Silencing long-descending inter-enlargement propriospinal neurons improves hindlimb stepping after contusive spinal cord injuries

Spinal locomotor circuitry is comprised of rhythm generating centers, one for each limb, that are interconnected by local and long-distance propriospinal neurons thought to carry temporal information necessary for interlimb coordination and gait control. We showed previously that conditional silencing of the long ascending propriospinal neurons (LAPNs) that project from the lumbar to the cervical rhythmogenic centers (L1/L2 to C6), disrupts right-left alternation of both the forelimbs and hindlimbs without significantly disrupting other fundamental aspects of interlimb and speed-dependent coordination (Pocratsky et al., 2020). Subsequently, we showed that silencing the LAPNs after a moderate thoracic contusive spinal cord injury (SCI) resulted in better recovered locomotor function (Shepard et al., 2021). In this research advance, we focus on the descending equivalent to the LAPNs, the long descending propriospinal neurons (LDPNs) that have cell bodies at C6 and terminals at L2. We found that conditional silencing of the LDPNs in the intact adult rat resulted in a disrupted alternation of each limb pair (forelimbs and hindlimbs) and after a thoracic contusion SCI significantly improved locomotor function. These observations lead us to speculate that the LAPNs and LDPNs have similar roles in the exchange of temporal information between the cervical and lumbar rhythm generating centers, but that the partial disruption of the pathway after SCI limits the independent function of the lumbar circuitry. Silencing the LAPNs or LDPNs effectively permits or frees-up the lumbar circuitry to function independently.

Silencing long ascending propriospinal neurons after spinal cord injury improves hindlimb stepping in the adult rat

Long ascending propriospinal neurons (LAPNs) are a subpopulation of spinal cord interneurons that directly connect the lumbar and cervical enlargements. Previously we showed, in uninjured animals, that conditionally silencing LAPNs disrupted left-right coordination of the hindlimbs and forelimbs in a context-dependent manner, demonstrating that LAPNs secure alternation of the fore- and hindlimb pairs during overground stepping. Given the ventrolateral location of LAPN axons in the spinal cord white matter, many likely remain intact following incomplete, contusive, thoracic spinal cord injury (SCI), suggesting a potential role in the recovery of stepping. Thus, we hypothesized that silencing LAPNs after SCI would disrupt recovered locomotion. Instead, we found that silencing spared LAPNs post-SCI improved locomotor function, including paw placement order and timing, and a decrease in the number of dorsal steps. Silencing also restored left-right hindlimb coordination and normalized spatiotemporal features of gait such as stance and swing time. However, hindlimb-forelimb coordination was not restored. These data indicate that the temporal information carried between the spinal enlargements by the spared LAPNs post-SCI is detrimental to recovered hindlimb locomotor function. These findings are an illustration of a post-SCI neuroanatomical-functional paradox and have implications for the development of neuronal- and axonal-protective therapeutic strategies and the clinical study/implementation of neuromodulation strategies.

Long ascending propriospinal neurons provide flexible, context-specific control of interlimb coordination

Within the cervical and lumbar spinal enlargements, central pattern generator (CPG) circuitry produces the rhythmic output necessary for limb coordination during locomotion. Long propriospinal neurons that inter-connect these CPGs are thought to secure hindlimb-forelimb coordination, ensuring that diagonal limb pairs move synchronously while the ipsilateral limb pairs move out-of-phase during stepping. Here, we show that silencing long ascending propriospinal neurons (LAPNs) that inter-connect the lumbar and cervical CPGs disrupts left-right limb coupling of each limb pair in the adult rat during overground locomotion on a high-friction surface. These perturbations occurred independent of the locomotor rhythm, intralimb coordination, and speed-dependent (or any other) principal features of locomotion. Strikingly, the functional consequences of silencing LAPNs are highly context-dependent; the phenotype was not expressed during swimming, treadmill stepping, exploratory locomotion, or walking on an uncoated, slick surface. These data reveal surprising flexibility and context-dependence in the control of interlimb coordination during locomotion.

RNA-seq data of soleus muscle tissue after spinal cord injury under conditions of inactivity and applied exercise

Reduced muscle mass and increased fatiguability are major complications after spinal cord injury (SCI), and often hinder the rehabilitation efforts of patients. Such detriments to the musculoskeletal system, and the concomitant reduction in level of activity, contribute to secondary complications such as cardiovascular disease, diabetes, bladder dysfunction and liver damage. As a result of decreased weight-bearing capacity after SCI, muscles undergo morphological, metabolic, and contractile changes. Recent studies have shown that exercise after SCI decreases muscle wasting and reduces the burden of secondary complications. Here, we describe RNA sequencing data for detecting chronic transcriptomic changes in the rat soleus after SCI at two levels of injury severity, under conditions of restricted in-cage activity and two methods of applied exercise, swimming or shallow water walking. We demonstrate that the sequenced data are of good quality and show a high alignment rate to the Rattus norvegicus reference assembly (Rn6). The raw data, along with UCSC Genome Browser tracks created to facilitate exploration of gene expression, are available in the NCBI Gene Expression Omnibus (GEO; GSE129694).

