Intracellular labeling study of neurons in the superficial part of the magnocellular layer of the medullary dorsal horn of the rat

Neuronal Cell Types in the Spinal Trigeminal Nucleus of the Camel Brain

Neurons in the spinal trigeminal nucleus of a camel were morphologically studied by the Golgi impregnation method. The neurons were classified based on the size and shape of their cell bodies, the density of their dendritic trees, and the morphology and distribution of their appendages. At least 12 morphological types of neurons were found in the camel spinal trigeminal nucleus, including the following: stalked, islets, octopus-like, lobulated, boat-like, pyramidal, multipolar, round, oval, and elongated neurons. These neurons exhibited large numbers of various forms of appendages that arise not only from their dendrites but also from their cell bodies. Moreover, neurons with unique large dilatations especially at their dendritic branching points were also reported. The neurons reported in this study displayed an array of different sizes and shapes and featured various forms of appendages arising from cell bodies and dendrites. Such morphologically distinctive neuronal cell types might indicate an evolutionary adaptation to pain and temperature processing pathways at the level of the spinal trigeminal nucleus in camels, which traditionally live in a very harsh climatic environment and are frequently exposed to painful stimuli.

Mechanisms of Peripheral and Central Pain Sensitization: Focus on Ocular Pain

Persistent ocular pain caused by corneal inflammation and/or nerve injury is accompanied by significant alterations along the pain axis. Both primary sensory neurons in the trigeminal nerves and secondary neurons in the spinal trigeminal nucleus are subjected to profound morphological and functional changes, leading to peripheral and central pain sensitization. Several studies using animal models of inflammatory and neuropathic ocular pain have provided insight about the mechanisms involved in these maladaptive changes. Recently, the advent of new techniques such as optogenetics or genetic neuronal labelling has allowed the investigation of identified circuits involved in nociception, both at the spinal and trigeminal level. In this review, we will describe some of the mechanisms that contribute to the perception of ocular pain at the periphery and at the spinal trigeminal nucleus. Recent advances in the discovery of molecular and cellular mechanisms contributing to peripheral and central pain sensitization of the trigeminal pathways will be also presented.

Postnatal Excitability Development and Innervation by Functional Transient Receptor Potential Vanilloid 1 (TRPV1) Terminals in Neurons of the Rat Spinal Sacral Dorsal Commissural Nucleus: an Electrophysiological Study

The sacral dorsal commissural nucleus (SDCN) in the spinal cord receives both somatic and visceral primary afferents. Transient receptor potential vanilloid 1 (TRPV1) channels are preferentially expressed in certain fine primary afferents. However, knowledge of the SDCN neurons postnatal excitability development and their contacts with TRPV1 fibers remains elusive. Here, whole-cell recordings were conducted in spinal cord slices to evaluate the postnatal development of SDCN neurons and their possible contacts with functional TRPV1-expressing terminals. SDCN neurons in neonatal (postnatal day (P) 1-2), young (P8-10), and adult rats (P35-40) have different electrophysiological properties. SDCN neurons in neonatal rats have higher frequency of spontaneous firing, higher resting membrane potential, and lower presynaptic glutamate release probability. However, no difference in quantal release was found. At all developmental stages, TRPV1 activation with the selective agonist capsaicin increases glutamate release in the presence of tetrodotoxin, which blocks action potential-dependent and polysynaptic neurotransmission, indicating that functional TRPV1 fibers innervate SDCN neurons directly. Capsaicin-induced presynaptic glutamate release onto SDCN neurons depends on external Ca²⁺ influx through TRPV1 channels; voltage-dependent calcium channels had a slighter impact. In contrast, capsaicin blocked C fiber-evoked synaptic transmission, indicating that TRPV1 activation has opposite effects on spontaneous asynchronous and action potential-dependent synchronous glutamate release. These data indicate that excitability of SDCN neurons undergoes a developmental shift, and these neurons receive functional TRPV1 terminals from early postnatal stage. The opposite action of capsaicin on asynchronous and synchronous glutamate release should be taken into account when TRPV1 channels are considered as therapeutic targets.

