Two major distinct subpopulations of neurokinin-3 receptor-expressing neurons in the superficial dorsal horn of the rat spinal cord

Molecular Anatomy of Synaptic and Extrasynaptic Neurotransmission Between Nociceptive Primary Afferents and Spinal Dorsal Horn Neurons

Sensory signals generated by peripheral nociceptors are transmitted by peptidergic and nonpeptidergic nociceptive primary afferents to the superficial spinal dorsal horn, where their central axon terminals establish synaptic contacts with secondary sensory spinal neurons. In the case of suprathreshold activation, the axon terminals release glutamate into the synaptic cleft and stimulate postsynaptic spinal neurons by activating glutamate receptors located on the postsynaptic membrane. When overexcitation is evoked by peripheral inflammation, neuropathy or pruritogens, peptidergic nociceptive axon terminals may corelease various neuropeptides, neurotrophins and endomorphin, together with glutamate. However, in contrast to glutamate, neuropeptides, neurotrophins and endomorphin are released extrasynaptically. They diffuse from the site of release and modulate the function of spinal neurons via volume transmission, activating specific extrasynaptic receptors. Thus, the released neuropeptides, neurotrophins and endomorphin may evoke excitation, disinhibition or inhibition in various spinal neuronal populations, and together with glutamate, induce overall overexcitation, called central sensitization. In addition, the synaptic and extrasynaptic release of neurotransmitters is subjected to strong retrograde control mediated by various retrogradely acting transmitters, messengers, and their presynaptic receptors. Moreover, the composition of this complex chemical apparatus is heavily dependent on the actual patterns of nociceptive primary afferent activation in the periphery. This review provides an overview of the complexity of this signaling apparatus, how nociceptive primary afferents can activate secondary sensory spinal neurons via synaptic and volume transmission in the superficial spinal dorsal horn, and how these events can be controlled by presynaptic mechanisms.

A neuropeptide code for itch

Itch is one of the most primal sensations, being both ubiquitous and important for the well-being of animals. For more than a century, a desire to understand how itch is encoded by the nervous system has prompted the advancement of many theories. Within the past 15 years, our understanding of the molecular and neural mechanisms of itch has undergone a major transformation, and this remarkable progress continues today without any sign of abating. Here I describe accumulating evidence that indicates that itch is distinguished from pain through the actions of itch-specific neuropeptides that relay itch information to the spinal cord. According to this model, classical neurotransmitters transmit, inhibit and modulate itch information in a context-, space- and time-dependent manner but do not encode itch specificity. Gastrin-releasing peptide (GRP) is proposed to be a key itch-specific neuropeptide, with spinal neurons expressing GRP receptor (GRPR) functioning as a key part of a convergent circuit for the conveyance of peripheral itch information to the brain.

Identifying functional populations among the interneurons in laminae I-III of the spinal dorsal horn

The spinal dorsal horn receives input from primary afferent axons, which terminate in a modality-specific fashion in different laminae. The incoming somatosensory information is processed through complex synaptic circuits involving excitatory and inhibitory interneurons, before being transmitted to the brain via projection neurons for conscious perception. The dorsal horn is important, firstly because changes in this region contribute to chronic pain states, and secondly because it contains potential targets for the development of new treatments for pain. However, at present, we have only a limited understanding of the neuronal circuitry within this region, and this is largely because of the difficulty in defining functional populations among the excitatory and inhibitory interneurons. The recent discovery of specific neurochemically defined interneuron populations, together with the development of molecular genetic techniques for altering neuronal function in vivo, are resulting in a dramatic improvement in our understanding of somatosensory processing at the spinal level.

Neuronal circuitry for pain processing in the dorsal horn

Neurons in the spinal dorsal horn process sensory information, which is then transmitted to several brain regions, including those responsible for pain perception. The dorsal horn provides numerous potential targets for the development of novel analgesics and is thought to undergo changes that contribute to the exaggerated pain felt after nerve injury and inflammation. Despite its obvious importance, we still know little about the neuronal circuits that process sensory information, mainly because of the heterogeneity of the various neuronal components that make up these circuits. Recent studies have begun to shed light on the neuronal organization and circuitry of this complex region.

