Ultrastructural and immunocytochemical study of lamina II islet cells in rat spinal dorsal horn

Calretinin-expressing islet cells are a source of pre- and post-synaptic inhibition of non-peptidergic nociceptor input to the mouse spinal cord

Unmyelinated non-peptidergic nociceptors (NP afferents) arborise in lamina II of the spinal cord and receive GABAergic axoaxonic synapses, which mediate presynaptic inhibition. However, until now the source of this axoaxonic synaptic input was not known. Here we provide evidence that it originates from a population of inhibitory calretinin-expressing interneurons (iCRs), which correspond to lamina II islet cells. The NP afferents can be assigned to 3 functionally distinct classes (NP1-3). NP1 afferents have been implicated in pathological pain states, while NP2 and NP3 afferents also function as pruritoceptors. Our findings suggest that all 3 of these afferent types innervate iCRs and receive axoaxonic synapses from them, providing feedback inhibition of NP input. The iCRs also form axodendritic synapses, and their targets include cells that are themselves innervated by the NP afferents, thus allowing for feedforward inhibition. The iCRs are therefore ideally placed to control the input from non-peptidergic nociceptors and pruritoceptors to other dorsal horn neurons, and thus represent a potential therapeutic target for the treatment of chronic pain and itch.

Analyses of the Mode of Action of an Alpha-Adrenoceptor Blocker in Substantia Gelatinosa Neurons in Rats

To elucidate why naftopidil increases the frequency of spontaneous synaptic currents in only some substantia gelatinosa (SG) neurons, post-hoc analyses were performed. Blind patch-clamp recording was performed using slice preparations of SG neurons from the spinal cords of adult rats. Spontaneous inhibitory and excitatory postsynaptic currents (sIPSCs and sEPSCs, respectively) were recorded. The ratios of the frequency and amplitude of the sIPSCs and sEPSCs following the introduction of naftopidil compared with baseline, and after the application of naftopidil, serotonin (5-HT), and prazosin, compared with noradrenaline (NA) were evaluated. First, the sIPSC analysis indicated that SG neurons reached their full response ratio for NA at 50 μM. Second, they responded to 5-HT (50 μM) with a response ratio similar to that for NA, but prazosin (10 μM) did not change the sEPSCs and sIPSCs. Third, the highest concentration of naftopidil (100 μM) led to two types of response in the SG neurons, which corresponded with the reactions to 5-HT and prazosin. These results indicate that not all neurons were necessarily activated by naftopidil, and that the micturition reflex may be regulated in a sophisticated manner by inhibitory mechanisms in these interneurons.

Neuropathic pain modeling: Focus on synaptic and ion channel mechanisms

Animal models of pain consist of modeling a pain-like state and measuring the consequent behavior. The first animal models of neuropathic pain (NP) were developed in rodents with a total lesion of the sciatic nerve. Later, other models targeting central or peripheral branches of nerves were developed to identify novel mechanisms that contribute to persistent pain conditions in NP. Objective assessment of pain in these different animal models represents a significant challenge for pre-clinical research. Multiple behavioral approaches are used to investigate and to validate pain phenotypes including withdrawal reflex to evoked stimuli, vocalizations, spontaneous pain, but also emotional and affective behaviors. Furthermore, animal models were very useful in investigating the mechanisms of NP. This review will focus on a detailed description of rodent models of NP and provide an overview of the assessment of the sensory and emotional components of pain. A detailed inventory will be made to examine spinal mechanisms involved in NP-induced hyperexcitability and underlying the current pharmacological approaches used in clinics with the possibility to present new avenues for future treatment. The success of pre-clinical studies in this area of research depends on the choice of the relevant model and the appropriate test based on the objectives of the study.

Dorsal Horn of Mouse Lumbar Spinal Cord Imaged with CLARITY

The organization of the mouse spinal dorsal horn has been delineated in 2D for the six Rexed laminae in our publication Atlas of the Spinal Cord: Mouse, Rat, Rhesus, Marmoset, and Human. In the present study, the tissue clearing technique CLARITY was used to observe the cyto- and chemoarchitecture of the mouse spinal cord in 3D, using a variety of immunohistochemical markers. We confirm prior observations regarding the location of glycine and serotonin immunoreactivities. Novel observations include the demonstration of numerous calcitonin gene-related peptide (CGRP) perikarya, as well as CGRP fibers and terminals in all laminae of the dorsal horn. We also observed sparse choline acetyltransferase (ChAT) immunoreactivity in small perikarya and fibers and terminals in all dorsal horn laminae, while gamma aminobutyric acid (GABA) and glutamate decarboxylase-67 (GAD67) immunoreactivities were found only in small perikarya and fibers. Finally, numerous serotonergic fibers were observed in all laminae of the dorsal horn. In conclusion, CLARITY confirmed the 2D immunohistochemical properties of the spinal cord. Furthermore, we observed novel anatomical characteristics of the spinal cord and demonstrated that CLARITY can be used on spinal cord tissue to examine many proteins of interest.

Noradrenaline modulates mechanically evoked responses in the rat spinal dorsal horn: an in vivo patch-clamp study

Purpose: We investigated the effects of noradrenaline (NA) on physiologically evoked synaptic responses of substantia gelatinosa (SG) neurons using anesthetized animals.

Methods: Male Sprague–Dawley rats (6–8 weeks, 200–300 g, n=21) were anesthetized. The lumbar spinal cord was exposed from L3 to L5; subsequently, the rats were fixed to a stereotaxic apparatus. The electrode was advanced at an angle of 30–45 degrees into the SG using a micromanipulator. We recorded excitatory post-synaptic currents (EPSC). Under these conditions, innocuous or noxious mechanical stimuli were applied to the receptive field of the ipsilateral hindlimb with or without NA, respectively.

Results: NA (50 μM) pre-application induced three types of responses for pinch-evoked EPSCs. The number of neurons showing inhibition, facilitation, and no-effect was 15 (71.4%), 2 (9.5%), and 4 (19%), respectively (n=21). Pre-treatment with NA also induced three different types of responses for puff-evoked EPSC (n=21). The number of neurons showing inhibition, facilitation, and no-effect was 9 (42.9%), 9 (42.9%), and 3 (14.2%), respectively. Further, there was a significant difference in the rate distribution (inhibition, facilitation, and no change) between puff- and pinch-evoked responses.

Conclusion: Our present data indicate that NA acts on noxious and innocuous mechanical transmission in the SG. Considering the distinct sensory inputs to the SG, the different actions of NA on the transmission of sensory information imply that NA exerts its analgesic effects in a manner more complicated than previously believed.

The histology, physiology, neurochemistry and circuitry of the substantia gelatinosa Rolandi (lamina II) in mammalian spinal cord

The substantia gelatinosa Rolandi (SGR) was first described about two centuries ago. In the following decades an enormous amount of information has permitted us to understand - at least in part - its role in the initial processing of pain and itch. Here, I will first provide a comprehensive picture of the histology, physiology, and neurochemistry of the normal SGR. Then, I will analytically discuss the SGR circuits that have been directly demonstrated or deductively envisaged in the course of the intensive research on this area of the spinal cord, with particular emphasis on the pathways connecting the primary afferent fibers and the intrinsic neurons. The perspective existence of neurochemically-defined sets of primary afferent neurons giving rise to these circuits will be also discussed, with the proposition that a cross-talk between different subsets of peptidergic fibers may be the structural and functional substrate of additional gating mechanisms in SGR. Finally, I highlight the role played by slow acting high molecular weight modulators in these gating mechanisms.