Effects of prostaglandin E2 on synaptic transmission in the rat spinal trigeminal subnucleus caudalis

The spinal trigeminal subnucleus caudalis (Vc) receives preferentially nociceptive afferent signals from the orofacial area. Nociceptive stimuli to the orofacial area induce cyclooxygenase both peripherally and centrally, which can synthesize a major prostanoid prostaglandin E2 (PGE2) that implicates in diverse physiological functions. To clarify the roles of centrally-synthesized PGE2 in nociception, effects of exogenous PGE2 on synaptic transmission in the Vc neurons were investigated in the rat brainstem slice. Spontaneously occurring excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) were recorded, respectively, under pharmacological blockade of inhibitory and excitatory transmission by whole-cell patch-clamp mode. Perfusion of PGE2 (1-5 μM) increased the frequency of sIPSCs in a concentration-dependent manner but had no significant effect on the amplitude. Similarly to the effects on sIPSCs, PGE2 increased the sEPSC frequency without any effect on the amplitude. These facilitatory effects of PGE2 on spontaneous synaptic transmissions were blocked by an EP1 antagonist SC19220 but not by an EP4 antagonist AH23848. Electrical stimulation of the trigeminal tract evoked short latency EPSCs (eEPSCs) in the Vc neurons. PGE2 (5 μM) was ineffective on the eEPSCs. The present study demonstrated that PGE2 facilitated spontaneous synaptic transmissions in the Vc neurons through activating the presynaptic EP1 receptors but had no effect on the trigeminal tract-mediated excitatory transmission. These results suggest that centrally-synthesized PGE2 modifies the synaptic transmission in the Vc region, thereby contributing to the processing of nociceptive signals originated from the orofacial area.

Trigeminal intersubnuclear neurons: morphometry and input-dependent structural plasticity in adult rats

Intersubnuclear neurons in the caudal division of the spinal trigeminal nucleus that project to the principal nucleus (Pr5) play an active role in shaping the receptive fields of other neurons, at different levels in the ascending sensory system that processes information originating from the vibrissae. By using retrograde labeling and digital reconstruction, we investigated the morphometry and topology of the dendritic trees of these neurons and the changes induced by long-term experience-dependent plasticity in adult male rats. Primary afferent input was either eliminated by transection of the right infraorbital nerve (IoN), or selectively altered by repeated whisker clipping on the right side. These neurons do not display asymmetries between sides in basic metric and topologic parameters (global number of trees, nodes, spines, or dendritic ends), although neurons on the left tend to have longer terminal segments. Ipsilaterally, both deafferentation (IoN transection) and deprivation (whisker trimming) reduced the density of spines, and the former also caused a global increase in total dendritic length and a relative increase in more complex arbors. Contralaterally, deafferentation reduced more complex dendritic trees, and caused a moderate decline in dendritic length and spatial reach, and a loss of spines in number and density. Deprivation caused a similar, but more profound, effect on spines. Our findings provide original quantitative descriptions of a scarcely known cell population, and show that denervation- or deprivation-derived plasticity is expressed not only by neurons at higher levels of the sensory pathways, but also by neurons in key subcortical circuits for sensory processing.

Pharmacological analysis of excitatory and inhibitory synaptic transmission in horizontal brainstem slices preserving three subnuclei of spinal trigeminal nucleus

Spinal trigeminal nucleus (Vsp) consists of three subnuclei: oralis (Vo), interpolaris (Vi) and caudalis (Vc). Previous anatomical studies using antero-/retro-grade tracers have suggested that intersubnuclear ascending/descending synaptic transmissions exist between subnuclei. However, pharmacological properties of the intersubnuclear synaptic transmission have not been studied yet. Since three subnuclei are located in Vsp along rostro-caudal axis, it will be necessary to prepare horizontal brainstem slices to perform pharmacological analysis of the intersubnuclear synaptic transmission. We here show horizontal brainstem slices retaining three subnuclei, and that, using blind whole-cell recordings in the slices, synaptic transmission may be abundantly retained between subnuclei in the horizontal slices, except for the transmission from Vo to Vc. Finally, pharmacological analysis shows that excitatory and inhibitory synaptic responses, respectively, are mediated by AMPA and NMDA receptors and by GABA(A) and glycine receptors, with a differential contribution to the synaptic responses between subnuclei. We therefore conclude that horizontal brainstem slices will be a useful preparation for studies on intersubnuclear synaptic transmission, modulation and plasticity between subnuclei, as well as, further, other brainstem nuclei.

