Philip J. Gatti et al- Can neurons in the nucleus ambiguus selectively regulate cardiac rate and atrio-ventricular conduction?

8/3/2019 Philip J. Gatti et al- Can neurons in the nucleus ambiguus selectively regulate cardiac rate and atrio-ventricular cond

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Journal of theAutonomicNervousSystemELSEVIER J ou rna l o f the A u tonom ic N ervous S ys tem 57 (1996) 12 3 -127Short Communication

C a n n e u r o n s i n t h e n u c l e u s a m b i g u u s s e l e c t i v e l y r e g u l a t e c a r d i a c r a t eand a tr io -ventr i cu lar conduct ion?

Phi l ip J . Gat t i a , * , Tanni s A . Johnson b , V . J o h n M a s s a r i aa Department o f Pharmacology, How ard University College of Medicine, 520 W Street, NW , Washington, DC 20059, U SA

b Department o f Veterinary Services, Howa rd University College of Medicine, W ashington, DC 20059, U SAReceiv ed 11 Apri l 1995; revis ion received 14 Septem ber 1995; accepted 15 Septem ber 1995

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Previous anatomic data have described the distribution of presumptive negative chronotropic and negative dromotropic neurons in theventro-lateral nucleus am biguus (N A-V L) following injections o f retrograde tracers into physiologically selective parasympatheticintracardiac ganglia. Negative dromotropic n eurons w ere preferentially distributed in the rostral NA -V L (rNA-VL ). N egative chronotropicneurons were preferentially distributed in the caudal NA-VL (cNA-VL). Significant numbers of both types of cardio-inhibitory neuronswere observed to overlap in an intermediate level of the NA-VL (iNA-VL). In the present report , we have examined the effects ofmicroinjections of the excitatory amino-acid glutamate (G LU ) into the cNA-VL and iNA-V L on cardiac rate and AV conduction whilerecording the electrocardiogram in paced and non-paced cat hearts. The data indicate that: (i) excitation of neurons in the cNA-VL causesa 58 + 17% reduction in cardiac rate, without influencing A V conduction; and (//) excitation o f neurons in the iNA-V L causes both areduction in hear t rate (68 + 1 2% ) and a decrease in the rate of AV conduction (38 + 7%). T hese ph ysiological results support theanatomical inference that neurons in the cNA-VL that are retrogradely labeled from physiologically selective parasympathetic intracardiacganglia selectively exhibit negative chronotropic properties. Furthermore, the data indicate that there is a longitudinal cardiotopicorganization of both negative chronotropic and negative dromotropic neurons in the NA-VL. This CNS organization mirrors theperipheral organization of functionally selective cardiac components of the vagus nerve. Finally, the data are consistent with thehypothesis that anatomically separated and functionally selective parasympathetic preganglionic vagal motoneurons in the NA indepen-dently control cardiac rate and AV conduction.Keywords: N ega t ive ch rono t rop ic neu ron ; N ega t ive d rom ot rop ic neu ron ; G lu tam ate ; V agus ne rve

D u r i n g t h e p a s t d e c a d e , W . C . R a n d a l l a n d c o l l a b o r a t o rshave i n t ens i ve l y s t ud i ed t he phys i o l og i ca l e f f ec t s o f s e l ec -t i ve ab l a t i on o f s pec i f i c i n t r aca r d i ac gang l i a on va r i ousa s pec t s o f c a r d i ac pe r f o r m ance [ 17 ,19 ,22 ] . The i r da t a i nd i -ca t e t ha t t he r e a r e ana t om i ca l l y s eg r ega t ed and phys i o -l o g i c a l l y i n d e p e n d e n t p a r a s y m p a t h e t i c g a n g l i a o n t h e s u r -f ace o f t he hea r t w h i ch a r e capab l e o f s e l ec t i ve con t r o l o fs i no - a t r i a l ( SA ) r a t e , a t r i o - ven t r i cu l a r ( A V ) conduc t i on anda t r i a l con t r ac t i l i t y . The nega t i ve ch r ono t r op i c e f f ec t s o fs t i m u l a t i n g t h e v a g u s n e r v e a r e m e d i a t e d b y a g r o u p o fgang l i a l oca t ed i n a f a t pad ove r l y i ng t he r i gh t pu l m ona r yve i ns a t t he j unc t i on o f t he r igh t a t r i um and s upe r i o r ven acava w h i ch w e r e f e r t o a s t he SA gang l i on . Su r g i ca lexc i s i on o f t he SA gang l i on , o r l oca l i n j ec t i ons o f gan -

" C orresponding author . Tel .: (1-20 2) 806-6311; Fax: (1-202) 806-7934.