Rebuilding CNS inhibitory circuits to control chronic neuropathic pain and itch

Cell transplantation offers an attractive alternative to pharmacotherapy for the management of a host of clinical conditions. Most importantly, the transplanted cells provide a continuous, local delivery of therapeutic compounds, which avoids many of the adverse side effects associated with systemically administered drugs. Here, we describe the broad therapeutic utility of transplanting precursors of cortical inhibitory interneurons derived from the embryonic medial ganglionic eminence (MGE), in a variety of chronic pain and itch models in the mouse. Despite the cortical environment in which the MGE cells normally develop, these cells survive transplantation and will even integrate into the circuitry of an adult host spinal cord. When transplanted into the spinal cord, the cells significantly reduce the hyperexcitability that characterizes both chronic neuropathic pain and itch conditions. This MGE cell-based strategy differs considerably from traditional pharmacological treatments as the approach is potentially disease modifying (i.e., the therapy targets the underlying etiology of the pain and itch pathophysiology).

Impaired excitatory drive to spinal GABAergic neurons of neuropathic mice

Adequate pain sensitivity requires a delicate balance between excitation and inhibition in the dorsal horn of the spinal cord. This balance is severely impaired in neuropathy leading to enhanced pain sensations (hyperalgesia). The underlying mechanisms remain elusive. Here we explored the hypothesis that the excitatory drive to spinal GABAergic neurons might be impaired in neuropathic animals. Transgenic adult mice expressing EGFP under the promoter for GAD67 underwent either chronic constriction injury of the sciatic nerve or sham surgery. In transverse slices from lumbar spinal cord we performed whole-cell patch-clamp recordings from identified GABAergic neurons in lamina II. In neuropathic animals rates of mEPSC were reduced indicating diminished global excitatory input. This downregulation of excitatory drive required a rise in postsynaptic Ca(2+). Neither the density and morphology of dendritic spines on GABAergic neurons nor the number of excitatory synapses contacting GABAergic neurons were affected by neuropathy. In contrast, paired-pulse ratio of Aδ- or C-fiber-evoked monosynaptic EPSCs following dorsal root stimulation was increased in neuropathic animals suggesting reduced neurotransmitter release from primary afferents. Our data indicate that peripheral neuropathy triggers Ca(2+)-dependent signaling pathways in spinal GABAergic neurons. This leads to a global downregulation of the excitatory drive to GABAergic neurons. The downregulation involves a presynaptic mechanism and also applies to the excitation of GABAergic neurons by presumably nociceptive Aδ- and C-fibers. This then leads to an inadequately low recruitment of inhibitory interneurons during nociception. We suggest that this previously unrecognized mechanism of impaired spinal inhibition contributes to hyperalgesia in neuropathy.

Morphological, neurochemical and electrophysiological features of parvalbumin-expressing cells: a likely source of axo-axonic inputs in the mouse spinal dorsal horn

Perception of normal bodily sensations relies on the precise regulation of sensory information entering the dorsal horn of the spinal cord. Inhibitory, axoaxonic, synapses provide a mechanism for this regulation, but the source of these important inhibitory connections remains to be elucidated. This study shows that a subpopulation of spinal interneurons that expresses parvalbumin and have specific morphological, connectivity and functional characteristics are a likely source of the inhibitory inputs that selectivity regulate non-noxious tactile input in the spinal cord. Our findings suggest that a loss of normal function in parvalbumin positive dorsal horn neurons may result in the development of tactile allodynia, where non-painful stimuli gain the capacity to evoke the sensation of pain.

GABA-receptor control of the amplitude and duration of the neuronal responses to formalin in the rat spinal cord

The GABAergic inhibitory system in the dorsal horn of the spinal cord has been implicated in the modulation of pain, including the control of nociceptive transmission during inflammation. This electrophysiological study examined the effects of the GABAA and GABAB receptor antagonists, bicuculline and CGP35348, on the magnitude and duration of the formalin response. The responses of spinal nociceptive dorsal horn neurones to subcutaneous injection of formalin into the hindpaw in the anaesthetized rat were recorded. Both phases of the formalin response were monitored, and the antagonists were administered either simultaneously with formalin or 50 min after injection of formalin. Bicuculline (50 microg), the GABAA antagonist, administered simultaneously with formalin significantly increased the magnitude of the overall response, especially the second phase, and also abolished the silent interphase period. In addition, 50 min after injection of formalin, bicuculline increased the duration of the second phase in a dose-dependent manner. CGP35348 (250 microg), the GABAB antagonist, administered 50 min after injection of formalin also increased the duration of the second phase significantly, but had no effect on the magnitude of the response or the silent interphase when administered simultaneously with formalin. These results show that GABAA- and GABAB-receptor-mediated inhibitions are involved in controlling the duration of the second phase of the formalin response, and that GABAA-receptor-mediated inhibition also contributes to the manifestation of the silent interphase period and the magnitude of the second phase. Thus, GABA neurones are critical in determining the level and duration of nociceptive information transmitted through the spinal cord during inflammation.

Acetylcholine and norepinephrine mediate GABAergic but not glycinergic transmission enhancement by melittin in adult rat substantia gelatinosa neurons

GABAergic and glycinergic inhibitory synaptic transmissions in substantia gelatinosa (SG; lamina II of Rexed) neurons of the spinal dorsal horn play an important role in regulating nociceptive transmission from the periphery. It has not yet been well known whether each of the inhibitory transmissions plays a distinct role in the regulation. We report an involvement of neurotransmitters in GABAergic but not glycinergic transmission enhancement produced by the PLA2 activator melittin, where the whole-cell patch-clamp technique is applied to the SG neurons of adult rat spinal cord slices. Glycinergic but not GABAergic spontaneous inhibitory postsynaptic current (sIPSC) was increased in frequency and amplitude by melittin in the presence of nicotinic, muscarinic acetylcholine, and α1-adrenergic receptor antagonists (mecamylamine, atropine, and WB-4101, respectively). GABAergic transmission enhancement produced by melittin was unaffected by the 5-hydroxytryptamine 3 receptor and P2X receptor antagonists (ICS-205,930 and pyridoxalphosphate-6-azophenyl-2′,4′-disulphonic acid, respectively). Nicotinic and muscarinic acetylcholine receptor agonists [(−)-nicotine and carbamoylcholine, respectively] and norepinephrine, as well as melittin, increased GABAergic sIPSC frequency and amplitude. A repeated application of (−)-nicotine, carbamoylcholine, and norepinephrine, but not melittin, at an interval of 30 min produced a similar transmission enhancement. These results indicate that melittin produces the release of acetylcholine and norepinephrine, which activate (nicotinic and muscarinic) acetylcholine and α1-adrenergic receptors, respectively, resulting in GABAergic but not glycinergic transmission enhancement in SG neurons. The desensitization of a system leading to the acetylcholine and norepinephrine release is slow in recovery. This distinction in modulation between GABAergic and glycinergic transmissions may play a role in regulating nociceptive transmission.

Postnatal refinement of proprioceptive afferents in the cat cervical spinal cord

Proprioceptive afferent (PA) information is integrated with signals from descending pathways, including the corticospinal tract (CST), by spinal interneurons in the dorsal horn and intermediate zone for controlling movements. PA spinal projections, and the reflexes that they evoke, develop prenatally. The CST projects to the spinal cord postnatally, and its connections are subsequently refined. Consequently, the tract becomes effective in transmitting control signals from motor cortex to muscle. This suggests sequential development of PAs and the CST rather than co-development. In this study we determined if there was also late postnatal refinement of PA spinal connections, which would support PA-CST co-development. We examined changes in PA spinal connections at 4 weeks of age, when CST terminations are immature, at 8 weeks, after CST refinement, and at 11 weeks, when CST terminations are mature. We electrically stimulated PA afferents in the deep radial nerve. Evoked PA responses were small and not localized at 4 weeks. By 8 and 11 weeks, responses were substantially larger and maximal in laminae VI and dorsal VII. We used intramuscular injection of cholera toxin β subunit to determine the distribution of PAs from the extensor carpii radialis muscle in the cervical enlargement at the same ages as in the electrophysiological studies. We found a reduction of the distribution of PAs with age that paralleled the physiological changes. This age-related sharpening of PA spinal connections also paralleled CST development, suggesting coordinated PA-CST co-development rather than sequential development. This is likely to be important for the development of adaptive motor control.

