Ian A. Paul

Adaptation of the NMDA receptor in rat cortex following chronic electroconvulsive shock or imipramine

Chronic (14 days) administration of either imipramine or electroconvulsive shock effected significant changes in the ligand binding properties of the NMDA (N-methyl-D-aspartate) receptor complex in rat cortex. These changes were manifested as: (1) a reduction in the potency of glycine to inhibit the binding of 5,7-dichloro[3H]kynurenic acid to strychnine-insensitive glycine receptors; and (2) a reduction in the proportion of high affinity, glycine-displaceable [3H]CGP-39653 binding to NMDA receptors. Chronic electroconvulsive shock, but not imipramine treatment also reduced the density of [3H]CGP-39653 binding sites in cortical membranes. These findings demonstrate that the ability of chronic antidepressant treatments to induce adaptive changes in the glycine and glutamate regulatory sites of the NMDA receptor is not species specific, since it obtains in rats as well as mice.

Effects of N6-cyclopentyl adenosine and 8-cyclopentyl-1,3-dipropylxanthine on N-methyl-D-aspartate induced seizures in mice.

The effect of the adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) and antagonist 8-cyclopentyl-1,3-dipropylxanthine (CPX) on N-methyl-D-aspartate (NMDA)-evoked seizures was studied in C57BL/6 mice (20/group). Animals were injected i.p. either with CPA (0.5, 1, 2 mg/kg) or CPX (1, 2 mg/kg) 15 min prior to administration of NMDA (30, 60, 125 mg/kg). Administration of NMDA alone resulted in a complete locomotor arrest at 30 mg/kg, while clonic/tonic seizures and progressively increasing mortality were seen at higher doses. Prior administration of CPA resulted either in a delay of seizure onset and unchanged mortality (0.5 mg/kg CPA, 60 mg/kg NMDA) or in elimination of tonic episodes and a significant reduction in postictal mortality (1, 2 mg/kg CPA; 60, 125 mg/kg NMDA). Pretreatment with CPX at either 1 or 2 mg/kg eliminated locomotor depression in animals injected with NMDA at 30 mg/kg. At 60 mg/kg NMDA, the effect of CPX administration resulted in mortality equivalent to that seen with 125 mg/kg NMDA administered alone. The results indicate that A1 receptor agonists may protect against NMDA-evoked seizures and that the adenosine A1 receptor may be directly involved in these actions.

Effects of chronic administration of adenosine A1 receptor agonist and antagonist on spatial learning and memory

Spatial memory acquisition in Morris water maze was tested in C57BL/6 mice. Animals were injected once daily with different doses of either N6-cyclopentyladenosine (CPA) or 8-cyclopentyl-1,3-dipropylxanthine (CPX). Drugs were administered for 9 days either concurrently with water maze testing (drugs injected 1 h after each trial), or prior to the entire block of trials. In the latter case, 1 day without injections preceded water maze experiments. Chronic administration of CPA resulted in a significant, dose-dependent reduction of target latencies, rapid development of spatial preference, and the absence of animals unable to perform the task. CPX treated animals did not show significant performance changes, and failed to develop spatial preference. Locomotor disturbances were not the cause of the observed effects. Our results indicate that chronic treatment with agents acting at adenosine A1 receptors results in behavioral effects that are significantly different from those observed following their acute administration. Therefore, particular caution is required in development of adenosine-based strategies targeted at neurodegenerative or cognitive disorders in which chronic treatment is advocated.

Down-regulation of cortical β-adrenoceptors by chronic treatment with functional NMDA antagonists

Down-regulation of cortical β-adrenoceptors is observed in laboratory animals following chronic treatment with many clinically effective antidepressant therapies. [3H]Dihydroalprenolol (DHA) binding to cortical β-adrenoceptors was examined in mice treated with the functional NMDA antagonists 1-aminocyclopropane-carboxylic acid (ACPC) and MK-801. ACPC and MK-801 reduced [3H]DHA binding by 19 (P>0.05) and 21% (P<0.05), respectively, while imipramine produced a 23% (P<0.05) reduction. No corresponding changes in the Kd of [3H]DHA were observed. These findings are consistent with the observation that functional NMDA antagonists are active in animal models commonly used to evaluate antidepressants and may represent a novel approach to the treatment of depression.

Chronic adenosine A1 receptor agonist and antagonist : effect on receptor density and N-methyl-D-aspartate induced seizures in mice

The effect of chronic administration of the adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) and the adenosine A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine (CPX) on N-methyl-D-aspartate (NMDA)-evoked seizures was studied in C57BL/6 mice (20/group). Animals were injected i.p. for 9 days with either 1.0 mg/kg CPA or 1.0 mg/kg CPX followed by 2 injection-free days (the washout period) and subsequent administration of a single dose of 60 mg/kg NMDA. As in our previous study, this dose of NMDA caused clonic/tonic seizures resulting in high (60%) mortality within 3 h after injection of the drug. Despite insignificant changes in seizure latency, chronic pretreatment with CPA increased the incidence of clonic/tonic episodes and end-point mortality. Conversely; chronic exposure to CPX completely eliminated clonic/tonic episodes, significantly increased average survival time, and reduced end-point mortality (P < 0.05). The results indicate that chronic treatment with adenosine A1 receptor antagonist may protect against NMDA-evoked seizures to the same degree as previously observed following a single, acute exposure to CPA. Since the density of adenosine receptor binding sites was unchanged after chronic treatment with either CPX or CPA, it is likely that the mechanism behind the observed protection may rest at the level of second messenger systems coupled to adenosine A1 receptors.