The neuropeptide tyrosine Y1R is expressed in interneurons and projection neurons in the dorsal horn and area X of the rat spinal cord

The localization of the neuropeptide tyrosine Y1 receptor was studied with immunohistochemistry in parasagittal and transverse, free-floating sections of the rat lumbar spinal cord. At least seven distinct Y1 receptor-positive populations could tentatively be recognized: Type 1) abundant small, fusiform Y1 receptor-positive neurons in laminae I-II, producing a profuse neuropil; Type 2) Y1 receptor-positive projection neurons in lamina I; Type 3) small Y1 receptor-positive neurons in lamina III, similar to Type 1 neurons, but less densely packed; Type 4) a number of large, multipolar Y1 receptor-positive neurons in the border area between laminae III-IV, with dendrites projecting toward laminae I-II; Type 5) a considerable number of large, multipolar Y1 receptor-positive neurons in laminae V-VI; Type 6) many large Y1 receptor-positive neurons around the central canal (area X); and Type 7) a small number of large Y1 receptor-positive neurons in the medial aspect of the ventral horns (lamina VIII). Many of the neurons present in laminae V-VI and area X produce craniocaudal processes extending for several hundred micrometers. Retrograde tracing using cholera toxin B subunit injected at the 9th thoracic spinal cord level shows that several Type 5 neurons in laminae V-VI, and at least a few Type 2 in lamina I and Type 6 in area X have projections extending to the lower segments of the thoracic spinal cord (and perhaps to supraspinal levels). The present results define distinct subpopulations of neuropeptide tyrosine-sensitive neurons, localized in superficial and deep layers of the dorsal, in the ventral horns and in area X. The lamina II neurons express somatostatin [The neuropeptide Y Y1 receptor is a somatic receptor on dorsal root ganglion neurons and a postsynaptic receptor on somatostatin dorsal horn neurons. Eur J Neurosci 11:2211-2225] and are presumably glutamatergic [Todd AJ, Hughes DI, Polgar E, Nagy GG, Mackie M, Ottersen OP, Maxwell DJ (2003) The expression of vesicular glutamate transporters VGLUT1 and VGLUT2 in neurochemically defined axonal populations in the rat spinal cord with emphasis on the dorsal horn. Eur J Neurosci 17:13-27], that is they are excitatory interneurons under a Y1 receptor-mediated inhibitory influence. The remaining Y1 receptor-positive spinal neurons need to be phenotyped, for example if the large Y1 receptor-positive laminae III-IV neurons (Type 5) are identical to the neurokinin (NK)1R-positive neurons previously shown to receive neuropeptide tyrosine positive dendritic contacts [Polgár E, Shehab SA, Watt C, Todd AJ (1999) GABAergic neurons that contain neuropeptide Y selectively target cells with the NK1 receptor in laminae III and IV of the rat spinal cord. J Neurosci 19:2637-2646]. If so, neuropeptide tyrosine could have an antinociceptive action not only via Y1 receptor-positive interneurons (Type 1) but also projection neurons. The present results show neuropeptide tyrosine-sensitive neuron populations virtually in all parts of the lumbar spinal cord, suggesting a role for neuropeptide tyrosine signaling in many spinal functions, including pain.

Characterization of neurons that express preprotachykinin B in the dorsal horn of the rat spinal cord

