Serotonergic axonal contacts on identified cat trigeminal motoneurons and their correlation with medullary raphe nucleus stimulation

Abstract

The innervation of the trigeminal motor nucleus by serotonergic fibers with cell bodies in the raphe nuclei pallidus and obscurus suggests that activation of this pathway may alter the excitability of trigeminal motoneurons. Thus, we recorded intracellular responses from cat jaw-closing (JC) andjaw-opening (JO) alpha-motoneurons evoked by raphe stimulation and used a combination of intracellular staining of horseradish peroxidase (HRP) and immunohistochemistry at the light and electron microscopic levels to examine the distribution of contacts made by serotonin (5-HT)-immunoreactive boutons on the two motoneurons types. Electrical stimulation applied to the nucleus raphe pallidus-obscurus complex induced a monosynaptic excitatory postsynaptic potential (EPSP) in JC (masseter) alpha-motoneurons and an EPSP with an action potential in JO (mylohyoid) alpha-motoneurons. The EPSP rise-times (time to peak) and half widths were significantly longer in the JC than in the JO motoneurons. The EPSPs were suppressed by systemic administration of methysergide (2 mg/kg). Six JC and seven JO alpha-motoneurons were well stained with HRP. Contacts were seen between 5-HT-immunoreactive boutons and the motoneurons. The JC motoneurons received a significantly larger number of the contacts than did the JO motoneurons. The contacts were distributed widely in the proximal three-fourths of the dendritic tree of JC motoneurons but were distributed on more proximal dendrites in the JO motoneurons. At the electron microscopic level, synaptic contacts made by 5-HT-immunoreactive boutons on motoneurons were identified. The present study demonstrated that JC motoneurons receive stronger 5-HT innervation, and this correlates with the fact that raphe stimulation caused larger EPSPs among these neurons than among JO motoneurons.

Behavioral correlates of the activity of serotonergic and non-serotonergic neurons in caudal raphe nuclei

“…Results obtained by Takatori et al (57) suggested that NRP and NRO exerted mainly a direct inhibitory influence on masseter motoneurons. Nagase et al (18) observed just a facilitatory influence of non-serotonergic (and serotonergic) neurons in NRP and NRO directly afferenting jaw-opener (mylohyoid) and jaw-closer (masseter) motoneurons. The cross-correlation involving the digastric muscle here was negative and that involving the masseter muscle was positive.…”

“…Well-known target structures are the trigeminal motor nucleus (1), facial nucleus (2), nucleus ambiguus (3), hypoglossus nucleus (4,5), the ventral horn of the spinal cord (6)(7)(8), the medial medul-lary reticular formation (9), the ventral respiratory group (10), the red nucleus (11), the dorsal motor nucleus of the vagus (12), the intermediolateral cell column of the spinal cord (13)(14)(15) and the rostral ventrolateral medulla and surrounding regions (9,16,17). At least part of the axons terminate directly in motoneurons (7,18) or preganglionic autonomic neurons (15).…”

“…Second, increased NRP or NRO activity induced by electrical or chemical stimulation facilitates trigeminal motoneurons (18), facilitates or depresses phrenic motoneurons (19)(20)(21), facilitates lumbar motoneurons (22,23), facilitates motor and secretory gastric preganglionic neurons in the dorsal motor nucleus of the vagus (12,24), and facilitates or depresses sympathetic preganglionic neurons to the heart and vessels (25,26).…”

“…In the cat, they represent about 50% of all neurons in the NRP and 35% of all neurons in the NRO (35). Correspondingly, target structures present many serotonergic terminals (18,36). Artificial activation of the NRO was shown to increase the release of serotonin in target structures (37) and blockade of serotonin receptors with antagonists considerably reduces the effects of artificial activation of NRP and NRO on target neurons (18)(19)(20)(21)(22).…”

Conexões e caracterização neuroquímica de vias neurais envolvidas com o controle dos movimentos mandibulares

“…Além disso, a região perifornicial da LH ainda possui eferências para o CeA (Touzani et al, 1996), e o CeA também se projeta para a LH (LeDoux et al, 1988) (Guridi et al, 82 1996) e humanos (Kumar et al, 1998) ) e gera um aumento do tônus do masseter (Sunden-Kuronen et al, 1983). A serotonina age na região perifornicial da LH (Kumar et al, 2007), além de atuar no Me5 e diretamente no Mo5 (Nagase et al, 1997).…”