Circadian activity rhythms in SHR and WKY rats: Strain differences and effects of clonidine

Physiology&Behavior,Vol. 53, pp. 23-29, 1993

0031-9384/93 $6.00 + .00 Copyright© 1993 PergamonPressLtd.

Printed in the USA,

Circadian Activity Rhythms in SHR and WKY Rats: Strain Differences and Effects of Clonidine A L A N M. R O S E N W A S S E R ~ A N D L O N N I E P L A N T E

Department of Psychology, University of Maine, Orono, M E 04469 Received 18 M a y 1992 ROSENWASSER, A. M. AND L. PLANTE. Circadian activity rhythms in SHR and WKY rats: Strain differences and effects of clonidine. PHYSIOL BEHAV 53(1) 23-29, 1993.--The spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) inbred rat strains have been subjected to extensive behavioral and neurochemical characterization. The present study examined free-running circadian activity rhythms in these two strains. Because previous studies indicated that free-running rhythms are altered during chronic clonidine administration, and that SHRs and WKYs may respond differentially to clonidine, the effects of this agent on rhythmicity were compared in the two strains. SHRs were hyperactive and showed shorter free-running periods than did WKYs. Cionidine administration altered free-running rhythms similarly in the two strains, but reduced activity levels only in the relatively hyperactive SHRs. These results are consistent with the hypothesis that central noradrenergic systems influence circadian locomotor activity rhythms. Circadian rhythms

Locomotor activity

CIonidine

SHR

THE spontaneously hypertensive (SHR) and norrnotensive Wistar-Kyoto (WKY) inbred rat strains have been subjected to extensive behavioral, physiological, and neurochemical characterization. SHRs have generally been reported to display both hyperactivity as well as hyperreactivity to arousing or stressful stimulation, when compared to the WKY parent strain (15). However, studies also indicate that SHRs show reduced emotionality, especially in behavioral tests purported to measure fear or anxiety (2,13,15,26,34). Similarly complex findings have emerged from studies comparing the central monoaminergic systems in SHRs and WKYs. The two strains have been reported to differ in noradrenergic neurotransmitter content and turnover, in spontaneous noradrenergic neuronal activity, and in noradrenergic receptor dynamics (11,32). However, this area is characterized by numerous conflicting reports. For example, SHRs have been reported to display both increased and decreased alpha-2-adrenergic receptor number and sensitivity using different functional and anatomical assays (6,8,9,21,34,44). The present study was designed to compare free-running circadian wheel-running activity rhythms in SHRs and WKYs. There were several reasons to expect that circadian periodicity might differ between the two strains. First, recent studies demonstrating that high levels of spontaneous behavioral activity can alter free-running circadian period (4,33,45) suggested that the differences in activity level between SHRs and WKYs might be associated with strain-dependent alterations in free-running period. Second, the circadian rhythm alterations that have been

WKY

Rats

described in patients with affective disorders (40,41) suggested that the dissimilar affective-behavioralprofiles of these two strains could be associated with strain differences in rhythmicity. Finally, recent findings in this laboratory that free-running activity rhythms are altered during chronic administration of the alpha2-noradrenergic agonist clonidine (27,28) suggested that differences in noradrenergic function between SHRs and WKYs could be associated with strain differences in rhythmicity. Therefore, the present study sought to characterize possible strain differences between SHRs and WKYs in the expression of free-running circadian activity rhythms. Since SHRs and WKYs have been reported to show differential behavioral and physiological responsiveness to clonidine (8,21,34), this study was also designed to extend our earlier observations on the rhythm-altering effects of chronic clonidine administration to these two strains. METHOD

Subjects and Apparatus Male SHR and WKY rats (Charles River Labs.), nine animals per strain, were maintained in standard running-wheel cages with attached home-cage compartments (Lafayette Instruments), beginning at about 8 weeks of age. The running wheels were housed within ventilated, light-shielded enclosures with either one, two, or four cages per enclosure. Each wheel revolution closed a microswitch and switch closures were continuously monitored and stored on the hard disk of a Zenith 158 computer

Requests for reprints should be addressed to A. M. Rosenwasser, Department of Psychology, 301 Little Hall, University of Maine, Orono, ME 04469.

