Interaction between disinhibited bursting and fictive locomotor patterns in the rat isolated spinal cord.
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Using a transverse barrier that allowed discrete application of neurochemicals to certain lumbar regions of the rat isolated spinal cord, we studied the intersegmental organization of rhythmic patterns recorded extracellularly from ventral roots and intracellularly from single motoneurons. Fictive locomotor patterns were elicited by serotonin (5-HT) and/or N-methyl-D-aspartate (NMDA) or high K(+) solution applied to the rostral or caudal lumbar region of the cord. Neither 4-aminopyridine nor Mg(2+)-free solution shared this property. Coapplication of strychnine and bicuculline (blockers of fast synaptic inhibition) to the caudal part elicited slow bursting in the whole cord. These bursts could trigger episodes of fictive locomotion patterns in the rostral roots. When the rostral region was exposed to 5-HT and/or NMDA (during continuous application of strychnine and bicuculline caudally) a standard locomotor-like pattern was generated during each interburst interval and was surprisingly expressed with its typical pattern alternation even in the caudal area despite the local presence of strychnine and bicuculline. Midsagittal splitting of the caudal region did not change this alternating pattern, indicating that it was driven by rostral regions above the surgical cut. Discrete changes in the concentrations of NMDA rostrally modulated the burst amplitude recorded in the same region after caudal application of strychnine and bicuculline. The period of fictive locomotor patterns changed bimodally depending on the temporal relation with disinhibited bursts, indicating a tight interaction between these two rhythmic activities. These results are interpreted on the basis of a model that assumes a modular arrangement for the locomotor central pattern generator, made up by a series of unit burst generators with serial and crossed connections. (+info)
Seizure-like events in disinhibited ventral slices of adult rat hippocampus.
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Epileptic discharges lasting 2-90 s, were studied in vitro in slices from the ventral hippocampus of adult rats, in which inhibition was blocked acutely with bicuculline methiodide (BMI, 5-30 microM) and potassium ([K(+)](o)) raised to 5 mM. These seizure-like events (SLEs) comprised three distinct phases, called here primary, secondary, and tertiary bursts. Primary bursts lasted 90-150 ms. Secondary bursts lasted a further 70-250 ms, comprising a short series of afterdischarges riding on the same depolarization as the primary burst. Finally a train of tertiary bursts started with a peak frequency of 5-10 Hz and could last >1 min. Slices from the ventral hippocampus showed significantly higher susceptibility to SLEs than did dorsal slices. SLEs proved sensitive to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonists. They were insensitive to N-methyl-D-aspartate (NMDA) receptor antagonists; 50 microl D-2-amino-5-phosphonopentanoic acid (D-AP5) did block the transient secondary bursts selectively. SLEs were restricted to the hippocampus proper even if the entorhinal cortex was present. Entorhinal bursts could last <2 s and were only coupled with hippocampal bursts in a minority of slices. Reentry of epileptic bursts occasionally occurred during interictal discharges, but not during the later stages of SLEs. Full-length SLEs always started in CA3 region and could be recorded in minislices containing CA3 plus dentate hilus. Ion-sensitive microelectrodes revealed that interictal discharges were followed by short (2-3 s) [K(+)](o) waves, peaking at approximately 7.5 mM. SLEs were always accompanied by increases in [K(+)](o) reaching approximately 8.5 mM at the start of tertiary bursts; [K(+)](o) then increased more slowly to a ceiling of 11-12 mM. After the end of each SLE, [K(+)](o) fell back to baseline within 10-15 s. SLEs were accompanied by significant increase in synaptic activity, compared with baseline and/or interictal activity, estimated by the variance of the intracellular signal in the absence of epileptic bursts and action potentials (0. 38 mV(2), compared with 0.13 mV(2), and 0.1 mV(2), respectively). No significant increases were observed in the interval preceding spontaneous interictal activity. These studies show that focal assemblies of hippocampal neurons, without long reentrant loops, are sufficient for the generation of SLEs. We propose that a key factor in the transition from interictal activity to SLEs is an increase in axonal and terminal excitability, resulting, at least in part, from elevations in [K(+)](o). (+info)
Effects of GABA receptor antagonist on trigeminal caudalis nociceptive neurons in normal and neonatally capsaicin-treated rats.
