In vivo labeling of phencyclidine (PCP) receptors with 3H-TCP in the mouse brain

Effects of the psychotomimetic benzomorphan N-allylnormetazocine (SKF 10,047) on prepulse inhibition of startle in mice

N-allylnormetazocine (NANM; SKF 10,047) is a benzomorphan opioid that produces psychotomimetic effects. (+)-NANM is the prototypical agonist for the sigma-1 (σ1) receptor, and there is a widespread belief that the hallucinogenic effects of NANM and other benzomorphan derivatives are mediated by interactions with σ1 sites. However, NANM is also an agonist at the κ opioid receptor (KOR) and binds to the PCP site located within the channel pore of the NMDA receptor, interactions that could potentially contribute to the effects of NANM. NMDA receptor antagonists such as phencyclidine (PCP) and ketamine are known to disrupt prepulse inhibition (PPI) of acoustic startle, a measure of sensorimotor gating, in rodents. We recently found that racemic NANM disrupts PPI in rats, but it is not clear whether the effect is mediated by blockade of the NMDA receptor, or alternatively whether interactions with KOR and σ1 receptors are involved. The present studies examined whether NANM and its stereoisomers alter PPI in C57BL/6J mice, and tested whether the effects on PPI are mediated by KOR or σ1 receptors. Racemic NANM produced a dose-dependent disruption of PPI (3-30mg/kg SC). (+)-NANM also disrupted PPI, whereas (-)-NANM was ineffective. Pretreatment with the selective KOR antagonist nor-binaltorphimine (10mg/kg SC) or the selective σ1 antagonist NE-100 (1mg/kg IP) failed to attenuate the reduction in PPI produced by racemic NANM. We also found that the selective KOR agonist (-)-U-50,488H (10-40mg/kg SC) had no effect on PPI. These findings confirm that NANM reduces sensorimotor gating in rodents, and indicate that the effect is mediated by interactions with the PCP receptor and not by activation of KOR or σ1 receptors. This observation is consistent with evidence indicating that the σ1 receptor is not linked to hallucinogenic or psychotomimetic effects.

Oxytocin as a natural antipsychotic: a study using oxytocin knockout mice

It has been previously suggested that oxytocin (Oxt) may act as a natural antipsychotic. To test this hypothesis, we investigated whether disruption of the oxytocin gene (Oxt-/-) made mice more susceptible to the psychosis-related effects of amphetamine (Amp), apomorphine (Apo) and phencyclidine (PCP). We examined drug-induced changes in the prepulse inhibition (PPI) of the startle reflex, a measure of sensorimotor gating deficits characteristic of several psychiatric and neurological disorders, including schizophrenia. We found that treatment with Amp, Apo and PCP all had effects on PPI. However, in Oxt-/- mice, but not Oxt+/+ mice, PCP treatment resulted in large PPI deficits. As PCP is a noncompetitive N-methyl-D-aspartic acid receptor antagonist, these findings suggest that the absence of Oxt alters the glutamatergic component of the PPI.

Cataleptic effects of gamma-hydroxybutyrate (GHB) and baclofen in mice: mediation by GABA(B) receptors, but differential enhancement by N-methyl-d-aspartate (NMDA) receptor antagonists

RATIONALE

Gamma-hydroxybutyrate (GHB) is a gamma-aminobutyric acid (GABA) analog that is used to treat narcolepsy but that is also abused. GHB has many actions in common with the GABA(B) receptor agonist baclofen, but their underlying GABA(B) receptor mechanisms may be different.

OBJECTIVE

The aim of this study is to further investigate a possible differential role of glutamate in GABA(B) receptor-mediated effects of GHB and baclofen.

MATERIALS AND METHODS

The experiments examined the effects of non-competitive antagonists at the N-methyl-d-aspartate (NMDA) subtype of glutamate receptors on GHB-induced catalepsy and compared these effects with those on baclofen-induced catalepsy.

RESULTS

In C57BL/6J mice, ketamine, phencyclidine (PCP), and dizocilpine (MK-801) all enhanced GHB-induced catalepsy. They did so with a potency order (i.e., MK-801 > PCP > ketamine) consistent with their relative potencies as NMDA antagonists but not as inhibitors of dopamine or organic cation transporters. Ketamine, PCP, and MK-801 enhanced catalepsy along inverted U-shaped dose-response curves likely because higher doses affected motor coordination, which limited their catalepsy-enhancing effects. Doses that were maximally effective to enhance GHB-induced catalepsy did not affect the cataleptic effects of baclofen.

CONCLUSIONS

The finding that NMDA receptor antagonists enhance the cataleptic effects of GHB but not those of baclofen is further evidence that the GABA(B) receptor mechanisms mediating the effects of GHB and GABA(B) agonists are not identical. Differential interactions of glutamate with the GABA(B) receptor mechanisms mediating the effects of GHB and baclofen may explain why GHB is effective for treating narcolepsy and is abused, whereas baclofen is not.

