Quantitative characterization and spinal pathway mediating inhibition of spinal nociceptive transmission from the lateral reticular nucleus in the rat

1. The modulation of spinal nociceptive transmission from the lateral reticular nucleus (LRN) was characterized for 47 spinal dorsal horn neurons in pentobarbital-anesthetized, paralyzed rats. All 47 units studied had receptive fields confined to the glabrous skin of the plantar surface of the ipsilateral hind foot and responded to mechanical stimulation as well as noxious heating (50 degrees C). Rostral projections contained in the ventrolateral quadrant of the cervical spinal cord were demonstrated for 15 of the 47 units by antidromic invasion. Glutamate- and stimulation-produced descending inhibition, the spinal pathway, and tonic descending inhibition from the LRN were systematically examined. 2. Inhibition of unit responses to heating of the skin by electrical stimulation in the LRN varied with the intensity, pulse duration (100 or 400 microseconds), and frequency (25-100 Hz) of stimulation. Greater inhibition was produced at lower intensities of stimulation with the 400-microseconds pulse duration and a frequency of 100 Hz. The effects of stimulation on spontaneous activity and responses to heat were compared in 16 experiments; inhibition of spontaneous activity was intensity dependent and did not differ significantly in magnitude from stimulation-produced inhibition of responses to heating of the skin. 3. Tracking experiments established that stimulation in the ipsilateral and contralateral ventrolateral medulla reliably attenuated unit responses to noxious heating of the skin and that stimulation in the LRN produced maximal inhibition at a low intensity of stimulation. Descending inhibition was quantitatively characterized from sites within (n = 32) and outside (n = 30) the LRN. Both the extrapolated mean stimulation threshold for inhibition and mean intensity inhibiting unit responses to heat to 50% of control were significantly lower for sites in the LRN. 4. The responses of seven spinal units to graded noxious heating of the skin were studied; all exhibited linear monotonic stimulus-response functions (SRFs) throughout the temperature range examined (42-50 degrees C). Electrical stimulation in the LRN significantly decreased the slope (42 +/- 4% of control) of the SRFs and increased the neuronal response threshold (2.0 +/- 0.7 degrees C). 5. S-glutamate (50 nmol, 0.5 microliter) was microinjected into stimulation sites within (n = 15) and distant from (n = 6) the LRN. Glutamate produced a transient (less than 7 min) but significant attenuation of neuronal responses to heat to 35 +/- 6% of control only when microinjected into the LRN.(ABSTRACT TRUNCATED AT 400 WORDS)     

Evidence that dopamine-beta-hydroxylase immunoreactive neurons in the lateral reticular nucleus project to the spinal cord in the rat.

The existence of noradrenergic projections from the lateral reticular nucleus (LRt) to the dorsal quadrant of cervical, thoracic, or lumbar spinal cord was investigated using a combined method of WGA-apo-HRP-gold retrograde tracing and dopamine-beta-hydroxylase (DBH) immunocytochemistry. Preliminary retrograde tracing studies indicated that LRt neurons projecting to cervical, thoracic, or lumbar spinal cord were characteristically located near the perimeter of the LRt. Double-labeling experiments demonstrated that a portion of these peripherally-located, spinal-projecting neurons were DBH-immunoreactive. Double-labeled neurons were also located at the parvocellular division of the contralateral LRt in the thoracic injection cases. Double-labeled neurons were not observed at the subtrigeminal division in cervical, thoracic, or lumbar injection case. The results suggest the possibility that the noradrenergic LRt-spinal pathway might be involved in a variety of pain processing and cardiovascular regulatory functions in the rat.     

Fluorescent double-label study of lateral reticular nucleus projections to the spinal cord and periaqueductal gray in the rat.

Following injections of WGA-HRP into either the spinal cord or periaqueductal gray, labeled neurons were observed bilaterally along the periphery of the lateral reticular nucleus (LRN) magnocellular division. The possibility that some of these neurons in the LRN provide collateral axonal branches to both the periaqueductal gray and the spinal cord was investigated in rats using a retrograde double-labeling method employing two different fluorescent tracers, True Blue and Nuclear Yellow. Following sequential injection of the two fluorescent axonal tracers into the spinal cord and periaqueductal gray in the same animal, a modest number of double-labeled neurons were observed bilaterally near the medial and dorsal perimeter of the magnocellular division of the LRN. The labeled neurons were distinctly multipolar in shape and measured approximately 15-18 mu in their greatest transverse diameter. No double-labeled neurons were observed in the parvocellular or subtrigeminal divisions of the LRN. Based upon these observations, it is suggested that collaterals of the LRN-spinal pathway provide feedback information to the periaqueductal gray that might then be used to modulate the participation of the latter cell group in a variety of pain processing and cardiovascular regulatory functions.     

