Light- and electron-microscopic analysis of neuropeptide Y-immunoreactive profiles in the cat spinal dorsal horn

The therapeutic potential of neuropeptide Y in central nervous system disorders with special reference to pain and sympathetically maintained pain

Neuropeptide Y (NPY), a widely distributed peptide, has been shown to have numerous effects in both the central and peripheral nervous systems. In particular, NPY has an important role in mediating analgesia and hyperalgesia by distinct central and peripheral mechanisms. At least six NPY receptor subtypes are known to exist and the development of subtype-specific ligands targeted at NPY receptors will offer novel therapeutic agents. This article will review the involvement of NPY in diverse pathologies of the nervous system, including pain, and will propose a role for NPY in the maintenance of sympathetically maintained pain.

Synaptic organization of tooth pulp afferent terminals in the rat trigeminal sensory nuclei

Previous studies provide evidence that a structure/function correlation exists in the distinct zones of the trigeminal sensory nuclei. To evaluate this relationship, we examined the ultrastructure of afferent terminals from the tooth pulp in the rat trigeminal sensory nuclei: the principalis (Vp), the dorsomedial part of oral nucleus (Vdm), and the superficial layers of caudalis (Vc), by using transganglionic transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). A total of 93 labeled boutons were serially sectioned, in which some sections were incubated with gamma-aminobutyric acid (GABA) antiserum. Almost all labeled boutons formed asymmetric contact with nonprimary dendrites, in which more than half of labeled boutons in the Vc made synapses with their spines. The labeled boutons could be divided into two types on the basis of numbers of dense-cored vesicles (DCVs) in a boutons: S-type and DCV-type. Almost all labeled boutons in the Vp and Vdm were S-type, whereas two types were distributed evenly in the Vc. In contrast to DCV-type boutons, the S-type was frequently postsynaptic to unlabeled axon terminals containing a mixture of round, oval, and flattened vesicles (p-endings) and forming symmetrical synapses. Most p-endings examined were immunoreactive to GABA. The frequency of axoaxonic contacts was higher for labeled boutons in the Vp than in the Vdm and Vc. These results suggest that the three structures of trigeminal sensory nuclei serve distinct functions in nociceptive processing.

Selective mediation of nerve injury-induced tactile hypersensitivity by neuropeptide Y

Prevention of nerve injury-induced tactile, but not thermal, hypersensitivity is achieved by ipsilateral lesions of the dorsal columns or lidocaine microinjection into the nucleus gracilis (n. gracilis). These and other data support the possibility that tactile hyperresponsiveness after nerve injury may be selectively mediated by a low-threshold myelinated fiber pathway to the n. gracilis. Here we identify a transmitter that might selectively mediate such injury-induced tactile hypersensitivity. Neuropeptide Y (NPY), normally not detected in the dorsal root ganglion (DRG) or in the n. gracilis of rats, became markedly upregulated at both sites and in the spinal cord after spinal nerve injury. Injury-induced NPY-IR occurred predominately in large-diameter DRG cells, and the NPY-IR in the n. gracilis was blocked by dorsal rhizotomy or dorsal column lesion. NPY microinjection into the n. gracilis of uninjured rats elicited reversible tactile, but not thermal, hypersensitivity only in the ipsilateral hindpaw. Administration of anti-NPY antiserum, but not control serum or preabsorbed serum, into the n. gracilis ipsilateral to nerve injury reversed tactile, but not thermal, hypersensitivity. Similarly, microinjection of the NPY antagonists NPY(18-36) and (R)-N-[[4-(aminocarbonylaminomethyl)-phenyl]methyl]-N2-(diphenylacetyl)-argininamide trifluoroacetate, into the n. gracilis ipsilateral to the injury reversed tactile, but not thermal, hypersensitivity. Antagonist administration into the contralateral n. gracilis had no effect on injury-induced hypersensitivity. These data suggest the selective mediation of nerve injury-induced tactile hypersensitivity by upregulated NPY via large fiber input to n. gracilis. Selective reversal of injury-induced tactile allodynia by NPY receptor antagonists would have significant implications for human neuropathic conditions.