23

24

ROSENWASSER AND P I A N I I

in 10-min time bins using a commercial interface system (Dataquest I/I; Mini-Mitter Co.). Food (Agway Prolab 3000) and water were freely available.

Circadian Rhythm Analysis The activity data were plotted in actogram format (Figs. 1 and 2) and subjected to spectral analysis (Fig. 3) using software supplied with the Dataquest system. This spectral analysis estimated various parameters of the free-running rhythm (freerunning period, circadian amplitude, spectral magnitude) by fitting sinusoidal functions of varying period to the data using an iterative least squares (cosinor) regression procedure. This procedure is one of the most widely applied statistical analyses of circadian rhythmicity when period cannot be specified in advance (i.e., under free-running conditions) [see (19) for further discussion]. The period of the sinusoidal function that best fits the data (i.e., minimizes the residual variance) is taken as the estimate of free-running period; the amplitude of this function is taken as the estimate of circadian amplitude; and the proportion of variance accounted for by this function provides an estimate of spectral intensity. Plotting the variance accounted for by each test period yields a spectral representation of the data. In the present study, continuous 21-day data samples were fit by sinusoidal functions with periods ranging from 3 to 30 h in 0.10h increments, and all period estimates reported in this paper represent fit functions that accounted for sufficient variance to satisfy a 0.01 significance criterion. Since the amplitude of the best-fit sinusoidal function is strongly correlated with its mean level, amplitude ratios were computed by dividing each amplitude by its mean level. This transformation provides a measure of rhythm amplitude that is uncorrelated with level of activity, and thereby allows unconfounded amplitude comparisons between animals, between strains, or between treatment conditions that differ in overall activity level. Activity levels are reported as wheel revolutions per 24 h.

Procedures All animals were maintained under constant light of moderate intensity (LL; approximately 25-50 lux depending on cage position) throughout the experiment. The experiment consisted of three phases; a predrug baseline, a drug treatment phase, and a postdrug recovery phase, each lasting from 26-35 days. During the drug treatment phase of the experiment, clonidine hydrochloride (Sigma) was added to the drinking water at a concentration of 5.0 txg/ml. Daily fluid intake was monitored during clonidine treatment in order to estimate actual drug doses. The free-running period, spectral profile, amplitude ratio, and level of activity were determined for each animal for each of the three conditions. With one exception (see Fig. 2 caption), the final 21 days of each condition were used in the analyses to exclude transients that frequently accompanied the introduction and termination of drug treatment. Group data were analysed using a 2 × 3 (SHR vs. WKY; predrug vs. clonidine treatment vs. postdrug conditions) repeated measures ANOVA, followed by separate one-factor ANOVAs when the two-factor analysis revealed significant interactions. RESULTS

Figures 1 and 2 present double-plotted circadian actograms obtained from three representative animals of each strain. During predrug baseline conditions, all animals in both strains displayed

