Raphe magnus-induced descending inhibition of spinal nociceptive neurons is mediated through contralateral spinal pathways in the cat

Serotonin targets inhibitory synapses to induce modulation of network functions

The cellular effects of serotonin (5-HT), a neuromodulator with widespread influences in the central nervous system, have been investigated. Despite detailed knowledge about the molecular biology of cellular signalling, it is not possible to anticipate the responses of neuronal networks to a global action of 5-HT. Heterogeneous expression of various subtypes of serotonin receptors (5-HTR) in a variety of neurons differently equipped with cell-specific transmitter receptors and ion channel assemblies can provoke diverse cellular reactions resulting in various forms of network adjustment and, hence, motor behaviour. Using the respiratory network as a model for reciprocal synaptic inhibition, we demonstrate that 5-HT(1A)R modulation primarily affects inhibition through glycinergic synapses. Potentiation of glycinergic inhibition of both excitatory and inhibitory neurons induces a functional reorganization of the network leading to a characteristic change of motor output. The changes in network operation are robust and help to overcome opiate-induced respiratory depression. Hence, 5-HT(1A)R activation stabilizes the rhythmicity of breathing during opiate medication of pain.

The role of the periaqueductal gray in the modulation of pain in males and females: are the anatomy and physiology really that different?

Anatomical and physiological studies conducted in the 1960s identified the periaqueductal gray (PAG) and its descending projections to the rostral ventromedial medulla (RVM) and spinal cord dorsal horn, as a primary anatomical pathway mediating opioid-based analgesia. Since these initial studies, the PAG-RVM-spinal cord pathway has been characterized anatomically and physiologically in a wide range of vertebrate species. Remarkably, the majority of these studies were conducted exclusively in males with the implicit assumption that the anatomy and physiology of this circuit were the same in females; however, this is not the case. It is well established that morphine administration produces greater antinociception in males compared to females. Recent studies indicate that the PAG-RVM pathway contributes to the sexually dimorphic actions of morphine. This manuscript will review our anatomical, physiological, and behavioral data identifying sex differences in the PAG-RVM pathway, focusing on its role in pain modulation and morphine analgesia.

Sex differences in the anatomical and functional organization of the periaqueductal gray-rostral ventromedial medullary pathway in the rat: a potential circuit mediating the sexually dimorphic actions of morphine

Previous studies have demonstrated that morphine, administered systemically or directly into the periaqueductal gray (PAG), produces a significantly greater degree of antinociception in males in comparison with females. Because the midbrain PAG and its descending projections to the rostral ventromedial medulla (RVM) constitute an essential neural circuit for opioid-based analgesia, the present studies were conducted to determine whether sex differences in the anatomical organization of the PAG-RVM pathway, and its activation during persistent inflammatory pain, could account for sex-based differences in opioid analgesia. In the rat, retrograde tracing was combined with Fos immunocytochemistry to investigate sexual dimorphism in the organization of the PAG-RVM circuit and its activation by persistent inflammatory pain induced by intraplantar injection of complete Freund's adjuvant (CFA). The ability of morphine to suppress the activation of the PAG-RVM circuit was also examined. Sexually dimorphic retrograde labeling was observed within the dorsomedial and lateral/ventrolateral PAG at all rostrocaudal levels, with females having significantly more PAG-RVM output neurons in comparison with males. While no sex differences were noted in the activation of the PAG by persistent inflammatory pain, significantly more PAG-RVM cells were activated in males in comparison with females. Systemic administration of morphine significantly suppressed CFA-induced Fos in the PAG in males only. The results of these studies demonstrate that both the anatomical organization and the functional activation of the PAG-RVM circuit are sexually dimorphic and may provide the anatomical substrate for sex-based differences in morphine analgesia.

Pain modulation by release of the endogenous cannabinoid anandamide

Synthetic cannabinoids produce behavioral analgesia and suppress pain neurotransmission, raising the possibility that endogenous cannabinoids serve naturally to modulate pain. Here, the development of a sensitive method for measuring cannabinoids by atmospheric pressure-chemical ionization mass spectrometry permitted measurement of the release of the endogenous cannabinoid anandamide in the periaqueductal gray (PAG) by in vivo microdialysis in the rat. Electrical stimulation of the dorsal and lateral PAG produced CB1 cannabinoid receptor-mediated analgesia accompanied by a marked increase in the release of anandamide in the PAG, suggesting that endogenous anandamide mediates the behavioral analgesia. Furthermore, pain triggered by subcutaneous injections of the chemical irritant formalin substantially increased the release of anandamide in the PAG. These findings indicate that the endogenous cannabinoid anandamide plays an important role in a cannabinergic pain-suppression system existing within the dorsal and lateral PAG. The existence of a cannabinergic pain-modulatory system may have relevance for the treatment of pain, particularly in instances where opiates are ineffective.

