Noradrenaline-mediated inhibition by locus coeruleus of spinal trigeminal neurons

Inhibitory effects of conditioning stimulation of the locus coeruleus (LC) on the neuron activity in spinal trigeminal nucleus (STN) were investigated in gallamine-immobilized cats. Field potentials of STN and spike potentials of single relay neurons in STN were orthodromically elicited by ipsilateral alveolar nerve stimulation and antidromically by stimulation of contralateral medial lemniscus. Conditioning stimuli were applied to LC and sensory cortex (SC) at various C-T intervals.

In tracking experiments near the LC region, conditioning stimulation of LC itself produced the most pronounced decrease in amplitude of the STN field potentials. Orthodromic spikes of STN single neurons were significantly reduced by conditioning stimulation of LC as well as SC. In reserpine-treated animals, however, conditioning stimulation of LC failed to produce a decrease in the number of orthodromic spikes, while the inhibitory effect of SC conditioning stimulation remained unaffected. Under these circumstances, intravenous L-dopa and intraventricular noradrenaline reproduced an inhibitory effect of LC conditioning stimulation on orthodromic spike generation, while such an effect was not seen with either dopamine or serotonin. Antidromic spike was unaltered by any of these treatments. Histochemically, catecholamine fluorescence in LC was entirely eliminated after reserpine-treatment, but was restored after L-dopa injection. These results strongly suggest that noradrenaline released from the terminals of neurons originating in LC produces an inhibition of transmission in the STN relay neurons.     

Beta-receptor involvement in locus coeruleus-induced inhibition of spinal trigeminal nucleus neurons: Microiontophoretic and HRP studies

Microiontophoretic and HRP studies were performed on cats anesthetized with alpha-chloralose to determine whether or not the locus coeruleus (LC)- and noradrenaline (NA)-induced inhibition of relay neurons in the subnucleus oralis of the spinal trigeminal nucleus (STN) is mediated by beta-adrenergic receptors. The inhibition of orthodromic spike generation upon intracranial trigeminal nerve stimulation by LC conditioning stimulation and microiontophoretically applied NA (100-200 nA) was antagonized during microiontophoretic application of sotalol, a beta-adrenergic antagonist, but not affected by phentolamine, an alpha-adrenergic antagonist. When HRP at doses of 300-500 nA was applied for 5-15 min to the immediate vicinity of the STN relay or interneuron, which was electrophysiologically identified by stimulating the ipsilateral trigeminal nerve and contralateral medial lemniscus, the injection site was localized to an area 0.3 mm in diameter and HRP-reactive cells were found in the ipsilateral LC, dorsal raphe nucleus and periaqueductal gray ventral to the aqueduct. These results strongly suggest that NA released from the nerve terminals of LC cells inhibits transmission in the STN relay neuron via beta-adrenergic receptors.     

Dorsal raphe stimulation produces inhibitory effect on trigeminal nucleus neurons.

Inhibitory effects of conditioning stimulation of the dorsal raphe nucleus (DR) on the neuron activity in the rostral part of spinal trigeminal nucleus (STN) were studied in cats for the purpose of comparison with the inhibition induced by locus coeruleus (LC) stimulation. DR conditioning stimulation reduced the orthodromic field potential in STN elicited by inferior alveolar nerve stimulation, and enhanced the antidromic field potential in the trigeminal nerve evoked by STN stimulation; but the inhibitory effects of DR stimulation were considerably weaker than those of LC stimulation. In tracking experiments near the raphe nucleus, conditioning stimulation of DR itself produced the most pronounced decrease in the STN field potential. Orthodromic spike number of STN relay neurons was significantly reduced by DR conditioning stimulation; however, the threshold for the conditioning stimulus to the DR was much higher than that to the LC. Antidromic spike generation of the STN neurons was unaltered by conditioning stimulation of both DR and LC. DR stimulation elicited a field potential in STN, which followed high frequency stimuli up to 200 HZ. A single fiber action potential was also obtained in STN by DR stimulation. STN stimulation produced a field potential in DR, which followed high frequency stimuli. It is suggested from these findings that conditioning stimulation of DR produces a direct inhibition of transmission in STN neurons; however, this stimulation has less effect on these neurons than does stimulation of the LC.     