016 5-18 38 /9 6 /$1 5 .00 1996 El s ev ie r S c ience B .V . A l l r igh t s r e s e rvedSSDI 0 1 6 5 - 1 8 3 8 ( 9 5 ) 0 0 1 0 4 - 2

g l i on i c b l ock i ng d r ugs s e l ec t i ve l y b l ock t he nega t i vech r ono t r op i c e f f ec t o f vaga l s t i m u l a t i on , w i t hou t i n f l uenc -i ng A V conduc t i on , o r a t r i a l con t r ac t i l i t y . C onve r s e l y ,w h e n a s e l e c t i v e p h a r m a c o l o g i c a l o r s u r g i c a l b l o c k a d e o fgang l i a l oca t ed a t t he j unc t i on o f t he i n f e r i o r vena cavaa n d t h e i n f e r i o r l e f t a tr i u m w a s p e r f o r m e d ( th e A V g a n -g l i on ) , the nega t i ve d r o m o t r op i c e f f ec t s o f vaga l s t im u l a -t i on w e r e s e l ec t i ve l y b l oc ked w i t hou t i n f l uenc i ng nega t i vech r ono t r op i c o r i no t r op i c e f f ec t s [ 4 ,22 ] . I t w as conc l udedf r o m t h e s e d a t a t h at t h e S A g a n g l i o n a n d t h e A V g a n g l i o na r e t he m a j o r ( i f no t nece s s a r i l y exc l us i ve [ 1 ,3 ] ) s ou r ce s o fpos t - gang l i on i c pa r a s ym pa t he t i c neu r ons i nne r va t i ng t heS A a n d A V n o d e s , r e s p e c t i v e l y .

T h e r e m a r k a b l e a n a t o m i c a l o r g a n i z a t i o n o f f u n c t io n a l lys e l e c t i v e c o m p o n e n t s o f t h e p e r i p h e r a l v a g u s n e r v e s u g -ges t s t he pos s i b i l i t y t ha t t he r e m ay be an ana l ogous cen t r a ln e r v o u s s y s t e m o r g a n i z a t i o n . R e c e n t a n a t o m i c a l a n d


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p h y s i o l o g i c a l d a t a i n d i c a t e t h a t n e g a t i v e d r o mo t r o p i c n e u -r o n s c a n b e f o u n d i n t h e r o s t r a l v e n t r o l a t e r a l n u c l e u sa m b i g u u s ( r N A - V L ) . T h e s e n e u r o n s c a n b e r e t r o g r a d e l yl a b e l e d f r o m t h e AV g a n g l i o n a n d i n h i b i t a t r i o v e n t r i c u l a rc o n d u c t i o n w i t h o u t i n f l u e n c i n g c a r d i a c r a t e [ 1 7] . I n l i g h t o ft h e s e r e s u l t s , i t ma y r e a s o n a b l y b e a s k e d i f a s i mi l a ro r g a n i z a t i o n o f n e g a t i v e c h r o n o t r o p i c n e u r o n s ma y e x i s t i no t h e r p a r t s o f th e NA . I n t h e p r e s e n t r e p o r t we d e s c r i b e t h ep h y s i o l o g i c a l e f f e c t s o f mi c r o i n j e c t i o n o f t h e e x c i t a t o r ya m i n o - a c i d g l u t a m a t e ( G l u ) i n to t w o r e g i o n s o f t h e N A - V Lc a u d a l t o t h e r NA- VL . T h e f i r s t r e g i o n i s i n a n i n t e r me d i -a t e a r e a o f t h e NA- VL ( i NA- VL ) , a t t h e l e v e l o f t h e a r e ap o s t r e m a . S i g n i f i c a n t n u m b e r s o f b o t h n e g a t i v ec h r o n o t ro p i c a n d n e g a t iv e d r o m o t r o p i c n e u r o n s h a v e b e e no b s e r v e d h e r e f o l l o wi n g mi c r o i n j e c t i o n s o f r e t r o g r a d e t r a c -e r s i n t o s p e c i f i c i n t r a c a r d i a c g a n g l i a [ 1 7 , 1 8 ] . T h e s e c o n dr e g i o n i s c a u d a l t o o b e x i n t h e c a u d a l N A - V L ( c N A - V L ) .I n t h i s a r e a n e g a t i v e c h r o n o t r o p i c n e u r o n s a r e mo r e f r e -q u e n t l y l a b e l e d t h a n a r e n e g a t i v e d r o mo t r o p i c n e u r o n s[17,18].

