Spinal premotor interneurons controlling antagonistic muscles are spatially intermingled

Elaborate behaviours are produced by tightly controlled flexor-extensor motor neuron activation patterns. Motor neurons are regulated by a network of interneurons within the spinal cord, but the computational processes involved in motor control are not fully understood. The neuroanatomical arrangement of motor and premotor neurons into topographic patterns related to their controlled muscles is thought to facilitate how information is processed by spinal circuits. Rabies retrograde monosynaptic tracing has been used to label premotor interneurons innervating specific motor neuron pools, with previous studies reporting topographic mediolateral positional biases in flexor and extensor premotor interneurons. To more precisely define how premotor interneurons contacting specific motor pools are organized, we used multiple complementary viral-tracing approaches in mice to minimize systematic biases associated with each method. Contrary to expectations, we found that premotor interneurons contacting motor pools controlling flexion and extension of the ankle are highly intermingled rather than segregated into specific domains like motor neurons. Thus, premotor spinal neurons controlling different muscles process motor instructions in the absence of clear spatial patterns among the flexor-extensor circuit components.

Neurotransmitters and Motoneuron Contacts of Multifunctional and Behaviorally Specialized Turtle Spinal Cord Interneurons

The spinal cord can appropriately generate diverse movements, even without brain input and movement-related sensory feedback, using a combination of multifunctional and behaviorally specialized interneurons. The adult turtle spinal cord can generate motor patterns underlying forward swimming, three forms of scratching, and limb withdrawal (flexion reflex). We previously described turtle spinal interneurons activated during both scratching and swimming (multifunctional interneurons), interneurons activated during scratching but not swimming (scratch-specialized interneurons), and interneurons activated during flexion reflex but not scratching or swimming (flexion reflex-selective interneurons). How multifunctional and behaviorally specialized turtle spinal interneurons affect downstream neurons was unknown. Here, we recorded intracellularly from spinal interneurons activated during these motor patterns in turtles of both sexes in vivo and filled each with dyes. We labeled motoneurons using choline acetyltransferase antibodies or earlier intraperitoneal FluoroGold injection and used immunocytochemistry of interneuron axon terminals to identify their neurotransmitter(s) and putative synaptic contacts with motoneurons. We found that multifunctional interneurons are heterogeneous with respect to neurotransmitter, with some glutamatergic and others GABAergic or glycinergic, and can directly contact motoneurons. Also, scratch-specialized interneurons are heterogeneous with respect to neurotransmitter and some directly contact motoneurons. Thus, scratch-specialized interneurons might directly excite motoneurons that are more strongly activated during scratching than forward swimming, such as hip-flexor motoneurons. Finally, and surprisingly, we found that some motoneurons are behaviorally specialized, for scratching or flexion reflex. Thus, either some limb muscles are only used for a subset of limb behaviors or some limb motoneurons are only recruited during certain limb behaviors.SIGNIFICANCE STATEMENT Both multifunctional and behaviorally specialized spinal cord interneurons have been described in turtles, but their outputs are unknown. We studied responses of multifunctional interneurons (activated during swimming and scratching) and scratch-specialized interneurons, filled each with dyes, and used immunocytochemistry to determine their neurotransmitters and contacts with motoneurons. We found that both multifunctional and scratch-specialized interneurons are heterogeneous with respect to neurotransmitter, with some excitatory and others inhibitory. We found that some multifunctional and some scratch-specialized interneurons directly contact motoneurons. Scratch-specialized interneurons may excite motoneurons that are more strongly activated during scratching than swimming, such as hip-flexor motoneurons, or inhibit their antagonists, hip-extensor motoneurons. Surprisingly, we also found that some motoneurons are behaviorally specialized, for scratching or for flexion reflex.

The mammalian spinal commissural system: properties and functions

Commissural systems are essential components of motor circuits that coordinate left-right activity of the skeletomuscular system. Commissural systems are found at many levels of the neuraxis including the cortex, brainstem, and spinal cord. In this review we will discuss aspects of the mammalian spinal commissural system. We will focus on commissural interneurons, which project from one side of the cord to the other and form axonal terminations that are confined to the cord itself. Commissural interneurons form heterogeneous populations and influence a variety of spinal circuits. They can be defined according to a variety of criteria including, location in the spinal gray matter, axonal projections and targets, neurotransmitter phenotype, activation properties, and embryological origin. At present, we do not have a comprehensive classification of these cells, but it is clear that cells located within different areas of the gray matter have characteristic properties and make particular contributions to motor circuits. The contribution of commissural interneurons to locomotor function and posture is well established and briefly discussed. However, their role in other goal-orientated behaviors such as grasping, reaching, and bimanual tasks is less clear. This is partly because we only have limited information about the organization and functional properties of commissural interneurons in the cervical spinal cord of primates, including humans. In this review we shall discuss these various issues. First, we will consider the properties of commissural interneurons and subsequently examine what is known about their functions. We then discuss how they may contribute to restoration of function following spinal injury and stroke.

Corticospinal and Reticulospinal Contacts on Cervical Commissural and Long Descending Propriospinal Neurons in the Adult Rat Spinal Cord; Evidence for Powerful Reticulospinal Connections

Descending systems have a crucial role in the selection of motor output patterns by influencing the activity of interneuronal networks in the spinal cord. Commissural interneurons that project to the contralateral grey matter are key components of such networks as they coordinate left-right motor activity of fore and hind-limbs. The aim of this study was to determine if corticospinal (CST) and reticulospinal (RST) neurons make significant numbers of axonal contacts with cervical commissural interneurons. Two classes of commissural neurons were analysed: 1) local commissural interneurons (LCINs) in segments C4-5; 2) long descending propriospinal neurons (LDPNs) projecting from C4 to the rostral lumbar cord. Commissural interneurons were labelled with Fluorogold and CST and RST axons were labelled by injecting the b subunit of cholera toxin in the forelimb area of the primary somatosensory cortex or the medial longitudinal fasciculus respectively. The results show that LCINs and LDPNs receive few contacts from CST terminals but large numbers of contacts are formed by RST terminals. Use of vesicular glutamate and vesicular GABA transporters revealed that both types of cell received about 80% excitatory and 20% inhibitory RST contacts. Therefore the CST appears to have a minimal influence on LCINs and LDPNs but the RST has a powerful influence. This suggests that left-right activity in the rat spinal cord is not influenced directly via CST systems but is strongly controlled by the RST pathway. Many RST neurons have monosynaptic input from corticobulbar pathways therefore this pathway may provide an indirect route from the cortex to commissural systems. The cortico-reticulospinal-commissural system may also contribute to functional recovery following damage to the CST as it has the capacity to deliver information from the cortex to the spinal cord in the absence of direct CST input.

