Retinal plasma extravasation in animals but not in humans: implications for the pathophysiology of migraine

High-intensity electrical stimulation of the trigeminal ganglion is accompanied by mast cell degranulation, vasodilatation, increased endothelial permeability and leakage of albumin from postcapillary venules within the dura mater. Overall, the histological appearance suggests an evolving sterile inflammatory response. This neurogenic inflammation within the meninges has been suggested as a model to explain the pain in migraine and cluster headache, and has been used to characterize the pharmacology of anti-migraine compounds. Using the rat model of neurogenic inflammation, the albumin extravasation ratio (stimulated : unstimulated side) in vehicle-treated animals in the dura and retina was 1.60 +/- 0.11 and 1.76 +/- 0.18, respectively (n = 10; values are mean +/- SEM). Pretreatment with sumatriptan (n = 9) produced a highly significant reduction in the ratio of extravasation within the dura to 1.10 +/- 0.06 (P = 0.002) and in the retina to 0.96 +/- 0.06 (P = 0.001), as did the neurokinin-1 receptor antagonist RP 67580 (n = 12) in the dura (1.04 +/- 0.11, P = 0.002) and retina (1.08 +/- 0.06, P = 0.001). These data demonstrate increased endothelial permeability and leakage of albumin not only in the dura but also in the retina. In a second stage we investigated possible extravasation in the human retina in acute migraine (n = 8) and cluster headache (n = 5) using fluorescein or indocyanine angiography. No increased endothelial permeability or leakage of dye could be found in the human retinal or choroidal vessels during headache attacks or in the headache-free interval in persons suffering from both migraine and cluster headache. These data raise the possibility that neurogenic inflammation is not a major factor in headache attacks in migraine or cluster headache.

Rizatriptan has central antinociceptive effects against durally evoked responses

The 5-HT(1B/1D) receptor agonist rizatriptan constricts intracranial, extracerebral blood vessels, inhibits neurogenic vasodilation and extravasation in the meninges and is effective clinically against migraine. The present study has investigated whether rizatriptan may also have activity at 5-HT(1B/1D) receptors within the central nervous system (CNS) that contributes to its antimigraine effects. Action potentials evoked by electrical stimulation of the dura-mater were recorded extracellularly from single neurones in the trigeminal nucleus caudalis in anaesthetized rats. Rizatriptan dose dependently inhibited these nociceptive dural responses by up to 63 +/- 9% after 3 mg/kg, i.v. Rizatriptan therefore has central activity which may contribute to its efficacy against migraine headache.

The novel anti-migraine agent rizatriptan inhibits neurogenic dural vasodilation and extravasation

These studies in anaesthetised rats showed, using intravital microscopy, that the novel anti-migraine agent, rizatriptan, significantly reduced electrically stimulated dural vasodilation but had no effect on increases in dural vessel diameter produced by exogenous substance P or calcitonin gene-related peptide (CGRP). Rizatriptan also significantly inhibited dural plasma protein extravasation produced by high intensity electrical stimulation of the trigeminal ganglion. We suggest that rizatriptan inhibits the release of sensory neuropeptides from perivascular trigeminal nerves to prevent neurogenic vasodilation and extravasation in the dura mater. These prejunctional inhibitory effects may be involved in the anti-migraine action of rizatriptan.

Sumatriptan inhibits neurogenic vasodilation of dural blood vessels in the anaesthetized rat--intravital microscope studies

This study used intravital microscopy to measure the diameter of dural arteries in anaesthetized rats. Electrical stimulation of the surface of a closed cranial window produced increases in dural vessel diameter which were blocked by the CGRP receptor antagonist human-alpha CGRP(8-37) but unaffected by the NK1 receptor antagonist RP67580. Sumatriptan (3 and 10 mg kg-1, i.v.) significantly reduced the response to electrical stimulation. In contrast, sumatriptan (3 mg kg-1) had no effects on the response to exogenously administered CGRP. These results indicate that neurokinins play no role in neurogenic vasodilation in this preparation and that neurogenic vasodilation in rat dural vessels is mediated predominantly by CGRP. Furthermore, the data indicate that sumatriptan attenuates neurogenic vasodilation, probably by inhibiting the release of CGRP from perivascular trigeminal nerve endings innervating the dura. These experimental data parallel the clinical findings that CGRP levels are elevated in migraine and normalized, concomitantly with headache relief, by sumatriptan.

Intravital microscope studies on the effects of neurokinin agonists and calcitonin gene-related peptide on dural vessel diameter in the anaesthetized rat

This study describes a novel intravital microscope technique for direct measurement of dural blood vessel diameter through a closed cranial window in anaesthetized rats. This technique avoids removal of the skull, which can lead to problems of altered vessel reactivity and brain swelling that are encountered with open cranial window techniques. Substance P and calcitonin gene-related (CGRP) evoked increases in dural vessel diameter, which were abolished by the NK1 receptor antagonist, RP67580 and the CGRP receptor antagonist, human-alpha CGRP(8-37) respectively. Neurokinin A produced increases in dural vessel diameter which were unaffected by the NK2 receptor antagonist SR 48968 but were blocked by RP67580, suggesting that neurokinin A can act through NK1 receptors to produce dural vasodilation in rats. The NK3 receptor agonist, senktide, had no effects on dural vessel diameter. All drugs were administered intravenously. In humans, vasodilation within the meningeal vasculature has been implicated in the pathogenesis of migraine, the present experiments indicate that substance P or neurokinin A (both acting through NK1 receptors) or CGRP may be responsible.

Neurokinin-receptor antagonists: pharmacological tools and therapeutic drugs

The mammalian tachykinins (substance P, neurokinin A, and neurokinin B) are widely distributed throughout the central and peripheral nervous systems, where they act as neurotransmitters or neuromodulators. Historically, the tachykinins have been implicated in a wide variety of biological actions such as pain transmission, neurogenic inflammation, smooth muscle contraction, vasodilation, secretion, and activation of the immune system. Their effects are mediated via specific G-protein-coupled receptors (NK1, NK2, and NK3 receptors). The development of nonpeptide receptor antagonists revealed species differences in neurokinin-receptor pharmacology, and the recent cloning of human neurokinin receptors has led to development of compounds with optimized affinity for the human target receptor. The neurokinin-receptor antagonists have been used in preclinical experiments to confirm the physiological roles of the tachykinins. Importantly, it is now recognised that these agents can inhibit the actions of tachykinins released from peripheral nerves, and for the NK1-receptor antagonists (the most widely studied class of neurokinin-receptor antagonists) central sites of action have also been demonstrated. These studies support the development of neurokinin-receptor antagonists as potentially exploitable drug therapies in humans, particularly in the treatment of pain and emesis.