1 1 a d u l t c a t s o f e i t h e r s e x we i g h i n g 2 . 5 - 4 . 0 k g we r ep r e m e d i c a t e d w i t h k e t a m i n e / a c e p r o m a z i n e s o l u t i o n ( 1 0m g / k g , i . m . ) p r i o r t o a d m i n i s t r a t i o n o f a - c h l o r a l o s e ( 7 0mg / k g , i . v . ) , a n d i n s e r t e d i n a s t e r e o t a x i c a p p a r a t u s . P u l -s a t i l e a r t e r i a l p r e s s u r e wa s o b t a i n e d f r o m t h e r i g h t f e mo r a la r t e r y . He a r t r a t e a n d c a r d i a c r h y t h m we r e o b t a i n e d f r o ml e a d I I o f t h e e l e c t r o c a r d i o g r a m. T h e r i g h t f e mo r a l v e i nwa s c a n n u l a t e d f o r t h e i n t r a v e n o u s i n j e c t i o n o f d r u g s . T h et r a c h e a wa s c a n n u l a t e d i n e a c h a n i ma l . T h e a n i ma l s we r et h e n p a r a l y z e d w i t h d e c a m e t h o n i u m ( 0 . 2 5 m g / k g , i . v . )a n d a r t i f i c i a l l y r e s p i r e d t o p r e v e n t a n y r e s p i r a t o r y c h a n g e sf r o m i n t e r f e r i n g w i t h c a r d i o v a s c u l a r re c o r d i n g . B l o o d g a s e sa n d p H we r e mo n i t o r e d p e r i o d i c a l l y a n d t h e r e s p i r a t o r yr a t e a n d t i d a l v o l u me we r e a d j u s t e d t o k e e p t h e s e v a l u e s i nt h e n o r ma l r a n g e f o r t h e c a t [ 7 ] . E a c h a n i ma l ' s c o r et e m p e r a t u r e w a s m a i n t a in e d a t 3 7 - 3 8 C u s i n g a s e r v o - c o n -t r o l l e d h e a t i n g p a d . T h e c h e s t wa s o p e n e d a l o n g wi t h t h ep e r i c a r d i u m, a n d a p a c i n g wi r e wa s s e wn o n t o t h e r i g h ta t r i u m. A t r i a we r e p a c e d a t 1 0 % a b o v e r e s t i n g h e a r t r a t e .P r o p r a n o l o l h y d r o c h l o r i d e ( 0 . 2 5 - 0 . 5 m g / k g , i . v . ; S i g m a ,S t . L o u i s , M O , U S A ) w a s i n j e c t e d t o b l o c k d i r e c t s y m p a -t h e t i c i n f l u e n c e s o n t h e h e a r t . T h e s e d o s e s o f p r o p r a n o l o lb l o c k e d b i l a t e r a l c a r o t i d o c c l u s i o n i n d u c e d r e f l e x t a c h y c a r -d i a . T h e d o r s a l s u r f a c e o f t h e b r a i n s t e m wa s t h e n e x p o s e d .Mi c r o i n j e c t i o n s we r e ma d e i n t o e i t h e r t h e l e f t o r t h e r i g h tNA u s i n g g l a s s mi c r o p i p e t t e s w i t h a n o u t s i d e t i p d i a me t e ro f a p p r o x . 3 0 , a m . A W P I N a n o p u m p ( W P I , O r la n d o , F L ,U S A ) w a s u s e d t o i n j e c t s m a l l v o l u m e s ( 2 0 - 4 0 n l ) o f 5 0m M L - g l u t a m i c a c i d ( m o n o s o d i u m s a l t ; S i g m a ) . E f f e c t i v ei n j e c t i o n s i t e s i n t h e NA- VL we r e d e t e c t e d o n e i t h e r s i d eo f t h e b r a i n , h o we v e r , t h e mo s t e f f e c t i v e s i d e v a r i e d f r o ma n i ma l t o a n i ma l .

T h e d e s i g n o f t h e e x p e r i me n t s wa s a s f o l l o ws : F i r s t ,b i l a t e r a l c a r o t i d o c c l u s i o n wa s p e r f o r me d . P r o p r a n o l o l wa st h e n i n j ec t e d ( 0 . 2 5 - 0 . 5 0 m g / k g , i .v . ). B l o c k a d e o f t h er e f l e x t a c h y c a r d i a e l i c i t e d b y b i l a t e r a l c a r o t i d o c c l u s i o n