Ultrastructural analysis of the synaptic connectivity of TRPV1-expressing primary afferent terminals in the rat trigeminal caudal nucleus

Trigeminal primary afferents that express the transient receptor potential vanilloid 1 (TRPV1) are important for the transmission of orofacial nociception. However, little is known about how the TRPV1-mediated nociceptive information is processed at the first relay nucleus in the central nervous system (CNS). To address this issue, we studied the synaptic connectivity of TRPV1-positive (+) terminals in the rat trigeminal caudal nucleus (Vc) by using electron microscopic immunohistochemistry and analysis of serial thin sections. Whereas the large majority of TRPV1+ terminals made synaptic contacts of an asymmetric type with one or two postsynaptic dendrites, a considerable fraction also participated in complex glomerular synaptic arrangements. A few TRPV1+ terminals received axoaxonic contacts from synaptic endings that contained pleomorphic synaptic vesicles and were immunolabeled for glutamic acid decarboxylase, the synthesizing enzyme for the inhibitory neurotransmitter γ-aminobutyric acid (GABA). We classified the TRPV1+ terminals into an S-type, containing less than five dense-core vesicles (DCVs), and a DCV-type, containing five or more DCVs. The number of postsynaptic dendrites was similar between the two types of terminals; however, whereas axoaxonic contacts were frequent on the S-type, the DCV-type did not receive axoaxonic contacts. In the sensory root of the trigeminal ganglion, TRPV1+ axons were mostly unmyelinated, and a small fraction was small myelinated. These results suggest that the TRPV1-mediated nociceptive information from the orofacial region is processed in a specific manner by two distinct types of synaptic arrangements in the Vc, and that the central input of a few TRPV1+ afferents is presynaptically modulated via a GABA-mediated mechanism.

In vivo patch-clamp analysis of dopaminergic antinociceptive actions on substantia gelatinosa neurons in the spinal cord

To elucidate the mechanisms of antinociception mediated by the dopaminergic descending pathway in the spinal cord, we investigated the actions of dopamine (DA) on substantia gelatinosa (SG) neurons by in vivo whole-cell patch-clamp methods. In the voltage-clamp mode (V(H)=-70mV), the application of DA induced outward currents in about 70% of SG neurons tested. DA-induced outward current was observed in the presence of either Na(+) channel blocker, tetrodotoxin (TTX) or a non-NMDA receptor antagonist, CNQX, and was inhibited by either GDP-β-S in the pipette solution or by perfusion of a non-selective K(+) channel blocker, Ba(2+). The DA-induced outward currents were mimicked by a selective D2-like receptor agonist, quinpirole and attenuated by a selective D2-like receptor antagonist, sulpiride, indicating that the DA-induced outward current is mediated by G-protein-activated K(+) channels through D2-like receptors. DA significantly suppressed the frequency and amplitude of glutamatergic spontaneous excitatory postsynaptic currents (EPSCs). DA also significantly decreased the frequency of miniature EPSCs in the presence of TTX. These results suggest that DA has both presynaptic and postsynaptic inhibitory actions on synaptic transmission in SG neurons. We showed that DA produced direct inhibitory effects in SG neurons to both noxious and innocuous stimuli to the skin. Furthermore, electrical stimulation of dopaminergic diencephalic spinal neurons (A11), which project to the spinal cord, induced outward current and suppressed the frequency and amplitude of EPSCs. We conclude that the dopaminergic descending pathway has an antinociceptive effect via D2-like receptors on SG neurons in the spinal cord.

TRPA1-expressing primary afferents synapse with a morphologically identified subclass of substantia gelatinosa neurons in the adult rat spinal cord

The TRPA1 channel has been proposed to be a molecular transducer of cold and inflammatory nociceptive signals. It is expressed on a subset of small primary afferent neurons both in the peripheral terminals, where it serves as a sensor, and on the central nerve endings in the dorsal horn. The substantia gelatinosa (SG) of the spinal cord is a key site for integration of noxious inputs. The SG neurons are morphologically and functionally heterogeneous and the precise synaptic circuits of the SG are poorly understood. We examined how activation of TRPA1 channels affects synaptic transmission onto SG neurons using whole-cell patch-clamp recordings and morphological analyses in adult rat spinal cord slices. Cinnamaldehyde (TRPA1 agonist) elicited a barrage of excitatory postsynaptic currents (EPSCs) in a subset of the SG neurons that responded to allyl isothiocyanate (less specific TRPA1 agonist) and capsaicin (TRPV1 agonist). Cinnamaldehyde evoked EPSCs in vertical and radial but not islet or central SG cells. Notably, cinnamaldehyde produced no change in inhibitory postsynaptic currents and nor did it produce direct postsynaptic effects. In the presence of tetrodotoxin, cinnamaldehyde increased the frequency but not amplitude of miniature EPSCs. Intriguingly, cinnamaldehyde had a selective inhibitory action on monosynaptic C- (but not Adelta-) fiber-evoked EPSCs. These results indicate that activation of spinal TRPA1 presynaptically facilitates miniature excitatory synaptic transmission from primary afferents onto vertical and radial cells to initiate action potentials. The presence of TRPA1 channels on the central terminals raises the possibility of bidirectional modulatory action in morphologically identified subclasses of SG neurons.

Morphophysiologic properties of islet cells in substantia gelatinosa of the rat spinal cord

Substantia gelatinosa (SG) neurons of the spinal cord are highly heterogeneous in their morphophysiologic properties and could be categorized on several subtypes. Here the properties of islet cells in rat SG (approximately 11%) are described with the use of confocal microscopy and patch-clamp recording. The cells had significantly longer and thicker dendritic trees among all other neurons. Only these cells expressed slow inward current activated by hyperpolarization, which could be blocked by Cs+ but not Ba2+, presumably representing H-current (Ih). Possibly due to Ih, islet cells had peculiar membrane and firing responses. Of note the membrane potential showed a sag in response to hyperpolarization while depolarization triggered action potentials (APs) in a tonic-like pattern. APs, however, occurred with larger maximal frequencies and in response to broader stimulation intensities than in other tonically firing neurons. Neuronal variability in SG and possible functional roles of islet cells are discussed.

Phospholipase A2 Activation Enhances Inhibitory Synaptic Transmission in Rat Substantia Gelatinosa Neurons

Phospholipase A2 (PLA2) activation enhances glutamatergic excitatory synaptic transmission in substantia gelatinosa (SG) neurons, which play a pivotal role in regulating nociceptive transmission in the spinal cord. By using melittin as a tool to activate PLA2, we examined the effect of PLA2 activation on spontaneous inhibitory postsynaptic currents (sIPSCs) recorded at 0 mV in SG neurons of adult rat spinal cord slices by use of the whole cell patch-clamp technique. Melittin enhanced the frequency and amplitude of GABAergic and glycinergic sIPSCs. The enhancement of GABAergic but not glycinergic transmission was largely depressed by Na+ channel blocker tetrodotoxin or glutamate-receptor antagonists (6-cyano-7-nitroquinoxaline-2,3-dione and/or dl-2-amino-5-phosphonovaleric acid) and also in a Ca2+-free Krebs solution. The effects of melittin on glycinergic sIPSC frequency and amplitude were dose-dependent with an effective concentration of ∼0.7 μM for half-maximal effect and were depressed by PLA2 inhibitor 4-bromophenacyl bromide or aristolochic acid. The melittin-induced enhancement of glycinergic transmission was depressed by lipoxygenase inhibitor nordihydroguaiaretic acid but not cyclooxygenase inhibitor indomethacin. These results indicate that the activation of PLA2 in the SG enhances GABAergic and glycinergic inhibitory transmission in SG neurons. The former action is mediated by glutamate-receptor activation and neuronal activity increase, possibly the facilitatory effect of PLA2 activation on excitatory transmission, whereas the latter action is due to PLA2 and subsequent lipoxygenase activation and is independent of extracellular Ca2+. It is suggested that PLA2 activation in the SG could enhance not only excitatory but also inhibitory transmission, resulting in the modulation of nociception.

