SYNAPTIC MECHANISMS ACTING ON LUMBAR MOTONEURONS DURING POSTURAL AUGMENTATION INDUCED BY SEROTONIN INJECTION INTO THE ROSTRAL PONTINE RETICULAR-FORMATION IN DECEREBRATE CATS

Intrapontine microinjections of serotonin in acutely decerebrated cats resulted in the bilateral augmentation of the postural muscle tone of the hindlimbs. Optimal injection sites were located in the dorsomedia l part of the rostral pontine reticular formation corresponding to the nucleus reticularis pontis oralis (NRPo). In this study, attempts wer e made to elucidate the cellular basis for the serotoninergically indu ced augmentation of postural muscle tone by recording the electromyogr aphic (EMG) activity of hindlimb extensor muscles, the monosynaptic re flex responses evoked by electrical stimulation of group la muscle aff erent fibres and the membrane potentials of hindlimb alpha-motoneurons (MNs). Serotonin injections resulted not only in the augmentation of the EMG activity of gastrocnemius soleus muscles, but also in the rest oration of EMG suppression, which was induced by previous injection of carbachol into the NRPo. Extensor and flexor monosynaptic reflex resp onses were facilitated by serotonin injections into the NRPo. Such ref lex facilitation was not induced by serotonin injections into the mese ncephalic or the medullary reticular formation. Intrapontine serotonin injections resulted in membrane depolarization of extensor and flexor MNs with decreases in input resistance and rheobase. Spontaneous depo larizing synaptic potentials (EPSPs) increased in both frequency and a mplitude. Peak voltage of la monosynaptic EPSPs also increased. Seroto nin injections which followed carbachol injections resulted in membran e depolarization of MNs along with an increase in the frequency of spo ntaneous EPSPs and a decrease in carbachol-induced inhibitory postsyna ptic potentials. Following pontine carbachol injections, antidromic an d orthodromic responses in MNs were suppressed. Discharges of MNs evok ed by intracellular current injections were also suppressed, but were restored following serotonin injections. These results indicate that p ostsynaptic excitation, presynaptic facilitation and disinhibition (wi thdrawal of postsynaptic inhibition) simultaneously act on the hindlim b MNs during serotonin-induced postural augmentation and restoration.