Habituation of the rat's respiratory response to auditory stimulation during sleeping and waking

Transcription

1 Psychobology 1987, Vol. 15 (3), Habtuaton of the rat's respratory response to audtory stmulaton durng sleepng and wakng KLAUS RUOOLPH Unversty of Tübngen, Tübngen, West Germany The rat's respratory response to audtory stmul s descrbed. Stmulaton wthn a sesson conssted of kHz tones of 60 db SPL and 3 16-kHz tones of 80 db SPL. Form and habtuaton ofthe respratory response were not affected by the rat's state ofvglance: Onset and offset of audtory stmulaton evoked a respratory pause whether the rat was awake or n slow-wave sleep. The response dsplayed shorterm, but not 10ng-term, habtuaton: t dsappeared wthn a sesson, but showed complete recovery when the stmulaton was repeated 1 week later. The data are dscussed wth respect to the queston of whether the respratory response evoked by audtory stmulaton consttutes an orentng response (OR) wthn the OR concept. The present study descrbes the rat's respratory response followng audtory stmulaton durng slow-wave sleep (SWS) and durng the awake state to answer the followng questons: (1) s there shor or long-term habtuaton durng SWS and the awake state? (2) s ths response an OR n terms of OR theory? A novel stmulus of moderate ntensty can cause an orentng reflex (OR) n anmals and humans, as frst descrbed by Pavlov (1927) and Sokolov (1963). Perpheral ORs, such as changes n heart rate and respraton, accompany central nformaton processng of an organsm and are thought to lower sensory tbresholds and enhance ntake of nformaton. One of the man characterstcs of the OR s that habtuaton of the response wll occur upon repeated presentaton of the same stmulus. Sokolov hypotheszed that elctaton of the OR s medated subcortca1ly, whereas habtuaton of the response requres cortcal or hppocampal actvty. Snce the nature of cortcal actvty durng the sleepng and awake states s qute dfferent, the nature ofthe OR and ts habtuaton (whch can be consdered a form oflearnng) could be expected to be dfferent durng wakng and sleepng. Ths appears to be true for the cardac response to tones n humans (Berg, Berg, & Graham, 1971; Berg, Jackson, & Graham, 1975). However, there are contradctory expermental fndngs, whch rase the queston of whether habtuaton can occur durng sleep. Jung (1954) and Sokolov and Paramanova (1961) reported habtuaton n humans durng sleep, and Johnen (1987) found habtuaton ofthe rat's hearrate response durng SWS. Other authors have faled to fnd habtuaton durng Ths work was supported by the Deutsche Forschungsgemenschaft Grant Sehn. 138/13-15 and by the SFB 300 of the Unversty oftübngen. Address reprnt requests to Klaus Rudolph, Department of Zoophysology, Unversty oftübngeo, Morgenstelle 28, 0-70 Tübngen, West Germany. sleep (Frth, 1973; Johnson, Townsend, & Wlson, 1975; Wllams, Hammack, Daly, Dement, & Lubn, 1964). Johnson and Lubn's (1967) data ndcated that sleep blocked habtuaton n the ease of autonome varables, whch led the authors to conclude that "sleep and habtuaton are ncompatble," at least for the varables they studed. Wllams (1973), n summarzng studes ofhabtuaton durng sleep, stated that "wth a few exceptons... the majorty of controlled studes appear to rule out sleep learnng" (p. 39). Comparatvely few authors have descrbed an anmal's respratory response to audtory stmulaton. Among them are Corbelle and Baldes (1929) and Crborn (1970), who reported changes n the breathng frequency of frogs, dogs, and mce followng audtory stmulaton. Such (1959) examned the nfluence of sound on the rabbt's heart rate and breathng rate. The data from these studes were not evaluated n a quanttatve manner, however, so they provde only vague nformaton about responses to audtory stmulaton. The present study offers a more dfferentated method for evaluatng the rat's audtoryevoked respratory