Pharmacological characteristics of cyclic homologues of glycine at the N-methyl-D-aspartate receptor-associated glycine site

In Xenopus oocytes, injected with mRNA from the brain of the rat, the characteristics of the cyclic homologues of glycine, ACPC, ACBC and cycloleucine have been examined. 1-Aminocyclopropane-1-carboxylate was a potent agonist at the NMDA-associated glycine site (EC50 = 0.09 +/- 0.02 microM) and exhibited characteristics consistent with a partial agonist. 1-Aminocyclobutane-1-carboxylate, in addition to its previously described antagonist properties, was found to possess agonist properties of low efficacy. Furthermore, ACBC did not completely block NMDA/glycine-induced currents, which is also consistent with partial agonist characteristics. In addition, small concentrations of glycine (less than 3 microM) did not alter the potency of ACBC, possibly suggesting that it is not simply a competitive glycine antagonist. Cycloleucine was a very weak glycine antagonist. These results suggest that as the size of the ring of cyclic homologues of glycine increases, there is a resulting transition from agonist to mixed agonist/antagonist to antagonist properties.     

Glycine antagonist action of 1-aminocyclobutane-1-carboxylate (ACBC) in Xenopus oocytes injected with rat brain mRNA

ACBC has been reported to have the binding profile of an antagonist at the glycine site of the NMDA receptor. In Xenopus oocytes injected with rat brain mRNA, we have confirmed the antagonist action of ACBC on NMDA responses. ACBC and HA-966, a known glycine antagonist, blocked NMDA responses in a non-competitive manner and blocked the potentiation of NMDA responses by glycine in a competitive manner. We conclude that ACBC blocks NMDA responses via a competitive interaction at the glycine modulatory site.     

Clozapine attenuates N-methyl-D-aspartate receptor complex-mediated responses in vivo: tentative evidence for a functional modulation by a noradrenergic mechanism.

Recent studies revealed a role for dopamine and noradrenaline in the etiology of ischemia-induced neuronal cell death. In the present investigation, the modulation by clozapine, an atypical antipsychotic agent that interacts with adrenergic receptors, of N-methyl-D-aspartate (NMDA) receptor complex-mediated events were studied by examining its effects on levels of cGMP in the cerebellum. Clozapine decreased basal levels of cGMP in the cerebellum and antagonized harmaline-, methamphetamine-, pentylenetetrazol- and D-serine-induced increases in levels of cGMP with ED50 values of 3.9, 2.36, 2.13 and 2.1 mg/kg (i.p.). However, clozapine (1.25-25 mg/kg) did not attenuate the quisqualate-induced increases in levels of cGMP, indicating a specific modulation of events modulated by the NMDA receptor complex. Antagonists of dopamine (D2), serotonin (5-HT)-5-HT1, 5-HT2 and 5-HT3 [haloperidol, propranolol, ritanserin, ICS 205-930 [(3-tropanyl-indole-3-carboxylate methiodide)] respectively], did not reverse the response to harmaline. However, WB-4101 [(2,6-dimethoxy-phenoxyethyl)aminomethyl-1,4-benzodioxane HCl], and alpha 1-adrenergic antagonist, reversed harmaline-, D-serine-, PTZ- and MA-induced increases in levels of cGMP, indicating an adrenergic modulation of the events mediated by the NMDA receptor complex. Intracerebellar and intracerebroventricular administration of clozapine and intracerebellar administration of WB-4101 reversed the D-serine-induced response, indicating a central locus of action. These results indicated that clozapine modulates levels of cGMP predominantly through its interactions with central adrenergic receptors.     