Effects of L-threo-DOPS, an L-noradrenaline precursor, on locus coeruleus-originating neurons in spinal trigeminal nucleus

Electrophysiological studies using reserpinized cats were performed to examine the effects of L-threo-3,4-dihydroxyphenylserine (L-threo-DOPS) on the noradrenergic pathway from the locus coeruleus (LC) to the spinal trigeminal nucleus (STN). The spike generation of STN relay neurons induced by trigeminal nerve stimulation was not affected by LC conditioning stimulation nor iontophoretic application of L-threo-DOPS. After intraventricular administration of L-threo-DOPS, the inhibition of the spike generation was seen with LC conditioning stimulation and blocked by iontophoretically applied sotalol, suggesting that L-noradrenaline converted from L-threo-DOPS inhibits transmission of STN relay neurons.     

Suppression of tooth pulp evoked responses by activation of raphe-trigeminal neurons in rat

IRIKI, A. AND K. TODA. Suppresion of tooth pulp evoked responses by activation of raphe-trigeminal neurons in rat. BRAIN RES. BULL. 8(6) 777–780, 1982.—Electrical stimulation (0.1 msec duration, 50 Hz, l min) applied to the nucleus raphe magnus markedly suppressed the tooth pulp evoked responses in caudal part of the spinal trigeminal nucleus but only slightly altered those in its rostral area. Raphe stimulation also greatly suppressed jaw opening reflex evoked by tooth pulp stimulation, which lasted longer than suppression of spinal trigeminal nucleus.     

Suppressive effect of vagal afferents on the activity of the trigeminal spinal neurons related to the jaw-opening reflex in rats: involvement of the endogenous opioid system

The purpose of the present study is to test the hypothesis that via the endogenous pain control system, vagal afferent input modulates the activity of the trigeminal spinal nucleus oralis (TSNO) related to the tooth pulp (TP)-evoked jaw-opening reflex (JOR). Extracellular single-unit recordings were made from 36 TSNO units responding to TP electrical stimulation with a constant temporal relationship to a digastric electromyogram (dEMG) signal in 26 pentobarbital-anesthetized rats. The activity of 36 TSNO neurons and the amplitude of the dEMG increased proportionally during 1.0–3.5 times the threshold for JOR. Some of these neurons (4 out of 5) were also excited by chemical stimulation (bradykinin, 1–2 μl, 1 mM) of TP. In 31 out of 36 TSNO neurons (86%), their activities during tooth pulp stimulation were suppressed by conditioning stimulation of the right vagus nerve. The suppressive effect of vagal afferent stimulation occurred at conditioning-test intervals of 20–150 ms after the onset of the stimulation, and its maximal suppressive effect occurred at approximately 50 ms. The mean time course of this suppressive effect paralleled that of the dEMG. After administration of naloxone (0.5 and 1.0 mg/kg, i.v.), an opiate receptor blocker, the suppressive effect on the activity of TSNO neurons (6 out of 8) was significantly attenuated at the conditioning-test interval of 50 ms compared to the control (p < 0.01). These results suggested that vagal afferent input inhibits nociceptive transmission in the TSNO related to TP-evoked JOR and this inhibitory effect may occur via the endogenous opioid system in rats.     

Effects of noxious stimuli on neurons of the lateral reticular nucleus region in rabbits

The responses of neurons in the lateral reticular nuclues region (LRN) to both noxious spinal (radiant heat) and trigeminal inputs (tooth pulp stimulation), were analyzed in 12 anesthetized and paralyzed rabbits. It was found that 20% of neurons tested were affected by one or both noxious stimuli and 58% of these cells could be antidromically activated by stimulation of the cerebellum. In the majority of cell (85% of the heat-responding, and 72% of the tooth pulp-responding) the effect of noxious stimuli was excitatory and in the remaining cells was inhibitory. Furthermore 53% of responding neurons showed a convergence to spinal and trigeminal input. The type of response was the same (excitation) in 63% of these units, but was different in the others. These data support the view of an involvement of the LRN region in the mechanisms of the nociceptive information and suggest the possibility that the cerebellum may also contribute to some aspects of these mechanisms.     

Convergence of trigeminal input with visceral and phrenic inputs on primate C1–C2 spinothalamic tract neurons

Trigeminal, spinal and vagal afferent fibers overlap in C1–C2 segments. We hypothesized that trigeminal input from the superior sagittal sinus (SSS) can excite C1–C2 spinothalamic tract (STT) neurons receiving thoracic visceral or phrenic inputs. Effects of SSS stimulation were evenly divided among cells responding to each nerve stimulus; magnitude of responses to ipsilateral vagal input was greater in neurons excited by SSS input. Somatic fields of 80% of neurons responding to SSS stimulation included face areas innervated by the trigeminal nerve, whereas somatic fields of 89% of neurons unaffected by SSS stimulation were located only on areas innervated by cervical spinal nerves. Results are consistent with the idea that pain referred to trigeminal areas could originate in thoracic organs.     