Immunohistochemical localisation of the voltage gated potassium ion channel subunit Kv3.3 in the rat medulla oblongata and thoracic spinal cord

Voltage gated K+ channels (Kv) are a diverse group of channels important in determining neuronal excitability. The Kv superfamily is divided into 12 subfamilies (Kv1-12) and members of the Kv3 subfamily are highly abundant in the CNS, with each Kv3 gene (Kv3.1-Kv3.4) exhibiting a unique expression pattern. Since the localisation of Kv subunits is important in defining the roles they play in neuronal function, we have used immunohistochemistry to determine the distribution of the Kv3.3 subunit in the medulla oblongata and spinal cord of rats. Kv3.3 subunit immunoreactivity (Kv3.3-IR) was widespread but present only in specific cell populations where it could be detected in somata, dendrites and synaptic terminals. Labelled neurones were observed in the spinal cord in laminae IV and V, in the region of the central canal and in the ventral horn. In the medulla oblongata, labelled cell bodies were numerous in the spinal trigeminal, cuneate and gracilis nuclei whilst rarer in the lateral reticular nucleus, hypoglossal nucleus and raphe nucleus. Regions containing autonomic efferent neurones were predominantly devoid of labelling with only occasional labelled neurones being observed. Dual immunohistochemistry revealed that some Kv3.3-IR neurones in the ventral medullary reticular nucleus, spinal trigeminal nucleus, dorsal horn, ventral horn and central canal region were also immunoreactive for the Kv3.1b subunit. The presence of Kv3.3 subunits in terminals was confirmed by co-localisation of Kv3.3-IR with the synaptic vesicle protein SV2, the vesicular glutamate transporter VGluT2 and the glycine transporter GlyT2. Co-localisation of Kv3.3-IR was not observed with VGluT1, tyrosine hydroxylase, serotonin or choline acetyl transferase. Electron microscopy confirmed the presence of Kv3.3-IR in terminals and somatic membranes in ventral horn neurones, but not motoneurones. This study provides evidence supporting a role for Kv3.3 subunits in regulating neuronal excitability and in the modulation of excitatory and inhibitory synaptic transmission in the medulla oblongata and spinal cord.

Postnatal development of excitation propagation in the trigeminal subnucleus caudalis evoked by afferent stimulation in mice

The postnatal development of nociceptive afferent activity expansion and its modulation features were examined in mice using an optical imaging technique. Developing mice (1-2 weeks old (N1-2 w), 3-4 weeks old (N3-4 w), 5-6 weeks old (N5-6 w) and 7-8 weeks old (N7-8 w)) and neonatally capsaicin-treated mice were used. The propagation of neuronal excitation was measured by changes in fluorescent intensity in horizontal brain stem slices evoked by electrical stimulation to the trigeminal spinal tract. A single-pulse stimulation evoked excitation propagation in the trigeminal caudalis (Vc). The propagation area was larger in N1-2 w than in N7-8 w, and no differences were observed between capsaicin-treated and naive mice in the same age groups. Repetitive stimulation (100 Hz, 30 pulses) elicited long-lasting and widespread excitation propagation. The excitation propagation area was significantly larger in N7-8 w than in N1-2 w, N3-4 w and N5-6 w. This propagation was suppressed by 5 microM L-703.606, an NK1-receptor antagonist, suggesting that the repetitive stimulation-elicited excitation may require substance-P releases. Morphological observations demonstrated that the neural network in the Vc had grown by postnatal week 5. These results suggest that nociceptive afferent activity co-operatively matures with development of the network structure in the Vc, and that a mechanism for prolonged increase in central excitability is established during a later postnatal period.

