2-chloro-5-hydroxyphenylglycine and 1-amino-1-3-dicarboxycyclopentane

Metabotropic glutamate receptors (mGluRs) have been popular drug targets for a variety of central nervous system (CNS) disease models, ranging from seizures to schizophrenia. The current study aimed to determine whether mGluRs participate in lateral hypothalamic (LH) stimulation of feeding. To this end, we used satiated adult male Sprague-Dawley rats stereotaxically implanted with indwelling bilateral LH guide cannulas to determine if injection of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), a broad mGluR group I and II agonist, would elicit feeding. Administration of 100 nmol ACPD induced feeding with a short latency. Similarly, unilateral LH injection of the selective mGluR group I agonist (S)-3,5-dihydroxyphenylglycine (DHPG) elicited significant feeding beginning 60 min postinjection and continuing until 4 h postinjection. Administration of the mGluR5 agonist, (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) produced a smaller delayed feeding response. These delayed but prolonged eating responses suggest that activation of LH mGluR1 and/or mGluR5 might be sufficient to elicit feeding. To determine which subtypes were involved, LH DHPG injections were preceded by LH injection of either the group I antagonist n-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxamide (PHCCC), the mGluR1 antagonist 6-amino-n-cyclohexyl-n,3-dimethylthiazolo[3,2-a]benzimi dazole-2-carboxamide hydrochloride (YM-298198) or the mGluR5 antagonist 3-((2-methyl-4-thiazolyl)ethynyl)pyridine (MTEP), and food intake was measured. PHCCC blocked DHPG-elicited feeding, and each of the other antagonists produced significant feeding suppression. These findings suggest roles for mGluR1 and/or mGluR5 in lateral hypothalamic circuits capable of stimulating feeding behavior.

Topics: Animals; Benzimidazoles; Benzopyrans; Cycloleucine; Dose-Response Relationship, Drug; Eating; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glycine; Hypothalamus; Male; Phenylacetates; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Resorcinols; Thiazoles; Time Factors

From previous work, it appears that glutamate can activate the hypothalamic-pituitary-adrenocortical (HPA) axis by an interaction at either ionotopic or metabotropic (G-protein coupled) receptors. For example, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD), a metabotropic glutamate (mGlu) receptor agonist, has been shown to increase the levels of serum corticosterone in rats. The present study was undertaken to further characterize which of the mGlu receptors are substantially involved in control of the HPA axis. The group I mGlu receptor agonists, 3,5-dihydroxyphenylglycine (DHPG), 1S,3R-ACPD, and 2-chloro-5-hydroxyphenylglycine (CHPG) but not the inactive isomer 1R,3S-ACPD were found to dose-dependently increase serum corticosterone 1 h after intracerebroventricular (i.c.v.) injection in male rats. The relative potency, DHPG (EC50 = 520 nmol) > 1S,3R-ACPD (1.4 micromol) = CHPG (2.7 micromol) >> 1R,3S-ACPD (>> 3 micromol) is consistent with activation of group I (mGlu1/5) receptors. The effects of DHPG were long lasting with substantial elevations in corticosterone remaining for at least 3 h. In a similar manner, the group III mGlu receptor agonists, L-AP4 (4-phosphono-2-aminobutyric acid) and L-SOP (serine-O-phosphate), were found to increase serum corticosterone levels at 1 h. In contrast, the mGlu group II selective agonists LY354740 (10 mg/kg, i.p.) and subtype-selective doses of the group II antagonist LY341495 (1 mg/kg, i.p.) did not significantly elevate serum corticosterone. Given the group I agonists results, it was surprising to find that group I selective and mGlu1 selective antagonists given alone also increased serum corticosterone. As with the agonists, the rise in serum corticosterone with LY393675 (an mGlu1/5 antagonist, EC50 = 20 nmol, i.c.v.) and LY367385 (an mGlu1 antagonist, 325 nmol, i.c.v.) were dose-dependent and consistent with their relative affinity for the group I mGlu receptors. The selective mGlu5 antagonist MPEP [2-methyl-6-(phenylethylnyl)pyridine] increased serum corticosterone but only at high doses (> 30 mg/kg, i.p.). A model involving the high glutamatergic tone on GABAergic interneurons in the paraventricular nucleus of the hypothalamus is discussed as a possible explanation for these results.

