Stimulation-induced responses of the trigeminal caudal neurons in the brainstem preparation isolated from newborn rats

Mechanisms of Peripheral and Central Pain Sensitization: Focus on Ocular Pain

Persistent ocular pain caused by corneal inflammation and/or nerve injury is accompanied by significant alterations along the pain axis. Both primary sensory neurons in the trigeminal nerves and secondary neurons in the spinal trigeminal nucleus are subjected to profound morphological and functional changes, leading to peripheral and central pain sensitization. Several studies using animal models of inflammatory and neuropathic ocular pain have provided insight about the mechanisms involved in these maladaptive changes. Recently, the advent of new techniques such as optogenetics or genetic neuronal labelling has allowed the investigation of identified circuits involved in nociception, both at the spinal and trigeminal level. In this review, we will describe some of the mechanisms that contribute to the perception of ocular pain at the periphery and at the spinal trigeminal nucleus. Recent advances in the discovery of molecular and cellular mechanisms contributing to peripheral and central pain sensitization of the trigeminal pathways will be also presented.

Excitatory Effects of Calcitonin Gene-Related Peptide (CGRP) on Superficial Sp5C Neurons in Mouse Medullary Slices

The neuromodulator calcitonin gene-related peptide (CGRP) is known to facilitate nociceptive transmission in the superficial laminae of the spinal trigeminal nucleus caudalis (Sp5C). The central effects of CGRP in the Sp5C are very likely to contribute to the activation of central nociceptive pathways leading to attacks of severe headaches like migraine. To examine the potential impacts of CGRP on laminae I/II neurons at cellular and synaptic levels, we performed whole-cell patch-clamp recordings in juvenile mouse brainstem slices. First, we tested the effect of CGRP on cell excitability, focusing on neurons with tonically firing action potentials upon depolarizing current injection. CGRP (100 nM) enhanced tonic discharges together with membrane depolarization, an excitatory effect that was significantly reduced when the fast synaptic transmissions were pharmacologically blocked. However, CGRP at 500 nM was capable of exciting the functionally isolated cells, in a nifedipine-sensitive manner, indicating its direct effect on membrane intrinsic properties. In voltage-clamped cells, 100 nM CGRP effectively increased the frequency of excitatory synaptic inputs, suggesting its preferential presynaptic effect. Both CGRP-induced changes in cell excitability and synaptic drives were prevented by the CGRP receptor inhibitor BIBN 4096BS. Our data provide evidence that CGRP increases neuronal activity in Sp5C superficial laminae by dose-dependently promoting excitatory synaptic drive and directly enhancing cell intrinsic properties. We propose that the combination of such pre- and postsynaptic actions of CGRP might underlie its facilitation in nociceptive transmission in situations like migraine with elevated CGRP levels.

Postnatal development of excitation propagation in the trigeminal subnucleus caudalis evoked by afferent stimulation in mice

The postnatal development of nociceptive afferent activity expansion and its modulation features were examined in mice using an optical imaging technique. Developing mice (1-2 weeks old (N1-2 w), 3-4 weeks old (N3-4 w), 5-6 weeks old (N5-6 w) and 7-8 weeks old (N7-8 w)) and neonatally capsaicin-treated mice were used. The propagation of neuronal excitation was measured by changes in fluorescent intensity in horizontal brain stem slices evoked by electrical stimulation to the trigeminal spinal tract. A single-pulse stimulation evoked excitation propagation in the trigeminal caudalis (Vc). The propagation area was larger in N1-2 w than in N7-8 w, and no differences were observed between capsaicin-treated and naive mice in the same age groups. Repetitive stimulation (100 Hz, 30 pulses) elicited long-lasting and widespread excitation propagation. The excitation propagation area was significantly larger in N7-8 w than in N1-2 w, N3-4 w and N5-6 w. This propagation was suppressed by 5 microM L-703.606, an NK1-receptor antagonist, suggesting that the repetitive stimulation-elicited excitation may require substance-P releases. Morphological observations demonstrated that the neural network in the Vc had grown by postnatal week 5. These results suggest that nociceptive afferent activity co-operatively matures with development of the network structure in the Vc, and that a mechanism for prolonged increase in central excitability is established during a later postnatal period.

