Median raphe, but not dorsal raphe, application of the 5-HT1A agonist 8-OH-DPAT stimulates rat motor activity

European Journal of Pharmacology, 160 (1989) 303-307

303

Elsevaer EJP 20297 Short communication

Median raphe, but not dorsal raphe, application of the 5-HTIA agonist 8-OH-DPAT stimulates rat motor activity V l v e k a H l l l e g a a r t 1 a n d S t e p h a n H j o r t h 2,. I Department of Psychology, Unwerstty of Goteborg~ POB 14158, S-400 20 Goteborg~ and 2 Department of Pharmacology, Umversttv of Goteborg, POB 33031, S-400 33 Goteborg, Sweden

Received 8 November 1988, accepted 6 December 1988

Local apphcation of the selective 5-HT1A agomst 8-hydroxy-2-(dl-n-propylanuno)tetrahn (8-OH-DPAT) in the median raphe of rats caused locomotor stimulation In contrast, dorsal raphe application of the compound induced flat body posture, winch was discontinuous and not dose-dependent, and therefore distinct from that characteristic for postsynaptic 5-HT receptor-mediated behaviour Injection of 8-OH-DPAT into the dorsal raphe or median raphe caused neither forepaw treading nor head-weaving, stiff tad and smfflng occurred inconsistently By activating somatodendntlc 5-HT1A autoreceptors in the median raphe, 8-OH-DPAT may dlslnhlblt locomotor-enforcing neural pathways that receive 5-HT afferents from tins nucleus The data suggest that medmn raphe and dorsal raphe 5-HT neurones have different roles in motor control Raphe nuclei, 5-HTIA agomsts, 8-OH-DPAT (8-hydroxy-2-(di-n-propylanuno)tetrahn), Motor control, (Rat)

1. I n t r o d u c t i o n

A l t h o u g h the role of braan s e r o t o m n ( 5 - H T ) systems in l o c o m o t i o n is n o t clearly d e f i n e d (for rev see e g G e r s o n a n d Baldessarlm, 1980), it is n o t a b l e that the 5 - H T cell b o d i e s in the braan stem r a p h e nuclei send p r o j e c t i o n s i n t o regions that are i m p o r t a n t for the i n i t i a t i o n a n d m a a n t e n a n c e of p r o p e r m o t o r function, e g the b a s a l g a n g h a a n d h m b l c f o r e b r a m a r e a s In a recent s t u d y we f o u n d t h a t 5 - H T r e f u s e d Into the d o r s a l or the m e d i a n r a p h e e h c l t e d d i f f e r e n t i a l effects with respect to m o t o r acuvaty in the rat, whale 5 - H T injection i n t o the d o r s a l r a p h e o n l y c a u s e d s u p p r e s s i o n of m o t o r acuvxty, a clear s t i m u l a t i o n was seen after its a d m l m s t r a t m n into the M R ( H l l l e g a a r t et a l , 1988, submatted) Based on in vitro r a d l o h g a n d b i n d i n g criteria, c e n t r a l 5 - H T r e c e p t o r s c a n be subclasslfied i n t o

* To whom all correspondence should be addressed

several p u t a t i v e l y distinct categories T h e 5-HT1A r e c e p t o r s u b t y p e - of p r o v e n f u n c t i o n a l relevance - a p p e a r s p a r t i c u l a r l y a b u n d a n t in the a n t e r i o r r a p h e nuclei ( M a r c l n l o e w l c z et a l , 1984) A l s o with respect to o u r p r e v i o u s findings with 5 - H T (cf above), we d e e m e d it of interest to s t u d y the b e h a v l o u r a l effects of l n t r a - d o r s a l r a p h e a n d lnt r a - m e d l a n r a p h e a d r m m s t r a t l o n o f the selective 5-HT agomst 8-hydroxy-2-(dl-n-propylanuno)tet r a h n ( 8 - O H - D P A T , H j o r t h et a l , 1982, 1987), the m o s t p o t e n t a n d specific h g a n d h i t h e r t o described for 5-HT1A sites ( e g M l d d l e n u s s a n d F o z a r d , 1983)

