The relative potencies of cholinomimetics and muscarinic antagonists on the rat iris in vivo: effects of pH on potency of pirenzepine and telenzepine

The effects of cholinomimetics and muscarinic antagonists were compared following topical administration to the eyes of anaesthetized rats. For tests with cholinomimetics, clonidine (0.3 mg/kg) was used to induce mydriasis via central inhibition of parasympathetic tone. Full, dose-dependent miosis was induced by acetylcholinesterase inhibitors [physostigmine greater than neostigmine greater than tetrahydroaminoacridine (THA)] and by membrane channel blockers (4-aminopyridine greater than 3,4-diaminopyridine). Oxotremorine was the most potent direct agonist tested [oxotremorine greater than arecaidine propargylester (APE) greater than arecoline greater than carbachol greater than ethoxyethyltrimethyl-ammonium iodide (EOE) greater than RS 86]. Some putative M1 selective agonists were weakly active or behaved as partial agonists (pilocarpine greater than AH6405 greater than Mc-A-343 greater than isoarecoline). Of the antagonists, compared in non-clonidine treated rats, scopolamine hydrochloride was the most potent. Of the receptor selective antagonists the M2 (ileal) selective compounds hexahydrosiladifenidol and 4-DAMP were more potent than either M1 selective (pirenzepine, telenzepine) or M2 (atrial) selective (AF DX 116) drugs. These data tentatively suggest the involvement of an M2 (ileal) type muscarinic receptor. Potency was lower for quaternary structures, probably due to impaired corneal penetration. The potency of pirenzepine and telenzepine was increased 60-fold at low pH following topical administration. Acid induced corneal damage does not appear to account for this potency shift as the effects of scopolamine and several agonists (oxotremorine, pilocarpine and McNA-343) were not substantially altered by acid media. For pirenzepine the potency shift appears to be related to protonation of the second amino group (N1) in the piperazine tail (pKa = 2.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative chemical structure and pharmacokinetics of MRI contrast agents

The blood clearance kinetics of five gadolinium complexes, Gd(L), were determined in rats and the results interpreted in terms of an open two-compartment pharmacokinetic model. The complexes were tested in vitro for stability in serum and in aqueous solutions of ions that they might encounter in vivo and that might be expected to react with the Gd(L) complexes to produce uncomplexed gadolinium. Reaction with serum was observed in two instances. Chemical structural differences among the chelating ligands appear to govern the overall reactivity of their Gd(L) complexes. It may be inferred from the results that a preferred structural feature of the ligand is the presence of a 12-membered 1,4,7,10-tetraaza macrocycle.

A predictive test for adverse reactions to contrast media. Preliminary results

In a prospective study, whole blood samples drawn from patients prior to their being injected with contrast media were incubated with zymosan to activate the complement cascade. The samples were tested for various analytes, including C3a, thromboxane B2 (TxB2), beta thromboglobulin and platelet factor 4 (PF4). Of 207 patients receiving contrast media, only eight experienced reactions, which were mild. Levels of the platelet constituents were generally elevated in these patients. Specificity and sensitivity were 89% and 83%, respectively, for the combined TxB2 and PF4 radioimmunoassay data. Using the Wilcoxon-Mann-Whitney rank sum test, both PF4 and TxB2 were collected with RCM reactions at the R less than .05 level. Although preliminary, the results suggest that RCM reactions are predictable by the in vitro test procedures described.

Place navigation in rats is impaired by lesions of medial septum and diagonal band but not nucleus basalis magnocellularis

The role of forebrain cholinergic projections in place navigation learning was assessed in two experiments. Following surgery, rats were required to learn the spatial location of an underwater platform on the basis of distal room cues. Bilateral injections of ibotenic acid into the nucleus basalis magnocellularis depleted choline acetyltransferase (ChAT) from the anterior and temporoparietal cortex but not the hippocampus. Separate histological studies confirmed the accuracy of the lesions and demonstrated a marked loss of cortical acetylcholinesterase. These rats subsequently showed no deficits in spatial learning or memory. In a second experiment, bilateral lesions of the vertical limb of the diagonal band of Broca and medial septum depleted ChAT from the hippocampus and posterior cortex but not the anterior cortex. Histological studies confirmed the accuracy of the lesion and showed a pronounced loss of acetylcholinesterase from the hippocampus. These rats were deficient in spatial learning and showed reduced spatial bias during transfer tests. The data are discussed in the light of the hypothesis that the cholinergic innervation of the hippocampus plays a key role in spatial reference memory processes involved in place navigation.

