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Effects of adrenergic agents on the ventral horn cells in spinal cord slices isolated from adult rats
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18. Motor system I. Spinal cord and proprioceptors EFFECTS ISOLATED
OF A D R E N E R G I C AGENTS FROM ADULT RATS
ON THE V E N T R A L
HORN CELLS
IN
SPINAL
CORD
SLICES
TAKASHI HIRAYAMA*, HIDEKI ONO and HIDEOMI FUKUDA, Department of T o x i c o l o g y and Pharmacology, Faculty of P h a r m a c e u t i c a l Sciences, The U n i v e r s i t y of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, J a p a n The e x i s t e n c e of n o r a d r e n e r g i c fibers o r i g i n a t i n g in the b r a i n stem nuclei and terminating in both dorsal and ventral horns of the spinal cord is well known. However, r e g u l a t i n g m e c h a n i s m s of the spinal motor function through a d r e n o c e p t o r s are not yet elucidated. In this experiment, the effects of a d r e n o c e p t o r agonists and antagonists on synaptic activities of the ventral horn neurons were investigated using spinal cord slices. The slice p r e p a r a t i o n (400-450 ~m) was prepared from the lumbar e n l a r g e m e n t (L4-L5) of the spinal cord isolated from adult rats (9-10 weeks) under urethane (ig/kg,i.p.) and ~-chloralose (25mg/kg,i.p.) anesthesia. An action potential driven trans-synaptically following stimulation of the motor nuclei of the ventral horn was recorded extracellularly from the area close to the stimulating electrode. When a stimulus intensity with a low p r o b a b i l i t y for cellular d i s c h a r g e induction (less than 2/10) was used, norepinephrine (NE) caused a concentration-dependent increase in the p r o b a b i l i t y of firing. The f a c i l i t a t i o n produced by NE was b l o c k e d by the ~ l - a d r e n o c e p t o r antagonist, p r a z o s i n but not by the B - a d r e n o c e p t o r antagonist, propranolol or ~ 2 - a d r e n o c e p t o r antagonist, yohimbine. When a stimulus intensity that c o n s i s t e n t l y induced an action potential (10/10) was used, NE had no effect alone yet d e c r e a s e d the p r o b a b i l i t y of firing in the presence of prazosin. The inhibitory effect produced by NE was a n t a g o n i z e d by yohimbine. Selective ~ l - a d r e n o c e p t o r agonists (phenylephrine and methoxamine) increased the p r o b a b i l i t y of cellular discharge. C l o n i d i n e had similar effects to those of NE. These results c o n f i r m e d that the f a c i l i t a t i o n and inhibition of neuronal a c t i v i t i e s p r o d u c e d by NE are m e d i a t e d through ~ i - and e 2 - a d r e n o c e p t o r s , respectively. Facilitatory actions through ~ l - a d r e n o c e p £ o r s may play a dominant role in the adrenergic t r a n s m i s s i o n in the motor nuclei of the ventral horn.
M O D U L A T I O N OF REFLEX RESPONSES F I C T I V E L O C O M O T I O N IN THE CAT MITSUYO TSUKUBA
OF E L B O W F L E X O R
MOTONEURONS
H I S H I N U M A * AND TAKASHI YAMAGUCHI*, INSTITUTE UNIVERSITY, TSUKUBA, IBARAKI 305, J A P A N
DURING
FORELIMB
OF BASIC M E D I C A L
SCIENCES,
In order to examine the synaptic linkage b e t w e e n the forelimb locomotor p a t t e r n g e n e r a t o r and spinal reflex pathways, reflex responses to m u s c l e and skin a f f e r e n t s t i m u l a t i o n were i n t r a c e l l u l a r l y recorded from flexor m o t o n e u r o n s (brachialis, Br) during fictive locomotion, evoked by s t i m u l a t i o n of the m e s e n c e p h a l i c locomotor r e g i o n (MLR) in immobilized d e c e r e b r a t e cats w i t h the lower thoracic cord transected. Before M L R s t i m u l a t i o n (control stage), afferent v o l l e y s in deep and superficial radial nerves (DR and SR, muscle and skin nerves, respectively) p r o d u c e d large, late p o l y s y n a p t i c EPSPs (segmental latency more than 3 msec) in Br motoneurons. Further, Br m o t o n e u r o n s received early EPSPs (segmental latencies shorter than 2.5 msec) from DR nerves. When M L R s t i m u l a t i o n started, prior to a c t i v a t i o n of fictive locomotion, Br m o t o n e u r o n s h y p e r p o l a r i z e d and the late EPSPs were r e m a r k a b l y suppressed (80% or more). However, the early EPSPs were not suppressed as m u c h (20% or less). Thus, s u p p r e s s i o n of the polysynaptic, late EPSPs is p r e s u m a b l y not due to a c o n d u c t a n c e increase of m o t o n e u r o n s o c c u r r i n g w i t h h y p e r p o l a r i z a t i o n , but rather to inhibition or d i s f a c i l i t a t i o n of interneurons. During fictive locomotion, reflex r e s p o n s e s were p h a s e - d e p e n d e n t . In the e x t e n s o r - a c t i v e phase, the reflex r e s p o n s e s were suppressed as above. In the f l e x o r - a c t i v e phase, the early EPSPs from DR and the late EPSPs from SR recovered; the former i n c r e a s e d r e m a r k a b l y in m a g n i t u d e (more than twice the control) w h i l e the latter only o c c a s i o n a l l y exceeded those of the control. The late EPSPs from DR were still inhibited. Thus, it was suggested that reflex pathways m e d i a t i n g early EPSPs from muscle a f f e r e n t s and late EPSPs from skin a f f e r e n t s are facilitated by the locomotor p a t t e r n generator, e s p e c i a l l y the f l e x i o n - g e n e r a t i n g part.
