Species differences in the distribution and coexistence ratio of serotonin and substance P in the monkey, cat, rat and chick spinal cord

Firing patterns of spontaneously active motor units in spinal cord-injured subjects

Involuntary motor unit activity at low rates is common in hand muscles paralysed by spinal cord injury. Our aim was to describe these patterns of motor unit behaviour in relation to motoneurone and motor unit properties. Intramuscular electromyographic activity (EMG), surface EMG and force were recorded for 30 min from thenar muscles of nine men with chronic cervical SCI. Motor units fired for sustained periods (>10 min) at regular (coefficient of variation ≤ 0.15, CV, n =19 units) or irregular intervals (CV>0.15, n =14). Regularly firing units started and stopped firing independently suggesting that intrinsic motoneurone properties were important for recruitment and derecruitment. Recruitment (3.6 Hz, SD 1.2), maximal (10.2 Hz, SD 2.3, range: 7.5-15.4 Hz) and derecruitment frequencies were low (3.3 Hz, SD 1.6), as were firing rate increases after recruitment (~20 intervals in 3 s). Once active, firing often covaried, promoting the idea that units received common inputs.Half of the regularly firing units showed a very slow decline (>40 s) in discharge before derecruitment and had interspike intervals longer than their estimated after hyperpolarisation potential (AHP) duration (estimated by death rate and breakpoint analyses). The other units were derecruited more abruptly and had shorter estimated AHP durations. Overall, regularly firing units had longer estimated AHP durations and were weaker than irregularly firing units, suggesting they were lower threshold units. Sustained firing of units at regular rates may reflect activation of persistent inward currents, visible here in the absence of voluntary drive, whereas irregularly firing units may only respond to synaptic noise.

Species-specific modulation of pattern-generating circuits

Phylogenetic comparison can reveal general principles governing the organization and neuromodulation of neural networks. Suitable models for such an approach are the pyloric and gastric motor networks of the crustacean stomatogastric ganglion (STG). These networks, which have been well studied in several species, are extensively modulated by projection neurons originating in higher-order ganglia. Several of these have been identified in different decapod species, including the paired modulatory proctolin neuron (MPN) in the crab Cancer borealis [Nusbaum & Marder (1989) J. Neurosci., 9,1501-1599; Nusbaum & Marder (1989), J. Neurosci., 9, 1600-1607] and the apparently equivalent neuron pair, called GABA (gamma-aminobutyric acid) neurons 1 and 2 (GN1/2), in the lobster Homarus gammarus [Cournil et al. (1990) J. Neurocytol., 19, 478-493]. The morphologies of MPN and GN1/2 are similar, and both exhibit GABA-immunolabelling. However, unlike MPN, GN1/2 does not contain the peptide transmitter proctolin. Instead, GN1/2, but not MPN, is immunoreactive for the neuropeptides related to cholecystokinin (CCK) and FLRFamide. Nonetheless, GN1/2 excitation of the lobster pyloric rhythm is similar to the proctolin-mediated excitation of the crab pyloric rhythm by MPN. In contrast, GN1/2 and MPN both use GABA but produce opposite effects on the gastric mill rhythm. While MPN stimulation produces a GABA-mediated suppression of the gastric rhythm [Blitz & Nusbaum (1999) J. Neurosci., 19, 6774-6783], GN1/2 activates or enhances gastric rhythmicity. These results highlight the care needed when generalizing neuronal organization and function across related species. Here we show that the 'same' neuron in different species does not contain the same neurotransmitter complement, nor does it exert all of the same effects on its postsynaptic targets. Conversely, a different transmitter phenotype is not necessarily associated with a qualitative change in the way that a modulatory neuron influences target network activity.

Immunohistochemical colocalization of serotonin, substance P and Met-enkephalin-Arg6-Gly7-Leu8 in the endocrine cells of the ruminant duodenum

The duodenum of various ruminants was examined by immunohistochemical staining for serotonin, substance P and Metenkephalin-Arg6-Gly7-Leu8 (MENK8). Serotonin- and substance P-immunoreactive endocrine cells were detected in samples from cow, calf, sheep, goat, and barbary sheep, MENK8-immunoreactive endocrine cells were detected exclusively in samples from cow and calf. Substance P and MENK8 immunoreactivity was found in serotonin-immunoreactive endocrine cells of cattle. Substance P- and MENK8-immunoreactive nervous elements were detected in all ruminants examined. The present results suggest that the expression and/or the mechanism that controls the expression of each peptide might differ between endocrine cells and nerve cells and might also depend on animal species.