In vitro and in vivo predictors of the anti-emetic activity of tachykinin NK1 receptor antagonists

The ability of tachykinin NK1 receptor antagonists to inhibit GR73632 (D-Ala-[L-Pro9,Me-Leu8]substance P-(7-11))-induced foot tapping in gerbils was employed as an indirect measure of brain penetration and this was compared with their ability to prevent acute emesis induced by cisplatin in ferrets. (+)-GR203040 ((2S,3S and 2R,3R)-2-methoxy-5-tetrazol-1-yl-benzyl-(2-phenyl-piperidin- 3-yl)-amine), CP-99,994 ((2S,3S)-cis-3-(2-methoxybenzylamino)-2-phenyl piperidine) dihydrochloride), and L-742,694 (2-(S)-(3,5-bis(trifluoromethyl)benzyloxy)-3-(S)-phenyl-4-(5-(3-oxo-1,2, 4-triazolo)methylmorpholine) potently inhibited GR73632-induced foot tapping (ID50 < or = 0.85 mg/kg), and acute retching induced by cisplatin (ID50 < or = 0.18 mg/kg). RPR100893 ((3aS,4S,7aS)-7,7-diphenyl-4-(2-methoxyphenyl)-2-[(S)-2-(2-m ethoxyphenyl)proprionyl] perhydroisoindol-4-ol) was not a potent antagonist of retching (ID50 4.1 mg/kg) or foot tapping (ID50 > 10 mg/kg). High doses (3-10 mg/kg) of CGP49823 ((2R,4S)-2-benzyl-1-(3,5-dimethylbenzoyl)-N-[(4-quinolinyl)methyl] -4-piperineamine) dihydrochloride), FK888 (N2-[(4R)-4-hydroxy-1-(1-methyl-1H-indol-3-yl)carbonyl-L-propyl]-N-methy l-N-phenylmethyl-L-3-(2-naphthyl)-alaninamide), and LY303870 ((R)-1-[N-(2-methoxybenzyl)acetylamino]-3-(1H-indol-3-yl)-2-[N-(2-(4-(pi peridinyl)piperidin-1-yl)acetyl)amino]propane) were required to inhibit foot tapping; these agents were not anti-emetic in this dose range. SR140333 ((S)-1-[2-[3-(3,4-dichlorphenyl)-1 (3-isopropoxyphenylacetyl)piperidin-3-yl] ethyl]-4-phenyl-1 azaniabicyclo [2.2.2]octane; 3-10 mg/kg) failed to inhibit foot tapping or emesis. Affinities for the human and ferret tachykinin NK1 receptor were highly correlated (r = 0.93, P = 0.0008). Inhibition of foot tapping in gerbils, but not NK1 receptor binding affinity, predicted anti-emetic activity in ferrets (r = 0.75, P < 0.01). These findings confirm that the anti-emetic activity of tachykinin NK1 receptor antagonists is dependent on brain penetration.

Differential effects of 5-HT1B/1D receptor agonists on neurogenic dural plasma extravasation and vasodilation in anaesthetized rats

These studies compared the effects of the 5-HT1B/1D receptor agonists sumatriptan, CP-122 288 ((R)-N-methyl-[3-(1-methyl-2-pyrrolidinylmethyl)-1H-indol-5-yl] methanesulphonamide succinate) and CP-93 129 (3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one dihydrochloride) on neurogenic dural extra-vasation and vasodilation in anaesthetized rats. Dural extravasation, evoked by high intensity (1.2 mA) stimulation of the trigeminal ganglion, was measured using the radioactive plasma marker 125I-labelled bovine serum albumin. Dural vasodilation produced by lower intensity (50-300 microA) stimulation of trigeminal fibres, was measured through a closed cranial window using intravital microscopy. All compounds inhibited dural extravasation (rank order of potency: CP-122 288 > > sumatriptan > CP-93 129) and dural vasodilation (rank order of potency: CP-93 129 > > sumatriptan = CP-122 288). Comparison of the potency of these compounds with their potencies in an in vitro functional model, agonist-induced [35S]GTP gamma S binding, suggests that blockade of dural extravasation was consistent with an action at rat 5-HT1D receptors, but activity at another, unknown, "extravasation receptor" could also be involved. In contrast, inhibition of dural vasodilation was consistent with an action at rat 5-HT1B receptors. We suggest that in our preparations, production of dural vasodilation involves activation of trigeminal A delta-fibres whereas production of dural extravasation involves activation of trigeminal C-fibres. The differential effects of compounds on dural extravasation and vasodilation may therefore be due to the different receptor subtypes involved and to the selective localization of these subtypes on different populations of trigeminal sensory fibre.

Comparison of the vasoconstrictor properties of the 5-HT1D-receptor agonists rizatriptan (MK-462) and sumatriptan in human isolated coronary artery: outcome of two independent studies using different experimental protocols

Rizatriptan (MK-462) is a novel 5-HT1D-receptor agonist and is effective in the treatment of migraine headache. As angiographic studies have shown that the prototypic 5-HT1D/1B-receptor agonist sumatriptan can cause coronary artery constriction in patients with mild coronary artery disease, we have compared the contractile effects of rizatriptan on human isolated coronary artery with those of sumatriptan and 5-HT. Two different experimental protocols were used. In Study 1 (to avoid agonist desensitisation and interaction effects), arterial segments were exposed to a single agonist (either 5-HT, sumatriptan or rizatriptan) and in Study 2 each arterial segment was exposed to all three agonists with randomised first exposure to sumatriptan or rizatriptan. In both these studies the maximum contractions evoked by sumatriptan and rizatriptan were found to be smaller than those evoked by 5-HT, and the maximum contraction evoked by rizatriptan was significantly smaller than that for sumatriptan.

L-733,060, a novel tachykinin NK1 receptor antagonist; effects in [Ca2+]i mobilisation, cardiovascular and dural extravasation assays

This study investigated the properties of a novel piperidine ether-based tachykinin NK1 receptor antagonist L-733,060, ((2S,3S)-3-((3,5-bis(trifluoromethyl)phenyl)methyloxy)-2-phenyl piperidine and its 2R,3R-enantiomer L-733,061 on [Ca2+]i mobilisation in Chinese hamster ovary cells transfected with human tachykinin NK1 receptors, compared to their effects in rodent cardiovascular and neurogenic plasma extravasation assays. Using FURA-2-imaging techniques, L-733,060 inhibited substance P-induced [Ca2+]i mobilisation with an estimated affinity of 0.8 nM whereas L-733,061 (30-300 nM) did not. No significant effects of L-733,060 were observed on mean arterial blood pressure or heart rate in conscious or anaesthetised rats at doses of < 3000 micrograms kg-1 i.v. L-733,060 also stereoselectively inhibited neurogenic plasma extravasation in rat dura produced by electrical stimulation of trigeminal nerves with an ID50 of 212 +/- 19 micrograms kg-1 i.v. Thus, L-733,060 is a novel antagonist of human tachykinin NK1 receptors which stereoselectively inhibits neurogenic plasma extravasation at doses that do not cause adverse cardiovascular effects.