18. Motor system I. Spinal cord and proprioceptors EFFECTS ISOLATED
OF A D R E N E R G I C AGENTS FROM ADULT RATS
ON THE V E N T R A L
HORN CELLS
IN
SPINAL
CORD
SLICES
TAKASHI HIRAYAMA*, HIDEKI ONO and HIDEOMI FUKUDA, Department of T o x i c o l o g y and Pharmacology, Faculty of P h a r m a c e u t i c a l Sciences, The U n i v e r s i t y of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, J a p a n The e x i s t e n c e of n o r a d r e n e r g i c fibers o r i g i n a t i n g in the b r a i n stem nuclei and terminating in both dorsal and ventral horns of the spinal cord is well known. However, r e g u l a t i n g m e c h a n i s m s of the spinal motor function through a d r e n o c e p t o r s are not yet elucidated. In this experiment, the effects of a d r e n o c e p t o r agonists and antagonists on synaptic activities of the ventral horn neurons were investigated using spinal cord slices. The slice p r e p a r a t i o n (400-450 ~m) was prepared from the lumbar e n l a r g e m e n t (L4-L5) of the spinal cord isolated from adult rats (9-10 weeks) under urethane (ig/kg,i.p.) and ~-chloralose (25mg/kg,i.p.) anesthesia. An action potential driven trans-synaptically following stimulation of the motor nuclei of the ventral horn was recorded extracellularly from the area close to the stimulating electrode. When a stimulus intensity with a low p r o b a b i l i t y for cellular d i s c h a r g e induction (less than 2/10) was used, norepinephrine (NE) caused a concentration-dependent increase in the p r o b a b i l i t y of firing. The f a c i l i t a t i o n produced by NE was b l o c k e d by the ~ l - a d r e n o c e p t o r antagonist, p r a z o s i n but not by the B - a d r e n o c e p t o r antagonist, propranolol or ~ 2 - a d r e n o c e p t o r antagonist, yohimbine. When a stimulus intensity that c o n s i s t e n t l y induced an action potential (10/10) was used, NE had no effect alone yet d e c r e a s e d the p r o b a b i l i t y of firing in the presence of prazosin. The inhibitory effect produced by NE was a n t a g o n i z e d by yohimbine. Selective ~ l - a d r e n o c e p t o r agonists (phenylephrine and methoxamine) increased the p r o b a b i l i t y of cellular discharge. C l o n i d i n e had similar effects to those of NE. These results c o n f i r m e d that the f a c i l i t a t i o n and inhibition of neuronal a c t i v i t i e s p r o d u c e d by NE are m e d i a t e d through ~ i - and e 2 - a d r e n o c e p t o r s , respectively. Facilitatory actions through ~ l - a d r e n o c e p £ o r s may play a dominant role in the adrenergic t r a n s m i s s i o n in the motor nuclei of the ventral horn.
M O D U L A T I O N OF REFLEX RESPONSES F I C T I V E L O C O M O T I O N IN THE CAT MITSUYO TSUKUBA
OF E L B O W F L E X O R
MOTONEURONS
H I S H I N U M A * AND TAKASHI YAMAGUCHI*, INSTITUTE UNIVERSITY, TSUKUBA, IBARAKI 305, J A P A N
DURING
FORELIMB
OF BASIC M E D I C A L
SCIENCES,
In order to examine the synaptic linkage b e t w e e n the forelimb locomotor p a t t e r n g e n e r a t o r and spinal reflex pathways, reflex responses to m u s c l e and skin a f f e r e n t s t i m u l a t i o n were i n t r a c e l l u l a r l y recorded from flexor m o t o n e u r o n s (brachialis, Br) during fictive locomotion, evoked by s t i m u l a t i o n of the m e s e n c e p h a l i c locomotor r e g i o n (MLR) in immobilized d e c e r e b r a t e cats w i t h the lower thoracic cord transected. Before M L R s t i m u l a t i o n (control stage), afferent v o l l e y s in deep and superficial radial nerves (DR and SR, muscle and skin nerves, respectively) p r o d u c e d large, late p o l y s y n a p t i c EPSPs (segmental latency more than 3 msec) in Br motoneurons. Further, Br m o t o n e u r o n s received early EPSPs (segmental latencies shorter than 2.5 msec) from DR nerves. When M L R s t i m u l a t i o n started, prior to a c t i v a t i o n of fictive locomotion, Br m o t o n e u r o n s h y p e r p o l a r i z e d and the late EPSPs were r e m a r k a b l y suppressed (80% or more). However, the early EPSPs were not suppressed as m u c h (20% or less). Thus, s u p p r e s s i o n of the polysynaptic, late EPSPs is p r e s u m a b l y not due to a c o n d u c t a n c e increase of m o t o n e u r o n s o c c u r r i n g w i t h h y p e r p o l a r i z a t i o n , but rather to inhibition or d i s f a c i l i t a t i o n of interneurons. During fictive locomotion, reflex r e s p o n s e s were p h a s e - d e p e n d e n t . In the e x t e n s o r - a c t i v e phase, the reflex r e s p o n s e s were suppressed as above. In the f l e x o r - a c t i v e phase, the early EPSPs from DR and the late EPSPs from SR recovered; the former i n c r e a s e d r e m a r k a b l y in m a g n i t u d e (more than twice the control) w h i l e the latter only o c c a s i o n a l l y exceeded those of the control. The late EPSPs from DR were still inhibited. Thus, it was suggested that reflex pathways m e d i a t i n g early EPSPs from muscle a f f e r e n t s and late EPSPs from skin a f f e r e n t s are facilitated by the locomotor p a t t e r n generator, e s p e c i a l l y the f l e x i o n - g e n e r a t i n g part.