6,7-Dinitroquinoxaline-2,3-dione and 6-nitro,7-cyanoquinoxaline-2,3-dione antagonize responses mediated by N-methyl-D-aspartate and NMDA-associated glycine recognition sites in vivo: measurements of cerebellar cyclic-GMP

Direct intracerebellar administration of quisqualate resulted in marked increases in levels of cGMP in the cerebellum of the mouse, with a Hill number of 2.0. Quinoxalinediones, DNQX (6,7-dinitroquinoxaline-2,3-dione) and CNQX (6-nitro,7-cyanoquinoxaline-2,3-dione) attenuated the quisqualate-induced response. 6,7-Dinitroquinoxaline-2,3-dione also attenuated the D-serine-induced increases in levels of cGMP in a competitive manner. Intracerebellar injection of DNQX also antagonized the response to parenterally-administered harmaline. Similar results were also obtained with CNQX. These results indicate that these quinoxalinediones can attenuate the responses, mediated through the NMDA-associated glycine recognition sites, as well as the NMDA receptor complex. However, the glycine antagonist HA-966 (3-amino-1-hydroxypyrrolidone-2), at doses which completely reversed the increases induced by D-serine, failed to alter the response to quisqualate, indicating a lack of effect of glycine antagonists on quisqualate-mediated synaptic events. These results further support the interaction of the quinoxalinediones, DNQX and CNQX, with the NMDA receptor complex as established in receptor binding and electrophysiological studies.     

BMY-14802 antagonizes harmaline- and D-serine-induced increases in mouse cerebellar cyclic GMP: neurochemical evidence for a sigma receptor-mediated functional modulation of responses mediated by the N-methyl-D-aspartate receptor complex in vivo

BMY-14802 [alpha-(4-flurophenyl)-4-(5-fluoro-pyramidinyl)-1-piperazine butanol], a potent sigma ligand with poor affinity for dopamine and phencyclidine receptors in vitro, attenuated parenteral harmaline- and direct intracerebellar D-serine-induced increases in mouse cerebellar cGMP. Intracerebroventricularly injected BMY-14802 also antagonized the effects of intracerebellar D-serine, indicating a central mechanism. However, direct co-injection of BMY-14802 into the cerebellum failed to antagonize the D-serine-induced increases in cGMP, indicating a locus of action outside the cerebellum. In contrast, quisqualate-induced cGMP increases were not attenuated by BMY-14802. These results indicate a functional modulation of the N-methyl-D-aspartate/glycine/phencyclidine/ion channel complex-mediated events by BMY-14802, possibly through a transsynaptic mechanism, thus representing the first in vivo demonstration of a sigma ligand modulation of a response mediated through the N-methyl-D-aspartate receptor complex.     

Neurochemical and behavioral profiling of the selective GlyT1 inhibitors ALX5407 and LY2365109 indicate a preferential action in caudal vs. cortical brain areas

Selective inhibitors of the glycine transporter 1 (GlyT1) have been implicated in central nervous system disorders related to hypoglutamatergic function such as schizophrenia. The selective GlyT1 inhibitors ALX5407 (NFPS) and LY2365109 {[2-(4-benzo[1,3]dioxol-5-yl-2-tert-butylphenoxy)ethyl]-methylamino}-acetic acid increased cerebrospinal fluid levels of glycine and potentiated NMDA-induced increases in dialysate levels of neurotransmitters in the prefrontal cortex (PFC) and the striatum. However, higher doses produced both stimulatory and inhibitory effects on motor performance and impaired respiration, suggesting significant involvement of cerebellar and brain stem areas. A dual probe microdialysis study showed that ALX5407 transiently elevated extracellular levels of glycine in the PFC with more sustained increases in the cerebellum. In support of these findings, immuno-staining with pan-GlyT1 and GlyT1a antibodies showed a higher abundance of immunoreactivity in the brain stem/cerebellum as compared to the frontal cortical/hippocampal brain areas in four different species studied, including the mouse, rat, monkey and human. In addition, the inhibitory effects of ALX5407 on cerebellar levels of cGMP in the mouse could be reversed by the glycine A receptor antagonist strychnine but not the glycine B receptor antagonist L-701324. We propose that the adverse events seen with higher doses of ALX5407 and LY2365109 are the result of high GlyT1 inhibitory activity in caudal areas of the brain with sustained elevations of extracellular glycine. High levels of glycine in these brain areas may result in activation of strychnine-sensitive glycine A receptors that are inhibitory on both motor activity and critical brain stem functions such as respiration.     