N-heteroaryl-2-phenyl-3-(benzyloxy)piperidines: a novel class of potent orally active human NK1 antagonists

The preparation of a series of N-heteroarylpiperidine ether-based human NK1 antagonists is described. Two of the compounds 3-[-(2S,3S)-3-(((3,5-bis(trifluoromethyl)phenyl)methyl)oxy)- 2-phenylpiperidino}methyl]-1,2,4-triazole (11) and 5-[¿(2S,3S)-3-(((3,5-bis(trifluoromethyl)-phenyl)methyl)oxy)-2- phenylpiperidino}methyl]-3-oxo-1,2,4-triazolone (12)), in particular, are orally bioavailable and exhibited significant improvements in potency, both in vitro and in vivo, over the lead (carboxamidomethyl)piperidine ether 1. Rat liver microsome studies on a selected number of compounds from this series show the triazolone heterocycle to be considerably more stable than the others. Furthermore, both 11 and 12 have been profiled in a number of assays that may be predictive of the clinical utility of substance P antagonists.

Effects of a partial agonist and a full antagonist acting at the glycine site of the NMDA receptor on inflammation-induced mechanical hyperalgesia in rats

1. NMDA receptor antagonists have previously been shown to have antinociceptive effects in behavioural experiments, but controversy remains as to the role of NMDA receptors in mechanical hyperalgesia. We have studied the effects on mechanical nociceptive thresholds in rats with carrageenin-induced paw inflammation of L-687,414, a low efficacy partial agonist which acts as a functional antagonist at the glycine modulatory site of the NMDA receptor and of L-701,324, a structurally novel, highly selective, full antagonist at this site. 2. Mechanical thresholds were measured for both hind paws 1 h before and 3 h after carrageenin or saline was injected into 1 hind paw. Dose-response curves were constructed for each test compound in separate experiments, with test compound or vehicle being given i.p. 1 h before the final test. 3. Both compounds produced selective dose-dependent and statistically significant reversal of mechanical hyperalgesia, with minimum effective doses of 100 mg kg-1 L-687,414 and 3 mg kg-1 L-701,324. Neither L-687,414 nor L-701,324 affected the response threshold of the contralateral non-inflamed paw over the dose-range producing reversal of carrageenin-induced hyperalgesia. Neither compound had any effect on the paw oedema produced by carrageenin injection. 4. These results show that both a full antagonist and a low efficacy partial agonist at the glycine modulatory site of the NMDA receptor complex reverse inflammation-induced mechanical hyperalgesia, thus supporting the argument that maximal activation of the glycine site is required for transmission via NMDA receptors, and showing that NMDA receptor-mediated actions are important in mechanical hyperalgesia induced by inflammation.

Tachykinin NK1 receptor antagonists act centrally to inhibit emesis induced by the chemotherapeutic agent cisplatin in ferrets

These studies have compared the pharmacological profile of two non-peptide human type neurokinin1 (hNK1) receptor selective antagonists, L-741,671 and a quaternised compound L-743,310. In radioligand binding studies L-741,671 and L-743,310 had high affinity for ferret and cloned hNK1 receptors [Ki (nM) ferret 0.7 and 0.1; human 0.03 and 0.06, respectively] but low affinity for rodent NK1 receptors [Ki (nM) 64 and 17, respectively] suggesting that ferret receptors have hNK1-like binding pharmacology. Studies in vivo showed that L-741,671 and L-743,310 had equivalent functional activity in the periphery (ID50s of 1.6 and 2 micrograms/kg i.v., respectively) as measured by inhibition of plasma protein extravasation evoked in the oesophagus of guinea pigs by resiniferatoxin (7 nmol/kg i.v.). Using an in situ brain perfusion technique in anaesthetised rats, L-741,671 was shown to be much more brain penetrant than the quaternary compound L-743,310 which had an entry rate similar to the poorly brain penetrant plasma marker inulin. These compounds thus provided an opportunity to compare the anti-emetic effects of equi-active hNK1 receptor antagonists with and without brain penetration to central NK1 receptor sites. When tested against cisplatin-induced emesis in ferrets, L-741,671 (0.3, 1 and 3 mg/kg i.v.) produced marked dose-dependent inhibition of retching and vomiting but L-743,310 was inactive at 3 and 10 micrograms/kg i.v. In contrast, direct central injection of L-741,671 and L-743,310 (30 micrograms) into the vicinity of the nucleus tractus solitarius or L-743,310 (200 micrograms) intracisternally was shown to inhibit retching and vomiting induced by i.v. cisplatin. L-741,671 and L-743,310 had equivalent functional activity, at the same dose, against cisplatin-induced emesis when injected centrally. These observations indicated that had L-743,310 penetrated into the brain after systemic administration it would have been active in the cisplatin-induced emesis assay and so show that brain penetration is essential for the anti-emetic action of systemically administered NK1 receptor antagonists.

Calcitonin gene-related peptide as an endogenous vasodilator: immunoblockade studies in vivo with an anti-calcitonin gene-related peptide monoclonal antibody and its Fab' fragment

1. Calcitonin gene-related peptide (CGRP) is localized in perivascular sensory neurons and is a potent vasodilator. We investigated the utility of immunoblockade as an in vivo technique for probing the role of CGRP as an endogenous vasodilator. 2. The effects of an anti-CGRP monoclonal antibody (MAb; coded C4.19) and its Fab' fragment on CGRP-induced changes in blood pressure and skin blood flow were studied in pentobarbitone-anaesthetized rats. Antidromic skin vasodilatation in the rat hind paw was measured by laser Doppler fluxmetry. 3. The dose-response relationship for the hypotensive effect of intravenous rat alpha CGRP (r alpha CGRP) was similarly shifted rightward by MAb C4.19 IgG (1 mg/rat; intravenously) and Fab' fragment (2 mg/rat; intravenously). The C-terminal fragment of human alpha CGRP (h alpha CGRP8-37) also blocked the hypotensive effect of r alpha CGRP. 4. MAb C4.19 Fab' fragment (2 mg/rat; intravenously) and h alpha CGRP8-37 (100 nmol/kg; intravenously), but not MAb C4.19 IgG (up to 3 mg/rat; intravenously) or normal mouse Fab' fragment (2 mg/rat; intravenously), blocked the increased skin blood flow response to antidromic stimulation of the saphenous nerve. 5. The mean percentage changes in skin blood flow parameters due to MAb C4.19 Fab' fragment were significantly different from those due to normal mouse Fab' fragment (unpaired t-test; P < 0.05) but not from those due to h alpha CGRP8-37. 6. The results demonstrate the pharmacokinetic advantage of Fab' fragment over IgG for immunoblockade studies in vivo and support the role of CGRP in mediating skin vasodilatation.