flee-running rhythms with periods longer than 24 tl and characterized by a clearly defined active phase. Although activity rhythms were generally quite stable throughout baseline observations, a few animals appeared to exhibit instability of freerunning period (e.g., Fig. 2B, C) or expression of weak secondar) activity components (e.g., Figs. 1C, 2C). It should be pointed out that the records shown in Figs. I and 2 were chosen to illustrate the full range of expressed behavior, and thus overrepresent the occurrence of rhythm anomolies actually observed. During clonidine administration, animals of both strains showed less well-organized activity patterns (Figs. 1 and 2). While most animals continued to express clear rhythmicity, one SHR showed a complex activity pattern without clear active and inactive phases during clonidine administration (Fig. I B). Most records also showed clearly detectable shortening of free-running period during drug treatment. After termination of clonidine treatment, activity patterns were again well organized, with an occasional secondary component (e.g., Fig. 1C), and were quite similar to those seen during baseline. These qualitative features of the activity records were confirmed by the quantitative analyses. Figure 3 presents least squares spectra averaged across animals for each strain and condition. As reflected in these averaged functions, individual rats of both strains displayed clear circadian spectral peaks in the pre- and postclonidine data. Many of the individual spectra also contained weak but significant secondary spectral peaks at approximately 12 and/or 8 h, as well as occasional higher frequency peaks. During clonidine administration, most individuals showed a clear but less pronounced circadian spectral peak, and secondary higher frequency spectral peaks were consistently absent. However, one animal (e.g., Fig. I B) showed two weak but statistically significant spectral peaks within the circadian range; this animal was excluded from the statistical analysis of freerunning period described below. Figure 4 presents group means for the circadian rhythm variables determined by the spectral analysis (free-running period, amplitude ratio, and spectral magnitude) and for activity level, for each strain across conditions. Two-factor ANOVA revealed that SHRs showed shorter free-running periods, F( 1, 15) = 15.53, p < 0.001, and that periods were shortened during clonidine administration, F(2, 30) - 20.91, p < 0.001, but there was no strain-by-drug treatment interaction. The amplitude ratio and the spectral magnitude each reflect different aspects of the strength or coherence of the activity rhythm. Two-factor ANOVA showed that amplitude ratios were reduced during clonidine treatment, F(2, 32) = 39.11, p < 0.001, but that the two strains did not differ. Like amplitude ratios, spectral magnitudes were reduced during clonidine treatment, F(2, 32) - 24.12, p < 0.001, but unlike amplitude ratios, spectral magnitudes were greater in SHRs than in WKYs, F(I, 16) = 6.18, p < 0.05. The strain-by-drug treatment interaction was not significant for either variable. Two-factor ANOVA on activity levels revealed significantly greater activity in SHRs than WKYs, F(1, 16) = 17.17, p < 0.001, and reduced activity during clonidine treatment, F(2, 32) = 10.51, p < 0.001. Furthermore, activity level was the only dependent variable that showed a significant strain-by-drug treatment interaction, F(2, 32) - 6.30, p < 0.01. This interaction was further explored by conducting separate one-factor ANOVAs for each strain, which revealed that clonidine significantly lowered activity levels only in the SHRs [SHR: F(2, 16) = 15.0 l, p < 0.001; WKY: F(2, 16) = 1.44, NS]. The failure of clonidine to significantly reduce activity levels in the WKY strain must be interpreted with caution, because the two strains differed in their levels ofpredrug activity. Indeed,

CIRCADIAN RHYTHMS IN SHR AND WKY

SHR A

25

B L ;1"

i

" .

.

.

.

.

. . . . .

.,.-,

: ,, ,. %':,_,

-,

. . . . . . . . . . .

,'.;,'.;,

~

....

L' ......... , ;.J.

,,

-""" N

,",

. . . . .

~

"~

i

u

.

. . . .

~,.', ..... Ix

M

•h i

l

l

l

i

.,l'lb,~a

.

.

.I

~

i

-

[ -al

. I

ii

i.

0

,~

.,a,

..

.

.

.

•

-

"

~

I

m

d

I

I

':

~

l

~

M

l l l i O - i

. . . . . . . • J illl. i l l . a d ~ 1....,

,_.muhL. roll .~ * .ILU~

~ J

...'[~c.a~,

24

,

=r -

**

[,

I. . . . . . . . . . . ml..'~...~.~ ktl .......... • i I J ~ J l . 'L *il 0. n~ . . . . . * . ,~'~1.,

,,c%,. .

,

:. --i~.L,::='-.

Jil~l i

d

,

=

. . . .

I .11

. .

; ,-..- .,.,-- .-..;~

. .I

,

Id

.

",,'";

L

.

~~.-.~,

tl.~

C

0

24

48h

O

~

48h

48h

FIG. 1. Double-plotted circadian actograms for three representative SHR rats over the entire course of the experiment. Arrows in the left margin of each plot indicate the days on which clonidine treatment was initiated and terminated. The lines superimposed on the data were fit using circadian period and phase information generated by the spectral analysis. Each fit was based on the last 21 days of each condition. Rat B showed two weak but significant circadian periods during clonidine administration.