Topics: Adrenalectomy; Adrenocorticotropic Hormone; Amino Acids; Animals; Benzoates; Bridged Bicyclo Compounds; Corticosterone; Cycloleucine; Dose-Response Relationship, Drug; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glutamic Acid; Glycine; Male; Neuroprotective Agents; Paraventricular Hypothalamic Nucleus; Phenylacetates; Propionates; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Resorcinols; Xanthenes

The ability of metabotropic glutamate receptor activation to mobilise intracellular calcium was investigated in cultured dorsal root ganglion (DRG) neurones from neonatal rats using the calcium sensitive fluorescent dye Fura-2. L-glutamate (10 microM) caused sustained and oscillatory increases in intracellular calcium concentration ([Ca2+]i) in a subpopulation of cultured DRG neurones. The oscillatory responses were not blocked by combined application of the ionotropic glutamate receptor antagonists MK 801 (2 microM) and CNQX (20 microM). Oscillations in [Ca2+]i were also observed following application of the nonselective metabotropic glutamate receptor (mGluR) agonist, trans-(1S,3R)-1-aminocyclopentane-1S, 3R-dicarboxylic acid (1S,3R)-ACPD, 20 microM) and the mGluR5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG, 500 microM). These responses were blocked by the selective Group I mGluR antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) (100 microM) and Ca2+ release channel inhibitors ryanodine (100 microM) and dantrolene (10 microM). The predominantly Group II agonist (2S,2'R,3'R)-2-(2'3'-dicarboxy-cyclopropyl)glycine (DCG-IV, 100 microM) failed to produce Ca2+ transients alone but suppressed responses to CHPG. Reverse transcriptase PCR techniques, using primers specific to Group I mGluRs, revealed the presence of mGluR5 but not mGluR1 mRNA in these cells. Therefore, glutamate can cause a slowly activating and reversible mobilisation of [Ca2+]i in sensory neurones by activation of ionotropic receptors, and can induce oscillatory calcium transients by selectively activating metabotropic glutamate receptors that are likely to be of the mGluR5 subtype.

Topics: Animals; Animals, Newborn; Calcium; Calcium Channel Blockers; Cells, Cultured; Cycloleucine; Cyclopropanes; Dantrolene; Fluorescence; Ganglia, Spinal; Glutamic Acid; Glycine; Indans; Phenylacetates; Rats; Rats, Sprague-Dawley; Rats, Wistar; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Reverse Transcriptase Polymerase Chain Reaction; RNA, Messenger; Ryanodine; Ryanodine Receptor Calcium Release Channel; Stereoisomerism