Embryonic brain precursors transplanted into kainate lesioned rat spinal cord

Embryonic day 14 rat cerebral cortex-derived precursors were expanded with FGF2 and labeled with BrdU prior to being transplanted into the kainic acid-lesioned adult rat spinal cord. While these precursors give rise to cells with neuronal, astrocytic and oligodendroglial phenotypes vitro, they remained largely undifferentiated up to 12 weeks in vivo. Numerous BrdU-labeled cells were found in injured gray matter, and also lining the dilated central canal that sometimes accompanies these lesions. BrdU-labeled cells never co-expressed Map2ab, rarely co-expressed GFAP but often co-expressed nestin, even after 12 weeks in vivo. These observations suggest that the environment of the kainic acid-injured spinal cord is not hostile to transplanted embryonic cerebral cortex-derived precursors, but also is not conducive to their neuronal differentation.

Lasting paraplegia caused by loss of lumbar spinal cord interneurons in rats: no direct correlation with motor neuron loss

OBJECT

The aims of this study were to investigate further the role played by lumbar spinal cord interneurons in the generation of locomotor activity and to develop a model of spinal cord injury suitable for testing neuron replacement strategies.

METHODS

Adult rats received intraspinal injections of kainic acid (KA). Locomotion was assessed weekly for 4 weeks by using the Basso, Beattie, and Bresnahan (BBB) 21-point locomotor scale, and transcranial magnetic motor evoked potentials (MMEPs) were recorded in gastrocnemius and quadriceps muscles at 1 and 4 weeks. No changes in transcranial MMEP latency were noted following KA injection, indicating that the descending motor pathways responsible for these responses, including the alpha motor neurons, were not compromised. Rats in which KA injections included much of the L-2 segment (10 animals) showed severe locomotor deficits, with a mean BBB score of 4.5 +/- 3.6 (+/- standard deviation). Rats that received lesions rostral to the L-2 segment (four animals) were able to locomote and had a mean BBB score of 14.6 +/- 2.6. Three rats that received only one injection bilaterally centered at L-2 (three animals) had a mean BBB score of 3.2 +/- 2. Histological examination revealed variable loss of motor neurons limited to the injection site. There was no correlation between motor neuron loss and BBB score.

CONCLUSIONS

Interneuron loss centered on the L-2 segment induces lasting paraplegia independent of motor neuron loss and white matter damage, supporting earlier suggestions that circuitry critical to the generator of locomotor activity (the central pattern generator) resides in this area. This injury model may prove ideal for studies of neuron replacement strategies.

Mitogen and substrate differentially affect the lineage restriction of adult rat subventricular zone neural precursor cell populations

The effects of specific mitogens and substrates on the proliferative capacity and the differentiated phenotypic plasticity of neural precursor cell populations isolated from the adult rat subventricular zone (SVZ) were examined. SVZ cells were grown on uncoated tissue culture plastic, extracellular matrix, or poly-D-ornithine with either laminin or fibronectin. SVZ neural precursor cells could not be generated with platelet-derived growth factor (PDGF), granulocyte macrophage colony stimulating factor, stem cell factor, heparin-binding epidermal growth factor (HB-EGF), granulocyte colony stimulating factor, or ciliary neurotrophic factor (CNTF), but could be with EGF, fibroblast growth factor 2 (FGF2), and FGF2 plus heparin. Varying combinations of substrate and mitogen resulted in very different expansion rates and/or lineage potential. Neurons, oligodendrocytes, and astrocytes differentiated from all cultures, but EGF-generated neural precursor cells were more restricted to an astrocytic lineage and FGF2-generated neural precursor cells had a greater capacity for neuronal differentiation. In both EGF- and FGF2-generated cell populations, CNTF increased the number of differentiated astrocytes, triiodothyronine oligodendrocytes, PDGF neurons, and brain-derived neurotrophic factor neurons only from EGF cells. Electrophysiological analysis of differentiated cells showed three distinct phenotypes, glial, neuronal, and presumed precursor cells, although the neuronal properties were immature. Collectively, these data indicate that CNS neural precursor cell populations isolated with different mitogens and substrates are intrinsically different and their characteristics cannot be directly compared.

