). Whereas i.v. treatment with phentolamine (2 mg/kg) or pentolinium (25 mg/kg + 50 mg/kg per h x 15 min) abolished the lung vascular permeability changes evoked by endothelin-1 (650) pmol), atropine (1 mg/kg), NG-nitro-L-arginine (50 mg/kg) or indomethacin (5 mg/kg) had no effect. Moreover, the effects of endothelin-1 were attenuated in capsaicin pretreated rats (125 mg/kg, 10 days earlier) and almost abolished in rats subjected to sympathectomy with 6-hydroxydopamine (100 mg/kg, 24-48 h earlier). All these treatments except atropine and NG-nitro-L-arginine prevented the endothelin-1-induced lung oedema and reduced the lethality by around 50%. These results suggest that the increases of pulmonary vascular permeability and oedema induced by i.t. endothelin-1 are due to an intense pulmonary vasoconstriction mediated by alpha-adrenoceptors following the release of catecholamines in response to the activation of endothelin ET(A) receptor in the spinal cord. This central phenomenon seems to be reflexogenic, including the involvement of primary afferent C-fibers and spinal cord ascending fibers to the brain. Thus, endothelin-1 could play a role in neurogenic pulmonary oedema through a central mechanism.

Passage of peptides across the blood-brain barrier: pathophysiological perspectives

Blood-borne peptides are capable of affecting the central nervous system (CNS) despite being separated from the CNS by the blood-brain barrier (BBB), a monolayer comprised of brain endothelial and ependymal cells. Blood-borne peptides can directly affect the CNS after they cross the BBB by nonsaturable and saturable transport mechanisms. The ability of peptides to cross the BBB to a meaningful degree suggests that the BBB may act as a modulatory pathway in the exchange of informational molecules between the brain and the peripheral circulation. The permeability of the BBB to peptides is a regulatory process affected by developmental, physiological, and pathological events. This regulation sets the stage for the relation between peptides and the BBB to be involved in pathophysiological events. For example, some of the classic actions of melanocortins on the CNS are explained by their abilities to cross the BBB, whereas aspects of feeding and alcohol-related behaviors are associated with the passage of other specific peptides across the BBB. The BBB should no longer be considered a static barrier but should be recognized as a regulatory interface controlling the exchange of informational molecules, such as peptides, between the blood and CNS.

Permeability of the blood-brain barrier to melanocortins

Adrenocorticotropin (ACTH)-related compounds, termed melanocortins, produce a large number of effects on the central nervous system (CNS) after their peripheral administration. Some of the CNS effects of ACTH are mediated through the release of glucocorticoids from the adrenal gland, but there are fragments and analogues of ACTH that do not act on the adrenals. This raises the possibility that some blood-borne melanocortins may be acting directly on the brain, which would necessitate their crossing the blood-brain barrier (BBB). We review here the literature showing that melanocortins can affect the BBB in several ways, including an alteration of the permeability of the BBB to other substances.

Kinins and kinin receptors in the nervous system

Kinins, including bradykinin and kallidin, are peptides that are produced and act at the site of tissue injury or inflammation. They induce a variety of effects via the activation of specific B1 or B2 receptors that are coupled to a number of biochemical transduction mechanisms. In the periphery the actions of kinins include vasodilatation, increased vascular permeability and the stimulation of immune cells and peptide-containing sensory neurones to induce pain and a number of neuropeptide-induced reflexes. Mechanisms for kinin synthesis are also present in the CNS where kinins are likely to initiate a similar cascade of events, including an increase in blood flow and plasma leakage. Kinins are potent stimulators of neural and neuroglial tissues to induce the synthesis and release of other pro-inflammatory mediators such as prostanoids and cytotoxins (cytokines, free radicals, nitric oxide). These events lead to neural tissue damage as well as long lasting disturbances in blood-brain barrier function. Animal models for CNS trauma and ischaemia show that increases in kinin activity can be reversed either by kinin receptor antagonists or by the inhibition of kinin production. A number of other central actions have been attributed to kinins including an effect on pain signalling, both within the brain (which may be related to vascular headache) and within the spinal dorsal horn where primary afferent nociceptors can be stimulated. Kinins also appear to play a role in cardiovascular regulation especially during chronic spontaneous hypertension. Presently, however, direct evidence is lacking for the release of kinins in pathophysiological conditions of the CNS and it is not known whether spinal or central neurones, other than afferent nerve terminals, are sensitive to kinins. A more detailed examination of the effects of kinins and their central pharmacology is necessary. It is also important to determine whether the inhibition of kinin activity will alleviate CNS inflammation and whether kinin receptor antagonists are useful in pathological conditions of the CNS.