BDNF-mediated modulation of GABA and glycine release in dorsal horn lamina II from postnatal rats

Recent studies show that excitatory glutamatergic transmission is potentiated by BDNF in superficial dorsal horn, both at the pre- and the postsynaptic site. The role of BDNF in modulating GABA and glycine-mediated inhibitory transmission has not been fully investigated. To determine whether the neurotrophin is effective in regulating the spontaneous release of the two neurotransmitters, we have recorded miniature inhibitory postsynaptic currents (mIPSCs) in lamina II of post-natal rats. We show that application of BDNF enhanced the spontaneous release of GABA and glycine, in presence of tetrodotoxin. The effect was blocked by the trk-receptor inhibitor k-252a. Amplitude and kinetics of mIPSCs were not altered. Evoked GABA and glycine IPSCs (eIPSCs) were depressed by BDNF and the coefficient of variation of eIPSC amplitude was significantly increased. By recording glycine eIPSCs with the paired-pulse protocol, an increase of paired-pulse ratio during BDNF application was observed. We performed parallel ultrastructural studies to unveil the circuitry involved in the effects of BDNF. These studies show that synaptic interactions between full length functional trkB receptors and GABA-containing profiles only occur at non peptidergic synaptic glomeruli of types I and II. Expression of trkB in presynaptic vesicle-containing dendrites originating from GABAergic islet cells, indicates these profiles as key structures in the modulation of inhibitory neurotransmission by the neurotrophin. Our results thus describe a yet uncharacterized effect of BDNF in lamina II, giving further strength to the notion that the neurotrophin plays an important role in pain neurotransmission.

Differential wiring of local excitatory and inhibitory synaptic inputs to islet cells in rat spinal lamina II demonstrated by laser scanning photostimulation

The substantia gelatinosa (lamina II) of the spinal dorsal horn contains inhibitory and excitatory interneurons that are thought to play a critical role in the modulation of nociception. However, the organization of the intrinsic circuitry within lamina II remains poorly understood. We used glutamate uncaging by laser scanning photostimulation to map the location of neurons that give rise to local synaptic inputs to islet cells, a major class of inhibitory interneuron in lamina II. We also mapped the distribution of sites on the islet cells that exhibited direct (non-synaptic) responses to uncaging of excitatory and inhibitory transmitters. Local synaptic inputs to islet cells arose almost entirely from within lamina II, and these local inputs included both excitatory and inhibitory components. Furthermore, there was a striking segregation in the location of sites that evoked excitatory versus inhibitory synaptic inputs, such that inhibitory presynaptic neurons were distributed more proximal to the islet cell soma. This was paralleled in part by a differential distribution of transmitter receptor sites on the islet cell, in that inhibitory sites were confined to the peri-somatic region while excitatory sites were more widespread. This differential organization of excitatory and inhibitory inputs suggests a principle for the wiring of local circuitry within the substantia gelatinosa.

Presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors modulate release of inhibitory amino acids in rat spinal cord dorsal horn

Local inhibition within the spinal cord dorsal horn is mediated by the neurotransmitters GABA and glycine and strongly influences nociceptive and temperature signaling. Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are expressed by inhibitory interneurons and have been shown to modulate GABA release in other regions of the CNS. In the spinal cord, there is morphological evidence for presynaptic AMPA receptor subunits in GABAergic dorsal horn neurons, but functional data are lacking. To determine if AMPA receptors are indeed functional at presynaptic terminals of inhibitory neurons, we recorded evoked and miniature inhibitory postsynaptic currents (mIPSPs) in the superficial dorsal horn of the rat spinal cord. We show that AMPA receptor activation enhances spontaneous release of inhibitory amino acids in the presence of tetrodotoxin onto both lamina II neurons and NK1 receptor-expressing (NK1R+) lamina I neurons. This effect is sensitive to the concentration of extracellular Ca2+, yet is not fully blocked in most neurons in the presence of Cd2+, suggesting possible Ca2+ entry through AMPA receptors. Postsynaptic Ca2+ elevation is not required for these changes. AMPA-induced increases in mIPSP frequency are also seen in more mature dorsal horn neurons, indicating that these receptors may play a role in nociceptive processing in the adult. In addition, we have observed AMPA-induced depression of evoked release of GABA and glycine onto lamina I NK1R+ neurons. Taken together these data support a role for presynaptic AMPA receptors in modulating release of GABA and glycine in the superficial dorsal horn. Because inhibition in the dorsal horn is important for controlling pain signaling, presynaptic AMPA receptors acting to modulate the inhibitory inputs onto dorsal horn neurons would be expected to impact upon pain signaling in the spinal cord dorsal horn.

Ionotropic glutamate receptors are expressed in GABAergic terminals in the rat superficial dorsal horn

Ionotropic glutamate receptors (IGR), including NMDA, AMPA, and kainate receptors, are expressed in terminals with varied morphology in the superficial laminae (I-III) of the dorsal horn of the spinal cord. Some of these terminals can be identified as endings of primary afferents, whereas others establish symmetric synapses, suggesting that they may be gamma-aminobutyric acid (GABA)-ergic. In the present study, we used confocal and electron microscopy of double immunostaining for GAD65, a marker for GABAergic terminals, and for subunits of IGRs to test directly whether IGRs are expressed in GABAergic terminals in laminae I-III of the dorsal horn. Although colocalization is hard to detect with confocal microscopy, electron microscopy reveals a substantial number of terminals immunoreactive for GAD65 also stained for IGRs. Among all GAD65-immunoreactive terminals counted, 37% express the NMDA receptor subunit NR1; 28% are immunopositive using an antibody for the GluR2/4 subunits of the AMPA receptor; and 20-35% are immunopositive using antibodies for the kainate receptor subunits GluR5, GluR6/7, KA1, or KA2. Terminals immunoreactive for IGR subunits and GAD65 establish symmetric synapses onto dendrites and perikarya and can be presynaptic to primary afferent terminals within both type 1 and type 2 synaptic glomeruli. Activation of presynaptic IGR may reduce neurotransmitter release. As autoreceptors in terminals of Adelta and C afferent fibers in laminae I-III, presynaptic IGRs may play a role in inhibiting nociception. As heteroreceptors in GABAergic terminals in the same laminae, on the other hand, presynaptic IGRs may have an opposite role and even contribute to central sensitization and hyperalgesia.

Opioid-Like Actions of Neuropeptide Y in Rat Substantia Gelatinosa: Y1 Suppression of Inhibition and Y2 Suppression of Excitation

Neuropathic pain that results from injury to the peripheral or CNS responds poorly to opioid analgesics. Y1 and Y2 receptors for neuropeptide Y (NPY) may, however, serve as targets for analgesics that retain their effectiveness in neuropathic pain states. In substantia gelatinosa neurons in spinal cord slices from adult rats, we find that NPY acts via presynaptic Y2 receptors to attenuate excitatory postsynaptic currents (EPSCs) and predominantly on presynaptic Y1 receptors to attenuate glycinergic and GABAergic inhibitory postsynaptic currents (IPSCs). Because NPY attenuates the frequency of TTX-resistant miniature EPSCs and IPSCs, perturbation of the neurotransmitter release process contributes to its actions at both excitatory and inhibitory synapses. These effects, which are reminiscent of those produced by analgesic opioids, provide a cellular basis for previously documented spinal analgesic actions mediated via Y1 and Y2 receptors in neuropathic pain paradigms. They also underline the importance of suppression of inhibition in spinal analgesic mechanisms.