Fig. 5 shows that individual differences in the effect of clonidine on activity level (i.e., activity level during clonidine minus activity level during baseline) were related to differences in baseline activity for both strains (SHR: r = -0.80, p < 0.01; WKY: r = -0.80, p < 0.01). In fact, the combined results from the two strains were well described by a single regression line fit to all the data (r - -0.87, p < 0.001). These results suggest that both strain differences and individual differences in the effects ofclonidine on activity level may be accounted for by differences in predrug activity levels. The two strains did not differ in their intake of clonidine solution over the course of treatment (mean daily intake: SHR: 31.96 ml; WKY: 32.59 ml). A 2 X 2 ANOVA (SHR vs. WKY; first half of treatment vs. second half of treatment) revealed that fluid intake increased over the course of treatment, F(1, 16) = 69.81, p < 0.001, but did not differ between strains. Furthermore, individual differences in fluid intake were not related to differences in responsiveness to clonidine for any of the dependent measures. Finally, there were no significant pairwise correlations

among the dependent measures during any of the experimental conditions. DISCUSSION

This study revealed differences between SHRs and WKYs in circadian period, spectral magnitude, activity level, and in the effect of clonidine on activity level. In contrast, the two strains did not differ in circadian amplitude or in the effects of donidine on circadian amplitude, spectral magnitude, or circadian period. Because the data were obtained under constant light, the strain difference in free-running period could reflect either a difference in the endogenous period of the circadian pacemaker or a difference in the pacemaker's response to light, or both. However, we have recently found that the period of the free-running drinking rhythm differs between SHRs and WKYs under constant light but not under constant darkness (29). This result implies that the two strains differ primarily in their response to the period-altering effects of light intensity.

26

ROSENWASSER

WKY A ~".'....;

: . -.

.~ , . . . . . . ~.~

~

~

,%~-,,'-,:~. .,,..-

,

~._:-

~ . . , - - -

•k

. . . . . . . .

___

+

j

,~1

..

lui

.

.

~ L

.

,Is.

. .

,

:-~ IW.+

.

,..

.

+

£ . . , . ~ - :.-

.....

.........

~ _

Ic__~x..=

k." ,

11".

~

L

, m +

. , ~ , , ~ ,

_.

+

,,

~t

i

., .%,. . . . ~ . , '

. ,-

~ , . .. -:. ~ ."-' .

.l

l

~

•

I__L J~ ~ ' _ hi

~.,'lk+. a ~ , : :'"-'~;.Z;

.

..

-

~.:_..;: . ,

.....

i Jr-

i . + ,L. ,, .l ,l ldl a l i l ~ &Jll.~lll~l

k,,

I . ,

i,,++ .I

•

.~ ~• lltmlSLJl

ii

•m a u a ~ , ~xt., •

t l , . i t ~

t

,,

•

~

x

.~

i,

;

~ ,.

,,- .,: : ;: .. ,,,

.

~

.,|.,

.i~•.

.....

|..

j

.

-,

g-~

J

~..tl.

llA

Eli

.ll

,~,."; '. ~ ' , - .

I

.-..'

':,~_'%'-:

~a,a.=l,,a,~

, ~.

.... f2Z122.

,.~L' A . . ' ""

} .... --

,

. .;'.,..~;%,'i

~...~,, ,-~,~$,:, I,,,.,

~

~,aml.l..

.

i .%'/]

'

,

, i

, • .. .. ... ... .. ... ..... . . ",

.

~. ~mll,

. . . . ~____-SS'

t. . . . . .

. . . . " . ; - . ' d ~

I k

JLIl~%,lli

.

.

.

C

all,,__--L

.... ,

PLANIE

~

, ,i . . . . . . . . . . .

,

B

>~:~

ANI)

.~

m,

ai

.i

.

I

.

, ......

...........

"

~_~',~,~

2.',_~.

I~1.

,

L,"".

,i

~ |

.

.

) %1.',_ .

, ,:.,

.

IiI .

II i l l l l ~ l l ~

z.~

I, Illl

iI.I

,llzl

-'" ',. - ~

I~L I I ~

I

~-L--*

~

a

~.L

. . . . . .

--

. . . . . .

14

__1 ~

,

l'-

I

.

M

x

\'-

~.--~

' ,

.lm4i

.

I

,

_

_

J

,~L.L

.

Jt~

alL_t

II

1111

a

- - ~ _ l k

L

. J i,

~,~'-;

0

24

48h

.

0

,,

24

..,.=._,. ;,',

•

L

~

L

L

,ILJ~

j

_ 48h

:

~L

0

_.JtllllLl

.

.

.

llZkL . . . . . .

.

.

.

.

-

.