Sigma receptors modulate nicotinic receptor function in adrenal chromaffin cells.

Neither the physiological function of sigma (σ) receptors nor the cellular mechanism responsible for the pharmacological effects of σ receptor ligands is known. We now report that σ receptor ligands noncompetitively inhibit nicotine‐stimulated catecholamine release from bovine adrenal chromaffin cells in a concentration‐dependent and reversible manner. The rank order of potency of ligands to inhibit nicotine‐stimulated catecholamine release is significantly correlated (P < 0.005) with that observed in radioligand binding assays selective for the σ1 receptor subtype. This naltrexone‐insensitive effect is paralleled by an inhibition of nicotine‐stimulated increases in [Ca2+]i. Sigma ligands were without effect on catecholamine release or [Ca2+]i in the absence of nicotine. In addition, nicotine accelerated the association of the σ receptor selective radioligand, [3H](+)pentazocine, to adrenal medullary homogenates while having no effect on the rate of ligand dissociation, consistent with a σ ligand binding site closely associated with and allosterically modulated by the nicotinic acetylcholine receptor. Thus, the actions of agonists at the nicotinic acetylcholine receptor in bovine chromaffin cells are modulated by σ1 receptor selective ligands.—Paul, I. A., Basile, A. S., Rojas, E., Youdim, M. B. H., De Costa, B., Skolnick, P., Pollard, H. B., Kuijpers, G. A. J. Sigma receptors modulate nicotinic receptor function in adrenal chromaffin cells. FASEB J. 7: 1171‐1178; 1993.

Ca2+ antagonists effect an antidepressant-like adaptation of the NMDA receptor complex

Chronic, but not acute treatment of mice with nimodipine and diltiazem produce significant increases in the IC50 of glycine to inhibit [3H]5,7-dichlorokynurenic acid binding in cerebral cortex. Such adaptive changes in the ligand binding properties of the NMDA receptor complex are also manifested following chronic treatment with antidepressants from every principal therapeutic class. These findings indicate voltage-dependent calcium channel antagonists would be strong candidates for rigorous clinical trials in depressive disorders.

Altered brain fyn kinase in a murine acquired immunodeficiency syndrome.

Mice infected with the replication‐defective virus (BM5def) in the LP‐BM5 murine leukemia virus (MuLV) mixture develop an immune deficiency syndrome and encephalopathy characterized by impaired spatial learning and memory as demonstrated in the Morris water maze. However, the molecular mechanism (or mechanisms) underlying this cognitive deficit remains unknown. Here we report that brain fyn kinase, which has been proposed to be involved in spatial learning and memory, was unresponsive to glutamatergic stimulation in mice with MAIDS. Thus, whereas application of glu‐tamate to hippocampal slices from control mice increased fyn protein tyrosine kinase (PTK) activity more than 2,5‐fold, these changes were significantly impaired in LP‐BM5 MuLV‐infected mice. Moreover, mice with MAIDS exhibited an abnormal histological distribution of fyn PTK in the hippocampus. These findings suggest that virus‐associated disruption of fyn kinase‐mediated signaling contributes to the cognitive deficits observed in mice with MAIDS and other retrovirus‐induced encephalopa‐thies.—Sei, Y., Whitesell, L., Kustova, Y., Paul, I A., Morse, H. C., III, Skolnick, P., Basile, A. S. Altered brain fyn kinase in a murine‐acquired immunodeficiency syndrome. FASEB J. 10,339‐344 (1996)

Down-regulation of dopamine1 (D1) receptors by chronic imipramine is species-specific

Chronic treatment with imipramine (15 mg/kg, twice daily for 14 days) induces a down-regulation of limbic D1 receptors in rats but not in mice. In this mouse strain, both chronic and acute imipramine treatment have been shown to produce clear behavioral effects in the forced swim test. While the data presented here are consistent with previously reported findings in rats, they demonstrate that the down-regulation of D1 receptors by chronic antidepressant treatment is species-specific. This phenomenon indicates that D1 receptor down-regulation is not critical to the therapeutic mechanism of action of antidepressants.

Multiple [3H]DTG binding sites in guinea pig cerebellum: evidence for the presence of non-specific binding

The characteristics of the low affinity component of 1,3-di(2-[5-3H]tolyl)guanidine binding to the guinea pig cerebellum were investigated. Saturation binding assays where sigma 1 receptors were masked with dextrallorphan indicated that 1,3-di(2-[5-3H]tolyl)guanidine bound to cerebellar membranes in a fashion best described by a 1 site+non-specific binding model with a low density of specific binding sites (Bmax approximately 200 fmol/mg protein). Boiling the cerebellar membranes before addition to the saturation assay had no effect on the density of 1,3-di(2-[5-3H]tolyl)guanidine binding. In contrast, both the Kd and Bmax for 1,3-di(2-[5-3H]tolyl)guanidine binding to liver membranes was significantly reduced by boiling, as was the density of [3H](+)-pentazocine binding to cerebellum and liver. Thus, a substantial component of 1,3-di(2-[5-3H]tolyl)guanidine binding in the guinea pig cerebellum is to non-specific, proteinaceous binding sites with some of the pharmacological characteristics of the sigma 2 binding site.