response, usng a breath-to-breath analyss. MEmOD Subjeds The subjects were 23 nave male Wstar rats wth a rnean body weght of 331 g ±8.3 g (SEM). The rats were housed ndvdually at 21 0 C, wth food pellets and water ad lb on a lght/dark (LD), 12 h: 12 h schedule, wth lghts on at 8 a.rn. Two groups of anmals were fonned accordng to ther tme of testng: 8 rats were tested between 8 a.rn. and 12 noon (SWS) and 15 rats between 8 p.rn. and 10 p.rn. (wakng state). Apparatus and Stmul. Respraton. The unrestraned anmals were measured nsde a transparent Plexglas cylnder 15-cm dameter, 25-cm heght. Ther breathng movernents were pcked up by a Bell & Howell pressure Copyrght 1987 Psychonomc Socety, nc. 272

2 HABTUATON N THE RAT 273 transducer wth a frequency range from 0 to 100 Hz. Tbe transducer was connected to the cylnder by a flexble rubber tube (wholebody plethysmography), and ts output was amplfed by a Bell & Howell brdge amplfer. Tbus, the anmals could move freely wthn the cylnder. Fresh ar was provded by a contnuous, moderate arflow. Electrocortcogram (ECoG). Electrcal bran actvty was measured va an FM mnature telemetry system, whch s descrbed elsewhere n detal (Neuhaus & Borbely, 1978). Brefly, two goldplated screws chroncally mplanted n the skull (one postoned above the rght paretal cortex, the other above the frontal cortex) served as electrodes. Tbey were connected va stanless steel wre to a mnature FM rado transmtter placed on the rat's head. Tbe omtted sgnals were receved by three antennas surroundng the anmal, each at a dstmce of 1.5 m. Tbe ECoG was bandpass fltered ( Hz), amplfed, and dsplayed on a Tektronx dual-beamstorage osclloscope. For techncal reasons, no quanttatve on-lne analyss of ECoG frequency could be carred out. t was possble, however, to dstngush dfferent states of vglance durng the experment by vsually scorng the ECoG ampltude dsplayed on a Tektronx dual-beam osclloscope. A sleep stage was judged to be SWS when the followng three crtera were fulflled: Hgh-voltage, low-frequency actvty n the ECoG, regularty of respraton, and motonlessness ofthe rat (slght body movements were seen as artfacts n the respraton sgnals). Fgure 1 shows synchronously regstered ECoG sgnals and breathng movements of a rat durng wakng and sleepng. Respraton and ECoG data were stored smultaneously on FM tape and wrtng paper (Gould 2200 two-channel recorder) for further evaluaton. Acoustc stmulaton. Stmulus ntensty nsde the Plexglas cylnder (2 cm above the bottom) was measured wth a lj-n. Brüel & Kjaer mcrophone and a HewletPackard frequency anayzer. Background nose was below db SPL (re: 20 JLPa). Stmul were generated by a Toellner 7706 frequency generator, attenuated wth a HewletPackard 350-D attenuator and amplfed by a Hewlett Packard 467 A amplfer. Tber duratons and rse and fall tmes were controlled by an electronc pulse generator va the frequency generator's VCA nput. Tbe loudspeaker (commercally avalable, hgh-frequency, hgh-fdelty) was postoned about 15 cm above the rat's head n the upper part of the Plexglas cylnder. All experments were carred out n a specal anechoc and soundproof room. Procedure Experments wth the rats n two dfferent states of vglance were earred out wth rat n SWS n Experment 1 and wth rats n the awake state n Experment 2. Each experment eomprsed two sessons. Tbere were kHz tones n Week 1, and they were repeated n Week 2. Tbe tones bad a duraton of 2 sec, an nterstmulus nterval (ls) of 60 sec, and a rse/fall tme of 5 msec. Tbe sound pressure level of Stmul 1-10 was 60 db and that of Stmul was 80 db. Experment 1 (SWS). Eght anmas were anesthetzed wth a mxture ofketavet (Parke-Davs, 100 mg/kg) and Rompun (Bayer, 2.3 mglkg) n order to fx electrodes and FM transmtter. One week later, daly tranng sessons were begun. On 5 eonsecutve days, eaeh anma was placed nto the Plexglas eylnder for 15 mn twce a day wthout