The spino-bulbar-cerebellar pathway: Activation of neurons projecting to the lateral reticular nucleus in the rat in response to noxious mechanical stimuli

It is now well established that the cerebellum receives input from nociceptors which may serve to adjust motor programmes in response to pain and injury. In this study, we investigated the possibility that spinoreticular neurons (SRT) which project to a pre-cerebellar nucleus, the lateral reticular nucleus (LRt), respond to noxious mechanical stimulation. Seven adult male rats received stereotaxic injections of the b subunit of cholera toxin in the LRt. Following a 5 day interval, animals were anesthetised with urethane and a noxious mechanical stimulus was applied to the right hind paw. Animals were fixed by perfusion 5min following application of the stimulus. Retrogradely labelled SRT neurons of the lumbar spinal cord were examined for immunoreactivity for phosphorylated ERK (pERK) and the neurokinin-1 (NK-1) receptor. Approximately 15% of SRT cells in deep laminae (IV-VII and X) expressed pERK ipsilateral to the site of the stimulus. Around 60% of SRT cells with the NK-1 receptor expressed pERK but 5% of pERK expressing cells were negatively labelled for NK-1. It is concluded that a significant proportion of SRT cells projecting to the LRt respond to noxious mechanical stimuli and that one of the functions of this pathway may be to provide the cerebellum with nociceptive information.

The spino-bulbar-cerebellar pathway: organization and neurochemical properties of spinal cells that project to the lateral reticular nucleus in the rat

In addition to classical spinocerebellar pathways, the cerebellum receives information from the spinal cord indirectly via spino-bulbar-cerebellar systems. One of the structures in this pathway is the lateral reticular nucleus (LRt). We performed series of experiments to investigate the organization and neurotransmitter content of spinoreticular tract (SRT) neurons in the lumbar spinal cord that project to the LRt. Three rats received injections of the b subunit of Cholera toxin (CTb) or Fluorogold (FG) within the left and right LRt. The majority of SRT cells (56–61%) were found within the contralateral medial intermediate gray matter where small numbers (7–10%) of double-labeled cells were also present on both sides of the cord. Six rats received unilateral spinal injections of CTb to label spinal projections to the LRt. Injections of FG were made also into the anterior lobe of the cerebellum to label LRt pre-cerebellar neurons. Terminals were found mainly ipsilateral to spinal injection sites within the central and ventrolateral regions of the LRt. Immunocytochemical analysis of SRT terminals revealed that the majority (75%) were contained vesicular glutamate transporter 2 but a minority (20%) contained the vesicular GABA transporter. The inhibitory subpopulation was found to be GABAergic, glycinergic, or contained both transmitters. Inhibitory and excitatory terminals were present within overlapping regions of the nucleus. Most CTb terminals contacting LRt pre-cerebellar neurons were excitatory (80%) whereas a minority were inhibitory and most cells (88%) received contacts from both inhibitory and excitatory terminals. This study shows that SRT axons in the LRt have the capacity to exert direct excitatory and inhibitory actions on LRt pre-cerebellar neurons. Thus spinal cord input has the capacity to facilitate or depress the activity of individual LRt cells which in turn adjust activity in the cerebellum to produce coordinated motor behaviors.

Origin and neurochemical properties of bulbospinal neurons projecting to the rat lumbar spinal cord via the medial longitudinal fasciculus and caudal ventrolateral medulla

Bulbospinal systems (BS) originate from various regions of the brainstem and influence spinal neurons by classical synaptic and modulatory mechanisms. Our aim was to determine the brainstem locations of cells of origin of BS pathways passing through the medial longitudinal fasciculus (MLF) and the caudal ventrolateral medulla (CVLM). We also examined the transmitter content of spinal terminations of the CVLM pathway. Six adult rats received Fluorogold (FG) injections to the right intermediate gray matter of the lumbar cord (L1–L2) and the b-subunit of cholera toxin (CTb) was injected either into the MLF or the right CVLM (3 animals each). Double-labeled cells were identified within brainstem structures with confocal microscopy and mapped onto brainstem diagrams. An additional 3 rats were injected with CTb in the CVLM to label axon terminals in the lumbar spinal cord. Double-labeled cells projecting via the MLF or CVLM were found principally in reticular regions of the medulla and pons but small numbers of cells were also located within the midbrain. CVLM projections to the lumbar cord were almost exclusively ipsilateral and concentrated within the intermediate gray matter. Most (62%) of terminals were immunoreactive for the vesicular glutamate transporter 2 while 23% contained the vesicular GABA transporter. The inhibitory subpopulation was glycinergic, GABAergic or contained both transmitters. The proportions of excitatory and inhibitory axons projecting via the CVLM to the lumbar cord are similar to those projecting via the MLF. Unlike the MLF pathway, CVLM projections are predominantly ipsilateral and concentrated within intermediate gray but do not extend into motor nuclei or laminia VIII. Terminations of the CVLM pathway are located in a region of the gray matter that is rich in premotor interneurons; thus its primary function may be to coordinate activity of premotor networks.

Ascending and descending propriospinal pathways between lumbar and cervical segments in the rat: evidence for a substantial ascending excitatory pathway

Precise mechanisms are required to coordinate the locomotor activity of fore- and hind-limbs in quadrupeds and similar mechanisms persist to coordinate movement of arms and legs in humans. Propriospinal neurons (PSNs) are major components of the networks that coordinate these mechanisms. The b subunit of cholera toxin (CTb) was injected unilaterally into either L1 or L3 segments in order to label ascending and descending propriospinal pathways. Labelled cells were examined with light or confocal microscopy. Cells projecting to lumbar segments were evenly distributed, bilaterally throughout all cervical segments. However many more cells were labelled from L1 injections than L3 injections. Roughly 15% of cells in both sides of the C2 segment was found to be immunoreactive for calretinin and a small number (4%) was immunoreactive for calbindin. Axons projecting from L1 to cervical segments formed predominant ipsilateral projections to the cervical intermediate grey matter and ventral horn. Very large numbers of terminals were concentrated within the ventrolateral motor (VLM) nuclei of C7-8 segments but there was sparse innervation of the contralateral nucleus. The vast majority (85%) of these axon terminals in the ipsilateral VML was immunoreactive for the vesicular glutamate transporter 2 (VGLUT2) and the remaining 15% was immunoreactive for the vesicular GABA transporter (VGAT); many of these contained GABA and/or glycine. Inhibitory and excitatory terminals were also found in the contralateral VLM. Most of the terminals in the VLM made contacts with motoneurons. The major finding of this study is the existence of a substantial excitatory propriospinal pathway that projects specifically to the VLM. Motoneurons in the VLM supply muscles of the axilla therefore this pathway is likely to have a profound influence on the activity of the shoulder joint. This pathway may synchronise lumbar and cervical pattern generators and hence the coordination of locomotor activity in the fore- and hind limbs.

Neurotransmitter phenotypes of descending systems in the rat lumbar spinal cord

Descending systems from the brain exert a major influence over sensory and motor processes within the spinal cord. Although it is known that many descending systems have an excitatory effect on spinal neurons, there are still gaps in our knowledge regarding the transmitter phenotypes used by them. In this study we investigated transmitter phenotypes of axons in the corticospinal tract (CST); the rubrospinal tract (RST); the lateral component of the vestibulospinal tract (VST); and the reticulospinal tract (ReST). They were labelled anterogradely by stereotaxic injection of the b subunit of cholera toxin (CTb) into the motor cortex, red nucleus, lateral vestibular nucleus and medial longitudinal fascicle (MLF) to label CST, RST, VST and ReST axons respectively. Neurotransmitter content of labelled axons was investigated in lumbar segments by using immunoflurescence; antibodies against vesicular glutamate transporters (VGLUT1 and VGLUT2) were used to identify glutamatergic terminals and the vesicular GABA transporter (VGAT) was used to identify GABA- and glycinergic terminals. The results show that almost all CST (96%) axons contain VGLUT1 whereas almost all RST (97%) and VST (97%) axons contain VGLUT2. Although the majority of ReST axons contain VGLUT2 (59%), a sizable minority contains VGAT (20%) and most of these terminals can be subdivided into those that are GABAergic or those that are glycinergic because only limited evidence for co-localisation was found for the two transmitters. In addition, there is a population of ReST terminals that apparently does not contain markers for the transmitters tested and is not serotoninergic. We can conclude that the CST, RST and VST are 'pure' excitatory systems whereas the ReST consists of a heterogeneous population of excitatory and inhibitory axons. It is anticipated that this information will enable inputs to spinal networks to be defined with greater confidence.