Kappa opioid receptors in rat spinal cord: sex-linked distribution differences

Activation of kappa opioid receptors (KORs) in the spinal cord can diminish nociception. Humans and rodents show sex differences in the analgesia produced by KOR agonists, and female rats show fluctuations in KOR density and sensitivity across the estrous cycle. However, it is unclear whether there are sex differences in the amount and/or distribution of spinal KORs. In the present study, immunocytochemically labeled KORs were examined in laminae I and II of the lumbosacral spinal dorsal horn of male and normally cycling female Sprague-Dawley rats. The basic pattern of KOR labeling was determined in both sexes using qualitative electron microscopy (EM), and sex-linked differences in the density and subcellular distribution of KOR immunoreactivity were determined with quantitative EM and light microscopy. KOR labeling was visualized with immunoperoxidase for optimally sensitive detection, or with immunogold for precise subcellular localization. By EM, the general pattern of KOR immunoreactivity was similar in males and females. KOR immunoreactivity was common in dendrites, axons, and axon terminals, and was in a few glia and neuronal somata. Most KOR-immunoreactive (-ir) axons were fine-diameter and unmyelinated. Most KOR-ir terminals were small or medium-sized, and a minority formed asymmetric or symmetric synapses with unlabeled dendrites. KOR immunoreactivity was associated both with the plasma membrane and with cytoplasmic organelles, notably including dense core vesicles in terminals. Light microscopic densitometry revealed that KOR immunoreactivity was significantly denser in estrus and proestrus females than in males. By EM, the distribution of KOR-immunogold labeling within axon terminals differed, with a greater proportion of cytoplasmic KOR labeling in estrus females compared with males. In contrast, the abundance and types of KOR-immunoperoxidase-labeled profiles did not show sex-linked differences. We conclude that in both sexes, KORs are positioned to influence both pre- and postsynaptic neurotransmission and are present in morphologically heterogeneous neuron populations. These findings are consistent with complex consequences of KOR activation in the spinal cord. In addition, the presence of increased KOR density and proportionally elevated intracellular KORs in proestrus/estrus females suggests a basis for sex-linked differences in KOR-mediated antinociception.

Actions of noradrenaline on substantia gelatinosa neurones in the rat spinal cord revealed by in vivo patch recording

To elucidate the mechanisms of antinociception mediated by the descending noradrenergic pathway in the spinal cord, the effects of noradrenaline (NA) on noxious synaptic responses of substantia gelatinosa (SG) neurones, and postsynaptic actions of NA were investigated in rats using an in vivo whole-cell patch-clamp technique. Under urethane anaesthesia, the rat was fixed in a stereotaxic apparatus after the lumbar spinal cord was exposed. In the current-clamp mode, pinch stimuli applied to the ipsilateral hindlimb elicited a barrage of EPSPs, some of which initiated an action potential. Perfusion with NA onto the surface of the spinal cord hyperpolarized the membrane (5.0-9.5 mV) and suppressed the action potentials. In the voltage-clamp mode (V(H), -70 mV), the application of NA produced an outward current that was blocked by Cs(+) and GDP-beta-S added to the pipette solution and reduced the amplitude of EPSCs evoked by noxious stimuli. Under the blockade of postsynaptic actions of NA, a reduction of the evoked and spontaneous EPSCs of SG neurones was still observed, thus suggesting both pre- and postsynaptic actions of NA. The NA-induced outward currents showed a clear dose dependency (EC(50), 20 microM), and the reversal potential was -88 mV. The outward current was mimicked by an alpha(2)-adrenoceptor agonist, clonidine, and suppressed by an alpha(2)-adrenoceptor antagonist, yohimbine, but not by alpha(1)- and beta-antagonists. These findings suggest that NA acts on presynaptic sites to reduce noxious stimuli-induced EPSCs, and on postsynaptic SG neurones to induce an outward current by G-protein-mediated activation of K(+) channels through alpha(2)-adrenoceptors, thereby producing an antinociceptive effect.

Glutamate and GABA: a painful combination

Regulation of release of inhibitory neurotransmitter is a key element of plasticity in dorsal horn function. In this issue of Neuron, Kerchner et al. report that neurotransmitter release from inhibitory dorsal horn neurons is affected by activation of presynaptic kainate-type glutamate receptors.

Topical bicuculline to the rat spinal cord induces highly localized allodynia that is mediated by spinal prostaglandins

The purpose of this study was to investigate the allodynic effect of bicuculline (BIC) given topically to the dorsal surface of the rat spinal cord, and to determine if spinal prostaglandins (PGs) mediate the allodynic state arising from spinal GABA(A)-receptor blockade. Male Sprague-Dawley rats (325-400 g) were anaesthetized with halothane and maintained with urethane for the continuous monitoring of blood pressure (MAP), heart rate (HR) and cortical electroencephalogram (EEG). A laminectomy was performed to expose the dorsal surface of the spinal cord. Unilateral application of BIC (0.1 microg in 0.1 microl) to the L5 or L6 spinal segment induced a highly localized allodynia (e.g. one or two digits) on the ipsilateral hind paw. Thus, hair deflection (brushing the hair with a cotton-tipped applicator) in the presence, but not absence of BIC, evoked an increase in MAP and HR, abrupt motor responses (MR; e.g. withdrawal of the hind leg, kicking, and/or scratching) on the affected side, and desynchrony of the EEG. BIC-allodynia was dose-dependent, yielding ED(50)'s (95% CI's) of 45 ng (31-65) for MAP; 68 ng (46-101) for HR and 76 ng (60-97) for MR. Allodynia was sustained for up to 2 h with repeated BIC application without any detectable change in the location or area of peripheral sensitization. Pretreatment with either the EP(1)- receptor antagonist, SC-51322, the cyclooxygenase (COX)-2 selective inhibitor, NS-398, or the NMDA-receptor antagonist, AP-7, inhibited BIC-allodynia in a dose-dependent manner. The results demonstrate: (a) BIC, applied to the dorsal surface of the spinal cord, induces highly localized allodynia; (b) this effect can be sustained with repeated BIC application; (c) it is evoked by NMDA-dependent afferent input; (d) spinal PGs are synthesized by constitutive COX-2 during BIC-allodynia; and (e) spinal PGs contribute to the abnormal processing of tactile input via spinal EP1-receptors.

