Behavioral effects of spinal cord transection in the developing rat

Albino rats, 0, 9, 12, 15, 18, 21 or greater than 90 days of age, were given a mid-thoracic spinal cord transection. Evaluation of responses of the hindlimbs to a variety of behavioral tasks was begun on the day of surgery and at intervals throughout the postoperative survival period (up to 300 days). Two investigators, independently and without knowledge of the animals' ages or survival times, rated the response data. Histological study showed all transections to be complete. Large differences in behavior are observed when animals trasected at the neonatal stage (0-4 days of age) are compared with animals transected at the weanling stage (21-26 days of age)37. Results of the present investigation indicate a critical period near 15 days of age; animals lesioned prior to this age (0, 9, 12 days of age) show response development and recovery similar to the neonatally lesioned animal, whereas those animals lesioned at a later age (18, 21, greater than 90 days of age) show little recovery and are behaviorally similar to the weanling transected animal. In animals lesioned prior to the fifteenth postnatal day, postural responses appear depressed for a brief period but recover rapidly while most responses of animals in the older groups are depressed for longer periods and never attain the degree of recovery characteristic of the neonatally transected animal. Finally, like the neonatally transected animal, rats lesioned on the ninth and twelfth postnatal day develop certain responses at appropriate times relative to normal response development. If, however, these responses are mature and supraspinal control is present at the time of lesioning, they appear to be permanently depressed and fail to recover.     

Localization of glutamate receptors in dorsal horn of rat spinal cord

Immunocytochemical localization of metabotropic glutamate receptors (mGluRs) and ionotropic glutamate receptors (NMDA-type: NMDAR1 and NMDAR2A-C; AMPA-type: GluR1-4) was performed on sections of rat dorsal horn. Immunoreactivity for mGluR1 alpha was detected in laminae I-III of the dorsal horn, whilst mGluR2/3 immunoreactivity was detected primarily in lamina III. Immunoreactivity for NMDAR1, GluR1, GluR2, GluR2/3, GluR4 and GluR5/6/7 was strongly localized in neuronal elements of laminae I-III. Immunoreactivity for NMDAR2B was localized in laminae I-III. No mGluR5, NMDAR2A and NMDAR2C immunoreactivity was detected. In addition, immunoreactivity for receptors was found to co-localize with immunoreactivity for glutamate in the dorsal horn. The present results indicate that glutamate receptors are differentially localized in neuronal elements of dorsal horn where receptor-neurotransmitter interaction takes place.     

Contribution of NMDA and non-NMDA glutamate receptors to locomotor pattern generation in the neonatal rat spinal cord

The motor programme executed by the spinal cord to generate locomotion involves glutamate-mediated excitatory synaptic transmission. Using the neonatal rat spinal cord as an in vitro model in which the locomotor pattern was evoked by 5-hydroxytryptamine (5-HT), we investigated the role of N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors in the generation of locomotor patterns recorded electrophysiologically from pairs of ventral roots. In a control solution, 5-HT (2.5-30 microM) elicited persistent alternating activity in left and right lumbar ventral roots. Increasing 5-HT concentration within this range resulted in increased cycle frequency (on average from 8 to 20 cycles min-1). In the presence of NMDA receptor antagonism, persistent alternating activity was still observed as long as 5-HT doses were increased (range 20-40 microM), even if locomotor pattern frequency was lower than in the control solution. In the presence of non-NMDA receptor antagonism, stable locomotor activity (with lower cycle frequency) was also elicited by 5-HT, albeit with doses larger than in the control solution (15-40 microM). When NMDA and non-NMDA receptors were simultaneously blocked, 5-HT (5-120 microM) always failed to elicit locomotor activity. These data show that the operation of one glutamate receptor class was sufficient to express locomotor activity. As locomotor activity developed at a lower frequency than in the control solution after pharmacological block of either NMDA or non-NMDA receptors, it is suggested that both receptor classes were involved in locomotor pattern generation.     