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We have recently demonstrated that significant increases in cutaneous mechanoreceptive field (RF) size and spontaneous activity occur in nociceptive neurons of trigeminal subnucleus caudalis (Vc, the medullary dorsal horn) of adult rats depleted of C-fiber afferents by neonatal treatment with capsaicin. These neuronal changes in capsaicin-treated (CAP) rats are suggestive of central neuroplasticity and involve N-methyl-D-aspartic acid (NMDA) receptor mechanisms. The present study examined whether the GABA(A) receptor antagonist bicuculline (BIC) or the GABA(B) receptor antagonist 2-hydroxysaclofen (SAC) can influence the RF properties and activity of Vc nociceptive neurons classified as either nociceptive-specific or wide-dynamic range in CAP adult rats or in neonatally vehicle-treated (CON) rats. C-fiber depletion was confirmed in the CAP rats by a significant decrease in plasma extravasation of Evans blue dye in a skin area receiving topical application of mustard oil, a small-fiber excitant and inflammatory irritant. As previously reported, marked increases in cutaneous RF size and spontaneous activity occurred in Vc nociceptive neurons of adult CAP rats, compared with CON rats. GABA(A) receptor blockade by BIC (i.t.) in CON rats produced a significant increase in spontaneous activity and in pinch RF size and tactile RF size (or appearance of a tactile area in the RF of nociceptive-specific neurons), as well as a significant lowering of the mechanical threshold and a significant enhancement of responses to pinch stimuli applied to the RF. In CAP rats, GABA(A) receptor blockade also produced significant changes similar to those documented in CON rats, except for a paradoxical and significant decrease in pinch RF size and no noticeable changes in responses to pinch stimuli. GABA(B) receptor blockade by SAC (i.t. ) did not produce any significant changes in Vc nociceptive neurons in either CON or CAP rats. These results suggest that GABA(A) receptor-mediated inhibition may be involved in maintaining the functional expression of Vc nociceptive neuronal properties in normal conditions, and that in animals depleted of their C-fiber afferents, some features of this GABA(A) receptor-mediated modulation may be disrupted such that a GABA(A) receptor-mediated excitation is manifested. (+info)
Dorsal root reflexes and cutaneous neurogenic inflammation after intradermal injection of capsaicin in rats.
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The role of dorsal root reflexes (DRRs) in acute cutaneous neurogenic inflammation induced by intradermal injection of capsaicin (CAP) was examined in anesthetized rats. Changes in cutaneous blood flow (flare) on the plantar surface of the foot were measured using a laser Doppler flowmeter, and neurogenic edema was examined by measurements of paw thickness. To implicate DRRs in neurogenic inflammation after CAP injection, the ipsilateral sciatic and femoral nerves were sectioned, dorsal rhizotomies were performed at L(3-)-S(1), and antagonists of GABA or excitatory amino acid receptors were administered intrathecally. Intradermal injection of CAP evoked a flare response that was largest at 15-20 mm from the injection site and that spread >30 mm. Acute transection of the sciatic and femoral nerves or dorsal rhizotomies nearly completely abolished the blood flow changes 15-20 mm from the CAP injection site, although there was only a minimal effect on blood flow near the injection site. These procedures also significantly reduced neurogenic edema. Intrathecal bicuculline, 6-cyano-7-nitroquinoxaline-2,3-dione, (CNQX) or D(-)-2-amino-7-phosphonoheptanoic acid (AP7), but not phaclofen, also reduced dramatically the increases in blood flow 15-20 mm from the CAP injection site, but had only a minimal effect on blood flow near the injection site. Neurogenic edema was reduced by the same agents that reduced blood flow. Multiunit DRRs recorded from the central stumps of cut dorsal rootlets in the lumbar spinal cord were enhanced after CAP injection. This enhanced DRR activity could be reduced significantly by posttreatment of the spinal cord with bicuculline, CNQX or AP7, but not phaclofen. It is concluded that peripheral cutaneous inflammation induced by intradermal injection of CAP involves central nervous mechanisms. DRRs play a major role in the development of neurogenic cutaneous inflammation, although a direct action of CAP on peripheral nerve terminals or the generation of axon reflexes also may contribute to changes in the skin near the injection site. (+info)
Electrophysiological properties and synaptic responses of cells in the trigeminal principal sensory nucleus of postnatal rats.
