Auditory deprivation modifies sleep in the guinea-pig

Neuroscience Letters 223 (1996) 1–4

Auditory deprivation modifies sleep in the guinea-pig Marisa Pedemonte a, Jose´ L. Pen˜a a, Pablo Torterolo b, Ricardo A. Velluti a ,* a Neurofisiologı´a, Departamento de Fisiologı´a, Facultad de Medicina, Gral. Flores 2125, 11800 Montevideo, Uruguay Neurofisiologı´a, Departamento de Fisiologı´a, Facultad de Ciencias, Universidad de la Repu´blica, Montevideo, Uruguay

b

Received 13 September 1996; revised version received 17 December 1996; accepted 2 January 1997

Abstract After destruction of both cochleae, a significant enhancement of both paradoxical sleep and slow wave sleep together with decreased wakefulness, were observed for up to 45 days. The sleep augmentation consisted of an increment in the number of episodes of both slow wave and paradoxical sleep rather than in the duration of single episodes. The partial isolation provoked by deafness is postulated as explanation. We suggest that the suppression of one input to a complex set of networks related to the sleep-waking cycle, introduce an imbalance that leads to sleep enhancement.  1997 Elsevier Science Ireland Ltd. Keywords: Wakefulness; Slow wave sleep; Paradoxical sleep; Deafness; Cochlear lesion; Auditory deprivation

The Bremer [2] passive approach to sleep-waking physiology was based on the role played by sensory systems in the maintenance of wakefulness, whereby reduced sensory activity leads to sleep. Sleep may also be induced during wakefulness by sensory stimulation such as repetitive, low intensity auditory [7] or somatosensory [16,18]. Other sound effects on sleep have been reported. High intensity white-noise continuous stimulation abolishes paradoxical sleep (PS) completely in rabbits [9], and partially (without decrease of slow wave sleep (SWS)) in rats [21]. Upon intense auditory stimulation specifically during PS, the number of PS episodes was increased without changing its total amount [5], although auditory stimulation carried out recently in rats, led to the conclusion that the pattern of PS occurrence was affected by stimulating during both PS and SWS, while the total amount of PS was substantially preserved (Parmeggiani, personal communication). The fact that external stimuli affect sleep implies that such information must somehow be processed to some extent by the sleeping brain, in animals and humans; evidence from the auditory nerve, brainstem auditory nuclei and auditory cortex show behavioral-dependent responses in the auditory system [11,13–15,22–24]. Moreover, the * Corresponding author. Fax: +598 2 948784; e-mail: rav@nfisfm. edu.uy

auditory channel is the only one that remains relatively ‘open’ during sleep in a micro-osmatic animal and may, teleologically, play a survival related function, e.g. monitoring predator noise. Thus, the sleep-waking cycle in deaf animals because it is pointing to the effects of a sensory system on a particular behavior, deserves further study. We target the effect of total auditory deprivation on the complex neuronal network underlying the sleep-waking system. Adult male guinea-pigs (Cavia porcellus, n = 15) were chronically implanted, under pentobarbital anesthesia (35 mg/k, i.p.), with electrodes to record the dorsal hippocampus electrogram (Hipp), parietal electrocorticogram (ECoG) and neck and masticatory muscles electromyogram (EMG). Bipolar electrodes were placed into the left and right auditory cortex to record the auditory evoked local-field potential in response to clicks. The preamplifier output signal was observed on a CRO and recorded on a Akai-VR10 Instrutech FM tape-recorder for off-line averaging. A second surgical procedure, the bilateral destruction of both cochleae was performed under anesthesia; after which the region was filled with dental cement and the bony-wall reconstructed. The evoked auditory cortex local-field potential was used as the pre- and post-lesion auditory electrophysiological test. The behavioral

0304-3940/97/$17.00  1997 Elsevier Science Ireland Ltd. All rights reserved PII S0304-3940 (97 )1 3392-4