Evidence for a functional coupling of the NMDA and glycine recognition sites in synaptic plasma membranes

Activation of the N-methyl-D-aspartate (NMDA) receptor complex is subject to modulation via interactions at a coupled [3H]glycine recognition site in rat brain synaptic plasma membranes (SPM). We examined the effect of the potent and specific glycine site antagonists, 1-hydroxy-3-amino-2-pyrrolidone (HA-966) and 1-aminocyclobutane-1-carboxylate (ACBC), on the NMDA recognition site. These glycine analogs were found to significantly stimulate the binding of the competitive NMDA antagonist, [3H]3-(2-carboxypiperazin-4-y1)propyl-1-phosphonate ([3H]CPP) in a dose-dependent fashion, whereas both compounds inhibited NMDA-specific L-[3H]glutamate (agonist) binding. Additionally, both glycine antagonists reduced the binding of [3H]1-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) to SPM, a functional assessment of activation of the NMDA receptor-channel complex. The glycine site agonists, glycine and serine reversed these effects in a dose-dependent manner, with the serine reversal being stereospecific for D-serine. The relative potencies of these compounds in reversing the glycine antagonist effects on the NMDA recognition site corresponded with their ability to competitively displace strychnine-insensitive [3H]glycine binding. These results provide evidence for a functional coupling between the glycine and NMDA recognition sites and further, may provide a mechanism by which compounds interacting at the glycine recognition site may modulate NMDA receptor activity.     

Characterization of a novel effect of serotonin 5-HT1A and 5-HT2A receptors: increasing cGMP levels in rat frontal cortex

Elucidating the mechanisms of action of hallucinogens has become an increasingly important area of research as their abuse has grown in recent years. Although serotonin receptors appear to play a role in the behavioral effects of the phenethylamine and indoleamine hallucinogens, the signaling pathways activated by these agents are unclear. Here it is shown that administration of serotonin (5-hydroxytryptamine, 5-HT) increased cyclic guanosine monophosphate (cGMP) production in frontal cortical slices of rat brain. The effect of 5-HT was greater than that of N-methyl-D-aspartate (NMDA), a stimulant of cGMP formation in the central nervous system. The 5-HT_2A/2C receptor phenethylamine agonist, 2,5-dimethoxy-4-methylamphetamine (DOM), increased cGMP content in the slices. Additionally 8-hydroxy-2-(di-n-propylamino)tetralin (DPAT), a 5-_HT1A/7 receptor agonist also increased cGMP production. Stimulation of cGMP formation by DOM was prevented by a 5-HT_2A/2C receptor antagonist, pirenperone, as well as by a 5-HT_2A receptor selective antagonist, MDL100907. A 5-HT_2C receptor antagonist, SB242084, did not block the effect of DOM. Stimulation of cGMP production by DPAT was blocked by the 5-HT_1A receptor antagonist, WAY100635. Stimulation of cGMP formation by serotonin could be prevented by pirenperone orWAY100635. In summary, activation of serotonin 5-HT_1A and 5-HT_2A receptors increase brain cGMP levels.     

Glycine, glycinamide and D-serine act as positive modulators of signal transduction at the N-methyl-D-aspartate (NMDA) receptor in vivo: differential effects on mouse cerebellar cyclic guanosine monophosphate levels

Direct intracerebellar (icb) administration of glycine, glycinamide and D-serine produced time- and dose-dependent changes in mouse cerebellar cGMP levels, indicating a modulation of ongoing neuronal activity through the NMDA receptor complex. Intracerebroventricular administration of glycinamide also produced a time-dependent change in cGMP levels, indicating a central mechanism of action. The icb dose-response data indicated a unimolecular interaction for these compounds. D-serine-, glycine-, and glycinamide-mediated increases in cGMP levels were reversed by the competitive NMDA antagonist, CPP and the NMDA-associated glycine receptor antagonist, HA-966, indicating mediation via the NMDA receptor complex. Glycine and D-serine were less effective than glycinamide at increasing cerebellar cGMP levels. In contrast, L- and D-serinamide did not affect cGMP levels. These results indicate that glycine receptor is not saturated under physiological conditions and also suggest possible existence of multiple glycine pools.     