i n d i c a t e d t h a t d i r e c t s y mp a t h e t i c e f f e c t s o n t h e h e a r t h a db e e n b l o c k e d . I n e a c h a n i ma l , t h e i n i t i a l mi c r o i n j e c t i o n swe r e ma d e i n t o t h e i NA- VL , a t t h e l e v e l o f t h e a r e ap o s t r e ma . A s l o wi n g i n c a r d i a c r a t e wa s i n d i c a t e d b y a ni n c r e a s e i n t h e R- R i n t e r v a l o n l e a d I I o f t h e E KG. 3 0 mi nl a t e r , h e a r t s we r e p a c e d , a n d Gl u wa s a g a i n i n j e c t e d .C h a n g e s i n A - V c o n d u c t i o n w e r e d e t e r m i n e d b y m e a s u r -i n g t h e s t i mu l u s ( $ ) - R i n t e r v a l i n t h e p a c e d h e a r t . W h e ns e c o n d d e g r e e h e a r t b l o c k w a s o b s e r v e d a f t e r G l u m i c r o in -j e c t io n s ( i . e. , c o m p l e t e a b s e n c e o f a Q R S c o m p l e x f o l lo w -ing a s t imulus a r t i f ac t ) , t he $-R in te rva l for tha t bea t wasd e f i n e d a s t w i c e t h a t o f a c o n t r o l b e a t . I n t wo a n i ma l s as e r i e s o f t wo Gl u i n j e c t i o n s wa s r e p e a t e d t w i c e a t 3 0 - mi ni n t e r v a l s t o c o n f i r m t h a t t h e p h y s i o l o g i c a l e f f e c t s n o t e dwe r e r e p r o d u c i b l e f r o m t h e s a me s i t e . T h e mi c r o e l e c t r o d ew a s t h e n p o s i t i o n e d 1 - 2 m m f u r t h e r c a u d a l l y i n t h e c N A -V L , a n d t h e e x p e r i m e n t a l p a r a d i g m w a s r e p e a t e d . C o n t r o lGl u i n j e c t i o n s we r e ma d e 1 . 5 mm me d i a l t o e f f e c t i v ei n j e c t io n s i te s . S u b s e q u e n t l y a n i ma l s r e c e i v e d a n i p s i l a t e r a lc e r v i c a l v a g o t o my , a n d t h e e n t i r e e x p e r i me n t a l p a r a d i g mwa s r e p e a t e d .

A t t h e e n d o f th e e x p e r i m e n t s , 2 0 - 4 0 n l o f b l u e i n k w a si n j e c t e d i n t o t h e c NA- VL f o r s u b s e q u e n t h i s t o l o g i c a l a n a l -y s i s o f t h e i n j e c ti o n s i t e s. Ca t s we r e k i l l e d b y t h e r a p i d i . v .i n f u s i o n o f a b o l u s o f c o n c e n t r a t e d KCI . B r a i n s we r ep r o m p t l y r e m o v e d a n d f r o z e n o n s o li d C O 2. S e r ia l c r y o s t a ts e c t i o n s we r e c o l l e c t e d o n s l i d e s, a i r d r ie d a n d s t a i n e d wi t hn e u t r a l r e d .

P h y s i o l o g i c a l d a t a a r e e x p r e s s e d a s t h e m e a n _+ S E M . Ap a i r e d t - t e s t wa s u s e d t o d e t e r mi n e t h e s t a t i s t i c a l s i g n i f i -c a n c e o f t h e d a t a . T h e c r i t e r i o n f o r s t a t i s t i c a l s i g n i f i c a n c ewa s P < 0 . 0 5 .

E f f e c t i v e i n j e c t i o n s i t e s we r e i d e n t i f i e d i n t h e i NA- VLa n d c N A - V L , b u t e x t e n d e d p a r t i a l l y i n t o b o t h t h e s e m i -c o mp a c t d i v i s i o n o f t h e NA d o r s a l l y , a n d t h e l a t e r a l r e t i c u -l a r n u c l e u s v e n t r a l l y . D i f f u s i o n o f t h e ma r k e r f o r o u ri n j e c t i o n s i t e s a p p e a r e d t o e n c o mp a s s a s p h e r i c a l a r e a o fa p p r o x . 0 . 9 - 1 . 2 m m i n d i a m e t e r ( F ig . 1 ).

W h e n Gl u wa s i n j e c t e d i n t o t h e c NA- VL , a s t a t i s t i c a l l ys i g n i f i c a n t 5 8 _+ 1 7 % d e c r e a s e i n c a r d i a c r a t e wa s o b s e r v e dd u r i n g t h e f i r s t 3 0 s ( F i g s . 2 a n d 3 A) . Ho we v e r , G l umi c r o i n j e c t i o n s d i d n o t c a u s e d e mo n s t r a b l e c h a n g e s i n AVc o n d u c t i o n a t t h i s s i t e ( F i g s . 2 a n d 3 B) . I n j e c t i o n s o f G l ui n t o t h e i N A - V L 1 - 2 m m r o s t r a l t o c N A - V L r e s u l t e d i n as t a t i s t i ca l ly s ign i f i ca n t 68 _+ 12% decre ase in ca rd iac r a t e(Figs . 2 an d 3C) , as w el l a s a s ign i f i can t 43 _+ 7% decr easei n AV c o n d u c t i o n ( F i g s . 2 a n d 3 D) . T h e t y p e o f AVb l o c k a d e o b s e r v e d v a r i e d w i t h t i m e a n d a c r o s s a n i m a l sa n d o c c a s i o n a l l y s h o we d t r a i n s o f 2 : 1 o r 3 : 1 b l o c k , o rc o mp l e t e h e a r t b l o c k . I p s i l a t e r a l c e r v i c a l v a g o t o my a b o l -i s h e d t h e e f f e c t s o f G l u mi c r o i n j e c t i o n s i n t o e i t h e r s i t e i nt h e N A - V L .