Enantiospecific inhibition of emesis induced by nicotine in the house musk shrew (Suncus murinus) by the neurokinin1 (NK1) receptor antagonist CP-99,994

Effects of the NK1 receptor antagonist CP-99,994 on nicotine-induced emesis were examined in Suncus murinus. CP-99,994 (3 and 10 mg/kg i.p.) attenuated emesis to (-)nicotine (4 mg/kg s.c.). CP-100,263 (3 and 10 mg/kg i.p.), the enantiomer of CP-99,994 with 1000 fold lower affinity for the NK1 receptor was without effect and RP67580 reduced emesis only at a dose of 30 mg/kg i.p. Responses to NK1 antagonists were ranked according to their affinities for the Suncus murinus NK1 receptor.

Acidic fibroblast growth factor stimulates motor and sensory axon regeneration after sciatic nerve crush in the rat

A time course study in untreated male Sprague-Dawley rats showed that there was no significant difference in the rate of regeneration of motor and sensory axons after a crush injury. Acidic fibroblast growth factor, given topically directly to the site of the crush injury (osmotic minipump for three days) or systemically (i.v. once daily for three days), stimulated the regeneration of motor axons and myelinated sensory axons in the sciatic nerve of the rat. Dose-dependent increases in regeneration distance were seen after 3.6, 36 or 360 ng/day were applied locally and 3 or 10 micrograms/kg per day were given systematically. The greatest effects were achieved with 36 ng/day locally or 10 micrograms/kg per day systematically, when the increase in regeneration distance over three days compared to untreated rats was 47% and 48%, respectively. Administration of heparin vehicle had no significant effect on regeneration. We conclude that since a crush injury has little effect on the endoneurial tubes and supporting cells, the stimulatory effects of acidic fibroblast growth factor on peripheral nerve regeneration seen in this study are likely to be due to a direct acceleration of axonal extension. This is in contrast with the axonal regeneration that occurs across a gap after nerve transection, where axonal extension may be secondary to stimulatory effects on non-neuronal cells providing a supporting "bridge" across the gap. These results suggest that acidic fibroblast growth factor may be clinically useful in the treatment of peripheral neuropathy in man, particularly since systemic treatment for short periods may be effective.

Comparison of the effects of sumatriptan and the NK1 antagonist CP-99,994 on plasma extravasation in Dura mater and c-fos mRNA expression in trigeminal nucleus caudalis of rats

Dural plasma extravasation produced by electrical stimulation of the trigeminal ganglion was measured in rats and the concomitant expression of c-fos mRNA produced in the trigeminal nucleus caudalis (NtV) was measured using in situ hybridization techniques. The non-peptide NK1 receptor selective antagonist CP-99,994 (1-3000 micrograms kg-1) and the 5HT1D receptor agonist sumatriptan (1-1000 micrograms kg-1) reduced dural plasma extravasation dose-dependently with ID50S of 52 micrograms kg-1 and 30 micrograms kg-1 respectively. CP-99,994 (1000 micrograms kg-1). a compound known to have good brain penetration, decreased c-fos mRNA expression in the NtV by 37 +/- 7% without disruption of the blood brain barrier (BBB). Sumatriptan (1000 micrograms kg-1), known to be poorly brain penetrant, had no significant effect on c-fos mRNA expression in the NtV unless the BBB was disrupted by infusion of a hyperosmolar mannitol solution after which sumatriptan decreased c-fos mRNA expression by 65 +/- 11%. The results suggest that brain penetrant NK1 receptor antagonists may have anti-migraine effects peripherally through blockade of dural extravasation and centrally by inhibition of nociceptive pathways. Furthermore the data indicates that the anti-migraine action of sumatriptan must be predominantly peripherally mediated, be it via inhibition of plasma extravasation or direct vasoconstriction, since it had little effect on the activation of neurones in the NtV unless the BBB was disrupted.

The neuroprotective effect of the glycine site antagonist 3R-(+)-cis-4-methyl-HA966 (L-687,414) in a rat model of focal ischaemia

3R-(+)-cis-4-Methyl-HA966 (L-687,414) is a novel and selective, low intrinsic activity, partial agonist at the glycine site of the N-methyl-D-aspartate (NMDA) receptor. Thus, while it acts primarily to block NMDA receptor function in the presence of glycine, it fails to produce a complete block of NMDA receptor activation. In this study, we have investigated its neuroprotective effects in a rat model of focal ischaemia, involving permanent occlusion of the left middle cerebral artery. L-687,414 was administered as a bolus dose of 17.6 mg/kg i.v. straight after the occlusion or as a bolus dose + infusion for 4 h. The doses of L-687,414 used for the infusion studies were 7 mg/kg i.v. + 7 mg/kg/h, 14 mg/kg + 14 mg/kg/h, or 30 mg/kg + 30 mg/kg/h. The 17.6-mg/kg dose gave an estimated peak plasma level of 24 micrograms/ml, which decayed with a t1/2 of 56 min. The three infusion dosing regimens gave mean plasma levels over the 4 h of 11, 25 and 61 micrograms/ml plasma, respectively. The 17.6-mg/kg dose of L-687,414 gave no significant protection against the volume of hemispheric, cortical, or caudate damage when compared with the control group of animals. The lowest infusion dosing regimen of L-687,414 which gave a plasma level of 11 micrograms/ml over the 4 h was also ineffective against the volume of infarction measured in the different brain regions.(ABSTRACT TRUNCATED AT 250 WORDS)

Biological properties of the benzodiazepine amidine derivative L-740,093, a cholecystokinin-B/gastrin receptor antagonist with high affinity in vitro and high potency in vivo