Excitatory inputs to four types of spinocerebellar tract neurons in the cat and the rat thoraco-lumbar spinal cord

The cerebellum receives information from the hindlimbs through several populations of spinocerebellar tract neurons. Although the role of these neurons has been established in electrophysiological experiments, the relative contribution of afferent fibres and central neurons to their excitatory input has only been estimated approximately so far. Taking advantage of differences in the immunohistochemistry of glutamatergic terminals of peripheral afferents and of central neurons (with vesicular glutamate transporters VGLUT1 or VGLUT2, respectively), we compared sources of excitatory input to four populations of spinocerebellar neurons in the thoraco-lumbar spinal cord: dorsal spinocerebellar tract neurons located in Clarke's column (ccDSCT) and in the dorsal horn (dhDSCT) and ventral spinocerebellar tract (VSCT) neurons including spinal border (SB) neurons. This was done on 22 electrophysiologically identified intracellularly labelled neurons in cats and on 80 neurons labelled by retrograde transport of cholera toxin b subunit injected into the cerebellum of rats. In both species distribution of antibodies against VGLUT1 and VGLUT2 on SB neurons (which have dominating inhibitory input from limb muscles), revealed very few VGLUT1 contacts and remarkably high numbers of VGLUT2 contacts. In VSCT neurons with excitatory afferent input, the number of VGLUT1 contacts was relatively high although VGLUT2 contacts likewise dominated, while the proportions of VGLUT1 and VGLUT2 immunoreactive terminals were the reverse on the two populations of DSCT neurons. These findings provide morphological evidence that SB neurons principally receive excitatory inputs from central neurons and provide the cerebellum with information regarding central neuronal activity.

Implementation and evaluation of a multisite drug usage evaluation program across Australian hospitals - a quality improvement initiative

BACKGROUND

With the use of medicines being a broad and extensive part of health management, mechanisms to ensure quality use of medicines are essential. Drug usage evaluation (DUE) is an evidence-based quality improvement methodology, designed to improve the quality, safety and cost-effectiveness of drug use. The purpose of this paper is to describe a national DUE methodology used to improve health care delivery across the continuum through multi-faceted intervention involving audit and feedback, academic detailing and system change, and a qualitative assessment of the methodology, as illustrated by the Acute Postoperative Pain Management (APOP) project.

METHODS

An established methodology, consisting of a baseline audit of inpatient medical records, structured patient interviews and general practitioner surveys, followed by an educational intervention and follow-up audit, is used. Australian hospitals, including private, public, metropolitan and regional, are invited to participate on a voluntary basis. De-identified data collected by hospitals are collated and evaluated nationally to provide descriptive comparative analyses. Hospitals benchmark their practices against state and national results to facilitate change. The educational intervention consists of academic detailing, group education, audit and feedback, point-of-prescribing prompts and system changes. A repeat data collection is undertaken to assess changes in practice.An online qualitative survey was undertaken to evaluate the APOP program. Qualitative assessment of hospitals' perceptions of the effectiveness of the overall DUE methodology and changes in procedure/prescribing/policy/clinical practice which resulted from participation were elicited.

RESULTS

62 hospitals participated in the APOP project. Among 23 respondents to the evaluation survey, 18 (78%) reported improvements in the documentation of pain scores at their hospital. 15 (65%) strongly agreed or agreed that participation in APOP directly resulted in increased prescribing of multimodal analgesia for pain relief in postoperative patients.

CONCLUSIONS

This national DUE program has facilitated the engagement and participation of a number of acute health care facilities to address issues relating to quality use of medicine. This approach has been perceived to be effective in helping them achieve improvements in patient care.

Organization and neurochemical properties of intersegmental interneurons in the lumbar enlargement of the adult rat

Intersegmental interneurons with relatively short axons perform an important role in the coordination of limb movement but surprisingly little is known about their organization and how they contribute to neuronal networks in the adult rat. We undertook a series of anatomical tract-tracing studies to label cell bodies and axons of intersegmental neurons in the lumbar cord and characterized their neurochemical properties by using immunocytochemistry. The b-subunit of cholera toxin was injected into L1 or L3 segments of seven rats in the vicinity of lateral or medial motor nuclei. In L5 lumbar segments, cells were found to be concentrated in contralateral lamina VIII, and in ipsilateral lamina VII and laminae V-VI following injections into the lateral and medial motor nuclei respectively. About 25% of labelled cells contained calbindin or calretinin or a combination of both. Calbindin positive cells were mainly distributed within the ipsilateral side of the L5 segment, especially within the ipsilateral dorsal horn whereas there was a concentration of calretinin cells in contralateral lamina VIII. A small population of cells around the central canal were cholinergic. We also examined axon terminals that projected from L1/3 to the L5 contralateral lateral motor nucleus. The majority of these axons were excitatory (75%) and made direct contacts with motoneurons. However, most inhibitory axons in L5 contained a mixture of GABA and glycine (20%) and about 22% of the total population of axons contained calbindin. In contrast, 19% of all intra-segmental axons in the L3 contralateral lateral motor nucleus were found to be purely glycinergic and 17% contained a mixture of GABA and glycine. This study shows that short range interneurons form extensive ipsi- and contralateral projections within the lumbar enlargement and that many of them contain calcium binding proteins. Those projecting contralaterally to motor nuclei are predominantly excitatory.

Properties of axon terminals contacting intermediate zone excitatory and inhibitory premotor interneurons with monosynaptic input from group I and II muscle afferents

The intermediate zone of the spinal grey matter contains premotor interneurons mediating reflex actions of group I and II muscle afferents. However, limited information is available on how activity of inhibitory versus excitatory interneurons in this population are modulated and how they contribute to motor networks. There were three aims of this study: (1) to characterize excitatory axonal contacts on interneurons; (2) to determine if contact patterns on excitatory and inhibitory interneurons are different; (3) to determine if there are differences in presynaptic inhibitory control of excitatory and inhibitory interneurons. We used intracellular labelling of electrophysiologically identified cells along with immunochemistry to characterise contacts formed by axons that contain vesicular glutamate transporters (VGLUT1 and VGLUT2) and contacts formed by VGLUT1 terminals which in turn were contacted by GABAergic terminals on cells that were characterised according to their transmitter phenotype. All 17 cells investigated were associated with numerous VGLUT1 contacts originating from primary afferents, and similar contact densities were found on excitatory and inhibitory cells, but VGLUT2-immunoreactive terminals originating from intraspinal neurons were less frequent, or were practically absent, especially on excitatory cells. Similar numbers of VGLUT1 contacts with associated GABAergic terminals were found on excitatory and inhibitory cells indicating a similar extent of presynaptic GABAergic control. However, scarce VGLUT2 terminals on intermediate zone excitatory premotor interneurons with input from muscle afferents suggest that they are not significantly excited by other spinal neurons but are under direct excitatory control of supraspinal neurons and, principally inhibitory, control of spinal neurons.

