Neurokinin actions on substantia gelatinosa neurones in an adult longitudinal spinal cord preparation

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.

Neurotrophins in spinal cord nociceptive pathways

Neurotrophins are a well-known family of growth factors for the central and peripheral nervous systems. In the course of the last years, several lines of evidence converged to indicate that some members of the family, particularly NGF and BDNF, also participate in structural and functional plasticity of nociceptive pathways within the dorsal root ganglia and spinal cord. A subpopulation of small-sized dorsal root ganglion neurons is sensitive to NGF and responds to peripheral NGF stimulation with upregulation of BDNF synthesis and increased anterograde transport to the dorsal horn. In the latter, release of BDNF appears to modulate or even mediate nociceptive sensory inputs and pain hypersensitivity. We summarize here the status of the art on the role of neurotrophins in nociceptive pathways, with special emphasis on short-term synaptic and intracellular events that are mediated by this novel class of neuromessengers in the dorsal horn. Under this perspective we review the findings obtained through an array of techniques in naïve and transgenic animals that provide insight into the modulatory mechanisms of BDNF at central synapses. We also report on the results obtained after immunocytochemistry, in situ hybridization, and monitoring intracellular calcium levels by confocal microscopy, that led to hypothesize that also NGF might have a direct central effect in pain modulation. Although it is unclear whether or not NGF may be released at dorsal horn endings of certain nociceptors in vivo, we believe that these findings offer a clue for further studies aiming to elucidate the putative central effects of NGF and other neurotrophins in nociceptive pathways.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Small sensory neurons in the rat dorsal root ganglia express functional NK-1 tachykinin receptor

The tachykinins substance P (SP) and neurokinin A, released by the C-type primary afferent fibre terminals of the small dorsal root ganglion (DRG) neurons, play important roles in spinal nociception. By means of non-radioactive in situ hybridization and whole-cell recording, we showed that the small rat DRG neurons also express the NK-1 tachykinin receptor. In situ hybridization demonstrated that the positive neurons in rat DRG sections were mainly small cells (85.9%) with diameters less than 25 microm. The remaining positive neurons (14.1%) were cells with medium diameters between 26 and 40 microm. No positive large neurons (diameters > 40 microm) were observed. Expression in small DRG neurons (diameter < 21 microm) was confirmed by in situ hybridization of isolated cells, which were demonstrated to express NK-1 receptor mRNA at a very high frequency (> 90% of small DRG neurons) and therefore were subjected to whole-cell recording. In 57 of 61 cells recorded, SP or the selective NK-1 receptor agonist [Sar9, Met(O2)11]SP (Sar-SP, 1 or 2 microM) produced a delayed vibrating inward current (50-300 nA) with a long duration of 0.5-2 h. These currents were blocked by co-application of the NK-1 receptor antagonist L-668, 169 (1 microM), but were not affected by the NK-2 antagonist L-659, 877 (2 microM). Both current-clamp recording and cell-attached single-channel recording demonstrated that the long-lasting response was due to the opening of a channel with an inward current. Employment of non-Ca2+ and Ca2+ + choline solutions revealed that this channel might be a Ca2+-permeable, non-selective cation channel. The prolonged NK-1 tachykinin response exhibited extreme desensitization. This work suggests that presynaptic NK-1 autoreceptors may be present on the primary afferent terminals in the spinal cord, where they could contribute to the chronic pain and hyperalgesia.

Profile of neuronal excitation following selective activation of the neurokinin-1 receptor in rat deep dorsal horn in vitro

The excitatory actions of the selective neurokinin-1 receptor (NK1R) agonist [Sar9,Met(O2)11]substance P (SP) were tested on a sample (n = 50) of deep dorsal horn neurones in the isolated and hemisected young rat spinal cord. Superfusion of the NK1R agonist (2 microM) elicited a prolonged membrane depolarisation (6.6 +/- 0.5 mV) and an increase in action potential firing in 41/50 (82%) neurones. These [Sar9,Met(O2)11]SP-induced depolarisations were attenuated by the selective NK1R antagonist GR82334 (1 microM). An increased neuronal excitability after [Sar9,Met(O2)11]SP application was indicated by an augmented spike frequency generated in response to long duration, step depolarisations. In order to assess whether a direct excitatory action existed, [Sar9,Met(O2)11]SP was re-tested on a sample of TTX-treated neurones (n = 14). The majority (9/14) retained agonist sensitivity although the amplitude of the depolarisation was reduced to 48% of the control value. A sample of neurones (n = 7) that responded to the NK1R agonist were morphologically characterised after filling with the intracellular dye, biocytin. Dorsal dendrites that clearly penetrated lamina II and that could receive a direct C-afferent input, were identified in only 2/7 neurones. These electrophysiological and neuroanatomical data demonstrate that deep dorsal horn neurones possess functional NK1Rs. The implications of the existence of these NK1Rs in the context of spinal somatosensory systems and SP is considered.