The effects of group I metabotropic glutamate (mGlu) receptors on excitatory transmission in the rat dorsal horn, but mostly substantia gelatinosa, neurons were investigated using conventional intracellular recording in slices. The broad spectrum mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S, 3R-ACPD), the group I mGlu receptor selective agonist (S)-3, 5-dihydroxyphenylglycine (DHPG), and the selective mGlu subtype 5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), all induce long-lasting depression of A primary afferent fibers-mediated monosynaptic excitatory postsynaptic potential (EPSP), and long-lasting potentiation of polysynaptic EPSP, and EPSP in cells receiving C-afferent fiber input. The DHPG potentiation of polysynaptic EPSP was partially or fully reversed by (S)-4-carboxyphenylglycine (S-4CPG), the mGlu subtype 1 preferring antagonist. 2-Methyl-6-(phenylethynyl)-pyridine, the potent and selective mGlu subtype 5 antagonist, partially reversed the CHPG potentiation of polysynaptic EPSP. The effects of DHPG on monosynaptic and polysynaptic EPSPs were reduced, or abolished, by the N-methyl-D-aspartate (NMDA) receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (AP5). A clear and pronounced facilitation of the expression of DHPG- and CHPG-induced enhancement of polysynaptic EPSP, and EPSP evoked at C-fiber strength, was seen in the absence of gamma-aminobutyric acid subtype A receptor- and glycine-mediated synaptic inhibition. Besides dual modulation of excitatory synaptic transmission, DHPG induces depression of inhibitory postsynaptic potentials evoked by primary afferent stimulation in dorsal horn neurons. In addition, group I mGlu receptor agonists produced a direct persistent excitatory postsynaptic effect consisting of a slow membrane depolarization, an increase in input resistance, and an intense neuronal discharge. Cyclothiazide and (S)-4-CPG, the mGlu receptor subtype 1 preferring antagonists, significantly attenuated the DHPG-induced depolarization. These results demonstrate that the pharmacological activation of group I metabotropic glutamate receptors induces long-term depression (LTD) and long-term potentiation (LTP) of synaptic transmission in the spinal dorsal horn. These types of long-term synaptic plasticity may play a functional role in the generation of post-injury hypersensitivity (LTP) or antinociception (LTD).

Topics: Animals; Benzoates; Bicuculline; Cycloleucine; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Female; Glycine; In Vitro Techniques; Kinetics; Magnesium; Male; Methoxyhydroxyphenylglycol; Nerve Fibers; Phenylacetates; Posterior Horn Cells; Rats; Rats, Sprague-Dawley; Receptors, Metabotropic Glutamate; Spinal Cord; Strychnine; Substantia Gelatinosa; Synaptic Transmission

The present report describes the effect of mGluR agonists and antagonists administration on phospholipase C activation by measuring accumulation of [3H] inositol monophosphates (IP) in rats pre-labeled with [3H]myo-inositol (i.c.v. 24 h pre-treatment). The levels of accumulated [3H]IP were then determined from clarified tissue homogenates using ion-exchange chromotography. Following lithium chloride treatment (10 mg/kg, s.c.), (R/S)-3, 5-dihydroxyphenylglycine (DHPG), a selective group I mGluR agonist was found to dose-dependently cause a maximal increase in the levels of [3H]IP at 0.3 to 3 micromol/8 microliter i.c.v. with lower doses resulting in less efficacious or no responses. This effect was temporal-dependent reaching a plateau at 2 h. The DHPG-induced increases in [3H]IP were most pronounced in the hippocampus where a 3- to 5-fold increase above vehicle was consistently found, but significant approximately 2-fold increases were also seen in the cerebellum, striatum and frontal cortex. The mixed group I and II agonist, (1S,3R)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (1S, 3R-t-ACPD), similarly resulted in dose-dependent increases in [3H]IP levels with doses of 1 to 3 micromol i.c.v. Furthermore, this effect was enantiomer specific since the less active 1R,3S-t-ACPD failed to alter phosphoinositol hydrolysis. Administration of the selective mGluR5 agonist (R/S)-2-chloro-5-hydroxyphenyl-glycine (CHPG) resulted in a dose-dependent increase in hippocampal but not cerebellar levels of [3H]IP, consistent with the receptor distribution of the two group I mGluRs. The Group II agonist LY354740 (1S,2S,5R,6S-2-aminobicycl[3.1.0]hexane-2,6-dicarboxylate monohydrate) and the group III agonist L-AP4 (L-(+)-2-amino-4-phosphonobutyric acid) failed to alter the levels of [3H]IP. LY341495 (2S-2-amino-2-(1S, 2S-2-carboxycycloprop-1-yl)-3-(xanth-9-yl)propanoic acid) is a nM potent Group II antagonist. However, LY341495 has also been found to have microM potency in inhibiting mGluR1 and 5. The stimulation of [3H]PI hydrolysis by 1 micromol DHPG was dose-dependently blocked by co-administration of the mGluR antagonists, LY341495 at doses that are constant with an interaction at Group I mGluR's. Taken together these results suggest that stimulation of group I mGluRs results in measurable increases in PI hydrolysis in vivo. This method could be quite useful in determining the doses and routes of administration of agonists and antagonists that are required to intera

