Inhibition of spinal nociceptive neurons by excitation of cell bodies or fibers of passage at various brainstem sites in the cat

Response Properties of Nucleus Reticularis Lateralis Neurons After Electroacupuncture Stimulation in Rats

A descending inhibitory mechanism from the periaqueductal gray (PAG) to the spinal cord through the nucleus raphe magnus (NRM) is strongly involved in endogenous analgesic system produced by acupuncture stimulation. In addition to the PAG to NRM system which descends in the medial pathway of the brain stem, the nucleus reticularis lateralis (NRL) situated in the lateral part of the brain stem is reported to play an important role in modulating centrifugal antinociceptive action. In the present study, to clarify the role of NRL in acupuncture analgesia, we investigated the response properties of NRL neurons to acupuncture stimulation. The majority of NRM-projecting NRL neurons were inhibited by electroacupuncture stimulation. This effect was antagonized by ionophoretic application of naloxone, indicating that endogenous opioids act directly onto these NRL neurons. By contrast, about half of spinal projecting NRL neurons were excited by electroacupuncture stimulation, suggesting that part of the NRL neurons may modulate pain transmission directly at the spinal level.

Descending control of spinal nociception from the periaqueductal grey distinguishes between neurons with and without C-fibre inputs

Information about noxious events in the periphery is conveyed to the spinal cord in A- and C-fibre nociceptive afferents, which have largely distinct electrical and chemical properties and which convey different qualities of the pain signal. Descending control that originates in the different functional columns of the midbrain periaqueductal grey (PAG) has important roles in the modulation of spinal nociception in different behavioural and emotional states and, it is now believed, in animal models of chronic pain. However, few studies of descending control have considered differential modulation of A- versus C-nociceptor-evoked responses. Here, we report that descending inhibitory control from the rostrocaudal extent of the dorsolateral/lateral and ventrolateral columns of the PAG preferentially targets Class 2 deep dorsal horn neurons with C-fibre inputs. Pinch-evoked responses of these neurons were depressed significantly by -37+/-4.2% (P<0.0001). In contrast, the pinch-evoked responses of Class 2 neurons without C-fibre inputs (presumably A-fibre mediated) were enhanced significantly by +34+/-11.8% (P<0.01). Further experiments indicated these facilitatory effects were at least partly due to a reduction in C-fibre-mediated segmental inhibition. We suggest this differential control of spinal nociception would be appropriate in many of the varied situations in which the PAG is believed to become active, whether short term (e.g. fight or flight) or long term (e.g. chronic pain). Additionally, the pro-nociceptive effects observed in a subset of spinal neurons may be related to the descending facilitation that has been reported in animal models of chronic pain.

Fos expression in the midbrain periaqueductal grey after trigeminovascular stimulation

There is an accumulating body of evidence suggesting that the periaqueductal grey (PAG) is involved in the pathophysiology of migraine. Positron emission tomography (PET) studies in humans have shown that the caudal ventrolateral midbrain, encompassing the ventrolateral PAG, has activations during migraine attacks. The PAG may well be involved not only through the descending modulation of nociceptive afferent information, but also by its ascending projections to the pain processing centres of the thalamus. In this study the intranuclear oncogene protein Fos was used to mark cell activation in the PAG following stimulation of the trigeminally-innervated superior sagittal sinus (SSS) in both cats and in nonhuman primates (Macaca nemestrina). Fos expression in the PAG increased following stimulation to a median of 242 cells (interquartile range 236-272) in the cat and 155 cells (range 104-203) in the monkey, compared with control levels of 35 cells (21-50) and 26 cells (18-33), respectively. Activation was predominantly in the ventrolateral area of the caudal PAG suggesting that the PAG is involved following trigeminally-evoked craniovascular pain.

