Tseggai Gherezghiher

Clonidine mydriasis in the rat

Pupillary responses to clonidine (3--100 micrograms/kg, i.v.) and epinephrine (1--30 micrograms/kg, i.v.) were observed in anesthetized rats. Clonidine caused a dose-dependent mydriasis which was effectively antagonized by pretreatment with yohimbine (1.5 mg/kg, i.v.). Pretreatment with phentolamine (5 mg/kg, i.v.) was less effective in antagonizing this clonidine-induced mydriasis. Phenoxybenzamine (2 mg/kg, i.v.) was almost without effect. In contrast, both phentolamine and phenoxybenzamine blocked the pupillary dilation produced by epinephrine while yohimbine pretreatment resulted in no antagonism of epinephrine-induced mydriasis. These results suggest that clonidine-induced mydriasis in the rat is mediated by a central adrenergic inhibitory mechanism.

CNS adrenergic inhibition of parasympathetic oculomotor tone

Inhibition of parasympathetic neural tone to the iris was produced by electrical stimulation of the afferent sciatic nerve, medullary reticular formation, and posterior hypothalamus in anesthetized cats in which only the parasympathetic nerves to the eye were intact. Stimulation of all 3 sites of activation produced a graded pupillary dilation and reduction of tonic nerve activity in the short ciliary nerves. Intravenous administration of the alpha-2-adrenoceptor antagonist, yohimbine hydrochloride, (0.03-1.0 mg/kg) produced a dose-dependent antagonism of the mydriasis elicited by activation of the ascending (sciatic nerve and medullary) mechanisms but did not block the pupillary dilation evoked by stimulation of the system descending from the hypothalamus. This differential action of yohimbine was confirmed directly by means of nerve recordings taken from the parasympathetic nerve to the eye. Depletion of CNS monoamines with reserpine and alpha-m-p-tyrosine reduced the norepinephrine concentration of the medulla and midbrain by 95% and 97%, respectively. In these depleted preparations, stimulation of the hypothalamus still produced the characteristic mydriasis and inhibition of parasympathetic tonic activity whereas activation of ascending mechanisms (sciatic or medullary) were no longer effective in producing these effects. Taken together, these results suggest that ascending parasympatho -inhibition is mediated by a monoamine (probably norepinephrine) and that inhibition descending from the hypothalamus is mediated by a non-monoaminergic mechanism.

Parasympathetic nervous control of intraocular pressure

Electrical or chemical activation of the oculomotor nucleus (ONC) was performed in pentobarbital anesthetized cats to determine the role of parasympathetic nervous input to the eye in modulating intraocular pressure (IOP). In all animals, the vagosympathetic nerve trunks were sectioned bilaterally at the mid-cervical level. Intracranial stimulation of the ONC produced miosis and a bilateral sustained rise in IOP with little or no cardiovascular response. During stimulation, IOP increased by approximately 35-40%, with an additional small rise seen when the current was turned off. The secondary rise in IOP probably represents the actual pressure level reached which was otherwise masked by intense accommodation due to stimulation. The rise in IOP was not antagonized with gallamine triethiodide. However, both IOP and pupillary responses were blocked by either hexamethonium (1.5 mg kg-1, i.v.) or atropine (0.5 mg kg-1, i.v.). These results provide evidence for a neural parasympathetic mechanism for increasing IOP in cats. In another group, activation of ONC following electrolytic lesion of the preganglionic parasympathetic nerve to one eye, produced a rise in IOP and miosis only on the side with intact oculomotor nerve indicating that the rise in IOP is in part mediated by nerve fibers that travel to the eye along the oculomotor nerve trunk. Microinjection of L-glutamate (2.5 x 10(-7) M) into the ONC produced a bilateral rise in IOP that lasted for more than 15 min. This increase in IOP was greater than 50% of control levels and was accompanied with miosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the mechanism of methyl-Dopa-induced mydriasis in the cat

