Transiently increased glutamate cycling in rat PFC is associated with rapid onset of antidepressant-like effects.

Several drugs have recently been reported to induce rapid antidepressant effects in clinical trials and rodent models. Although the cellular mechanisms involved remain unclear, reports suggest that increased glutamate transmission contributes to these effects. Here, we demonstrate that the antidepressant-like efficacy of three unique drugs, with reported rapid onset antidepressant properties, is coupled with a rapid transient rise in glutamate cycling in the medial prefronal cortex (mPFC) of awake rats as measured by ex vivo 1H-[13C]-nuclear magnetic resonance spectroscopy. Rats were acutely pretreated by intraperitoneal injection with a single dose of ketamine (1, 3, 10, 30 and 80 mg kg-1), Ro 25-6981 (1, 3 and 10 mg kg-1), scopolamine (5, 25 and 100 µg kg-1) or vehicle (controls). At fixed times after drug injection, animals received an intravenous infusion of [1,6-13C2]glucose for 8 min to enrich the amino-acid pools of the brain with 13C, followed by rapid euthanasia. The mPFC was dissected, extracted with ethanol and metabolite 13C enrichments were measured. We found a clear dose-dependent effect of ketamine and Ro 25-6981 on behavior and the percentage of 13C enrichment of glutamate, glutamine and GABA (γ-aminobutyric acid). Further, we also found an effect of scopolamine on both cycling and behavior. These studies demonstrate that three pharmacologically distinct classes of drugs, clinically related through their reported rapid antidepressant actions, share the common ability to rapidly stimulate glutamate cycling at doses pertinent for their antidepressant-like efficacy. We conclude that increased cycling precedes the antidepressant action at behaviorally effective doses and suggest that the rapid change in cycling could be used to predict efficacy of novel agents or identify doses with antidepressant activity.

Sedative but not anxiolytic properties of benzodiazepines are mediated by the GABA(A) receptor alpha1 subtype.

Inhibitory neurotransmission in the brain is largely mediated by GABA(A) receptors. Potentiation of GABA receptor activation through an allosteric benzodiazepine (BZ) site produces the sedative, anxiolytic, muscle relaxant, anticonvulsant and cognition-impairing effects of clinically used BZs such as diazepam. We created genetically modified mice (alpha1 H101R) with a diazepam-insensitive alpha1 subtype and a selective BZ site ligand, L-838,417, to explore GABA(A) receptor subtypes mediating specific physiological effects. These two complimentary approaches revealed that the alpha1 subtype mediated the sedative, but not the anxiolytic effects of benzodiazepines. This finding suggests ways to improve anxiolytics and to develop drugs for other neurological disorders based on their specificity for GABA(A) receptor subtypes in distinct neuronal circuits.

Carbon isotope fractionation by anoxygenic phototrophic bacteria in euxinic Lake Cadagno.

Anoxygenic phototrophic bacteria utilize ancient metabolic pathways to link sulfur and iron metabolism to the reduction of CO2 . In meromictic Lake Cadagno, Switzerland, both purple sulfur (PSB) and green sulfur anoxygenic phototrophic bacteria (GSB) dominate the chemocline community and drive the sulfur cycle. PSB and GSB fix carbon utilizing different enzymatic pathways and these fractionate C-isotopes to different extents. Here, these differences in C-isotope fractionation are used to constrain the relative input of various anoxygenic phototrophs to the bulk community C-isotope signal in the chemocline. We sought to determine whether a distinct isotopic signature of GSB and PSB in the chemocline persists in the settling fraction and in the sediment. To answer these questions, we also sought investigated C-isotope fractionation in the water column, settling material, and sediment of Lake Cadagno, compared these values to C-isotope fractionation of isolated anoxygenic phototroph cultures, and took a mass balance approach to investigate relative contributions to the bulk fractionation signature. We found a large C-isotope fractionation between dissolved inorganic carbon (DIC) and particulate organic carbon (POC) in the Lake Cadagno chemocline. This large fractionation between the DIC and POC was also found in culture experiments carried out with anoxygenic phototrophic bacteria isolated from the lake. In the Lake Cadagno chemocline, anoxygenic phototrophic bacteria controlled the bulk C-isotope fractionation, but the influence of GSB and PSB differed with season. Furthermore, the contribution of PSB and GSB to bulk C-isotope fractionation in the chemocline could be traced in the settling fraction and in the sediment. Taken together with other studies, such as lipid biomarker analyzes and investigations of other stratified lakes, these results offer a firmer understanding of diagenetic influences on bacterial biomass.