The effect of conduction block on the spinal release of immunoreactive-neuropeptide Y (ir-NPY) in the neuropathic rat

Peripheral nerve injury may result in significant changes in neuropeptide production and the development of neuropathic pain behaviour. Rats with a chronic constriction injury of one sciatic nerve were used to study the spinal release of immunoreactive neuropeptide Y (ir-NPY), using the antibody-coated microprobe technique. Previous work has shown an increase in NPY synthesis by large to medium-sized primary afferent neurones, as well as a new area of ir-NPY release in the deep dorsal horn on the side of nerve injury, when compared to uninjured rats. The stimulus for spontaneous ir-NPY release was unclear, but may have been due to ectopic neuronal discharges developing after nerve injury. This study used local anaesthetic to block all electrical input from the injured nerve. No change was found in the new zone of spontaneous release of ir-NPY in the deep dorsal horn ipsilateral to nerve injury. It appears therefore, that ir-NPY is released from the central termination of primary afferent neurones, without regulation from neuronal activity in the primary afferent neurones themselves.

K(+)-Evoked [(3)H]D-aspartate release in rat spinal cord synaptosomes: modulation by neuropeptide Y and calcium channel antagonists

This study was conducted to investigate mechanisms regulating the release of [(3)H]D-aspartate (or endogenous glutamate) in the rat spinal cord. Presynaptic modulation of glutamate release was studied in superfused synaptosomes depolarized with 20 mM KCl. Calcium-channel antagonists, omega-conotoxin GVIA (omega-CgTx GVIA; N-type), nifedipine (L-type), and omega-conotoxin MVIIC (omega-CmTx MVIIC; P/Q type), were used to characterize the voltage-operated Ca(2+) channels (VOCCs) involved in this release. Nifedipine had no significant effect on the K(+)-evoked release of [(3)H]D-aspartate, but the omega-conotoxins GVIA and MVIIC produced dose-dependent inhibitory effects that were additive. The most substantial reduction (54.30% +/- 4.40%) was seen with omega-CgTx GVIA, indicating that N-type channels play a major role in the release of glutamate in this tissue. We investigated the effects of neuropeptide Y (NPY), NPY(13-36), and [Leu(31)][Pro(34)]NPY on Ca(2+)-dependent, K(+)-evoked [(3)H]D-aspartate release. NPY and NPY(13-36) equipotently inhibited the release of glutamate in a concentration-dependent manner. The half-maximal response was observed at about 12 nM; maximal inhibition of 44.22% +/- 4.60% was achieved with 0.3 microM. The selective GABA(B) agonist (-)baclofen inhibited K(+)-evoked [(3)H]D-aspartate release from superfused spinal cord synaptosomes by 50.00% +/- 4.80% at 10 microM. When NPY(13-36) and (-)baclofen were used together at maximal doses, their release-inhibiting effects were not additive. In addition, neither of the agonists was able to enhance the inhibition produced by pretreating the synaptosomes with the selective inhibitor of N-type VOCCs omega-CgTx GVIA. These results are consistent with the hypothesis that presynaptic Y(2)-like and GABA(B) receptors regulate glutamate release by blocking Ca(2+) currents through N-type VOCCs. Characterization of the receptors that can inhibit the release of glutamate may provide useful information for treatment of conditions characterized by excessive glutamatergic transmission in the spinal cord.

GABAergic neurons that contain neuropeptide Y selectively target cells with the neurokinin 1 receptor in laminae III and IV of the rat spinal cord

Neuropeptide Y (NPY) is contained in a population of GABAergic interneurons in the spinal dorsal horn and, when administered intrathecally, can produce analgesia. We previously identified a strong monosynaptic link between substance P-containing primary afferents and cells in lamina III or IV with the neurokinin 1 (NK1) receptor. Because some of these cells belong to the spinothalamic tract, they are likely to have an important role in pain mechanisms. In this study, we used confocal microscopy to examine the input to lamina III/IV NK1 receptor-immunoreactive neurons from NPY-containing axons. All of the cells studied received a dense innervation from NPY-immunoreactive axons, and electron microscopy revealed that synapses were often present at points of contact. Most NPY-immunoreactive boutons were also GABAergic, which supports the suggestion that they are derived from local neurons. The association between NPY-containing axons and NK1 receptor-immunoreactive neurons was specific, because postsynaptic dorsal column neurons (which were located in laminae III-V but did not possess NK1 receptors) and lamina I neurons with the NK1 receptor received significantly fewer contacts from NPY-immunoreactive axons. In addition, the NK1 receptor-immunoreactive lamina III/IV cells received few contacts from nitric oxide synthase-containing axons (which belong to a different population of GABAergic dorsal horn neurons). The NPY-containing axons appeared to be targeted to the NK1 receptor-immunoreactive neurons themselves rather than to their associated substance P-immunoreactive inputs. The dense innervation of these cells by NPY-containing axons suggests that they may possess receptors for NPY and that activation of these receptors may contribute to NPY-mediated analgesia.