Opipramol, a potent sigma ligand, is an anti-ischemic agent: neurochemical evidence for an interaction with the N-methyl-D-aspartate receptor complex in vivo by cerebellar cGMP measurements

Opipramol, a potent sigma ligand and a tricyclic antidepressant compound, provided significant neuronal protection (P less than 0.0001) against ischemia-induced neuronal cell loss in the hippocampus in Mongolian gerbils, at a dose of 50 mg/kg (30 min pretreatment). However, opipramol did not offer protection when given 60 min after the ischemic insult. Opipramol decreased basal levels of cGMP in the cerebellum of the mouse and harmaline-induced increases in levels of cGMP, with approximate ED50 values of 4 and 27 mg/kg. Opipramol antagonized methamphetamine- and pentylenetetrazol-induced increases in levels of cGMP. Parenteral administration of opipramol also antagonized the increases in levels of cGMP in the cerebellum of the mouse after the local administration of D-serine, an agonist at the N-methyl-D-aspartate (NMDA)-associated, strychnine-insensitive glycine receptor. These results indicate that opipramol attenuates responses mediated through the NMDA receptor complex. These results further support the functional modulation of the NMDA receptor complex by sigma ligands and provide a neurochemical correlate for the observed anti-ischemic properties of opipramol.     

P物质增强大鼠鞘内注射D—丝氨酸诱发的热痛过敏

目的：研究脊髓伤害性信息传递中P物质（SP）与N－甲基－D－天冬氨酸（NMDA）受体甘氨酸位点激动剂D－丝氨酸（D-serine)之间的功能联系。方法：在浅麻大鼠,采用行为学方法,测定甩尾反射潜伏期（TFL）并结合鞘内给药途径观察药物作用。结果：鞘内注射D-serine 1000nmol后1．5分钟,TEL明显缩短;在注射D-serine 10nmol前6分钟鞘内施加SP 0.05nmol,明显增强D-serine 10nmol引起的TEL缩短效应;选择性NMDA受体甘氨酸位点拮抗剂7－氯犬尿酸1pmol及非选择性PKC抑制剂H－7 10μmol均可阻断这种增强作用。结论：SP可使D－丝氨酸诱发的热痛过敏明显加强,NMDA受体甘氨酸位点及胞内蛋白激酶系统参与了脊髓SP与NMDA受体的相互作用。     

GABA(A), but not NMDA, receptors modulate in vivo NO-mediated cGMP synthesis in the rat cerebral cortex

We have investigated the functional relationships between NMDA receptors and the NOS/sGC system in the rat pre-frontal cortex in vivo by microdialysis. cGMP basal levels were sensitive to NOS or sGC inhibitors (L-NARG or ODQ) or NO donors (SNAP) when enzymatic breakdown was blocked by the phosphodiesterase inhibitor IBMX, indicating that basal cGMP production derives, at least in part, from the NOS/sGC pathway activity and that the pre-frontal cortex possesses a very efficient degradation system for cGMP. The glutamate receptor agonist NMDA did not alter extracellular cGMP either in absence or presence of IBMX. cGMP was not augmented when NMDA was co-infused with the NOS substrate L-arginine, the glycine site agonist d-serine or the glutamate receptor agonist AMPA. Interestingly, the selective GABA(A) receptor antagonist bicuculline enhanced cGMP production, revealing that the cortical NOS/sGC system is tonically inhibited by endogenous GABA. However, in the presence of bicuculline, NMDA did not increase extracellular cGMP. In the presence of bicuculline, blockade of 5-HT1/2 receptors, known to inhibit the NMDA/NOS/sGC pathway, with the antagonist methiothepin did not unmask cGMP elevations by NMDA. Thus, it would seem that NMDA receptors do not regulate cortical NOS/sGC activity that, on the other hand, is modulated by endogenous GABA acting at GABA(A) receptors.     

Agonist-like character of the (R)-enantiomer of 1-hydroxy-3-amino-pyrrolid-2-one (HA-966).