Cholinergic terminals in the ventral horn of adult rat and cat: evidence that glutamate is a cotransmitter at putative interneuron synapses but not at central synapses of motoneurons

Until recently it was generally accepted that the only neurotransmitter to be released at central synapses of somatic motoneurons was acetylcholine. However, studies on young mice (P0-10) have provided pharmacological evidence indicating that glutamate may act as a cotransmitter with acetylcholine at synapses between motoneurons and Renshaw cells. We performed a series of anatomical experiments on axon collaterals obtained from intracellularly labeled motoneurons from an adult cat and labeled by retrograde transport in adult rats to determine if glutamate is co-localized with acetylcholine by these terminals. We could find no evidence for the presence of vesicular glutamate transporters in motoneuron axon terminals of either species. In addition, we were unable to establish any obvious relationship between motoneuron terminals and the R2 subunit of the AMPA receptor (GluR2). However we did observe a population of cholinergic terminals in lamina VII which did not originate from motoneurons but were immunoreactive for the vesicular glutamate transporter 2 and formed appositions to GluR2 subunits. These were smaller than motoneuron terminals and, unlike them, formed no relationship with Renshaw cells. The evidence suggests that glutamate does not act as a cotransmitter with acetylcholine at central synapses of motoneurons in the adult cat and rat. However, glutamate is present in a population of cholinergic terminals which probably originate from interneurons where its action is via an AMPA receptor.

Commissural interneurons with input from group I and II muscle afferents in feline lumbar segments: neurotransmitters, projections and target cells

The aim of this study was to analyse neurotransmitter content, projection areas and target cells of commissural interneurons with input from group I and/or II muscle afferents in lumbar segments in the cat. Axonal projections of 15 intracellularly labelled commissural interneurons were reconstructed. Ten interneurons (nine located in laminae VI-VII, one in lamina VIII) were glutamatergic; only one interneuron (located in lamina VIII) was glycinergic. Contralateral terminal projections were found both in motor nuclei and within laminae VI-VIII. In order to identify target cells of commissural interneurons, effects of stimulation of contralateral group I and II muscle afferents were investigated on interneurons within these laminae. Three tests were used: intracellular records from individual interneurons, modulation of probability of activation of extracellularly recorded interneurons and modulation of their actions on motoneurons using disynaptic PSPs evoked in motoneurons as a measure. All these tests revealed much more frequent and/or stronger excitatory actions of contralateral afferents. The results indicate that commissural interneurons with input from contralateral group I and II afferents target premotor interneurons in disynaptic pathways from ipsilateral group I and II afferents and that excitatory disynaptic actions of contralateral afferents on these interneurons are mediated primarily by intermediate zone commissural interneurons. A second group of commissural interneurons activated by reticulospinal neurons, previously described, frequently had similar, but occasionally opposing, actions to the cells described here, thus indicating that these two subpopulations may act on the same premotor interneurons and either mutually enhance or counteract each other's actions.

Excitatory and inhibitory intermediate zone interneurons in pathways from feline group I and II afferents: differences in axonal projections and input

The aim of the present study was to compare properties of excitatory and inhibitory spinal intermediate zone interneurons in pathways from group I and II muscle afferents in the cat. Interneurons were labelled intracellularly and their transmitter phenotypes were defined by using immunocytochemistry. In total 14 glutamatergic, 22 glycinergic and 2 GABAergic/glycinergic interneurons were retrieved. All interneurons were located in laminae V-VII of the L3-L7 segments. No consistent differences were found in the location, the soma sizes or the extent of the dendritic trees of excitatory and inhibitory interneurons. However, major differences were found in their axonal projections; excitatory interneurons projected either ipsilaterally, bilaterally or contralaterally, while inhibitory interneurons projected exclusively ipsilaterally. Terminal projections of glycinergic and glutamatergic cells were found within motor nuclei as well as other regions of the grey matter which include the intermediate region, laminae VII and VIII. Cells containing GABA/glycine had more restricted projections, principally within the intermediate zone where they formed appositions with glutamatergic axon terminals and unidentified cells and therefore are likely to be involved in presynaptic as well as postsynaptic inhibition. The majority of excitatory and inhibitory interneurons were found to be coexcited by group I and II afferents (monosynaptically) and by reticulospinal neurons (mono- or disynaptically) and to integrate information from several muscles. Taken together the morphological and electrophysiological data show that individual excitatory and inhibitory intermediate zone interneurons may operate in a highly differentiated way and thereby contribute to a variety of motor synergies.

A population of large neurons in laminae III and IV of the rat spinal cord that have long dorsal dendrites and lack the neurokinin 1 receptor

The dorsal horn of the rat spinal cord contains a population of large neurons with cell bodies in laminae III or IV, that express the neurokinin 1 receptor (NK1r) and have long dorsal dendrites that branch extensively within the superficial laminae. In this study, we have identified a separate population of neurons that have similar dendritic morphology, but lack the NK1r. These cells also differ from the NK1r-expressing neurons in that they have significantly fewer contacts from substance P-containing axons and are not retrogradely labelled following injection of tracer into the caudal ventrolateral medulla. We also provide evidence that these cells do not belong to the postsynaptic dorsal column pathway or the spinothalamic tract. It is therefore likely that these cells do not have supraspinal projections. They may provide a route through which information transmitted by C fibres that lack neuropeptides is conveyed to deeper laminae. The present findings demonstrate the need for caution when attempting to classify neurons solely on the basis of somatodendritic morphology.

Premotor interneurones contributing to actions of feline pyramidal tract neurones on ipsilateral hindlimb motoneurones

The aim of the study was to analyse the potential contribution of excitatory and inhibitory premotor interneurones in reflex pathways from muscle afferents to actions of pyramidal tract (PT) neurones on ipsilateral hindlimb motoneurones. Disynaptic EPSPs and IPSPs evoked in motoneurones in deeply anaesthetized cats by group Ia, Ib and II muscle afferents were found to be facilitated by stimulation of the ipsilateral, as well as of contralateral, PT. The ipsilateral actions were evoked by either uncrossed or double-crossed pathways. The results show that interneurones mediating reflex actions of muscle afferents may be activated strongly enough by PT stimulation to contribute to movements initiated by ipsilateral PT neurones and that PT actions relayed by them might be enhanced by muscle stretches and/or contractions. However, in some motoneurones disynaptic IPSPs and EPSPs evoked from group Ib or II afferents were depressed by PT stimulation. In order to analyse the basis of this depression, the transmitter content in terminals of 11 intracellularly labelled interneurones excited by PT stimulation was defined immunohistochemically and their axonal projections were reconstructed. The interneurones included 9 glycinergic and 2 glutamatergic neurones. All but one of these neurones were mono- or disynaptically excited by group I and/or II afferents. Several projected to motor nuclei and formed contacts with motoneurones. However, all had terminal projections to areas outside the motor nuclei. Therefore both inhibitory and excitatory interneurones could modulate responses of other premotor interneurones in parallel with direct actions on motoneurones.