Electrophysiological properties of mitogen-expanded adult rat spinal cord and subventricular zone neural precursor cells

Growth factor-expanded neural precursor cells isolated from the mammalian central nervous system can differentiate into neurons and glia. Although the morphological and neurochemical development of these neural precursor cells has been investigated, little attention has been paid to their electrophysiology. This study examined the electrophysiological properties of neurons and glia derived from neural precursor cells isolated from the adult rat spinal cord (SC) and subventricular zone (SVZ). Cells were cultured in medium containing epidermal growth factor and/or fibroblast growth factor-2. After at least two passages, spheres of neural precursor cells were plated on coated coverslips and maintained in culture for up to 6 weeks. Whole-cell patch recordings were made using standard current clamp techniques. Immature action potentials were observed within hours of plating for both SC and SVZ cells. Input resistance and time constants decreased over the first week after plating and no further changes were found at later times. At similar times following plating, however, SVZ cells had a lower input resistance and shorter time constant compared to SC cells. SVZ cells also had higher resting membrane potentials and smaller after hyperpolarizations than those of SC cells, despite no significant difference in the amplitude of action potentials. Neither the SC nor the SVZ cells were capable of eliciting more than a single action potential in response to injected current. While all SC cells tested were depolarized by glutamate, the response of SVZ cells to glutamate varied considerably. This study revealed that neural precursor cells from SC and SVZ differ in both active and passive membrane properties. It appears also that the electrophysiological development of SC and SVZ precursor-derived neurons is incomplete under the conditions used. These observations suggest that the neural precursor cells from different anatomical locations may be physiologically diverse and may exhibit some differences in commitment toward neuronal or glial phenotypes.

Comparing deficits following excitotoxic and contusion injuries in the thoracic and lumbar spinal cord of the adult rat

The majority of human spinal cord injuries involve gray matter loss from the cervical or lumbar enlargements. However, the deficits that arise from gray matter damage are largely masked by the severe deficits due to associated white matter damage. We have developed a model to examine gray matter-specific deficits and therapeutic strategies that uses intraspinal injections of the excitotoxin kainic acid into the T9 and L2 regions of the spinal cord. The resulting deficits have been compared to those from standard contusion injuries at the same levels. Injuries were assessed histologically and functional deficits were determined using the Basso, Beattie, and Bresnahan (BBB) 21-point open field locomotor scale and transcranial magnetic motor evoked potentials (tcMMEPs). Kainic acid injections into T9 resulted in substantial gray matter damage; however, BBB scores and tcMMEP response latencies were not different from those of controls. In contrast, kainic acid injections into L2 resulted in paraplegia with BBB scores similar to those following contusion injuries at either T9 or L2, without affecting tcMMEP response latencies. These observations demonstrate that gray matter loss can result in significant functional deficits, including paraplegia, in the absence of a disruption of major descending pathways.

Neuronal excitatory properties of human immunodeficiency virus type 1 Tat protein

Neuronal dysfunction and cell death in patients with human immunodeficiency virus type-1 (HIV-1) infection may be mediated by HIV-1 proteins and products released from infected cells. Two HIV-1 proteins, the envelope glycoprotein gp120 and nonstructural protein Tat, are neurotoxic. We have determined the neuroexcitatory properties of HIV-1 tat protein using patch-clamp recording techniques. When fmoles of Tat were applied extracellularly, it elicited dose-dependent depolarizations of human fetal neurons in culture and rat CA1 neurons in slices, both in the absence and presence of tetrodotoxin. These responses were voltage-dependent, reversed at approximately 0 mV, and were significantly increased by repetitive applications with no evidence of desensitization. That these responses to Tat were due to direct actions on neurons was supported by observations that Tat dose-dependently depolarized outside-out patches excised from cultured human neurons. Removal of extracellular Ca2+ decreased the responses both in neurons and membrane patches. This is the first demonstration that an HIV-1 protein can, in the absence of accessory cells, directly excite neurons and leads us to speculate that Tat may be a causative agent in HIV-1 neurotoxicity.

Locomotor rhythm evoked by ventrolateral funiculus stimulation in the neonatal rat spinal cord in vitro