Subpopulations of GABAergic and non-GABAergic rat dorsal horn neurons express Ca2+-permeable AMPA receptors

Subpopulations of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors that are either permeable or impermeable to Ca2+ are expressed on dorsal horn neurons in culture. While both mediate synaptic transmission, the Ca2+ -permeable AMPA receptors provide a Ca2+ signal that may result in a transient change in synaptic strength [Gu, J.G., Albuquerque, C., Lee, C.J. & MacDermott, A.B. (1996) Nature, 381, 793]. To appreciate the relevance of these receptors to dorsal horn physiology, we have investigated whether they show selective expression in identified subpopulations of dorsal horn neurons. Expression of Ca2+-permeable AMPA receptors was assayed using the kainate-induced cobalt loading technique first developed by Pruss et al. [Pruss, R.M., Akeson, R.L., Racke, M.M. & Wilburn, J.L. (1991) Neuron, 7, 509]. Subpopulations of dorsal horn neurons were identified using immunocytochemistry for gamma-aminobutyric acid (GABA), glycine, substance P receptor (NK1 receptor) and the Ca2+-binding proteins, calretinin and calbindin D28K. We demonstrate that, in dorsal horn neurons in culture, kainate-induced cobalt uptake is selectively mediated by Ca2+-permeable AMPA receptors, and that a majority of GABA and NK1 receptor-expressing neurons express Ca2+-permeable AMPA receptors. GABAergic dorsal horn neurons are important in local inhibition as well as in the regulation of transmitter release from primary afferent terminals. NK1 receptor-expressing dorsal horn neurons include many of the projection neurons in the nociceptive spino-thalamic pathway. Thus, we have identified two populations of dorsal horn neurons representing important components of dorsal horn function that express Ca2+-permeable AMPA receptors. Furthermore, we show that several subpopulations of putative excitatory interneurons defined by calretinin and calbindin expression do not express Ca2+-permeable AMPA receptors.

The neuropeptide Y Y1 receptor is a somatic receptor on dorsal root ganglion neurons and a postsynaptic receptor on somatostatin dorsal horn neurons

Using indirect immunofluorescence, neuropeptide Y Y1 receptor (Y1 receptor)-like immunoreactivity (LI) was localized close to the plasmalemma of small neurons in lumbar dorsal root ganglia (DRGs) and neurons in the inner lamina II of the lumbar spinal cord of the rat. Using confocal microscopy, colocalization of Y1 receptor-LI and transferrin receptor-LI, a marker for endosomes and coated vesicles, was observed in dot-like structures along the plasmalemma. Under the electron microscope, Y1 receptor-LI was localized in coated vesicles and endosomes, in the membrane of tubular cisternae, sometimes connected to multivesicular bodies, and in the plasmalemma. These complex distribution patterns may reflect receptor turnover and internalization processes. In the lamina II of the spinal dorsal horn, Y1 receptor-LI was localized in the plasmalemma of neurons without any apparent association with paramembrane structures, as described above for the DRG neurons. Many dendrites were Y1 receptor-positive, and some of them made synaptic contacts with unstained axonal terminals. In general, Y1 receptor-LI was localized in the membrane outside the postsynaptic density. Double-immunofluorescence staining showed that most Y1 receptor-immunoreactive neurons in lamina II contained somatostatin-LI. Both in DRG and dorsal horn neurons, the Y1 receptor thus seems to represent a postjunctional/postsynaptic receptor.

Nitric oxide-producing islet cells modulate the release of sensory neuropeptides in the rat substantia gelatinosa

The substantia gelatinosa of the spinal cord (lamina II) is the major site of integration for nociceptive information. Activation of NMDA glutamate receptor, production of nitric oxide (NO), and enhanced release of substance P and calcitonin gene-related peptide (CGRP) from primary afferents are key events in pain perception and central hyperexcitability. By combining reduced nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase histochemistry for NO-producing neurons with immunogold labeling for substance P, CGRP, and glutamate, we show that (1) NO-producing neurons in lamina IIi are islet cells; (2) these neurons rarely form synapses onto peptide-immunoreactive profiles; and (3) NADPH diaphorase-positive dendrites are often in close spatial relationship with peptide-containing terminals and are observed at the periphery of type II glomeruli showing glutamate-immunoreactive central endings. By means of confocal fluorescent microscopy in acute spinal cord slices loaded with the Ca2+ indicator Indo-1, we also demonstrate that (1) NMDA evokes a substantial [Ca2+]i increase in a subpopulation of neurons in laminae I-II, with morphological features similar to those of islet cells; (2) a different neuronal population in laminae I-IIo, unresponsive to NMDA, displays a significant [Ca2+]i increase after slice perfusion with either substance P and the NO donor 3morpholinosydnonimine (SIN-1); and (3) the responses to both substance P and SIN-1 are either abolished or significantly inhibited by the NK1 receptor antagonist sendide. These results provide compelling evidence that glutamate released at type II glomeruli triggers the production of NO in islet cells within lamina IIi after NMDA receptor activation. The release of substance P from primary afferents triggered by newly synthesized NO may play a crucial role in the cellular mechanism leading to spinal hyperexcitability and increased pain perception.

Electron microscopic analysis of gamma-aminobutyric acid and glycine colocalization in rat trigeminal subnucleus caudalis

Postembedding immunogold methods were used to examine the distribution of gamma-aminobutyric acid (GABA) and glycine and especially their colocalization in glomerular neuronal profiles adjacent to trigeminal primary afferent profiles in lamina II of rat subnucleus caudalis. We found that 60% of the profiles adjacent to the trigeminal primary afferent terminals exhibited colocalization of GABA and glycine. GABA alone was found to localize in 17% of the adjacent profiles. Glycine alone was found to localize in 18% of the adjacent profiles. Of interest, 10% of the trigeminal primary afferent fibers showed glycine localization. All the profiles with colocalization of GABA and glycine were identified as presynaptic axonal terminals, suggesting a possible cumulative effect by these two inhibitory neurotransmitters in presynaptic inhibition. These findings show that GABA and glycine colocalize in a subpopulation of presynaptic axonal terminals within lamina II of the subnucleus caudalis. The possible origins of these axons are discussed, as well as their potential involvement in presynaptic inhibition of orofacial nociception.

GABA- and glycine-mediated inhibitory postsynaptic potentials in neonatal rat rostral ventrolateral medulla neurons in vitro

Whole-cell patch recordings were made from rostral ventrolateral medulla neurons of two in vitro preparations: (i) brainstem spinal cords of two- to five-day-old rats, and (ii) coronal brainstem slices of eight- to 12-day-old rats, and the inhibitory synaptic activities in these neurons have been studied. In brainstem spinal cord preparations, Lucifer Yellow was diffused into the recording neurons at the end of experiments. Medullary neurons were characterized as: (i) spinally projecting by the appearance of an antidromic spike following electrical stimulation of the spinal tract between T2 and T3 segments, and (ii) adrenergic by the detection of phenylethanolamine-N-methyltransferase immunoreactivity in Lucifer Yellow-filled neurons. Of the 13 spinally projecting and phenylethanolamine-N-methyltransferase-positive medullary neurons, focal stimulation elicited in the presence of glutamate receptor antagonists an inhibitory postsynaptic potential in nine neurons. Inhibitory synaptic potentials were reversibly eliminated by the GABA(A) receptor antagonist bicuculline (10-20 microM) in six of nine neurons, by the glycine receptor antagonist strychnine (0.1-1 microM) in two and by a combination of bicuculline and strychnine in one neuron. In brainstem slice preparations, focal stimulation elicited three types of synaptic potential: (i) an excitatory postsynaptic potential, (ii) an inhibitory postsynaptic potential and (iii) a biphasic synaptic potential consisting of an excitatory synaptic potential followed by an inhibitory synaptic potential. Inhibitory synaptic potentials had a reversal potential between -70 and -80 mV, reversed their polarity in a low (6.7 mM) Cl- Krebs' solution, and suppressed or blocked by either bicuculline or strychnine or both. Elimination of inhibitory synaptic potentials unmasked in some cells an excitatory synaptic potential or enhanced the excitatory synaptic potential component in medullary neurons with a biphasic response, indicating a marked convergence of excitatory and inhibitory inputs onto a single neuron. A population of medullary neurons appeared to be pacemaker neurons whereby they discharged spontaneously. When discharges were suppressed by membrane hyperpolarization, focal stimulation elicited inhibitory synaptic potentials in 8/23 neurons tested. Our results suggest that inhibitory synaptic potentials in medullary neurons are mediated by either GABA and/or glycine which open primarily Cl- channels. The prevalence of inhibitory synaptic potentials in medullary neurons indicates an essential role of inhibitory transmission in controlling the input and output ratio of these neurons.