An excitatory GABAergic plexus in developing neocortical layer 1

Layer 1 of the developing rodent somatosensory cortex contains a dense, transient GABAergic fiber plexus. Axons arising from the zona incerta (ZI) of the ventral thalamus contribute to this plexus, as do axons of intrinsic GABAergic cells of layer 1. The function of this early-appearing fiber plexus is not known, but these fibers are positioned to contact the apical dendrites of most postmigratory neurons. Here we show that electrical stimulation of layer 1 results in a GABA(A)-mediated postsynaptic current (PSC) in pyramidal neurons. Gramicidin perforated patch recording demonstrates that the GABAergic layer 1 synapse is excitatory and can trigger action potentials in cortical neurons. In contrast to electrical stimulation, activation of intrinsic layer 1 neurons with a glutamate agonist fails to produce PSCs in pyramidal cells. In addition, responses can be evoked by stimulation of layer 1 at relatively large distances from the recording site. These findings are consistent with a contribution of the widely projecting incertocortical pathway, the only described GABAergic projection to neonatal cortex. Recording of identified neonatal incertocortical neurons reveals a population of active cells that exhibit high frequencies of spontaneous action potentials and are capable of robustly activating neonatal cortical neurons. Because the fiber plexus is confined to layer 1, this pathway provides a spatially restricted excitatory GABAergic innervation of the distal apical dendrites of pyramidal neurons during the peak period of cortical synaptogenesis.

Inhibitory effects evoked from both the lateral and ventrolateral periaqueductal grey are selective for the nociceptive responses of rat dorsal horn neurones

In rats anaesthetized with alphaxalone/alphadolone a comparative study was made of the inhibitory effects on dorsal horn neurones evoked by chemical stimulation at identified pressor and depressor sites in the lateral and ventrolateral periaqueductal grey (PAG). Stimulating micropipettes were inserted stereotaxically into the lateral or ventrolateral PAG at sites where microinjection of DL-homocysteic acid (DLH) evoked increases or decreases respectively in mean arterial blood pressure. The effects of DLH microinjection at these sites were tested against the responses of dorsal horn neurones to noxious and innocuous stimuli applied to their cutaneous receptive fields. Single unit extracellular recordings were made from 15 Class 1 (low-threshold) and 37 Class 2 (wide dynamic range) dorsal horn neurones in laminae II-VI of the lower lumbar spinal cord. The responses of Class 1 neurones to innocuous prodding of their receptive fields were unaffected by neuronal activation in either the lateral or ventrolateral PAG. The nociceptive (noxious pinch/heat) responses of most Class 2 neurones were strongly inhibited by chemical stimulation in either sector of the PAG. The low threshold (prod) responses of the same neurones were generally unaffected or only weakly inhibited by identical stimulation, regardless of stimulation site. No significant differences were found between the effects of lateral vs. ventrolateral PAG stimulation on the responses of dorsal horn neurones. These results do not support the view that dorsal horn neurones may be inhibited with different selectivities by hyper- and hypotensive regions of the PAG.

The organization and function of endogenous antinociceptive systems

Much progress has been made the understanding of endogenous pain-controlling systems. Recently, new concepts and ideas which are derived from neurobiology, chaos research and from research on learning and memory have been introduced into pain research and shed further light on the organization and function of endogenous antinociception. These most recent developments will be reviewed here. Three principles of endogenous antinociception have been identified, as follows. (1) Supraspinal descending inhibition: the patterns of neuronal activity in diencephalon, brainstem and spinal cord during antinociceptive stimulation in midbrain periaqueductal gray (PAG) or medullary nucleus raphe magnus have now been mapped on the cellular level, using the c-Fos technique. Results demonstrate that characteristic activity patterns result within and outside the PAG when stimulating at its various subdivisions. The descending systems may not only depress mean discharge rates of nociceptive spinal dorsal horn neurons, but also may modify harmonic oscillations and nonlinear dynamics (dimensionality) of discharges. (2) Propriospinal, heterosegmental inhibition: antinociceptive, heterosegmental interneurons exist which may be activated by noxious stimulation or by supraspinal descending pathways. (3) Segmental spinal inhibition: a robust long-term depression of primary afferent neurotransmission in A delta fibers has been identified in superficial spinal dorsal horn which may underlie long-lasting antinociception by afferent stimulation, e.g. by physical therapy or acupuncture.