Prekallikrein activation, C1 esterase inhibitor, and factor XII as predictors of adverse reaction to contrast media. A prospective study

The susceptibility of 152 patients to idiosyncratic reactions resulting from the administration of radiographic contrast media was studied. The rate of activation of plasma prekallikrein was measured in samples taken from these patients before they received contrast agents. Kallikrein inhibitor and factor XII levels were also determined. The tests were of no value in selecting the ten patients who subsequently experienced mild reactions. However, the possibility remains that one or more of the tests may have predictive value for patients who experience moderate or severe reactions.

Spatial learning in the rat: impairment induced by the thiol-proteinase inhibitor, leupeptin, and an analysis of [3H]glutamate receptor binding in relation to learning

Rats were given continuous intraventricular infusion of saline or the thiol-proteinase inhibitor leupeptin, via subcutaneously implanted osmotic minipumps, while being trained on a spatial learning water task using spaced trials. Leupeptin caused overnight forgetting during training, but performance eventually reached asymptote in both groups. A retention test conducted 48 h later to assess spatial memory revealed no significant group differences, but did cause, in saline-treated rats only, a disruption of subsequent retraining back to the correct spatial location. The groups showed no differences in Cl-dependent [3H]glutamate receptor binding to hippocampal or entorhinal cortex membranes subsequent to training. In a second experiment, normal rats trained on the same task also showed no differences in Cl-dependent [3H]glutamate binding relative to rats exposed to the water task but given random spatial position training and handled controls. The results are discussed in relation to the hypothesis of Lynch and Baudry (Science (1984) 224, 1057-1063) that a calcium-dependent thiol proteinase is involved in memory formation through its ability to modify glutamate receptor distribution and dendritic spine shape.

Facilitation of amphetamine-induced rotation by muscarinic antagonists is correlated with M2 receptor affinity

This study examined the relationship between the affinity of cholinergic drugs for muscarinic receptor subtypes and their potency in potentiating or inhibiting amphetamine-induced rotation. The ascending nigrostriatal dopaminergic pathway was unilaterally lesioned in male Wistar rats using 6-hydroxydopamine. In these rats, ipsiversive rotation induced by amphetamine sulphate (1 mg/kg, s.c.) was dose-dependently inhibited by the cholinergic agonists oxotremorine, RS86 and pilocarpine and by the acetylcholinesterase inhibitor physostigmine. In contrast the cholinergic antagonists scopolamine, secoverine and dicyclomine facilitated amphetamine-induced rotation. Agonist and antagonist potencies were then compared with M1 and M2 binding site affinities estimated by displacing [3H]pirenzepine from forebrain and [3H]QNB from brainstem homogenates. The data suggest a relationship between antagonist potency and M2 binding site affinity.

Cholinergic stimulation of drinking from the lateral hypothalamus: indications for M2 muscarinic receptor mediation

Available evidence suggests that muscarinic receptor binding sites may exist in at least two heterogenous subclasses (M1 and M2), distinguished by their affinity for the antagonist pirenzepine. In order to evaluate the role of these receptors in comsummatory behaviour a series of conventional and putatively receptor selective drugs were tested for their effects on water consumption following injection (0.5 microliter/30 sec) into the perifornical hypothalamic area of non-deprived rats. Of the conventional agonists tested, carbachol and oxotremorine were approximately equipotent and arecoline was about 16 X weaker. Of the putative M1 agonists tested, pilocarpine was about 50 X weaker than carbachol and the remainder (MCNA343, AHR602, AH6405) were inactive. Inhibition of carbachol (1 microgram) induced drinking was subsequently measured. The most potent inhibition was found using scopolamine, a non selective antagonist. 4-DAMP was approximately 7 X weaker than scopolamine, but was more potent than the putative M1 antagonists pirenzepine, telenzepine or dicyclomine. In a separate series of experiments the affinity of these drugs for [3H]pirenzepine forebrain receptors (M1) and [3H]QNB brainstem receptors (M2) was determined to confirm their receptor binding selectivity. No systematic relationship was found between agonist potency and M1 or M2 affinities. M2 receptor involvement was indicated by the antagonist data which show a close relationship between rank potency order and M2 receptor affinity. An important role for M1 receptors is excluded by the absence of a clear relationship between potency order and M1 affinity. The data therefore suggest an important role for M2 receptors in mediating drinking stimulated by muscarinic receptor activation.