A novel series of 5-amino-1,4-benzodiazepin-2-one derivatives (amidines), which contain a cationic solubilizing group and which are antagonists for the cholecystokinin (CCK)-B receptor, have been identified. Optimization of this series led to the identification of an azabicyclononane amidine, L-740,093 [N-[(3R)-5-(3-azabicyclo[3.2.2]nonan-3-yl)-2,3-dihydro-1-methyl-2- oxo- 1H-1,4-benzodiazepin-3-yl]-N'-(3-methylphenyl)urea], that bound with high affinity of CCK-B receptors from guinea pig cerebral cortex (IC50 of 0.1 nM) and had a CCK-B/CCK-A receptor selectivity of 16,000. In comparison, L-365,260 had 85-fold lower affinity (8.5 nM) and was only 87-fold selective for CCK-B over CCK-A receptors. L-740,093 bound with high affinity to guinea pig gastrin receptors in vitro (IC50 of 0.04 nM). Electrophysiological studies on slices of rat ventromedial hypothalamic nucleus showed that L-740,093 produced rightward shifts of the concentration-response curve for the CCK-B receptor agonist pentagastrin (Kb of 0.06 nM). L-740,093 blocked pentagastrin-induced gastric acid secretion in anesthetized rats with a 50% inhibitory dose of 0.01 mg/kg, intraperitoneally, showing 100-fold greater activity, compared with L-365,260 (50% inhibitory dose of 1 mg/kg, intraperitoneally). An ex vivo binding assay in mice was used to investigate the interaction of L-740,093 with central CCK binding sites. After intravenous administration, L-740,093 inhibited ex vivo binding dose dependently, with a 50% effective dose of 0.2 mg/kg. These studies demonstrate that L-740,093 is the most potent and selective CCK-B antagonist yet described and that it has excellent central nervous system penetration.

Synthesis and biological activity of 3-[2-(dimethylamino)ethyl]-5-[(1,1-dioxo-5-methyl-1,2,5-thiadiazolidin- 2-yl)-methyl]-1H-indole and analogues: agonists for the 5-HT1D receptor

A novel series of 5-(1,1-dioxo-1,2,5-thiadiazolidin-2-yl)tryptamines was designed, synthesized, and evaluated as 5-HT1D receptor agonists. Compounds such as 8d,f,k were identified which had comparable affinity, potency, and receptor selectivity to that of the antimigraine drug sumatriptan. Both 8d,k were found to be well absorbed in the rat with oral bioavailabilities of 66% and 62%, respectively. Additionally, 8d was found to be selective over other non-serotonergic receptors and exhibited relatively low central nervous system penetration.

A second generation of non-peptide cholecystokinin receptor antagonists and their possible therapeutic potential

The profile of an acidic series of benzodiazepine CCK-B receptor antagonists is described. The tetrazolyl urea derivative L-368,935 had high affinity (CCK-B IC50 0.1 nM) and was one of the most selective (CCK-B/CCK-A 10,000) CCK-B antagonists known. L-368,935 was a CCK-B antagonist with high affinity on the rat ventromedial hypothalamic slice preparation (Kb 0.6 nM) and also blocked pentagastrin-induced calcium mobilization in GH3 cells. L-368,935 had potent in vivo activity and antagonized pentagastrin-induced gastric acid secretion in the anesthetized rat and CCK-8S-induced aspartate release using microdialysis in the striatum of conscious rats. Activity within the central nervous system was confirmed by a mouse ex vivo binding assay and by direct measurement of the compound within the central nervous system using an HPLC assay. A second generation of CCK-B receptor antagonists such as L-368,935 will be important in determining the therapeutic potential of this class of compound in man.

Enantioselective inhibition of apomorphine-induced emesis in the ferret by the neurokinin1 receptor antagonist CP-99,994

These studies have examined the effects of the selective neurokinin1 (NK1) receptor antagonist CP-99,994 on the retching and vomiting response to apomorphine. CP-99,994 (1-3 mg/kg i.p.) attenuated retching and vomiting induced by apomorphine (0.25 mg/kg s.c.) with complete inhibition of retching and vomiting at the 3 mg/kg dose. In contrast CP-100,263 (3 mg/kg i.p.), the enantiomer of CP-99,994 with 1000-fold lower affinity for the NK1 receptor, was without effect.

Neuroprotective NMDA antagonists: the controversy over their potential for adverse effects on cortical neuronal morphology

It has been reported that several uncompetitive NMDA receptor ion channel blocking agents (phencyclidine, ketamine, dizocilpine, dextrorphan) cause transient reversible vacuolation in neurons in the posterior cingulate cortex of rats. Similar effects have also been observed with competitive glutamate antagonists such as CPP, CGS 19755 and CGP 37849. This transient morphological change has been noted to be coincident anatomically with brain regions showing hypermetabolism after administration of uncompetitive NMDA receptor ion channel blockers and competitive glutamate antagonists. These results therefore indicate that the functional consequences of NMDA receptor blockade with competitive glutamate and uncompetitive channel antagonists are ultimately the same. These changes do not appear to be a prelude to irreversible damage except after relatively high doses of the receptor ion channel antagonists but they have given rise to concern over the safety in use of NMDA antagonists as neuroprotective agents. In contrast, vacuolation has not yet been demonstrated with agents acting at the glycine (L-687,414) or polyamine (eliprodil) modulatory sites of the NMDA receptor complex suggesting that agents acting at these sites may have a greater potential therapeutic window.

Competitive as well as uncompetitive N-methyl-D-aspartate receptor antagonists affect cortical neuronal morphology and cerebral glucose metabolism

The studies examined the effects of three antagonists (CPP, CGS 19755, and CGP 37849) that act competitively at the glutamate recognition site of the NMDA receptor complex on cortical neuronal morphology and cerebral limbic glucose metabolism. Responses were compared to the effects of dizocilpine, an uncompetitive NMDA receptor ion channel antagonist as a positive control. CGS 19755 and CGP 37849 (100 mg kg-1 i.p.) caused vacuolation in cortical pyramidal neurons in the posterior cingulate cortex four hours after dosing and this dose of CGP 37849 caused a pattern of limbic glucose metabolism activation similar to that seen after dizocilpine. CPP was without effect at 100 mg/kg i.p. probably due to poor brain penetration. The data indicates that the functional consequences (structural and metabolic) of NMDA receptor blockade with NMDA antagonists acting competitively at the glutamate recognition site and uncompetitively in the receptor ion channel are ultimately the same. Comparisons of the potential therapeutic window for CGS 19755 and CGP 37849 with dizocilpine (neuroprotection versus vacuolation) suggests that the window for the competitive antagonists is greater. This indicates that the potential therapeutic window for the different classes of NMDA antagonists may vary with the site in the receptor complex at which they interact.

The tachykinin NK1 receptor antagonist CP-99,994 attenuates cisplatin induced emesis in the ferret

The tachykinin NK1 receptor antagonist CP-99,994 ((+)-(2S,3S)-3-(2-methoxybenzylamino)-2-phenylpiperidine) (0.3-3 mg/kg i.v.), but not its inactive enantiomer CP-100,263, attenuated the retching and vomiting induced by cisplatin (10 mg/kg i.v.) in the ferret. CP-99,994, 3 mg/kg i.v., prevented vomiting in all ferrets tested. Since substance P is the preferred ligand at the NK1 receptor subtype these data support a role for the release of this peptide during the emetic response induced by cytotoxic chemotherapeutic agents.