Fluorescent double‐label study of lateral reticular nucleus projections to the spinal cord and periaqueductal gray in the rat

Following injections of WGA‐HRP into either the spinal cord or periaqueductal gray, labeled neurons were 7observed bilaterally along the periphery of the lateral reticular nucleus (LRN) magnocellular division. The possibility that some of these neurons in the LRN provide collateral axonal branches to both the periaqueductal gray and the spinal cord was investigated in rats using a retrograde double‐labeling method employing two different fluorescent tracers, True Blue and Nuclear Yellow. Following sequential injection of the two fluorescent axonal tracers into the spinal cord and periaqueductal gray in the same animal, a modest number of double‐labeled neurons were observed bilaterally near the medial and dorsal perimeter of the magnocellular division of the LRN. The labeled neurons were distinctly multipolar in shape and measured approximately 15–18 μ in their greatest transverse diameter. No double‐labeled neurons were observed in the parvocellular or subtrigeminal divisions of the LRN. Based upon these observations, it is suggested that collaterals of the LRN‐spinal pathway provide feedback information to the periaqueductal gray that might then be used to modulate the participation of the latter cell group in a variety of pain processing and cardiovascular regulatory functions. Anat Rec 256:91–98, 1999. © 1999 Wiley‐Liss, Inc.     

Projections from the lateral vestibular nucleus to the spinal cord in the mouse

The present study investigated the projections from the lateral vestibular nucleus (LVe) to the spinal cord using retrograde and anterograde tracers. Retrogradely labeled neurons were found after fluoro-gold injections into both the cervical and lumbar cord, with a smaller number of labeled neurons seen after lumbar cord injections. Labeled neurons in the LVe were found in clusters at caudal levels of the nucleus, and a small gap separated these clusters from labeled neurons in the spinal vestibular nucleus (SpVe). In the anterograde study, BDA-labeled fiber tracts were found in both the ventral and ventrolateral funiculi on the ipsilateral side. These fibers terminated in laminae 6–9. Some fibers were continuous with boutons in contact with motor neurons in both the medial and lateral motor neuron columns. In the lumbar and sacral segments, some collaterals from the ipsilateral vestibulospinal tracts were found on the contralateral side, and these fibers mainly terminated in laminae 6–8. The present study reveals for the first time the fiber terminations of the lateral vestibular nucleus in the mouse spinal cord and therefore enhances future functional studies of the vestibulospinal system.     

Inhibition of spinal nociceptive information by stimulation in midbrain of the cat is blocked by lidocaine microinjected in nucleus raphe magnus and medullary reticular formation

The organization in the brain stem of descending inhibitory control of spinal nociceptive information was studied in anesthetized, paralyzed cats by quantitatively evaluating the effects of reversible blocks produced by lidocaine microinjected in the medial and/or lateral medulla. Spinal neuronal inhibition produced by stimulation in the nucleus raphe magnus (NRMS) was compared to the inhibition of the same dorsal horn neurons produced by stimulation 2 mm lateral in the medullary reticular formation (MRFS). When the inhibition produced by NRMS and/or MRFS was blocked by lidocaine microinjected in those medullary sites, the efficacy of spinal neuronal inhibition produced by stimulation in the midbrain periaqueductal gray (PAGS) and 4 mm lateral in the reticular formation (LRFS) was evaluated and compared with the inhibition produced before the intramedullary microinjection of lidocaine. All 32 spinal dorsal horn neurons studied responded to hindlimb cutaneous nerve stimulation at strengths supramaximal for activation of A-alpha,delta- and C-fibers, to mechanical stimuli applied to the skin, and 27 also responded to noxious radiant heating (50 degrees C, 10 s) of the skin of the foot- or toepads (5 units had receptive fields in the hairy skin of the hindlimb). The noxious heat-evoked responses of all units studied were inhibited by NRMS or MRFS. The mean threshold currents for spinal inhibition, the mean maximal inhibition produced, and the mean stimulation currents producing an attenuation to 50% of the control response to 50 degrees C skin heating did not differ between NRMS and MRFS. When quantitatively compared on the same spinal units, NRMS produced the same mean magnitude of inhibition as the same intensities of MRFS, and both NRMS and MRFS produced the same mean percent increment in inhibition per 100-microA increase in the intensity of brain stimulation. The responses of the spinal units studied to graded noxious heating of the skin was a monotonic linear function throughout the temperature range employed (42-50 degrees C). MRFS shifted this stimulus response function (SRF) to the right, raising significantly the threshold of response a mean 2.2 degrees C to noxious heating of the skin without significantly affecting the slope of the SRF. MRFS reduced the number of discharges of spinal units evoked by electrical A-alpha,beta-fiber stimulation of hindlimb cutaneous nerves in 4 of 10 units studied. NRMS similarly inhibited the A-alpha,beta-fiber-evoked responses of two of the same four units affected by MRFS but also affected two of the remaining six units not affected by MRFS.(ABSTRACT TRUNCATED AT 400 WORDS)     