Nu me r o u s s t u d i e s h a v e a t t e mp t e d t o i d e n t i f y t h e o r i g i no f p a r a s y mp a t h e t i c p r e g a n g l i o n i c n e u r o n s i n n e r v a t i n g t h eh e a r t . P r e v i o u s s t u d i e s h a v e u t i l iz e d i n j e c ti o n s o f r e t r o -g r a d e t r a c e r s i n t o c a r d i a c n e r v e s [ 1 0 , 1 6 ], i n j e c t io n s i n t o t h e


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F i g . 1 . T h e h a t c h e d a r e a i n P 1 3 s c h e m a t i c a l l y i n d i c at e s th e l o c a t i o n o fm i c ro p i p e t t e t ip s i n t h e c a u d a l N A a t t h e s i t e wh e re a s e l e c t i v e n e g a t i v ec h ro n o t ro p i c e f f e c t wa s s e e n fo l l o wi n g g l u t a m a t e i n j e c t i o n . Ha t c h e d a r e ai n d i c a t e s t h e s i t e i n t h e i n t e rm e d i a t e NA wh e re m i x e d n e g a t i v ec h ro n o t ro p i c a n d n e g a t i v e d ro m o t ro p i c e f f e c t s we re o b s e rv e d fo l l o wi n gg l u t a m a t e i n j e c ti o n . T h i s f i g u re i s m o d i f i e d f ro m a n a r i as o f t h e c a t b r a i nwi t h t h e p e rm i s s i o n o f t h e a u t h o r s [2 2 ] a n d t h e p u b l i s h e r . Ab b re v i a t i o n s :AM B , n u c l e u s a m b i g u u s ; C U, n . c u n e a t u s ; C X, e x t e rn a l c u n e a t e n . ;DM V, d o r s a l m o t o r n . o f t h e v a g u s ; d s c , d o r s a l s p i n o c e re b e l l a r t r a c t; c a ,exte rna l a rcua te f ibe rs ; FFL, la te ra l t egmenta l f i e ld ; GR, n . grac i l i s ; IN,n . i n t e r e a l a t u s ; IOD, d o r s a l a c c e s s o ry i n f e r i o r o l i v a ry n . ; IOM , m e d i a la c c e s s o ry i n f e r i o r o l i v a ry n . ; IOP , p r i n c i p a l i n f e r io r o l i v a ry n ; L R , l a t e r a lr e t i c u l a r n . ; m l , m e d i a l l e m n i s c u s ; m l f , m e d i a l l o n g i t u d i n a l f a s c i c u l u s ;mrs , med ia l re t iculospina l t ract ; 12n, 12th ne rve ; 12N, hypoglo ssa l n . ; P ,p y ra m i d a l t r a c t ; P H , n . p r e p o s i t u s h y p o g l o s s i ; P R , p a ra m e d i a n r e t i c u l a rn . , r b , r e s t if o rm b o d y ; R O, n . r a p h e o b s c u ru s ; R P , n . r a p h e p a l l i d u s; S L ,la te ra l n . of the so l i t a ry t rac t ; SM, media l n . of the so l i t a ry t rac t ; 5SP ,a l a m i n a r s p i n a l t r i g e m i n a l n . , p a rv o c e Uu l a r d i v is i o n ; 5 s t , s p i n a l t r i g e m i n a lt rac t ; s t , so l i t a ry t rac t ; VIN, infe r ior ves t ibula r n . ; VMN, media l ves t ibu-la r n .

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2 0 .

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m i I ~ - V I .c~ ~-VL~

R -R interval,U n l a C e d

1 1S -R Interval,P a c e d

F i g . 2 . T h e e f f e c t o f u n i la t e r al i n j e c t i o n o f g l u t a m a t e i n t o t h e i NA-V L o rc NA-VL o n s i n u s r a t e a n d AV c o n d u c t i o n . As t e r i s k s i n d i c a t e P < 0 . 0 5 ,p a i r e d S t u d e n t t - t es t .