Blockade of spatial learning by the M1 muscarinic antagonist pirenzepine

Two experiments were conducted to determine the effects of the M1 muscarinic receptor antagonist pirenzepine on place navigation in a water maze. In the first experiment rats were required to learn the location of a hidden platform following intracerebroventricular injections of equimolar doses of pirenzepine or scopolamine methylbromide. Both drugs dose-dependently impaired spatial learning according to both escape latency data and transfer test analysis. Pirenzepine was approximately 3 times less potent than scopolamine, a potency ratio which suggests M1 receptor mediation of the impairment. In the second experiment pirenzepine (1 approximately 92.3 micrograms/rat ICV) was injected prior to training on a simultaneous place discrimination task in the water maze. Impairments of choice accuracy were found with a dose of 20 micrograms/rat in the absence of any marked increases in either errors of omission or choice latency. These data suggest that M1 receptor blockade impairs processes which are involved in spatial learning.

Lack of task specificity and absence of posttraining effects of atropine on learning

Three experiments are reported whose purpose was to examine the effect of the cholinergic antagonist atropine on the acquisition of different learning tasks known to be sensitive or insensitive to impairment by hippocampal lesions; on the retention of performance acquired in the absence of the drug; and on memory consolidation immediately after daily training trials. In Experiment 1, atropine sulfate (10 or 50 mg/kg, ip), injected 30 min prior to training, severely impaired learning of both spatial and nonspatial discrimination tasks when compared with saline or atropine methylnitrate (50 mg/kg). In Experiment 2, atropine sulfate (50 mg/kg) also impaired spatial discrimination accuracy in rats previously trained to asymptote under drug-free conditions. These deficits were not due to either peripheral drug effects or gross sensorimotor impairments. In Experiment 3, daily posttraining injections of atropine sulfate (50 mg/kg) failed to influence either learning or subsequent retention of place navigation in rats that were trained to find a single hidden escape platform. The data confirm that profound learning deficits occur when training is conducted under atropine but offer no support to the hypothesis that cholinergic neurons play an important role in memory consolidation or other posttraining processes. Furthermore, these results point to dissimilarities between the behavioral impairments induced by cholinergic blockade and hippocampal lesions under appropriate test regimes.

Lack of task specificity and absence of posttraining effects of atropine on learning

Three experiments are reported whose purpose was to examine the effect of the cholinergic antagonist atropine on the acquisition of different learning tasks known to be sensitive or insensitive to impairment by hippocampal lesions; on the retention of performance acquired in the absence of the drug; and on memory consolidation immediately after daily training trials. In Experiment 1, atropine sulfate (10 or 50 mg/kg, ip), injected 30 min prior to training, severely impaired learning of both spatial and nonspatial discrimination tasks when compared with saline or atropine methylnitrate (50 mg/kg). In Experiment 2, atropine sulfate (50 mg/kg) also impaired spatial discrimination accuracy in rats previously trained to asymptote under drug-free conditions. These deficits were not due to either peripheral drug effects or gross sensorimotor impairments. In Experiment 3, daily posttraining injections of atropine sulfate (50 mg/kg) failed to influence either learning or subsequent retention of place navigation in rats that were trained to find a single hidden escape platform. The data confirm that profound learning deficits occur when training is conducted under atropine but offer no support to the hypothesis that cholinergic neurons play an important role in memory consolidation or other posttraining processes. Furthermore, these results point to dissimilarities between the behavioral impairments induced by cholinergic blockade and hippocampal lesions under appropriate test regimes.

Vasopressin prolongs bradycardiac response during orientation

Adult male rats were implanted with transcutaneous ECG electrodes and habituated to a dark chamber with elevated background noise levels. ECG was recorded prior to, immediately after, and 3 min after sudden elimination of background noise. The orienting response to the stimulus offset was accompanied by transient bradycardia. Neither AVP (1 microgram/rat) nor oxytocin (1 microgram/rat) injected subcutaneously 1 hr prior to testing altered baseline heart rate or the immediate bradycardiac response to stimulus offset. However, AVP, and to a lesser extent oxytocin, prolonged the bradycardia induced by stimulus offset. The results show that neurohypophyseal peptide hormones enhance the cardiovascular component of orienting to stimulus change.