Relationship of NK3 receptor-immunoreactivity to subpopulations of neurons in rat spinal cord

The distribution of immunoreactivity to the neurokinin3 receptor (NK3R) was examined in segments C7, T11-12, L1-2, and L4-6 of the rat spinal cord. NK3R immunoreactivity was visualized by using two antisera generated against sequences of amino acids contained in the C-terminal region of the NK3R. NK3R-immunoreactive cells were numerous in the substantia gelatinosa of all spinal segments examined as well as the dorsal commissural nucleus of spinal segments L1-2. Isolated, immunoreactive cells were scattered throughout other regions of the spinal cord. The relationship of NK3R-immunoreactivity with neurons was demonstrated by colocalization with microtubule associated protein 2-immunoreactivity in individual cells. Within neurons, NK3R-immunoreactivity was associated predominately with the plasma membrane of cell bodies and dendrites. Within the substantia gelatinosa, 86% of nitric oxide synthase (NOS)-immunoreactive neurons were also NK3R-immunoreactive. Although NOS-immunoreactive neurons were found throughout all other regions of the spinal cord in the segments examined, these were not NK3R-immunoreactive. When preganglionic sympathetic neurons in spinal segments T11-12 and L1-2 were visualized by intraperitoneal injection of Fluorogold, less than 1% of the Fluorogold-labeled neurons were also immunoreactive for NK3R. The large number of NK3R-immunoreactive neurons in the substantia gelatinosa suggests that some effects of tachykinins on somatosensation may be mediated by NK3R.

Tachykinin actions on deep dorsal horn neurons in vitro: an electrophysiological and morphological study in the immature rat

To assess whether functional neurokinin receptors exist in the deep dorsal horn of the rat, the actions of the selective neurokinin-1 receptor (NK1R) agonist [Sar9,Met(O2)11]substance P ([Sar9,Met(O2)11]SP), the neurokinin-2 receptor (NK2R) agonists [beta-Ala8]NKA(4-10) and GR64349 and the neurokinin-3 receptor (NK3R) agonist senktide were examined intracellularly in vitro. [Sar9,Met(O2)11]SP (1-4 microM) and senktide (1-2 microM) elicited slow depolarizations (<10 mV) associated with increased synaptic activity and cell firing. [beta-Ala8]NKA(4-10) (10-20 microM) and GR64349 (0.25-10 microM) caused small depolarizations (<2.0 mV) and no firing. Neurons were categorized as either 'tonic' or 'phasic' depending on their firing response to direct current step depolarizations. Tonic neurons, which, unlike phasic neurons, display no spike firing accommodation, generated a significantly larger depolarization to the NK1R and NK3R agonists. The putative contribution of these receptors to primary afferent-mediated synaptic transmission was assessed by testing the NK1R antagonist GR82334 (1 microM), the NK2R antagonist MEN10,376 (1 microM) and the NK3R antagonist [Trp7,beta-Ala8]NKA(4-10) (1 microM) against the dorsal root-evoked excitatory postsynaptic potential (DR-EPSP). GR82334 and [Trp7,beta-Ala8]NKA(4-10) significantly reduced (P < or = 0.05) the duration but not the amplitude of the DR-EPSP. MEN10,376 (1 microM) had no effect on DR-EPSP amplitude or duration. Morphological detail was obtained for seven biocytin-filled deep dorsal horn neurons tested with [Sar9,Met(O2)11]SP. Five neurons responded to the NK1R agonist, and two of these had dorsally directed dendrites into the substantia gelatinosa. The other three [Sar9,Met(O2)11]SP-sensitive neurons had dendrites within deeper laminae. These data support the existence of functional NK1Rs and NK3Rs in the deep dorsal horn which may be involved in mediating sensory afferent inputs from nociceptors.

The role of spinal neurokinin-2 receptors in the processing of nociceptive information from the joint and in the generation and maintenance of inflammation-evoked hyperexcitability of dorsal horn neurons in the rat

In spinal cord neurons in anesthetized rats, the role on neurokinin A and neurokinin-2 receptors in the processing of nociceptive information from the knee joint was studied. The specific non-peptide antagonist at the neurokinin-2 receptor, SR48968, its inactive R-enantiomer, SR48965, neurokinin A, substance P and (RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), were administered ionophoretically close to neurons with input from the knee joint. SR48968 reduced the effects of exogenous neurokinin A, but not those of exogenous substance P and AMPA, indicating selective blockade of neurokinin-2 receptors. In most neurons with input from the normal knee joint, SR48968 reduced dose-dependently the responses to noxious pressure with applied to the knee, and in approximately 50% of the neurons the responses to innocuous pressure. The administration of SR48968 during the induction of an experimental joint inflammation markedly attenuated the development of inflammation-evoked hyperexcitability. In hyperexcitable neurons with input from the inflamed joint, SR48968 reduced the responses to noxious and innocuous pressure. The relative reduction of the responses was more pronounced than in neurons with input from the normal joint. None of the effects of SR48968 was mimicked by SR48965. These data show that neurokinin-2 receptors are involved in the spinal processing of nociceptive information from the normal joint. Furthermore, neurokinin-2 receptors must be coactivated at an early stage of inflammation, to allow the generation of hyperexcitability. Finally, neurokinin-2 receptors are involved in maintenance of hyperexcitability during inflammation. In summary, spinal neurokinin-2 receptors are important in the generation of pain in the normal and inflamed joint.