Peptidergic transmission: from morphological correlates to functional implications

Like non-peptidergic transmitters, neuropeptides and their receptors display a wide distribution in specific cell types of the nervous system. The peptides are synthesized, typically as part of a larger precursor molecule, on the rough endoplasmic reticulum in the cell body. In the trans-Golgi network, they are sorted to the regulated secretory pathway, packaged into so-called large dense-core vesicles, and concentrated. Large dense-core vesicles are preferentially located at sites distant from active zones of synapses. Exocytosis may occur not only at synaptic specializations in axonal terminals but frequently also at nonsynaptic release sites throughout the neuron. Large dense-core vesicles are distinguished from small, clear synaptic vesicles, which contain "classical' transmitters, by their morphological appearance and, partially, their biochemical composition, the mode of stimulation required for release, the type of calcium channels involved in the exocytotic process, and the time course of recovery after stimulation. The frequently observed "diffuse' release of neuropeptides and their occurrence also in areas distant to release sites is paralleled by the existence of pronounced peptide-peptide receptor mismatches found at the light microscopic and ultrastructural level. Coexistence of neuropeptides with other peptidergic and non-peptidergic substances within the same neuron or even within the same vesicle has been established for numerous neuronal systems. In addition to exerting excitatory and inhibitory transmitter-like effects and modulating the release of other neuroactive substances in the nervous system, several neuropeptides are involved in the regulation of neuronal development.

Immunohistochemical examination of intraspinal serotonin neurons and fibers in the chicken lumbar spinal cord and coexistence with Leu-enkephalin

Intraspinal serotonin-positive cells and fibers were examined in the chicken lumbar spinal cord following removal of descending serotonin fibers by spinal transection. Co-localization of Leu-enkephalin immunoreactivity in intraspinal serotonin cells was also examined using a double immunofluorescence labeling technique. By one or two weeks after spinal transection, virtually all supraspinal serotonin fibers were eliminated. Intraspinal serotonin cells were located ventral or ventrolateral to the central canal corresponding to laminae VII, VIII, and IX, and the anterior funiculus. Intraspinal serotonin cells sent fibers to (1) the pia mater on the ventral or ventrolateral surface of the spinal cord; (2) vessels in the spinal cord; (3) sympathetic preganglionic column of Terni; (4) other intraspinal serotonin neurons; (5) the central canal. Some 30%-50% of the intraspinal serotonin cells co-localized with Leu-enkephalin. Intraspinal serotonin fibers co-containing Leu-enkephalin were observed in the pia mater located on the most lateral surface of the spinal cord.

Immunohistochemical analysis of the relation between 5-hydroxytryptamine- and neuropeptide-immunoreactive elements in the spinal cord of an amphibian (Xenopus laevis)

In mammals, a large proportion of the bulbospinal 5-hydroxytryptamine (5-HT) neurons also contain neuropeptides, such as substance P (SP) and galanin (GAL). To examine whether a similar coexistence occurs in an amphibian, an immunofluorescence double-labelling technique was employed on sections of the Xenopus laevis spinal cord. Antisera raised against SP, GAL, enkephalin (ENK), corticotropin-releasing factor (CRF), calcitonin gene-related peptide (CGRP), and cholecystokinin (CCK) produced a labelling of fibers at all rostrocaudal levels of the spinal cord, with the highest fiber densities for SP and ENK and intermediate densities for GAL, CCK, and CGRP, while CRF-immunoreactive fibers were barely detectable in intact animals. 5-HT-immunoreactive fibers were widely distributed in the spinal cord, and they often occurred in the vicinity of different types of peptide-immunoreactive fibers. However, no coexistence between 5-HT and the different peptide immunoreactivities could be detected, although SP and GAL immunoreactivities were sometimes found to be colocalized in the same fiber. Similar negative results were obtained when 5-HT+SP- and 5-HT+GAL-labelled sections were examined in single focal planes with a confocal microscope. After a spinal transection, (survival period 6 weeks to 4 months), almost all 5-HT-immunoreactive fibers below the lesion were lost, and a build-up of immunoreactive material occurred in fibers just rostral to the cut. In contrast, no significant loss of peptide-immunoreactive fibers occurred, although some swollen SP-, GAL-, ENK-, CRF-, and CCK-immunoreactive fibers were present rostral to the cut. The distribution of swollen peptide-immunoreactive fibers did not overlap with that of the swollen 5-HT-immunoreactive fibers. Although negative immunohistochemical data must be interpreted with caution, in conjunction with previous studies (Brodin et al. [1988] J. Comp. Neurol. 271:1-18; Sakamoto and Atsumi [1991] Cell Tissue Res. 264:221-230), the present results indicate that bulbospinal 5-HT neurons in nonmammalian vertebrates cocontain neuropeptides to a lesser extent than in mammals.