Spinal cords from 2- to 8-day-old rats, maintained in vitro, were used to investigate the effects of discrete electrical stimuli applied to the ventrolateral funiculus (VLF) on motor neuron activity recorded from the lumbar ventral roots. Short trains of stimuli (1-3 s) delivered to one VLF in the low cervical region elicited rhythmic activity that persisted for up to 30 s. Responses consisted of short periods of activity (1-5 s) occurring simultaneously in the ipsilateral L5 and contralateral L3 ventral roots that alternated with similar bursts of activity in the ipsilateral L3 ventral root, a pattern consistent with locomotion. The rhythmicity of the ventral root responses to VLF stimulation was not affected by midsagittal sectioning of the preparation rostral to T10 and/or caudal to L4. Midsagittal sectioning of the lower thoracic or upper lumbar segments, however, disrupted the rhythmicity of the ventral root responses, leaving only long-duration simultaneous activation of the ipsilateral roots following VLF stimulus trains. The minimum lesion that effectively abolished the rhythmicity was one that divided only the L2 and L3 segments. In preparations rendered arrhythmic to VLF stimulation by an L2/L3 midsagittal lesion, rhythmicity could still be induced by N-methyl-D-aspartate (NMDA; 2-5 microM) and serotonin (5-HT; 20-50 microM), a drug combination commonly used to induce locomotor-like rhythmicity and air-stepping in vitro. Field potentials recorded following single stimuli delivered to the VLF revealed short-latency, large-amplitude responses in the ventral horn and intermediate gray both ipsilateral and contralateral to the stimulus site at T12 and L2. These observations suggest that 1) the discrete pathway under study may be an important descending locomotor command pathway and 2) this pathway has a strong bilateral projection in the lower thoracic and upper lumbar segments that is crucial for the initiation of VLF-induced rhythmic motor output. The induction of rhythmicity by NMDA/5-HT in an L2/L3-lesioned preparation suggests that these two rhythmogenic mechanisms may represent different levels within the circuitry that comprises the central pattern generator for locomotion. The rhythmic activity resulting from VLF stimulation is dependent on a bilateral projection that can be bypassed by the generalized excitation and subsequent rhythmicity that results from bath application of the NMDA/5-HT combination.

Identification of a human immunodeficiency virus type 1 Tat epitope that is neuroexcitatory and neurotoxic

Tat is an 86- to 104-amino-acid viral protein that activates human immunodeficiency virus type 1 expression, modifies several cellular functions, and causes neurotoxicity. Here, we determined the extent to which peptide fragments of human immunodeficiency virus type 1 BRU Tat1-86 produced neurotoxicity, increased levels of intracellular calcium ([Ca2+]i), and affected neuronal excitability. Tat31-61 but not Tat48-85 dose dependently increased cytotoxicity and levels of [Ca2+]i in cultured human fetal brain cells. Similarly, Tat31-61 but not Tat48-85 depolarized rat hippocampal CA1 neurons in slices of rat brain. The neurotoxicity and increases in [Ca2+]i could be significantly inhibited by non-N-methyl-D-aspartate excitatory amino acid receptor antagonists. Shorter 15-mer peptides which overlapped by 10 amino acids each and which represented the entire sequence of Tat1-86 failed to produce any measurable neurotoxicity. Although it remains to be determined if Tat acts directly on neurons and/or indirectly via glial cells, these findings do suggest that Tat neurotoxicity is conformationally dependent, that the active site resides within the first exon of Tat between residues 31 to 61, and that these effects are mediated at least in part by excitatory amino acid receptors.

Neurons derived from P19 embryonal carcinoma cells develop responses to excitatory and inhibitory neurotransmitters

Cells of the P19 line of embryonal carcinoma cells differentiate into neurons, astrocytes and oligodendrocytes following treatment with retinoic acid. The neurons from these differentiating P19 cultures synthesize a pattern of neurotransmitters that resembles that of neurons of the forebrain. We treated P19 cells with retinoic acid and then implanted them into the striatum of adult rats. After times ranging from 1 to 15 weeks post-implantation, brain slices containing the implanted tissue were prepared and used for intracellular recording of electrical activity and responsiveness to application of neurotransmitters. Within 2 weeks of implantation, the P19-derived neurons had developed responsiveness to the excitatory neurotransmitter glutamate and the inhibitory transmitters gamma-aminobutyric acid and glycine. These neurons also exhibited spontaneous synaptic potentials. The responses to glutamate appear to be mediated by N-methyl-D-aspartic acid as well as non-N-methyl-D-aspartic acid receptor subtypes. Gamma-aminobutyric acid evoked bicuculline-sensitive depolarizing responses in the younger grafts and biphasic depolarizing/hyperpolarizing responses in older ones. Responses to glycine were strychnine sensitive and also showed age-related changes from depolarizing to biphasic character. Synaptic potentials in the younger grafts were exclusively depolarizing, but in older ones both depolarizing and hyperpolarizing events were observed. The synaptic potentials appear to arise from synaptic connections between P19-derived neurons within the grafts. Many of the features of P19-derived neurons are similar to those of neurons in the developing forebrain.