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 Ca(2+) and the release of Ca(2+) 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.

Cannabinoid-induced presynaptic inhibition at the primary afferent trigeminal synapse of juvenile rat brainstem slices

Systemic or intraventricular administration of cannabinoids causes analgesic effects, but relatively little is known for their cellular mechanism. Using brainstem slices with the mandibular nerve attached, we examined the effect of cannabinoids on glutamatergic transmission in superficial trigeminal caudal nucleus of juvenile rats. The exogenous cannabinoid receptor agonist WIN 55,212-2 (WIN), as well as the endogenous agonist anandamide, hyperpolarized trigeminal caudal neurones and depressed the amplitude of excitatory postsynaptic potentials (EPSPs) or currents (EPSCs) monosynaptically evoked by stimulating mandibular nerves in a concentration-dependent manner. The inhibitory action of WIN was blocked or fully reversed by the CB1 receptor antagonist SR 141716A. WIN had no effect on the amplitude of miniature excitatory postsynaptic currents (mEPSCs) recorded in the presence of tetrodotoxin or cadmium. The inhibitory effect of WIN on EPSCs was greater for those with longer synaptic latency, suggesting that cannabinoids have a stronger effect on C-fibre EPSPs than on Adelta-fibre EPSPs. Ba2+ (100 microm) blocked the hyperpolarizing effect of cannabinoids, but did not affect their inhibitory effect on EPSPs. The N-type Ca2+ channel blocker omega-conotoxin GVIA (omega-CgTX) occluded the WIN-mediated presynaptic inhibition, whereas the P/Q-type Ca2+ channel blocker omega-agatoxin TK (omega-Aga) had no effect. These results suggest that cannabinoids preferentially activate CB1 receptors at the nerve terminal of small-diameter primary afferent fibres. Upon activation, CB1 receptors may selectively inhibit presynaptic N-type Ca2+ channels and exocytotic release machinery, thereby attenuating the transmitter release at the trigeminal nociceptive synapses.

Effect of neonatal capsaicin treatment on neural activity in the medullary dorsal horn of neonatal rats evoked by electrical stimulation to the trigeminal afferents: an optical, electrophysiological, and quantitative study

To elucidate which glutamate receptors, NMDA or non-NMDA, have the main role in synaptic transmission via unmyelinated afferents in the trigeminal subnucleus caudalis (the medullary dorsal horn), and to examine the early functional effects of neonatal capsaicin treatment to the subnucleus caudalis, optical recording, field potential recording, and quantitative study using electron micrographs were employed. A medulla oblongata isolated from a rat 5--7 days old was sectioned horizontally 400-microm thick or parasagittally and stained with a voltage-sensitive dye, RH482 or RH795. Single-pulse stimulation with high intensity to the trigeminal afferents evoked optical responses mainly in the subnucleus caudalis. The optical signals were composed of two phases, a fast component followed by a long-lasting component. The spatiotemporal properties of the optical signals were well correlated to those of the field potentials recorded simultaneously. The fast component was eliminated by 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX; 10 microM), while the long-lasting component was not. The latter increased in amplitude under a condition of low Mg(2+) but was significantly reduced by DL-2-amino-5-phosphonovaleric acid (AP5; 30 microM). Neonatal capsaicin treatment also reduced the long-lasting component markedly. In addition, the decreases in the ratio of unmyelinated axons to myelinated axons and in the ratio of unmyelinated axons to Schwann cell subunits of trigeminal nerve roots both showed significant differences (P<0.05, Student's t-test) between the control group and the neonatal capsaicin treatment group. This line of evidence indirectly suggests that synaptic transmission via unmyelinated afferents in the subnucleus caudalis is mediated substantially by NMDA glutamate receptors and documented that neonatal capsaicin treatment induced a functional alteration of the neural transmission in the subnucleus caudalis as well as a morphological alteration of primary afferents within several days after the treatment.