The lateral reticular nucleus in the cat

Summary Intracellular recording from neurones in the lateral reticular nucleus (LRN) demonstrated that, in addition to the previously identified excitatory ipsilateral forelimb tract (iF tract) (Clendenin et al. 1974c) there is an inhibitory tract mediating information from the ipsilateral forelimb to the LRN. The excitatory and inhibitory tracts were similarly organized. The tract neurones were monosynaptically activated by affcrents in the ipsilateral forelimb and projected to the same area of the LRN. They will be considered as excitatory and inhibitory components of the iF tract and denoted the excitatory and inhibitory iF tract (EiF and IiF tracts). Stimulation of the descending ipsilateral dorsolateral funiculus (iDLF) in the C3 segment evoked disynaptic EPSPs and IPSPs in LRN neurones contacted by the EiF and IiF tracts. The responses in individual LRN neurones evoked from the iDLF were similar to the responses evoked from the forelimb nerves suggesting that the EiF and IiF tracts are monosynaptically activated by fibres in the iDLF. The dorsal portion of the magnocellular part of the LRN constituted the main termination area of both the EiF and IiF tracts. Neurones in this area have previously been shown to project ipsilaterally to lobule V in the pars intermedia of the cerebellar anterior lobe and to the paramedian lobule (Clendenin et al. 1974a). IPSPs evoked from the IiF tract in LRN neurones outside the main termination area had smaller amplitudes and longer latencies. This finding suggests that these responses were generated by thin axon collaterals given off from dorsally located stem axons.     

Spinal projections to the lateral reticular nucleus in the rat

Summary The synaptic relationships and the distribution of the afferent terminals of the spinal pathway to the lateral reticular nucleus (LRN) of the rat were examined following induced degeneration. After high cervical hemisections, the spino-LRN projection was first examined with the Fink-Heimer silver impregnation method. Degeneration was confined primarily to the ipsilateral LRN and all three divisions of the nucleus were involved. Maximum degeneration was observed in the caudal regions of the parvocellular division. The magnocellular division, except for the extreme dorsomedial area, showed substantial degeneration as well. The subtrigeminal division throughout its entire length contained only sparse degeneration.Electron microscopic examination following spinal cord lesions revealed both round and pleomorphic-vesicle terminals in various stages of electron dense degeneration. The majority of the degenerating terminals were of the round-vesicle variety. Both types of terminals contacting somata were also observed to degenerate but their number was small in comparison to those on dendritic profiles. Terminals in synaptic contact with two dendritic profiles were also observed to degenerate. Some of the large terminals belonging to synaptic configurations (glomeruli) underwent degeneration and were therefore of spinal origin as well.     