Lamina VII neurons are rhythmically active during locomotor-like activity in the neonatal rat spinal cord

The midsagittally-sectioned lumbar spinal cord with thoracic segments intact retains the capacity for locomotor-like activity. Intracellular recordings were used to characterize the activity and concurrently label lumbar neurons in lamina VII, an area previously implicated in the generation of locomotion. Sharp electrodes were shown to preferentially impale larger neurons. These neurons undergo rhythmic voltage oscillations, presumably synaptically driven, during locomotor-like activity induced by bath application of N-methyl-D-aspartate and 5-hydroxytryptamine. This supports the hypothesis that synaptic activity recruits neurons in lamina VII that are associated with locomotor behavior.

Long-duration, frequency-dependent motor responses evoked by ventrolateral funiculus stimulation in the neonatal rat spinal cord

Three variations of the in vitro neonatal rat spinal cord preparation were used to investigate motor responses to stimulation of the ventrolateral funiculus (VLF). In a partially hemisected spinal cord preparation, stimuli elicited frequency-dependent activity in lumbar ventral roots that outlasted the stimulus train by up to 30 s. In a spinal cord-hindlimb preparation, trains of VLF stimuli elicited slow, step-like flexor-extensor hindlimb movement that also persisted for up to 30 s beyond the stimulus. Finally, in a partially hemisected spinal cord preparation where 5-hydroxytryptamine/N-methyl-D-aspartate was used to induce locomotor-like rhythmic activity, short trains of VLF stimuli were capable of perturbing the locomotor rhythm, transiently altering its frequency. Application of pharmacological antagonists suggests that these responses may be the result of stimulation of a descending pathway that includes glutamatergic and catecholaminergic fibres comprising part of a descending locomotor command path.

Human immunodeficiency virus type 1 tat activates non-N-methyl-D-aspartate excitatory amino acid receptors and causes neurotoxicity

The human immunodeficiency virus type 1 (HIV-1) protein Tat is known to be released from HIV-1-infected cells. We show that micromolar concentrations of Tat depolarized young rat and adult human neurons. In addition, Tat, at similar concentrations, was toxic to human fetal neurons in culture. Tat-induced responses were insensitive to the Na+ channel blocker tetrodotoxin, suggesting a direct effect of Tat on neurons. Tat-induced depolarizations and cytotoxicity were blocked by the excitatory amino acid antagonist kynurenate. The N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovalerate had little effect on Tat-induced depolarizations but did provide protection from Tat neurotoxicity. These results suggest that Tat, released from HIV-1-infected cells, may be an important mediator of neurotoxicity observed in HIV-1 encephalopathy.

In vivo electrophysiological maturation of neurons derived from a multipotent precursor (embryonal carcinoma) cell line

The multipotent embryonal carcinoma (EC) P19 cell line differentiates into neurons, glia and smooth muscle following exposure to retinoic acid (RA). RA-induced differentiation is irreversible and the neurons that develop are abundant, post-mitotic, and survive for prolonged periods in culture or when grafted into the CNS of adult rats. Striatal slices containing grafted P19 cells were studied with intracellular recording and labelling techniques to examine the development of electrophysiological and morphological properties of P19-derived neurons over a period of 6 to 120 days after grafting into ibotenic acid lesioned striatum. Cells from 1-week-old grafts had a range of immature electrophysiological characteristics including unstable resting membrane potentials (RMP's) and very high membrane input resistances (Rin's). Many were not able to produce action potentials (AP's). In contrast, the majority of cells recorded from 2- and 3-week-old grafts had stable RMP's, moderate Rin's, and were able to produce regenerative AP's. In grafts over 4 weeks of age, the majority of P19-derived neurons had mature neuronal electrophysiological characteristics including RMP's of -60 mV, Rin's of 100-300 M omega, and overshooting AP's. Morphologically, P19 derived neurons increase in soma size from 12-15 mu in diameter in 7-14-day-old grafts, to 25-35 mu in diameter in grafts 50-120 days old. Developing neurons exhibited a variety of morphotypes with increasingly complex processes and lengths of process extension. Our results demonstrate a developmental progression of the electrophysiology of P19-derived neurons, culminating in mature characteristics closely resembling those of adult rodent hippocampal or cortical pyramidal neurons. The ability to easily alter these cells genetically provides a powerful model for addressing issues specific to neuronal development.

Murine embryonal carcinoma-derived neurons survive and mature following transplantation into adult rat striatum

P19 embryonal carcinoma cells are pluripotent and can be efficiently induced to differentiate in culture into neurons and astroglia by brief treatment with retinoic acid. Retinoic acid-treated P19 cells survive after grafting into the adult rat striatum and differentiate into neurons and glia within the transplantation site. No tumours develop from the grafted cells which continue to express foreign genes that had been transfected into the parental P19 cells. The neurons in these grafts express a variety of neurotransmitters similar to those formed in retinoic acid-treated P19 cell cultures and they mature to acquire the electrophysiological properties expected of fully developed neurons. These results suggest that P19 cells may be used for studies related to neuronal cell development and maturation and that P19 cells may be considered for cell replacement strategies in neurodegenerative disorders of the central nervous system.

