Inhibitory control of excitatory C3-C4 propriospinal neurones with projection to forelimb motoneurones in the cat and macaque monkey.

	Inhibitory control of excitatory C3-C4 propriospinal neurones with projection to forelimb motoneurones in the cat and macaque monkey.
Research field:	Central nervous system
Authors:	Alstermark B, Isa T, Ohki Y and Saito Y
Address of presenting author:	Department of Integrative Medical Biology, Section of Physiology Umeå University 901 87 Umeå
E-mail:	Bror.Alstermark@physiol.umu.se
Phone:	+46-90-7866921
Fax:	+46-90-7866683
Text of abstract	Introduction C3-C4 propriospinal interneurones (PNs) mediate disynaptic pyramidal excitation to forelimb motoneurones in the cat (Illert et al. 1977) and behavioural experiments have shown that these C3-C4 PNs can issue the command from the brain for visually guided target reaching movements with the forelimb. The C3-C4 PNs are controlled by feed-forward and feed-back inhibition, which is essential for the selection of subpopulations of C3-C4 PNs subserving different motor components and to terminate the movement (for review cf. Alstermark and Lundberg 1992). However, in the monkey, Maier et al. (1998) concluded that their "results provide little evidence for significant corticospinal excitation of motoneurones via a system of C3-C4 propriospinal neurones." We have now re-investigated this issue with special focus on the possibility that the phylogenetical difference between the cat and monkey is not in the presence of C3-C4 PNs, but in the strength of inhibitory control of the C3-C4 PNs. Methods The experiments were performed in cats and macaque monkeys during deep anaesthesia with a-chloralose (100 mg/kg). Corticospinal fibres were stimulated electrically in the contralateral pyramid with a tungsten electrode. Intracellular recordings were made with glass microelectrodes filled with 2 M potassium citrate. Intravenous injections of strychnine were regularly performed during intracellular recordings using a concentration of 1 mg/ml. The standard dose used was 0.1mg/kg. Results We confirmed the low frequency of disynaptic pyramidal EPSPs in forelimb motoneurones observed by Maier et al. (1998) in monkeys. However, after intravenous injection of strychnine, disynaptic EPSPs could be evoked in 95% of forelimb motoneurones recorded after lesion of the corticospinal fibres in C5. After a corresponding lesion in C2, disynaptic pyramidal EPSPs were observed in only 4% of the motoneurones, indicating that the disynaptic EPSPs were mediated via C3-C4 PNs. In cats, we confirmed the results by Illert et al. (1977) and in addition found that the disynaptic pyramidal EPSPs, present after the corticospinal transection in C5, were increased in amplitude after injection of strychnine in all tested motoneurones. Conclusions In contrast to previous reports, we conclude that C3-C4 PNs can mediate disynaptic pyramidal excitation in high frequency of occurrence to forelimb motoneurones in the C6-8 segments in the macaque monkey and that this transmission is under a stronger inhibitory control as compared to the cat. References Alstermark, B.& Lundberg, A. 1992 In: L. Jami, E. Pierrot-Deseilligny and D. Zytnicki (eds.) Muscle afferents and spinal control of movement pp. 327-354. Pergamon Press, IBRO Series Illert, M., Lundberg, A. and Tanaka R. 1977, Exp Brain Res 28;29: 323-346. Maier, M. A., Illert, M., Kirkwood, P.A., Nielsen, J. and Lemon, R. N 1998 J Physiol (Lond) 15: 191-212.
Keywords:	Inhibition, C3-C4 Propriospinal neurones, Motoneurones, Cat, Macaque Monkey

Created 2000-05-02