2. M a t e r i a l s and m e t h o d s

T w e n t y - s e v e n m a l e W l s t a r rats ( M o l l e g a a r d , Vejle, D e n m a r k ) , weighing 280-320 g, were gaven i n t r a c e r e b r a l injections F i v e a d d i t i o n a l rats f r o m the same b a t c h served as u n t r e a t e d c o n r o l s T h e rats a r r i v e d o n e week p r i o r to surgery, so as to

0014-2999/89/$03 50 © 1989 Elsexaer Science Pubhshers B V (Btomechcal Dlwslon)

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adapt to the laboratory conditions (temperature 25°C, hurmdlty 60-65%, 12/12 h h g h t / d a r k cycle hghts off 10 00-22 00, standard laboratory chow (R34, Ewos, Sodertalje, Sweden) and tap water ad hbltum) For surgery, the ammals were anaesthetlsed with pentobarbltone (60 m g / k g i p ) and a guide cannula (25 G) was stereotaxacally placed into the skull bone, 7 8 mm posterior to bregma and either 2 3 of 3 4 mm (left) lateral from the rmdllne suture (coordinates accordmg to Paxmos and Watson, 1986) The grade cannulas were implanted at an angle of 30 o to the vertical, and reached, but did not penetrate, the dura (cf Hlllegaart et al, 1988) After guide cannula implantation, the rats were housed singly one week before the behavloural experiment was performed The rats were also kept in their home cages d u n n g the experimental session, being removed only for injection 8 - O H - D P A T × HBr (RBI, Wayland, MA, USA) was freshly prepared in physiological sahne The drugs were injected into the raphe nuclei via the grade with an injection cannula (31 G) extending either 6 8 mm dorsal raphe or 8 5 mm (medium raphe) below the dura Drug or vehicle was infused in a volume of 0 5 /~1 over a period of 90 s and the cannula was left in situ for another 60 s Ten minutes later each ammal was observed for 60 s in its home cage by an observer who was unaware of the treatment The behavioural items scored were locomotor stlmulatxon, flat body posture, muscular tone (manually assessed), forepaw treading, sniffing, head-weaving and stiff tail (Straub) The following scoring system was used 0 = absent, 1 = mild, 2 = moderate and 3 = prominent Muscular tone was scored only quahtatlvely Increase ( + ) or decrease ( - ) as compared to normal (0) controls Statistical analysis of the experimental data was performed by means of Mann-Whitney U-test or by Flsher's exact probablhty test Probabdltles of < 5% were considered statistically sigmflcant

3. Results In a separate study in which the same rejection techmque was used, gross histologtcal mspecUon of the brains postmortem demonstrated that the

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8-OH-DPAT, ug Fig 1 I n f u s i o n of 8 - O H - D P A T i n t o the m e d i a n ( M R ) , b u t n o t i n t o the d o r s a l ( D R ) r a p h e n u c l e u s s t i m u l a t e s m o t o r a c t i v i t y T h e m o t o r a c t i v i t y s t i m u l a t i o n scores are s h o w n m e d i a n s o f 4-5 o b s e r v a t i o n s m e a c h t r e a t m e n t g r o u p S t a u s U c s M a n n W h i t n e y U-test, * * P < 0 01 vs c o r r e s p o n d i n g vetucle c o n t r o l injection ( = 0 # g ) n s = n o t s l g m f i c a n t U n t r e a t e d c o n t r o l s (n = 5 ) d i d n o t d~splav m o t o r activity s t i m u l a t i o n m e d i a n ( r a n g e ) = 0 (0-0), n s

median raphe and dorsal raphe rejection sites were within the limits of the intended target areas in 17 out of 20 cases (Hlllegaart et al, subrmtted) As shown in fig 1, mjecuons of both 1 and 5 /~g of 8-OH-DPAT into the MR caused a clearcut, significant (P < 0 01) stimulation of locomotor activity, which was occasionally accompanied by rearing (not scored gross behavloural observations) In contrast, application of 8 - O H - D P A T in the dorsal raphe faded to increase locomotion Indeed, some of these ammals even appeared less active than the vehicle-injected ammals although, since the motor actiwty of rats maintained in their home cages ~s already very low, a decrease in mouhty could not be proven under the present test conditions Forepaw treading and head-weaving were not detected after any treatment (0 _+ 0 for all groups, n = 4-5) Stiff tail and sniffing occurred occasionally in some animals after either intra-dorsal raphe or mtra-medlan raphe 8-OHDPAT, but was not observed in the vehicle-injected nor m the untreated control rats In no case was there any consistent trend or slgmficant effect of 8-OH-DPAT compared with vehicle (data not shown)