Convergence of trigeminal input with visceral and phrenic inputs on primate C1-C2 spinothalamic tract neurons.

Trigeminal, spinal and vagal afferent fibers overlap in C1-C2 segments. We hypothesized that trigeminal input from the superior sagittal sinus (SSS) can excite C1-C2 spinothalamic tract (STT) neurons receiving thoracic visceral or phrenic inputs. Effects of SSS stimulation were evenly divided among cells responding to each nerve stimulus; magnitude of responses to ipsilateral vagal input was greater in neurons excited by SSS input. Somatic fields of 80% of neurons responding to SSS stimulation included face areas innervated by the trigeminal nerve, whereas somatic fields of 89% of neurons unaffected by SSS stimulation were located only on areas innervated by cervical spinal nerves. Results are consistent with the idea that pain referred to trigeminal areas could originate in thoracic organs.     

Locus coeruleus modulation of the motor thalamus: Inhibition in nuclei ventralis lateralis and ventralis anterior

Single-cell recordings were made from 693 cells in thalamic nuclei ventralis lateralis and ventralis anterior (VL-VA). Cells were identified as thalamocortical projection cells by antidromic firing from motor cortex or classified according to responsiveness to stimulation of the brachium conjunctivum (BC), entopeduncular nucleus, and motor cortx. Only 14% of the cells tested responded to entopeduncular nucleus stimulation, whereas BC and motor cortex (orthodromic) stimulation each evoked responses in 31% of the VL-VA cells tested. The most common sources of convergent input to VL-VA cells were motor cortex and BC. In 30% of the VL-VA population tested, spontaneous firing was inhibited by stimulation of the locus coeruleus (LC). This inhibition had a long latency to onset which varied from cell to cell (100 to 1000 ms or more) and a long duration (mean = 1183 ms). The inhibition of spontaneous firing by LC was associated with a variable effect upon BC-evoked excitatory responses in VL-VA cells. In some cases, BC evoked responses were suppressed, but not abolished. In other cells, the excitatory response to BC was unaffected despite complete cessation of VL-VA cell spontaneous firing after LC stimulation. The inhibitory action of LC was not limited to any class of VL-VA cells, but occurred most frequently in neurons receiving an input from the BC. The LC inhibition of VL-VA is not related to changes in systemic blood pressure or an action at the level of the cerebellar cortex. However, LC also produces inhibitory and excitatory effects in centrum medianum neurons, which could account for some of the long-latency responses observed in VL-VA. This electrophysiological study of the action of locus coeruleus upon cellular activity in the motor thalamus argues against involvement in phasic movement and associated postural adjustments. Rather, the locus coeruleus projection to thalamus has properties which suggest a role in longer-term tonic regulation of motor activity.     

Stimulation of the locus coeruleus suppresses trigeminal sensorimotor function in the rat

The nucleus locus coeruleus (LC) has been implicated in the modulation of the spinal sensorimotor function. The aim of the present study was to examine the effect of electrical stimulation of the LC on sensorimotor function in the trigeminal system. The following two cases of sensorimotor behaviors mediated by the trigeminal brainstem sensory nuclear complex were examined: (1) the activity of the masseter muscle evoked by pressure on the region of the temporomandibular joint (TMJ); and (2) the activity of the digastric muscle evoked by electrical stimulation of the tooth pulp, resulting in the jaw-opening reflex. In the first case, LC stimulation at 10, 30 and 50 microA resulted in a 70%, 68% and 55% reduction in the magnitude of electromyogram (EMG) activity of the masseter muscle compared with the control (without LC stimulation), respectively. The threshold intensity for the onset of masseter EMG activity increaced to 106%, 111% and 121% of the control with 10, 30 and 50 microA LC stimulation, respectively. In the second case, EMG magnitude in response to the digastric muscle decreased to 42% of the control when 30 microA of LC stimulation was delivered. These results suggest that descending influences from the LC can act in suppression of the trigeminal sensorimotor function.     

