Characterization of long-term potentiation of primary afferent transmission at trigeminal synapses of juvenile rats: Essential role of subtype 5 metabotropic glutamate receptors

Recent work has demonstrated that a brief high-frequency conditioning stimulation to the primary afferent nerve fibers can induce a long-term potentiation (LTP) of synaptic transmission in neurons in the superficial layer of the trigeminal caudal nucleus; however, the cellular and molecular mechanisms underlying this synaptic potentiation remain unclear. Using both extracellular field potential and whole-cell patch-clamp recordings in brainstem parasagital slices of juvenile rat with the mandibular nerve attached, we show here that the induction of trigeminal primary afferent LTP: (1) does not require the activation of ionotropic glutamate receptors; (2) is dependent on extracellular Ca²⁺ and the release of Ca²⁺ from intracellular stores; (3) is specifically prevented by the metabotropic glutamate receptor subtype 5 (mGluR5) antagonist 2-methyl-6-(phenylethynyl)pyridine but not the mGluR1 antagonist LY367385, group II mGluR antagonist LY341495 or group III mGluR antagonist MAP4; (4) is mimicked by the bath-applied group I mGluR agonist (S)-3,5-dihydroxyphenylglycine and mGluR5 agonist (RS)-2-chloro-5- hydroxyphenylglycine; (5) requires the activation of phospholipase C (PLC) and protein kinase C (PKC); and (6) is concomitantly with a decrease in paired-pulse depression. These results demonstrate that the activation of mGluR5 and in turn triggering a PLC/PKC-dependent signaling cascade may contribute to the induction of LTP of primary afferent synaptic transmission in the superficial layer of trigeminal caudal nucleus of juvenile rats. This may be relevant to the processing of nociceptive information.