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In the rodent brain stem trigeminal complex, select sets of neurons form modular arrays or "barrelettes," that replicate the patterned distribution of whiskers and sinus hairs on the ipsilateral snout. These cells detect the patterned input from the trigeminal axons that innervate the whiskers and sinus hairs. Other brain stem trigeminal cells, interbarrelette neurons, do not form patterns and respond to multiple whiskers. We examined the membrane properties and synaptic responses of morphologically identified barrelette and interbarrelette neurons in the principal sensory nucleus (PrV) of the trigeminal nerve in early postnatal rats shortly after whisker-related patterns are established. Barrelette cell dendritic trees are confined to a single barrelette, whereas the dendrites of interbarrelette cells span wider territories. These two cell types are distinct from smaller GABAergic interneurons. Barrelette cells can be distinguished by a prominent transient A-type K(+) current (I(A)) and higher input resistance. On the other hand, interbarrelette cells display a prominent low-threshold T-type Ca(2+) current (I(T)) and lower input resistance. Both classes of neurons respond differently to electrical stimulation of the trigeminal tract. Barrelette cells show either a monosynaptic excitatory postsynaptic potential (EPSP) followed by a large disynaptic inhibitory postsynaptic potential (IPSP) or just simply a disynaptic IPSP. Increasing stimulus intensity produces little change in EPSP amplitude but leads to a stepwise increase in IPSP amplitude, suggesting that barrelette cells receive more inhibitory input than excitatory input. This pattern of excitation and inhibition indicates that barrelette cells receive both feed-forward and lateral inhibition. Interbarrelette cells show a large monosynaptic EPSP followed by a small disynaptic IPSP. Increasing stimulus intensity leads to a stepwise increase in EPSP amplitude and the appearance of polysynaptic EPSPs, suggesting that interbarrelette cells receive excitatory inputs from multiple sources. Taken together, these results indicate that barrelette and interbarrelette neurons can be identified by their morphological and functional attributes soon after whisker-related pattern formation in the PrV. (+info)
Development of neuronal activity and activity-dependent expression of brain-derived neurotrophic factor mRNA in organotypic cultures of rat visual cortex.
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We have analyzed in organotypic rat visual cortex cultures the way in which expression of brain-derived neurotrophic factor (BDNF) mRNA depends on synaptically generated spontaneous bioelectric activity (SBA) as monitored by recordings of pyramidal cells. SBA was initially low, but from the fourth week onwards 83% of the neurons fired action potentials at 0.2-1.2 impulses/s in a well-balanced state of excitation and inhibition. BDNF mRNA expression increased during the second week to a level surprisingly similar to the adult visual cortex in vivo, despite the fact that activity rates in vitro were approximately 10-fold lower than rates reported in vivo. Thus, SBA generated by a cortical neuronal network in the absence of sensory input is sufficient to elicit and maintain BDNF expression. The transient BDNF peak occurring after eye opening in vivo did not occur in vitro. A blockade of SBA seems not to alter the expression of neurotrophin (NT)-3 and -4/5, and tyrosine kinase receptor C and B mRNA. However, BDNF expression remained extremely low. A recovery of SBA after a period of blockade concurred with a transient hyperexcitability. BDNF immediately increased, driven by calcium influx through voltage-gated channels in synergy with NMDA receptors. Expression transiently reached high levels in neurons of supragranular layers. Infragranular neurons, although firing action potentials, recovered BDNF expression much slower. After 5 days in vitro recovery, the network had de novo established a balanced state of excitation and inhibition. Distribution and expression level of BDNF mRNA had returned to control. Even in 'adult' cultures an acute blockade of SBA downregulated BDNF, and a subsequent recovery of SBA restored BDNF expression. We conclude that BDNF mRNA expression depends on and responds with a fast kinetic to changes of the SBA. Steady-state levels do not depend on the absolute levels of activity, but more likely on the balance between excitation and inhibition, suggesting a role for BDNF in activity homeostasis. (+info)
Regulation of action potential size and excitability in substantia nigra compacta neurons: sensitivity to 4-aminopyridine.