any stmulaton. Ths served to shorten the latency of sleep onset by famlarzng the anmas wth the expermental stuaton. Subsequent to ths tranng phase, stmulaton experments were begun. A rat was placed n the Plexglas cylnder wthn the frst 4 h of the lght (L) phase, because total sleep tme and SWS densty are hghest at ths tme (Borbely, 1982). Tbe stmul were presented only when the anma's sleepng state was dentfed as SWS usng the prevously mentoned crtera. f the anmal woke up durng the experment or swtched to another sleepng state (e.g., REM sleep, characterzed by a low-voltage, hgh frequency ECoG and a eurled-up, motonless body poston wth eyes dosed), the stmulaton was stopped untl SWS reappeared. Ths was rarely the ease, however. Tbe number of ehanges n the state ofvglanee per sesson and anma ranged from 0 (.e., 13 stmul wth an S of 60 sec were delvered wthout nterrupton) to 5 (Le., there were 5 Ss longer than 60 sec), wth a mean of 2. Experment 2 (awake state). Ffteen nave anmas were used. Tbe procedure was smlar to that of Experment 1, except that no tranng was carred out before the experment. Tbus, the anmas n the awake state were not as famlar wth ther surroundngs as were those n SWS (for more on ths pont, see Dscusson). Tbe rat was placed n the measurng cylnder at the begnnng of the dark (D) phase, when total sleep tme and SWS densty are low (Borbely, 1982). All experments were carred out n drn red lght (-W red bulb, approxmately 12 x). Stmulaton began after an adaptaton perod of about 10 rnn. Norrnally, the rat sat wth eyes open n a sphnx-lke poston. No stmul were delvered durng snffng and exploratory behavor. Although no ECoG measurements were taken n ths experment, behavoral observatons showed that the anmas were n the awake state durng at least 90% of the tme they spent n the eylnder. Evaluaton of Data Prelmnary experments had shown that the rat's respratory response could be characterzed by a lengthened breathng eyde, that s, arespratory pause followng the breathng eyde mmedately before stmulus onset and offset. Accordngly, a breathng cyde was defmed as the horzontal dstance between two maxma (end ofnspraton) ofthe breathng curve (see Fgures 1 and 2 for orgnal data). Ths respratory pause s expressed n pereent response magntude, whch represents the percentage of ncrease n the duraton of the breathng cyde followng stmulus onset or offset as compared wth the cyde before onset or offset. For an example, see Fgure 2, whch shows the regstraton of a sleepng rat's respratory response to audtory stmulaton. n addton to a respratory pause, a dmnshed breathng ampltude was notced followng stmulaton. Because ths response occurred n a rather unsystematc manner, however, these results are not reported n ths study. Statstcal Analyss Tme courses of responses to Stmul 1-10 were anayzed usng a polynornal regresson based on an analyss of varance. Comparsons of two means were made wth Student's two-taled t test for ether pared or unpared values. Data not normaly dstrbuted were evaluated wth the nonparametrc Wleoxon U test and the Wlcoxon matched-pars sgned-ranks test. All values are expressed as mean ±SEM unless stated otherwse. RESULTS Fgure 1 shows synchronously regstered ECoG and respraton of an anmal durng sleepng and wakng. The hgh-voltage, low-frequency ECoG together wth regular breathng sgnals was scored to be SWS, whereas a low-voltage, hgh-frequency ECoG wth rregular breathng sgnals was scored to be the awake state. The rat's respratory response to audtory stmulaton was smlar durng sleepng and wakng, and can be characterzed by arespratory pause followng both tone onset and tone offset. Fgure 2 shows the audtory-evoked respratory response of a sleepng rat. The respratory pause followng tone offset s shown to be larger than that followng tone onset. The mean breathng frequency mmedately before stmulaton was 80±29.5 breathng cycles/mn for