The lateral reticular nucleus in the cat

The afferent paths from the spinal cord and from trigeminal afferents to the lateral reticular nucleus (LRN) were investigated by intracellular recording from 204 LRN neurones in preparations with a spinal cord lesion at C3 that spared only the ipsilateral ventral quadrant. Stimulation of nerves in the limbs evoked EPSPs and JPSPs in 201 of 204 tested LRN neurones. The strongest input was from the ipsilateral forelimb (iF) which evoked EPSPs in 49% and IPSPs in 73% of the LRN neurones. Each of the other limbs evoked EPSPs in approximately 20% and IPSPs in approximately 25% of the neurones. Stimulation of the ipsilateral trigeminal nerve (iTrig) evoked EPSPs in 32% and IPSPs in 46% of the neurones. The shortest latencies of the EPSPs and IPSPs indicated a disynaptic connection between primary afferents in the iF and iTrig and the LRN. The most direct pathways for excitatory and inhibitory responses from the other limbs were trisynaptic. Stimulation of the ventral part of the ipsilateral funiculus (iVLF) at C3 (C3iVLF) evoked monosynaptic responses in 189 of 201 tested LRN neurones. Monosynaptic EPSPs were recorded in 104 neurones and monosynaptic IPSPs in 126 neurones. Monosynaptic EPSPs and IPSPs were encountered in all parts of the LRN. Stimulation of the iVLF at L1 (L1iVLF) evoked monosynaptic EPSPs and IPSPs in the ventrolateral part of the LRN. The termination areas of excitatory and inhibitory fibres appeared to be the same. LRN neurones without monosynaptic EPSPs or IPSPs from the L1iVLF were located mainly in the dorsal part of the magnocellular division. Stimulation of the dorsal funiculi (DF) at C2 and the ipsilateral trigeminal nerve (iTrig) evoked excitatory and inhibitory responses in the LRN. The shortest latencies of EPSPs and IPSPs indicated disynaptic connections.     

The lateral reticular nucleus in the cat

Intracellular recordings were obtained from 204 neurones in the lateral reticular nucleus (LRN). LRN neurones contacted by the bVFRT were identified by the responses evoked on stimulation of descending fibres in the contralateral ventral quadrant of the spinal cord (cVQ) at cervical (C5cVQ) and lumbar (L2cVQ) levels. Stimulation of the cVQ evoked excitatory or inhibitory responses in 124 of the 204 LRN neurones. EPSPs were evoked in 45, IPSPs in 52 and both EPSPs and IPSPs in 27 LRN neurones. The shortest latencies of the responses evoked from the cVQ indicated that both EPSPs and IPSPs were disynaptic. This finding was confirmed by direct stimulation of the ascending fibres in the ipsilateral ventrolateral funiculus at C3 (C3iVLF) or L1 (L1iVLF). In most LRN neurones activated or inhibited from the cVQ, stimulation of the iVLF evoked similar responses at a monosynaptic latency. These results indicate that the bVFRT consists of roughly equally large groups of excitatory and inhibitory neurones monosynaptically connected with the LRN. Excitatory and inhibitory bVFRT neurones had similar peripheral receptive fields and termination areas in the LRN. LRN neurones were divided into those contacted by cervical bVFRT neurones and lumbar bVFRT neurones. The former group consisted of LRN neurones responding to C5cVQ stimulation at latencies below 5 ms, whereas the latter group contained LRN neurones responding to stimulation of the L2cVQ. Cervical bVFRT neurones projected to most parts of the LRN whereas the projection of lumbar bVFRT neurones were confined to the ventrolateral part of the nucleus. Excitatory and inhibitory vVFRT neurones of each group had similar termination areas.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of the nociceptive jaw opening reflex by the lateral reticular nucleus (LRN) in the rabbit

The effect of stimulation of the lateral reticular nucleus (LRN) on the jaw opening reflex (JOR), evoked by tooth pulp or by palatal tissue stimulation, was studied in anesthetized rabbits. The results show an inhibitory effect on the JOR evoked by tooth pulp stimulation, which is considered a nociceptive reflex, while no effect is shown on the JOR evoked by palatal tissue stimulation. The maximum effect was observed at intervals of 30-50 ms between the two stimuli and the more effective site of inhibition in the nucleus was found to be the ventrolateral portion. The result of a selective effect of LRN stimulation on nociceptive reflexes supports the view of a role of this nucleus in the central mechanisms of pain control.     

Evidence that dopamine‐beta‐hydroxylase immunoreactive neurons in the lateral reticular nucleus project to the spinal cord in the rat

The existence of noradrenergic projections from the lateral reticular nucleus (LRt) to the dorsal quadrant of cervical, thoracic, or lumbar spinal cord was investigated using a combined method of WGA‐apo‐HRP‐gold retrograde tracing and dopamine‐beta‐hydroxylase (DBH) immunocytochemistry. Preliminary retrograde tracing studies indicated that LRt neurons projecting to cervical, thoracic, or lumbar spinal cord were characteristically located near the perimeter of the LRt. Double‐labeling experiments demonstrated that a portion of these peripherally‐located, spinal‐projecting neurons were DBH‐immunoreactive. Double‐labeled neurons were also located at the parvocellular division of the contralateral LRt in the thoracic injection cases. Double‐labeled neurons were not observed at the subtrigeminal division in cervical, thoracic, or lumbar injection case. The results suggest the possibility that the noradrenergic LRt‐spinal pathway might be involved in a variety of pain processing and cardiovascular regulatory functions in the rat. Anat Rec 263:269–279, 2001. © 2001 Wiley‐Liss, Inc.     