24

48

h

FIG. 2. Double-plotted actograms for three representativeWKY rats. Conventions as in Fig. 1 except that only the last 17 rather than 21 days were used in the spectral analysis for rat C in order to avoid including a large and apparently spontaneous period change•

Consistent with earlier results (15), SHRs were dramatically more active than WKYs, raising the possibility that the strain difference in free-running period was a secondary consequence of the differential activity levels (4,33,45). However, we failed to detect any within-strain correlation between free-running period and activity level. Furthermore, we have recently found that SHRs show shorter periods than WKYs under constant light even when drinking rhythms are monitored in small cages providing little opportunity for exercise (29). Therefore, it appears that the strain differences in period and activity level are not causally related. Both SHRs and WKYs showed shortened free-running period and reduced circadian amplitude during clonidine administration, confirming and extending previous studies from this laboratory using Long-Evans females (27,28) and males (30). Also consistent with our prior studies, clonidine administration generally reduced activity level, and this effect was most pronounced in animals showing the highest baseline activity. However, a strain-by-drug treatment interaction revealed that clonidine significantly reduced activity only in the relatively hyperactive SHR strain. This apparent strain difference in responsiveness to clonidine is similar to the individual differences described above:

clonidine reduced activity most consistently in the most active individuals and in the more active strain. It has recently been reported that at least some drug effects on circadian rhythms are indirectly mediated by drug-induced alterations in activity level (39). Therefore, it could be hypothesized that the effects of clonidine on free-running period are secondary to its effects on activity. Several observations argue against this hypothesis: 1. WKYs showed signifcant shortening of free-running period during clonidine treatment even in the absence of a significant effect on activity level; 2. individual differences in response to clonidine within each strain were not correlated across the different dependent measures; and 3. shortening of free-running period is generally associated with increased activity level, a relationship that is opposite in direction to the covariation of period and activity level seen during clonidine administration• Therefore, we conclude that the effects of clonidine on freerunning period and on activity level, like the strain differences

CIRCADIAN RHYTHMS IN SHR AND WKY

PRE

WKY

SHR

30

20

27 26,5 25.0

PRE

Bo J:

2o~

40.0

PRE ~'m CLON r ~ l POST m m

lnm

25.5

20.0

10

10.0

24.5 0

0.0

24.0 l

5

10

15

20

25

30

35

0

l

5

,

10

,

l

15

WKY

SHR

,

20

25

WKY

SHR

30

i,i Iz

~°I2o,oCLON

CLON

(.9 <

20

__J

10

< EE

L~ L~J Q_ U~

8.o

2.0

30

x

2.0

o ~ ~

0 5

10

15

20

25

30

0

35

0il 1hi in/ I

£

1.5

0.5

I

I

I

I

I

J

5

10

15

20

25

30

35

0.0

o.o

WKY

SHR

POST 20

20

10

1

WKY

SHR

FIG. 4. Group mean free-running period (upper left), amplitude ratio (lower left), spectral magnitude (upper fight), and activity level (lower right) for both strains across experimental conditions (predrug baseline, during clonidine administration, and postdrug condition). Error bars indicate one standard error of the mean (SEM).

30

0 5

10

15

20

25

30

PERIOD (h)

35

0

, 5

, 10

, 15

, 20

, 25

i 30

35

PERIOD (h)

FIG. 3. Least squares spectral analyses averaged across individual animals for both strains and each experimental condition (predrug baseline, during clonidine administration, and postdrug condition). The curves have been displaced above the horizontal axis for clarity. in period and activity level, represent two independent effects mediated by distinct mechanisms. SHRs and WKYs have been reported to differ in several aspects of noradrenergic function (11,32), and in their physiological responsiveness to clonidine (6,34). Therefore, noradrenergic mechanisms could underlie both the strain difference in baseline rhythmicity and the strain-differentiated effects of clonidine on activity rhythms reported here. For example, since spontaneous noradrenergic neuronal activity is reduced in SHRs (6) and during chronic clonidine treatment (5), it is reasonable to speculate that reduced noradrenergic activity may underlie both the short free-running periods seen in SHRs and the period-shortening effects of clonidine. This hypothesis is consistent with our preliminary finding that DSP-4, a selective noradrenergic neurotoxin, produces alterations in free-running period that are similar to those seen with clonidine (30), and with previous findings of lengthened free-running period during chronic treatment with antidepressants (3,14,43), amphetamine (10), and stress [(35,36); see (42)]. However, reduced noradrenergic neuronal activity cannot account for both strain differences and clonidine effects on activity level, because activity is increased in the SHRs but decreased during clonidine administration. Indeed, the acute activity-suppressing effects of clonidine are probably not mediated by reduced noradrenergic activity (20). Alterations in circadian rhythmicity have been described in patients with affective disorders (40,41) and in various animal