Post training lysine-vasopressin injections increase conditioned suppression in rats

Male rats were trained to lever press for food reward on a variable interval schedule of reinforcement. When stable response rates had been achieved they were trained to avoid footshock (UCS) in a two way shuttle box using a compound conditioning stimulus (CS) of tone and light. Having reached the learning criterion of ten consecutive avoidance responses they were returned to their home cage and injected with saline or lysine vasopressin (LVP 1 microgram/rat/SC) after 30 min. Twenty four hours later, the suppressive effect of the avoidance CS on the appetitive baseline was tested. Rats which had been injected with LVP after avoidance training showed significantly more suppression of the operant response than saline controls. The results are discussed in terms of the behavioural substrate underlying the long term effects of vasopressin on behaviour.

The effects of endorphins on cardiac responses during an emotional stress

Male rats were punished (0.6 mA/2 sec) for entering the shock compartment of a passive avoidance chamber. Twenty four hours later they were returned to the apparatus and confined to the grid floor of the shock compartment for 5 min (forced exposure). The bradycardic response found in control rats during forced exposure was not altered by either alpha-endorphin (beta-LPH 61-76) or (des-tyrl)-alpha-endorphin (beta-LPH 62-76) (1.5 micrograms/rat) injected subcutaneously 60 min prior to forced exposure. In contrast (des-tyrl)-gamma-endorphin (beta-LPH 62-77) (1.5 micrograms/rat/SC) enhanced the bradycardic response of shocked rats during forced exposure without altering heart rate in non-shocked rats. (Des-tyrl)-gamma-endorphin did not affect the rats' bradycardic response to a sudden reduction in background noise. The results conform to a pattern whereby C-terminal beta-LPH fragments may be distinguished in a number of behavioural tests and show that (des-tyrl)-gamma-endorphin treatment facilitates the rat's cardiac response to aversive but not novel experiences.

The effects of central catecholamine depletions on spatial learning in rats

The role of forebrain catecholamine systems in spatial learning was assessed using a recently described place navigation task. Rats were required to learn the spatial location of a platform hidden 1 cm below the surface in order to escape from a large pool filled with opaque water. Circumscribed destruction of dopamine or noradrenaline neurones was achieved with bilateral stereotaxic injections of 6-hydroxydopamine. Rats with neostriatal dopamine depletions showed regulatory impairments, mild akinesia and increased latencies to escape during training on both the spatial task and on a non-spatial control task. However their normal transfer test performance indicated that their deficit was not due to impaired spatial learning. Similarly, spatial learning and transfer test performance were unimpaired by mesocorticolimbic dopamine depletions or by extensive noradrenaline loss in cortex and hippocampus following dorsal noradrenergic bundle lesions. These findings suggest that considerable spatial learning can occur in the absence of each of the major telencephalic catecholamine systems.

Lysine vasopressin fails to alter (3H)-noradrenaline uptake or release from hippocampal tissue in vitro

Lysine vasopressin (5.2 and 10.4 microM) failed to significantly alter the spontaneous release of (3H)-noradrenaline from hippocampal slices in vitro or the release observed after stimulation with potassium chloride (15 and 25 mM). Uptake of (3H)-noradrenaline by synaptosomes prepared from hippocampal tissue was not significantly changed by incubation with vasopressin (5.2 and 10.4 mu M). Our results fail to support the hypothesis, derived from behavioural and in vivo biochemical studies, that vasopressin modulates the activity of noradrenergic nerve terminals in this part of the limbic system.

Hippocampal electrical activity and gamma-aminobutyrate metabolism in brain tissue following administration of homocysteine

The concentration of gamma-aminobutyrate (GABA) and the activity of glutamate decarboxylase and GABA-transaminase were measured in extracts of mouse brain before the onset and during the course of generalized seizures induced by systemic administration of homocysteine thiolactone. The results indicate that whole brain GABA metabolism is unaffected by subconvulsive and convulsive doses of homocysteine at all stages of the generalized seizure. Electroencephalographic monitoring of rat brain electrical activity via hippocampal electrode implantation allowed the course homocysteine-induced seizures to be followed and afforded a means of quantifying such seizures.