Acta Oto-Laryngologica
ISSN: 0001-6489 (Print) 1651-2251 (Online) Journal homepage: http://www.tandfonline.com/loi/ioto20
The Vestibulo-Ocular Reflex in Newborn Rats Ian S. Curthoys To cite this article: Ian S. Curthoys (1979) The Vestibulo-Ocular Reflex in Newborn Rats, Acta Oto-Laryngologica, 87:3-6, 484-489, DOI: 10.3109/00016487909126456 To link to this article: http://dx.doi.org/10.3109/00016487909126456
Published online: 08 Jul 2009.
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Citing articles: 3 View citing articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ioto20 Download by: [University of California, San Diego]
Date: 29 February 2016, At: 13:56
Acta Otolaryngol 87: 484489, 1979 THE VESTIBULO-OCULAR REFLEX IN NEWBORN RATS Ian S. Curthoys From the Department of Psychology, University of Sydney, Sydney, N e w South Wales, Australia
Downloaded by [University of California, San Diego] at 13:56 29 February 2016
(Received August 8, 1978)
Abstract. To study the development of vestibular functioning, the vestibulo-ocular reflex was tested in rats of increasing age. High frequency electrical stimulation of the vestibular nerve elicited characteristic conjugate eye movements in newborn rats well before the eyes were open (about day 15 after birth) and before the semicircular canals had reached full adult size (about day 22 after birth). The threshold currents decreased during growth.
Studies of slow eye deviations in response to head angular acceleration have shown that the vestibulo-ocular reflex is functioning at birth in cat, guinea pig and man (Carmichael, 1934; Fish & Windle, 1932; Galebsky, 1927; Groen, 1963; Tibbling, 1969). However, in each of these species the gross morphology of the semicircular canals is already adult size by birth and a considerable proportion of fine vestibular receptor and afferent development has also occurred (Altmann, 1950; Bast & Anson, 1949; Elliott & Elliott, 1964; Heywood, Pujol & Hilding, 1976; Romand et al., 1976; Sato, 1903; Curthoys, unpublished observations). In order to gain detailed understanding of the entire course of vestibular functional development in relation to anatomical growth changes, the ideal preparation would appear to be one whose semicircular canal system is not fully developed at birth. In an extensive study of such a species, the pike, ten Kate (1969) found that the minimum thresholds for the slow phase of post-rotatory eye deviations were independent of the animal size. Indeed, he was able to elicit these deviations in the smallest pikes studied. Results from the pike are difficult to apply to mammals because, unlike mammals, the dimensions of the pike A ( I(I O t o l o r \ n ~ o l 8 7
labyrinth continue to increase throughout life. Although rarely used in vestibular research, the rat seems to be a good candidate for studying the development of vestibular functioning. Clark (1973) has shown that the rat labyrinth is not mature at birth. The gross morphological dimensions continue to increase for about 22 days after birth. Other studies have shown that horizontal semicircular canal primary afferents in newborn rats have a lower average resting rate and are less sensitive than comparable neurons in adults (Curthoys, 1977, 1978). The aim of the present experiment was to determine whether the vestibulo-ocular reflex could be elicited in newborn rats and if so, whether the eye movements were conjugate. It is stressed that this study was only concerned with the slow compensatory eye movements: the important related question of the development of the quick phase of vestibular nystagmus was not addressed. The problem was studied by using high frequency electrical stimulation of the vestibular nerve, measuring the threshold current needed to elicit eye movements in lightly anesthetized rats of various ages. In adult cats and monkeys, high frequency electrical stimulation of branches of the vestibular nerve produce “compensatory” eye movements in characteristic planes and directions which depend upon the branches of the nerve stimulated (Cohen & Suzuki, 1963; Cohen et al., 1964). It was considered that use of direct electrical stimulation of the vestibular nerve in this study would bypass any developmental
Downloaded by [University of California, San Diego] at 13:56 29 February 2016
Vestibulo-ocular reflex in newborn rats
changes in the receptor mechanism and may indicate the stage of development of the more central neuronal pathways and ocular motoneurons. A related consideration is that the rats' eyes do not open until well after birth. It seemed possible that the development of the vestibuloocular reflex may be more closely locked to eye-opening, prior to which the image-stabilizing function of the reflex would be unnecessary. Accordingly, we also sought to date accurately the time of eye-opening in rats. This information may also be important in studies of developmental changes in visual-vestibular interaction.