Electrophysiological identification of neurons in locus coeruleus

In 17 urethane-anesthetized rats 35 neurons, histologically verified as being situated in the locus coeruleus, were driven antidromically (latency, 44 msec) by electrical stimulation of the supracallosal bundle. Neurons of the locus coeruleus were also activated antidromically by stimulation of sites along the dorsal noradrenergic bundle in the midbrain (8-msec latency) and the hypothalamus (12-msec latency), and by stimulation of sites in the olfactory bulb (latency, 39 msec). Conduction velocity from these sites to the locus coeruleus was estimated to be 0.4 to 0.6 m/sec. Refractory periods of fibers in the dorsal noradrenergic bundle were determined at twice threshold and in the supracallosal bundle at intensities just above threshold; refractory periods ranging from 4 to 20 msec were observed. Because neurons both in and near the locus coeruleus were antidromically activated by stimulation of the dorsal noradrenergic bundle whereas stimulation of the supracallosal bundle antidromically activated only neurons in the locus coeruleus, stimulation of the dorsal noradrenergic bundle could not be used to identify locus coeruleus neurons. It is concluded that a subpopulation of neurons in the locus coeruleus can be identified by their slow, steady firing rate (2.6 per second) and long-latency antidromic response to stimulation of the supracallosal bundle. The electrophysiological properties of locus coeruleus neurons are considered in relation to neuroanatomical and functional studies of the locus coeruleus.     

Noradrenergic responses of spinal neurons in locus coeruleus-spinal cord co-cultures

Locus coeruleus (LC) explants were co-cultured with dissociated spinal neurons of mice. Nerve fibers exhibiting catecholamine fluorescence radiated from the explants and frequently invested spinal cord (SC) neurons close to the explants. Electrical stimulation of the explant and iontophoretic application of norepinephrine evoked a spectrum of slow depolarizing, hyperpolarizing, and biphasic responses in the SC cells. The responses to LC stimulation and to application of norepinephrine were usually similar in a given cell. The depolarizing responses were associated with an increase in apparent input resistance and pharmacologic tests indicated that the responses were mediated by alpha-receptors. Neurons in regions innervated by catecholamine-containing fibers usually gave depolarizing responses to LC stimulation and such neurons had a very high probability of exhibiting depolarizing responses to applied norepinephrine. It would appear that either locus coeruleus explants favored the survival of cells with alpha-receptors or expression of these receptors in SC neurons was induced by innervation of the neurons by locus coeruleus axons.     

Evidence of facilitatory coerulospinal action in lumbar motoneurons of cats

Functional connectivity of the feline coerulospinal projection was delineated by utilizing the combined approaches of antidromic activation and electrical stimulation. We isolated 25 locus coeruleus (LC) neurons that were electrophysiologically identified and histologically verified and that could be driven by stimulating the spinal cord. Antidromicity of the spike potentials was confirmed by the constant latency, the high frequency (100 Hz) following, fractionation of the initial segment-somatodendritic potential, and collision between the antidromic and the spontaneous orthodromic spikes. The mean conduction speed was20 ± 8m/sec(range= 7to32m/sec). Intracellular studies revealed facilitatory LC actions in 22 lumbar motoneurons (MNs), In 13 MNs, LC activation alone produced slow-rising excitatory postsynaptic potentials (EPSPs) of3 ± 1mV amplitude that lasted 4–30 msec. Six of the 13 MNs discharged action potentials upon LC stimulation. In the remaining 9 MNs, no observable potential change was registered after LC activation. Antecedent LC stimulation consistently potentiated the synaptic efficacy of testing dorsal root shocks. The enhancement of synaptic activation was antagonized by systemic injection of phenoxybenzamine (3 mg/kg). These results suggest that facilitation of MNs by the LC is at least in part mediated by distal dendritic depolarization. Those MNs that exhibited augmented excitability but no demonstrable EPSPs may have been activated by norepinephrine-mediated synaptic modulation.     

Evidence for peripheral, but not central modulation of trigeminal cold receptive cells in the rat

The effects of locus coeruleus (LC), periaqueductal grey (PAG) and segmental stimulation (all of which are known to inhibit convergent nociceptive cells), were tested on the activity of cold receptive cells in the trigeminal system of the rat. LC and PAG stimulation from sites which inhibited convergent nociceptive cells had no effect on cells with cold receptive input in the trigeminal nucleus caudalis. Electrical or mechanical segmental stimulation caused suppression of activity in cold receptive trigeminal nucleus neurons. Recording from the trigeminal ganglion showed this suppression to be a property of the primary afferent cold receptors themselves and therefore it is not analogous to the proposed mechanism for the segmental inhibition of convergent nociceptive neurons.     