HA-966 (1-hydroxy-3-amino-pyrrolid-2-one), an antagonist at the strychnine-insensitive glycine site on the N-methyl-D-aspartate (NMDA) receptor complex, only partially inhibits the binding of noncompetitive antagonists to the NMDA receptor but enhances the binding of the NMDA competitive antagonist CPP (3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid). Here we report that the IC50 of the active (R)-enantiomer of HA-966 for displacement of [3H]glycine binding is decreased in the presence of spermine, suggesting that spermine increases the affinity of (R)-HA-966 at the [3H]glycine binding site. The IC50 values of the agonist glycine and the partial agonist 1-aminocyclopropane-1-carboxylate are also decreased. The IC50 values of glycine antagonists 6,7-dinitroquinoxalin-2,3-dione and 7-chlorokynurenic acid are not significantly altered. The spermine shift represents the first demonstration of the agonist-like character of the (R)-enantiomer of HA-966 at the glycine site.     

In vivo antagonism of agonist actions at N-methyl-D-aspartate and N-methyl-D-aspartate-associated glycine receptors in mouse cerebellum: studies of 1-hydroxy-3-aminopyrrolidone-2

Intracerebellar injections of either NMDA or D-serine dramatically elevated levels of cGMP in the cerebellum of the mouse, in vivo. These actions were both antagonized by simultaneous injection of the NMDA-associated glycine receptor antagonist, HA-966. Intracerebellar injections of D-serine were also antagonized by peripheral (s.c.) injections of HA-966, demonstrating the bioavailability of this glycine receptor antagonist. Parenteral administration of HA-966 was also effective in antagonizing the actions of intravenously injected harmaline, an activator of the cerebellar climbing fiber pathway, on cGMP in the cerebellum. An evaluation of the parenteral dose-response curve for HA-966, revealed no effect on basal activity within the cerebellum. This contrasts sharply with the abilities of both competitive and non-competitive NMDA antagonists to decrease basal levels of cGMP in the cerebellum. In summary, these studies demonstrate that HA-966 is a bioavailable antagonist of the NMDA-associated glycine receptor and that this compound can limit excessive stimulation of the NMDA receptor by exogenous application of agonist, with minimal effects on basal activity. These data suggest that antagonists of the NMDA-associated glycine receptor may be optimal therapies in the treatment of stroke and epilepsy.     

Kynurenic acid derivatives. Structure-activity relationships for excitatory amino acid antagonism and identification of potent and selective antagonists at the glycine site on the N-methyl-D-aspartate receptor

Derivatives of the nonselective excitatory amino acid antagonist kynurenic acid (4-oxo-1,4-dihydroquinoline-2-carboxylic acid, 1) have been synthesized and evaluated for in vitro antagonist activity at the excitatory amino acid receptors sensitive to N-methyl-D-aspartic acid (NMDA), quisqualic acid (QUIS or AMPA), and kainic acid (KA). Introduction of substituents at the 5-, 7-, and 5,7-positions resulted in analogues having selective NMDA antagonist action, as a result of blockade of the glycine modulatory (or coagonist) site on the NMDA receptor. Regression analysis suggested a requirement for optimally sized, hydrophobic 5- and 7-substituents, with bulk tolerance being greater at the 5-position. Optimization led to the 5-iodo-7-chloro derivative (53), which is the most potent and selective glycine/NMDA antagonist to date (IC50 vs [3H]glycine binding, 32 nM; IC50's for other excitatory amino acid receptor sites, greater than 100 microM). Substitution of 1 at the 6-position resulted in compounds having selective non-NMDA antagonism and 8-substituted compounds were inactive at all receptors. The retention of glycine/NMDA antagonist activity in heterocyclic ring modified analogues, such as the oxanilide 69 and the 2-carboxybenzimidazole 70, suggests that the 4-oxo tautomer of 1 and its derivatives is required for activity. Structurally related quinoxaline-2,3-diones are also glycine/NMDA antagonists, but are not selective and are less potent than the 1 derivatives, and additionally show different structure-activity requirements for aromatic ring substitution. On the basis of these results, a model accounting for glycine receptor binding of the 1 derived antagonists is proposed, comprising (a) size-limited, hydrophobic binding of the benzene ring, (b) hydrogen-bond acceptance by the 4-oxo group, (c) hydrogen-bond donation by the 1-amino group, and (d) a Coulombic attraction of the 2-carboxylate. The model can also account for the binding of quinoxaline-2,3-diones, quinoxalic acids, and 2-carboxybenzimidazoles.     