Afferent ingrowth and onset of activity in the rat trigeminal nucleus

A novel in vitro preparation, consisting of the rat brainstem with the trigeminal ganglion attached, has been used to study the anatomical and functional development of the trigeminal nucleus from embryonic day (E)13 to postnatal day (P)6. Neurobiotin injections into the trigeminal ganglion showed that primary afferents had reached the trigeminal tract by E13 and had grown simple, mainly unbranched, collaterals into all levels of the nucleus by E15. By E17, these collaterals were extensively branched, with occasional boutons present. Patches of intense neurobiotin-labelled terminals, corresponding to whisker-related patterns, were first seen at E20 and became clearer over the next few days. Terminal arbours at this stage were relatively localized and densely branched, with many boutons. Responses from the trigeminal nucleus were recorded with suction electrodes, following stimulation of the trigeminal ganglion. Recordings from the main sensory nucleus showed a postsynaptic response was first present at E15. At E16, bath application of AP5 and DNQX showed that the response contained both NMDA and AMPA components, with NMDA predominating (75%). The NMDA : AMPA ratio remained high until P1, then gradually declined to 50% by P6. The postsynaptic response was also reduced by bath application of bicuculline, indicating the presence of a GABAA-mediated excitatory component. GABAergic excitation was present at all ages but was maximal from E20 to P1, the age at which whisker-related patterns are developing. It is hypothesized that both GABAergic excitation and NMDA receptor activation play a role in the consolidation of trigeminal connections, and are thus important in the development of whisker-related patterns.

Primary afferent synaptic responses recorded from trigeminal caudal neurons in a mandibular nerve-brainstem preparation of neonatal rats

1. Whole-cell patch-clamp recordings were made from the neurons in the superficial trigeminal caudal nucleus (substantia gelatinosa) visually identified in a parasagittal brainstem slice of neonatal rat with the mandibular nerve attached. 2. Stimulation of the mandibular nerve at 0.03 Hz evoked compound excitatory postsynaptic potentials (EPSPs) or currents (EPSCs) in trigeminal caudal neurons. When stimulated at higher frequency (> 0.5 Hz), compound synaptic responses were largely attenuated and a small component remained. This component had a monosynaptic nature, following high-frequency stimulation (33-50 Hz) with a stable synaptic latency. 3. The N-methyl-D-aspartate (NMDA) receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP5, 50 microM) largely attenuated the slow polysynaptic EPSCs. The AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) largely attenuated monosynaptic EPSCs, but only weakly attenuated slow polysynaptic EPSCs. Simultaneous application of CNQX and D-AP5 completely abolished EPSCs. The monosynaptic EPSCs isolated by repetitive stimulation had both NMDA and non-NMDA components. 4. Monosynaptic EPSCs having high threshold had a relatively long latency. During repetitive stimulation (0.5-5.0 Hz), EPSCs having high threshold and long latency underwent a stepwise potentiation in an activity-dependent manner. The conduction velocity estimated for these EPSCs fell into the range of C-fibres. The activity-dependent potentiation was observed for both non-NMDA and NMDA EPSCs and was accompanied by a significant decrease in the coefficient of variation of EPSC amplitude. 5. We suggest that the activity-dependent potentiation of EPSCs is induced presynaptically and that it may underlie the wind-up phenomenon, an activity-dependent hyperexcitability of the primary afferent C-fibres.