On coupling and decoupling of spinal interneuronal networks

This review addresses the question of interrelations between spinal interneuronal networks. On the basis of electrophysiological, pharmacological, morphological and immunohistochemical analysis of interneurones mediating various reflex actions from muscle receptors and of reticulospinal neurones a considerable degree of interweaving between networks of these neurones has been established. The coupling has been found to occur at the level of several sites of these networks but the review focuses on two of these sites. The first is between dorsal horn interneurones with group II input and their target ipsilaterally and contralaterally projecting intermediate zone and commissural interneurones. The second is between commissural interneurones with input from reticulospinal neurones and their target interneurones. Several ways of both strengthening and weakening of coupling between various interneuronal networks are also briefly reviewed.

Morphology of inhibitory and excitatory interneurons in superficial laminae of the rat dorsal horn

If we are to stand any chance of understanding the circuitry of the superficial dorsal horn, it is imperative that we can identify which classes of interneuron are excitatory and which are inhibitory. Our aim was to test the hypothesis that there is a correlation between the morphology of an interneuron and its postsynaptic action. We used in vitro slice preparations of the rat spinal cord to characterize and label interneurons in laminae I-III with Neurobiotin. Labelled cells (n = 19) were reconstructed in 3D with Neurolucida and classified according to the scheme proposed by Grudt & Perl (2002). We determined if cells were inhibitory or excitatory by reacting their axon terminals with antibodies to reveal glutamate decrboxylase (for GABAergic cells) or the vesicular glutamate transporter 2 (for glutamatergic cells). All five islet cells retrieved were inhibitory. Of the six vertical (stalked) cells analysed, four were excitatory and, surprisingly, two were inhibitory. It was noted that these inhibitory cells had axonal projections confined to lamina II whereas excitatory vertical cells projected to lamina I and II. Of the remaining neurons, three were radial cells (2 inhibitory, 1 excitatory), two were antennae cells (1 inhibitory, 1 excitatory), one was an inhibitory central cell and the remaining two were unclassifiable excitatory cells. Our hypothesis appears to be correct only for islet cells. Other classes of cells have mixed actions, and in the case of vertical cells, the axonal projection appears to be a more important determinant of postsynaptic action.

A new approach to reconstruction models of dendritic branching patterns

Quantitative models for characterising the detailed branching patterns of dendritic trees aim to explain these patterns either in terms of growth models based on principles of dendritic development or reconstruction models that describe an existing structure by means of a canonical set of elementary properties of dendritic morphology, which when incorporated into an algorithmic procedure will generate samples of dendrites that are statistically indistinguishable in both canonical and emergent features from those of the original sample of real neurons. This article introduces a conceptually new approach to reconstruction modelling based on the single assumption that dendritic segments are built from sequences of units of constant diameter, and that the distribution of the lengths of units of similar diameter is independent of location within a dendritic tree. This assumption in combination with non-parametric methods for estimating univariate and multivariate probability densities leads to an algorithm that significantly reduces the number of basic parameters required to simulate dendritic morphology. It is not necessary to distinguish between stem and terminal segments or to specify daughter branch ratios or dendritic taper. The procedure of sampling probability densities conditioned on local morphological features eliminates the need, for example, to specify daughter branch ratios and dendritic taper since these emerge naturally as a consequence of the conditioning process. Thus several basic parameters of previous reconstruction algorithms become emergent parameters of the new reconstruction process. The new procedure was applied successfully to a sample of 51 interneurons from lamina II/III of the spinal dorsal horn.

Differential projections of excitatory and inhibitory dorsal horn interneurons relaying information from group II muscle afferents in the cat spinal cord

Dorsal horn interneurons with input from group II muscle spindle afferents are components of networks involved in motor control. Thirteen dorsal horn interneurons with monosynaptic group II input were characterized electrophysiologically and labeled intracellularly with Neurobiotin. Their axonal projections were traced, and neurotransmitter content was established by using immunocytochemistry. Two subpopulations were identified: five interneurons had axons that contained vesicular glutamate transporter 2 and hence were glutamatergic and excitatory. Terminals of the remaining eight interneurons were immunoreactive for the glycine transporter 2 or were apposed to gephyrin but did not contain the GABA-synthesizing enzyme glutamic acid decarboxylase and were therefore glycinergic and inhibitory. Excitatory cells were located mainly in the central region of lamina IV and had relatively small somata and restricted dendritic trees. In contrast, inhibitory interneurons were located more ventrally, in lamina V and had relatively larger somata and more extensive dendritic trees. Axonal projections of the two subpopulations differed considerably. Excitatory interneurons predominantly projected ipsilaterally, whereas most inhibitory interneurons projected both ipsilaterally and contralaterally. Three inhibitory axons formed contacts with large cholinergic cells in motor nuclei, thus revealing a novel direct coupling between inhibitory dorsal horn interneurons and motoneurons. The organization of the excitatory interneurons is consistent with current knowledge of reflex pathways to motoneurons, but the existence and connections of the inhibitory subpopulation could not be predicted from previous data. Our results indicate that these latter interneurons exercise widespread inhibitory control over a variety of cell types located on both sides of the spinal cord.

Empiric management of community‐acquired pneumonia in Australian emergency departments

OBJECTIVE

To describe empiric community-acquired pneumonia (CAP) management in Australian hospital emergency departments (EDs) and evaluate this against national guidelines, including use of the pneumonia severity index and antibiotic selection.

DESIGN

A multicentre, cross-sectional, retrospective audit, April 2003 to February 2005.

SETTING

37 Australian hospitals: 22 principal referral hospitals, six large major city hospitals, four large regional hospitals, four medium hospitals and one private hospital.

PARTICIPANTS

Adult patients with a diagnosis of CAP made in the ED. Data on 20 consecutive CAP ED presentations were collected in participating hospitals.

OUTCOME MEASURES

Documented use of the pneumonia severity index, initial antibiotic therapy prescribed in the ED, average length of stay, inpatient mortality, and concordance with national guidelines.

RESULTS

691 CAP presentations were included. Pneumonia severity index use was documented in 5% of cases. Antibiotic therapy covering common bacterial causes of CAP was prescribed in 67% of presentations, although overall concordance with national guidelines was 18%. Antibiotic prescribing was discordant due to inadequate empiric antimicrobial cover, allergy status (including contraindication to penicillin), inappropriate route of administration and/or inappropriate antibiotic choice according to recommendations. There was no significant difference between concordant and discordant antibiotic prescribing episodes in average length of stay (5.0 v 5.7 days; P = 0.22) or inpatient mortality (1.6% v 4.1%; chi2 = 1.82; P = 0.18).

CONCLUSIONS

Antibiotic therapy for CAP prescribed in Australian EDs varied. Concordance with national CAP guidelines was generally low. Targeted interventions are required to improve concordance.