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Slow, pacemaker-like firing is due to intrinsic membrane properties in substantia nigra compacta (SNc) neurons in vitro. How these properties interact with afferent synaptic inputs is not fully understood. In this study, intracellular recordings from SNc neurons in brain slices showed that spontaneous action potentials (APs) were attenuated when generated from lower than normal threshold. Such APs were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and could be related to non-N-methyl-D-aspartate (NMDA) receptor-mediated spontaneous excitatory postsynaptic potentials (EPSPs). The AP attenuation was reproduced by stimulus-evoked EPSPs and by current injections to the soma. APs evoked from holding potentials between -40 and -60 mV were reduced in width by Cd(2+) (0. 2 mM). Tetraethylammonium chloride (TEA, 10 mM) or 4-aminopyridine (4-AP, 5 mM) increased the AP width. However, at more negative holding potentials, Cd(2+) and TEA were inefficacious, whereas 4-AP enlarged the AP, partly via induction of a Cd(2+)-sensitive component. A monophasic afterhyperpolarization (AHP), following attenuated APs, was little affected by either Cd(2+) or TEA, but inhibited by 4-AP, which induced an additional, slow component, sensitive to Cd(2+) or apamin (100 nM). The AP delay showed a discontinuous relation to the amplitude or slope of the injected current (delay shift), which was sensitive to low doses of 4-AP (0. 05 mM). The initial time window before the delay shift was longer than the rise time of EPSPs. It is suggested that a 4-AP-sensitive current prevents or postpones discharge during slow depolarization's, but allows direct excitation by fast EPSPs. Fast excitation leads to AP attenuation, primarily due to strong activation of 4-AP-sensitive current. This seems to cause inhibition of the Ca(2+) current during the AP and reduction of Ca(2+)-dependent K(+) currents. Together, these properties are likely to influence the excitability and the local, somatodendritic effects of the AP, in a manner that discriminates between firing induced by the intrinsic pacemaker mechanism and fast synaptic potentials. (+info)
Selective modulation of excitatory transmission by mu-opioid receptor activation in rat supraoptic neurons.
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Opioid peptides have profound inhibitory effects on the production of oxytocin and vasopressin, but their direct effects on magnocellular neuroendocrine neurons appear to be relatively weak. We tested whether a presynaptic mechanism is involved in this inhibition. The effects of mu-opioid receptor agonist D-Ala(2), N-CH(3)-Phe(4), Gly(5)-ol-enkephalin (DAGO) on excitatory and inhibitory transmission were studied in supraoptic nucleus (SON) neurons from rat hypothalamic slices using whole cell recording. DAGO reduced the amplitude of evoked glutamatergic excitatory postsynaptic currents (EPSCs) in a dose-dependent manner. In the presence of tetrodotoxin (TTX) to block spike activity, DAGO also reduced the frequency of spontaneous miniature EPSCs without altering their amplitude distribution, rising time, or decaying time constant. The above effects of DAGO were reversed by wash out, or by addition of opioid receptor antagonist naloxone or selective mu-antagonist Cys(2)-Tyr(3)-Orn(5)-Pen(7)-NH(2) (CTOP). In contrast, DAGO had no significant effect on the evoked and spontaneous miniature GABAergic inhibitory postsynaptic currents (IPSCs) in most SON neurons. A direct membrane hyperpolarization of SON neurons was not detected in the presence of DAGO. These results indicate that mu-opioid receptor activation selectively inhibits excitatory activity in SON neurons via a presynaptic mechanism. (+info)