SEROTONERGIC AXONAL CONTACTS ON IDENTIFIED CAT TRIGEMINAL MOTONEURONSAND THEIR CORRELATION WITH MEDULLARY RAPHE NUCLEUS STIMULATION

The innervation of the trigeminal motor nucleus by serotonergic fibers with cell bodies in the raphe nuclei pallidus and obscurus suggests t hat activation of this pathway may alter the excitability of trigemina l motoneurons. Thus, we recorded intracellular responses from cat jaw- closing (JC) and jaw-opening (JO) alpha-motoneurons evoked by raphe st imulation and used a combination of intracellular staining of horserad ish peroxidase (HRP) and immunohistochemistry at the light and electro n microscopic levels to examine the distribution of contacts made by s erotonin (5-HT)-immunoreactive boutons on the two motoneurons types. E lectrical stimulation applied to the nucleus raphe pallidus-obscurus c omplex induced a monosynaptic excitatory postsynaptic potential (EPSP) in JC (masseter) alpha-motoneurons and an EPSP with an action potenti al in JO (mylohyoid) alpha-motoneurons. The EPSP rise-times (time to p eak) and half widths were significantly longer in the JC than in the J O motoneurons. The EPSPs were suppressed by systemic administration of methysergide (2 mg/kg). Six JC and seven JO alpha-motoneurons were we ll stained with HRP. Contacts were seen between 5-HT-immunoreactive bo utons and the motoneurons. The JC motoneurons received a significantly larger number of the contacts than did the JO motoneurons. The contac ts were distributed widely in the proximal three-fourths of the dendri tic tree of JC motoneurons but were distributed on more proximal dendr ites in the JO motoneurons. At the electron microscopic level, synapti c contacts made by 5-HT-immunoreactive boutons on motoneurons were ide ntified. The present study demonstrated that JC motoneurons receive st ronger 5-HT innervation, and this correlates with the fact that raphe stimulation caused larger EPSPs among these neurons than among JO moto neurons. (C) 1997 Wiley-Liss, Inc.