Angiotensin II in the spinal cord of the rat and its sympatho-excitatory effects

Immunohistochemical studies have shown there is a dense angiotensin-like immunoreactivity of terminals in the sympathetic region of the thoracic and lumbar spinal cord. In the present study measurements were made of the concentration of angiotensin in the spinal cord of rats using radioimmunoassay following two different extraction procedures. These gave concentrations of angiotensin as mean of 108 and 161 pg.g-1 tissue wet weight. Angiotensin II given intrathecally or microinjected into the spinal cord caused an increase in postganglionic sympathetic nerve activity which was blocked by prior application of saralasin. Angiotensin III was without effect. Intracellular recordings from sympathetic preganglionic neurones in-vitro in slices of neonate rat spinal cord showed that angiotensin II produced an increase of excitability of the neurones by a slow depolarisation without the generation of action potentials. This effect still occurred in the presence of TTX. Angiotensin II also could increase synaptic activity, both EPSPs and IPSPs as well as a synaptically induced slow depolarisation being observed suggesting that presympathetic interneurones are also sensitive to the peptide. The evidence indicates that if angiotensin is released from nerve terminals surrounding sympathetic neurones it will enhance the gain of the neurone so that it could more easily be discharged by other excitatory inputs.     

Effect of spinal cord transection on N-methyl-D-aspartate receptors in the cord

Spinal cord injury can lead to an exaggeration of transmission through spinal pathways, resulting in muscle spasticity, chronic pain, and abnormal control of blood pressure and bladder function. These conditions are mediated, in part, by N-methyl-D-aspartate (NMDA) receptors on spinal neurons, but the effects of cord injury on the expression or function of these receptors is unknown. Therefore, antibodies to the NMDA-R1 receptor subunit and binding of [3H]MK-801 were used to assess NMDA receptors in the spinal cord. Receptor density in rats with intact spinal cords was compared to that in rats 1 and 2 weeks after spinal cord transection (SCT) at the mid-thoracic level. At 1 and 2 weeks after SCT, [3H]MK-801 binding was reduced in most laminae in cord segments caudal to the injury, whereas no decrease in amount of R1 subunit immunoreactivity was observed. No significant changes in [3H]MK-801 binding and NMDA-R1 immunoreactivity could be seen rostral to the transection. Since [3H]MK-801 binding requires an open ion channel, the discrepancy between [3H]MK-801 binding and immunocytochemistry may indicate a loss of functional receptors without a consistent change in their total number. Therefore, the exaggerated reflexes that are well established in rats 2 weeks after cord injury must be mediated by a mechanism that withstands attenuation of NMDA receptor function.     

The cellular localization of 5-HT2A receptors in the spinal cord and spinal ganglia of the adult rat

Cortical inhibitory mechanisms were investigated with the technique of paired transcranial magnetic stimulation in 10 patients with dystonia of the right arm: six patients had focal, task-specific dystonia (writer's cramp) and three had segmental and one had generalized dystonia. Paired stimuli were delivered in a conditioning-test design during slight voluntary activation of the target muscle, with subthreshold conditioning stimuli at short intervals (3-20 ms) and suprathreshold conditioning stimuli at long intervals (100-250 ms). The amount of inhibition at short interstimulus intervals did not differ significantly between patients and normal subjects. With long interstimulus intervals, patients showed more inhibition of the test response, which was significant at the 150-ms interval. The cortical silent period following a single suprathreshold magnetic stimulus was slightly shorter in patients. No significant difference was detected between the affected side and the unaffected side in patients with unilateral task-specific dystonia, neither in the duration of the silent period nor in the response to paired magnetic stimuli. These results indicate that the different types of motor cortical inhibition are produced by different inhibitory circuits. We propose that the alterations observed in patients with dystonia are the result of impaired feedback from the basal ganglia to motor cortical areas, with the ultimate effect of a flattening of the excitability curve of the cortical motoneuron pool during voluntary muscle activation.     