Synaptic loss following removal of serotoninergic fibers in newly hatched and adult chickens

Neurotransmitters such as serotonin (5HT) may have nontransmitter, trophic-like functions in the developing and adult nervous system. In order to examine this possibility in the avian spinal cord, we have quantified synapse numbers on spinal neurons following treatment with drugs that result in the destruction of 5HT positive axons. Either p-chlorophenylalanine or reserpine was injected into newly hatched or adult chickens. Following treatment for 7 days the density of nonserotoninergic synapses was considerably decreased in the targets of 5HT fibers. By contrast, neither change was observed in the dendritic structures of spinal motoneurons or in the distribution of substance P and enkephalin positive fibers. These data suggest that 5HT may play an important role in the normal increase and maintenance of synapses in developing and adult animals. A lesion of 5HT neurons may not only alter neurochemistry but also alter the general synaptic structures of the brain. While 5HT containing fibers were depleted in a dose-dependent fashion we cannot rule out the possibility that other neurotransmitter systems were depleted at higher dose of PCPA and reserpine.

Immunohistochemical study on development of serotonin-, substance P-, and enkephalin-positive fibers in the rat spinal motor nucleus

Developmental changes in the coexistence of serotonin and substance P/enkephalin within single fibers of lamina IX of the rat lumbar spinal cord were examined by the use of a double-labelling immunohistochemical technique. On postnatal day (P) 0, 65.0% of immunoreactive varicosities contained only serotonin, and 21.3% of them had both serotonin and substance P. The coexistent ratio of serotonin and substance P in single fibers increased with development: 61.9% of serotonin positive varicosities co-contained substance P on P28, similar to the ratio found in adult animals (67.4%). The ratios of varicosities containing only substance P remained the same from P0 to adult stage (about 15%). Enkephalin positive immunoreactivity was not co-localized with serotonin positive varicosities at any stage of development. Although numerous serotonin positive fibers were found in lamina IX, only a few substance P and enkephalin positive fibers were observed in the same area on P0. The density of serotonin positive varicosities increased slightly by P28, whereas substance P and enkephalin positive fibers increased considerably by this age. Between P28 and the adult stage, the density of serotonin positive fibers decreased by about 50%. The cross sectional area of axonal varicosities containing serotonin- and substance P-like immunoreactivity was similar in both P0 and adult animals, whereas that of enkephalin positive fibers was different. We also examined the coexistence of serotonin and substance P within single neurons of the caudal raphe nuclei in P7 and adult animals, and found that the coexistent ratio significantly increased with development.

Substance P-containing pyramidal neurons in the cat somatic sensory cortex

Light and electron microscopic immunocytochemical methods were used to verify the possibility that neocortical pyramidal neurons in the first somatic sensory cortex of cats contain substance P. At the light microscopic level, substance P-positive neurons accounted for about 3% of all cortical neurons, and the vast majority were nonpyramidal cells. However, 10% of substance P-positive neurons had a large conical cell body, a prominent apical dendrite directed toward the pia, and basal dendrites, thus suggesting they are pyramidal neurons. These neurons were in layers III and V. At the electron microscopic level, the majority of immunoreactive axon terminals formed symmetric synapses, but some substance P-positive axon terminals made asymmetric synapses. Labelled dendritic spines were also present. Combined retrograde transport-immunocytochemical experiments were also carried out to study whether substance P-positive neurons are projection neurons. Colloidal gold-labelled wheat germ agglutinin conjugated to enzymatically inactive horseradish peroxidase was injected either in the first somatic sensory cortex or in the dorsal column nuclei. In the somatic sensory cortex contralateral to the injection sites, a few substance P-positive neurons in layers III and V also contained black granules, indicative of retrograde transport. This indicates that some substance P-positive neurons project to cortical and subcortical targets. We have therefore identified a subpopulation of substance P-positive neurons that have most of the features of pyramidal neurons, are the probable source of immunoreactive axon terminals forming asymmetric synapses on dendritic spines, and project to the contralateral somatic sensory cortex and dorsal column nuclei. These characteristics fulfill the criteria required for classifying a cortical neuron as pyramidal.