strychnine and milacemide

The blockade of spinal glycine receptors with intrathecal (i.t.) strychnine (STR) produces reversible, segmentally localized allodynia in the rat. The purpose of this study was: (1) to investigate the effect of the anticonvulsant agent, milacemide, a glycine pro-drug on STR-allodynia; (2) to compare this effect with that of milacemide on normal nociception (without STR); and (3) to determine the sensitivity of the anti-allodynic effect of milacemide to pretreatment with selective monoamine oxidase (MAO)-A (clorgyline) and MAO-B (L-deprenyl) inhibitors. Male Sprague-Dawley rats, fitted with chronic i.t. catheters, were lightly anesthetized with urethane. Hair deflection (HD) evoked maximum changes in blood pressure and heart rate were recorded from left carotid artery, and cortical electroencephalographic (EEG) activity was continuously monitored using subdermal needle electrodes before and after i.t. STR (40 microg). Rats were pretreated with a single intravenous (i.v.) injection of milacemide (100-600 mg/kg), 1 h before i.t. STR. To sustain the allodynic state, STR was injected every hour for up to 4 h. HD was applied to the affected dermatomes (2 min duration) using a cotton-tipped applicator at 5-min intervals for the duration of the STR effect. Normally innocuous HD elicited a marked increase in mean arterial blood pressure and heart rate, an immediate motor responses, and desynchronisation of EEG when applied to the cutaneous dermatomes affected by i.t. STR. Milacemide (100-600 mg/kg, i.v.) dose-dependently inhibited the heart rate and pressor responses (ED50 = 398 mg/kg; 95%CI = 196-873) and the motor responses (ED50 = 404 mg/kg; 95%CI = 275-727). Maximum inhibition was observed approximately 2 h after i.v. injection. The duration of action ranged from 3 h (400 mg/kg) to 4 h (600 mg/kg). Milacemide had no effect on the percent synchrony in the EEG. At the time of maximum inhibition of STR-allodynia (2 h post-infusion), responses evoked by noxious pinch were unaffected by milacemide. Pretreatment with L-deprenyl (3 mg/kg, i.p.), but not clorgyline (10 mg/kg, i.p.) significantly blocked the anti-allodynic effect of milacemide (600 mg/kg i.v). These data indicate that i.v. milacemide significantly attenuates the allodynia arising from spinal glycine receptor blockade, and are consistent with: (1) the selective modulation of low threshold afferent input by STR-sensitive, glycine interneurons in the rat spinal cord; and (2) the pharmacological actions of

Topics: Acetamides; Animals; Anticonvulsants; Clorgyline; Glycine; Hyperalgesia; Injections, Intravenous; Injections, Spinal; Male; Monoamine Oxidase Inhibitors; Nociceptors; Prodrugs; Rats; Rats, Sprague-Dawley; Reference Values; Selegiline; Strychnine

[3H]Glycine is actively taken up into bovine isolated adrenal medulla chromaffin cells with the subsequent catecholamine release. [3H]Glycine uptake has two interaction sites based on relative Km measurements. These sites are inherently distinct since the effects of strychnine and temperature on glycine binding are significantly different. The high affinity site (Km = 6 x 10(-7) M) is strychnine-sensitive and its activity is unaltered at 4 degrees C. These results point to a receptor-like function. The low-affinity site (Km = 1.4 x 10(-3) is strychnine-insensitive and is significantly inhibited (75%) by low temperature (4 degrees C), by low Na+ concentration and 50% by ouabain (10(-4) M). Compounds structurally similar to glycine and known to antagonize its uptake to neuronal cells, such as beta-alanine, N-methyl-d,l-alanine and sarcosine, also inhibit the low affinity site which indicate a glycine uptake function for this site. The relative activity of the uptake inhibitors indicate that in the adrenal chromaffin cell, glycine uptake is carried out by a System A amino acid transporter mechanism. GABA does not affect glycine binding or uptake in the chromaffin cells, suggesting that these two inhibitory amino acid neurotransmitters act via different mechanisms in the adrenal medulla. The results for glycine activity in adrenal medulla chromaffin cells are remarkably similar to those seen in CNS neuronal cells, and thus support the use of chromaffin cells as a model system for studying the mechanism of action of glycine in the central nervous system.

Topics: Acetamides; Adrenal Medulla; Alanine; Animals; beta-Alanine; Cattle; Cell Membrane; Choline; Cysteine; gamma-Aminobutyric Acid; Glutamates; Glutamic Acid; Glycine; Kinetics; Ouabain; Receptors, Glycine; Sarcosine; Sodium; Strychnine; Taurine; Temperature

The anticonvulsant activity of a new drug, milacemide (2-(pentylamino)-acetamide), has been studied in animal models of convulsions like those induced by bicuculline, pentylenetetrazol, picrotoxin, strychnine, inhibitors of GABA synthesis as 3-mercaptopropionic acid, allylglycine, isoniazid and thiosemicarbazide and electroshock. Milacemide is particularly effective in inhibiting the convulsions induced by bicuculline. The ED50 is 5.7 mg/kg by oral route and the activity lasts for more than 48 hr. It is less active against pentylenetetrazol and only marginally active against electroshock. It has not be found active against the other types of convulsions. Milacemide has a low toxicity (LD50: 2585 mg/kg in the mouse) and alters the behaviour of mouse, rat and monkey, only at high doses (greater than or equal to 1000 mg/kg). Milacemide seems to be specially free of sedative potential.

Topics: Acetamides; Animals; Anticonvulsants; Behavior, Animal; Bicuculline; Electroshock; gamma-Aminobutyric Acid; Lethal Dose 50; Male; Mice; Pentylenetetrazole; Rats; Rats, Inbred Strains; Seizures; Strychnine