Spinal dorsal horn neurone targets for nociceptive primary afferents: do single neurone morphological characteristics suggest how nociceptive information is processed at the spinal level

It has become increasingly clear that nociceptive information is signalled by several anatomically distinct populations of primary afferents that target different populations of neurones in the spinal cord. It is probable that these different systems all give rise to the sensation pain and hence, an understanding of their separate roles and the processes that they employ, may offer ways of selectively targeting pain arising from different causes. The review focuses on what is known of the anatomy of neurones in LI-III of the spinal dorsal horn that are implicated in nociception. The dendritic geometry and synaptic input of the large LI neurones that receive input from primary afferents containing substance P that express neurokinin 1 (NK(1)) receptors suggests that these neurones may monitor the extent of injury rather than the specific localisation of a discrete noxious stimulus. This population of neurones is also critically involved in hyperalgesia. In contrast neurones in LII with the morphology of stalked cells that receive primary afferent input from glomerular synapses may be more suitable for fine discrimination of the exact location of a noxious event such as a sting or parasite attack. The review focuses as far as possible on precisely defined anatomy in the belief that only by understanding these anatomical relationships will we eventually be able to interpret the complex processes occurring in the dorsal horn. The review attempts to be an accessible guide to a sometimes complex and highly specialised literature in this field.

Spinal lamina I neurones that express neurokinin 1 receptors: II. Electrophysiological characteristics, responses to primary afferent stimulation and effects of a selective mu-opioid receptor agonist

Intracellular recordings were made from neurones in laminae I and II of the dorsal horn of a longitudinal, parasagittal spinal cord slice from the neonatal rat. Their responses to peripheral nerve stimulation were first tested. Then the responses to bath application of [Sar(9),Met(O(2))(11)]-substance P and [D-Ala(2),N-MePhe(4),Gly-ol(5)]-enkephalin, neurokinin 1 (NK(1)) and mu-opioid receptor agonists respectively, were studied. Finally, the structure of each neurone was investigated by injecting neurobiotin intracellularly following recording, and immunocytochemical studies were performed on post-fixed tissues to reveal whether they expressed the NK(1) receptor. Nine lamina I neurones where shown to express NK(1) receptor and these were depolarised by [Sar(9),Met(O(2))(11)]-substance P. These neurones typically received a powerful C-fibre input that was strongly inhibited, presynaptically, by the mu-opioid receptor agonist.The structure, afferent input, opioid sensitivity and intrinsic properties of these neurones are all consistent with the view that they are a major relay for nociceptive information leading to intense pain. The characteristics of 10 other neurones studied in which the NK(1) receptor was not found to be expressed at levels detectable by immunocytochemistry are briefly described for comparison. These results contribute to the emergent view that the large neurones in the most dorsal neuronal layer (lamina I) of the spinal cord, which express the principal receptor for substance P (NK(1)) over their entire soma and dendrites, are a major relay for information leading to intense pain. Inhibition of the relay of information by these neurones would be predicted to result in analgesia and hence, a detailed knowledge of their unique neurochemical characteristics is of paramount importance.

Lamina-specific membrane and discharge properties of rat spinal dorsal horn neurones in vitro

Membrane and discharge properties determine the input-output relationship of neurones and are therefore of paramount importance for the functions of neural circuits. Here, we have tested the hypothesis that neurones in different laminae of the spinal dorsal horn differ in their electrophysiological properties. Whole-cell patch-clamp recordings from dorsal horn neurones in a rat transverse spinal cord slice preparation were used to record active and passive membrane properties. Neurones from superficial dorsal horn laminae had higher membrane resistances and broader action potentials than deep dorsal horn neurones. Action potential thresholds were highest in lamina II neurones, representing low membrane excitability. Five types of firing patterns were identified in response to depolarising current injections. Tonic-firing neurones discharged action potentials at regular intervals throughout the current pulse. Delayed-firing neurones showed a delayed onset of firing in response to current injections that was due to activation of a transient voltage-dependent outward current, presumably an A-current. Another group of neurones fired a short initial burst of action potentials. Single-spiking neurones discharged only one action potential at the onset of a depolarising pulse. Phasic-bursting neurones showed irregular bursts of action potentials. Firing patterns were unequally distributed among laminae. Tonic-firing neurones were numerous in lamina I and deeper laminae but were not found in lamina II. Delayed-firing neurones were encountered in laminae I and II but not in deeper laminae. Most of the neurones showing an initial burst were found in lamina II. These differences in membrane and discharge properties probably contribute to lamina-specific processing of sensory, including nociceptive, information.