Spinal effects of bicuculline: modulation of an allodynia-like state by an A1-receptor agonist, morphine, and an NMDA-receptor antagonist

Single-unit recordings were made in the intact anesthetized rat of the responses of dorsal horn neurons to C-, Adelta-, and Abeta-fiber stimulation. The postdischarge and windup responses of the same cells along with responses to innocuous stimuli, prod and brush, also were measured. The effects of (-)-bicuculline-methobromide (0.5, 5, 50, and 250 microg) were observed on these neuronal responses. The C- and Adelta-fiber-evoked responses were facilitated significantly in a dose-dependent manner. The input was facilitated, but as the final overall response was not increased by the same factor, windup appeared to be reduced. However, postdischarge, resulting from the increase in the excitability produced by windup, tended to be facilitated. After doses of >/=5 microg bicuculline, stimulation at suprathreshold Abeta-fiber-evoked activity caused enhanced firing, mainly at later latencies corresponding to Adelta-fiber-evoked activity in normal animals. Few cells responded consistently to brush and so no significant change was observed. Responses evoked by innocuous pressure (prod) always were observed in cells that concurrently responded to electrical stimulation with a C-fiber response. This tactile response was facilitated significantly by bicuculline. The effects of N6-cyclopentyladenosine (N6-CPA), an adenosine A1-receptor agonist, was observed after pretreatment with 50 microg bicuculline, as were the effects of morphine and 7-chlorokynurenate (7-CK). N6-CPA inhibited prod, C- and Adelta-fiber-evoked responses as well as the initial and overall final response to the train of C-fiber strength stimuli. Inhibitions were reversed with 8(p-sulphophenyl) theophylline. Morphine, the mu-receptor agonist, also inhibited the postbicuculline responses to prod, C-, and Adelta-fiber responses and initial and final responses to a train of stimuli. Inhibitory effects of morphine were reversed partly by naloxone. 7-CK, an antagonist at the glycine site on the N-methyl-D-aspartate-receptor complex, inhibited the responses to C- and Adelta-fiber-evoked activity as well as prod. The postdischarges were inhibited by this drug. Again both the initial and overall responses of the cell were inhibited. To conclude, bicuculline caused an increase in the responses of deep dorsal horn cells to prod, Adelta-fiber-evoked activity, increased C-fiber input onto these cells along with the appearance of responses at latencies normally associated with Adelta fibers, but evoked by suprathreshold Abeta-fiber stimulation. These alterations may be responsible for some aspects of the clinical phenomenon of allodynia and hyperalgesia. These altered and enhanced responses were modulated by the three separate classes of drugs, the order of effectiveness being 7-CK, N6-CPA, and then morphine.

Biphasic response of spinal GABAergic neurons after a lumbar rhizotomy in the adult rat

The expression of gamma-aminobutyric acid (GABA) and of the isoforms of the enzyme involved in its synthesis, glutamic acid decarboxylase (GAD), is modified in several rat brain structures in different injury models. The aim of the present work was to determine whether such plasticity of the GABAergic system also occurred in the deafferented adult rat spinal cord, a model where a major reorganization of neural circuits takes place. GABAergic expression following unilateral dorsal rhizotomy was studied by means of non-radioactive in situ hybridization to detect GAD67 mRNA and by immunohistochemistry to detect GAD67 protein and GABA. Three days following rhizotomy the number of GAD67 mRNA-expressing neurons was decreased in the superficial layers of the deafferented horn, while GABA immunostaining of axonal fibres located in this region was highly increased. Seven days after lesion, on the other hand, many GAD67 mRNA-expression neurons were bilaterally detected in deep dorsal and ventral layers, this expression being correlated with the increased detection of GAD67 immunostained somata and with the reduction of GABA immunostaining of axons. GABA immunostaining was frequently found to be associated with reactive astrocytes that exhibited intense immunostaining for glial fibrillary acidic protein (GFAP) but remained GAD67 negative. These results indicate that degeneration of afferent terminals induces a biphasic response of GABAergic spinal neurons located in the dorsal horn and show that many spinal neurons located in deeper regions re-express GAD67, suggesting a possible participation of the local GABAergic system in the reorganization of disturbed spinal networks.

Effects of GABA and glycine receptor antagonists on the activity and PAG-induced inhibition of rat dorsal horn neurons

The effects of bicuculline and strychnine on the activity and periaqueductal gray (PAG)-induced inhibition of rat dorsal horn neurons of the lumbar spinal cord were tested. Extracellular single unit recordings were from 36 dorsal horn neurons near a microdialysis fiber passed through the spinal cord for drug application. The GABAA receptor antagonist, bicuculline, was tested on 19 cells, whereas the glycine receptor antagonist, strychnine, was tested on 17 cells. Both bicuculline and strychnine increased the background activity and responses to mechanical stimulation (BRUSH, PRESS, and PINCH) of the skin.06 They also significantly blocked the PAG-induced inhibition of responses to peripheral mechanical stimuli. This experiment suggests that the mechanism of PAG-induced descending inhibition of dorsal horn neuron activity involves GABA and/or glycine release in the spinal cord and that there is tonic release of these inhibitory neurotransmitters.

Distribution of glycine-immunoreactive profiles in the monkey spinal cord: a light microscopic and ultrastructural study

The present study analyzed the relationships of glycine (GLY)-immunoreactive (-IR) and unlabeled profiles in the primate spinal cord. Light microscopic analysis demonstrated GLY-IR profiles in laminae III-VII, with fewer labeled profiles in laminae I, II, VIII, IX and X. The dorsal part of the lateral funiculus and the dorsal funiculus contained few labeled axons, in contrast to all other areas of white matter, which were heavily labeled. At the electron microscopic level, GLY-IR terminals in monkeys contained mainly round, with occasional pleomorphic, clear vesicles; however, F-type GLY-IR terminals synapsing on motoneurons contained pleomorphic vesicles. This seems to be an important species difference because vesicles in GLY-IR terminals in rat and cat are predominantly oval or elliptical. GLY-IR terminals synapsed on unlabeled as well as GLY-IR cell bodies and dendrites. This is morphological evidence that GLY may be both an inhibitor (GLY-IR terminals synapse on and presumably inhibit non-GLY cell bodies and dendrites) and a disinhibitor (GLY-IR terminals synapse on and presumably inhibit other GLY elements) of spinal activity. Also noteworthy was the conspicuous absence of axoaxonic interactions involving GLY-IR terminals. A related finding was that GLY profiles were always postsynaptic, never presynaptic, to glomerular primary afferent terminals. The functional implications would seem to be that primary afferent input can activate the spinal GLY system but that there is little GLY presynaptic control of afferent input in monkeys. This is in contrast to rats and cats, in which axoaxonic interactions involving GLY-IR terminals have been observed and where it is common to find GLY-IR terminals presynaptic to glomerular primary afferent terminals.