The benzodiazepine antagonist Ro 15-1788 reverses the effect of methyl-beta-carboline-3-carboxylate but not of harmaline on cerebellar cGMP and motor performance in mice

The cerebellar cGMP level in mice was decreased in a dose-dependent manner 30 min after diazepam (ED50 = 2 mg/kg p.o.). This effect was reversed by the specific benzodiazepine antagonist Ro 15-1788. Methyl-beta-carboline-3-carboxylate (beta-CCM) and harmaline increased cGMP. Ro 15-1788 dose dependently counteracted the beta-CCM- but not the harmaline-induced increase in cGMP. In the horizontal wire test Ro 15-1788 antagonized the impairment of motor performance induced by beta-CCM, but not that induced by harmaline. These findings further support the view that harmaline in contrast to beta-carboline-3-carboxylates does not act through benzodiazepine receptors, and that Ro 15-1788 antagonizes only those convulsants and stimulants that act through specific benzodiazepine receptors.     

The anticonvulsant and behavioural profile of L-687,414, a partial agonist acting at the glycine modulatory site on the N-methyl-D-aspartate (NMDA) receptor complex.

1. The anticonvulsant and behavioural effects of the glycine/NMDA receptor partial agonist, L-687,414 (R(+)-cis-beta-methyl-3-amino-1-hydroxypyrrolid-2-one) have been investigated in rodents. 2. L-687,414 dose-dependently antagonized seizures induced by N-methyl-D,L- aspartic acid (NMDLA, ED50 = 19.7 mg kg-1), pentylenetetrazol (PTZ, ED50 = 13.0 mg kg-1) and electroshock (ED50 = 26.1 mg kg-1) when given intravenously 15 min before test, in male Swiss Webster mice but was most potent against audiogenic seizures induced by a 120 dB bell in DBA/2 mice (ED50 = 5.1 mg kg-1, i.p., 30 min before test). 3. L-687,414 also induced impairments of performance in a rotarod test in both Swiss Webster and DBA/2 mice and the ratio [rotarod MED:anticonvulsant ED50] varied between 0.9 and 5, depending on the convulsant used. 4. Similar behaviours to those seen after administration of the non-competitive NMDA receptor antagonist, MK-801 (head weaving, body rolling, hyperlocomotion) were seen in the mouse after giving L-687,414, although the peak effect occurred at a dose (100 mg kg-1) which was 5-20 times the anticonvulsant ED50S, depending on the convulsant used. Unlike MK-801, however, doses of L-687,414 that were behaviourally stimulant did not increase dopamine turnover in the nucleus accumbens. 5. Consistent with the interaction of L-687,414 with the glycine/NMDA receptor, the anticonvulsant, ataxic and motor stimulant effects of the compound were significantly attenuated by the glycine/NMDA receptor agonist, D-serine (10-100 micrograms per mouse, i.c.v.).(ABSTRACT TRUNCATED AT 250 WORDS)     

Interaction of angiotensin II and adenosine A1 and A2A receptor ligands on the writhing test in mice

The effects of adenosine A1 and A2A receptor agonists and antagonists administered intraperitoneally (i.p.) and their interaction with angiotensin II (Ang II) administered intracerebroventricularly (i.c.v.) were studied in mice using the acetic acid-induced abdominal constriction test. Ang II (0.1 microg/mouse) induced antinociception in this model. The adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA; 0.05, 0.25 and 0.5 mg/kg) also showed a well-developed antinociceptive effect. Ang II (0.1 microg/mouse) administered 5 min before CPA (0.25 mg/kg) decreased the number of writhes, i.e., it enhanced the antinociceptive effect of CPA. Losartan, an AT1 receptor antagonist (25 microg/mouse i.c.v.), enhanced the antinociceptive effect of CPA, while the AT2 receptor antagonist 1-[-4-(dimethylamino)-3-methylphenylmethyl]-5-diphenylacetyl)-4,5,6,7-tetrahydro 1H-4-imidazol [4,5c]pyridine-6 carboxylic acid, ditrifluoroacetate, dihydrate (PD 123319; 10 microg/mouse) had less effect. 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX; 0.1 mg/kg), an adenosine A1 receptor antagonist, exhibited a pronociceptive effect and did not change the antinociceptive effect of Ang II. The adenosine A2A receptor agonist PD-125944 (DPMA; 0.1, 0.5 and 1 mg/kg) showed pronounced antinociceptive effect. Ang II (0.1 microg/mouse) did not significantly influence the antinociceptive effect of DPMA (0.1 mg/kg). The A2A receptor antagonist 3,7-dimethyl-1-propargilxanthine (DMPX; 0.1 mg/kg) had no effect on the number of writhes and did not influence the effect of Ang II. These data indicate that the antinociceptive effect of Ang II interacts with that produced by adenosine A1 receptor agonist.     