Modulation of [3H]MK-801 binding to NMDA receptors in vivo and in vitro

[3H]MK-801 binding in vivo was used to determine the occupancy of NMDA receptor ligands shown to allosterically modulate binding in vitro. ED(50) values (mg/kg) were obtained for the channel blockers (+)-5-methyl-10,11-dihydro-5,4-dibenzo[a,d]cyclohepten-5,10-imine maleate ((+)-MK-801, 0.2), 1-(1-phenylcyclohexyl)piperidine (phencyclidine, PCP, 1.7) and ketamine (4.4). Antagonists at the glutamate (DL-(2-carboxypiperazine-4-yl)propyl-1-phosphonate (DL-CPP, 5.7)) and glycine site (7-Chloro-4-hydroxy-3-(3-phenoxy)-phenyl-2(H)quinolinone (L-701,324, 14.1), 3R(+)cis-4-methyl-pyrrollid-2-one (L-687,414, 15.1)) inhibited [3H]MK-801 binding in vivo to varying maximum levels (69%, 103% and 45%, respectively). NR2B subunit-selective compounds acting at the ifenprodil site inhibited [3H]MK-801 in vivo by a maximum of 52-72% and gave ED(50) values (mg/kg) of: (+/-)-(1S*, 2S*)-1-(4-hydroxyphenyl)-2-(4-hydroxy-4-phenylpiperidino)-1-propanol ((+/-)CP-101,606), 1.9; (+/-)-(3R, 4S)-3-[4-(4-fluorophenyl)-4-hydroxypiperidin-1-yl]chroman-4,7-diol ((+/-)CP-283,097), 1.8; (+/-)-(R*, S*)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperidine propanol ((+/-)Ro 25-6981), 1.0; ifenprodil, 6.0. The glycine site agonist D-serine stimulated binding to 151% of control with an ED(50) of 1.7 mg/kg. Results show that [3H]MK-801 binding in vivo may be used to measure receptor occupancy of ligands acting not only within the ion channel but also at modulatory sites on the NMDA receptor complex.

Modulation by neurosteroids of the in vivo (+)-[3H]SKF-10,047 binding to sigma 1 receptors in the mouse forebrain

Recent reports suggest an interaction between neuro-(active)steroids and sigma1 (sigma 1) receptors, affecting biochemical parameters as well as physiological responses mediated by sigma 1 ligands in the rodent brain. In this study, we examined the modulation by neurosteroids of the haloperidol-sensitive in vivo (+)-[3H]SKF-10,047 binding to sigma 1 sites in the mouse hippocampus and cortex. Progesterone (PROG; 2-40 mg/kg), pregnenolone sulfate (PREGS; 10-40 mg/kg), and dehydroepiandrosterone sulfate (DHEAS; 10-40 mg/kg) were administered systemically 10 min before the radioactive tracer. The total amount of (+)-[3H]SKF-10,047 bound in each structure was significantly affected by PROG and PREGS only at the highest dose tested and was unaffected by DHEAS. However, bound to free (B/F) radioactivity ratios were highly significantly decreased by 30-40% in each structure by PROG and PREGS. DHEAS, at 40 mg/kg, induced a significant 20% decrease in the hippocampus. Furthermore, the in vivo (+)-[3H]SKF-10,047 binding parameters were diminished in pregnant female mice compared to non-pregnant or male mice. These results confirm the in vitro binding results, bring a direct in vivo demonstration of the interaction between neurosteroids and sigma 1 receptors, and show that physiologic modulations of the steroidal concentrations affect the sigma 1 systems.

Beneficial effects of sigma agonists on the age-related learning impairment in the senescence-accelerated mouse (SAM)

A beneficial effect of sigma (sigma) agonists was previously described on several pharmacological models of learning impairments. We examined this effect in senescence-accelerated mice (SAM), which has been developed as a murine model of aging and cognitive dysfunction. SAMP8/Ta (P8, senescence-prone substrain), 10-12 months of age, showed significant impairments in mnemonic capacities, as compared to age-matched SAMR1/Ta controls (R1, senescence-resistant substrain). Tests included open-field behavior, spontaneous alternation performances in the Y-maze, step-down passive avoidance and place learning after repetitive training in a water-maze. Pretreatment with the sigma agonists JO-1784 (igmesine) or PRE-084, at 0.1-3 mg/kg, s.c., significantly improved spontaneous alternation and passive avoidance performances in P8. JO-1784 or PRE-084, at 1 mg/kg, also improved place learning in the water-maze, and retention, in term of escape latency. The implication of sigma sites was indicated by the lack of significant effect of JO-1783, the inactive enantiomer of JO-1784, and by the ability of BMY-14802 (5 mg/kg, i.p.) to antagonize the effects on passive avoidance of JO-1784 (0.5 mg/kg) or PRE-084 (1 mg/kg). Subchronic treatments with JO-1784 (0.5 mg/kg/day) or PRE-084 (1 mg/kg/day) during 10 days, allowed a significant improvement of learning during training in the water-maze, but retention was not significantly ameliorated. These results confirmed the interest of the SAM substrains as an experimental model for senile memory impairment and showed that sigma agonists could improve the quality of learning, although they seem less effective on long-term memory retrieval upon chronic administration.