Schizophrenia and L-745,870, a novel dopamine D4 receptor antagonist.

The discovery of a novel high-affinity and selective dopamine D4 receptor antagonist, L-745,870, and the results of clinical trials with this compound are reviewed. Despite several lines of evidence which suggest that a selective D4 receptor antagonist may be an effective antipsychotic agent with a lower propensity to induce extrapyramidal side-effects, L-745,870 was ineffective as an antipsychotic in humans.

Genetic knockout and pharmacological blockade studies of the 5-HT7 receptor suggest therapeutic potential in depression.

The affinity of several antidepressant and antipsychotic drugs for the 5-HT7 receptor and its CNS distribution suggest potential in the treatment of psychiatric diseases. However, there is little direct evidence of receptor function in vivo to support this. We therefore evaluated 5-HT7 receptors as a potential drug target by generating and assessing a 5-HT7 receptor knockout mouse. No difference in assays sensitive to potential psychotic or anxiety states was observed between the 5-HT7 receptor knockout mice and wild type controls. However, in the Porsolt swim test, 5-HT7 receptor knockout mice showed a significant decrease in immobility compared to controls, a phenotype similar to antidepressant treated mice. Intriguingly, treatment of wild types with SB-258719, a selective 5-HT7 receptor antagonist, did not produce a significant decrease in immobility unless animals were tested in the dark (or active) cycle, rather than the light, adding to the body of evidence suggesting a circadian influence on receptor function. Extracellular recordings from hypothalamic slices showed that circadian rhythm phase shifts to 8-OH-DPAT are attenuated in the 5-HT7 receptor KO mice also indicating a role for the receptor in the regulation of circadian rhythms. These pharmacological and genetic knockout studies provide the first direct evidence that 5-HT7 receptor antagonists should be investigated for efficacy in the treatment of depression.

Competitive and glycine/NMDA receptor antagonists attenuate withdrawal-induced behaviours and increased hippocampal acetylcholine efflux in morphine-dependent rats.

The present study has examined the glycine/N-methyl-D-aspartate (NMDA) receptor antagonist, R-(+)-3-amino-1-hydroxypyrrolid-2-one (R-(+)-HA-966) and the competitive NMDA receptor antagonist, cis-4-(phosphonomethyl)piperidine-2-carboxylic acid (CGS 19755) on the behavioural syndrome and increased hipppocampal acetylcholine efflux induced during morphine-withdrawal in the rat. Subcutaneous naltrexone (1 mg/kg) injection, 48 hr after implantation of a 75 mg morphine pellet, induced a robust withdrawal syndrome consisting of wet dog shakes, ejaculations, mouth movement, ptosis, irritability to touch and diarrhoea. Pretreatment with the alpha2-adrenoceptor agonist, clonidine (0.1-0.4 mg/kg), R-(+)-HA-966 (10-60 mg/kg) or CGS 19755 (5 or 10 mg/kg) significantly reduced the incidence of withdrawal behaviours. In addition, all three compounds significantly attenuated the increase in hippocampal acetylcholine efflux induced following naltrexone (1 mg/kg, s.c.) injection in morphine-dependent rats. These results provide further evidence demonstrating that NMDA receptor antagonists attenuate both the behavioural and neurochemical effects observed during morphine withdrawal in the rat.

Role of metabotropic glutamate receptor subtype 5 (mGluR5) in the maintenance of cold hypersensitivity following a peripheral mononeuropathy in the rat.