Intrathecally administered D-cycloserine produces nociceptive behavior through the activation of N-methyl-D-aspartate receptor ion-channel complex acting on the glycine recognition site

Intrathecal (i.t.) administration of D-cycloserine (100 and 300 fmol), a partial agonist of the glycine recognition site on the N-methyl-D-aspartate (NMDA) receptor ion-channel complex, produced a behavioral response mainly consisting of biting and/or licking of the hindpaw and the tail along with slight hindlimb scratching directed toward the flank in mice, which peaked at 5 - 10 min and almost disappeared at 15 min after the injection. The behavior induced by D-cycloserine (300 fmol) was dose-dependently inhibited by an intraperitoneal injection of morphine (0.5-2 mg/kg), suggesting that the behavioral response is related to nociception. The nociceptive behavior was also dose-dependently inhibited by i.t. co-administration of 7-chlorokynurenic acid (0.25-4 nmol), a competitive antagonist of the glycine recognition site on the NMDA receptor ion-channel complex; D-(-)-2-amino-5-phosphonovaleric acid (62.5-500 pmol), a competitive NMDA receptor antagonist; MK-801 (62.5-500 pmol), an NMDA ion-channel blocker; ifenprodil (0.5-8 nmol); arcaine (31-125 pmol); and agmatine (0.1-10 pmol), all being antagonists of the polyamine recognition site on the NMDA receptor ion-channel complex. However, [D-Phe7,D-His9]-substance P(6-11), a specific antagonist for substance P (NK1) receptors, and MEN-10,376, a tachykinin NK2-receptor antagonist, had no effect on D-cycloserine-induced nociceptive behavior. These results in the mouse spinal cord suggest that D-cycloserine-induced nociceptive behavior is mediated through the activation of the NMDA receptor ion-channel complex by acting on the glycine recognition site and that it does not involve the tachykinin receptor mechanism.     

Differential regulation of observational fear and neural oscillations by serotonin and dopamine in the mouse anterior cingulate cortex

Rationale

The aberrant regulation of serotonin (5-HT) and dopamine (DA) in the brain has been implicated in neuropsychiatric disorders associated with marked impairments in empathy, such as schizophrenia and autism. Many psychiatric drugs bind to both types of receptors, and the anterior cingulate cortex (ACC) is known to be centrally involved with empathy. However, the relationship between the 5-HT/DA system in the ACC and empathic behavior is not yet well known.

Objectives

We investigated the role of 5-HT/DA in empathy-like behavior and in the regulation of ACC neural activity.

Methods

An observational fear learning task was conducted following microinjections of 5-HT, DA, 5-HT and DA, methysergide (5-HT receptor antagonist), SCH-23390 (DA D₁ receptor antagonist), or haloperidol (DA D₂ receptor antagonist) into the mouse ACC. The ACC neural activity influenced by 5-HT and DA was electrophysiologically characterized in vitro and in vivo.

Results

The microinjection of haloperidol, but not methysergide or SCH-23390, decreased the fear response of observing mice. The administration of 5-HT and 5-HT and DA together, but not DA alone, reduced the freezing response of observing mice. 5-HT enhanced delta-band activity and reduced alpha- and gamma-band activities in the ACC, whereas DA reduced only alpha-band activity. Based on entropy, reduced complexity of ACC neural activity was observed with 5-HT treatment.

Conclusions

The current results demonstrated that DA D₂ receptors in the ACC are required for observational fear learning, whereas increased 5-HT levels disrupt observational fear and alter the regularity of ACC neural oscillations.