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response to high intensity sudden sounds was another test. The post-mortem anatomical study verified the total destruction of the lesioned cochleae. The animals were maintained in light-dark 12:12 (light 0800–2000 h) schedule before and during the recording period. During the experimental period they were housed, single or in pairs, in a plexiglass cage (20 × 40 × 50 cm, for each one) placed in a sound-attenuated (one-way mirror) chamber with food and water ad libitum. The chamber temperature was 20–22°C. The cage cleaning was done every day at 0830 h and, at the same time, the animals were weighed. Fifteen animals were used to establish the control sleep parameters of the species, eight of them were employed in the lesion experiments as well as their own control. The eight animals used in the lesion study showed no signs of vestibular damage and only those with less than 10% of the initial weight were used. After the surgical recovery of 5 days, several 24 h sleep recordings were carried out for up to 45 days on both control, not yet operated, and lesioned operated animals. A computerized Polygraph (Nautilus Plus-PSG) was used for the recordings as follows: (1) the sleep-waking characteristics of the 15 animals used; (2) every lesioned gui-

nea-pigs (n = 8) were recorded as their own control, i.e. their sleep-waking parameters were studied before and after deafening; (3) as another control experiment, two guinea-pigs, one control previously to lesion and the other already deaf, were recorded simultaneously (four out eight animals). The bio-electrical recordings were complemented with visual behavioral control. Quantification of the time spent in wakefulness (W), SWS and PS was divided into 20 s epochs, and carried out by two investigators using visual scoring. A PC computer calculated the total sleep time (TST), showed the stage distribution during the 24 h periods (hypnogram), the average stage duration. The two-tail paired t-test was used for comparison of a guinea-pig sleep-waking behavior previous and after the lesion (n = 8), while the Student’s t-test was applied for global comparison between the control (n = 15) and the lesioned animals (n = 8). All data is presented as the mean ± SD. The averaged percentages of sleep and waking duration of the control guinea-pigs (n = 15), calculated from visually scored 24 h recordings and totaling 288 h, were: SWS, 28.8 ± 9.4%, PS, 4.1 ± 1.1% and W, 67.1 ± 10%. All the lesioned animals (deaf) demonstrated shifts in their sleep parameters, as shown in Fig. 1A (bars indicate a

Fig. 1. Total time spent in sleep and waking (control or normal animals before lesion and lesioned, deaf guinea-pigs, n = 8). After cochlear lesion (A) the open bars (lesioned animals) show the decrement in W time while both, SWS and PS, increased significantly. The inset shows the PS changes in a different scale. The total W decrement in the lesioned (deaf) animals, was due to decreased episodes duration and the increased frequency of episodes (B). The total augmentation of sleep was due to the significant increment in the number of episodes with no change in the episodes duration (C,D). W, Waking time; SWS, slow wave sleep; PS, paradoxical sleep.

M. Pedemonte et al. / Neuroscience Letters 223 (1997) 1–4

significant decrease of the W time; from 67.1 to 56.4 ± 10.2%; percentage of decrement, 15.9%; P , 0.05), associated to the increase in the time spent in both, SWS (from 28.8 to 38.2 ± 9.3%; percentage of increase, 32.6%; P , 0.05) and PS (inset; from 4.1 to 5.4 ± 1.1%; percentage of increase, 31.7%; P , 0.005). The total W decreased because of the diminution of single episodes duration although the number of episodes increased (Fig. 1B). The SWS episodes showed no change in duration but an increase in frequency (Fig. 1C). The total PS increment was due to the statistically significant augmentation of the frequency of episodes (control, 2.0 ± 0.6 episodes/h; lesioned, 3.2 ± 0.5 episodes/h; P , 0.005) with a non-significant decrease of individual episode duration (Fig. 1D). TST of the eight lesioned guinea-pigs significantly increased (P , 0.05) when compared to their own controls as can be observed in Fig. 2A. The total PS in 24 h increased as well as the SWS, although with different slopes (Fig. 2B,C). The total PS versus TST (Fig. 2D), also presented an increase in the deaf animals; the plot

Fig. 2. Averaged sleep parameters observed in eight animals, control condition (C) and lesioned, deaf (L). In (A) all the lesioned (deaf) animals show an increment in TST. The TST increase is due to the increase of PS (B) as well as SWS (C). In six out of the eight lesioned animals (D), the PS increment was greater than the SWS increase. The other two (diamonds, open and full), showed similar increases as during SWS. TST, Total sleep time; SWS, slow wave sleep; PS, paradoxical sleep.