Bulbar reticular neurons relaying somatosensory information to the mesencephalic parabrachial area of the cat

Somatosensory neurons projecting to the mesencephalic parabrachial area (MPBA), which is located ventral to the inferior colliculus and dorsal to the brachium conjunctivum, were recorded from the bulbar reticular formation of adult cats anesthetized with alpha-chloralose. The majority (41 of 50 neurons) were nociceptive-specific neurons responding only to noxious mechanical and/or thermal stimuli to the skin, cornea and/or oral mucosa. The size of their receptive fields was smaller than that of the intrinsic MPBA-neurons, but larger than that of the trigeminal sensory nucleus neurons. Twenty-three neurons received input from the tooth pulp nerve and 10 of 32 neurons tested responded to electrical stimulation of the vagal nerve. These results indicate that these bulbar reticular neurons receive noxious inputs and transmit them to the MPBA, which also receives input from spinal or trigeminal sensory nucleus neurons projecting directly to the MPBA.     

Interactions between nucleus centrum medianum and gigantocellular nociceptive neurons

Cells in nucleus reticularis gigantocellularis (NGC) and nucleus centrum medianum (CM) are known to respond to tooth pulp stimulation, which is a nociceptive trigeminal stimulus. We examined the effect of stimulation of CM on this class of neurons in NGC. Thirty-nine percent of the 57 neurons tested were antidromically activated by stimulation of the ipsilateral CM, while 28% of the 40 neurons tested were antidromically activated from the contralateral CM. In addition, the spontaneous activity of seven NGC neurons was altered by stimulation of the ipsilateral CM, while five cells were affected by stimulation of the contralateral CM. These data suggest a complex and reciprocal interaction between neurons in NGC and CM.     

Activation of ascending antinociceptive system by vagal afferent input as revealed in the nucleus ventralis posteromedialis

Thalamic nociceptive neurons receiving afferent input from the tooth pulp (TP) were recorded from the nucleus ventralis posteromedialis proper (VPM) in cats anesthetized with urethane and chloralose. Effects of cervical vagus nerve stimulation on responses of TP neurons in the VPM were investigated. Twenty-one tooth pulp specific (TPS) and eight wide dynamic range (WDR) neurons with TP input were obtained from the periphery (shell region) of the posterior half of the VPM. Of these, many were also excited by electrical stimulation of trigeminothalamic tract (TTT) fibers in the trigeminal medial lemniscus. A conditioning-test paradigm was used to examine effects of vagal stimulation on responses of VPM neurons to electrical stimulation of TP and TTT. Inhibition of the responses was observed in 12 TPS and seven WDR neurons. Local anesthetic block of the mesencephalic periaqueductal gray (PAG) and/or nucleus raphe dorsalis (NRD) eliminated the inhibitory effects of vagal stimulation on the responses of both classes of TP neurons to TTT stimulation. In contrast, the inhibitory effects on responses to TP stimulation were insignificantly affected. These data suggest that vagal afferents can activate the ascending antinociceptive pathway from PAG/NRD onto VPM, in addition to activating the descending antinociceptive system acting upon the lower brain stem.     

Projection of tooth pulp afferents to the cat trigeminal nucleus caudalis

Unit activity was recorded extracellularly from cat medullary neurons following electrical stimulation of the canine tooth pulp. Response characteristics of the neurons quickly stabilized at specific suprathreshold stimulus intensities but such properties as spike latency, interspike interval and spike density varied systematically as intensity was raised to maximally effective values. Receptive fields were principally unilateral. The majority included both canines and extended into other oro-facial areas. Suppression of a pulpal response could be effected by preceding tooth stimulation with a conditioning stimulus applied to some other point in the receptive field of the responding cell at an appropriate interstimulus interval. In contrast, a pulpal response could be enhanced by presenting two stimuli successively to the same canine at such intervals. Similar enhancing effects followed simultaneous stimulation of spatially segregated loci in a field. The pulp-responsive neurons were localized histologically in, or in the immediate vicinity of, the nucleus caudalis of the spinal trigeminal complex where the possibility of their existence has been questioned previously. Most of the cells were situated along the ventromedial border of the nucleus, a region reported to contain other pain-related neurons with trigeminal fields.     

Two physiologically distinct populations of neurons in the ventrolateral medulla innervate the locus coeruleus

Recent anatomic studies indicate that the nucleus paragigantocellularis (PGi), located in the rostral ventrolateral medulla, strongly innervates the locus coeruleus (LC) while no such input derives from the more caudally located lateral reticular nucleus (LRN). In the present study, focal electrical stimulation of the LC was used to antidromically activate neurons in the ventrolateral medulla. A substantial number of PGi neurons were antidromically driven from the ipsilateral LC, while antidromic activation was virtually absent in LRN. Furthermore, several physiologic properties of antidromically driven cells in PGi define two populations within this group of neurons afferent to LC. These findings provide physiologic confirmation of an anatomically identified input to LC.