The present series of experiments were designed to examine the contribution of metabotropic glutamate receptor subtype 5 (mGluR5) to neuropathic pain by determining the effects of the selective mGluR5 antagonist MPEP (2-methyl-6-(phenylethynyl)-pyridine) on neuropathy-induced cold hypersensitivity. Unilateral chronic constriction injury (CCI) to the sciatic nerve in rats produced an increase in the number of hind paw withdrawals from a cold surface (4 +/- 2 degrees C) which was dose-dependently inhibited by systemic (i.p.) injection of MPEP (ID(50) = 11.3 mg/kg). In vivo brain mGluR5 receptor occupancy following systemic (i.p.) MPEP revealed that >90% occupancy is required for behavioral efficacy. Intracerebroventricular (i.c.v.) injection of MPEP dose-dependently inhibited CCI-induced cold hypersensitivity (ID(50) = 123.5 nmol), while microinjection of MPEP directly into the rostral ventromedial medulla (RVM) potently inhibited this hypersensitivity (ID(50) = 1.3 pmol). A role for mGluR5 in the RVM was further supported by the observation that intra-RVM injection of the mGluR5 agonist CHPG (10 nmol; 2-chloro-5-hydroxyphenylglycine) produced cold hypersensitivity in naïve rats that was blocked by pretreatment with intra-RVM MPEP (3 nmol). Intrathecal (500 nmol; i.t.) or intraplantar (300 nmol; i.pl.) injection of MPEP was ineffective in reversing CCI-induced cold hypersensitivity. These results demonstrate that mGluR5 contributes to cold hypersensitivity following peripheral neuropathy exclusively at supraspinal sites in the CNS. Additionally, mGluR5 in the RVM significantly contributes to the maintenance of cold hypersensitivity, likely via activation of descending nociceptive facilitatory systems.

Evidence for accelerated desensitisation of 5-HT(2C) receptors following combined treatment with fluoxetine and the 5-HT(1A) receptor antagonist, WAY 100,635, in the rat.

Both pre-clinical and clinical studies suggest that additional treatment with 5-HT(1A) receptor antagonists may accelerate the antidepressant efficacy/onset of selective serotonin re-uptake inhibitors (SSRIs). Given that chronic SSRI treatment has been shown to desensitise 5-HT(2C) receptor mediated responses, we have used the rat social interaction test to determine if combined treatment with WAY 100,635, a selective 5-HT(1A) receptor antagonist, will accelerate this effect. In pairs of unfamiliar rats, acute administration of the 5-HT(2C) receptor agonist m-chlorophenylpiperazine (mCPP) or fluoxetine decreased the time spent in social interaction, responses which were reversed by the 5-HT(2C/2B) receptor antagonists SB 200646A and SB 221284. Similar reductions in social interaction were observed in rats treated with fluoxetine (10 mg/kg, i.p. daily) for 4, 7 and 14 days but was no longer apparent after 28 days of treatment. In contrast, only 7 days of combined treatment with WAY 100,635 (1 mg/kg/s.c./day) and fluoxetine were needed to reverse this response. The decrease in social interaction induced by an acute challenge of mCPP (1 mg/kg, i. p.) was also reduced after 6 days co-treatment with WAY 100,635 and fluoxetine. Thus, WAY 100,635 accelerates SSRI-induced desensitisation of 5-HT(2C) receptors, suggesting that this response might contribute towards the therapeutic effects of SSRIs in man.

Discriminative stimulus properties of the putative dopamine D3 receptor agonist, (+)-PD 128907: role of presynaptic dopamine D2 autoreceptors.

The putative D3 receptor agonist, (+)-PD 128907, is widely used to study the functional relevance of D3 receptors in vivo. Given that non-selective D2/3/4 receptor agonists serve as effective discriminative stimuli in rats we have trained animals to discriminate (+)-PD 128907 (30 microg kg(-1), s.c.) from saline and examined the pharmacological specificity of the response. Consistent with a D3 receptor mediated response, the non-selective D2/3 receptor agonist apomorphine and the D3 preferring agonists 7-OH-DPAT and (-) quinpirole generalised to the cue whilst the D2/3 receptor antagonists haloperidol, raclopride, spiperone and (+)-butaclamol antagonised drug lever responding. In contrast, the D1 selective agonist (+/-)-SKF 81297 and D1/5 selective antagonist, R-(+)-SCH 23390 had no effect. Results also suggest that presynaptic dopamine receptors are involved. Thus the dopamine depleting agent alpha-methyl-p-tyrosine potentiated the effects of a submaximal dose of (+)-PD 128907 whereas amphetamine failed to generalise per se and blocked (+)-PD 128907 lever selection. However, studies using subtype selective antagonists argue against a role for the D3 receptor. Thus the 10-fold selective D2 receptor antagonist L-741,626 blocked the (+)-PD 128907 discriminative stimulus whereas L-745,829 and GR 103,691, antagonists > 40 and > 100-fold selective for D3 receptors, failed to modify the response. These results suggest that presynaptic D2 receptors mediate the discriminative stimulus properties of (+)-PD 128907 and highlight the lack of selectivity of (+)-PD 128907 for D3 receptors in vivo.