Bidirectional neurones in the cervical enlargement of the cat spinal cord with axons descending to sacral segments and ascending to the cerebellum and the lateral reticular nucleus

Neurones located in cervical segments (C6/C7) of the spinal cord were investigated electrophysiologically in cats deeply anaesthetized with alpha-chloralose. Extracellular recordings of antidromic action potentials were performed in order to establish whether long descending propriospinal neurones projecting to sacral segments could have collateral axonal branches ascending to supraspinal centres. The effects of stimulation of the lateral reticular nucleus (LRN) and the inferior cerebellar peduncle (restiform body, RB), as well as the thirteenth thoracic (Th13) and sacral (S1/S2) segments of the spinal cord were tested in 93 cells. Two main groups of cells were identified: 54 % of the total sample were classified as purely propriospinal and 46 % as bidirectional neurones. Various patterns of projections, as well as the ipsi-, contra- or bilateral courses of axons in the lateral funiculi of the spinal cord, enabled several types of neurones to be distinguished within the above groups. Comparison between particular types showed no significant difference with respect to location in the grey matter (predominantly Rexed's laminae VII-VIII) and the conduction velocities of descending axons. However, the mean axonal conduction velocities of branches ascending to LRN and/or RB were significantly lower in comparison to those measured for spinal collaterals. The hypothetical function of the neurones examined is discussed. Since the same information can be conveyed simultaneously by these branching neurones to lower spinal segments and supraspinal centres, an integrative role in the system of motor control is suggested.     

Afferents to the lateral reticular nucleus from the oculomotor region

Summary By means of retrograde axonal transport of the wheat germ agglutinin — horseradish peroxidase complex, a projection from the Edinger-Westphal nucleus to the lateral reticular nucleus was demonstrated in the cat. Following small tracer ejections in the main part of the lateral reticular nucleus, a significant number of labelled neurons were found bilaterally throughout the Edinger-Westphal nucleus. Most of the labelled cells were located on the ipsilateral side. The projecting neurons are spindle-shaped to round with a maximum diameter of the cell body between 15 and 30 m. The findings are discussed in relation to other Edinger-Westphal efferent projections, and some comments are made concerning the cytoarchitecture and delineation of the feline Edinger-Westphal nucleus.     

Afferents to the lateral reticular nucleus from the oculomotor region

Summary By means of retrograde transport of the wheat germ agglutinin-horseradish peroxidase complex, afferent fibres to the lateral reticular nucleus from the oculomotor and accessory oculomotor nuclei were demonstrated in the cat. Small iontophoretic ejections were made into the main part of the lateral reticular nucleus from a ventral approach. Significant numbers of retrogradely labelled neurons were found bilaterally in all parts of the oculomotor nucleus. The majority was of small size and distributed along the dorsal and lateral boundaries of the nucleus. Some labelled neurons were located just outside these boundaries, in the periaqueductal gray and the adjacent mesencephalic reticular formation. Retrogradely labelled neurons were also found in the accessory oculomotor nuclei: The interstitial nucleus of Cajal featured a substantial number of labelled neurons. Some labelled neurons were consistently found also in the nucleus of the posterior commissure, but no labelled neurons were found in the nucleus of Darkschewitch. The labelled neurons in the interstitial nucleus of Cajal were of different sizes and located bilaterally, mainly in its rostral part. Caudal as well as rostral parts of the main lateral reticular nucleus appear to receive the descending afferents from the oculomotor region, but higher numbers of labelled neurons were found subsequent to ejections in the rostral part. The findings are discussed and some comments are made concerning the lateral reticular nucleus as a possible relay nucleus for oculomotor input to the cerebellum.     

An ipsilateral projection from the red nucleus to the lateral reticular nucleus in the cat

Summary Injections of the wheat germ agglutinin — horse-radish peroxidase complex into the lateral reticular nucleus reveal that in addition to the well known contralateral rubroreticular connection, there is also a small but clear cut ipsilateral projection. Cells of various sizes participate in this ipsilateral pathway, and the retrogradely labelled neurons lie dispersed throughout the entire red nucleus.