Membrane receptors involved in modulation of responses of spinal dorsal horn interneurons evoked by feline group II muscle afferents

Modulatory actions of a metabotropic 5-HT1A&7 membrane receptor agonist and antagonist [(+/-)-8-hydroxy-2-(di-n-propylamino)-tetralin; N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane-carboxamide] and an ionotropic 5-HT3 membrane receptor agonist and antagonist [2-methyl-serotonin (2-Me 5-HT); N-(1-azabicyclo[2.2.2]oct-3-yl)-6-chloro-4-methyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-8-carboxamide hydrochloride] were investigated on dorsal horn interneurons mediating reflex actions of group II muscle afferents. All drugs were applied ionophoretically in deeply anesthetized cats. Effects of agonists were tested on extracellularly recorded responses of individual interneurons evoked by electrical stimulation of group II afferents in a muscle nerve. Effects of antagonists were tested against the depression of these responses after stimulation of raphe nuclei. The results show that both 5-HT1A&7 and 5-HT3 membrane receptors are involved in counteracting the activation of dorsal horn interneurons by group II afferents. Because only quantitative differences were found within the sample of the tested neurons, these results suggest that modulatory actions of 5-HT on excitatory and inhibitory interneurons might be similar. The relationship between 5-HT axons and axons immunoreactive for the 5-HT3A receptor subunit, which contact dorsal horn interneurons, was analyzed using immunofluorescence and confocal microscopy. Contacts from both types of axons were found on all interneurons, but their distribution and density varied, and there was no obvious relationship between them. In two of six interneurons, 5-HT3A-immunoreactive axons formed ring-like arrangements around the cell bodies. In previous studies, axons possessing 5-HT3 receptors were found to be excitatory, and as 2-Me 5-HT depressed transmission to dorsal horn interneurons, the results indicate that 5-HT operates at 5-HT3 receptors presynaptic to these neurons to depress excitatory transmission.

P boutons in lamina IX of the rodent spinal cord express high levels of glutamic acid decarboxylase-65 and originate from cells in deep medial dorsal horn

Presynaptic inhibition of primary muscle spindle (group Ia) afferent terminals in motor nuclei of the spinal cord plays an important role in regulating motor output and is produced by a population of GABAergic axon terminals known as P boutons. Despite extensive investigation, the cells that mediate this control have not yet been identified. In this work, we use immunocytochemistry with confocal microscopy and EM to demonstrate that P boutons can be distinguished from other GABAergic terminals in the ventral horn of rat and mouse spinal cord by their high level of the glutamic acid decarboxylase (GAD) 65 isoform of GAD. By carrying out retrograde labeling from lamina IX in mice that express green fluorescent protein under the control of the GAD65 promoter, we provide evidence that the cells of origin of the P boutons are located in the medial part of laminae V and VI. Our results suggest that P boutons represent the major output of these cells within the ventral horn and are consistent with the view that presynaptic inhibition of proprioceptive afferents is mediated by specific populations of interneurons. They also provide a means of identifying P boutons that will be important in studies of the organization of presynaptic control of Ia afferents.

Conditional rhythmicity of ventral spinal interneurons defined by expression of the Hb9 homeodomain protein

The properties of mammalian spinal interneurons that underlie rhythmic locomotor networks remain poorly described. Using postnatal transgenic mice in which expression of green fluorescent protein is driven by the promoter for the homeodomain transcription factor Hb9, as well as Hb9-lacZ knock-in mice, we describe a novel population of glutamatergic interneurons located adjacent to the ventral commissure from cervical to midlumbar spinal cord levels. Hb9+ interneurons exhibit strong postinhibitory rebound and demonstrate pronounced membrane potential oscillations in response to chemical stimuli that induce locomotor activity. These data provide a molecular and physiological delineation of a small population of ventral spinal interneurons that exhibit homogeneous electrophysiological features, the properties of which suggest that they are candidate locomotor rhythm-generating interneurons.

Transmitter content, origins and connections of axons in the spinal cord that possess the serotonin (5-hydroxytryptamine) 3 receptor

Recent evidence suggests that serotonin has pronociceptive actions in the spinal cord when it acts through 5-hydroxytryptamine (5-HT)(3) receptors. Cells and axon terminals which are concentrated in the superficial dorsal horn possess this receptor. We performed a series of immunocytochemical studies with an antibody raised against the 5-HT(3A) subunit in order to address the following questions: 1) Are axons that possess 5-HT(3) receptors excitatory? 2) Are 5-HT(3) receptors present on terminals of myelinated primary afferents? 3) What is the chemical nature of dorsal horn cells that possess 5-HT(3) receptors? 4) Do axons that possess 5-HT(3) receptors target lamina I projection cells? Approximately 45% of 5-HT(3A) immunoreactive boutons were immunoreactive for the vesicular glutamate transporter 2 and almost 80% formed synapse-like associations with GluR2 subunits of the AMPA receptor therefore it is principally glutamatergic axons that possess the receptor. Immunoreactivity was not present on myelinated primary afferent axons labeled with the B-subunit of cholera toxin or those containing the vesicular glutamate transporter 1. Calbindin (which is associated with excitatory interneurons) was found in 44% of 5-HT(3A) immunoreactive cells but other markers for inhibitory and excitatory cells were not present. Lamina I projection cells that possessed the neurokinin-1 receptor were associated with 5-HT(3A) axons but the density of contacts on individual neurons varied considerably. The results suggest that 5-HT(3) receptors are present principally on terminals of excitatory axons, and at least some of these originate from dorsal horn interneurons. The relationship between lamina I projection cells and axons possessing the 5-HT(3) receptor indicates that this receptor has an important role in regulation of ascending nociceptive information.

Antibiotic guidelines in NSW hospitals

THIS PAPER REPORTS THE RESULTS of a preliminary survey which investigated antibiotic guideline use and the strategies used to implement and evaluate guideline use in NSW hospitals. We were interested in assessing the extent to which well established guidelines of proven value are being used in a sample of hospitals, as one way to measure implementation of the principles of clinical governance.

The actions of monoamines and distribution of noradrenergic and serotoninergic contacts on different subpopulations of commissural interneurons in the cat spinal cord

Modulatory actions of monoamines were investigated on spinal commissural interneurons which coordinate left-right hindlimb muscle activity through direct projections to the contralateral motor nuclei. Commissural interneurons located in Rexed lamina VIII, with identified projections to the contralateral gastrocnemius-soleus motor nuclei, were investigated in deeply anaesthetized cats. Most interneurons had dominant input from either the reticular formation or from group II muscle afferents; a small proportion of neurons had input from both. Actions of ionophoretically applied serotonin and noradrenaline were examined on extracellularly recorded spikes evoked monosynaptically by group II muscle afferents or reticulospinal tract fibres. Activation by reticulospinal fibres was facilitated by both serotonin and noradrenaline. Activation by group II afferents was also facilitated by serotonin but was strongly depressed by noradrenaline. To investigate the possible morphological substrates of this differential modulation, seven representative commissural interneurons were labelled intracellularly with tetramethylrhodamine-dextran and neurobiotin. Contacts from noradrenergic and serotoninergic fibres were revealed by immunohistochemistry and analysed with confocal microscopy. There were no major differences in the numbers and distributions of contacts among the interneurons studied. The findings suggest that differences in modulatory actions of monoamines, and subsequent changes in the recruitment of subpopulations of commissural interneurons in various behavioural situations, depend on intrinsic interneuron properties rather than on the patterns of innervation by monoaminergic fibres. The different actions of noradrenaline on different populations of interneurons might permit reconfiguration of the actions of the commissural neurons according to behavioural context.

