Visceral inputs to sensory pathways in the spinal cord

Chronic daily intrathecal injections of a large volume of fluid increase mast cells in the thalamus of mice

Mast cells are found in the central nervous system (CNS) as well as in the periphery. In the brain of mice, they are localized primarily in the thalamus and meninges. Although their numbers increase in response to stress, the mediator of their recruitment is not known. During studies in which drugs were delivered intrathecally in a volume sufficiently large to distribute to the brain, we discovered that repeated daily injections of this large volume increased the number of mast cells in the thalamus. The increase was not due to changes in electrolyte composition of the cerebrospinal fluid (CSF) as chronically administered artificial CSF produced similar effects. Repeated injections of even small volumes (2 mul) increased mast cells in the medial intralaminar (Med), ventral posterior (VP) and posterior (Po) nuclei. Increasing the volume injected daily to 20 mul increased mast cells in the lateral intralaminar (Lat), laterodorsal (LD), ventrolateral (VL) and lateral geniculate (LG) nuclei and further increased those in the lateral extension of the Po nucleus. Thus, small and large volumes augment distinct populations of mast cells. While stem cell factor (SCF) is abundant in the CNS and is chemotactic to mast cells in the periphery, thalamic mast cells in the rodent do not express c-kit, the SCF receptor, suggesting that this factor may not be responsible for the effect. Consistent with this, centrally injected SCF was incapable of increasing thalamic mast cell populations after either single or chronic (21 days) daily injections compared to the effect of saline alone. Although the mechanism is not known, repeated injections of a large volume of fluid dramatically increase mast cells in the CNS, a phenomenon that may be relevant to clinical conditions of increased CSF pressure or volume.

Branching and/or collateral projections of spinal dorsal horn neurons

Branching and/or collateral projections of spinal dorsal horn neurons is a common phenomenon. Evidence is presented for the existence of STTm/STTl, STTc/STTi, STT/SMT, STT/SRT, SCT/DCPS, SST/DCPS, SCT/SST, STT/SHT, STeT/SHT, STeTs and other doubly or multiply projecting spinal neurons that have been anatomically and physiologically identified and named based on the locations of the cells of origin and their terminations in the brain. These newly discovered spinal projection neurons are characterized by a single cell body and branched axons and/or collaterals that project to two or more target areas in the brain. These novel populations of neurons seem to be a fuzzy set of spinal projection neurons that function as an intersection set of the corresponding single projection spinal neurons and to be at an intermediate stage phylogenetically. Identification strategies are discussed, and general concluding remarks are made in this review.

Projections from the renal nerve to the cat's lateral somatosensory thalamus

The representation of the kidney in the lateral somatosensory thalamus was mapped using electrical stimulation of the renal nerve in pentobarbitone-anesthetized cats. Ninety-five of 197 thalamic neurons studied responded to renal nerve stimulation. The responsive neurons were located in the periphery of the ventral posterolateral nucleus (42%; VPLp) and the neighboring dorsal and lateral aspects of the posterior complex (58%; POd and POl). No visceroceptive neurons were found within VPL proper. The mean response latency of the thalamic neurons to electrical nerve stimulation was 9.5 +/- 2.6 ms (mean +/- S.D.), suggesting an involvement of A delta, and possibly A beta fibers in the primary afferent pathway. The visceroceptive neurons were further characterized with innocuous mechanical stimulation of the body surface, and for 94 of the 95 neurons a somatic receptive field could be determined. Of these, 35% were located on the lower back and belly, i.e., the dermatomes of the lower thoracic and upper lumbar spinal projection areas of the renal nerve. 52% of the somatic receptive fields were located on the contralateral foot, thigh, tail, or hind leg (lower lumbar, sacral and coccygeal dermatomes) and 13% covered the arm and upper body (upper thoracic and lower cervical dermatomes). Comparison between the thalamic representations of the renal and pelvic nerves showed that both covered comparable areas adjacent and around, but not within VPL proper. It is concluded that VPLp, POd and POl play a role in processing visceral, possibly including nociceptive, information from the kidney of the cat.