A comparison of the effects of glial cell line-derived neurotrophic factor on spinal cord and cortex cerebri grafts

Glial cell line-derived neurotrophic factor (GDNF) has trophic effects on developing dopamine neurons, enhances survival of embryonic motoneurons in vitro and prevents axotomy-induced motoneuron atrophy in vivo. Here we investigate effects of GDNF on grafts of cortex cerebri tissue from E18, P1 and P8 donors and on spinal cord tissue for P8 and adult animals transplanted to the anterior chamber of the eye of host rats. Grafts were treated with GDNF or cytochrome C on days 0, 5, 10, 15, 20 and 25 (total amounts 0.5 microgram GDNF/eye/injection). Spinal cord grafts from P8 donors treated with GDNF grew to sizes larger than controls, had higher numbers of neuron-like cells and showed increased areas of neurofilament immunoreactivity and decreased glial fibrillary acidic protein immunoreactivity. In contrast to the P8 spinal cord grafts, there were no such effects observed in adult spinal cord grafts or in E18, P1 or P8 cerebral cortex grafts. To determine if an endogenous source of GDNF might exert similar effects on spinal cord grafts, we transplanted spinal cord tissue from P1 together with pieces of developing kidney, known to express high levels of GDNF mRNA. Spinal cord cografted with kidney tissue grew to a slightly larger extent then controls. We conclude that GDNF exerts a powerful trophic effect on P8 spinal cord grafts, although GDNF appears unable to support survival of grafted adult spinal cord tissue. Grafts of cortex cerebri from several different stages of development were not affected.     

NMDA and AMPA receptors evoke transmitter release from noradrenergic axon terminals in the rat spinal cord

The organic extracts of soil collected at parks in residential areas in Osaka and neighboring cities in the Kansai area, Japan, showed mutagenicity in Salmonella typhimurium strain TA98 in the presence or absence of a mammalian metabolic activation system (S9 mix). The soil extracts from Ibaraki and two different sites in Osaka, i.e., Sumiyoshi-ku and Minato-ku, were mutagenic in strain TA100 as well as in strain TA98. Direct-acting mutagenicity of soil extracts from Sumiyoshi-ku and Minato-ku toward strain TA98 were 66 or more times higher than that of the other cities. Both extracts exerted stronger mutagenicity in strains YG1021 and YG1024 than TA98 and TA100, and the potency was especially high in strain YG1024: Sumiyoshi-ku, 153 000 revertants/g of soil; and Minato-ku, 246 000 revertants/g of soil. Two mutagenic compounds (I and II) were isolated from the Soxhlet extract of soil from the park in Sumiyoshi-ku by repetitive separation using normal-phase and reversed-phase column chromatography. By comparing the mass and UV spectra and retention times for HPLC on two individual ODS columns of compounds I and II with those of authentic chemicals, we identified these two compounds as 1,6- and 1,8-dinitropyrene (DNPy) isomers. Amounts of DNPy isomers in soil from Sumiyoshi-ku and Minato-ku were 1.7-2.2 ng/g. Forty-three percent and 40% of the mutagenicity of soil from Sumiyoshi-ku and Minato-ku could be attributed to these DNPy isomers, respectively.     

Oxytocin acts at V1 receptors to excite sympathetic preganglionic neurones in neonate rat spinal cord in vitro

