Mechanism of dexamphetamine-induced mydriasis in the anaesthetized rat

The Effects of Systemic Alpha-1 Adrenergic Antagonists on Pupil Diameter in Rats

PURPOSE

To investigate the effects of alpha-1 adrenergic blockers on pupil diameter (PD) in rats.

METHODS

Four groups, with 10 rats each, were designed to receive terazosin, tamsulosin, doxazosin, and no medication. Dilated pupil diameter (PD) measurements were performed 24 hr before, 24 hr after, and 30 days after the initiation of medication, and after 30 days of washout. The intergroup and intragroup differences in PD were evaluated using one-way ANOVA and Wilcoxon signed rank test, respectively.

RESULTS

In day 1, PD decreased in both eyes significantly only in tamsulosin and doxazosin groups, but these effects became insignificant at 30 days of treatment (p > 0.05). The control group showed no significant difference in PD (p > 0.05). PD values returned to baseline after the washout period in all groups.

CONCLUSIONS

A significant reduction in PD occurred in two of the three groups with alpha-1 adrenergic blockers (tamsulosin and doxazosin), but this effect was not sustained at 30 days. Further functional and structural studies of the iris are needed to determine the clinical significance of these findings.

Rilmenidine produces mydriasis in cats by stimulation of CNS alpha2-adrenoceptors

1. Experiments were undertaken to determine if the imidazoline/alpha2-adrenoceptor agonist, rilmenidine, would produce mydriasis in cats and, if so, to delineate its site of action and determine if this effect is mediated by imidazoline receptors or alpha2-adrenoceptors. 2. Rilmenidine produced dose-related pupillary dilator responses in pentobarbital anaesthetized cats that were independent of sympathetic innervation to the iris but were dependent upon intact parasympathetic neuronal tone. The ED50 for rilmenidine-induced pupillary dilation was approximately 200 microg kg(-1), i.v., and was sustained for at least 1 h. 3. The highly selective alpha2-adrenoceptor antagonist, RS-79948, administered either before or after rilmenidine, antagonized rilmenidine-induced mydriasis. Neuronally induced reflex inhibition of parasympathetic nerve activity was also inhibited by administration of RS-79948. 4. These results suggest that rilmenidine acts like clonidine to produce pupillary dilation by inhibition of parasympathetic tone to the iris sphincter and that this central nervous system parasympatho-inhibition is mediated by alpha2-adrenoceptors, rather than imidazoline receptors.

Kinins and peritoneal exudates induced by carrageenin and zymosan in rats

1. Kinins were measured by a radioimmunoassay in the inflammatory exudates induced by carrageenin or zymosan in the peritoneal cavity of normal Wistar rats and of kininogen-deficient Brown Norway rats. 2. After administration of carrageenin to normal rats, levels of immunoreactive kinins showed a single peak during the first two hours and then decreased. The presence of kinins preceded and accompanied the exudation of 125I-labelled albumin. Kinins were identified as bradykinin by chromatography. 3. Captopril, an inhibitor of kininase 2, increased the level of kinins and the volume of the exudates after carrageenin treatment. In Brown Norway rats, the volume of the exudates was small and contained little or undetectable amounts of immunoreactive kinins. 4. During zymosan-induced peritonitis, the exudates were devoid of immunoreactive kinins in both species. The volume of the exudates was larger in kininogen-deficient rats than in normal rats. 5. We conclude that in rats, the kinin system is a major factor responsible for the development of the inflammatory reactions induced by carrageenin, but is not involved in the reactions induced by zymosan.

Modulation by antidepressant drugs of CNS postsynaptic alpha 2-adrenoceptors mediating mydriasis in the rat

The modulation of central postsynaptic alpha 2-adrenoceptors mediating mydriasis in the pentobarbitone-anaesthetized rat was studied after the acute and short/long-term administration of antidepressant treatments (drugs, electroshock). The acute administration of cyclic antidepressant drugs (2.5 mg/kg, i.v.) resulted in different mydriatic effects (amitriptyline greater than protriptyline approximately imipramine greater than clomipramine greater than nortriptyline greater than desipramine approximately maprotiline) which were attenuated (17-55%) by idazoxan (1 mg/kg, i.v., 5 min) and reserpine (5 mg/kg, s.c., 18 h) indicating that, besides the well-known anticholinergic properties of some of these drugs, their mydriatic effects are due in part to activation of alpha 2-adrenoceptors (through endogenous noradrenaline). In contrast, the long-term (7-21 days) but not the short-term (1-4 days) administration of cyclic antidepressant drugs (2.5-10 mg/kg, i.p.), MAO inhibitors (1 mg/kg, i.p.), lithium (20 mg/kg, i.p.) and electroshock (150 mA, 63 Hz, 8 ms during 300 ms) resulted in dose- and time-dependent reductions of the dose-pupillary response curve for clonidine (ED50 increased 1.2-2.0-fold; Emax decreased by 9-29%) which indicated desensitization of postsynaptic alpha 2-adrenoceptors. In line with these findings, treatment for 7 days with clonidine (0.1-1 mg/kg, i.p.) or idazoxan (3-10 mg/kg, i.p.) led to an opposite modulation (down- and up-regulation) of the dose-pupillary response curve for clonidine. The main results demonstrate that cyclic antidepressant drugs, through indirect mechanisms which involve endogenous noradrenaline, can modulate the sensitivity of brain postsynaptic alpha 2-adrenoceptors mediating mydriasis in the rat.