Neuropeptide Y-immunoreactive terminals form axo-axonic synaptic arrangements in the substantia gelatinosa (lamina II) of the cat spinal dorsal horn

Axo-axonic synapses: Diversity in neural circuit function

The chemical synapse is the principal form of contact between neurons of the central nervous system. These synapses are typically configured as presynaptic axon terminations onto postsynaptic dendrites or somata, giving rise to axo-dendritic and axo-somatic synapses, respectively. Beyond these common synapse configurations are less-studied, non-canonical synapse types that are prevalent throughout the brain and significantly contribute to neural circuit function. Among these are the axo-axonic synapses, which consist of an axon terminating on another axon or axon terminal. Here, we review evidence for axo-axonic synapse contributions to neural signaling in the mammalian nervous system and survey functional neural circuit motifs enabled by these synapses. We also detail how recent advances in microscopy, transgenics, and biological sensors may be used to identify and functionally assay axo-axonic synapses.

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.

Inhibitory amino acid transmitters associated with axons in presynaptic apposition to cutaneous primary afferent axons in the cat spinal cord

The purpose of the present study was to characterize the transmitter content of structures in presynaptic apposition to the central terminals of cutaneous afferent fibers in the dorsal horn of the spinal cord. Axons in the Aalphabeta conduction velocity range were identified in adult cats, stained intra-axonally with horseradish peroxidase, and prepared for combined light and electron microscopy. In total, we labeled two slowly adapting (Type 1) axons, two hair-follicle afferents, and one rapidly adapting (Krause) afferent. Ninety-nine labeled boutons were examined through complete series of serial sections. Approximately 80% of boutons originating from rapidly adapting and hair-follicle afferents were postsynaptic to other axons, but only 50% of boutons from slowly adapting axons were associated with this type of arrangement. Postembedding immunogold reactions revealed that between 80% (for slowly adapting axons) and 100% (for rapidly adapting axons) of boutons presynaptic to primary afferents were immunoreactive for gamma-aminobutyric acid (GABA). The vast majority of these terminals (in excess of 80%) were also enriched with glycine. Therefore, presynaptic inhibition of these three functional classes of Aalphabeta cutaneous primary afferents is mediated principally by the subgroup of GABAergic interneuron that also contains glycine.

The effect of intrathecal selective agonists of Y1 and Y2 neuropeptide Y receptors on the flexor reflex in normal and axotomized rats

We have examined the effects of intrathecal (i.t.) administration of [Leu31,Pro34]-neuropeptide Y (NPY) or NPY-(13-36), selective agonists of NPY Y1 or Y2 receptors, respectively, on the excitability of the flexor reflex in normal rats and after unilateral transection of the sciatic nerve. In rats with intact and sectioned sciatic nerves, i.t. [Leu31,Pro34]-NPY induced a similar biphasic effect on the flexor reflex with facilitation at low doses and facilitation followed by depression at high doses. In contrast, i.t. NPY-(13-36) only facilitated the flexor reflex in normal rats, and at high dose it caused ongoing discharges in the electromyogram. NPY-(13-36) caused dose-dependent depression of the flexor reflex in rats after sciatic nerve transection, in addition to its facilitatory effect. Topical application of [Leu31,Pro34]-NPY or NPY-(13-36) caused a moderate and brief reduction in spinal cord blood flow. No difference was noted between the vasoconstrictive effect of [Leu31,Pro34]-NPY and NPY-(13-36). It is suggested that activation of Y1 receptors may be primarily responsible for the reflex depressive effect of i.t. neuropeptide Y in rats with intact sciatic nerves, whereas both Y1 and Y2 receptors may be involved in mediating the depressive effect of NPY after axotomy.