MATERIALS AND METHODS
To measure day of eye-opening, birth-dated albino Wistar rats were examined and weighed on successive days where possible. Day of eye opening was taken as the day on which at least one eye was open. Sixty-one albino rats were used for the eye movement experiment. Five were classed as adult rats and weighed in excess of 170 g; the remaining 56 were newborn and young rats ranging in weight from 4.4 to 35.4 g. These weights correspond to ages of from 0 to about 23 days after birth. Each animal had a tracheal cannula inserted under ether anesthesia and was maintained under ether for the remainder of the experiment. ECG was monitored continuously to ensure depth of anesthesia was adequate. The animals were placed in specially designed head-holders in a Kopf stereotaxic device which was mounted of turntable. Adult rats were held by a modified palate clamp. For young rats the head was held by a machine screw cemented to the skull by cyanoacrylate glue and dental cement. The techniques for preparation of very young rats have been described in detail elsewhere (Curthoys & Webber, 1977). For animals of all ages the head was pitched nose down by about 45" to 32-792951
485
ensure the horizontal canal was in an earthhorizontal plane. Body temperature was maintained by a heating pad under the animal. In each rat the lateral half of the left cerebellum was removed to reveal the VIII nerve. Using an operating microscope, a bipolar stimulating electrode was placed on the dorsal surface of the nerve as close to the temporal bone as possible. The electrode consisted of two 80 p m insulated stainless steel wires cemented together and cut flush. The cut ends were slightly separated by about 0.1-0.2 mm to prevent short circuiting. The skin covering both eyes was removed if necessary and the eyes were kept moist with Ringers. Electrical stimulation consisted of a train of 0.1 msec duration constant current pulses at a rate of 400/sec lasting for 100 msec and repeated once per second (Phipps & Bird Constant Current Stimulator). Current was calibrated by measuring the voltage drop across a 100 ohm resistor in series with the electrode. Attempts at recording eye movements by electronystagmographic techniques in very young animals were not successful. The electrodes usually dragged on the animal or themselves generated eye movements. Also, signal to noise ratios were poor in newborn rats. Because of these problems, a technique for studying eye movements was sought which would be uniformly applicable and sensitive at all ages. Direct observation of eye movements by the operating microscope is unaffected by any developmental changes in the corneo-retinal potential which is the basis of electronystagmography. Eye movements were observed using the operating microscope on high power (32x). A sharg feature of the eye, such as a blood vessel or the edge of the iris was focused and the electrical stimuli delivered. Stimuluslocked eye movements smaller than I" were easily directed by this method. A few minutes prior to the electrical stimulation, the level of anesthesia was made as light as possible, since the elicitation of the vestibulo-ocular reflex has been shown to be Actu Otolaryngol87
486
I . S. Curthoys
birth to 15 days exhibit small spontaneous eye movements without any stimulation. High frequency electrical stimulation of the vestibular nerve usually caused the ipsilateral eye to move in a forward and upward direction whereas the contralateral eye moved in a backward and downward direction. There was one eye movement for each train of pulses. At all ages these movements were conjugate. Fig. 2 shows the lowest thresholds for ipsi9 b 1. li 13 i4 15 16 17 18 19 POSTNATAL AGE IN DAYS lateral eye movements for animals of increasFig. I . The percentage of rats with at least one eye open ing body weight. The upper abscissa shows at various ages after birth. The upper row shows the numthe estimated postnatal age in days correber of animals examined on each postnatal day and the sponding to body weight, derived from earlier next row shows their average weight. measurements of age and weight in young rats from this population (Curthoys & Webber, very sensitive to anesthesia in other prepara- 1977). There was a statistically significant tions (e.g. Eckmiller & Mackeben, 1976). negative correlation between weight and The initial current delivered was about 100 threshold ( r = -0.44, n=40, p
ISSN: 0001-6489 (Print) 1651-2251 (Online) Journal homepage: http://www.tandfonline.com/loi/ioto20
The Vestibulo-Ocular Reflex in Newborn Rats Ian S. Curthoys To cite this article: Ian S. Curthoys (1979) The Vestibulo-Ocular Reflex in Newborn Rats, Acta Oto-Laryngologica, 87:3-6, 484-489, DOI: 10.3109/00016487909126456 To link to this article: http://dx.doi.org/10.3109/00016487909126456
Published online: 08 Jul 2009.