Essential organization of the sympathetic nervous system

The sympathetic nervous system consists of efferent neurones supplying the viscera. The cell bodies of preganglionic neurones are located in four areas in the thoracolumbar cord; however, the majority are found in the IML. Various tracing techniques have provided information concerning the location of the cell bodies of sympathetic preganglionic neurones projecting into various nerves and ganglia and regulating the adrenal gland, the kidney and the sympathetic supply to skeletal muscle. Numerous supraspinal neurones project to the neuropil surrounding sympathetic preganglionic neurones and may form synaptic contacts with these neurones. The areas of the brain that project to the IML appear to be part of a network of reciprocally connected supraspinal cell groups. Although much emphasis has been placed on the importance of the RVLM in the mediation of tonic and phasic inputs to sympathetic preganglionic neurones, it appears that other areas are of significant import; the RVLM should not be considered to be 'the vasomotor centre'. Spinal and cranial afferents influence the sympathetic nervous system. Baroreceptor afferents terminate in the NTS and may utilize an excitatory amino acid as their neurotransmitter. However, a number of neuropeptides are also associated with these afferents. Neurones within the NTS project to a number of brain stem areas thought to be involved in the regulation of sympathetic activity; consequently the baroreceptor reflex may be mediated over a number of parallel pathways involving both supraspinal and spinal sites of inhibition. Many neurotransmitters are thought to regulate the activity of sympathetic preganglionic neurons: monoamines, peptides and amino acids. Matching the chemical content of the cell bodies of neurones within a particular cell group with physiological characteristics is a challenging task; some barosensitive neurones of the RVLM do not appear to be adrenergic although they are in the midst of the C1 adrenergic cell group. Besides acetylcholine and noradrenaline, neurotransmission in the periphery appears to involve numerous peptides and ATP.

A quantitative light and electron microscopic analysis of taurine-like immunoreactivity in the dorsal horn of the rat spinal cord

Taurine has been proposed as an inhibitory neurotransmitter or neuromodulator in the vertebrate central nervous system. Within the spinal cord, taurine has been shown to have a direct inhibitory effect on spinal neurons and to have a selective antinociceptive effect on chemically induced nociception. Although sufficient data exists to suggest that taurine plays a neurotransmitter or neuromodulatory role in the spinal cord, it is not known whether this amino acid is present in axon terminals nor if this amino acid has a unique pattern of distribution within spinal tissue. To address these questions a monoclonal antibody against taurine was employed to localize taurine-like immunoreactivity in the dorsal horn of the rat spinal cord by using both light and electron microscopic techniques. Taurine-like immunoreactivity was most dense and most prominent in laminae I and II of the dorsal horn. A moderate amount of immunoreactivity was also present in laminae VIII and IX and X while the remaining laminae were only lightly stained. In laminae I and II taurine-like immunostaining was evident within neuronal cell bodies, dendrites, myelinated and unmyelinated axons, axon terminals, and astrocytes and their processes. Cell counts of these two laminae indicated that approximately 30% of neuronal perikarya at the C2 level, 52% of neuronal perikarya at the T6 level, and 18% of neuronal perikarya at the L2 level of the cord exhibited taurine-like immunoreactivity. With preembedding diaminobenzidine staining, approximately 20% of the axons examined in laminae I and II were found to be immunoreactive for taurine. Using postembedding immunogold staining in combination with quantitative procedures, the highest densities of gold particles were found in axon terminals containing pleomorphic vesicles and forming symmetrical synapses (36.8 particles/micron2), in a subpopulation of myelinated axons (34.2 particles/micron2), in a subpopulation of neuronal dendrites (32.6 particles/micron2), and in capillary endothelial cells (39.8 particles/micron2). Moderate labeling occurred in astrocytes (20.9 particles/micron2) and neuronal perikarya (18.7 particles/micron2). The localization of taurine to presumptive inhibitory axon terminals provides anatomical support for the hypothesis that taurine may serve an inhibitory neurotransmitter role in the superficial dorsal horn of the spinal cord. On the other hand, its localization to astrocytes and endothelial cells within both the dorsal ventral horns implies that it serves other nonneuronal functions as well.

