Sympathetic preganglionic efferent and afferent neurons mediated by the greater splanchnic nerve in rabbit

Ultrasound-Guided Dorsolateral Approach for Quadratus Lumborum Block in Rabbits (Oryctolagus cuniculus): A Prospective, Randomized, Blinded, Cadaveric Study Comparing Four Different Injectate Volumes

Ultrasound-guided (US-guided) loco-regional anesthesia can provide significant analgesia and anesthetic-sparing effects when used in rabbits. The aims of this study were to investigate the thoraco-lumbar anatomy of the rabbits, particularly the quadratus lumborum (QL) muscle, to design an appropriate US-guided quadratus lumborum block (QLB) specific for rabbits, and to define the most adequate volume of injectate required to consistently cover the ventral branches of T11 to L3 without affecting the pelvic limb innervation (L4, L5 and L6). Sixteen adult rabbit cadavers were included in the study. After randomization, four different volumes of injectate (0.1 mL/kg, 0.2 mL/kg, 0.3 mL/kg and 0.4 mL/kg) were tested, with these volumes additionally randomized to two sites of injection (right or left QL fascia). An ultrasound-guided QLB was performed with a solution of lidocaine, iodinated contrast and tissue dye (in a proportion of 3:1:1 volume, respectively), with subsequent computed tomography (CT) and anatomical dissection, to evaluate the spread of the injectate. In all but one case, the US-guided QLB performed with a dorsolateral approach using 0.3 mL/kg was adequate, while a dose of 0.4 mL/kg consistently reached the targeted nerves but also extended to L4 and caudally. This may suggest that an injectate volume of 0.3 mL/kg may be the most appropriate to produce adequate spread while not affecting pelvic limb innervation.

Neural pathways that control the glucose counterregulatory response

Glucose is an essential metabolic substrate for all bodily tissues. The brain depends particularly on a constant supply of glucose to satisfy its energy demands. Fortunately, a complex physiological system has evolved to keep blood glucose at a constant level. The consequences of poor glucose homeostasis are well-known: hyperglycemia associated with uncontrolled diabetes can lead to cardiovascular disease, neuropathy and nephropathy, while hypoglycemia can lead to convulsions, loss of consciousness, coma, and even death. The glucose counterregulatory response involves detection of declining plasma glucose levels and secretion of several hormones including glucagon, adrenaline, cortisol, and growth hormone (GH) to orchestrate the recovery from hypoglycemia. Low blood glucose leads to a low brain glucose level that is detected by glucose-sensing neurons located in several brain regions such as the ventromedial hypothalamus, the perifornical region of the lateral hypothalamus, the arcuate nucleus (ARC), and in several hindbrain regions. This review will describe the importance of the glucose counterregulatory system and what is known of the neurocircuitry that underpins it.

The viscerosympathetic response in rabbits is mediated by GABAergic and glutamatergic inputs into the sympathetic premotor neurons of the rostral ventrolateral medulla

Neurons in the rostral ventrolateral medulla (RVLM) receive inputs from various sources, including baroreceptors, and then regulate the activity of sympathetic preganglionic neurons in the spinal cord. Whether RVLM neurons mediate the viscerosympathetic reflex has yet to be clarified. In the present study, we investigated the role of RVLM neurons in the viscerosympathetic reflex in anaesthetized and vagotomized rabbits. Electrical stimulation of the greater splanchnic nerve (SplN) evoked reflex responses in renal sympathetic activity that were composed of inhibitory and/or excitatory components. Bilateral microinjection of muscimol, a GABA(A) receptor agonist, into the RVLM blocked the reflex responses. Bilateral microinjection of bicuculline, a GABA(A) receptor antagonist, largely attenuated the inhibitory component, whereas kynurenic acid, a glutamate receptor antagonist, eliminated the excitatory component. The activities of 21 RVLM barosensitive bulbospinal neurons were recorded. Twenty of the neurons responded to the SplN stimulation. The responses also consisted of inhibitory and/or excitatory components. The excitatory component of these neurons preceded that of the renal sympathetic nerve activity by about 100 ms. This latency difference was almost the same as that of the inhibitory responses evoked by aortic nerve stimulation. Therefore, the renal sympathetic reflex responses evoked by SplN stimulation are mediated by RVLM neurons, and GABAergic and glutamatergic transmission in the RVLM are related to this reflex.

The role of the RVLM neurons in the viscero-sympathetic reflex: a mini review

Neurons in the rostral ventrolateral medulla (RVLM neurons) receive inputs from various sources, including baroreceptors, and then regulate activity of sympathetic preganglionic neurons. Though RVLM neurons are assumed to mediate the viscero-sympathetic reflex, it has not been clarified yet. Here we give a brief overview of the participation of RVLM neurons in the viscero-sympathetic reflex. We conclude that RVLM neurons show excitatory and inhibitory responses to stimulation of sympathetic afferents and mediate multi-phase reflex responses of the sympathetic nerve.