Modulation of the activity of midbrain central gray substance neurons by calcium channel agonists and antagonists in vitro

Changes in the background impulse activity of midbrain central gray substance neurons have been studied on slice preparations from the rat midbrain upon application of calcium-free solution, an activator of calcium channels, BAY-K 8644 (10 nM), organic (verapamil, 40 microM; D600, 10 microM; nifedipine, 1-10 microM; amiloride, 1 microM) and inorganic (Co2+, 1.5 mM) calcium channel blockers. Besides BAY-K 8644, all the substances inhibited most of the neurons studied. Verapamil, BAY-K 8644 and Co2+ also revealed facilitatory effects. Facilitatory action of BAY-K was most effective in silent neurons and in those previously inhibited by amiloride. Latent period values of inhibition in calcium-free solution and upon application of organic and inorganic blockers have the following sequence: D600 > amiloride > verapamil > Co2+ > nifedipine > calcium-free solution. Maximum rise time had the following order: amiloride > D600 > nifedipine > verapamil > Co2+ > calcium-free solution. Complete suppression of the neuronal activity induced by amiloride lasted twice as long as that induced by calcium-free solution, Co2+ and nifedipine, and six times as long as verapamil-induced suppression. Preliminary application of calcium channel blockers reduced facilitatory and increased inhibitory effects of serotonin and substance P. Data obtained are discussed with the supposition in mind that inhibition of the function of calcium channels in central gray substance neurons could be one of the mechanisms underlying the analgesic effect of a series of neurotropic agents after their introduction into this structure.

Functional characteristics of the midbrain periaqueductal gray

The major functions of the midbrain periaqueductal gray (PAG), including pain and analgesia, fear and anxiety, vocalization, lordosis and cardiovascular control are considered in this review article. The PAG is an important site in ascending pain transmission. It receives afferents from nociceptive neurons in the spinal cord and sends projections to thalamic nuclei that process nociception. The PAG is also a major component of a descending pain inhibitory system. Activation of this system inhibits nociceptive neurons in the dorsal horn of the sinal cord. The dorsal PAG is a major site for processing of fear and anxiety. It interacts with the amygdala and its lesion alters fear and anxiety produced by stimulation of amygdala. Stimulation of PAG produces vocalization and its lesion produces mutism. The firing of many cells within the PAG correlates with vocalization. The PAG is a major site for lordosis and this role of PAG is mediated by a pathway connecting the medial preoptic with the PAG. The cardiovascular controlling network within the PAG are organized in columns. The dorsal column is involved in pressor and the ventrolateral column mediates depressor responses. The major intrinsic circuit within the PAG is a tonically-active GABAergic network and inhibition of this network is an important mechanism for activation of outputs of the PAG. The various functions of the PAG are interrelated and there is a significant interaction between different functional components of the PAG. Using the current information about the anatomy, physiology, and pharmacology of the PAG, a model is proposed to account for the interactions between these different functional components.

Inhibition of spinal nociceptive neurons by microinjections of somatostatin into the nucleus raphe magnus and the midbrain periaqueductal gray of the anesthetized cat

The effects of somatostatin (SOM) after intravenous application and intracerebral microinjection into the medullary nucleus raphe magnus (NRM) or into the periaqueductal gray (PAG) on the spinal nociceptive transmission was quantitatively studied in the anesthetized cat. Noxious heat-evoked responses of multireceptive lumbar spinal dorsal horn neurons were reversibly depressed to 56.6 +/- 9.7% of the control after systemically applied SOM (7 micrograms/kg i.v.; 7 micrograms/kg per h infusion rate). At 11 of 14 brainstem microinjection sites in the NRM and PAG, SOM (2.5 micrograms/microliter) attenuated the heat-evoked responses to 58.9 +/- 6.2% (n = 5) (NRM) and 64.4 +/- 6.3% (n = 6) (PAG) of the control. After microinjection, maximal inhibition was reached within 8-14 min (NRM) or 23-29 min (PAG), respectively. Inhibition was reversible within 60 min after the injection. Thus, SOM has an antinociceptive potency by activating descending inhibition of nociceptive dorsal horn neurons from the NRM and PAG.

Characteristics of propriospinal modulation of nociceptive lumbar spinal dorsal horn neurons in the cat