models of depression (31). Depressed patients have most commonly been reported to show phase-advanced circadian rhythms (thought to be indicative of a short underlying free-running period) and reduced circadian amplitude, as well as reduced activity level (psychomotor retardation). These changes are quite s i m i l e to those seen during clonidine administration, consistent with the suggestion that chronic clonidine administration may provide a pharmacological animal model of depression (7). Traditionally, behavioral differences between SHRs and WKYs are generally interpreted as reflecting pathology in the SHRs. However, recent studies suggest that WKYs, not SHRs, show unusual emotional reactivity in comparison to more standard laboratory strains (22,23), and it has been suggested that the WKY strain may provide an animal model of anxious

2000

1000

a I z

o

O -1000 -2000 -3000 -4000 0 SHR • WKY

-5000 -6000 O

' 1000

, 2000

0 n

a

3000

BASELINE

4000

0

5000

l

I

6000

7000

8000

ACTIVITY (TURNS/DAY)

FIG. 5. Relationship between clonidine-induced changes in activity level and baseline activity level for both SHRs (open circles) and WKYs (filled circles).The regressionline was fit to the combined data set by the method of least squares.

28

ROSENWASSER A N D P I , A N l l

depression. Therefore, our finding of lengthened free-running period in WKYs is consistent with the period lengthening and phase delays seen in another putative animal model of anxious depression, the olfactory, bulbectomized animal (16,18,24,25), in a strain of mice showing increased irritable aggression (1), and in human depressive syndromes associated with anxiety or agitation ( 12,17,37). Possibly, both depressed patients and animal depression models may be similarly categorized into phase-ad-

vanced (short period) and phase-delayed 0ong period) subtypes (31,38). In conclusion, the strain differences and drug eflkcts described in this study are consistent with the hypothesis that noradrenergic systems influence the circadian period and level of spontaneous locomotor activity. Altered noradrenergic transmission may also mediate relationships between circadian rhghmicity and normal and abnormal affective state.