Submit your article to this journal
Article views: 10
View related articles
Citing articles: 3 View citing articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ioto20 Download by: [University of California, San Diego]
Date: 29 February 2016, At: 13:56
Acta Otolaryngol 87: 484489, 1979 THE VESTIBULO-OCULAR REFLEX IN NEWBORN RATS Ian S. Curthoys From the Department of Psychology, University of Sydney, Sydney, N e w South Wales, Australia
Downloaded by [University of California, San Diego] at 13:56 29 February 2016
(Received August 8, 1978)
Abstract. To study the development of vestibular functioning, the vestibulo-ocular reflex was tested in rats of increasing age. High frequency electrical stimulation of the vestibular nerve elicited characteristic conjugate eye movements in newborn rats well before the eyes were open (about day 15 after birth) and before the semicircular canals had reached full adult size (about day 22 after birth). The threshold currents decreased during growth.
Studies of slow eye deviations in response to head angular acceleration have shown that the vestibulo-ocular reflex is functioning at birth in cat, guinea pig and man (Carmichael, 1934; Fish & Windle, 1932; Galebsky, 1927; Groen, 1963; Tibbling, 1969). However, in each of these species the gross morphology of the semicircular canals is already adult size by birth and a considerable proportion of fine vestibular receptor and afferent development has also occurred (Altmann, 1950; Bast & Anson, 1949; Elliott & Elliott, 1964; Heywood, Pujol & Hilding, 1976; Romand et al., 1976; Sato, 1903; Curthoys, unpublished observations). In order to gain detailed understanding of the entire course of vestibular functional development in relation to anatomical growth changes, the ideal preparation would appear to be one whose semicircular canal system is not fully developed at birth. In an extensive study of such a species, the pike, ten Kate (1969) found that the minimum thresholds for the slow phase of post-rotatory eye deviations were independent of the animal size. Indeed, he was able to elicit these deviations in the smallest pikes studied. Results from the pike are difficult to apply to mammals because, unlike mammals, the dimensions of the pike A ( I(I O t o l o r \ n ~ o l 8 7
labyrinth continue to increase throughout life. Although rarely used in vestibular research, the rat seems to be a good candidate for studying the development of vestibular functioning. Clark (1973) has shown that the rat labyrinth is not mature at birth. The gross morphological dimensions continue to increase for about 22 days after birth. Other studies have shown that horizontal semicircular canal primary afferents in newborn rats have a lower average resting rate and are less sensitive than comparable neurons in adults (Curthoys, 1977, 1978). The aim of the present experiment was to determine whether the vestibulo-ocular reflex could be elicited in newborn rats and if so, whether the eye movements were conjugate. It is stressed that this study was only concerned with the slow compensatory eye movements: the important related question of the development of the quick phase of vestibular nystagmus was not addressed. The problem was studied by using high frequency electrical stimulation of the vestibular nerve, measuring the threshold current needed to elicit eye movements in lightly anesthetized rats of various ages. In adult cats and monkeys, high frequency electrical stimulation of branches of the vestibular nerve produce “compensatory” eye movements in characteristic planes and directions which depend upon the branches of the nerve stimulated (Cohen & Suzuki, 1963; Cohen et al., 1964). It was considered that use of direct electrical stimulation of the vestibular nerve in this study would bypass any developmental
Downloaded by [University of California, San Diego] at 13:56 29 February 2016
Vestibulo-ocular reflex in newborn rats
changes in the receptor mechanism and may indicate the stage of development of the more central neuronal pathways and ocular motoneurons. A related consideration is that the rats' eyes do not open until well after birth. It seemed possible that the development of the vestibuloocular reflex may be more closely locked to eye-opening, prior to which the image-stabilizing function of the reflex would be unnecessary. Accordingly, we also sought to date accurately the time of eye-opening in rats. This information may also be important in studies of developmental changes in visual-vestibular interaction.