Biophysical and histological determinants underlying natural firing behaviors of splanchnic sympathetic preganglionic neurons in neonatal rats

Isolated thoracic spinal cords of neonatal rats spontaneously generate splanchnic sympathetic nerve discharge (SND) with a quasiperiodic rhythm approximately 1-Hz. Using in vitro nerve-cord preparations that retained T6-T12 spinal segments, we investigated whether the natural firing behavior of sympathetic preganglionic neurons (SPNs) encoded the SND rhythm and what were the main biophysical and histological determinants of SPN firing. Under extracellular recording conditions, electrical stimulation of splanchnic nerves elicited antidromic responses in 212 SPNs. Among them, 92 SPNs were quiescent; 120 active SPNs had an average firing rate of 0.72+/-0.04 Hz, which was close to the quasiperiodic rhythm of SND. SPNs with rhythmic burst firing were rare. Probability plots of interspike intervals were constructed to extract mathematical features underlying SPN firing. Most active SPNs (88%) had a firing well described by unimodal Gaussian, suggesting a predominantly tonic pattern with normal variations. Biophysical properties of 112 SPNs were measured under whole-cell recording conditions. The charging time constant, tau, is positively correlated with the average firing rate. Histological properties were examined in 45 SPNs with intracellular diffusion of Lucifer Yellow or biocytin. SPNs with pyramidal somata and multipolar dendrites tend to be spontaneously active. In contrast, those with bipolar somata and fewer dendritic branches were quiescent in firing. These observations suggest that activity levels of SPNs are correlated with their capacity for temporal and spatial summation of synaptic inputs. How the seemingly tonic firing of individual SPNs is integrated into whole-nerve SND with quasiperiodic rhythms is discussed.

Identification of active thoracic spinal segments responsible for tonic and bursting sympathetic discharge in neonatal rats

The isolated thoracic cord of a neonatal rat in vitro generates tonic sympathetic activities in the splanchnic nerves. This tonic sympathetic nerve discharge (SND) has a prominent quasi-periodic oscillation at approximately 1-2 Hz. Bath application of bicuculline and strychnine, which removes endogenous GABA(A) and glycine receptor activities, transforms the quasi-periodic tonic SND into synchronized bursts (bSND). Picrotoxin, another GABA(A) receptor antagonist, also induces bSND. Serial transections of the thoracic cord (T1-12) were performed to identify the cord segments responsible for these tonic and bursting SNDs. Removal of T1-5 did not affect tonic SND. Nerve-cord preparation with either T6-8 or T10-12 segments could generate a substantial amount of tonic SND that retained comparable oscillating patterns. On the other hand, removal of T1-5 significantly reduced bSND amplitude without affecting its rhythmicity. Either T6-8 or T10-12 segments alone could generate bSND. Mid-point transection of T6-12 at T9 might split bSND rhythmogenesis, leading to the occurrence of bSND that could be attributed to two independent oscillators. Our results demonstrated that three segments within the T6-12 cord were sufficient to generate a rudimentary tonic and bursting SNDs. The thoracic cord segments, however, are dynamically interacting so that a full size bSND could only be produced with the intact thoracic cord.

Central projections of thoracic splanchnic and somatic nerves and the location of sympathetic preganglionic neurons in Xenopus laevis

The central and peripheral organization of thoracic visceral and somatic nervous elements was studied by applying dextran amines to the proximal cut ends of the thoracic splanchnic and somatic nerves in Xenopus laevis. Many labeled dorsal root ganglion cells of visceral afferents, and all somatic afferents, were located in a single ganglion of one spinal segment, and the two types of cells were distributed topographically within the ganglion. The labeled sympathetic preganglionic neurons were located predominantly in the same area of the thoracic spinal gray as in other frogs and in mammals. The labeled visceral afferents projected to Lissauer's tract and the dorsal funiculus. The visceral fibers of the tract ascended to the level of the subcerebellar area, supplying collateral branches to the lateral one-third of the dorsal horn and to the area of brainstem nuclei, including lateral cervical and descending trigeminal nucleus, and descended to the filum terminale. The visceral fibers of the dorsal funiculus were distributed to the dorsal column nucleus and the solitary tract. A similar longitudinal projection was also seen in the somatic afferents. The dual central pathway of thoracic primary afferents in the anuran spinal cord is a property held in common with mammals, but the widespread rostrocaudal projection through Lissauer's tract may be a characteristic of the anuran central nervous system. In frogs, the direct transmission of primary afferent information to an extremely wide area of the central nervous system may be important for prompt assessment of environmental factors and control of body functions.