Splanchnic input to thoracic spinal neurons and its supraspinal modulation in the rat

The urinary responses of 62 T8-T11 spinal neurons were recorded extracellularly following electrical stimulation of the greater splanchnic nerve (GSN) in chloralose-anesthetized rats. Recorded neurons were found in both the dorsal and ventral horns. Fifty-seven neurons increased their firing rate in response to GSN stimulation; 8 of these exhibited biphasic responses consisting of excitations followed by inhibitions. Excitatory responses to GSN stimulation consisted of either one or two bursts with latencies consistent with activation by either A delta or C fibers. GSN stimulation inhibited 5 neurons. The effects of reversible spinalization on spontaneous activity and on both synchronous and non-synchronous (afterdischarge) GSN-evoked responses were investigated using a cooling probe on the spinal cord between C1 and C2. Of 19 neurons tested in this way, 9 exhibited opposite directional changes in their spontaneous activities and their GSN-evoked responses upon spinalization. Differential effects of cold-block on first and second bursts, or on A delta- and C-fiber mediated responses, were not usually observed. However, differential effects of cold-block on synchronous and non-synchronous portions of the overall GSN-evoked response were often observed in that their magnitudes often changed independently of one another. Supraspinal pathways contributed to GSN-evoked responses of several neurons because their responses were diminished during cooling while spontaneous activity was increased or unchanged. These decreases in the magnitude of the GSN-evoked response were not always accounted for by decreases in the synchronous portions of the responses. However, most neurons did exhibit decreases in the number of non-synchronous responses, or afterdischarges, during spinal cooling, exhibiting in some cases biphasic responses. This study provides evidence for strong supraspinal regulation of splanchnic afferent input to the spinal cord of the rat. Further, this regulation exhibits some specificity toward different portions of splanchnic-evoked responses in spinal neurons.

Bilateral inputs and supraspinal control of viscerosomatic neurones in the lower thoracic spinal cord of the cat

1. Single-unit activity has been recorded from eighty-three viscero-somatic neurones in the lower thoracic spinal cord (T9-T11) of chloralose-anaesthetized cats. These neurones were driven by natural and/or electrical stimulation in their somatic receptive fields and gave excitatory responses to electrical stimulation of the ipsilateral splanchnic nerve. Contralateral visceral inputs were tested by electrical stimulation of the contralateral splanchnic nerve. Tonic and phasic descending influences were tested by reversible spinalization with cold block at T7 and by electrical stimulation in nucleus raphe magnus and the immediately adjacent reticular formation. 2. Most viscero-somatic neurones (89%) gave an excitatory response to stimulation of the contralateral splanchnic nerve and were therefore considered to have bilateral visceral inputs. In this group of neurones three categories of cells were identified depending on whether their responses to ipsilateral splanchnic nerve stimulation were decreased (50%), increased (42%) or unchanged (8%) in the spinal state. Only one cell with an exclusively ipsilateral visceral input was tested for the effects of reversible spinalization. Stimulation of contralateral splanchnic nerve failed to evoke activity in this cell in the spinal state. 3. Sixty-four viscero-somatic neurones with bilateral visceral inputs and four neurones with exclusively ipsilateral visceral inputs were tested with electrical stimulation in nucleus raphe magnus and the adjacent reticular formation. Seventy-eight per cent gave an initial excitatory response which was followed by a period of reduced responsiveness to stimulation of visceral and somatic afferents. Three of the four neurones with an exclusively ipsilateral visceral input had no excitatory drive from the brain stem but their responses to stimulation of visceral and somatic afferents were depressed. 4. The majority (77%) of neurones with bilateral inputs were located in laminae VII and VIII with the remainder in the dorsal horn, predominantly laminae I and V, whereas all but one of the neurones with an exclusively ipsilateral visceral input were located in the superficial dorsal horn, predominantly lamina I, and none in laminae VII and VIII. 5. These results show that the majority of viscero-somatic neurones in the cat's lower thoracic spinal cord receive bilateral visceral inputs and that the transfer of this information is subjected to descending control which includes excitation as well as inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)