e n t i r e my o c a r d i u m [ 1 3 ] , i n j e c t i o n s i n t o t h e a t r i a o r v e n t r i -c l e s s e p a r a t e l y [ 9 , 1 1 , 1 5 , 2 5 , 2 6 ] o r i n j e c t i o n s i n t o i n t r a c a r -d i a c g a n g l i a [ 1 7 , 1 8 , 2 1 ] . M o s t a u t h o r s a g r e e t h a t i n t r a c a r -d i a c i n j e c t i o n s o f t r a c e r s l e a d t o t h e l a b e l i n g o f c e l l s i n t h eN A , t h e d o r sa l m o t o r n u c le u s o f t h e v a g u s ( D M V ) , a n d ani n t e r m e d i a t e z o n e i n b e t w e e n t h e s e n u c l e i . T w o r e c e n tr e p o r t s h a v e e x a m i n e d t h e d i s t r i b u t i o n o f r e t r o g r a d e l yl a b e l e d c e l l s i n t h e m e d u l l a o f t h e c a t f o l l o w i n g i n j e c t i o no f a t r ac er i n t o e i t h er t h e S A o r A V g a n g l i o n [ 1 7 , 1 8 ] .P l e c h a e t a l . [ 2 1 ] h a v e p e r f o r me d a s i mi l a r a n a l y s i s i n t h ed o g b r a i n . T h e s e d a t a i n d i c a t e d t h a t t h e d i s t r i b u t i o n o fp r e s u m p t i v e n e g a t i v e c h r o n o t r o p i c n e u r o n s l a b e le d f r o mt h e S A g a n g l i o n w a s d i f fe r e n t f r o m t h e d i s t ri b u t io n o fp r e s u m p t i v e n e g a t i v e d r o m o t r o p i c n e u r o n s l a b e l e d f r o m

F i g . 3 . T h e e f f e c t o f m i c ro i n j e c t i o n o f g l u t a m a t e i n t o t h e c NA-VL o ri NA-VL o n c a rd i a c r a t e a n d AV c o n d u c t i o n . Af t e r i n j e c t i o n s i n t h ec NA -VL , n o t e t h e p ro n o u n c e d s l o wi n g i n s i n u s r a t e in p a n e l A . Ho w e v e r ,n o t e t h e f a i l u r e t o e f f e c t AV c o n d u c t i o n i n p a n e l B . Af t e r i n j e c ti o n s i n t h ei NA -VL , n o t e t h e p ro n o u n c e d s l o w i n g i n s i n u s ra t e i n p a n e l C , a s we l l a st h e s l o wi n g o f AV c o n d u c t i o n i n p a n e l D . T ra c e s a r e c o n t i n u o u s . P a p e rs p e e d is 25 m m / s i n p an e l s A , B a n d C , an d 5 0 m m / s i n p an e l D .


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126 P.J. Gatti et al. / Journal of the A utoru)rnic Nervous System 57 (1996) 123- 127

t h e AV g an g l i o n . Th e an a to mica l d a t a su g g es t ed t h a t t h ed i s t r i b u t i o n o f n eg a t i v e ch ro n o t ro p i c an d d ro mo t ro p i c n eu -ro n s ap p ro x imated two p ar t i a l l y o v er l ap p in g cu rv es wi thm o d e s a t t h e c N A - V L a n d r N A - V L , r e s p e c ti v e l y . T h e s ed i f f e r en ces su g g es t ed t h a t t h e re mig h t b e a ca rd io to p i co rg an i za t i o n o f fu n c t i o n a l l y se l ec t i v e n eu ro n s i n t h e NA-VL. I t co u ld b e a rg u ed , h o wev er , t h a t t h e d i f f e r en ces i nl ab e l i n g o b se rv ed mere ly r e f l ec t d i f f e r en ces i n t h e e f f i -c i en cy o f r e t ro g rad e l ab e l i n g o f n eu ro n s f ro m th e S A an dAV g an g l i a . I n o rd er t o t e s t t h ese two co mp et in g i n t e rp re -t a t io n s , we d ec id ed t o i n j ec t t h e ex c i t a t o ry amin o -ac id G luin to d i f f e r en t r eg io n s o f t h e NA-VL in o rd er t o s t imu la t ea ll ca rd io - i n h ib i t o ry n eu ro n s i n t h ese r eg io n s . I f s i g n i f ican tn u m b er s o f n eg a t i v e ch ro n o t ro p i c n eu ro n s a r e t o b e fo u n din th e d i f f e r en t p a r ts o f N A-V L, i n t e r sp er sed wi th n eg a t i v ed ro mo t ro p i c n eu ro n s , t h en l o ca l mic ro in j ec t i o n o f Glush o u ld cau se b o th n eg a t i v e ch ro n o t ro p i c an d n eg a t i v e d ro -mo t ro p i c e f f ec t s . Co n v er se ly , i f a ca rd io to p i c o rg an i za t i o no f n eg a t i v e ch ro n o t ro p i c an d n eg a t i v e d ro mo t ro p i c n eu ro n sd o es ex i s t i n t h e NA-VL, t h en l o ca l mic ro in j ec t i o n o f Glush o u ld g iv e e i t h e r a n eg a t i v e ch ro n o t ro p i c o r a n eg a t i v ed ro mo t ro p i c e f f ec t , b u t n o t b o th . We ch o se t o i n j ec t G lub e c a u se : ( i ) v a g a l c a r d i o m o t o r n e u r o n s a r e e x c i t e d b y G l u[24]; ( i i ) NM DA recep to r s a r e fo u n d i n t h e NA [2 8 ] ; ( i i i )Glu ac t i v a t es so mato d en d r i t i c memb ran es , b u t n o t ax o n s o fp assag e [6 ] ; an d ( i v ) v ag a l b a ro re f l ex es a r e r ed u ced b ymicro in j ec t i o n o f a Glu r ecep to r an t ag o n i s t i n to t h e NA[ 8 ] .