Electron microscopy of immunoreactivity patterns for glutamate and gamma-aminobutyric acid in synaptic glomeruli of the feline spinal trigeminal nucleus (Subnucleus Caudalis)

We studied the ultrastructure of the synaptic organization in the feline spinal trigeminal nucleus, emphasizing specific neurotransmitter patterns within lamina II of the pars caudalis/medullary dorsal horn. Normal adults were perfused, and Vibratome sections from pars caudalis were processed for electron microscopy. Ultrathin sections were reacted with antibodies for the excitatory neurotransmitter glutamate (Glu) and for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) by using postembedding immunogold techniques. Both single- and double-labeled preparations were examined. Results with single labeling show that Glu-immunoreactive terminals have round synaptic vesicles and form asymmetric synaptic contacts onto dendrites. GABA-immunoreactive axon terminals and vesicle-containing dendrites have pleomorphic vesicles, and the axon terminals form symmetric contacts onto dendrites and other axons. Double labeling on a single section shows glomeruli with central Glu-immunoreactive terminals that are presynaptic to dendrites, including GABA+ vesicle-containing dendrites. These Glu+ terminals are also postsynaptic to GABA+ axon terminals, and these GABA-immunoreactive terminals may also be presynaptic to the GABA+ vesicle-containing dendrites. Quantitative analyses confirm the specificity of the Glu and GABA immunoreactivities seen in the various glomerular profiles. The results suggest that a subpopulation of Glu-immunoreactive primary afferents (excitatory) may be under the direct synaptic influence of a GABA-immunoreactive intrinsic pathway (inhibitory) by both presynaptic and postsynaptic mechanisms.

Glutamatergic and GABAergic input to rat spinothalamic tract cells in the superficial dorsal horn

The distribution of synaptic terminals onto spinothalamic tract cells (types I and II) of the superficial dorsal horn was determined with special reference to the amino acid transmitters glutamate and gamma-aminobutyric acid. Fifteen spinothalamic cells retrogradely labeled from the thalamus with the neuroanatomical tracer wheatgerm agglutinin conjugated to horseradish peroxidase were sectioned for electron microscopy. Serial sections from several levels through each cell were immunostained for glutamate and gamma-aminobutyric acid using a postembedding immunogold technique. Perimeter measurements of spinothalamic cell somata and dendrites and the lengths of apposition for all terminal profiles in contact with the spinothalamic cells were obtained from electron micrographs using a digitizing tablet. These data were used to determine the density of terminals on the soma and dendrites. In addition, the terminal population on these cells was categorized by transmitter content (glutamate, gamma-aminobutyric acid, or unlabeled). The results demonstrate that terminal density increased on dendrites relative to their distance from the soma. Glutamatergic and GABAergic input composed 37% and 20% of the terminal population, respectively, and these percentages remained uniform for the soma and dendrites. There were no significant differences among the 15 cells analyzed for this study. The results, therefore, suggest that both type I and type II STT cells of the superficial DH have similar synaptic organizations.

Galanin is contained in GABAergic neurons in the rat spinal dorsal horn

In order to determine which types of neuron in laminae I-III of the rat spinal dorsal horn contain the peptide galanin, pre-embedding immunocytochemistry with antiserum to galanin was combined with post-embedding detection of GABA- and glycine-like immunoreactivities. Sixty-eight galanin immunoreactive neurons in laminae I-III selected from four rats were examined, and in each case semi-thin sections through the cell body were tested with a monoclonal antibody to GABA and an antiserum to glycine. All of the 68 galanin-immunoreactive neurons tested were GABA-immunoreactive, while only one of them (in lamina III) was glycine-immunoreactive. This suggests that galanin is contained in inhibitory interneurons, and that (like enkephalin, neuropeptide Y and thyrotropin-releasing hormone) it is mainly restricted to GABAergic neurons which do not use glycine as a co-transmitter.

Some inhibitory neurons in the spinal cord develop c-fos-immunoreactivity after noxious stimulation

In order to determine which types of spinal neuron produce c-fos in response to noxious stimulation, we have combined pre-embedding detection of c-fos-like immunoreactivity with post-embedding immunocytochemistry using antibodies against GABA and glycine, 2 h after subcutaneous injection of formalin into a hindpaw of anaesthetized rats. Throughout the spinal cord, the majority of c-fos-immunoreactive neurons (72-81%) did not possess GABA- or glycine-like immunoreactivity, while the remaining cells contained one or both types of immunoreactivity. In the superficial dorsal horn (laminae I and II) and dorsal white matter, between 14 and 20% of c-fos-immunoreactive neurons were GABA-immunoreactive, and some of these were also glycine-immunoreactive. A single neuron in lamina I in one animal was glycine- but not GABA-immunoreactive. In the remainder of the spinal cord, between 21 and 35% of the c-fos-immunoreactive cells were GABA- or glycine-immunoreactive, and the majority of these neurons contained both types of immunoreactivity. These results suggest that some inhibitory neurons in both the superficial and deep parts of the dorsal horn are activated by noxious stimuli. It is known that some of the cells which produce c-fos in response to noxious stimulation are projection neurons, with axons ascending to the brainstem or thalamus, however, because of the large number of c-fos-immunoreactive cells in the dorsal horn, it is likely that many are interneurons, and some of these are probably excitatory cells which use glutamate as a transmitter. It therefore appears that after noxious stimulation c-fos is produced in several types of spinal neuron, including projection cells and both excitatory and inhibitory interneurons.

Immunocytochemical characterization of the synaptic innervation of a single spinal neuron, the electric catfish electromotoneuron

The electric catfish, Malapterurus electricus, possesses electric organs that are innervated by a pair of identifiable electromotoneurons located within the cervical spinal cord. The pattern of synaptic innervation of the electromotoneurons can be revealed by an antibody against the synaptic vesicle protein SV2. Both somata and proximal dendrites are densely innervated. Several transmitters contribute to this innervation. Glutamate, the neurotransmitter of the dorsal root sensory fibers, reveals a weak punctuate immunoreactivity. The previously described electrical synapses of the electromotoneurons were visualized by an antibody against a gap-junctional protein. In contrast to the electromotoneurons of other electric fish, the electric catfish electromotoneurons possess many inhibitory synapses. With antibodies against glycine and against the glycine receptor, a dense immunoreactivity of the surface of the somata and proximal dendrites can be revealed. The glycine receptor-like immunoreactivity exhibits a patch-like distribution similar to that revealed by the anti-SV2 antibody. gamma-Aminobutyric acid (GABA)-immunopositive terminals contribute to the inhibitory electromotoneuron innervations to a lesser degree. The chemical characteristics of the electromotoneuron innervations of Malapterurus resemble those of other spinal motoneurons rather than spinal electromotoneurons of other electric fish. Thus our immunocytochemical study supports the view that the pattern of electromotoneuron innervations in Malapterurus reveals little specialization. The capacity for information processing required for the control of the electric organ discharge appears to be achieved by the increased integrational capacity of the newly evolved multiple dendrites and not by an additional parallel channel specific for the electromotor system.

Coexistence of NADPH diaphorase with GABA, glycine, and acetylcholine in rat spinal cord

The enzyme NADPH diaphorase is present in many spinal neurons, and is thought to correspond to nitric oxide synthase. In order to determine which types of neuron in the spinal cord contain this enzyme, we have carried out a combined enzyme histochemical and immunocytochemical study with antibodies to GABA, glycine, and choline acetyltransferase. Two hundred rats were tested for GABA- and glycine-like immunoreactivity. The majority of these neurons (207/224) were GABA-immunoreactive and 139 were also glycine-immunoreactive. NADPH diaphorase-positive neurons in laminae I and II generally showed both types of immunoreactivity, while those in deeper laminae of the dorsal horn and around the central canal either showed both types or else were only GABA-immunoreactive. Since GABA and acetylcholine are thought to coexist in spinal neurons, NADPH diaphorase staining was combined with immunostaining for choline acetyltransferase. Immunoreactive neurons in laminae III and IV were all NADPH diaphorase-positive, while only some of those around the central canal and in the deeper laminae of the dorsal horn were positive. Choline acetyltransferase-immunoreactive neurons in the intermediolateral cell column (presumed sympathetic preganglionic neurons) were often NADPH diaphorase-positive, whereas those in the ventral horn (presumed motoneurons) were not. NADPH diaphorase-positive cells in the intermediolateral cell column were not immunoreactive with GABA or glycine antibodies.