Intracellular recordings were made from sympathetic preganglionic neurones (SPNs) in transverse slices of thoraco-lumbar spinal cord of young rats (12-20 days old). A small group of SPNs generally having higher membrane potentials (-70 mV) compared to a remaining group (-66 mV) showed spontaneous oscillations of their membrane potential. Oxytocin superfused in concentrations of 0.1-30 microM had four effects on SPNs, inducing slow depolarisation, EPSPs, IPSPs and brief rhythmic oscillations. The slow depolarisation was unaffected by TTX whereas this abolished the other changes. The oxytocin-induced depolarisation was associated with a slow inward current and was not reversed at membrane potentials negative to EK, it increased at more positive potentials and was still present in low Ca2+ and high Mg2+ solutions. These features of the oxytocin induced current are similar to those of the TTX resistant voltage dependent Na+ current described in brainstem autonomic neurones. Vasopressin superfused at concentrations of 0.1 microM to 30 microM had similar effects on SPNs to those of oxytocin. A comparison of the effects of oxytocin and vasopressin on the same neurones revealed that oxytocin was almost 10 times less potent than vasopressin. The effects of oxytocin were not mimicked by a selective oxytocin agonist but were mimicked by a selective vasopressin V1a agonist and blocked by a selective V1a antagonist. Therefore it is concluded that the effects of oxytocin on SPNs are mediated by the vasopressin V1a receptor. It is suggested that oxytocin and vasopressin terminals in the lateral horn are part of a descending system controlling oscillating networks of SPNs in the spinal cord.     

The effects of aging on mu and delta opioid receptors in the spinal cord of Fischer-344 rats

The lordosis-inhibiting effects of the 5-HT1A receptor agonist, (+/-)8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), were examined in ovariectomized rats, hormone primed with 2.5, 7.5, or 25 micrograms estradiol benzoate plus 500 micrograms progesterone. 8-OH-DPAT (50, 100 or 200 ng per bilateral site) infused into the ventromedial nucleus of the hypothalamus (VMN), inhibited lordosis behavior in all hormone-treated conditions. However, animals primed with 2.5 micrograms estradiol benzoate were significantly more affected by the infusion than rats primed with 7.5 or 25 micrograms of the hormone. These findings strengthen prior speculations that 5-HT1A receptor function is modulated by estrogen.     

Met-enkephalin inhibits 5-hydroxytryptamine release from the rat ventral spinal cord via delta opioid receptors

The effect of opioid receptor agonists and antagonists on the electrically evoked release of endogenous serotonin (5-hydroxytryptamine, 5-HT) was studied in superfused slices of the rat ventral lumbar spinal cord. Met-ENK (1 x 10(-8)M-1 x 10(-6)M) and DPDPE (1 x 10(-8)M-1 x 10(-6)M) reduced the evoked 5-Ht release in a concentration dependent fashion. DAMGO (1 x 10(-8)-1 x 10(-6)) and (-)-trans-(1S,2S)-U-50488 (1 x 10(-6)M) had no effect on the 5-HT release. The inhibitory effect of met-ENK was completely abolished by ICI-174,864, but neither by naloxonazine nor nor-binaltorphimine. Following i.c.v. treatment with 5,7-dihydroxytryptamine (5,7-DHT), the tissue concentration of 5-HT was reduced by 97%, whereas the concentration of noradrenaline was reduced by only 5%. The tissue concentration of met-ENK, as measured by radioimmunoassay, was not significantly altered. The results suggest that met-ENK is present in the rat ventral spinal cord mainly in non-serotonergic nerve terminals and exerts an inhibitory action on 5-HT release via delta opioid receptors.     

Postnatal development of multiple opioid receptors in the spinal cord and development of spinal morphine analgesia

The postnatal ontogeny of mu, delta and kappa opioid receptor binding sites in the spinal cord of rat pups at various postnatal days was determined using in vitro autoradiographical methods. The functional effect of spinal morphine was also assessed using in vivo electrophysiological methods in rats at P14, P21 and adults (P56). Both mu and kappa opioid receptor binding-sites are present from P0 and spread relatively diffusely throughout the spinal cord. Overall binding peaks at P7 and subsequently decreases to adult levels with the mu opioid receptor binding sites regressing to become denser in the superficial dorsal horn. delta Opioid receptor binding was first seen at P7, and no distinction between superficial and deeper laminae was seen. In the adult, the relative proportions of the opiate receptors in the superficial dorsal horn are 63%, 22% and 15%, for mu, delta and kappa receptor binding sites, respectively. C-fibre evoked dorsal horn neuronal responses recorded from anaesthetized rat pups were highly sensitive to spinal morphine at P21, (EC50 0.005 microgram), compared to the adult (EC50 0.9 microgram). However, the EC50 (0.2 microgram) at P14 was greater than at P21 despite the fact that mu receptor binding was greater at P14. Opioid receptor binding is developmentally regulated and undergoes substantial postnatal reorganization. However, the number of mu receptor binding sites appears not to be the only determinant of functional sensitivity to spinal morphine. Other factors, such as coupling of the receptors are likely to be important.     