In t h e p resen t ex p er imen t s i t h as b een sh o wn th a tu n i l a t e r a l G lu mic ro in j ec t i o n s i n to t h e cNA-VL cau se ase l ec t i v e n eg a t i v e ch ro n o t ro p i c e f f ec t . Th e o n se t o f t h ee f f ec t was a lmo s t i n s t an tan eo u s , an d w as p ro lo n g ed fo rab o u t 3 0 s . A l th o u g h t h i s p o wer fu l n eg a t i v e ch ro n o t ro p i ce f f e c t w a s r e a d il y o b s e r v e d , n o c h a n g e i n A V c o n d u c t i o nw a s d e t e c t e d . W e c o n c l u d e t h e r e f o r e t h a t th e c N A - V Lco n ta in s a se l ec t i v e p o p u l a t i o n o f n eg a t i v e ch ro n o t ro p i cn eu ro n s , a l t h o u g h i t i s n o t p o ss ib l e t o ex c lu d e t h e p o ss ib i l-i t y t h a t a f ew n eg a t i v e d ro mo t ro p i c n eu ro n s a r e p resen t ,b u t w e r e n o t d e t e c t e d b y t h e m e t h o d o f e l e c t r o c a r d i o -g rap h i c d a t a an a ly s i s wh ich was u sed . Wh en Glu wasin j ec t ed i n to t h e iNA-VL, b o th n eg a t i v e ch ro n o t ro p i c an dn eg a t i v e d ro mo t ro p i c e f f ec t s were d e t ec t ed . Th ese p h y s io -l o g i ca l r esu l t s a r e co n s i s t en t w i th t h e an a to mica l d a t ain d i ca t i n g t h a t su b s t an t i a l n u mb er s o f n eu ro n s p ro j ec t i n gto b o th t h e S A an d AV g an g l i a a r e fo u n d i n t h e iNA-VL[17 ,1 8 ]. I t h as p rev io u s ly b een sh o wn th a t u n i l a te r a l G lumicro in j ec t i o n s i n to t h e rNA-VL cau sed a se l ec t i v e n eg a-t i v e d ro m o t ro p i c e f f ec t [ 1 7 ]. Th ese d a t a i n d i ca t e t h a t fu n c-t i o n a l l y se l ec t i v e ca rd io - i n h ib i t o ry n eu ro n s i n t h e NA-VLare ca rd io to p i ca l l y o rg an i zed i n a l o n g i t u d in a l p lan e . T h ed a t a a r e a l so co n s i s t en t w i th t h e h y p o th es i s t h a t an a to mi -ca l l y sep ara t ed an d fu n c t i o n a l l y se l ec t i v e p a rasy mp ath e t i cp reg an g l i o n i c v ag a l mo to n eu ro n s i n t h e NA in d ep en d en t l yco n t ro l ca rd i ac r a t e an d AV co n d u c t i o n . Th e r esu l t s o fo th e r i n v es t i g a to rs i n h u ma n s a l so su p p o r t t h i s h y p o th es i s .F o r e x a m p l e , p a ti e n ts w i t h ' s w a l l o w s y n c o p e ' c a n d e v e l o p

p aro x y smal AV b lo ck an d f a in t i n g d u r in g o r immed ia t e lya f t e r t h e ac t o f swa l l o win g wi th o u t ch an g es i n ca rd i ac r a t e[20 ]. C o n v er s e ly , ca ro t i d s i n u s m assag e [23 ] o r ' ca rd io in -h ib i t o ry ca ro t i d s i n u s sy n co p e ' [ 1 4 ] can r esu l t i n f a i n t i n gan d ch an g es i n ca rd i ac r a t e wi th o u t ch an g es i n AV co n -d u c t i o n . Th ese d a t a su g g es t t h a t t h e re a r e sep ara t e n eu -ro n a l p a th way s wh ich r eg u l a t e ca rd i ac r a t e an d AV co n -d u c t i o n wh ich may b e o rg an i zed ev en a t t h e l ev e l o f t h ep r imary a f f e r en t n eu ro n .