The effect of intrathecal neuropeptide Y on the flexor reflex in rats after carrageenan-induced inflammation

We examined the effects of intrathecal (i.t.) administration of neuropeptide Y (NPY) on the excitability of the flexor reflex in normal rats and 24 h after inflammation induced by subcutaneous carrageenan. In normal rats, i.t. NPY at low doses (10 and 100 ng) caused a brief facilitation of the flexor reflex with no subsequent depression. At higher doses (1 and 10 microg), the effect of NPY was mainly inhibitory, causing substantial and usually prolonged depression of the flexor reflex. At 24 h after the injection of carrageenan, when inflammation was at its peak, the magnitude of the reflex was increased and discharge duration became prolonged. I.t. NPY produced similar pattern of dose-dependent facilitatory and depressive effects on the flexor reflex. The facilitatory effect of i.t. NPY, particularly for the higher doses, was significantly enhanced in inflamed rats compared to normals. In contrast, the depressive effect of high doses of i.t. NPY was unchanged. These data suggest that the changes in levels of NPY and NPY receptors in the spinal cord known to occur after inflammation, are associated with an increased excitatory effect of this peptide.

The release of immunoreactive neuropeptide Y in the spinal cord of the anaesthetized rat and cat

The release of immunoreactive (ir-) neuropeptide Y (NYP) was studied in the anaesthetized rat and cat by means of microprobes bearing immobilized antibodies to the C terminus of NPY. An extensive basal release of ir-NYP was detected throughout the dorsal and upper ventral horn of the rat. This spontaneous release was not significantly altered by sectioning the spinal cord at the thoraco-lumbar junction nor by electrical stimulation of peripheral nerves. Since NPY is virtually absent in primary afferents it is probable that spontaneous release within the spinal cord comes from active NPY-containing intrinsic spinal neurones. In the spinal cat spontaneous release of ir-NPY was detected in the mid-dorsal horn and this was unaltered by peripheral noxious thermal or noxious mechanical stimuli. As in the rat, release from intrinsic spinal neurones is most probable. The extensive spontaneous release of ir-NPY in both species suggests a widespread role in spinal cord function.

Ultrastructural studies on peptides in the dorsal horn of the rat spinal cord--IV. Effects of peripheral axotomy with special reference to neuropeptide Y and vasoactive intestinal polypeptide/peptide histidine isoleucine

Using immunofluorescence histochemistry and pre- and post-embedding immunoelectron microscopy the rat lumbar dorsal horn was analysed in normal rats and 14 days after unilateral transection of the sciatic nerve. A marked increase in neuropeptide Y-like immunoreactivity was observed in the ipsilateral, superficial dorsal horn, especially in laminae III and IV, of the lumbar 4-5 spinal cord segments after peripheral axotomy. In the ipsilateral lamina II two types of neuropeptide Y-immunoreactive, presumably primary afferent terminals could be identified at the ultrastructural level. The first type contained many large dense-core vesicles (100-155 nm in diameter), whereas a second, more common type had only a few and smaller large dense-core vesicles (80-100 nm in diameter), plus synaptic vesicles of varying diameter (50-85 nm), large empty vesicles and tubular structures. Only occasionally were neuropeptide Y-positive terminals in lamina II involved in the formation of axonal labyrinths. In the ipsilateral lamina III, the number of neuropeptide Y-positive nerve terminals markedly increased after axotomy, with a moderate increase in lamina IV. These neuropeptide Y-positive terminals were morphologically similar to the second type of neuropeptide Y-positive terminal in lamina II, i.e. contained many synaptic vesicles (45-50 nm in diameter), a few small large dense-core vesicles (80-100 nm in diameter), electron-dense granular matrix and a few tubular structures. Fusion of synaptic vesicles with the plasma membrane was often observed at these synapses. These terminals frequently formed glomeruli but were not involved in axonal labyrinths. With regard to local neurons, neuropeptide Y-like immunoreactivity was observed in many dendrite-like profiles mostly making synaptic contacts with neuropeptide Y-negative dendrites and only rarely contacting the central terminal of the glomeruli. Neuropeptide Y-positive nerve endings were mainly seen in lamina I and the outer third of lamina II. After peripheral axotomy the number of vasoactive intestinal polypeptide/peptide histidine isoleucine immunoreactive terminals was increased in laminae I and II. They contained many large dense-core vesicles (100-120 nm in diameter), and some of them were positive for vasoactive intestinal polypeptide/peptide histidine isoleucine. Morphologically, the terminals were characterized by a granular matrix, tubular structures, empty vesicles, reduction in synaptic vesicles and absence of postsynaptic densities. Vasoactive intestinal polypeptide/peptide histidine isoleucine-like immunoreactivities were often found in association with labyrinth formation.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of intrathecal neuropeptide Y on the spinal nociceptive flexor reflex in rats with intact sciatic nerves and after peripheral axotomy