The influence of injection of oestradiol to female rats on changes in alpha 2- and beta-adrenoceptor function induced by repeated administration of desipramine or electroconvulsive shock.

Repeated daily administration to female rats of either an electroconvulsive shock (110 V, 1 sec) or desipramine (DMI; 5 mg/kg x 2) caused a progressive decrease in presynaptic alpha 2-adrenoceptor function assessed by the hypoactivity (sedation) response to clonidine (0.5 mg/kg). This reduction was maximal after approximately seven electroshocks or 8-12 days of injection of DMI. Daily administration of oestradiol (100 micrograms s.c.), starting one day prior to the commencement of administration of DMI or treatment with electroshock, markedly accelerated the onset of decreased hypoactivity responses to clonidine, but did not alter the maximum reduction induced by repeated injection of DMI or administration of electroshock. Injection of oestradiol alone had no effect on the responses to clonidine. Administration of DMI for 14 days decreased the number of both alpha 2- and beta-adrenoceptors in the cortex. Cortical beta-, but not alpha 2-adrenoceptors, were also decreased after 4 days of injection of DMI. Two and ten electroshocks moderately increased and decreased cortical alpha 2-adrenoceptors, respectively. beta-Adrenoceptors were also decreased by ten electroshocks, but two were without effect. Simultaneous administration of oestradiol had little influence on the changes in the binding of alpha 2- or beta-adrenoceptors induced by repeated administration of DMI or treatment with electroshock. Oestradiol increased the numbers of cortical alpha 2- and beta-adrenoceptors 3 and 15 days after injection, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of L-triiodothyronine (T3) on the effects of repeated administration of desipramine or electroconvulsive shock on alpha 2- and beta-adrenoceptor function in the brain of the rat: implications for the potentiation of antidepressant therapy by T3.

Repeated, daily administration of either an electroconvulsive shock (ECS; 110 V, 1 sec) or desipramine (DMI; 5 mg/kg X 2) to rats caused a progressive decrease in the function of presynaptic alpha 2-adrenoceptors, assessed by the hypoactivity (sedation) response to clonidine (0.2 mg/kg). This attenuation required approximately 7 days' administration of either treatment for maximum effect. A single injection of triiodothyronine (T3; 100 micrograms/kg) on day 1 of the treatment markedly accelerated the decreased responses to clonidine induced by DMI or electroconvulsive shock, but did not alter the maximum attenuation. By itself T3 did not affect the hypoactivity responses. alpha 2-Adrenoceptors, measured by the binding of [3H]idazoxan in the cortex, which are believed to be predominantly postsynaptic, were decreased by 14 days of DMI or electroconvulsive shock for 10 days, but not 2 days of either treatment. Triiodothyronine did not influence the decreased number of alpha 2-adrenoceptors induced by DMI or electroconvulsive shock but may have delayed the onset produced by DMI. Binding to beta-adrenoceptors in the cortex was measured using [3H]dihydroalprenolol. This was significantly decreased by 14 days administration of DMI, but not significantly by electroconvulsive shock for 10. Down-regulation of beta-adrenoceptors, induced by DMI was rapid, being observed after 1 day of treatment. Injection of T3 did not influence the final decreases produced by DMI or electroconvulsive shocks but moderately delayed their onset. Triiodothyronine alone caused a 25% reduction in cortical beta-adrenoceptors 24 hr after injection.(ABSTRACT TRUNCATED AT 250 WORDS)

The behavioural and neurochemical profile of the putative dopamine D3 receptor agonist, (+)-PD 128907, in the rat.