L-694,247: a potent 5-HT1D receptor agonist

1. The 5-hydroxytryptamine (5-HT) receptor binding selectivity profile of a novel, potent 5-HT1D receptor agonist, L-694,247 (2-[5-[3-(4-methylsulphonylamino)benzyl-1,2,4-oxadiazol-5-yl ]- 1H-indole-3-yl]ethylamine) was assessed and compared with that of the 5-HT1-like receptor agonist, sumatriptan. 2. L-694,247 had an affinity (pIC50) of 10.03 at the 5-HT1D binding site and 9.08 at the 5-HT1B binding site (sumatriptan: pIC50 values 8.22 and 5.94 respectively). L-694,247 retained good selectivity with respect to the 5-HT1A binding site (pIC50 = 8.64), the 5-HT1C binding site (6.42), the 5-HT2 binding site (6.50) and the 5-HT1E binding site (5.66). The pIC50 values for sumatriptan at these radioligand binding sites were 6.14, 5.0, < 5.0 and 5.64 respectively. Both L-694,247 and sumatriptan were essentially inactive at the 5-HT3 recognition site. 3. L-694,247, like sumatriptan, displayed a similar efficacy to 5-HT in inhibiting forskolin-stimulated adenylyl cyclase in guinea-pig substantia nigra although L-694,247 (pEC50 = 9.1) was more potent than sumatriptan (6.2) in this 5-HT1D receptor mediated functional response. L-694,247 (pEC50 = 9.4) was also more potent than sumatriptan (6.5) in a second 5-HT1D receptor mediated functional response, the inhibition of K(+)-evoked [3H]-5-HT release from guinea-pig frontal cortex slices. 4. The excellent agreement observed for L-694,247 between the 5-HTlD radioligand binding affinity and the functional potency confirm that the two functional models (the inhibition of forskolin-stimulated adenylyl cyclase in guinea-pig substantia nigra and the inhibition of K+-evoked [3H]-5-HT release from guinea-pig frontal cortex) do indeed reflect 5-HTID-mediated events.5. L-694,247 is a novel, highly potent 5-HTID/5-HTIB receptor ligand which should prove useful for the exploration of the physiological role of these receptors in animals.

Lack of effect of L-687,414 ((+)-cis-4-methyl-HA-966), an NMDA receptor antagonist acting at the glycine site, on cerebral glucose metabolism and cortical neuronal morphology

1. N-methyl-D-aspartate (NMDA) receptor ion channel antagonists have been reported to cause pronounced increases in cerebral glucose metabolism (CMRglc) and transient reversible vacuolation within pyramidal cortical neurones. The present studies examined in rats the effects of the NMDA receptor antagonist, L-687,414 (R-(+)-cis-4-methyl-3-amino-l-hydroxypyrolid-2-one (+)-cis-4-methyl-HA-966) on regional CMRglc and cortical neuronal morphology. 2. L-687,414 was given as a steady state intravenous infusion for 4 h in a neuroprotective dose regime of 17.5 mg free base kg-1 bolus followed by 225 micrograms kg-1 min-1 (n = 8) or at the higher dose rate of 35 mg kg-1 bolus followed by 440 micrograms kg-1 min-1 (n = 10). Data were compared to a parallel series of experiments in rats given the NMDA receptor ion channel antagonist, dizocilpine for 4 h in the optimum intravenous neuroprotective dose-regime of 0.12 mg kg-1 bolus followed by 1.8 micrograms kg-1 min-1 (n = 8) or at the higher dose rate of 0.4 mg kg-1 bolus followed by 6 micrograms kg-1 min-1 (n = 4; morphology only studied). A saline-infused group of rats (n = 8) were used as controls. 3. CMRglc was studied by use of [14C]-2-deoxyglucose and autoradiography (n = 4 each group) whilst plasma drug levels were in a steady state during the final 45 min of the 4 h drug infusion. Effects on cortical neuronal morphology were assessed at the end of the 4 h infusion period using light microscopic techniques (n = 4-6 each group).4. The results showed a selective activation of limbic CMRglc by dizocilpine at optimal neuroprotective dose levels and showed that this dose was at the threshold for the neuronal vacuolation response as I of 4 rats showed morphological changes in the pyramidal neurones in the posterior cingulate and retrosplenial cortices. At the higher dose rate of dizocilpine, all 4 animals showed extensive morphological changes in these cortical neurones. In contrast, L-687,414 did not increase limbic CMRglc, nor evoke vacuolation when given in the neuroprotective dose-regime or at the higher dosage rate.5. The findings of the present study suggest that neuroprotection mediated through the NMDA receptor complex can be achieved without changes in CMRglc or cortical neuronal morphology by antagonism at the glycine modulatory site.

Effect of RP 67580, a non-peptide neurokinin1 receptor antagonist, on facilitation of a nociceptive spinal flexion reflex in the rat

1. In order to examine the contribution of neurokinin1 (NK1) receptors to facilitation of a spinal nociceptive reflex in the rat, we have investigated the effects of RP 67580 (3aR, 7aR)-7,7-diphenyl-2(1-imino-2-(2-methoxyphenyl/ethyl)perhydrois oindole)), a non-peptide neurokinin1 (NK1) receptor antagonist, selective for the rodent receptor sub-type, on the activity of individual motorunits. These results were compared with the effects of RP 68651, the inactive 3aS, 7aS enantiomer of RP 67580, as a control for non-specific activity. 2. Experiments were performed on 15 rats anaesthetized with a continuous i.v. infusion of alphaxalone/alphadalone and spinalized at T9-10. Single unit recordings of motorunit activity from biceps femoris/semitendinosus were made with a concentric needle electrode. In each experiment, a vehicle dose followed by 4 sequential rising doses of either RP 67580 or RP 68651 were given at 15 min intervals. High intensity electrical stimuli were applied to the hindlimb receptive field of the motorunit at a rate of 1 per 60 s throughout the experiment to establish a baseline. A conditioning stimulus (20 of these stimuli at 1 Hz) was delivered 5 min after each dose and the effect of the size of the baseline response examined. 3. The conditioning stimulus evoked a facilitation of the baseline at the start of all experiments (mean increase +/- s.e. mean = 151 +/- 20%). RP 67580 attenuated this facilitation, with an ID50 (+/- s.e. mean) of 2.5 +/- 4.2 micrograms kg-1, i.v., whereas RP 68651 at doses of up to 3 mg kg-1, i.v. did not. There was no statistically significant effect of drug on the baseline reflex, nor on the response to the conditioning stimulus. Doses of 300 and 3000 microg kg-1 of both RP 67580 and RP 68651 evoked small depressor effects on systemic arterial blood pressure.4. We conclude that the facilitation of a spinal flexor reflex by noxious conditioning stimuli in the rat is mediated by NK1 receptors whereas the baseline reflex is not. The results suggest that brain penetrantNK1 receptor antagonists may have central anti-nociceptive effects.