The segmental and laminar origin of propriospinal antinociceptive systems in the cat spinal cord and the modes to activate them are characterized. The experiments were performed on pentobarbital-anesthetized cats with a high cervical spinalization. Recordings were made from single lumbar spinal dorsal horn neurons responding to noxious radiant skin heating and to innocuous mechanical skin stimuli. The segmental and laminar origin of heterosegmental, propriospinal neurons modulating background activity and nociceptive responses were identified and the conditions to activate them were characterized. Conditioning noxious front paw stimulation and superfusion of the cervical enlargement with L-glutamate, but not with substance P, reduced noxious heat-evoked responses of about 50% of all lumbar neurons tested. Glutamate superfusions of the lower thoracic or upper sacral spinal cord enhanced background activity and reduced nociceptive responses of most lumbar spinal dorsal horn neurons. Superfusions with substance P or somatostatin were ineffective. Glutamate microinjections into the superficial layers of the thoracic, upper lumbar or sacral dorsal horn ipsi- or contralateral to the recording sites or into lamina VIII of the ipsilateral thoracic or upper lumbar cord reduced noxious heat-evoked responses with or without changes in the level of background activity. It is concluded that propriospinal neurons originating from circumscribed areas of the cervical, thoracic, lumbar or sacral spinal cord independently modulate background activity and noxious heat-evoked responses of multireceptive lumbar spinal dorsal horn neurons. The incidence and efficacy of propriospinal antinociceptive stimulation sites was found to be as high as for the classical region of endogenous antinociception, the midbrain periaqueductal gray.

Analgesia from the periaqueductal gray in the developing rat: focal injections of morphine or glutamate and effects of intrathecal injection of methysergide or phentolamine

The aim of these experiments was to examine the changes in antinociception elicited by morphine or glutamate stimulation of the periaqueductal gray of the midbrain (PAG) during the postnatal development of the rat. Pups, aged 3, 10, and 14 days, were implanted with cannulas aimed at either the dorsal or the ventral aspect of the PAG, and glutamate (vehicle, 60 mM or 180 mM) or morphine (vehicle, 2 micrograms or 6 micrograms) was microinjected into one of those two sites. Pups were tested for analgesia against noxious thermal and mechanical stimuli. Morphine produced analgesia at 3 and 10 days of age only when administered to the ventral part of the PAG and the thermal noxious stimulus was tested. Conversely, analgesia induced by glutamate was seen at 3 and 10 days of age only when glutamate was given to the dorsal aspect of the PAG and the mechanical stimulus was used. In 14-day-old pups, both drugs produced analgesia against both types of noxious stimuli regardless of their site of administration within the PAG. Systemically administered naloxone attenuated the analgesic effects of both drugs when they were administered to the ventral PAG, but did not consistently attenuate the analgesic effect of either compound given to the dorsal aspect of the PAG. When either morphine or glutamate was injected into the ventral PAG, intrathecal injections of methysergide attenuated analgesia against the thermal stimulus to a significantly greater degree than the mechanical stimulus and intraspinal injection of phentolamine attenuated analgesia against the mechanical stimulus more potently. When glutamate was given to the dorsal PAG, analgesia against both stimulus types was significantly attenuated. These results indicate that the morphine- and glutamate-induced analgesia mediated by the PAG are developmentally differentiated. These ontogenetic differences most likely reflect differences in the mechanism of action by which these drugs produce analgesia when administered to the PAG, as well as neuroanatomical differences within the dorsal and the ventral regions of the PAG.

GABA-mediated inhibition in rostral ventromedial medulla: role in nociceptive modulation in the lightly anesthetized rat

Local microinjection of GABAA receptor agonists and antagonists was used to characterize the role of GABA-mediated inhibitory processes in the nociceptive modulatory functions of the rostral ventromedial medulla (RVM) in the lightly anesthetized rat. Microinjection of selective GABAA receptor antagonists bicuculline methiodide and SR95531 produced a significant increase in tail-flick (TF) latency. This antinociception was dose related, showed recovery and was attenuated by prior injection of the GABAA receptor agonist THIP at the same site. Microinjection of saline or the glycine receptor antagonist strychnine did not significantly affect TF latency. In contrast, administration of GABAA receptor agonists THIP and muscimol resulted in a significant decrease in TF latency. Microinjections at sites surrounding the RVM did not significantly affect TF latency. These results demonstrate that a GABA-mediated process within the RVM is crucial in permitting execution of the TF and, presumably, other spinal nociceptive reflexes.