Electrophysiological changes accompanying DSP-4 lesions of rat locus coeruleus neurons

The electrophysiological characteristics of intracellularly recorded locus coeruleus (LC) neurons in brain stem slices from DSP-4 treated animals have been compared to those from untreated controls. LC neurons from DSP-4 treated animals had action potentials and Ca2+ spikes (elicited in the presence of TTX) of significantly reduced duration compared to controls. These observations suggest that chemical axotomy with DSP-4 reduces Ca2+ conductance in neurons of the locus coeruleus.

Lamina-specific effects of morphine and naloxone in dorsal horn of rat spinal cord in vitro

1. Extracellular recordings were made from 95 dorsal horn neurons in a sagittal slice preparation of rat spinal cord. Neurons were synaptically activated by electrical stimulation of the dorsal root entry zone (n = 72) or driven by pressure ejection of L-glutamate into the substantia gelatinosa (SG; n = 23). For the majority of neurons low-intensity electrical stimuli evoked a burst of spikes with short latency (early firing). An increase in the stimulus intensity evoked the early firing followed by a characteristic prolonged period of activity (late firing). 2. The patterns of synaptically induced activity observed from neurons located in the SG (n = 45) and in deeper laminae (n = 27) were similar. Early and late firing of both SG and deep neurons was reduced by the nonspecific excitatory amino acid antagonist kynurenate. Raising [Mg2+] in the superfusate to 5 from 2.0 mM selectively reduced the late firing of both SG and deep neurons. These findings suggest that fibers present in the dorsal root zone make excitatory amino acid-mediated synapses with dorsal horn neurons. 3. The majority of deep neurons showed reduced responses to electrical stimuli in the presence of morphine, mimicking the findings reported in vivo. Naloxone reversed morphine inhibitions or, when applied to morphine-naive slices, caused modest increases in the responses of some deep neurons. 4. Most neurons located in the SG had their responses enhanced by morphine (late firing: 86.5 +/- 19.6%, mean +/- SE) and were inhibited by naloxone (-78.3 +/- 22.7%). Morphine-induced enhancements often persisted long after the morphine had washed out of the bath. Inhibitions by naloxone, whether pre- or postmorphine, were short lived; and responses generally returned to either control or morphine-enhanced levels on washout of the naloxone. 5. The gamma-aminobutyric acid (GABA) antagonist bicuculline, applied to GABA-naive slices, caused an increase in response of SG neurons while slightly depressing activity of deep neurons. A tonic release or presence of endogenous GABA, affecting neurons that are inhibited by exogenously applied GABA, may be responsible for the observed action of bicuculline. 6. Intracellular recordings were made from a further 32 dorsal horn neurons located in the translucent band of the spinal cord slices. Of 11 neurons examined, 5 showed increased input resistance (Rin) and were depolarized in response to morphine applied in the superfusate. One neuron showed no change in Rin or potential with morphine, whereas 5 of the 11 were hyperpolarized with decreased Rin.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential interactions of cholecystokinin and FLFQPQRF-NH2 with mu and delta opioid antinociception in the rat spinal cord

An in vivo preparation of the rat spinal cord was used to investigate the electrophysiological actions of two non-opioid peptides, cholecystokinin (CCK8) and FLFQPQRF-NH2 (FMRFamide-like peptide) applied intrathecally. These compounds were examined alone and as a pretreatment before DAGO, a mu opioid agonist, and DSTBULET, a delta opioid agonist, both which selectively reduce C-fibre evoked dorsal horn neurone activity elicited by transcutaneous electrical stimulation. Given alone, CCK8 (1 microgram) elicited a modest enhancement of C-fibre induced activity which returned to control levels after 20 min, while FLFQPQRF-NH2 (10 micrograms) had no significant effect on C-fibre evoked firing. As a pretreatment, however, both peptides selectively prevented the inhibition of C-fibre evoked activity normally resulting from intrathecal DAGO, while having no effect on that resulting from DSTBULET. Further, CCK8 enhanced the facilitation of C-fibre evoked firing normally observed with low doses of DAGO. These data indicate that the anti-opioid roles suggested for CCK8 and FLFQPQRF-NH2 may be specific for neural elements utilizing the mu opioid receptor.