Topics: Amino Acids; Animals; Brain Chemistry; Cerebellum; Cycloleucine; Dose-Response Relationship, Drug; Excitatory Amino Acid Antagonists; Glycine; Hippocampus; Hydrolysis; Injections, Intraventricular; Male; Neuroprotective Agents; Phenylacetates; Phosphatidylinositols; Rats; Rats, Sprague-Dawley; Receptors, Metabotropic Glutamate; Resorcinols; Tritium; Type C Phospholipases; Xanthenes

Our previous work has shown that Group I mGlu receptors participate in thalamic sensory processing in vivo. However, unequivocal demonstration of mGlu5 participation has not been possible due to the lack of specific ligands. We have therefore made a preliminary study of the in vivo actions of the agonist (R,S)-2-Chloro-5-hydroxyphenylglycine [CHPG] and the novel mGlu5 antagonist 6-methyl-2-(phenylethynyl)-pyridine [MPEP] in order to characterize their suitability for functional studies. Iontophoretically administered MPEP selectively antagonized excitatory responses of single rat thalamic neurones to CHPG compared to the broad-spectrum mGlu agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate. In contrast, the established mGlu1 and mGlu5 antagonist (S)-4-carboxyphenylglycine reduced responses to both agonists. These findings are the first demonstration of an in vivo action of CHPG and its antagonism by a selective mGlu5 antagonist. Furthermore MPEP appears to be a good tool for functional studies of mGlu5.

Topics: Animals; Cycloleucine; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glycine; Phenylacetates; Pyridines; Rats; Rats, Wistar; Receptors, Metabotropic Glutamate; Thalamus

The specific metabotropic glutamate receptor (mGluR)5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) is able to potentiate NMDA and AMPA responses recorded from ventral roots of the isolated hemisected baby rat spinal cord. Previously we have demonstrated that activation of group I mGluRs (mGluR1 and mGluR5) with the broad spectrum mGluR agonist 1S,3R-1-amino-1,3-cyclopentanedicarboxylate (ACPD) produced potentiation of ionotropic glutamate responses. In contrast to ACPD-induced potentiation, however, no evidence for an involvement of protein kinase C (PKC) is found in the CHPG-induced potentiation of both NMDA and AMPA depolarization because the PKC blockers chelerythrine chloride or calphostin C did not antagonize this effect. Moreover, in the absence of Ca2+ in the perfusing medium or depleting intracellular Ca2+ stores with thapsigargin or dantrolene did not modify the CHPG-induced enhancement of NMDA depolarizations. Phorbol-12,13-diacetate (PDA), on the other hand, was able to attenuate this effect, which was reversed by chelerythrine chloride. These results suggest that both mGluR5 and mGluR1 may act to enhance ionotropic glutamate responses but the two types of mGluRs may have different intracellular mechanisms of action.

Topics: Alkaloids; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Animals; Animals, Newborn; Benzophenanthridines; Calcium; Cycloleucine; Drug Synergism; Excitatory Amino Acid Antagonists; Glycine; In Vitro Techniques; Membrane Potentials; Motor Neurons; N-Methylaspartate; Phenanthridines; Phenylacetates; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Spinal Cord

