Serotonergic modulation of respiratory motoneurons and interneurons in brainstem slices of perinatal rats

Respiration-related membrane potential fluctuations were recorded in hypoglossal (XII) motoneurons and pre-B?tzinger complex (pre-B?tC) interneurons in medullary slices from perinatal rats. Bath application of serotonin (5-HT) evoked a ketanserine-sensitive depolarization (approximately 11 mV) and tonic spike discharge in XII motoneurons, whereas pre-B?tC neurons responded with a <6 mV depolarization and no tonic discharge. The membrane effects were accompanied by an increase in respiratory frequency by up to 260% in 64% of preparations. A frequency decrease leading to block of respiratory activity could also occur (20%) as well as an initial acceleration that turned into a frequency depression (16%). In contrast, iontophoresis of 5-HT into the pre-B?tC exclusively increased respiratory frequency by 30-220%, whereas iontophoresis into the XII nucleus did not change respiratory frequency but induced tonic nerve discharge. The effects of local iontophoretic administration of 5-HT on membrane properties of XII and pre-B?tC cells were very similar to those upon bath application. Bath application and iontophoresis of the 5-HT2 receptor agonist -methyl-hydroxytryptamine mimicked the effects of 5-HT. Bath application of the 5-HT1A receptor agonist 8-hydroxydipropylaminotetralin hydrobromide did not affect XII nerve bursting or pre-B?tC neurons. Iontophoresis of 8-hydroxydipropylaminotetralin hydrobromide had almost no effect on respiratory frequency and induced in the interneurons either a depolarization or hyperpolarization (<5 mV) which was blocked by the 5-HT1A receptor antagonist N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)N-2-pyridinylcyclohexane carboxamide. In conclusion, 5-HT-evoked tonic excitation of respiratory XII motoneurons is mediated by postsynaptic 5-HT2 receptors. The excitatory effects on respiratory rhythm are also primarily attributable to postsynaptic 5-HT2 receptors of pre-B?tC neurons. Additional modulatory effects on the interneurons appear to be mediated by postsynaptic 5-HT1A receptors.     

Serotonin Modulates Oscillations of the Membrane Potential in Isolated Spinal Neurons from Lampreys

Studies were performed on spinal neurons from lampreys isolated by an enzymatic/mechanical method using pronase. The effects of 100 M serotonin (5-HT) on membrane potential oscillations induced by a variety of excitatory amino acids were studied. 5-HT was found to depolarize branched cells (presumptive motoneurons and interneurons) by 2–6 mV without inducing membrane potential oscillations. However, when oscillations were already present because of an excitatory amino acid, 5-HT changed the parameters of these oscillations, increasing the amplitudes of all types of oscillations, increasing the frequency of irregular oscillations, and increasing the duration of the depolarization plateaus accompanied by action potentials. Serotonin modulation of the effects of excitatory amino acids and the electrical activity of cells in the neural locomotor network facilitates motor activity and leads to increases in the contraction of truncal muscles and more intense movements by the animal. The possible mechanisms of receptor coactivation are discussed, along with increases in action potential frequency and changes in the parameters of the locomotor rhythm.     

Respiratory interneurons of the lower cervical (C4-C5) cord: membrane potential changes during fictive coughing,vomiting, and swallowing in the decerebrate cat

The possible roles of interneurons in the C4-C5 cervical spinal cord in conveying central drives to phrenic motoneurons during different behaviour patterns were investigated using intracellular recordings in decerebrate, paralysed, artificially ventilated cats. Eleven cells were tentatively classified as respiratory interneurons since they: (i) could not be antidromically activated from the ipsilateral whole intrathoracic phrenic nerve, and (ii) exhibited large membrane potential changes during eupnea (7.3 mV±3.6, range 2–13.5 mV) or non-respiratory behaviour patterns. Six neurons depolarized in phase with phrenic discharge; four others depolarized during the expiratory phase; one neuron exhibited depolarization during the end of both expiration and inspiration. A variety of responses was observed during fictive coughing, vomiting, and swallowing. The results are consistent with C4-C5 expiratory interneurons conveying inhibition to phrenic motoneurons during different behaviour patterns. The responses of inspiratory and multiphasic neurons suggest that the roles of these interneurons are mode complex than simply relaying central excitatory or inhibitory drive to phrenic motoneurons.     