3 274 RUDOLPH ECOG 200 JV RESPRATON ECOO 200 JV RESPRATON SLOW 1 see AWAKE WAVE SLEEP wth an acceleraton. A trend analyss consderng OnlY Stmul 2-10, however, dd not yeld sgnfcant results. n Week 2, the regresson analyss showed sgnfcant lnear [F(1,88) = 9.5, p <.005] and quadratc [F(2,87) = 6.4, P <.005] trends of habtuaton across Stmul The 80-dB tones followng the 6O-dB tones had no effect on the habtuated response: There were no sgnfcant dfferences between responses to Stmulus 10 (60 db) and Stmulus 11 (80 db). Respratory response to tone ottset. Short -term habtuaton of the respratory pause followng tone offset can clearly be seen n the response decrement across the 10 6O-dB tones, whch are shown n the lower part of Fgure 3. n Week 1, response magntude decreased from 48 % (frst 6O-dB tone) to 10 % (last 6O-dB tone). There are both lnear [F(1,74) = 21.2,p <.OO]andquadratc [F(2,73) = 13.9, P <.001] trends of habtuaton. One week later, the response had recovered completely but agan showed shorterm habtuaton, whch can be seen n the decrease from 57% (frst 6O-dB tone) to nearly zero (last 6O-dB tone), wth F(1,75) = 26.3 for lnear and F(2,74) = 25.7 for quadratc trends ofhabtuaton. The 10 6O-dB tones were followed by 3 80-dB tones (Stmul 11-13). These tones caused no recovery of the habtuated respratory response, that s, the response remaned near zero. Tone offset evoked a larger respratory pause than dd tone onset [for Week 1, R(8) = 3, P <.05; for Week 2, R(8) = 0, P <.05]. 1 see Fgure. Smultaneously recorded eectrocortcog (ECoG) and breatbng movements of a rat durng a sleepng state tbat was judged as slow-wave sleep and n the wakng state. Note tbat durng sleep there s a bgh-voltage, low-frequency ECoG together wth reguar breatbng movements, and n the wakng state there s a ow-voltage, bgb-frequency ECoG together wth rreguar breatbng movements. nspraton upward. the rats n the awake state and 86± 12.8 breathng cycles/mn for the sleepng rats. The dfference between the two means was not sgnfcant (Student's t test), whereas the standard devatons were sgnfcandy dfferent [F(443, 216) = 5.3, p <.001]. Ths ndcates that respraton was more regular durng sleep than durng the awake state. Experment 1 (Sleepog Rats) Respratory response to tone onset. As shown n the upper part of Fgure 3, tone onset was followed by a respratory pause consstng n a 10%-30% ncrease n breathng cycle length compared wth the precedng breathng cycle. n Week 1, ths respratory pause showed no sgnfcant response decrement across Stmul 1-10 [F(1,75) =.46, P >.05, for lnear-trend analyss; F(2,74) =.36, p >.05, for quadratc trends]. The fgure shows that n both weeks the response to Stmulus 1 was consderably smaller than that to Stmulus 2, because only half of the anmals responded to the onset of the frst tone wth a respratory pause, whereas the other half responded Experment 2 (Rats n the Awake State) Respratory response to tone onset. Sgnfcant lnear [F(1,142) = 12.9, P <.001] and quadratc [F(2, 141) = 9.61, P <.001] trends of habtuaton to tone onset were seen n Week 1, but not n Week 2 [F(1,130) = 1.3, p>.05,forlneartrends;f(2,129) = 1.7,p >.05,for quadratc trends]. The results for both weeks are shown n the upper part of Fgure 4. Respratory response to tone ottset. The responses to tone offset n Weeks 1 and 2 are shown n the lower part of Fgure 4. Smlar to Experment 1, there was shorterm, but no long-term habtuaton. Shorterm habtuaton to tone offset was sgnfcant n Week 1 [F(1,139) = 67.6, p <.001, for lnear trends; F(2, 138) = 47.1, P <.001, for quadratc trends] as wel as n ONSET PRE POST '-'-' 1 : 1 LUS- OFFSET PRE POST :-1-; : Fgure 2. Orgnal data showng a sleepng rat's respratory response to a 6O-dB, 16-kHz tone. 80th stmulus onset and off set are followed by a respratory pause. nspraton upward. Pre = prestmulus level; Post = poststmulus level.