Blood-borne interleukin-1 alpha is transported across the endothelial blood-spinal cord barrier of mice

1. Previous work has shown that one mechanism by which blood-borne interleukin-1 alpha (IL-1) may be able to affect the central nervous system (CNS) is by direct transport into the brain across the blood-brain barrier (BBB). The BBB of the brain consists of endothelial (between blood and interstitial fluid) and ependymal (between blood and cerebrospinal fluid) barriers. Which of these barriers IL-1 can cross has not previously been investigated. At the spinal cord, which could be the site of action for some of the effects of IL-1 such as analgesia, the BBB consists only of the endothelial barrier. 2. We show here that IL-1 labelled with 125I (I-IL) is transported across the BBB of the spinal cord by a saturable system similar to the one previously described for the brain. High performance liquid chromatography (HPLC) showed that most of the material entering the spinal cord represented intact I-IL. The BBB of the spinal cord was no more leaky to radioactively labelled albumin than the BBB of the brain and was not disrupted by 50 micrograms kg-1 of IL-1. 3. Capillary depletion showed that most of the I-IL entered the parenchymal-interstitial fluid space of the spinal cord with only a modest amount being sequestered by the endothelial cells of its BBB. 4. I-IL entered the cervical, thoracic and lumbar regions of the spinal cord equally well. I-IL entering at the brain and diffusing caudally was estimated only to account for about 1% of the total radioactivity found in the spinal cord after i.v. injection.(ABSTRACT TRUNCATED AT 250 WORDS)

Possible existence of a new tachykinin receptor subtype in the guinea pig ileum

The guinea pig ileum possesses NK-1 and NK-3 tachykinin receptors. As expected, [Pro9]SP and senktide, which are selective agonists of NK-1 and NK-3 receptors, respectively, were found to be highly potent in contracting the guinea pig ileum. Surprisingly, similar observations were made with septide, SP-O-CH3, [Apa9-10]SP, or [Pro9,10]SP although, in contrast to [Pro9]SP, these four peptides showed a low affinity for 3H-[Pro9]SP-specific NK-1 binding sites on membranes from the guinea pig ileum. They were also devoid of affinity for NK-2 and NK-3 binding sites. GR 71251, a compound which has been described as a NK-1 antagonist, was more potent in inhibiting the septide- than the [Pro9]SP-evoked contracting response. Altogether, these results suggest that septide, [Apa9-10]SP, and [Pro9,10]SP exert their high contracting activity in the guinea pig ileum by acting on a new subtype of tachykinin receptors.

Cardiovascular responses elicited by intrathecal kinins in the conscious rat

In the conscious, unrestrained rat, intrathecal (i.t.) injection of 0.81 pmol-81 nmol bradykinin (BK), kallidin (KD) and T-kinin at the T-9 spinal cord level produced transient (less than 10 min) increases in mean arterial pressure (MAP) and longer lasting decreases in heart rate (HR). These effects were dose-dependent and similar with respect to intensity and time course for the three kinins. The des-Arg9-BK fragment, a selective agonist for B1 receptors, was active only at 81 nmol. The pressor response induced by BK was enhanced by propranolol and by transection of the cervical spinal cord but was converted to a vasodepressor effect by prazosin. The bradycardia was converted to tachycardia by prazosin, atropine, pentolinium, capsaicin and in spinal transected rats. However, the cardiovascular responses to BK remained unaffected by diphenhydramine plus cimetidine, morphine, indomethacin, adrenal medullectomy, i.t. idazoxan and after bulbospinal noradrenaline deafferentation with 6-hydroxydopamine. These results suggest that the increase in MAP induced by i.t. BK is mediated by the sympathoadrenal system while the decrease in HR is ascribable to a vagal reflex involving sensory C-fibers and a spinobulbar pathway. This pharmacological evidence therefore supports a role for kinins in cardiovascular regulation in the spinal cord.