REFERENCES 1. Benus, R. F.; Koolhass, J. M.; van Oortmerssen, G. A. Aggression and adaptation to the light-dark cycle: Role of intrinsic and extrinsic control. Physiol. Behav. 43: t 31- 137; 1988. 2. Danysz, W.; Plaznik, A.; Pucilowski, O.: Plewako, M.; Obersztyn, M.; Kostowski, W. Behavioral studies in spontaneously hypertensive rats. Behav. Neural Biol. 39:22-29; 1983. 3. Duncan, W. C.; Tamarkin, L.: Sokolove, P. G.; Wehr, T. A. Chronic clorgyline treatment of Syrian hamsters: An analysis of effects on the circadian pacemaker. J. Biol. Rhythms 3:305-322; 1988. 4. Edgar, D. M.; Martin, C. E.; Dement, W. C. Activity feedback to the mammalian circadian pacemaker: Influence on observed measures of period length. J. Biol. Rhythms 6:185-199; 1991. 5. Engberg, G.; Elam, M.; Svensson, T. H. Clonidine withdrawal: Activation of brain noradrenergic neurons with specifically reduced alpha-2 receptor sensitivity. Life Sci. 30:235-243; 1982. 6. Engberg, G.; Oreland, L.; Thoren, P.; Svensson, T. Locus coeruleus neurons show reduced alpha2-receptor responsiveness and decreased basal activity in spontaneously hypertensive rats. J. Neural Transm. 69:71-83; 1987. 7. Enginar, N.; Eroglu, L. The long term clonidine treatment induced behavioral depression in rats. Pol. J. Pharmacol. Pharm. 42:409415; 1990. 8. Eriksson, E.; Dellborg, M.; Soderpalm, B.; Carlsson, M.; Nilsson, C. Growth hormone responses to clonidine and GRF in spontaneously hypertensive rats: Neuroendocrine evidence for an enhanced responsiveness of brain alpha2-adrenoceptors in genetical hypertension. Life Sci. 39:2103-2109; 1986. 9. Gehlert, D. R.; Wamsley, J. K. Quantitative autoradiography of alpha2 agonist binding sites in the spontaneously hypertensive rat brain. Brain Res. 409:308-315; 1987. 10. Honma, S.; Honma, K.; Hiroshige, T. Methamphetamine effects on rat circadian clock depend on actograph. Physiol. Behav. 49:787795; 1991. 11. Howes, L. G. Central catcholaminergic neurons and spontaneously hypertensive rats. J. Autonom. Pharmacol. 4:207-217; 1984. 12. Kaspar, S.; Rosenthal, N. E. Anxiety and depression in seasonal affective disorders. In: Kendal, P. C.; Watson, D., eds. Anxiety and depression: Distinctive and overlapping features. San Diego: Academic Press; 1989:341-375. 13. Krauchi, K.; Wirz-Justice, A.; Willener, R.; Campbell, I. C.; Feer, H. Spontaneous hypertensive rats: Behavioral and corticosterone response depend on circadian phase. Physiol. Behav. 30:35-40; 1983. 14. Kripke, D. F.; Mullaney, D. J.; Gabriel, S. The chronopharmacology of antidepressant drugs. Annu. Rev. Chronopharmacol. 2:275-289; 1987. 15. Leaton, R. N.; Cassella, J. V.; Whitehorn, D. Locomotor activity, auditory startle and shock thresholds in spontaneously hypertensive rats. Physiol. Behav. 3 l:103-109; 1983. 16. Leonard, B. E.; Butler, J.; O'Neill, B.; O'Connor, W. T. Bilaterally olfactory bulbectomized rat model of depression. In: Lerer, B.; Gershon, S., eds. New directions in affective disorders. New York: Springer; 1989:13-16. 17. Lewy, A. L.; Sack, R. L.; Miller, S.; Hoban, T. M. Antidepressant and circadian phase-shifting effects of light. Science 235:352-354; 1987. 18. Lumia, A. R.; Salchli, F. J.; Ayers, E. L.; McGinnis, M. Y.; McEwen, B. S. Olfactory bulbectomy model of depression: Ejaculatory and circadian dysfunction and their response to antidepressant treatment. Soc. Neurosci. Abstr. 13:1035; 1987.