Axon terminals possessing α2C-adrenergic receptors densely innervate neurons in the rat lateral spinal nucleus which respond to noxious stimulation

The lateral spinal nucleus (LSN) in the rat spinal cord contains projection neurons that are densely innervated by peptidergic varicosities which probably originate from spinal interneurons. The alpha2C-adrenoceptor (alpha2C-AR) is present on axon terminals in this nucleus and therefore norepinephrine is likely to modulate input to LSN neurons. We investigated the involvement of LSN neurons in nociceptive transmission and their relationship with axons that possess alpha2C-ARs. Double-labeling immunostaining experiments showed that alpha2C-ARs are present on axon terminals of excitatory and inhibitory interneurons that frequently contain colocalised peptides. Electron microscopy revealed that alpha2C-AR terminals are presynaptic to dendrites and somata of LSN neurons and predominantly form asymmetric synapses. We retrogradely labeled LSN neurons that project to the caudal ventrolateral medulla and combined this with induction of c-Fos expression by peripheral noxious thermal stimulation along with immunolabelling for the alpha2C-AR and the substance P (neurokinin-1) receptor. This enabled us to identify neuronkinin-1 projection neurons in the LSN that express c-Fos and to determine if such cells receive contacts from alpha2C-AR terminals. The results show that some LSN neurons are activated by noxious stimulation and that this input is likely to be modulated by norepinephrine acting on alpha2C-ARs which are present on axon terminals that are presynaptic to LSN neurons.

Networks of inhibitory and excitatory commissural interneurons mediating crossed reticulospinal actions

Axonal projections and neurotransmitters used by commissural interneurons mediating crossed actions of reticulospinal neurons were investigated in adult cats. Eighteen interneurons, located in or close to lamina VIII in midlumbar segments, that were monosynaptically excited by reticulospinal tract fibres and projected to contralateral motor nuclei were labelled by intracellular injection of tetramethylrhodamine-dextran and Neurobiotin. The nine most completely labelled interneurons were analysed with combined confocal and light microscopy. None of the stem axons gave off ipsilateral axon collaterals. Seven cells had axon collaterals that arborized in the contralateral grey matter in the ventral horn of the same segments. Transmitters were identified by using antibodies raised against vesicular glutamate transporters 1 and 2, glutamic acid decarboxylase and the glycine transporter 2. The axons of two cells were immunoreactive for the glycine transporter 2 and hence were glycinergic. Three cells were immunoreactive for the vesicular glutamate transporter 2 and hence were glutamatergic. None of the axons displayed immunoreactivity for glutamic acid decarboxylase. Electron microscopy of two cells revealed direct synaptic connections with motoneurons and other neurons. Axonal swellings of one neuron formed synapses with profiles in motor nuclei whereas those of the other formed synapses with other structures, including cell bodies in lamina VII. The results show that this population of commissural interneurons includes both excitatory and inhibitory cells that may excite or inhibit contralateral motoneurons directly. They may also influence the activity of motoneurons indirectly by acting through interneurons located outside motor nuclei in the contralateral grey matter but are unlikely to have direct actions on interneurons in the ipsilateral grey matter.

Networks of inhibitory and excitatory commissural interneurons mediating crossed reticulospinal actions

Axonal projections and neurotransmitters used by commissural interneurons mediating crossed actions of reticulospinal neurons were investigated in adult cats. Eighteen interneurons, located in or close to lamina VIII in midlumbar segments, that were monosynaptically excited by reticulospinal tract fibres and projected to contralateral motor nuclei were labelled by intracellular injection of tetramethylrhodamine-dextran and Neurobiotin. The nine most completely labelled interneurons were analysed with combined confocal and light microscopy. None of the stem axons gave off ipsilateral axon collaterals. Seven cells had axon collaterals that arborized in the contralateral grey matter in the ventral horn of the same segments. Transmitters were identified by using antibodies raised against vesicular glutamate transporters 1 and 2, glutamic acid decarboxylase and the glycine transporter 2. The axons of two cells were immunoreactive for the glycine transporter 2 and hence were glycinergic. Three cells were immunoreactive for the vesicular glutamate transporter 2 and hence were glutamatergic. None of the axons displayed immunoreactivity for glutamic acid decarboxylase. Electron microscopy of two cells revealed direct synaptic connections with motoneurons and other neurons. Axonal swellings of one neuron formed synapses with profiles in motor nuclei whereas those of the other formed synapses with other structures, including cell bodies in lamina VII. The results show that this population of commissural interneurons includes both excitatory and inhibitory cells that may excite or inhibit contralateral motoneurons directly. They may also influence the activity of motoneurons indirectly by acting through interneurons located outside motor nuclei in the contralateral grey matter but are unlikely to have direct actions on interneurons in the ipsilateral grey matter.

Distribution and colocalisation of glutamate decarboxylase isoforms in the rat spinal cord

The inhibitory neurotransmitter GABA is synthesized by glutamic acid decarboxylase (GAD), and two isoforms of this enzyme exist: GAD65 and GAD67. Immunocytochemical studies of the spinal cord have shown that whilst both are present in the dorsal horn, GAD67 is the predominant form in the ventral horn. The present study was carried out to determine the pattern of coexistence of the two GAD isoforms in axonal boutons in different laminae of the cord, and also to examine the relation of the GADs to the glycine transporter GLYT2 (a marker for glycinergic axons), since many spinal neurons are thought to use GABA and glycine as co-transmitters. Virtually all GAD-immunoreactive boutons throughout the spinal grey matter were labelled by both GAD65 and GAD67 antibodies; however, the relative intensity of staining with the two antibodies varied considerably. In the ventral horn, most immunoreactive boutons showed much stronger labelling with the GAD67 antibody, and many of these were also GLYT2 immunoreactive. However, clusters of boutons with high levels of GAD65 immunoreactivity were observed in the motor nuclei, and these were not labelled with the GLYT2 antibody. In the dorsal horn, some GAD-immunoreactive boutons had relatively high levels of labelling with either GAD65 or GAD67 antibody, whilst others showed a similar degree of labelling with both antibodies. GLYT2 immunoreactivity was associated with many GAD-immunoreactive boutons; however, this did not appear to be related to the pattern of GAD expression. It has recently been reported that there is selective depletion of GAD65, accompanied by a loss of GABAergic inhibition, in the ipsilateral dorsal horn in rats that have undergone peripheral nerve injuries [J Neurosci 22 (2002) 6724]. Our finding that some boutons in the superficial laminae showed relatively high levels of GAD65 and low levels of GAD67 immunoreactivity is therefore significant, since a reduction in GABA synthesis in these axons may contribute to neuropathic pain.