MATERIALS AND METHODS
To measure day of eye-opening, birth-dated albino Wistar rats were examined and weighed on successive days where possible. Day of eye opening was taken as the day on which at least one eye was open. Sixty-one albino rats were used for the eye movement experiment. Five were classed as adult rats and weighed in excess of 170 g; the remaining 56 were newborn and young rats ranging in weight from 4.4 to 35.4 g. These weights correspond to ages of from 0 to about 23 days after birth. Each animal had a tracheal cannula inserted under ether anesthesia and was maintained under ether for the remainder of the experiment. ECG was monitored continuously to ensure depth of anesthesia was adequate. The animals were placed in specially designed head-holders in a Kopf stereotaxic device which was mounted of turntable. Adult rats were held by a modified palate clamp. For young rats the head was held by a machine screw cemented to the skull by cyanoacrylate glue and dental cement. The techniques for preparation of very young rats have been described in detail elsewhere (Curthoys & Webber, 1977). For animals of all ages the head was pitched nose down by about 45" to 32-792951
485
ensure the horizontal canal was in an earthhorizontal plane. Body temperature was maintained by a heating pad under the animal. In each rat the lateral half of the left cerebellum was removed to reveal the VIII nerve. Using an operating microscope, a bipolar stimulating electrode was placed on the dorsal surface of the nerve as close to the temporal bone as possible. The electrode consisted of two 80 p m insulated stainless steel wires cemented together and cut flush. The cut ends were slightly separated by about 0.1-0.2 mm to prevent short circuiting. The skin covering both eyes was removed if necessary and the eyes were kept moist with Ringers. Electrical stimulation consisted of a train of 0.1 msec duration constant current pulses at a rate of 400/sec lasting for 100 msec and repeated once per second (Phipps & Bird Constant Current Stimulator). Current was calibrated by measuring the voltage drop across a 100 ohm resistor in series with the electrode. Attempts at recording eye movements by electronystagmographic techniques in very young animals were not successful. The electrodes usually dragged on the animal or themselves generated eye movements. Also, signal to noise ratios were poor in newborn rats. Because of these problems, a technique for studying eye movements was sought which would be uniformly applicable and sensitive at all ages. Direct observation of eye movements by the operating microscope is unaffected by any developmental changes in the corneo-retinal potential which is the basis of electronystagmography. Eye movements were observed using the operating microscope on high power (32x). A sharg feature of the eye, such as a blood vessel or the edge of the iris was focused and the electrical stimuli delivered. Stimuluslocked eye movements smaller than I" were easily directed by this method. A few minutes prior to the electrical stimulation, the level of anesthesia was made as light as possible, since the elicitation of the vestibulo-ocular reflex has been shown to be Actu Otolaryngol87
486
I . S. Curthoys
birth to 15 days exhibit small spontaneous eye movements without any stimulation. High frequency electrical stimulation of the vestibular nerve usually caused the ipsilateral eye to move in a forward and upward direction whereas the contralateral eye moved in a backward and downward direction. There was one eye movement for each train of pulses. At all ages these movements were conjugate. Fig. 2 shows the lowest thresholds for ipsi9 b 1. li 13 i4 15 16 17 18 19 POSTNATAL AGE IN DAYS lateral eye movements for animals of increasFig. I . The percentage of rats with at least one eye open ing body weight. The upper abscissa shows at various ages after birth. The upper row shows the numthe estimated postnatal age in days correber of animals examined on each postnatal day and the sponding to body weight, derived from earlier next row shows their average weight. measurements of age and weight in young rats from this population (Curthoys & Webber, very sensitive to anesthesia in other prepara- 1977). There was a statistically significant tions (e.g. Eckmiller & Mackeben, 1976). negative correlation between weight and The initial current delivered was about 100 threshold ( r = -0.44, n=40, p