The functional relevance of the dopamine D3 receptor is still unresolved, largely because of the absence of selective D3 receptor ligands. In the present study we have examined the in vivo profile of (+)-PD 128907, a potent and functionally selective D3 receptor agonist. Low doses of (+)-PD 128907 reduced spontaneous locomotor activity in the rat (ED50 = 13 +/- 3 micrograms/kg, s.c.) a response which was comparable with the non-selective D2,3 receptor agonist apomorphine (ED50 = 13 +/- 1.6 micrograms/kg, s.c.). In addition (+)-PD 128907 impaired prepulse inhibition of the acoustic startle response, with significant effects observed at doses of 30 micrograms/kg when appropriate prepulse intensities were used. Higher doses reversed gamma-butyrolactone-induced catecholamine synthesis (ED50 = 95 +/- 22 and 207 +/- 37 micrograms/kg in accumbens and striatum respectively) and induced yawning (100-300 micrograms/kg), penile grooming (30-1000 micrograms/kg) and sniffing (> or = 300 micrograms/kg) although doses 3- to 10-fold greater than apomorphine were required to produce maximal effects. In contrast to apomorphine, however, (+)-PD 128907 failed to induce intense stereotyped licking and biting in the rat. In view of the potency and selectivity of (+)-PD 128907 for the D3 receptor, a role in the control of locomotor activity is suggested. In addition, the observation that (+)-PD 128907 disrupts prepulse inhibition, a phenomenon which is also impaired in schizophrenic subjects, may indicate the pathological importance of this receptor subtype.

Activation of mesolimbic dopamine function by phencyclidine is enhanced by 5-HT(2C/2B) receptor antagonists: neurochemical and behavioural studies.

Administration of the non-competitive NMDA receptor antagonists phencyclidine (PCP) (0.6-5 mg/kg s.c.) and MK-801 (0.1-0.8 mg/kg s.c. ) dose-dependently increased locomotor activity in the rat. Pre-treatment of rats with SB 221284 (0.1-1 mg/kg, i.p.) a 5-HT(2C/2B) receptor antagonist or SB 242084 (1 mg/kg, i.p.) a selective 5-HT(2C) receptor antagonist, doses shown to block mCPP induced hypolocomotion, significantly enhanced the hyperactivity induced by PCP or MK-801. Neither compound altered locomotor activity when administered alone. Furthermore, systemic administration of PCP (5 mg/kg s.c.) increased nucleus accumbens dopamine efflux in the rat to a maximum of approximately 220% of basal, 40-60 min after administration. Pre-treatment with the 5-HT(2C/2B) receptor antagonist SB 221284 (1 mg/kg, i.p.) and the 5-HT(2C) receptor antagonist SB 242084 (1 mg/kg i.p.) failed to affect nucleus accumbens dopamine efflux per se but significantly enhanced the magnitude and duration of the increase induced by PCP. However, the time course of the neurochemical and behavioural effects were qualitatively and quantitatively different, suggesting the potential involvement of other neurotransmitter pathways. Nevertheless, the present results provide behavioural and neurochemical evidence which demonstrate that, in the absence of effects per se, blockade of 5-HT(2C) receptors enhanced the activation of mesolimbic dopamine neuronal function by the non-competitive NMDA receptor antagonists PCP and MK-801.

The effects of GR127935, a putative 5-HT1D receptor antagonist, on brain 5-HT metabolism, extracellular 5-HT concentration and behaviour in the guinea pig.

Studies of neurotransmitter release in guinea pig and human brain indicate that the 5-HT terminal autoreceptor is the 5-HT1D subtype and that it regulates the depolarization evoked release of 5-HT. Thus, blockade of the terminal 5-HT autoreceptor should enhance 5-HT release in vivo. In the present study, we have used the recently described, selective and potent 5-HT1D receptor antagonist, GR127935, to determine if blockade of the terminal 5-HT autoreceptor enhanced 5-HT neurotransmission in the guinea pig. Neurochemical studies showed that GR127935 (0.1, 0.3 and 1.0 mg/kg i.p.) significantly increased 5-HT metabolism in forebrain regions but not in the raphe nucleus of the guinea pig. However, using in vivo dialysis, GR127935 did not significantly increase cortical 5-HT efflux when given either systemically (1 and 5 mg/kg i.p.) or by infusion via the probe directly into the cortex (10, 33 and 100 microM). Fast cyclic voltammetry studies in the guinea pig dorsal raphe slice in vitro failed to observe any significant effects of GR127935 (0.01-1 microM) on electrically evoked 5-HT release. Behavioural studies in the guinea pig were also unable to demonstrate any effects of GR127935 (0.1-3.0 mg/kg i.p.) per se or in combination with the 5-HT precursor 5-hydroxytryptophan. Taken together, results from the present neurochemical and behavioral studies in the guinea pig provide little substantial evidence that blockade of the terminal 5-HT autoreceptor following the acute administration of GR127935 increased brain 5-HT neurotransmission in vivo.

N-Arylsulfonylindole derivatives as serotonin 5-HT(6) receptor ligands.