4 m ) L4. S - e3 50J \ "..D..,U P ö -EY TRAL TRAL Fgure 3. Respratory pause fouowng onset (top) and ofjset (bottom) of 16-kBz tones (sleepng rats, n = 8). Trals 1-10: 60 db SPL; Trals 11-13: 80 db SPL. Week2 [F(l,125) = 24.3,p <.001, forlneartrends; [F(2,124) = 18.9, p <.001, for quadratc trends]. Tbe frst 6O-dB tone was followed by a response magntude of 82 % n Week 1 and by one of 87 % n Week 2, whch ndcates that the habtuated response had completely recovered wthn 1 week wthout any sgns of long-term habtuaton. n both weeks, tone offset of the frst 6O-dB tone caused sgnfcantly larger response magntudes than tone onset [for Week, R(15) = 3, P <.001; for Week 2, R(O) = 0, p <.01). Just as wth tone onset, the offset response to the 80-dB tones was not dfferent from that to the 6O-dB tones. Thus, the 20-dB ncrease followng 10 presentatons of 6O-dB tones evoked no change n responsvty ndependent of the anmals ' state of vglance. DSCUSSON The results show that (1) the rat's respratory response to moderate-ntensty pure tones conssts n arespratory pause followng tone onset and offset, (2) there s hab HABTUATON N THE RAT 275 uaton ofths response wthn a sesson (shorterm habtuaton) but not between sessons (long-term habtuaton), and (3) shorterm habtuaton occurs durng sleepng as wel as durng wakng. Although there were dfferences n responsvty between the two states of vglance (a hgher response magntude durng the awake state), these dfferences could also be due to dfferences n famlarty wth the test envronment. As prevously mentoned, only the sleepng rats had been made famlar wth ther surroundngs pror to testng. There were no dfferences between the two groups n rate of habtuaton. Tbs s n accordance wth the results of Johnen (1987), who found dear habtuaton ofthe sleepng rat's hearrate response to audtory stmulaton. t thus appears that the rat's habtuaton to audtory stmulaton does not depend on ts state of vglance. Tbe 80-dB stmul followng the seres of 1060-dB stmul faled to evoke an addtonal OR once habtuaton had occurred. Tber lack of effect when presented at the end of aseres of 6O-dB tones could be due to stmulus generalzaton. As already mentoned, sleepng rats dd not respond to the frst onset, of audtory stmulaton (Fgure 3). Ths was ) ) t;:; S 80! ) 30 20!! 10 f3, 1 '2 ' TRAL week 2 V; TRAL Fgure 4. Respratory pause fouowng onset (top) and ofjset (bottom) of 16-kBz tones (rats n the wakng state, n = 15). Trals 1-10, 60 db SPL; Trals 11-13, 80 db SPL.

5 276 RUDOLPH not due to nonrespondng, but to the fact that half of the anmals responded wth a respratory pause, whereas the other half responded wth an acceleraton, that s, breathng-cycle length became shorter. Caspers, Grueter, and Lerche (1958) found that 60 db at 16 khz was an ntensty below the rat's awake threshold. Ths was confnned by the present experments, snce ths ntensty evoked no behavoral (motor) arousal durng SWS, despte the descrbed respratory responses. Stmulus ntenstes n the range of 60 db are approprate for the evocaton of an OR (Sokolov, 1963). Accordng to Sokolov, an OR s senstve to habtuaton, and the response should be smlar to turnng a stmulus on or off. As descrbed above, the rat's respratory response to audtory stmulaton was senstve to habtuaton durng sleepng and wakng. Onset as wel as offset of the tones evoked arespratory pause. Barry (l977b) examned, n humans, respratory responses to l-khz tones of20-50 db SPL. He reported a 10%-20% ncrease n respratory perod at stmulus onset, whch habtuated upon repeated stmulaton (Barry, 1977a, 1977b, 1982). n terms of Barry's proposed four-way fractonaton of response ndcators, the respratory OR s controlled by the "novelty regster" and therefore s senstve to habtuaton. Thus, the present data are n accordance wth Barry' s as wel as Sokolov's OR theory, and the form of the rat's respratory response to 6O-dB 16-kHz tones s smlar to a human's respratory OR to dB l-khz tones. A drect comparson can be made between Barry' s (1977a, 1977b, 1982) results and those of the present study, because the rat' s sensaton level at 16 khz s smlar to that ofhumans at 1 khz, a commonly used frequency n OR experments (see Kelly & Masterton, 1977, for the rat's hearng threshold, and Zwcker & Henz, 1955, for that of humans). A plausble explanaton for the respratory pause followng low-ntensty audtory stmulaton was frst gven by Woodworth (1936), who wrote: "Sudden stmul wll make the subject 'catch hs breath.' f he s lstenng to a fant sound, arrested breathng elmnates dsturbng respratory sound" (p. 260). Snce habtuaton ofthe rat's respratory OR was seen durng both sleepng and awake states, our results are n contrast to most of the fndngs n experments wth humans, where habtuaton s a functon ofthe subjects' state of vglance. Johnson and Lubn (1967), for example, came to the concluson that habtuaton and sleep were ncompatble. They found that the average respratory response to l-khz tones near threshold was a slowng n breathng rate, regardless of the state of vglance. Ths response habtuated durng the awake state, but retumed wth sleep onset and remaned durng sleep. 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