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Fig 2 lnodence of flat body posture after mfuston of 8-OHDPAT into the median (MR) or the dorsal (DR) raphe nucleus The percent lnodence of flat body posture m the different treatment groups (n = 4-5) ~s shown Statlsttcs Flsher's exact probablhty test, * P = 0 04 and n s = not slgmficant vs corresponding vehicle control reJection ( = 0 p,g) Untreated controls (n = 5) dxd not dtsplay flat body posture (incidence = 0% ns)

Frequent discontinuous, non dose-dependent bouts of flattened body posture were observed in the two groups of rats receiving 8 - O H - D P A T in the dorsal raphe (incidence 50 and 80% after 1 (n = 4) and 5 #g ( n - - 5 ) 8-OH-DPAT, respecuvely), but was only rarely observed after median raphe injection (only one out of a total of nine rats), and never in the vehacle-lnjected or in the untreated animals (fig 2) This behavlour, in which the rats periodically lie down flat and rest 5-10 s, was quite different from the (prolonged and hypot o n i c ) flat body posture displayed as part of the ' 5 - H T syndrome', and which is seen after systemic injections of high dose ( > 0 1 m g / k g s c ) of 8O H - D P A T (cf e g Hjorth et a l , 1982) There were no changes in muscular tone in any of the experimental groups

4. Discussion

The results of tlus study demonstrate distinct and differential effects on motor activity in a farmllar environment of the selective 5-HT1A agonist 8 - O H - D P A T after infusion into either of

the two major 5-HT cell body-containing anterior raphe nuclei Thus, while there was a clear stimulation of locomotor activity after median raphe application of 8-OH-DPAT, no such effect was evident following its apphcatlon in the dorsal raphe In the latter cases, however, the ammals frequently but discontinuously, assumed a flattened body position (cf Results section) It should be pointed out that this behaviour was not slrmlar to the characteristic continuous and dosedependent flat body posture induced by postsynaptlc 5-HT receptor stimulation, and that the ratlngs for (other) components of the ' 5 - H T syndrome' ( e g forepaw treading, Straub tail) were negative in all the treatment groups Interestingly, in a recent study on the behavloural effects of lntra-raphe (dorsal or median raphe) injections of 5-HT, findings that are practically identical to those now described with 8 - O H - D P A T were obtained (Hlllegaart et al, 1988), and subnutted) Furthermore, Carh and Samanln (1988) investigating the effect of 8 - O H - D P A T in animal anxiety models, observed increased exploration in a novel environment (two-compartment exploratory test) after median raphe, but not after dorsal raphe, infusion of the compound These authors also cited their own unpublished findings that xntra-dorsal raphe 8 - O H - D P A T reduced a motor activity measure used in this test, which they suggested was due to the simultaneous occurrence of flat body posture Available data thus indicate that stimulation of 5-HT, tentatively 5-HTIA, receptors in the median raphe area can elicit increased motor activity both in a famlhar (homecage, present findings, Hillegaart et al, 1988. and submatted) and in an unfanuhar environment (Carh and Samanin, 1988) By contrast, stimulation of such receptors in the dorsal raphe does not stimulate locomotion, but may induce vegetative changes that negatively affect normal body posture without causing muscular h y p o t o m a Electrophyslologlcal studies have shown that nucrolontophoresis of 8 - O H - D P A T (like of 5-HT Itself) onto dorsal raphe 5-HT cells suppresses neuronal impulse flow, presumably via activation of somatodendntlC 5 - H T 1 A autoreceptors (Sprouse and Aghajaman, 1987) Furthermore, slrmlar to the effects of systemic 8 - O H - D P A T injections