The role of metabotropic glutamate receptors (mGluRs) in long-term potentiation (LTP) has remained controversial. However, it has recently been shown that group I mGluR activation, prior to high frequency stimulation (HFS), can facilitate or 'prime' LTP in the area CA1 of the hippocampus. Here we report that, in the dentate gyrus in vitro, activation of both group I and group II mGluRs primes LTP. Control LTP, 60 min after HFS was 145.4+/-3.6% of control. The group I mGluR agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG, 100 microM), resulted in LTP of 180.1+/-12.1% of control, which was significantly greater than control LTP (n=4; P<0.05). The group I/II mGluR agonist 1S, 3R-1-aminocyclopentate-1,3-dicarboxylic acid (1S,3R-ACPD, 10 microM), and the group II mGluR agonist (2S,3S, 4S)-alpha-(carboxy-cyclopropyl)-glycine (L-CCG-1, 20 microM) also produced LTP that was significantly greater than control LTP (177. 7+/-11.5% and 183.2+/-9.1% of control respectively; n=5; P<0.05). The group III mGluR agonist l-2-amino-4-phosphonobutyric acid (L-AP4, 20 microM), failed to significantly prime LTP (153.8+/-5.9% of control; n=5). It also proved difficult to depotentiate the primed LTP. Following low frequency stimulation (LFS), control LTP was reduced to 101.1+/-3.6% of control, and to 145.0+/-2.1%, 141.2+/-14. 7% and 134.0+/-8.7% of control for CHPG, ACPD and L-CCG-1 primed LTP respectively. We conclude that LTP may be primed by mGluR activation in the dentate gyrus and that this priming is mediated through group I and II mGluRs.

Topics: Amino Acids, Dicarboxylic; Animals; Cycloleucine; Dentate Gyrus; Excitatory Amino Acid Agonists; Excitatory Postsynaptic Potentials; Glycine; In Vitro Techniques; Long-Term Potentiation; Membrane Potentials; Neuroprotective Agents; Phenylacetates; Propionates; Rats; Receptors, Metabotropic Glutamate

A combined study of behavioural and electrophysiological tests was carried out in order to assess the role of metabotropic glutamate receptors (mGluRs) in mediating sensory inputs to the spinal cord of the rat. In the behavioural study the responses of conscious animals, with or without carrageenan-induced inflammation, to noxious mechanical and thermal stimuli were observed both before and after the intrathecal administration of mGluR antagonists L(+)-2-amino-3-phosphonopropionic acid (L-AP3) and (S)-4-carboxy-3-hydroxyphenylglycine (CHPG). It was found that the mGluR antagonist (S)-CHPG was capable of increasing both mechanical threshold and thermal latency in both groups of animals, and L-AP3 did so in those with inflammation induced in their hindpaw. Following this study, the responses of single lamina III-V dorsal horn neurons to an innocuous A beta fibre brush stimulus and a noxious C fibre (mustard oil) stimulus were extracellularly recorded and the effect of ionophoretically applied drugs was examined. Cyclothiazide (CTZ), a selective antagonist at mGluR1, markedly reduced the activity evoked by mustard oil, but not that elicited by brushing of the receptive field. Activity induced in dorsal horn neurons by ionophoresing various mGluR subgroup agonists was examined. CTZ successfully inhibited the activity evoked by group I mGluR agonist 3,5-dihydroxyphenylglycine (DHPG). In comparison to the neurons which responded to the ionophoresis of DHPG, less were activated by the selective mGluR5 agonist trans-azetidine dicarboxylic acid (t-ADA). Together these results indicate that group I mGlu receptors, in particular mGluR1, play a crucial role in mediating nociception, particularly following a sustained noxious input.

Topics: Alanine; Animals; Antihypertensive Agents; Behavior, Animal; Benzothiadiazines; Cycloleucine; Electrophysiology; Excitatory Amino Acid Antagonists; Female; Glycine; Male; Mustard Plant; Neuroprotective Agents; Nociceptors; Pain; Phenylacetates; Plant Extracts; Plant Oils; Plants, Medicinal; Rats; Rats, Inbred Strains; Receptors, Metabotropic Glutamate; Resorcinols; Spinal Cord