We examined the effects of intrathecally administered neuropeptide Y on the spinal nociceptive flexor reflex in decerebrate, spinalized, unanesthetized rats with intact sciatic nerves, or 11-39 days after unilateral transection of the sciatic nerve. In rats with intact sciatic nerve, intrathecal neuropeptide Y at low doses (10 and 100 ng) caused a brief facilitation of the flexor reflex. At a dose of 300 ng, the effect of neuropeptide Y on the flexor reflex was biphasic, i.e. a brief facilitation followed by slight depression. At higher doses (1 and 10 micrograms), the effect of neuropeptide Y was mainly inhibitory, causing substantial and usually prolonged depression of the flexor reflex magnitude. The reflex depression caused by intrathecal neuropeptide Y was not reversed by the opioid antagonist naloxone or the alpha 2 adrenoceptor antagonist atipamezole. Intrathecal neuropeptide Y at doses up to 1 and 10 micrograms had no effect on reflex facilitation caused by conditioning stimulation of C-fibers, intrathecal substance P or neurokinin A. Topical application of neuropeptide Y (1 microgram/microliter) failed to influence the monosynaptic reflex in normal rats. Eleven to 16 days after peripheral axotomy, the initial excitation of the flexor reflex to intrathecal neuropeptide Y was significantly enhanced in axotomized compared with normal rats. However, the depressive effect of neuropeptide Y on the flexor reflex was unchanged. Neuropeptide Y did not influence the monosynaptic reflex in axotomized rats at this period. In experiments performed on rats in which the sciatic nerve had been transected 31-39 days previously, the facilitatory effect of neuropeptide Y on the flexor reflex remained enhanced compared with normal rats. Furthermore, the inhibitory effect of neuropeptide Y also increased as 100 ng intrathecal neuropeptide Y was able to produce reflex depression in a similar fashion as 300 ng neuropeptide Y normally and the reflex depression caused by 1 microgram neuropeptide Y was stronger and longer lasting than in normal rats. Intrathecal neuropeptide Y (100 ng-10 micrograms) in rats with intact sciatic nerves caused a moderate decrease in spinal cord dorsal surface blood flow as measured with a laser Doppler flowmeter. This effect of neuropeptide Y was unchanged in axotomized rats. The present results support previous observations that spinal application of neuropeptide Y in normal rats caused antinociception. As the depressive effect of neuropeptide Y is independent of spinal opioid and alpha 2-adrenergic systems, it may be mediated by its own receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Lack of effect of microinjection of noradrenaline or medetomidine on stimulus-evoked release of substance P in the spinal cord of the cat: a study with antibody microprobes

1. Experiments were performed on barbiturate anaesthetized, spinalized cats to investigate the effect of microinjected noradrenaline or medetomidine on the release of immunoreactive substance P in the dorsal spinal cord following peripheral nerve stimulation. The presence of immunoreactive substance P was assessed with microprobes bearing C-terminus-directed antibodies to substance P. 2. Noradrenaline or medetomidine were microinjected into the grey matter of the spinal cord, near microprobe insertion sites, at depths of 2.5, 2.0, 1.5 and 1.0 mm below the spinal cord surface with volumes of approximately 0.125 microliters and a concentration of 10(-3) M. 3. In the untreated spinal cord, electrical stimulation of the ipsilateral tibial nerve (suprathreshold for C-fibres) elicited release of immunoreactive substance P which was centred in and around lamina II. Neither noradrenaline nor medetomidine administration in the manner described produced significant alterations in this pattern of nerve stimulus-evoked release. 4. In agreement with recent ultrastructural studies these results do not support a control of substance P release by catecholamines released from sites near to the central terminals of small diameter primary afferent fibres.