Summaryalpha-methyl-Dopa (10–100 mg/kg, i.v.) produced a dose-dependent mydriasis in cats anaesthetized with pentobarbital (30 mg/kg, i.p.). The onset was gradual, reaching a maximum plateau in 2–2.5 h. Intracerebroventricular administration of 1 or 3 mg of alpha-methyl Dopa (MD) also produced pupillary dilation with a similar time course. These dosages were without effect when given intravenously. Pretreatment with the alpha2-adrenoceptor antagonist, yohimbine (0.5 mg/kg, i.v.), blocked the pupillary response to MD. The alpha1-adrenoceptor antagonist, prazosin (1.0 mg/kg, i.p.), was ineffective. Selective enzymatic blockade with 3-hydroxy-benzyl-hydrazine (NSD-1015; 25 mg/kg, i.p.), a Dopa-decarboxylase enzyme inhibitor, as well as with bis (4-methyl-homopiperazinyl-thiocarbonyl) disulfide (FLA-63; 2.5 mg/kg, i.p.), a dopamine-beta-hydroxylase blocker, prevented the mydriatic effect of MD. These results support the hypothesis that MD produces a clonidine-like, CNS mediated mydriasis in the cat, primarily by action of its metabolite alpha-methyl-noradrenaline acting on alpha2-adrenoceptors.

α-adrenoceptor activation of nictitating membrane and iris in cats

SummaryIntra-arterial administration of (+)- and (−)isomers of adrenaline and noradrenaline produced doserelated contraction of the nictitating membrane (NM) and dilation of the pupil in anesthetized cats. The relative potencies were (−)-adrenaline > (+)-adrenaline = (−)-noradrenaline > (+)-noradrenaline. Observations of the effects of α-adrenoceptor antagonists on (−)-noradrenaline activation of these two effectors were made simultaneously. All of the α1-adrenoceptor antagonists tested produced a doserelated blockade of the NM with the relative potencies being prazosin > WB-4101 > phentolamine > phenoxybenzamine. In contrast, the iris dilator was blocked by WB-4101 and phenoxybenzamine but was refractory to antagonism by doses of prazosin and phentolamine that reduced the (−)-noradrenaline evoked NM response by 75–80% in the same animals. The α2-adrenoceptor antagonist, yohimbine, produced significant inhibition of the NM only at high dose (1 mg/kg) but even at this level had no effect on pupil diameter. These results suggest that activation of the NM by exogenous noradrenaline is due solely to stimulation of α1-adrenoceptors. α2-adrenoceptors do not seem to significantly contribute to noradrenaline induced activation of either the NM or iris dilator muscle in vivo. In contrast, the α-adrenoceptors on the iris dilator muscle that are stimulated by exogenous noradrenaline can not easily be classified pharmacologically as either α1- or α2-adrenoceptors.

Alpha-methyl-DOPA induced mydriasis in the cat. Relationship between pupillary response and the oculomotor perfusate concentration of methyldopa and its metabolites.

SummaryIn pentobarbital anaesthetized cats, intravenous administration of 30 mg/kg, alpha-methyl-DOPA produced mydriasis that reached a maximum plateau in 2–2.5 h. The oculomotor nucleus was perfused with saline using a pushpull cannula system chronically implanted over the nucleus. Perfusate samples were collected and subsequently analyzed by liquid chromatography with electrochemical detection (LC-EC). Alpha-methyl-DOPA administration resulted in a gradual build up of alpha-methyl-noradrenaline to approximately 32 μmoles over the 3 h sampling period. In contrast, the concentration of alpha-methyl-dopamine was below the detection level for the first 90 min with peak leves of less than 6 μmoles after 3 h. A linear regression analysis demonstrated a negative correlation (R=0.90) between the pupil size and the perfusate concentration of alpha-methyl-DOPA and a positive correlation for both alpha-methyl-dopamine and alpha-methyl-noradrenaline (R=0.88 and 0.94 respectively). Pretreatment with the DOPA-decarboxylase inhibitor, 3-hydroxy-benzyl-hydrazine (NSD-1015; 25 mg/kg, i.p.) completely blocked the mydriatic response to alpha-methyl-DOPA, with neither alpha-methyl-dopamine nor alpha-methyl-noradrenaline reaching detectable levels in the oculomotor perfusate. After pretreatment with the dopamine-beta-hydroxylase inhibitor, bis (4-methyl-homopiperazinyl thiocarbonyl) disulfide (FLA-63; 2.5 mg/kg, i.p.) there was a significant accumulation of alpha-methyl-dopamine when compared to that obtained in the alpha-methyl-DOPA controls but with no apparent alpha-methyl-dopamine related pupillary dilation. However, the correlation between alpha-methyl-noradrenaline concentration and the increase in the diameter of the pupil was maintained. The results of the pupillary response-neurochemical correlation studies conclusively show that pupillary responses to alpha-methyl-DOPA administration are not correlated with alterations of endogenous amine concentration, nor to the CNS levels of alpha-methyl-DOPA or alpha-methyl-dopamine. There is however, a very high and consistent correlation between alpha-methyl-noradrenaline oculomotor nucleus perfusate concentration and pupil size. As the correlation was maintained after three different doses of alpha-methyl-DOPA as well as after the administration of selective blockers of the enzymes involved in catecholamine biosynthesis, these results suggest that alpha-methyl-noradrenaline is the active metabolite responsible for the mydriatic effect of alpha-methyl-DOPA in the cat.