The non-peptide neurokinin1 receptor antagonist, RP 67580, blocks neurogenic plasma extravasation in the dura mater of rats

A non-peptide neurokinin1 (NK1) receptor antagonist, RP 67580, that is selective for the rodent subtype of the NK1 receptor, dose-dependently reduced plasma extravasation in the dura mater produced by electrical stimulation of the trigeminal ganglion in rats, with an ID50 of 0.6 micrograms kg-1. Its enantiomer RP 68651 was some 400 fold less active. The results indicate that neurogenic plasma extravasation within the dura mater is NK1 receptor-mediated and suggest that NK1 receptor antagonists may have a role as antimigraine agents.

The design of novel muscarinic partial agonists that have functional selectivity in pharmacological preparations in vitro and reduced side-effect profile in vivo

Antagonist/agonist binding ratios (NMS/Oxo-M ratio) were used as an index of the efficacy of novel compounds acting at muscarinic receptors. These binding ratios have been used with a range of functional pharmacological assays to investigate the effects of varying the efficacy of muscarinic agonists. This strategy has been used as a means of obtaining functional receptor selectivity by exploiting differences in effective receptor reserves. The oxadiazole and pyrazine muscarinic agonists L-670,548 (NMS/Oxo-M ratio 1100) and L-680,648 (NMS/Oxo-M ratio 690) are amongst some of the most potent and efficacious agonists known. Decreasing the efficacy of compounds from these series, resulted in compounds with functional selectivity. The chloropyrazine L-689,660 (NMS/Oxo-M ratio 28) was an agonist on the rat superior cervical ganglion (M1), a partial agonist on the guinea-pig ileum (M3), but was an antagonist in the guinea-pig atria (M2). Synthesis of compounds with even lower predicted efficacy, such as the cyclopropyloxadiazole L-687,306 (NMS/Oxo-M ratio 15), maintained agonist activity in the ganglion, but showed antagonist activity in the M3 ileal, as well as the M2 atrial preparations. When tested in vivo these compounds did not produce many of the side effects associated with more efficacious agonists, particularly those associated with the cardiovascular system. However, they were active in reversing scopolamine-induced deficits in a variety of behavioural paradigms. This approach shows how functional selectivity for muscarinic receptor subtypes can be achieved in vitro, that in vivo reduces the dose-limiting side effects normally associated with muscarinic agonists.

L-689,660, a novel cholinomimetic with functional selectivity for M1 and M3 muscarinic receptors

1. L-689,660, 1-azabicyclo[2.2.2]octane, 3-(6-chloropyrazinyl)maleate, a novel cholinomimetic, demonstrated high affinity binding (pKD (apparent) 7.42) at rat cerebral cortex muscarinic receptors. L-689,660 had a low ratio (34) of pKD (apparent) values for the displacement of binding of the antagonist ([3H]-N-methylscopolamine ([3H]-NMS) compared with the displacement of the agonist [3H]-oxotremorine-M ([3H]-Oxo-M), in rat cerebral cortex. Low NMS/Oxo-M ratios have been shown previously to be a characteristic of compounds that are low efficacy partial agonists with respect to stimulation of phosphatidyl inositol turnover in the cerebral cortex. 2. L-689,660 showed no muscarinic receptor subtype selectivity in radioligand binding assays but showed functional selectivity in pharmacological assays. At M1 muscarinic receptors in the rat superior cervical ganglion, L-689,660 was a potent (pEC50 7.3 +/- 0.2) full agonist in comparison with (+/-)-muscarine. At M3 receptors in the guinea-pig ileum myenteric plexus-longitudinal muscle or in trachea, L-689,660 was again a potent agonist (pEC50 7.5 +/- 0.2 and 7.7 +/- 0.3 respectively) but had a lower maximum response than carbachol. In contrast L-689,660 was an antagonist at M2 receptors in guinea-pig atria (pA2 7.2 (95% confidence limits 7, 7.4)) and at muscarinic autoreceptors in rat hippocampal slices. 3. The putative M1-selective muscarinic agonist, AF102B (cis-2-methylspiro-(1,3-oxathiolane 5,3')-quinuclidine hydrochloride) was found to have a profile similar to L-689,660 but had up to 100 times less affinity in binding and functional assays.RS-86 (2-ethyl-8-methyl-2,8-diazospiro[4,5]decan 1,3-dionehydrochloride) also had lower affinity than L-689,660, and had no binding selectivity for muscarinic receptor subtypes. RS-86 had a higher NMS/Oxo-M ratio than L-689,660 and was a full agonist at MI,M2 and M3 receptors in the functional pharmacological assays.4. The functional selectivity of L-689,660 in muscarinic pharmacological assays is consistent with the effects of a low efficacy partial agonist in tissues with different effective receptor reserves.

The effect of chronic low level lead exposure on blood-brain barrier function in the developing rat

Blood-brain barrier (BBB) function was assessed in 19-21-day-old rats exposed to low level lead from birth. Newborn rats received lead via milk from lactating dams given drinking water containing 0.1% lead acetate [Pb(Ac)2]. The treatment regime produced lead levels in the neonates within the range 20-80 micrograms dl-1 blood, without affecting growth. Cerebrovascular permeability (PS-product) to the diffusion-limited solute mannitol was unchanged in six regions of the cerebral hemisphere, the cerebellum and the brainstem, suggesting that barrier integrity was not affected by the low dose lead treatment. Regional cerebrovascular permeability to nutrient tracers representing seven BBB transport classes was not impaired by lead treatment. However, the PS estimates for the amino acids lysine and histidine and for thiamine were greater than control in some regions of the cerebral hemisphere. These alterations in nutrient supply to the brain may reflect altered substrate utilization associated with repair processes or delayed maturation of the CNS.