Long-term potentiation of primary afferent neurotransmission at trigeminal synapses of juvenile rats

Primary afferent monosynaptic and polysynaptic excitatory postsynaptic currents (EPSCs) were recorded from brainstem trigeminal neurons by stimulation of the mandibular nerve attached to the brainstem preparation of juvenile rats. A high-frequency conditioning stimulus induced long-term potentiation (LTP) of high-threshold EPSCs in the majority of trigeminal caudal neurons in substantia gelatinosa, where both A- and C-fibres terminate. However, the same conditioning stimulus did not potentiate low-threshold EPSCs in caudal neurons or EPSCs recorded from neurons in the middle part of trigeminal interpolar nucleus, where C-fibres rarely terminate. LTP in caudal neurons could be induced after blocking N-methyl-D-aspartate (NMDA) receptors with D(-)-2-amino-5-phosphonopentanoic acid (D-AP5, 50 microM), after postsynaptic loading of the Ca2+ chelator BAPTA (10 mM), or even after completely blocking excitatory transmission with kynurenic acid during conditioning. However, LTP was blocked by the metabotropic glutamate receptor antagonist (+)-alpha-methyl-4-carboxyphenylglycine (1 mM). We suggest that LTP of the trigeminal primary afferent EPSCs is induced preferentially in the C-fibre inputs and that the induction mechanism involves metabotropic glutamate receptors, possibly at the presynaptic terminals.

Newborn rat brainstem preparation with the trigeminal nerve attached for pain study

Studies using a brainstem-spinal cord preparation isolated from newborn rats have provided substantial information on neuro-physiology, -pharmacology and -anatomy of the respiratory center, such as mechanisms of respiratory rhythm generation, development of a respiratory center or respiratory reflex [T. Murakoshi, T. Suzue, S. Tamai, A pharmacological study on respiratory rhythm in the isolated brainstem-spinal cord preparation from newborn rat, Br. J. Pharmac. 86 (1985) 95-104 [5]; H. Onimaru, A. Arata, I. Homma, Primary respiratory rhythm generator in the medulla of brainstem-spinal cord preparations from newborn rats, Brain Res. 445 (1988) 314-324 [6]; H. Onimaru, I. Homma, Whole cell recordings from respiratory neurons in the medulla of brainstem-spinal cord preparations isolated from newborn rats, Pflügers Arch. 420 (1992) 399-406 [7]; J.C. Smith, K. Ballanyi, D.W. Richter, Whole-cell patch clamp recordings from respiratory neurons in neonatal rat brainstem in vitro, Neurosci. Lett. 314 (1992) 153-156 [10]; T. Suzue, Respiratory rhythm generation in the in vitro brainstem-spinal cord preparation of the neonatal rat, J. Physiol. (London) 354 (1984) 173-183 [11], reviewed in H. Onimaru, A. Arata, I. Homma, Neuronal mechanisms of respiratory rhythm generation: An approach using in vitro preparation, Jpn. J. Physiol. 47 (1997) 385-403 [8]]. Recently, the dissecting method of the preparation was modified to introduce a brainstem preparation with the trigeminal primary afferents attached for pain studies [M. Hamba, Repetitive stimulation potentiated the stimulus-evoked firing in the trigeminal caudalis-in vitro study. Neurosci. Res. 20 (1996) s163 [2]; M. Hamba, Stimulation-induced responses of the trigeminal caudal neurons in the brainstem preparation isolated from newborn rats, Brain Res. 785 (1998) 66-74 [3]]. As reported previously [3], the activity-dependent change in the excitability of pain-processing neurons, wind-up phenomenon, was studied in the trigeminal caudalis by stimulating the mandibular nerve, using a modified brainstem preparation isolated from newborn rats. The caudalis, the medulla dorsal horn, is known as the center for processing pain and sensory information from the cranio-facial area. The results indicated that the brainstem preparation is applicable for studies on the neuroplasticity at the pain-processing synapses. Here, we describe the method for isolation of a brainstem preparation with the trigeminal mandibular nerve attached and for recording the synaptic response evoked in the caudal neurons, using a whole-cell patch clamp technique. In the present study, we demonstrated repetitive stimulation-induced responses of caudal neurons at postnatal day 1 as an example showing the feasibility of the preparation for pain studies.