H e r e , w e h a v e e m p h a s i z e d t h e c o n s i d e r a b l e b o d y o fd a t a wh ich su p p o r t s t h e h y p o th es i s t h a t b o th p a rasy mp a-th e t i c p r eg an g l i o n i c , an d p arasy mp ath e t i c p o s tg an g l i o n i cn eu ro n s a r e o rg an i zed i n to fu n c t i o n a l l y se l ec t i v e u n i t s . I tsh o u ld b e n o t ed , h o wev er , t h a t n o t a l l i n v es t i g a to r s ag reeth a t t h e S A an d AV g an g l i a a r e fu n c t i o n a l l y se l ec t i v e ,esp ec i a l l y i n t h e can in e mo d e l . Armo u r an d co l l eag u es[2 ,5 ,2 9 ] h av e p resen t ed d a t a , f o r ex amp le , wh ich su g g es tt h a t e i t h e r e l ec t r i ca l o r ch emica l (n i co t i n e ) s t imu la t i o n o fsp ec i f i c i n t r aca rd i ac g an g l i a can cau se mu l t i p l e e f f ec t s o ncard i ac r a t e , AV co n d u c t i o n an d my o card i a l co n t r ac t i l i t y .Th ese d a t a , h o wev er , h av e b een d i f f i cu l t t o i n t e rp re t b e -cau se : ( i ) e l ec t r i ca l s t imu la t i o n i s l i k e ly t o cau se ac t i v a t i o no f n e a r b y a x o n s o f p a s s a g e ; a n d ( i i ) a l t h o u g h i n j ec t i o n s o fn i co t i n e d o n o t a c t i v a t e ax o n s o f p assag e , n i co t i n e is ap o we r fu l s t imu lan t o f a f f e r en t n e rv e t e rmin a l s , t h e reb yp o t en t i a l l y l ead in g t o n u m ero u s r e f l ex iv e ch an g es i n ca r-d i ac fu n c t i o n . F u r th e rmo re , n i co t i n e a l so h as b o th s t imu -l an t ( ex c i t a t o ry ) an d d ep ress iv e (d ep o l a r i za t i o n b lo ck ad e)p h ases o f ac t i o n [2 7 ] wh ich fu r t h e r co m p l i ca t e t h e i n t e rp re -t a t i o n o f i t s e f f ec t s . I n su mmary , a l t h o u g h t h e d eg ree o fp h y s io lo g i ca l s e l ec t i v i t y o f t h e AV an d S A g an g l i a m ayh av e t o b e r eev a lu a t ed i n t h e fu tu re , a s mo re d a t a b eco meav a i l ab l e o n t h e r eg io n a l e f f ec t s o f i n d iv id u a l i n t r aca rd i acg a n g l i a u p o n v a r i o u s p a r a m e t e r s o f c a r d i a c p e r f o r m a n c e ,( a l so in c lu d in g mea su res su ch as co ro n ary f l o w an d my -o card i a l co n t r ac t i l i t y ) , n o n e th e l es s , we f ee l t h a t t h e ir p o w-er fu l an d ap p aren t l y se l ec t i v e imp ac t u p o n a t r i o -v en t r icu l a rco n d u c t i o n an d s i n u s r a t e mer i t t h e t e rms 'AV g an g l i o n 'a n d ' S A g a n g l i o n ' r e s p e c t i v e l y .

AcknowledgementsTh i s r esea rch was su p p o r t ed i n p a r t b y g ran t s f ro m th e

Amer i can Hear t Asso c i a t i o n , t h e Na t i o n a l In s t i t u t es o fH e a l t h ( H L 4 4 9 2 2 ) , t h e H o w a r d U n i v e r s i ty G r a d u a te S c h o o lC o l l a b o r a ti v e C o r e U n i t P r o g r a m a n d a n a w a r d f r o m t h eO f f i c e o f th e D e a n o f t h e M e d i c a l S c h o o l , H o w a r d U n i v e r-s i t y . Th e au th o r s wi sh t o t h an k Dr . B .E . Jo n es , Dr . A .Beau d e t an d Jo h n Wi l ey an d S o n s , I n c . f o r p e rmi t t i n g u sto ex cerp t a p o r t i o n o f t h e i r a t l a s o f t h e ca t b r a in s t em [ 1 2 ]in F ig. 1 o f th i s man u scr ip t . T h e au th o r s a l so wi sh t o t h an kP ro f . R .A. Gi l l i s , Geo rg e to wn Un iv er s i t y S ch o o l o fM e d ic in e , f o r h i s h e lp wi th t h e e l ec t ro card io g rap h i c an a ly -s i s o f th e d a t a . F in a l l y , w e w o u ld l i k e t o t h an k M s . J . -M .Lau en s t e in fo r ex ce l l en t t ech n i ca l a s s i s t an ce .


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