Ascending connections from the caudal part to the oral part of the spinal trigeminal nucleus in the rat

The brainstem trigeminal somatosensory complex, while sharing many common aspects with the spinal somatosensory system, displays features specific to orofacial information processing. One of those is the redundant representation of peripheral structures within the various subnuclei of the complex. A functional redundancy also exists since a single sensory modality, e.g. nociception, may be processed within different subnuclei. In the present study, we addressed the question whether anatomical connections from the caudal part to the oral part of the spinal trigeminal nucleus may support topographical and functional redundancy within the rat trigeminal somatosensory complex. The retrograde tracer tetramethylrhodamine-dextran was injected iontophoretically into the oral subnucleus of anaesthetised rats. Cell bodies labelled retrogradely from the oral subnucleus were observed in laminae III-IV and V of the ipsilateral caudal subnucleus consistently, and to a lesser degree in lamina I. Such a distribution of retrogradely labelled cells suggested that specific subsets of neurones may relay nociceptive information, and others non-nociceptive information. Furthermore, intratrigeminal connections conserved the somatotopic distribution of primary afferents in the two subnuclei. First, injections of tracer in the dorsomedial and ventrolateral parts of the oral subnucleus resulted in retrograde labelling of the dorsal and ventral parts of the caudal subnucleus respectively. Second, animals that received tracer into the ventrolateral oral subnucleus displayed more caudal labelling than animals that were injected into the dorsomedial oral subnucleus. These findings show the existence of anatomical connections from the caudal part to the oral part of the spinal trigeminal nucleus in the rat. The connections conserve the somatotopic distribution of primary afferents in the two subnuclei. They provide an anatomical substrate for the indirect activation of trigeminal oral subnucleus neurones by somatosensory stimuli through the caudal subnucleus.

Synaptic connections between trigemino-parabrachial projection neurons and gamma-aminobutyric acid- and glycine-immunoreactive terminals in the rat

The synaptic connections between gamma-aminobutyric acid (GABA)- and glycine-immunoreactive terminals and neurons projecting to the lateral parabrachial region were examined by a combination of retrograde tracing and immunohistochemical staining in the rat medullary dorsal horn. After injection of horseradish peroxidase (HRP) into the right lateral parabrachial region, HRP retrogradely labeled neurons were observed bilaterally in laminae I, II and III of the medullary dorsal horn with an ipsilateral predominance. GABA- and glycine-like immunoreactive terminals were found in laminae I, II and III. Some of these GABA- and glycine-like immunoreactive terminals were observed chiefly to make symmetric synapses with HRP-labeled neuronal cell bodies and dendritic processes. The present results indicate that neurons in the medullary dorsal horn projecting to the lateral parabrachial region might be modulated by GABAergic and glycinergic inhibitory intrinsic neurons, which might be significantly involved in the regulation of the noxious information transmission.

Morphological features and electrophysiological properties of serotonergic and non-serotonergic projection neurons in the dorsal raphe nucleus. An intracellular recording and labeling study in rat brain slices

The morphology and electrophysiological properties of serotonergic and non-serotonergic projection neurons in the dorsal raphe nucleus (DRN) of the rat were examined in frontal brain slices. Biocytin was injected intracellularly into the intracellularly recorded neurons. Then the morphology of the recorded neurons was observed after histochemical visualization of biocytin. The recorded neurons extending their main axons outside the DRN were considered as projection neurons. Subsequently, serotonergic nature of the neurons was examined by serotonin (5-HT) immunohistochemistry. The general form of the dendritic trees is radiant and poorly branching in both 5-HT- and non-5-HT neurons. However, the dendrites of the 5-HT neurons were spiny, whereas those of the non-5-HT neurons were aspiny. The main axons of both 5-HT- and non-5-HT neurons were observed to send richly branching axon collaterals to the DRN, ventrolateral part of the periaqueductal gray and the midbrain tegmentum. In response to weak, long depolarizing current pulses, the 5-HT neurons displayed a slow and regular firing activity. The non-5-HT neurons fired at higher frequencies even when stronger current was injected. Some other differences in electrophysiological properties were also observed between the 5-HT-immunoreactive spiny projection neurons and the 5-HT-immunonegative aspiny projection neurons.