A series of N(1)-arylsulfonyltryptamines were found to be potent ligands of the human serotonin 5-HT(6) receptor with the 5-methoxy-1-benzenesulfonyl analogue (19) having the highest affinity. Additionally, it was discovered that a group such as 3-(3-methoxybenzyl)-1,2,4-oxadiazol-5-yl in the 2-position of the indole ring (43) can replace the arylsulfonyl substituent in the 1-position with no loss of affinity. This suggested that the binding conformation of the aminoethyl side chain at this receptor was toward the 4-position of the indole ring and was supported by the fact that the 4-(aminoethyl)indoles (45) also displayed high affinity, as did the conformationally rigid 1,3,4,5-tetrahydrobenz[c,d]indole (49). Molecular modeling showed that 19, 43, and 45 all had low-energy conformers that overlaid well onto 49. Both 19 and 49 had good selectivity over other serotonin receptors tested, with 49 also showing excellent selectivity over all dopamine receptors. In a functional adenylate cyclase stimulation assay, 19 and 49 had no agonist activity, whereas 45 behaved as a partial agonist. Finally, it was shown that 19 had good activity in the 5-HT(2A) centrally mediated mescaline-induced head twitch assay, which implies that it is brain-penetrant.

Identification of 3,5-dihydro-2-aryl-1H-pyrazolo[3,4-c]quinoline-1,4(2H)-diones as novel high-affinity glycine site N-methyl-D-aspartate antagonists.

Almost all of the existing known antagonists at the glycine site of the N-methyl-D-aspartate (NMDA) receptor have a low propensity for crossing the blood-brain barrier. It has been suggested that in many cases this may be due to the presence of a carboxylic acid which is a common feature of most of the potent full antagonists at this receptor. In this study, 2-aryl-1H-pyrazolo[3,4-c]quinoline-1,4(2H)-diones were found to have high-affinity binding at the glycine receptor. In particular, structure-activity studies identified 7-chloro-3,5-dihydro-2-(4-methoxyphenyl)-1H-pyrazolo[3,4-c]quinoline- 1,4(2H)-dione as the most potent of a series of analogues with an IC50 of 3.3 nM. The measured pKa values in this class of compounds (typically 4.0) indicate they are of equivalent acidity to carboxylic acids. Functional antagonism was demonstrated by inhibition of NMDA-evoked responses in rat cortical slices. Anticonvulsant activity in DBA/2 mice was achieved after dosing by direct injection into the cerebral ventricles, but no activity was seen after systemic administration, suggesting low brain penetration with this class of antagonists.

Effects of ketamine and N-methyl-D-aspartate on glutamate and dopamine release in the rat prefrontal cortex: modulation by a group II selective metabotropic glutamate receptor agonist LY379268.

Previous studies have shown that the metabotropic glutamate receptor (mGluR)2/3 agonist LY354740 attenuated glutamate release in medial prefrontal cortex (mPFC) induced by the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist phencyclidine. In the present study we examined the effects of the more potent mGluR2/3 selective agonist LY379268 on ketamine-evoked glutamate and dopamine (DA) release in mPFC of male rats. Subjects were implanted with a unilateral microdialysis probe in the mPFC and were tested 12-24 h after implantation. Ketamine (18 mg/kg, s.c.) evoked a significant release of glutamate and DA, although the glutamate response was slower in onset compared with DA. Pretreatment with either systemic (3 mg/kg s.c.) or local (1 microM, in the probe) LY379268 blocked ketamine-evoked glutamate, but not DA, release. When applied directly to the mPFC via the dialysis probe, ketamine (1 mM in the probe) had no effect on glutamate release but did significantly enhance the release of DA. Application of NMDA (500 microM in the probe), on the other hand, decreased DA while increasing glutamate release. The effect of NMDA on evoking glutamate release was blocked by systemic but not local administration of LY379268. These findings indicate that systemic ketamine increases both glutamate and DA release in mPFC and that the effect on glutamate can be blocked by stimulating mPFC group II mGluR receptors. Local ketamine, on the other hand, does not increase glutamate but does increase DA release. This suggests that ketamine acts outside of the mPFC to enhance glutamate, but within the mPFC to enhance DA release. The origin of the ketamine effect on mPFC glutamate is currently not known.

Analysis of the neurotrophic effects of GPI-1046 on neuron survival and regeneration in culture and in vivo.