Neurokinin-1 projection cells in the rat dorsal horn receive synaptic contacts from axons that possess alpha2C-adrenergic receptors

Thealpha2C subclass of adrenergic receptor (alpha2C-AR) mediates some of the antinociceptive actions of norepinephrine in the spinal cord. Axon terminals, which possess this receptor, are concentrated in the superficial dorsal horn and originate from spinal interneurons. We performed a series of combined tract-tracing and immunocytochemical studies to determine whether alpha2C-AR-immunoreactive axons target projection neurons that possess the neurokinin-1 (NK-1) receptor because such cells are likely to transmit nociceptive information to the brain. Spinomedullary neurons were labeled by stereotaxic injection of the B-subunit of cholera toxin (CTb) into the caudal ventrolateral medulla of three anesthetized adult rats. After 3 d, the animals were anesthetized again and fixed by perfusion. Sections were cut from midlumbar segments and reacted with antibodies to reveal alpha2C-ARs, CTb, and NK-1 receptors. Retrogradely labeled neurons possessing the NK-1 receptor (n = 45) were examined with confocal microscopy to investigate their relationship with alpha2C-AR-immunoreactive axons. Numerous alpha2C-AR axons were apposed to cell bodies and proximal dendrites of cells in lamina I and also to distal dendrites that originate from labeled cell bodies in lamina III/IV. A combined confocal and electron microscopic method confirmed that these appositions were synaptic. Additional experiments showed that virtually all alpha2C-AR terminals in contact with labeled cells are also immunoreactive for the vesicular glutamate transporter 2 and therefore are glutamatergic. These data suggest that norepinephrine can modulate excitatory synaptic transmission from spinal interneurons to projection cells by acting at alpha2C-ARs. This could be one of the mechanisms that underlie the antinociceptive actions of norepinephrine.

Distribution of and organisation of dorsal horn neuronal cell bodies that possess the muscarinic m2 acetylcholine receptor

Cholinergic systems in the dorsal horn are involved in antinociception but little is known about the organisation of receptors that mediate this process. In this study we examined immunocytochemical properties of dorsal horn neuronal cell bodies that express the m2 muscarinic acetylcholine receptor. Tissue was examined with confocal laser scanning microscopy and quantitative analysis performed. Immunoreactive cells were found throughout the dorsal horn and in lamina X. Quantitative analysis revealed that 22% of neuronal somata in the dorsal horn possess the receptor. The greatest concentration of cells was found in deeper laminae (IV-VI) and around lamina X. A proportion of cholinergic cells (labelled with an antibody against choline acetyltransferase) were immunoreactive for the receptor (approximately, 40% of dorsal horn cells and 44% of lamina X cells). Populations of presumed inhibitory interneurons also displayed immunoreactivity for the receptor. Between 27-34% of cells immunoreactive for GABA, nitric oxide synthase and the somatostatin receptor(2A) expressed the receptor but only 8% of parvalbumin-immunoreactive cells displayed receptor immunoreactivity. Cells labelled with neurotensin, which belong to a subgroup of excitatory neurons, displayed no receptor immunoreactivity. A small number neurokinin-1 receptor-immunoreactive cells in lamina I possessed m2 immunoreactivity but 42% of laminae III/IV neurokinin-1 cells possessed it. This study shows that a significant proportion of cell bodies in the dorsal horn express the muscarinic m2 acetylcholine receptor. The receptor is present on some cholinergic neurons and therefore may function as an autoreceptor. It is associated with inhibitory local circuit neurons and may have a role in the modulation of specific inhibitory systems. It is also found on a proportion of projection cells that possess the neurokinin-1 receptor. This could be the basis of some of the antinociceptive actions of acetylcholine.

Selective loss of spinal GABAergic or glycinergic neurons is not necessary for development of thermal hyperalgesia in the chronic constriction injury model of neuropathic pain

GABA and glycine are inhibitory neurotransmitters used by many neurons in the spinal dorsal horn, and intrathecal administration of GABA(A) and glycine receptor antagonists produces behavioural signs of allodynia, suggesting that these transmitters have an important role in spinal pain mechanisms. Several studies have described a substantial loss of GABA-immunoreactive neurons from the dorsal horn in nerve injury models, and it has been suggested that this may be associated with a loss of inhibition, which contributes to the behavioural signs of neuropathic pain. We have carried out a quantitative stereological analysis of the proportions of neurons in laminae I, II and III of the rat dorsal horn that show GABA- and/or glycine-immunoreactivity 2 weeks after nerve ligation in the chronic constriction injury (CCI) model, as well as in sham-operated and nai;ve animals. At this time, rats that had undergone CCI showed a significant reduction in the latency of withdrawal of the ipsilateral hindpaw to a radiant heat stimulus, suggesting that thermal hyperalgesia had developed. However, we did not observe any change in the proportion of neurons in laminae I-III of the ipsilateral dorsal horn that showed GABA- or glycine-immunoreactivity compared to the contralateral side in these animals, and these proportions did not differ significantly from those seen in sham-operated or nai;ve animals. In addition, we did not see any evidence for alterations of GABA- or glycine-immunostaining in the neuropil of laminae I-III in the animals that had undergone CCI. Our results suggest that significant loss of GABAergic or glycinergic neurons is not necessary for the development of thermal hyperalgesia in the CCI model of neuropathic pain.

Axon terminals possessing the alpha 2c-adrenergic receptor in the rat dorsal horn are predominantly excitatory

In this study we used confocal microscopy to show that most (83.67%) alpha(2C)-adrenergic receptor (alpha(2C)-AR)-immunoreactive terminals in the superficial dorsal horn contain the vesicular glutamate transporter 2 and hence are glutamatergic. Few (11.33%) terminals contain glutamic acid decarboxylase (a marker for GABAergic axons) and none were associated with the B subunit of cholera toxin (a marker for myelinated primary afferents) or the vesicular glutamate transporter 1. These data indicate that most dorsal horn axons possessing the alpha(2C)-AR are excitatory and add further support to the suggestion that they originate principally from spinal interneurons.

Characterisation of axon terminals in the rat dorsal horn that are immunoreactive for serotonin 5-HT3A receptor subunits

Serotonin 5-HT(3) receptors are abundant in the superficial dorsal horn and are likely to have an involvement in processing of nociceptive information. It has been shown previously that 5-HT(3) receptors are present on primary afferent terminals and some dorsal horn cells. The primary aim of the present study was to determine what classes of primary afferent possess 5-HT(3)A receptor subunits. We performed a series of double- and triple-labelling immunofluorescence experiments. Subunits were labelled with an anti-peptide antibody and primary afferent axons were identified by the presence of calcitonin gene-related peptide (CGRP) and binding of the lectin IB4. Quantitative confocal microscopic analysis revealed that approximately 10% of axons displaying 5-HT(3)A immunoreactivity were also labelled for CGRP but that only 3% of these fibres bind IB4. We also investigated the relationship between immunoreactivity for the subunit and descending serotoninergic systems, axons originating from inhibitory neurons that contain glutamic acid decarboxylase, and axons of a subpopulation of excitatory neurons that contain neurotensin. None of these types of axon was associated with immunoreactivity for receptor subunits. Ultrastructural studies confirmed that punctate immunoreactive structures observed with the light microscope were axon terminals. These terminals invariably formed asymmetric synaptic junctions with dendritic profiles and often contained a mixture of granular and agranular vesicles. Some terminals formed glomerular-like arrangements. Immunoreactive cells were also examined and were found to contain intense patches of reaction product within the cytoplasm. We conclude that the majority (about 87%) of dorsal horn axons that are immunoreactive for 5-HT(3)A receptor subunits do not originate from the subtypes of primary afferent fibres that bind IB4 or contain CGRP. It is likely that most of these axons have an excitatory action and they may originate from dorsal horn interneurons and/or fine myelinated primary afferent fibres.