306 ( H j o r t h et a l , 1982). local a p p h c a t m n of the drug i n t o the dorsal raphe results m a reduced synthesis a n d r e l e a s e / t u r n o v e r of the t r a n s n u t t e r in the terminals, a n d thus m a t t e n u a t e d 5 - H T synaptxc transmassmn m the relevant projection areas, e g the s t r l a t u m ( H j o r t h et a l , 1987, H j o r t h a n d Magn u s s o n , m press) Interestingly, m tlus context the t r y p t o p h a n hydroxylase mhabltor para-chlorop h e n y l a l a m n e (PCPA) reduces m o t o r excitation per se (see G e r s o n a n d B a l d e s s a n m . 1980), a n d does n o t i n h i b i t the h y p e r l o c o m o t m n reduced by systermc 8 - O H - D P A T ( T n c k l e b a n k et a l , 1984) While it c a n n o t be entirely excluded that the m o t o r effects of 8 - O H - D P A T seen m the present study revolve a n a c t m n on p o s t s y n a p t l c 5 - H T receptors &stal to short projection serotonerglc n e u r o n e s t e r n u n a t m g in the raphe region, the present data are c o m p a t i b l e with the m t e r p r e t a t m n that the c o m p o u n d stimulates m o t o r actlwty by activating s o m a t o d e n d n t l c 5-HTxA autoreceptors in the m e & a n raphe, t h e r e b y d l s l n h l b l t l n g l o c o m o t o r - e n f o r c i n g n e u r a l pathways that receive 5 - H T afferents from tlus nucleus M a j o r projecu o n s of the m e d m n a n d dorsal raphe 5 - H T nucle~ i n c l u d e the Imablc f o r e b r a m regmns a n d the basal g a n g h a (strlatal as well as mgral sites), respectwely (e g Molhver, 1987) I n terms of f u n c t m n , it is well-estabhshed that these areas are assocmted with the ~mtmtmn of m o v e m e n t a n d the m a i n t e n a n c e of posture d u r i n g the execution of movements Since s t i m u l a t i o n of s o m a t o d e n d r m c 5-HTaA autoreceptors suppresses the o u t p u t of 5 - H T n e u rones from the m e d m n raphe a n d dorsal raphe pathways, these projections m a y play an i n h i b i tory a n d a possible excitatory, respectively, role in the m a i n t e n a n c e of p r o p e r m o t o n c f u n c t m n This could be relevant to the controversy m the hterature a b o u t 5 - H T a n d m o t o r control (see G e r s o n a n d Baldessarlnl, 1980) I n c o n c l u s m n , the present study c o n f i r m s a n d extends previous ( H d l e g a a r t et a l , 1988), subn u t t e d , C a r h a n d S a m a m n , 1988) findings that i n d i c a t e that m e d m n raphe a n d dorsal raphe 5 - H T n e u r o n e s have &fferent, perhaps even opposing. roles m m o t o r c o n t r o l Thus, i n f u s i o n of the selective 5-HTIA agomst 8 - O H - D P A T m the raphe nuclex leads to either m o t o r s t i m u l a t i o n ( m e d i a n

raphe Injection) or lmpaxrment (dorsal raphe mjectlon) A similar dlfferentml p a t t e r n of m o t o r effects after m e d m n raphe vs dorsal r a p h e l n f u stun of 5 - H T was described previously ( H d l e g a a r t et a l , 1988, subnutted), i m p l y i n g that 5-HT1A receptors were involved in thts case also Needless to say, verification of the present i n t e r p r e t a t i o n s has to await the ldentlficat~on of selective a n d specific 5-HT, m particular 5-HT1A, receptor antagomsts Finally, the present findings add to the growing b o d y of data that d e m o n s t r a t e s the distinct n e u r o a n a t o n u c a l , morphological, p h a r m a c o l o g i c a l a n d f u n c u o n a l properties of the m e d m n a n d dorsal raphe serotonergtc pathways (cf Molllver, 1987)

Acknowledgements The financial support from the Swedish Medical Research Council (Grant No 7486), the Swedish Humamtles and Social Sciences Research Council, the Bank of Sweden Tercentenary Foundation, 'T & R Soderbergs Fonder' and "Anna Ahrenbergs Fond' is gratefully acknowledged

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Sprouse, J S and G K Aghajaman, 1987, Electrophyslologlcal responses of happocampal pyramidal cells to serotomn 5HT-1A and 5-HT-1B selective agonlsts a comparative study with dorsal raphe neurons, Soc Neuroscl Abstr 13, 1650 (No 459 9) Tncklebank, M , C Forler and J R Fozard, 1984, The involvement of subtypes of the 5-HT 1 receptor and of catecholarmnerglc systems in the behavloural response to 8-hydroxy-2-(&-n-propylarmno)tetrahn m the rat, European J Pharmacol 106, 271