19. Minors, D. S.; Waterhouse, J. M. Mathematical and statistical analysis of circadian rhythms. Psychoneuroendocrinology 13:443-464: 1988. 20. Nassif, S.; Kempf, E.; Cardo, B.: Velley, L. Neurochemical lesions of the locus coeruleus of the rat does not suppress the sedative effect ofclonidine. Eur. J. Pharmacol. 91:69-76; 1983. 21. Olmos, G.; Miralles, A.; Barturen, F.; Garcia-Sevilla, J. A. Decreased density and sensitivity of alpha2-adrenoceptors in the brain of spontaneously hypertensive rats. Eur. J. Pharmacol. 205:93-96; 1991. 22. Pare, W. P. "Behavioral despair" test predicts stress ulcer in WKY rats. Physiol. Behav. 46:483-487; 1989. 23. Pare, W. P. Stress ulcer susceptibility and depression in WistarKyoto (WKY) rats. Physiol. Behav. 46:993-998; 1989. 24. Pieper, D. R.; Lobocki, C. A. Olfactory bulbectomy lengthens circadian period of locomotor activity in golden hamsters. Am. J. Physiol. 261:R973-R978; 1991. 25. Possidente, B.; Lumia, A. R.: McGinnis, M. Y.: Teicher, M. H.; deLemos, E.; Sterner, L.; Deros, L. Olfactory bulb control of circadian activity rhythm in mice. Brain Res. 513:325-328: 1990. 26. Rogers, L. J.; Sink, H. S.; Hambley, J. W. Exploration, fear and maze learning in spontaneously hypertensive and normotensive rats. Behav. Neural Biol. 49:222-233; 1988. 27. Rosenwasser, A. M. Effects of chronic clonidine administration and withdrawal on flee-running circadian activity rhythms. Pharmacol. Biochem. Behav. 33:291-297: 1989. 28. Rosenwasser, A. M. Free-running circadian activity rhythms during long-term clonidine administration in rats. Pharmacol. Biochem. Behav. 35:35-39; 1990. 29. Rosenwasser, A. M. Circadian rhythmicity in SHR and WKY rats: Effects of light intensity. Soc. Neurosci. Abstr. 16:1334; 1990. 30. Rosenwasser, A. M. Pharmacological manipulation of the noradrenergic system alters circadian activity rhythms in the rat. Montreal, Canada: Abstracts of the Third IBRO World Congress of Neuroscience: 1991. 31. Rosenwasscr, A. M. Circadian rhythms and depression: Animal models? Light Treat. Biol. Rhythms 4:35-39; 1992. 32. Saavedra, J. M. Central biogenic amines and neuropeptides in genetic hypertension. In: Buckley, J. P.: Ferrario, C. M., eds. Central nervous system mechanisms in hypertension. New York: Raven Press; 198 t : 129-139. 33. Shiori, T.; Takahashi, K.; Yamada, N.; Takahashi, S. Motor activity correlates negatively with flee-running period, while positively with serotonin contents in SCN in flee-running rats. Physiol. Behav. 49: 779-786; 1991. 34. Soderpalm, B. The SHR exhibits less "anxiety" but increased sensitivity to the anticonflict effect of clonidine compared to normotensive controls. Pharmacol. Toxicol. 65:381-386; 1989. 35. Stewart, K. T.; Rosenwasser, A. M.; Hauser, H.; Volpicelli, J. R.; Adler, N. T. Circadian rhythmicity and behavioral depression: I. Effects of stress. Physiol. Behav. 48:149-155; 1990. 36. Stewart, K. T.; Rosenwasser, A. M.; Levine, J. D.; McEachron, D. L.; Volpicelli, J. R.; Adler, N. T. Circadian rhythmicity and behavioral depression: II. Effects of lighting schedules. Physiol. Behav. 48:157-164; 1990. 37. Teicher, M. H.; Lawrence, J. M.; Barber, N. I.; Finklestein, S. P.; Lieberman, H. R.; Baldessarini, R. J. Increased activity and phase delay in circadian motility rhythms in geriatric depression. Arch. Gen. Psychiatry 45:913-917; 1988. 38. Teicher, M. H.; Barber, M. I.; Lawrence, J. M.; Baldessarini, R. J. Motor activity and antidepressant drugs: A proposed approach to

C I R C A D I A N R H Y T H M S IN S H R A N D W K Y

categorizing depression syndromes and their animal models. In: Koob, J. F.; Ehlers, C. L.; Kupfer, D. J., eds. Animal models of depression. Boston: Birkhauser; 1989:135-161. 39. Van Reeth, O,; Turek, F. W. Stimulated activity mediates phase shifts in the hamster circadian clock induced by dark pulses or benzodiazepines. Nature 339:49-51; 1989. 40. Wehr, T. A.; Gillin, J. C.; Goodwin, F. K. Sleep and circadian rhythms in depression. In: Chase, M.; Weitzman, E. D., eds. Sleep disorders: Basic and clinical research. New York: Spectrum; 1983: 195-226. 41. Wehr, T. A. Biological rhythms and manic-depressive illness. In: Post, R. M.; Ballenger, J. C., eds. Neurobiology of mood disorders. Baltimore: Williams and Wilkins; 1984:190-206.

29 42. Weiss, J. M. Stress-induced depression: Critical neurochemieal and electrophysiologieal changes. In: Madden, J., ed. Neurobiology of learning, emotion and affect. New York: Raven; 1991:123-154. 43. Wirz-Justice, A.; Campbell, I. C. Antidepressant drugs can slow or dissociate circadian rhythms. Experientia 38:1301-1309; 1982. 44. Wirz-Justice, A.; Krauchi, K.; Campbell, I. C.; Feer, H. Adrenoceptor changes in spontaneous hypertensive rats: A circadian approach. Brain Res. 262:233-242; 1983. 45. Yamada, N.; Shimoda, K., Ohi, K.; Takahashi, S.; Takahashi, K. Free access to a running wheel shortens the period of free-running rhythm in blinded rats. Physiol. Behav. 42:87-91; 1988.