A lateralized deficit in morphine antinociception after unilateral inactivation of the central amygdala

The amygdala is a forebrain region that is receiving increasing attention as a modulator of pain sensation. The amygdala contributes to antinociception elicited by both psychological factors (e.g., fear) and exogenous opioid agonists. Unlike the midbrain periaqueductal gray matter (PAG) or rostral ventromedial medulla, the amygdala is a pain-modulating region that has clear bilateral representation in the brain, making it possible to determine whether pain-modulating effects of this region are lateralized with respect to the peripheral origin of noxious stimulation. Unilateral inactivation of the central nucleus of the amygdala (Ce) plus adjacent portions of the basolateral amygdaloid complex (with either the excitotoxin NMDA or the GABAA agonist muscimol) reduced the ability of morphine to suppress prolonged, formalin-induced pain derived from the hindpaw ipsilateral, but not contralateral, to the inactivated region. This effect was evident regardless of the nociceptive scoring method used (weighted scores or flinch-frequency method) and was not accompanied by a concurrent reduction in morphine-induced hyperlocomotion. Unilateral lesions restricted to the basolateral amygdaloid complex (i.e., not including the Ce) did not reduce the ability of morphine to suppress formalin-induced pain derived from either hindpaw. The results constitute the first report of a lateralized deficit in opioid antinociception after unilateral inactivation of a specific brain area and show the first clear neuroanatomical dissociation between antinociceptive and motor effects of systemically administered morphine in the rat. The amygdala appears to modulate nociceptive signals entering the ipsilateral spinal dorsal horn, probably through monosynaptic connections with ipsilateral portions of the PAG.

Central nervous system pathways for pain transmission and pain control: issues relevant to the practicing clinician

This review considers two areas important for the clinician; ascending pathways for pain transmission and endogenous pain control systems. The classic three-neuron pathway for pain transmission is presented and contrasted with more detailed, contemporary views on pain transmission found in the literature. The focus of the review is a discussion of spinal cord and brainstem pain control systems that modulate pain transmission. Evidence is presented which supports not only the existence of these pain modulation mechanisms but also that selected clinical procedures used by clinicians are capable of activating these mechanisms to control their patients' pain.

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.

Differential effects of spinalization on discharge patterns and discharge rates of simultaneously recorded nociceptive and non-nociceptive spinal dorsal horn neurons

Recordings were made simultaneously from 2-5 neurons at the same site in the lumbar spinal dorsal horn of pentobarbital-anesthetized rats. Neurons were classified as low-threshold (LT) or multireceptive (MR) according to their responses to non-noxious mechanical or noxious radiant heat stimuli of the skin. At the same recording sites neurons could be encountered which belong to different classes and/or which had mechanoreceptive fields which did not overlap. Cold blocks of the upper or lower thoracic cord or transsections of the upper cervical cord were made to evaluate the effects of spinalization on both the rate and pattern of background activity and/or noxious heat-evoked responses of different dorsal horn neurons under identical experimental conditions. At 24 of 27 recording sites, spinalization had qualitatively or quantitatively different effects on the rate of background activity of simultaneously recorded neurons. Interspike interval (ISI) means of background activity were significantly reduced in 29 of 65 (44.6%) neurons, prolonged in 23 of 65 (35.4%) neurons, or unchanged in 13 of 65 (20%) neurons. MR neurons displayed a significantly higher incidence of decreased background activity 17 of 45 (37.8%) and a lower incidence of increased background activity (18 of 45, 40%) during spinalization than the LT neurons from which 1 of 12 (8.3%) decreased and 8 of 12 (66.6%) increased background activity. Almost all (95.4%) neurons changed their discharge patterns after spinalization. At 9 of 27 recording sites, the discharge patterns of simultaneously recorded neurons were affected differently by spinalization as revealed by the coefficient of dispersion of the interspike intervals (ISI), indicating changes in the tendency to discharge action potential in clusters (bursts). At the same recording sites the level of noxious heat-evoked responses of simultaneously recorded MR neurons was also differentially affected by spinalization. Nociceptive responses were significantly enhanced in 19 of 37 (51.4%) neurons (137.8 +/- 142.6% of control, mean +/- SD), reduced in 13 of 37 neurons (35.1%) (by 58.9 +/- 20.9%) and/or unchanged in 5 of 37 (13.5%) neurons. It is concluded that no general 'tone' of descending antinociception exists and that tonic descending excitatory and inhibitory systems may be active simultaneously modulating both the level and pattern of neuronal discharges.