Subpopulations of GABAergic neurons in laminae I-III of rat spinal dorsal horn defined by coexistence with classical transmitters, peptides, nitric oxide synthase or parvalbumin

GABAergic neurons in laminae I-III of the spinal dorsal horn may contain one or more of the following compounds: glycine, acetylcholine, neuropeptide Y, enkephalin, nitric oxide synthase or parvalbumin. Although the pattern of co-localization of some of these compounds is understood, it is not known which types of GABAergic neurons contain parvalbumin, or whether nitric oxide synthase coexists with peptides, acetylcholine or parvalbumin in any of these neurons, and in this study we have used immunocytochemistry and enzyme histochemistry to resolve these issues. Parvalbumin-immunoreactivity was restricted to those GABA-immunoreactive neurons that also showed glycine-immunoreactivity and was not co-localized with neuropeptide Y-immunoreactivity or NADPH diaphorase activity. By combining NADPH diaphorase histochemistry with immunocytochemistry with an antiserum to nitric oxide synthase, we were able to show that NADPH diaphorase activity was a reliable marker for nitric oxide synthase in the spinal cord. Neurons that possess GABA- but not glycine-immunoreactivity may contain neuropeptide Y, enkephalin, acetylcholine or NADPH diaphorase, and all of the cholinergic neurons appear to contain NADPH diaphorase. By combining immunofluorescent detection of neuropeptide Y or enkephalin with NADPH diaphorase histochemistry, we showed that peptide-immunoreactivity did not coexist with NADPH diaphorase. This suggests that neither of these peptides coexists with nitric oxide synthase or with acetylcholine in neurons in the superficial dorsal horn. Several phenotypically distinct groups of GABA-immunoreactive neuron can therefore be identified in laminae I-III of the dorsal horn, and these may represent different functional types of inhibitory neuron.

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

The organization of neuropeptide Y-containing profiles in the dorsal horn of cat lumbosacral spinal cord was examined in an immunocytochemical study employing a specific antiserum against neuropeptide Y. Light-microscopic inspection revealed heavy concentrations of immunoreactive axons and varicosities within the superficial layers of the dorsal horn (laminae I and II) and only low to moderate numbers of positive terminals in the deeper layers (laminae III-VI). Neuropeptide-Y immunoreactivity in the superficial laminae occurred primarily as single punctate terminals, although in sagittal sections long rostrocaudally orientated fibres were also found. Immunoreactive fibres in the deeper layers were usually long and beaded. Two-hundred and eight neuropeptide Y-immunoreactive profiles throughout laminae I-VI were examined through serial sections with the electron microscope, and the overwhelming majority (n = 194) was confirmed to be axon terminals, most of which (95%) formed synaptic junctions. These terminals were packed with small irregularly shaped agranular vesicles, together with a number of large dense-core vesicles. Immunoreactivity was homogeneously scattered throughout the cytoplasm, and was also associated with the dense-core vesicles. A few neuropeptide Y-containing profiles (n = 14) were difficult to classify but they could have been vesicle-containing dendrites. The postsynaptic targets of neuropeptide Y-positive terminals were similar throughout each dorsal horn lamina. Most frequently, neuropeptide Y-positive boutons formed axodendritic and axosomatic synaptic junctions (range = 64% of synapses in laminae V/VI to 83% in lamina III). A smaller proportion of synapses were found upon other axon terminals and in laminae I-III the postsynaptic axon terminals were sometimes the central boutons of glomeruli. A number of terminals, especially those in lamina II, formed multiple synapses which often comprised a triadic arrangement. These findings suggest that neuropeptide Y regulates spinal sensory transmission through both a postsynaptic action upon dorsal horn neurons and a presynaptic action upon primary afferent terminals.