Structural requirements for activation of excitatory amino acid receptors in the rat spinal cord in vitro

The conformational requirements for activation of N-methyl-D-aspartate (NMDA) and quisqualate (QUIS) excitatory amino acid receptors on rat spinal neurones in vitro have been examined using a number of conformationally restricted compounds related to L-glutamate (L-GLU). The excitants were assigned to a receptor type on the basis of their susceptibility to blockade by D (-)-2-amino-5-phosphonvalerate (DAPV) and kynurenate (KYNA). When iontophoretically applied to unidentified neurones in the dorsal horn of spinal cord slices maintained in vitro, three of the isomers of 1-amino-1,3-cyclopentane dicarboxylate (ACPD) evoked excitations which were DAPV-sensitive and therefore were probably elicited via NMDA receptors. The fourth isomer (D-trans-(1R,3S)-ACPD) resembled quinolinate (QUIN) in its actions, and differed from both NMDA and QUIS. Several pyridine derivatives in addition to QUIN were tested, and both the 2,5- and 2,6-pyridine dicarboxylates evoked excitations which, like those produced by QUIS and L-GLU, were largely unaffected by both DAPV and KYNA and thus appeared due to activation of the QUIS receptor. 2,4-Pyridine dicarboxylate acted as a weak and unselective antagonist of amino acid-induced excitations. The results support an earlier conclusion that compounds reacting with the NMDA receptor do so in an extended configuration whereas the QUIS receptor has a more folded template. The possibility that QUIN reacts with a receptor different from those activated by other amino acids is considered.

Synthesis, resolution, and absolute configuration of the isomers of the neuronal excitant 1-amino-1,3-cyclopentanedicarboxylic acid

The endogenous amino acids glutamate and aspartate depolarize mammalian neurons to produce excitation, and the rigid glutamate analogue 1-amino-1,3-cyclopentanedicarboxylic acid also has this effect. This compound exists as two pairs of geometric isomers, and in the present study the absolute configuration of the four isomers is assigned. The known (+)-S and (-)-R isomers of 3-oxocyclopentanecarboxylic acid were used as the basis for the synthesis. The cis and trans amino acids were obtained by fractional crystallization. Spectral data, including optical rotation, circular dichroism, and 13C nuclear magnetic resonance, are presented. The compounds were evaluated as excitants by microiontophoretic ejection into the dendritic region of impaled CA1 pyramidal neurons of rat hippocampal slices. One isomer, cis-1R,3R, mimicked completely the actions elicited by N-methyl-D-aspartic acid; the other three isomers were alpha-kainic acid like.

The action of quinolinate in the rat spinal cord in vitro

The responses of dorsal horn neurones to the excitatory amino acids quisqualate, kainate, N-methyl-D-aspartate (NMDA), and quinolinate have been examined in an in vitro preparation of the rat spinal cord. The antagonism of these responses by iontophoretically applied D-(-)-2-amino-5-phosphonovalerate (DAPV), kynurenate, and acridinate was tested, and the results were compared with data obtained from the spinal cord in vivo. The pattern of antagonism was similar in both preparations, although the potencies of agonists and antagonists were found to be significantly greater in vitro. The antagonism of amino acid induced firing of neurones was also recorded during the application of DAPV and kynurenate in the bathing medium. Dose-response curves and IC50 values were determined for these antagonists against all four agonists. The responses to quinolinate were antagonized differently from those to NMDA, quisqualate, or kainate, suggesting that quinolinate does not act specifically through the NMDA receptor as it does in other regions, nor does it appear to act via two or more of the three archetypal amino acid receptors. These findings suggest that a fourth amino acid receptor responsible for quinolinate's action in the spinal cord may exist.

Excitation of rat hippocampal neurones by the stereoisomers of cis- and trans-1-amino-1,3-cyclopentane dicarboxylate

Intracellular recordings were obtained from rat hippocampal neurons during the microiontophoretic ejection of the stereoisomers of cis- and trans-1-amino-1,3-cyclopentane dicarboxylate into the dendritic region (stratum radiatum) of the impaled cells. L-(+)-cis-1-Amino-1,3-cyclopentane dicarboxylate, D(+)-trans-1-amino-1,3-cyclopentane dicarboxylate, and L-(-)-trans-1-amino-1,3-cyclopentane dicarboxylate all evoked patterns of excitation resembling that elicited by kainate. All of these responses were unaffected by D-(-)-2-amino-5-phosphonovalerate but were antagonized at comparable currents by kynurenate. The excitation produced by D-(-)-cis-1-amino-1,3-cyclopentane dicarboxylate was similar to that evoked by N-methyl-D-aspartate. At low ejection currents a slow depolarization triggered rhythmic burst firing, each burst consisting of a depolarizing shift in membrane potential upon which were superimposed four to five action potentials. These responses were antagonized both by D-(-)-2-amino-5-phosphonovalerate and by kynurenate. The results are discussed with respect to the conformational requirements considered to be necessary for interaction at the kainate and N-methyl-D-aspartate receptors on CA1 pyramidal neurones. It is important to note that the isopropylene side chain of kainate is absent from the 1-amino-1-3-cyclopentane dicarboxylate molecule.