Suppression of a hind limb flexion withdrawal reflex by microinjection of glutamate or morphine into the periaqueductal gray in the rat

Microinjection into the midbrain periaqueductal gray (PAG) or lateral reticular formation (LRF) of the neuronal excitant glutamate produces analgesia, and suppresses the responses of a fraction of spinal dorsal horn neurons to noxious heat applied to ventral hind paw skin. Microinjection of morphine into the PAG also produces analgesia, but has been reported to frequently facilitate, as well as to suppress or have no effect, on nociceptive spinal neurons. In anesthetized rats, we tested whether (a) glutamate microinjections into PAG or LRF, and (b) morphine microinjections into PAG, affected the isometric force of hind limb withdrawal elicited by the same noxious heat stimuli on the hind paw as used in single-unit studies of dorsal horn neurons. Glutamate (0.5 M; 0.1-0.5 microliter) microinjected at 9/12 PAG and 8/10 LRF sites suppressed the reflex, and had no effect or facilitated the reflex from the remaining sites. Morphine (5 micrograms in 0.5 microliter) microinjected at each of 10 PAG sites suppressed the reflex in a naloxone-reversible manner. Suppression usually began shortly after morphine, peaked at 20-40 min, and lasted greater than 60 min. The integrated flexion reflex thus appears to be more susceptible to chemical midbrain stimulation under these experimental conditions, compared to previous studies of single dorsal horn neurons.

Comparison of the influence of rostral and caudal raphe neurons on the adrenal secretion of catecholamines and on the release of adrenocorticotropin in the cat

Neuroendocrine and autonomic responses were assessed in chloralose-anesthetized cats after chemical stimulation of medial brain-stem regions, including those that influence nociceptive input to the medullary or spinal dorsal horn. Microinjections of L-glutamate (0.5 M, 160 nl) were directed at the following rostral and caudal raphe nuclei: the periaqueductal gray (PAG), the dorsal raphe nucleus (DR), the raphe magnus (RM), and the raphe obscurus/raphe pallidus (Ro/Rpa). Activation of DR neurons evoked a significant increase in the adrenal secretion of epinephrine (+2.6 +/- 1.1 ng/min, P less than 0.01) that returned towards prestimulus values by 6 min, whereas microinjections into other raphe nuclei had no consistent effect. Activation of Ro/Rpa neurons evoked an increase in the plasma concentration of adrenocorticotropin (ACTH, +47.9 +/- 12.3 pg/ml, P less than 0.01), whereas microinjections into other raphe nuclei did not affect ACTH. Arterial pressure increased significantly after activation of PAG (+7.5 +/- 2.1 mm Hg, P less than 0.01) or of DR (+4.8 +/- 2.0 mm Hg, P less than 0.05) neurons, whereas heart rate increased significantly (P less than 0.05) after stimulation of cells within the Ro/Rpa. Glutamate microinjections within the RM, a raphe nucleus that exerts a significant descending influence on nociceptive input to the medullary and to the spinal dorsal horns, had no consistent effect on any measured variable. No evidence was seen to suggest that chemical activation of neurons within raphe nuclei inhibited the adrenal secretion of catecholamines or inhibited the release of ACTH. The results indicated that glutamate activation of neurons within different raphe nuclei evoked non-uniform effects on neuroendocrine and autonomic function. Further, these data suggested that the neural substrate underlying the control of the adrenal secretion of catecholamines and of the release of ACTH in response to activation of raphe neurons is likely distinct from that which contributes to the descending influence on nociceptive input to the medullary and spinal dorsal horn.

Microinjections of glutamate or morphine at coincident midbrain sites have different effects on nociceptive dorsal horn neurons in the rat

Responses of single lumbar spinal neurons to noxious skin heating (50 degrees C, 10 s) were electrophysiologically recorded in barbiturate-anesthetized rats. Responses of all neurons were suppressed by electrical stimulation in the midbrain periaqueductal gray (PAG) or lateral reticular formation (LRF). Microinjection of glutamate (GLU, 0.1-0.3 microliter, 0.5 M) into the PAG rapidly (within 15 s) suppressed (to 13-55% of control) the responses of 6/16 neurons with recovery within 8 min. The remainder were affected less at even higher doses (0.5-1 microliter). Responses of 4/10 neurons were suppressed following GLU microinjected into the LRF. We also tested effects of microinjection of morphine (MOR, 5 micrograms/0.5 microliter) into GLU-sensitive and insensitive PAG sites. Responses of 4 neurons were unaffected, 4 were enhanced (to 130-155%), and 2 suppressed (to 43 and 57%) following MOR in PAG, with enhancement or suppression beginning within 12-20 min and lasting 40 to over 70 min. The differing effects of GLU and MOR may reflect different mechanisms for the descending modulation of spinal nociceptive transmission.