Behavioral evidence for a modulating role of sigma ligands in memory processes. I. Attenuation of dizocilpine (MK-801)-induced amnesia

The potentiating effect of low doses of sigma ligands on the N-methyl-D-aspartate (NMDA)-induced excitation of pyramidal CA3 dorsal hippocampal neurons has recently been reported. In the present study, we investigated behavioral effects relevant to these findings in the experimental amnesia induced by the non-competitive NMDA antagonist, dizocilpine (MK-801), in mice. At doses below 1 mg/kg s.c., the sigma ligands, 1,3-di-(2-tolyl)guanidine (DTG), (+)-SKF 10,047, and (+)-pentazocine, but not their (-)-isomers, significantly decreased MK-801 (100 microgram/kg s.c.)-induced impairment of spontaneous alternation performances in 8-min sessions of a Y-maze exploration, an index of spatial working memory, without affecting the concomitant hyperlocomotion. The effect of DTG (100 micrograms/kg s.c.) was completely antagonized by the simultaneous administration of BMY 14802 (10 mg/kg i.p.) and NE-100 (1 mg/kg i.p.), two putative sigma antagonists, which had no effect by themselves. In long-term memory tests (step-down and step-through types of passive avoidance, elevated plus-maze), DTG exhibited a significant attenuation of MK-801-induced amnesia, at doses of 10 and 100 micrograms/kg s.c. In all tests of short- and long-term memory, the effects exhibited by the sigma ligands tested had a bell-shaped curve; no effect was seen at 1 mg/kg. DTG did not affect the impairment of alternation induced by CPP (5 mg/kg i.p.): the modulation may selectively target the blockade of NMDA receptor-associated ion channels. Moreover, DTG (1-1000 micrograms/kg) did not affect the impairment induced by scopolamine (1 mg/kg i.p.) or diazepam (4 mg/kg i.p.), but significantly prevented the impairment induced by mecamylamine (10 mg/kg i.p.). These results suggest that the potentiating effect of sigma ligands on NMDA receptor-mediated glutamatergic neurotransmission, already demonstrated electrophysiologically, may have some relevance to learning and memory processes in the hippocampus. A similar modulation may also affect cholinergic nicotinic systems.

Regional differences in the effect of N-[1-(2-benzo[b]thiophenyl)cyclohexyl]piperidine (BTCP) on extracellular dopamine levels: an in vivo microdialysis study

N-[1-(2-Benzo[b]thiophenyl)cyclohexyl]piperidine (BTCP) is a phencyclidine derivative highly selective for the dopamine (DA) uptake complex. Its effect on extracellular DA levels was studied by in vivo microdialysis on freely moving rats. In the striatum, BTCP induced a dose-dependent increase in DA levels, without affecting DA metabolites. In the nucleus accumbens, a lower increase in DA was observed, but with concomitant decreases in 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). These experiments show that the effects of BTCP on extracellular DA levels are significantly different on extrapyramidal and mesolimbic dopaminergic terminals.

Effect of amygdaloid kindling on the high- and low-affinity [3H]TCP binding sites of the rat CNS

The regulation of the binding sites of [3H]TCP, a non-competitive ligand of the N-methyl-D-aspartate (NMDA) receptor, was studied on membranes prepared from different CNS regions of amygdaloid-kindled rats. The high-affinity binding sites (KdH = 4.2-7.4 nM), identified as the NMDA-gated ion channels, were not affected by kindling or by a daily injection of TCP (5 mg/kg before each electrical stimulation) which prevented kindling. These results suggest that the NMDA receptors participate to the establishment and not to the permanence of kindling. Kindling increases the number of low affinity [3H]TCP binding sites in the hippocampus (+21%, P less than 0.01) without change of the affinity (KdL = 340 nM). In the striatum both KdL and BmaxL were increased (3.3-4.4 fold, P less than 0.001) in animals pretreated with TCP before each electrical stimulation for 20 days. These last results argue in favour of a function of the low-affinity [3H]TCP binding sites, the nature of which remains to be determined.

Desipramine and the phencyclidine derivative BTCP differently inhibit [3H]TCP binding to high- and low-affinity sites

The effects of N-[1-(2-benzo(b)thiophenyl)cyclohexyl]piperidine (BTCP) and desipramine on [3H]N-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) binding were investigated in vivo and in vitro. In the cerebrum, both drugs were competitive inhibitors of high-affinity [3H]TCP binding. Conversely, in the cerebellum, they were non-competitive inhibitors of low-affinity [3H]TCP binding. These results imply that the different [3H]TCP binding sites have distinct pharmacological properties, and show that, although chemically related to TCP, BTCP has an effect similar to that of desipramine.