Modulation of neuropeptide FF receptors by guanine nucleotides and cations in membranes of rat brain and spinal cord

Using a radioligand binding assay, we examined ionic modulation and G protein coupling of neuropeptide FF (NPFF) receptors in membranes of rat brain and spinal cord. We found that NaCl (but not KCl or LiCl) and MgCl2 increased specific 125I-YLFQPQRFamide (125I-Y8Fa) binding to NPFF receptors in both tissues in a dose-dependent manner, with optimal conditions being 60 mM NaCl and 1 mM MgCl2. Guanine nucleotides dose-dependently inhibited specific 125I-Y8Fa binding to rat brain and spinal cord membranes with maximal effects of 64 +/- 6 and 71 +/- 2%, respectively. The order of potency was nonhydrolyzable GTP analogues > GTP > or = GDP > GMP, ATP. The guanine nucleotide inhibition was observed in the absence and presence of NaCl and MgCl2. The mechanism of inhibition in spinal cord membranes appeared to be a reduction in the number of NPFF receptors; in one experiment, control KD and Bmax values were 0.068 nM and 7.2 fmol/mg of protein, respectively, and with 0.1 microM guanylylimidodiphosphate the respective values were 0.081 nM and 4.9 fmol/mg, a 32% reduction in receptor number. Similar results were obtained with guanosine 5'-O-(3-thiotriphosphate). Our data suggest that 125I-Y8Fa binding sites in rat CNS are G protein-coupled NPFF receptors regulated by GTP and cations.     

A novel antagonist, phenylbenzene omega-phosphono-alpha-amino acid, for strychnine-sensitive glycine receptors in the rat spinal cord

Leukaemia-specific proteins may be recognized by T-lymphocytes as neoantigens if peptides corresponding to mutated sequences bind to major histocompatibility complex (MHC) molecules on leukaemic cells. We studied the ability of a series of synthetic peptides corresponding to the junctional sequences of BCR/ABL proteins to bind to class I molecules in two human cell lines, LBL 721.174 (T2) (HLA-A2, B5) and BM36.1 (HLA-A1, B35), and one murine cell line RMA-S (H-2Kb, Db). These cell lines are defective in intracellular peptide loading of class I molecules, resulting in markedly reduced cell surface class I expression: class I expression can be rescued by provision of peptides binding to the alleles expressed by the mutant cell. Eighteen peptides spanning the junctional sequences of the b2a2 and b3a2 proteins were tested for their ability to rescue expression of the class I alleles borne by these cells using flow cytometry. Allele-specific control peptides known to bind HLA-A2, HLA-B35, H-2Kb and H-2Db increased expression of these alleles 2- to 3-fold: 0/18 BCR/ABL peptides enhanced HLA-A2, HLA-B35 or H-2Kb expression, but three b2a2 peptides consistently increased H-2Db expression. These results suggest that BCR/ABL junctional peptides are unlikely to be presented to T-cells in association with HLA-A2, HLA-B35 or H-2Kb. Conversely, the finding that some b2a2 peptides bind specifically to H-2Db suggests that a murine model of graft-versus-leukaemia (GVL) could be constructed.     