Serotonin-1A receptor activation in hippocampal CA1 neurons by 8-hydroxy-2-(di-n-propylamino)tetralin, 5-methoxytryptamine and 5-hydroxytryptamine

Serotonin (5-HT) usually induced a slow hyperpolarization lasting several minutes on first drop-application onto CA1 neurons. Subsequent applications always caused a briefer (less than 2 min) hyperpolarization, usually followed by a depolarization. 8-Hydroxy-2(di-n-propylamino)tetralin, a 5-HT1A receptor agonist, and 5-methoxytryptamine, a 5-HT1 receptor agonist, produced only the long-lasting hyperpolarization. The application of 5-HT agonists caused a persistent prolongation of the post-spike train afterhyperpolarization. These observations suggest that the long-lasting hyperpolarization produced by 5-HT may be mediated by the activation of the 5-HT1A receptor subtype.     

Development and serotonergic modulation of NMDA bursting in rat trigeminal motoneurons

The development of N-methyl-D-aspartate (NMDA)-induced burst discharge in rat trigeminal motoneurons (TMNs) between postnatal days P1 and P10 was examined using whole cell patch-clamp recording methods in brain slices. Bath application of NMDA (50 microM) induced a Mg(2+)-dependent rhythmical bursting activity starting around P8. Prior to the onset of bursting, the membrane potential depolarized and the input resistance increased. Hyperpolarization of the membrane potential with extrinsic current demonstrated a narrow window of membrane potential where maintained rhythmical burst discharge was evident. In P1-P4 neurons, NMDA application produced membrane depolarization and a minimal change in input resistance, but no burst activity at any membrane potential. Voltage-clamp analysis indicated that the bursting activity was related to the presence or absence of a voltage-dependent Mg(2+) block and induction of a negative slope conductance (NSC) region in the I(NMDA)-V relationship. Regardless of age, reduction of extracellular Mg(2+) from 1 mM to 30 microM enhanced I(NMDA) at voltages negative to -60 mV. However, in 1 mM Mg(2+), P1-P4 neurons were devoid of a prominent NSC region compared with P8-P10 neurons, suggesting that the efficacy of depolarization in unblocking the NMDA receptors increased with age. NMDA bursting was not dependent on calcium influx through voltage-gated calcium channels (VGCC) but did require a minimal concentration of Ca(2+) in the bath. Intracellular bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid application suppressed burst discharge completely, suggesting that intracellular Ca(2+) directly, or via second-messenger systems, regulates NMDA receptor activity and bursting. Interestingly, NMDA bursting could be induced in P1-P4 neurons by simultaneous bath application of serotonin (5-HT, 10 microM), which by itself did not produce bursting, suggesting an "enabling" role for 5-HT. Voltage-clamp analysis demonstrated that the NMDA/5-HT bursting resulted from induction of an NSC in the I-V relationship of total membrane current. 5-HT by itself produced no such effect. The mechanisms for this effect were due to an enhancement of the NSC region of the I(NMDA)-V relationship and reduction of a presumed leak current by 5-HT. These data indicate that NMDA bursting in trigeminal motoneurons is developmentally regulated and subject to neuromessenger modulation. Control of the Mg(2+) sensitivity of the NMDA receptor and voltage-dependent block by neuromessengers could be an effective means to control the efficacy of glutamatergic synaptic drive to motoneurons during rhythmical oral-motor activity at early postnatal ages.     

Angiotensin AT(1)-receptors depolarize neonatal spinal motoneurons and other ventral horn neurons via two different conductances