Arborization of single axons of the spinal dorsolateral funiculus to the contralateral superficial dorsal horn

This study provides an anatomical basis for the observation that a unilateral lesion of the spinal dorsolateral funiculus (DLF) can reduce the inhibitory effect of electrical stimulation of the nucleus raphe magnus (NRM) on dorsal horn nociceptive neurons located caudal and contralateral to the lesion. We injected the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) into the NRM and traced the arborization of single DLF axons in the spinal gray matter. Although the majority of DLF axons arborized in the dorsal horn ipsilaterally, we found some axons which entered the spinal gray matter, traversed the gray matter to the central canal and then abruptly changed direction and coursed to the contralateral superficial dorsal horn. Very few branches were given off en route. These data indicate that some raphe-spinal axons may selectively influence the firing of neurons of the superficial dorsal horn, contralateral to the DLF in which they descend the spinal cord.

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

The relative contribution of cells of origin and fibers of passage to the inhibition of spinal nociceptive neurons from various brainstem sites is not known. The present study therefore quantitatively compares the descending inhibition produced by focal electrical stimulation which indiscriminatively excites all neuronal elements with the inhibition evoked by glutamate microinjections which selectively excite cell bodies at the same sites within the midbrain periaqueductal gray (PAG) and neighboring lateral reticular formation (LRF) and within the medullary nucleus raphe magnus (NRM). In pentobarbital anesthetized cats extracellular recordings were made from 22 lumbar dorsal horn neurons responding to innocuous mechanical skin stimuli and to noxious radiant heating of the glabrous skin at the ipsilateral hindpaw. Glutamate microinjections (1 microliter, 0.5 M) into 6 of 10 sites within the NRM reduced heat-evoked responses in 6 different cats to 69.2 +/- 3.8% of control. Electrical stimulation (180-600 microA) at the same sites and two additional sites in the NRM reduced responses to 50.0 +/- 8.7% of control. At 4 of 10 sites within the PAG glutamate reduced spinal neuronal responses to heat in 4 different cats to 69.4 +/- 6.1% of control. Electrical stimulation at 3 of the same sites and 6 additional sites within the PAG reduced heat-evoked responses to 52.7 +/- 3.5% of control. Microinjections of glutamate into the LRF failed to affect heat-evoked responses in any of the 8 experiments tested, while electrical stimulation at 6 of the same sites in the LRF reduced neuronal responses to heat to 52.4 +/- 7.1% of control.(ABSTRACT TRUNCATED AT 250 WORDS)

The use of local anaesthetic microinjections to identify central pathways: a quantitative evaluation of the time course and extent of the neuronal block

The time course and extent of local anaesthetic blocks within the spinal cord of cats were evaluated. A monopolar stimulation electrode with the tip lowered into the dorsal columns (DC) 1000 microns below cord surface was used to activate antidromically DC fibers at the T13 level and evoke cord dorsum potentials at the level of the lumbar spinal cord. The amplitude of the negative deflection, the N-wave, was determined for various stimulation intensities (stimulation-response-function, SRF). Lidocaine (1%) was microinjected in volumes of 0.5 or 1.0 microliter into the DC from a glass micropipette 1 mm caudal to the stimulation site. Conduction block was characterized by a reversible shift of the SRFs to higher stimulation intensities. The diameter of the blocked area in the transverse plane was evaluated from threshold intensities and was found to be 0.9 +/- 0.1 mm 4 to 30 min after the injection of 0.5 microliter lidocaine and 1.6 +/- 0.36 mm 10 to 45 min after the injection of 1.0 microliter lidocaine. In the sagittal plane, the diameter of the blocked area following 1.0 microliter lidocaine was found to be up to 2.8 mm. The DC-block was reversible within 92 min following injection of 1.0 microliter and 69 min after the injection of 0.5 microliter lidocaine. The application of the present findings for blocks in other CNS structures is discussed.