Dual ultrastructural immunocytochemical labeling of mu and delta opioid receptors in the superficial layers of the rat cervical spinal cord

The delta opioid receptor (DOR) and mu opioid receptor (MOR) are abundantly distributed in the dorsal horn of the spinal cord. Simultaneous activation of each receptor by selective opiate agonists has been shown to result in synergistic analgesic effects. To determine the cellular basis for these functional associations, we examined the electron microscopic immunocytochemical localization of DOR and MOR in single sections through the superficial layers of the dorsal horn in the adult rat spinal cord (C2-C4). From a total of 270 DOR-labeled profiles, 49% were soma and dendrites, 46% were axon terminals and small unmyelinated axons, and 5% were glial processes. 6% of the DOR-labeled soma and dendrites, and < 1% of the glial processes also showed MOR-like immunoreactivity (MOR-LI). Of 339 MOR-labeled profiles, 87% were axon terminals and small unmyelinated axons, 12% were soma and dendrites, and 2% were glial processes. 21% of the MOR-labeled soma and dendrites, but none of the axon terminals also contain DOR-LI. The subcellular distributions of MOR and DOR were distinct in axon terminals. In axon terminals, both DOR-LI and MOR-LI were detected along the plasmalemma, but only DOR-LI was associated with large dense core vesicles. DOR-labeled terminals formed synapses with dendrites containing MOR and conversely, MOR-labeled terminals formed synapses with DOR-labeled dendrites. These results suggest that the synergistic actions of selective MOR- and DOR-agonists may be attributed to dual modulation of the same or synaptically linked neurons in the superficial layers of the spinal cord.     

Down-regulation of mu-opioid receptors in rat and monkey dorsal root ganglion neurons and spinal cord after peripheral axotomy

To understand the role of opioids and their receptors in chronic pain following peripheral nerve injury, we have studied the mu-opioid receptor in rat and monkey lumbar 4 and 5 dorsal root ganglion neurons and the superficial dorsal horn of the spinal cord under normal circumstances and after peripheral axotomy. Our results show that many small neurons in rat and monkey dorsal root ganglia, and some medium-sized and large neurons in rat dorsal root ganglia, express mu-opioid receptor-like immunoreactivity. Most of these neurons contain calcitonin gene-related peptide. The mu-opioid receptor was closely associated with the somatic plasmalemma of the dorsal root ganglion neurons. Both mu-opioid receptor-immunoreactive nerve fibers and cell bodies were observed in lamina II of the dorsal horn. The highest intensity of mu-opioid receptor-like immunoreactivity was observed in the deep part of lamina II. Most mu-opioid receptor-like immunoreactivity in the dorsal horn originated from spinal neurons. A few mu-opioid receptor-positive peripheral afferent terminals in the rat and monkey dorsal horn were calcitonin gene-related peptide-immunoreactive. In addition to pre- and post-junctional receptors in rat and monkey dorsal horn neurons, mu-opioid receptors were localized on the presynaptic membrane of some synapses of primary afferent terminals in the monkey dorsal horn. Peripheral axotomy caused a reduction in the number and intensity of mu-opioid receptor-positive neurons in the rat and monkey dorsal root ganglia, and of mu-opioid receptor-like immunoreactivity in the dorsal horn of the spinal cord. The decrease in mu-opioid receptor-like immunoreactivity was more pronounced in the monkey than in the rat dorsal root ganglia and spinal cord. It is probable that there was a parallel trans-synaptic down-regulation of mu-opioid-like immunoreactivity in local dorsal horn neurons of the monkey. These data suggest that one factor underlying the well known insensitivity of neuropathic pain to opioid analgesics could be due to a marked reduction in the number of mu-opioid receptors in the axotomized sensory neurons and in interneurons in the dorsal horn of the spinal cord.     