The putative neurotrophic effects of the immunophilin ligand GPI-1046 were evaluated in established experimental systems of neuron survival and axon growth in vitro and in vivo. GPI-1046 marginally increased neurite outgrowth of chick dorsal root ganglia in culture under conditions where a very robust effect of nerve growth factor was seen. GPI-1046 failed to protect dopaminergic neurons from 1-methyl-4-phenylpyridinium in culture or to protect cultured cortical neurons from experimentally induced apoptosis in vitro. In adult rats in vivo, daily administration of GPI-1046 (10 mg/kg, s.c.) for three days enhanced the maximal regeneration distance of both motor and large myelinated sensory axons measured using an electrophysiological assay. However, detailed morphometric analysis of these animals failed to provide evidence for an increase in axon numbers in GPI-1046-treated animals. The ability of GPI-1046 to promote the recovery of dopaminergic function following unilateral 6-hydroxydopamine lesions of the substantia nigra was also tested in rats. In the first study, the duration of amphetamine (3 mg/kg, s.c.)-induced circling, but not the maximal number of rotations, was significantly reduced in animals treated with GPI-1046 for five days (10 mg/kg/day). In a second study, testing the effects of delayed GPI-1046 administration, chronic treatment with GPI-1046 (10 mg/kg/day) for two weeks, beginning one month after surgery, did not alter circling responses. Morphometric analysis failed to reveal any changes in either the density of tyrosine hyroxylase-positive fibres in dopaminergic target areas or in cell numbers in the substantia nigra in both experiments. Thus, while GPI-1046 produced marginal effects on neurite outgrowth in dorsal root ganglia cultures and on functional paramaters of nerve regeneration in vivo, we failed to obtain evidence in support of the notion of a general neuroprotective effect of the compound or for an effect on morphologic nerve regeneration in vivo.

Biphasic effects of intra-accumbens histamine administration on spontaneous motor activity in the rat; a role for central histamine receptors.

1. The effect of intra-accumbens injection of histamine and related compounds on the spontaneous motor activity of the rat has been investigated. 2. Microinjections of histamine (1-200 micrograms) induced dose-dependent, biphasic changes in rat activity consisting of an initial brief hypoactivity response followed by a marked hyperactivity phase. The histamine metabolite, n-tele-methylhistamine was without effect. 3. Pretreatment with the H1-receptor antagonist mepyramine (10 micrograms) blocked the hypoactivity response and markedly attenuated histamine-induced hyperactivity. In contrast, pretreatment with the H2-receptor antagonist SKF93479 had no effect on histamine-induced behaviour. 4. Microinjection of the H1-receptor agonist 2-thiazolylethylamine induced a marked hyperactivity response, but unlike the response to histamine, there was no initial hypoactivity. The H2-receptor agonist dimaprit had no apparent behavioural effects following intra-accumbens injection. 5. Intra-accumbens injection of the non-selective histamine agonists n alpha-methylhistamine or n alpha, n alpha-dimethylhistamine induced both marked hypoactivity and hyperactivity responses which were comparable with the effects of histamine. 6. The present results demonstrate a histamine, H1-receptor-mediated arousal in the nucleus accumbens which follows transitory hypoactivity, possibly due to activation of presynaptic H3-receptors.

Effect of chronic intra-accumbens administration of the TRH analogue CG3509 on histamine-induced behaviour in the rat.

1. The present study has investigated the effect of chronic intra-accumbens administration of the thyrotrophin-releasing hormone (TRH) analogue, CG3509, on CG3509- and histamine-induced spontaneous motor activity and brain TRH-like immunoreactive (TRH-LI) levels in the rat. 2. Chronic intra-accumbens administration of CG3509 (5 x 5 micrograms over 3 days) induced: (a) a significant (P less than 0.05) reduction in intra-accumbens CG3509 (0.5 micrograms)-induced hyperactivity, (b) reduced levels of TRH-LI in the nucleus accumbens but not other brain regions, (c) a marked increase (107%, P less than 0.01) in histamine-induced non-locomotor hyperactivity. 3. The present results demonstrate that alteration of central TRH function following treatment with a TRH analogue enhances the effect of intra-accumbens histamine on behavioural hyperactivity, possibly via changes in H1 receptors and suggest that the neuropeptide, TRH and histamine interact in behavioural arousal mechanisms in rat brain.