Distribution of immunoreactivity for enkephalin, substance P and vasoactive intestinal peptide in fibres surrounding splanchnic sympathetic preganglionic neurons in rats

The distribution of substance P, enkephalin and vasoactive intestinal peptide in fibres and cells was examined in the autonomic nuclei of the lower thoracic and lumbar segments of the rat spinal cord. Attention was focussed on the location of the peptides in sympathetic preganglionic neurons contributing to the greater and lesser splanchnic nerves and in fibres surrounding these neurons. To identify splanchnic preganglionic neurons, Fluoro-Gold was applied to the left splanchnic nerve in anaesthetized rats and some of these animals received intrathecal administration of colchicine at thoracic segments 6, 9 and 12, 24-48 h before perfusion with fixative. Immunoreactivity for substance P, enkephalin and vasoactive intestinal peptide in fibres and cells of the sixth thoracic to second lumbar spinal cord was detected with fluorescent immunocytochemical techniques. Most retrogradely labelled cells (90%) were located in the intermediolateral nucleus and the rest were situated in the nucleus intercalatus and the central autonomic nucleus of the gray matter. Terminals of fibres containing immunoreactivity to all three peptides were found in all autonomic regions. Fibres immunoreactive for substance P and enkephalin were seen projecting in the white matter to the region of the intermediolateral nucleus and extending from this nucleus to the central autonomic nucleus. Terminals containing each of the three peptides were also found surrounding the retrogradely labelled cells in the intermediolateral nucleus. Approximately two cells immunoreactive for vasoactive intestinal peptide were found per section and 80% were located in the autonomic regions. Fewer cells immunoreactive for substance P and enkephalin were observed (approximately one per section) and 70% were outside laminae VII and X. Although cells immunoreactive for substance P, enkephalin and vasoactive intestinal peptide were located in all autonomic regions of the spinal cord, cells doubly labelled with retrograde dye and with the antisera to either of the peptides could not be identified. The data suggest that (i) substance P, enkephalin and vasoactive intestinal peptide are contained in fibres of neurons regulating preganglionic sympathetic control of the abdominal viscera and its vasculature; and (ii) these peptides may not be major transmitters within splanchnic preganglionic neurons.

The intermediolateral nucleus: an 'open' or 'closed' nucleus?

The sympathetic preganglionic neurons located in the intermediolateral nucleus (IML) that project to the superior cervical ganglion of the rabbit were observed to have two major dendritic orientations after retrograde labeling with horseradish peroxidase. One projection extends longitudinally within IML. The second projection courses medially and presents a triangular shape in horizontal sections. The labeled processes that project medially arise from cells in IML and project through the intercalated nucleus towards the central autonomic area and follow the contour of the central canal. Medially oriented dendrites intruding into other areas of the intermediate grey matter show that IML is an 'open' rather than a 'closed' nucleus as has been recently suggested. The location and distribution of the sympathetic preganglionic neurons projecting to the superior cervical ganglion in the rabbit are compared with those reported for other species.

The central projections of primary afferent neurons of greater splanchnic and intercostal nerves in the rat. A horseradish peroxidase study

The central projections of primary afferent fibers of the greater splanchnic nerve of the rat were investigated using the transganglionic horseradish peroxidase transport technique. In addition, the corresponding spinal ganglion cells and the preganglionic sympathetic neurons were demonstrated. For comparing visceral and somatic afferents, intercostal nerve afferents were labelled by the same technique. Splanchnic afferent dorsal root ganglion cells were found at segments T3 to T13 ipsilaterally, with the greatest density at T8 to T12. Labelled cells represented about 10%-15% of all neurons in the ganglia at maximal projection levels. They were randomly distributed within individual ganglia. The great majority were medium to small sized and round to slightly oval in shape. In the spinal cord, labelled visceral afferent axons were found maximally at T8 to T11, but could be detected in decreasing density up to T1 and down to L1. They were distributed over Lissauer's tract and the dorsal funiculus to a medial and lateral collateral pathway (MCP and LCP, respectively). The MCP, somewhat more prominent than the LCP, was destined primarily to clustered presumptive terminal fields in medial lamina I and outermost lamina IIa. Only a few axons continued further to laminae V and X. Splanchnic afferent axons, most likely derived from the MCP, formed a longitudinal bundle ventral to the central canal. The LCP consisted of more or less well-defined axon bundles emanating from the lateral Lissauer's tract and curving round the lateral edge of the dorsal horn and through the dorsolateral funiculus. Presumptive terminal sites of LCP axons are the lateral laminae I and IIa, the nucleus of the dorsolateral funiculus and the dorsal part of lamina V. A few LCP axons were seen in the vicinity of lateral dendrites of preganglionic sympathetic axons. Visceroafferent terminals were absent from laminae IIb-IV and VII. The possible consequences of the MCP/LCP duality for the central connections of splanchnic afferents are discussed. Some splanchnic afferents ascended to the gracile and cuneate nuclei, and rarely to the spinal trigeminal nucleus. These results fit into the general concept of visceroafferent terminal organization that has emerged during the last few years. Differences to other reports in the detailed arrangement of fibers and terminals are discussed. Somatoafferent cell bodies represented the vast majority of neurons in the respective spinal ganglia. Cell sizes encompassed the whole range from very small to very large without a clear predominance of one particular size class.(ABSTRACT TRUNCATED AT 400 WORDS)