Expression of TNF alpha in central neurons of Lewis rat spinal cord after EAE induction

Experimental allergic encephalomyelitis (EAE), an animal model for multiple sclerosis (MS), is a demyelinating autoimmune disease of the central nervous system (CNS). The proinflammatory cytokine TNF alpha, as an endogenous mediator of inflammation, plays an important role in the pathogenesis of EAE disease. In this study, we demonstrate the presence of TNF alpha in spinal cord of Lewis rats, during the critical phase of EAE. The expression of TNF alpha is observed mainly in the gray matter of thoracic and lumbar levels of the spinal cord, in the motoneurons and interneurons of the ventral horn. Surprisingly, one month after recovery, we still found an intense TNF alpha-neuronal expression, including in the cervical region, and this positivity lasted up to 40 days after recovery, with, however, a decrease in its intensity. These results suggest that central neurons respond directly to massive infiltration of lymphocytes and macrophages after the breakdown of the blood-brain barrier (BBB), by producing TNF alpha cytokine. In addition, neuronal-TNF alpha detection in the recovery stage of EAE may suggest a role other than its classical action in promoting inflammatory processes.     

Suicide in a spinal cord injured population: its relation to functional status

OBJECTIVE: To determine the relation between functional status and risk of suicide among individuals with spinal cord injury (SCI). DESIGN: A follow-up study of all individuals who survived a traumatic SCI during the period from 1953 through 1990. SETTING: An SCI center in eastern Denmark. SUBJECTS: A total of 888 individuals with SCI, including 236 who died, 23 of whom committed suicide. MAIN OUTCOME MEASURES: Standardized Mortality Ratios (SMRs) of suicides among individuals with SCI. RESULTS: A 100% follow-up was established January 1, 1993. The total suicide rate among individuals with SCI was nearly five times higher than expected in the general population and lower for men than for women. The suicide rate doubled from an early inclusion period (1953-1971) to a later period (1972-1990). An unexpected finding was that the suicide rate in the group of marginally disabled persons was nearly twice as high as the group of functionally complete tetraplegic individuals. CONCLUSION: Given the high frequency of suicide, there is a need for increased awareness by rehabilitation staff and general practitioners regarding depression and psychological adjustment difficulties. Such conditions should be given special attention during rehabilitation and follow-up, especially among women with SCI and the marginally disabled.     

Development of spontaneous synaptic transmission in the rat spinal cord

Dorsal root afferents form synaptic connections on motoneurons a few days after motoneuron clustering in the rat lumbar spinal cord, but frequent spontaneous synaptic potentials are detected only after birth. To increase our understanding of the mechanisms underlying the differentiation of synaptic transmission, we examined the developmental changes in properties of spontaneous synaptic transmission at early stages of synapse formation. Spontaneous postsynaptic currents (PSCs) and tetrodotoxin (TTX)-resistant miniature PSCs (mPSCs) were measured in spinal motoneurons of embryonic and postnatal rats using whole cell patch-clamp recordings. Spontaneous PSC frequencies were higher than mPSC frequencies in both embryonic and postnatal motoneurons, suggesting that even at embryonic stages, when action-potential firing rate was low, presynaptic action potentials played an important role in triggering spontaneous PSCs. After birth, the twofold increase in spontaneous PSC frequency was attributed to an increase in action-potential-independent quantal release rather than to a higher rate of action-potential firing. In embryonic motoneurons, the fluctuations in peak amplitude of spontaneous PSCs were normally distributed around single peaks with modal values similar to those of mPSCs. These data indicated that early in synapse differentiation spontaneous PSCs were primarily composed of currents generated by quantal release. After birth, mean mPSC amplitude increased by 50% but mean quantal current amplitude did not change. Synchronous, multiquantal release was apparent in postnatal motoneurons only in high-K+ extracellular solution. Comparison of the properties of miniature excitatory and inhibitory postsynaptic currents (mEPSCs and mIPSCs) demonstrated that mean mEPSC frequency was higher than mIPSC frequency, suggesting that either excitatory synapses outnumbered inhibitory synapses or that the probability of excitatory transmitter release was higher than the release of inhibitory neurotransmitters. The finding that mIPSC duration was several-fold longer than mEPSC duration implied that despite their lower frequency, inhibitory currents could modulate motoneuron synaptic integration by shunting incoming excitatory inputs for prolonged time intervals.