Comparison of the effects of ascorbyl palmitate and L-ascorbic acid on paracetamol-induced hepatotoxicity in the mouse

The effects of ascorbyl palmitate (ASCP) and free L-ascorbic acid (LAA) on the hepatotoxicity of paracetamol (acetaminophen) and the in vivo covalent binding of reactive paracetamol metabolites to hepatic proteins has been studied in male MF1 mice. The oral administration of [3H(G)]paracetamol (600 mg/kg) resulted in covalent binding to hepatic proteins, a depletion of hepatic non-protein sulphydryl (NPS) groups after 2 h, and a marked elevation of plasma alanine aminotransferase (ALAT) activity after 24 h. The co-administration of paracetamol and ASCP (1412 mg/kg, equivalent to 600 mg/kg free LAA), but not paracetamol and LAA (600 mg/kg), significantly reduced covalent binding of paracetamol metabolites at 2 and 4 h after treatment. In addition ASCP, but not LAA, significantly reduced the depletion of NPS groups and the elevation of plasma ALAT activity. ASCP also completely prevented the 35% mortality observed at 24 h in paracetamol treated mice. These results demonstrate that ASCP, but not LAA, when co-administered orally with the analgesic is an effective inhibitor of paracetamol-induced hepatotoxicity in the mouse. The mechanism by which ASCP prevents liver injury appears to involve destruction of reactive paracetamol metabolites which is associated with a sparing action on hepatic reduced glutathione levels.

Regional cerebral glucose metabolism and blood flow during the silent phase of methylmercury neurotoxicity in rats

Methylmercuric chloride was given to rats in a neurotoxic dose regimen (six daily doses of 8 mg kg-1 p.o.). During the silent (asymptomatic) phase of intoxication, the rates of cerebral glucose influx and cerebral glucose phosphorylation were measured simultaneously using 2-deoxyglucose. Regional cerebral blood flow was also measured using iodoantipyrine. The unidirectional flux of glucose into brain was not affected by methylmercury, and differences in the rates of glucose phosphorylation from region to region remained coupled to the regional cerebral blood flow. However, the blood flow was reduced throughout the brain, an observation suggesting that the operational level of metabolically regulated blood flow had been reset. Thus, in spite of a generalised reduction in blood flow, there was no indication of impaired cerebral glucose supply or utilization during the silent phase of methylmercury intoxication.

Persistent mercury in nerve cells 16 years after metallic mercury poisoning

A male subject, after exposure to mercury metal at work in 1968, developed classical signs of mercurialism from which he made a slow clinical recovery. He subsequently developed psychoneurotic symptoms and became an alcoholic; he never returned to work and died in 1984. No histological changes relevant to mercury intoxication were found in the brain, but staining by Danscher & Schroeder's method for mercury showed many positively staining lysosomal dense bodies in a large proportion of nerve cells, and the presence of mercury was confirmed by elemental X-ray analysis. The mercury content of the brain was increased, much of it being present in colloidal form.

Studies on the relationship between cerebral glucose transport and phosphorylation using 2-deoxyglucose

Regional rates of blood-brain glucose transfer and phosphorylation have been measured in anaesthetized fasted and conscious fed and fasted rats using a dual-label 2-deoxyglucose technique that exploits differences in the early-time distribution of analogue and native glucose between blood and brain. Regional cerebral blood flow was also measured in comparable groups of rats. Estimates of glucose influx in the anaesthetized group were compared with those calculated from previously published kinetic constants obtained using [14C]D-glucose as tracer. The close agreement of these two sets of results served to validate estimates of influx obtained using the glucose analogue. Comparisons between all three groups showed that regional rates of glucose influx were maintained at levels appropriate to the rate of cerebral glucose phosphorylation. This occurred despite wide variations in plasma glucose concentration. The results indicate that at least two factors are involved in the adaptation of glucose supply to meet metabolic demand. One is related to blood flow, and probably reflects changes in the surface area of the capillary endothelium perfused. The second involves changes in the blood-brain barrier permeability to glucose and could reflect changes in the density of functioning glucose transporters within capillary endothelial cell membranes.

The effect of various dietary fibres on tissue concentration and chemical form of mercury after methylmercury exposure in mice

The whole-body retention of mercury after exposure of BALB/c mice to methylmercury was measured in animals fed fibre-free, 5% pectin, 5% cellulose or 5, 15 or 30% wheat bran diets. The rate of elimination of mercury was dependent on the diet fed, with dietary bran increasing the rate of elimination. The incorporation of 15 or 30% bran in the diet of the mice decreased the total mercury concentration in the brain, blood and small intestine, although the effects were significant only in those animals on 30% bran diet. The fibres had little effect on mercury levels in other tissues. The proportion of mercury found in the mercuric form was significantly greater in liver, kidneys and gut of mice fed bran. The results suggest that dietary bran may reduce the levels of mercury in the brain after methylmercury exposure and may therefore reduce the neurotoxic effects of the organomercurial. We suggest that wheat bran exerts its effects on mercury retention and brain level via a modification of the metabolic activity of the gut microflora.

Regional blood-brain glucose transfer in the rat: a novel double-membrane kinetic analysis

Regional blood-brain glucose transfer was studied in pentobarbitone-anaesthetized rats using a programmed intravenous infusion technique that maintained steady levels of unlabeled (up to 55 mM) and tracer D-glucose in the circulating plasma. Regional cerebral blood flow, glucose phosphorylation rate, and tissue glucose content were also measured under comparable conditions. Data were analysed in terms of irreversible Michaelis-Menten kinetics assuming independent influx and efflux (Type I) and reversible Michaelis-Menten kinetics (Type II) across both the luminal and the abluminal membranes of the endothelial cell. The latter analysis corresponds to simple stereospecific membrane pores. The mathematical model allowed for changes in tissue glucose content and back-diffusion of tracer during the experiments. Type I analyses gave Kt values of approximately 6.6 mM, whereas those by Type II were consistently lower. Interregional differences were not significant using either scheme. Comparison of Type II with Type I analyses revealed a possible explanation for discrepancies in the estimates of nonsaturable glucose transfer by different methods and highlighted the importance of tissue glucose measurements in studies of unidirectional glucose influx. Since the experimental data may be described equally well by either scheme and some interaction between influx and efflux across the endothelial cell might be expected, consideration of this alternative approach is suggested.

Changes in the distribution of histochemically localized mercury in the CNS and in tissue levels of organic and inorganic mercury during the development of intoxication in methylmercury treated rats

The development of neurotoxicity in rats after exposure to methylmercuric chloride was monitored using behavioural indices. At selected time points the cellular localization of mercury and the relative amounts of organic and inorganic mercury were determined in several regions of the CNS, and in some non-neural tissues. The CNS showed an affinity for organic mercury, the levels of inorganic mercury remaining low throughout symptomatic intoxication. Histopathological changes were not closely related to the regional tissue content of the organic or inorganic forms, nor to mercury localized histochemically at the cellular level. The stained deposits, which had focal cytoplasmic distribution, appeared in glial cells initially then in larger neurones as the intoxication progressed. These observations may represent changes in the mercury content of different cell types or reflect differences in the way that they handle a similar burden of mercury. A transitory accumulation of mercury in glial cells may be a factor contributing to the occurrence of a latent period and sequestration of mercury in cytoplasmic organelles may serve to protect some cell types from injury.