Pharmacological studies on pupillary reflex dilatation

Alpha2-adrenoceptors do not mediate reflex mydriasis in rabbits

The aim of this study was to identify receptors that mediate reflex mydriasis in pentobarbital-anesthetized rabbits, in which the cervical sympathetic nerve was sectioned unilaterally. Voltage-response curves of pupillary dilation were generated bilaterally by stimulation of the sciatic nerve. Evoked mydriatic responses were mediated mainly by efferent parasympathetic innervation, and, to a lesser extent, by sympathetic innervation. The alpha-adrenergic antagonist, phenoxybenzamine (0.3 mg/kg, intravenously (i.v.)), antagonized mydriasis of the neurally intact eye, but not that on the sympathectomized side. The alpha2-adrenergic antagonist, RS 79948 (0.3 mg/kg, i.v.), potentiated mydriasis of the normal eye, but was without either a potentiating or inhibitory effect on the mydriasis of the sympathectomized eye. In addition, the dopamine-receptor antagonist, haloperidol (1 mg/kg, i.v.), inhibited evoked mydriasis of the sympathectomized eye. These results suggest that, unlike some other species (cats and rats), alpha2-adrenoceptors do not mediate reflex mydriasis elicited by sciatic-nerve stimulation in the rabbit, and support the previous finding in humans that dopamine receptors may mediate this response.

Cutaneous afferents producing a reflex pupil dilation in anesthetized rats

Cutaneous afferents producing a reflex pupil dilation were examined using natural mechanical stimulation of the hindlimb skin and electrical stimulation of a sural nerve in anesthetized, artificially ventilated rats. Pupil diameter was continuously recorded after magnification using a microscope connected to a charge coupled device camera. Innocuous brushing, or weak pressing of the skin, did not have any effect on pupil diameter, while pressing the skin more than 720 g/cm2 produced a pressure-dependent pupil dilation. Pinching rather than pressing the skin induced the larger pupil dilation. Electrical stimulation of a sural afferent nerve with weak intensity, which was supra-threshold for Abeta-afferents and sub-threshold for Asigma-afferents, induced a reflex pupil dilation. This dilation continued to increase with further increases in stimulus intensity which involved excitation of Asigma afferents and C afferents. It is concluded that Abeta, Asigma and C afferents in the skin can work as afferents in eliciting reflex pupil dilation in anesthetized rats.

Neural mechanism of pupillary dilation elicited by electro-acupuncture stimulation in anesthetized rats

The neural mechanisms to reflex dilation elicited by electro-acupuncture stimulation were investigated in anesthetized rats. Two needles, with 160 microns diameter and about 5 mm apart, were inserted into the skin and underlying muscle of a hindpaw. Repetitive 20 Hz, 0.5 ms electrical pulses at various intensities were used for stimulation for 30s. The pupil size was magnified about 44 times via a microscope and was continuously recorded on a videotape. Electro-acupuncture stimulation at more than 0.5 up to 6 mA induced stimulus intensity-dependent pupil dilation. These responses were abolished by the severance of the sciatic and saphenous nerve of the stimulated hindlimb. Compound action potentials were recorded from the distal cut end of the tibial of a saphenous nerve following electro-acupuncture stimulation of the hindpaw. The mean threshold of the compound action potentials of the myelinated fibers in saphenous nerves was 0.18 mA, while that of unmyelinated fibers was 3.0 mA. The mean threshold of the compound action potentials of the myelinated fibers in the tibial nerve was 0.20 mA of unmyelinated fibers was 3.3 mA. Severance of bilateral trunks did not affect the response, while severance of the third cranial nerves abolished the responses. In conclusion, electro-acupuncture stimulation applied to the hindpaws of the anesthetized rats induced excitation of myelinated or of both myelinated and unmyelinated afferent fibers of the tibial and saphenous nerve, and involved a reflex response of pupil dilation through the third cranial parasympathetic efferent nerve.

Interactions of apomorphine and a novel neuroleptic dibenzodioxazocine derivative, as evidenced by changes of somato-autonomic reflexes and spontaneous sympathetic activity in cats

The apomorphine-antagonistic effects of EGYT-2509, a novel neuroleptic compound, has been studied by two different methods suitable for investigating the dopaminergic modulation of sympathetic output. (1) In cats lightly anaesthetized with urethane (600 mg kg-1 i.p.), blood pressure (BP) reflexes evoked by electrical stimulation of the sciatic nerve were inhibited by apomorphine (0.2 mg kg-1 i.v.) at low frequencies of stimuli (2-8 Hz), while the BP reflexes were facilitated by apomorphine at higher frequencies of stimulation; the evoked contractions of the nictitating membrane were depressed in the entire range of frequencies applied. EGYT-2509 (1.5 mg kg-1 i.v.) antagonized both the inhibition and facilitation of pressor reflexes induced by apomorphine. EGYT-2509, given alone, in doses exceeding 1.5 mg kg-1 either did not influence or inhibited the responses of nictitating membrane and of BP; the inhibition could be antagonized by haloperidol. The apomorphine-induced sustained hypotension was inhibited by EGYT-2509 (18.5 mg kg-1): after EGYT-2509, higher doses of apomorphine (0.7 vs 0.2 mg kg-1) were required for the effect. Sustained hypotension could be elicited by EGYT-2509, too; after apomorphine, smaller doses of EGYT-2509 (8.5 vs 18.5 mg kg-1) were enough to decrease BP. (2) In cats anaesthetized with chloralose and urethane (50 and 400 mg kg-1 i.p., respectively), apomorphine (0.2 mg kg-1) inhibited the spontaneous activity of the postganglionic renal sympathetic nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

The pupillary effects of opioids

Morphine's miotic action on the pupil is an easily recognizable and quantifiable effect in man. The neural pathways responsible for regulating pupil size are reasonably well defined. Yet, the mechanisms behind this and related effects of opioids on the eye in humans and laboratory animals have just begun to be explored. In this review, we have attempted to organize the available information on pupillary actions of opioids, emphasizing the dynamic nature of the responses, their species specificity, possible mechanisms of action, and the recently discovered development of tolerance to these actions. Our current knowledge regarding differences among the opioids, the effects of endogenous opioid peptides and the role of the various opiate receptor subtypes in pupillary effects is also summarized.