Quantitative assessment of ocular norepinephrine concentration in the rabbit using liquid chromatography with electrochemical detection

An assay technique that utilizes liquid chromatography with electrochemical detection to determine the norepinephrine (NE) concentration of the rabbit aqueous humor (AH) and iris-ciliary body (ICB) is discussed. NE concentration was monitored in control preparations as well as in experiments involving stimulation of the cervical sympathetic nerve (CSN) or 24 h following extirpation of the superior cervical ganglion. In 15 control rabbits, the baseline AH NE concentration was determined to be 7.2 and 7.8 ng/ml in the left and right eyes, respectively. Prolonged unilateral electrical stimulation of the CSN increased the AH NE concentration to 59.0 +/- 1.2 ng/ml on the stimulated side. Pretreatment with cocaine (12.5 mg/kg) and subsequent stimulation of the CSN increased the NE in the AH to 93.7 +/- 1.3 ng/ml on the stimulated side, and raised the baseline AH NE of the control (non-stimulated) side to 25.9 +/- 5.0 ng/ml. The baseline concentration of NE in the ICB was 5.4 +/- 0.8 micrograms/g. 24 h after a unilateral removal of the superior cervical ganglion, the NE content of the ICB was depleted by more than 90% in the ganglionectomized eye with no apparent change in the level of NE in the AH. The accurate analysis of catecholamines in the AH, ICB and other parts of the eye may be useful in providing information as to the extent of innervation and the possible role of sympathetic nerve supply in the overall control of AH dynamics.(ABSTRACT TRUNCATED AT 250 WORDS)

Design of New Contact Lens for YAG Laser Filtering Procedures

A contact goniolens has been developed specifically for YAG laser filtering procedures both for use in unblocking failed surgical filtering procedures and for performing two-stage or one-stage YAG filtering procedures. The design concentrates the YAG energy and maintains a constant small angle with the cornea.

Ocular effects of dipivalyl esters of epinephrine and α-methylepinephrine

Abstract The intraocular pressure (IOP) and pupillary effects of dipivalyl esters of epinephrine and α-methylepinephrine were compared after topical application in conscious rabbits. Both dipivalyl-α-methylepinephrine (DPαmeE) and dipivalylepinephrine (DPE) produced a dose-dependent pupillary dilation (PD) and decrease in IOP. The onset of PD was approximately 30 min for both agents and reached maximal plateau within 1 and 2 hr for DPE and DPαmeE, respectively. Duration of mydriatic effect was also dose-related, although more prolonged with DPαmeE. This probably reflects differences in rate of inactivation of these compounds. The onset of IOP lowering effect of DPαmeE was more rapid (35–45 min) when compared with DPE (1·5-2 hr) which may be due to the initial ocular hypertensive response seen with DPE. The initial rise in IOP was prevented by transection of three rectus muscles. DPαmeE produced initial ocular hypertension only at the highest doses. The decrease of IOP lasted more than 6 hr for both drugs, returning to normal by 24 hr. No pupillary or IOP effects were seen in the contralateral eye. Denervation supersensitivity to both the pupillary and IOP responses to DPαmeE was seen after superior cervical ganglionectomy. These findings are consistent with the hypothesis that the pupillary and IOP responses to DPαmeE do not require intact adrenergic innervation to the eye, and that these effects are mediated by activation of postjunctional α-adrenoceptors. It is concluded that DPαmeE is a potent adrenergic ocular hypotensive agent. In comparison to DPE, DPαmeE has a more rapid onset, longer duration of action, less pupillary effect, and produces ocular hypotension with less initial ocular hypertension.

Synaptic Organization in the Oculomotor Nucleus

The interpretation of neurochemical analysis in the intact brain is facilitated by utilizing a well-defined system. One of the better defined and experimentally approachable central autonomic regulating systems is that controlling the pupil. Physiological, anatomical, and pharmacological studies will be briefly described to establish that there is an adrenergic synapse in the oculomotor nucleus of the cat. Implantation of a push-pull cannula permits measurement of both endogenous and exogenous neurochemicals in the oculomotor nucleus under various experimental conditions. The kinetic analysis of these measurements suggests certain features about that synapse for comparison with the traditional adrenergic synaptic organization.