Investigating the modulation of neonatal rat facial motoneurone excitability by monoamine neurotransmitters: Postsynaptic mechanisms and presynaptic modulation of glutamate release.

摘要

The activity patterns of 5-HT-releasing neurons can be positively correlated withudbehavioural state and motor function and the central 5-HT system modulates motorudactivity at the cellular level. The rat facial motor nuclei are densely innervated by 5-udHT releasing afferents and 5-HT-mediated modulation of ion channels on the somaudand dendrites can markedly influence the excitability of facial motoneurones andudtheir integration of excitatory postsynaptic potentials (EPSPs). 5-HT facilitates facialudmotoneuron excitation by inhibiting a ‘leak’ potassium (K+) conductance (gK+udLeak)udand enhancing the hyperpolarisation-activated cation current, Ih. These actions of 5-udHT have been confirmed using whole-cell voltage-clamp recordings from visuallyudidentified facial motoneurones in an acute brainstem slice preparation.udPharmacological approaches have been used to identify the receptors which mediateudthe actions of 5-HT in facial motoneurones. The inhibition of gK+udLeak by 5-HT canudbe blocked by the 5-HT2A receptor antagonist, R96544 (0.3 – 1μM) and theudenhancement of Ih by 5-HT is sensitive to the 5-HT7 receptor antagonist, SB269970ud(0.3 – 10 μM).udNoradrenaline was also found to inhibit gK+udLeak, via activation of a1 adrenoceptors,udand the molecular identity of the amine-sensitive ‘leak’ K+ channels has beenudinvestigated. TASK-1 and TASK-3 are pH-sensitive two-pore domain K+ channelsudthat can be modulated by amines and provide ‘leak’ K+ conductances in severaludcentral neurones. The mRNAs for these channels have been reported to be present inudthe rat facial motor nucleus. The gK+udLeak in facial motoneurones is sensitive toudchanges in external pH and has a pK of ~7.1, which is intermediate between theudvudvalues for homomeric TASK-1 and TASK-3 channels (7.5 and 6.8 respectively).udThe TASK-1 selective inhibitor anandamide (10 μM), its stable analogueudmethanandamide (10 μM), the TASK-3 selective inhibitor ruthenium red (10 μM)udand Zn2+ (100-300 μM) all failed to alter the actions of noradrenaline or changingudexternal pH. These findings argue against principal contributions to gKLeak byudhomomeric TASK-1 or TASK-3 channels. Isoflurane, a volatile anaesthetic thatudenhances heteromeric TASK-1 / TASK-3 currents, potentiated gKLeak supporting audpredominant role for heterodimeric TASK-1 / TASK-3 channels in the gKLeak inudfacial motoneurones.udEvoked fast excitatory synaptic transmission in the facial motor nucleus has beenudcharacterised and NMDA and non-NMDA receptor-mediated components of thisudsynaptic transmission have been identified. Through a combination of analysis of theudpaired pulse ratio, rate of failure to generate a response and the frequency andudamplitude of miniature excitatory postsynaptic currents (mEPSCs) this studyudprovides evidence to suggest that glutamate release from pre-synaptic terminals inudthe facial motor nucleus is depressed by 5-HT. This action of 5-HT is mediated byudactivation of presynaptic 5-HT1B receptors as this effect is mimicked by the 5-HT1Budreceptor agonist, CP93129 (10 μM) and can be blocked by the 5-HT1B receptorudantagonist, isamoltane (1 μM).udThese studies indicate that the modulation of synaptic integration in the facial motorudnucleus involves activation of distinct pre- and post-synaptic 5-HT receptor subtypes.udThese findings not only increase our understanding of the cellular mechanisms forudviudthe 5-HT modulation of motor activity but may also be relevant to the role of 5-HTudin the control of other central neurones.