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of animals numbers 37 and 35 (diamonds) showed no percentage increase because the same animals presented a great increase in SWS/24 h (Fig. 2C, diamonds). The simultaneous recording of two animals, one normal and one lesioned, led to the same results. The deaf animals’ enhanced SWS and PS and decreased W, were observed for a period of 45 days post-lesion; the sleep augmentation was present on the fifth day after lesion and persisted throughout the whole period. Neither auditory cortex evoked potentials nor the behavioral reaction to the noise test were present after the surgical lesion, indicating that the animals were completely deaf. Results support the notion that bilateral lesions of the cochlea that include receptors and first-order neurons producing total deafness, lead to significant changes in total sleep and waking duration. To exclude possible female hormonal cyclic effects [10] only male animals were used. The cochlear surgical lesions were carried out opening only the bulla, thus, no direct damage on the brain could occur. The lack of vestibular deficits were also used to ensure the lesions were restricted to the cochleae, thus, the deaf animals were otherwise normal. The variability of our own animals’ sleep-waking cycle parameters were determined. The simultaneous recording of a normal and a deaf animal was carried out, leading to the same results obtained when using only the lesioned one, demonstrating the changes were not related to any non-auditory environmental conditions, stress, time lapsed from the last general anesthesia, etc. Several form of stress may provoke sleep rebound, although the repetition of the same stressful stimuli (e.g. immobilization stress) finally did not affect significantly neither SWS nor PS [17]. The phenomena described in our results was observed up to 45 days of deafness, suggesting the possible stress may be already over. The guinea-pig has been described as either a diurnal or a nocturnal animal [3,8,10,12,19,20]. Its 24 h sleep-wake rhythm, under a 12 h light and 12 h dark cycle, was 1.1:1; i.e. the physiological sleep and waking distribution was almost equal during the 24 h period. However, depending on the author, the reported data shows great variations of the normal amount of SWS, whose total duration ranges from 24 [12] to 50% [8], and of PS whose total duration ranges from 2.5 [8] to 5.4% [20]. The data from our control experiments about the total SWS duration confirmed the one reported by Tobler et al. [20], i.e. 28.8%, while our control of PS total amount was 4.1%, in accord to Malven and Sawyer’s [10] and Pellet and Be´raud’s [12] results. These values were used as the physiological controls for recognizing changes imposed by deafness. The difference observed regarding PS total duration, between Tobler et al.’s [20] results (5.4%, the highest so far reported) and the one presented here (4.1%), may relate

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to the different methods of data analysis, an automatic one based on spectra analysis [20] versus a visual one scored by two judges. Conflicting results have been reported about the consequences of sound stimulation during sleep [9,21]. On the one hand, the duration of individual paradoxical sleep episodes and the number of ponto-geniculo-occipital spikes were found to be enhanced in cats stimulated with high intensity sound during PS [1,4,5]; however the total PS amount remained constant. This can be explained by more recent data; the sound stimuli delivered during any of both sleep phases, SWS or PS, would lead to similar results without changing the PS total duration, suggesting a less specific, more general phenomenon restricted to the frequency of episodes (Parmeggiani, personal communication). The present results with deaf animals, points to a separate action that prolongs the total sleep time due to shifts in the number of episodes. An important feature emerging is the relative isolation from the outside world imposed by deafness that could contribute to the W reduction and the sleep lengthening, stressing the relevance of such sensory input for sleep organization. Although sleep is clearly an active process it is not fair to disregard the sensory influences, particularly a meaningful input that is normally open during sleep. The brainstem pontine auditory nuclei are located in close proximity to PS executive networks; sleep related changes in the auditory unitary activity were reported in the pathway from brainstem up to auditory cortex [11,13– 15,23,24], besides their well known sensory contribution to activity in the reticular nuclei. Thus, the auditory system may produce actions on sleep-related central nervous system loci. The sleep-waking balance may depend upon the coordinated function of multiple networks whose synaptic processes and other neuronal properties exhibit ‘cooperative interactions’ [6]. Changing an input to such networks would introduce functional differences providing that it is significant functionally, when such an input is lacking, the observed sleep-waking imbalance would arise. We are grateful to Jose´ P. Segundo (UCLA, CA, USA) for the valuable comments and suggestions on the manuscript. This work was partially supported by a Commission of the European Communities CI1*-CT930002 Grant (Brussels), and Pedeciba, Uruguay. [1] Ball, W.A., Morrison, A.R. and Ross, R.J., The effects of tones on PGO waves in slow wave sleep and paradoxical sleep, Exp. Neurol., 104 (1989) 251–256. [2] Bremer, F., Cerveau ‘isole´’ et physiologie du sommeil, C.R. Soc. Biol., 118 (1935) 1235–1241. [3] Campbell, S.S. and Tobler, I., Animal sleep: a review of sleep duration across phylogeny, Neurosci. Biobehav. Rev., 8 (1984) 269–300.

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