The N-methyl-D-aspartate receptor and burst firing of CA1 hippocampal pyramidal neurons

Previous intracellular investigations in the rat hippocampus have demonstrated that N-methyl-D-aspartate, ibotenate and 2,3-pyridine dicarboxylate (quinolinate) all evoke burst firing of CA1 pyramidal neurons, whereas kainate and quisqualate, which are thought to react with different receptors, do not. The purpose of the present study has been to investigate the ability of a series of compounds either to trigger burst firing or to antagonize this pattern of excitation. We report here that N-methyl-L-aspartate, 1,2-benzene dicarboxylate (phthalate) and methylene succinate (itaconate) are also capable of evoking burst firing. The results of this investigation suggest that since both quinolinate and phthalate are rigid planar molecules and only the 2 and 3 positioning of the carboxylates of pyridine was active, a cis configuration of the carboxyls with respect to the 2,3 carbon bond appears to be necessary for excitation. While a nitrogen atom is not necessary for activity (this is absent in phthalate and itaconate) a third functional group, bearing at least a partial positive charge, and in a position alpha to one of the carboxyl groups is required. The requirements for pyridine derivatives to trigger burst firing is similar to that reported as necessary for evoking convulsions and neurotoxicity after intrahippocampal infusion and a correlation between N-methyl-D-aspartate-like burst firing and depolarization and this neuropathology is considered. An important observation has been that the addition of a benzene ring to either quinolinate or phthalate to yield 2,3-quinoline dicarboxylate and 2,3-napthalene dicarboxylate, respectively, converted these excitants into antagonists of burst firing.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel spinal cord slice preparation from the rat

A spinal cord slice preparation is described. The lumbar enlargement of weanling rats is exposed by laminectomy, the dorsal and ventral roots cut and a portion of cord consisting of about 4 segments removed and chilled to 3 degrees C. A modified tissue chopper is used to produce 400 micron dorsoventral longitudinal slices. The slices are maintained in a low-volume, continuously perfused tissue chamber at the interface between warm moist 95% O2/5% CO2 and oxygenated artificial cerebrospinal fluid, and remain viable for over 8 h. An extracellular recording of excitatory amino acid-induced activity of a dorsal horn lamina IV cell is shown. Excitatory amino acid antagonists applied iontophoretically and in the perfusate have actions similar to those in the adult spinal cord in vivo. This in vitro preparation of the cord has permitted stable extracellular recordings from single cells of 2 h or more, and has the potential for intracellular investigation of spinal cord neurones.

Acridinic acid: a new antagonist of amino acid-induced excitations of central neurones

The actions of acridinic acid (2,3-quinoline dicarboxylic acid), a new derivative of quinolinic acid, as an antagonist of amino acid-induced excitations are described. Acridinate, like kynurenate, in the cerebral cortex reduced the effects of all amino acids equally, but in the spinal cord was significantly less active against quisqualate.

Ca2+-dependent depolarization and burst firing of rat CA1 pyramidal neurones induced by N-methyl-D-aspartic acid and quinolinic acid: antagonism by 2-amino-5-phosphonovaleric and kynurenic acids

The excitatory effects of microiontophoretically applied quisqualic (QUIS), N-methyl-D-aspartic (NMDA), and quinolinic (QUIN) acids were investigated using intracellular recording from CAl pyramidal neurones in slices of rat hippocampus. QUIS evoked only simple action potentials superimposed upon a depolarization which attained a clear plateau. When this level had been reached, increased ejecting currents did not produce further depolarization. By contrast, with low currents NMDA and QUIN elicited small membrane depolarizations which triggered bursts of action potentials superimposed upon rhythmically occurring depolarizing shifts. Larger currents caused depolarization which if sufficiently large completely blocked spike activity. Tetrodotoxin (TTX) prevented the spikes evoked by QUIS and the bursts of action potentials seen with NMDA and QUIN, and the rhythmic depolarizing shifts then appeared as broad spikes of up to 50 mV in amplitude. These and the underlying membrane depolarization were blocked by Co2+, by the NMDA antagonist D(-)-2-amino-5-phosphonovaleric acid (DAPV), and by kynurenic acid (KYNU). It thus appears that the depolarization and burst firing of rat CAl pyramidal neurones elicited by NMDA and QUIN are Ca2+ dependent while the actions of QUIS are not.