Although it is established that neurokinin B is expressed by some neurons in laminae I-III of the rat spinal dorsal horn, little is known about the proportions of cells in these laminae that express neurokinin B, or whether these are excitatory or inhibitory neurons. Neurokinin B is derived from preprotachykinin B, and we have used an antibody against preprotachykinin B to address these issues. We found that preprotachykinin B-immunoreactive neurons were present throughout laminae I-III, constituting 10-11% of the neuronal population in laminae I-II, and 4% of that in lamina III. They formed a prominent band in the ventral half of lamina II (where they made up 16% of the population) and the dorsalmost part of lamina III. The great majority (99%) of preprotachykinin B-immunoreactive axonal boutons contained the vesicular glutamate transporter 2, while none contained glutamic acid decarboxylase. Since most of these boutons are likely to be derived from local preprotachykinin B-expressing cells, these observations suggest that most of the latter are excitatory interneurons. Although 9% of preprotachykinin B-labeled axonal varicosities were substance P-immunoreactive, none contained calcitonin gene-related peptide, which is consistent with reports that neurokinin B is not expressed by primary afferent axons. Many of the preprotachykinin B-immunoreactive cells contained compounds that are present in putative excitatory neurons in laminae I-III: calbindin (84%), protein kinase Cgamma (76%) or somatostatin (31%). However, there was little or no overlap between preprotachykinin B and three other markers associated with excitatory neurons in these laminae: the mu opioid receptor MOR-1, the neurokinin 1 receptor and neurotensin. These results suggest that neurokinin B is expressed by specific populations of excitatory neurons in the superficial dorsal horn. By examining expression of Fos protein in response to intraplantar injection of formaldehyde we provide evidence that many of the preprotachykinin B cells in lamina I and the outer part of lamina II respond to noxious stimulation.

NK1 receptor activation leads to enhancement of inhibitory neurotransmission in spinal substantia gelatinosa neurons of mouse

Substance P (SP) is a well-established pain messenger in the spinal cord, although its role in substantia gelatinosa (lamina II) still remains elusive. We carried out patch-clamp recordings on lamina II neurons from transverse mouse spinal cord slices (P8-12), using the selective NK1 receptor agonist [Sar9,Met(O2)11]-SP (SM-SP, 3-5 microM) in the presence of NBQX. Activation of NK1 receptors was confirmed after pre-incubation with selective NK1 antagonist L732,138 (4 microM) that consistently blocked the effects of SM-SP (nine neurons). After SM-SP challenge and spontaneous inhibitory post-synaptic current (sIPSC) analysis, 50% of recorded neurons (15 out of 30) were found to display a transient increase in frequency; in five neurons this was also associated with increase of peak amplitude. Five out of eight neurons displayed pure GABAA microM) receptor-mediated sIPSCs, whereas the remaining ones showed mixed GABAergic/glycinergic events. After miniature IPSC analysis, a significant increase in frequency was observed in three out of 14 SM-SP responsive neurons. At least four different morphological types were apparent among NK1-responsive neurons after filling with Lucifer Yellow/biocytin: fusiform with dorso-ventral dendritic arbors (i); round-to-oval with dendritic arborization mainly directed to lamina I (ii) or III (iii), and round-to-oval with dendrites sparsely distributed all around the cell body (iv). Thus, there was no correlation between morphology and electrophysiological properties of responsive neurons. Our observations provide new insights on the processing of sensory neurotransmission in spinal cord, and indicate that activation of NK1 receptors is involved in the maintenance of the inhibitory tone of substantia gelatinosa interneurons.

Intestinal anti-nociceptive behaviour of NK3 receptor antagonism in conscious rats: evidence to support a peripheral mechanism of action

The involvement of neurokinin receptors in visceral nociception is well documented. However, the role and localization of NK3 receptors is not clearly established. This study was designed to determine whether NK3 receptor antagonists crossing (talnetant) or not (SB-235375) the blood-brain barrier reduce the nociceptive response to colo-rectal distension (CRD) and whether NK3 antagonism reduces inflammation- or stress-induced hypersensitivity to rectal distension. Isobaric CRD and isovolumic rectal distensions were performed in rats equipped with intramuscular electrodes to record abdominal muscle contractions. In controls, CRD induced a pressure-related (15-60 mmHg) increase in the number of abdominal contractions. Both talnetant and SB-235375 [50 mg x kg-1, per oral (p.o.)], which had no effect on colo-rectal tone, reduced the number of contractions associated with CRDs from 30 to 60 mmHg. Three days after rectal instillation of TNBS, abdominal contractions were increased for rectal distension volume of 0.4 mL. This effect was not modified by talnetant (30 mg x kg-1, p.o.). Partial restraint stress increased abdominal contractions at all distension volumes (0-1.2 mL). Talnetant (10 mg kg-1, p.o.) abolished the increase observed for 0.8 and 1.2 mL. These results indicate that peripheral NK3 receptor antagonism reduced nociception associated with CRD and hypersensitivity induced by stress but not inflammation.