Angiotensin receptors are highly expressed in neonatal spinal cord. To identify their influence on neuronal excitability, we used patch-clamp recordings in spinal cord slices to assess responses of neonatal rat (5-12 days) ventral horn neurons to bath-applied angiotensin II (ANG II; 1 microM). In 14/34 identified motoneurons tested under current clamp, ANG II induced a slowly rising and prolonged membrane depolarization, blockable with Losartan (n = 5) and (Sar(1), Val(5), Ala(8))-ANG II (Saralasin, n = 4) but not PD123319 (1 microM each; n = 4). Under voltage clamp (V(H) -65 mV), 7/22 motoneurons displayed an ANG-II-induced tetrodotoxin-resistant inward current (-128 +/- 31 pA) with a similar time course, an associated reduction in membrane conductance and net current reversal at -98.8 +/- 3.9 mV. Losartan-sensitive ANG II responses were also evoked in 27/78 tested ventral horn "interneurons." By contrast with motoneurons, their ANG-II-induced inward current was smaller (-39.9 +/- 5.2 pA) and analysis of their I-V plots revealed three patterns. In eight cells, membrane conductance decreased with net inward current reversing at -103.8 +/- 4.1 mV. In seven cells, membrane conductance increased with net current reversing at -37.9 +/- 3.6 mV. In 12 cells, I-V lines remained parallel with no reversal within the current range tested. Intracellular dialysis with GTP-gamma-S significantly prolonged the ANG II effect in seven responsive interneurons and GDP-beta-S significantly reduced the ANG II response in four other cells. Peak inward currents were significantly reduced in all 13 responding neurons recorded in slices incubated in pertussis toxin (5 microgram/ml) for 12-18 h or in 12 neurons perfused with N-ethylmaleimide. Of 29 interneurons sensitive to pertussis toxin or N-ethylmaleimide treatment, 9 cells displayed a decrease in membrane conductance that reversed at -101.3 +/- 3.8 mV. In eight cells, membrane conductance increased and reversed at -38.7 +/- 3.4 mV. In 12 cells, the I-V lines remained parallel with no reversal within the current range tested, suggesting that both conductances are modulated by pertussis toxin-sensitive G proteins. These observations reveal a direct, G-protein-mediated depolarizing action of ANG II on neonatal rat ventral horn neurons. They also imply involvement of two distinct conductances that are differentially distributed among different cell types.     

Age-related changes in the effects of 5-hydroxytryptamine on substantia gelatinosa neurons of the trigeminal subnucleus caudalis

The trigeminal subnucleus caudalis (Vc) is the critical brainstem relay site of orofacial nociceptive processing to higher brain centers. The descending serotonergic pathway from the brainstem exerts inhibitory or facilitatory effects on nociceptive transmission in the spinal dorsal horn and the Vc, and SG neurons of the Vc exhibit hyperpolarization, no response or depolarization in response to 5-hydroxytryptamine (5-HT) application. In this study, we examined age-related changes in the effects of 5-HT on SG neurons of the Vc using immature, peripubertal and adult male mice and gramicidin-perforated patch recordings under the current-clamp mode. In the three age groups, hyperpolarization was the major response in SG neurons exhibiting membrane potential changes in response to 5-HT application. The proportion of the SG neurons responding to 5-HT by hyperpolarization was significantly higher in the immature (20/27) than in the adult mice (10/26; P<0.05). The proportion of SG neurons showing no response to 5-HT was significantly higher in the peripubertal (11/21) and the adult mice (13/26) compared with the immature mice (5/27). The amplitude of 5-HT-induced hyperpolarization significantly decreased with increasing postnatal age (correlation coefficient=-0.43, P<0.05). The mean amplitude of 5-HT-induced hyperpolarization was significantly higher in the immature mice (-9.7±1.1 mV, n=20) than in the peripubertal (-5.3±1.0 mV, n=10) and the adult mice (-5.4±0.9 mV, n=10; both P<0.05). These results suggest that the descending serotonergic modulatory influence over the orofacial nociceptive processing in the Vc may change during postnatal development and postnatal age of three weeks is a critical period for changes in 5-HT-induced hyperpolarizing effects in mice.     

Slow excitatory post-synaptic potentials in myenteric AH neurons of the guinea-pig ileum are reduced by the 5-hydroxytryptamine7 receptor antagonist SB 269970

Serotonin (5-HT) is a key modulator of neuronal excitability in the central and peripheral nervous system. In the enteric nervous system, 5-HT causes a slow depolarization in the intrinsic sensory neurons, but the receptor responsible for this has not been correlated with known gene products. The aim of this study was to determine whether the newly characterized 5-HT7 receptor may participate in the 5-HT-mediated depolarization of, and synaptic transmission to, the intrinsic sensory neurons of the guinea-pig ileum. Intracellular electrophysiological recordings were made from intrinsic sensory neurons identified as myenteric AH neurons from guinea-pig ileum. 5-HT (5 microM) applied to the cell body evoked both a fast depolarization (5-HT3 mediated) and/or a slow depolarization (5-HT1P-like). The 5-HT1/5/7 receptor agonist 5-carboxamidotryptamine (5-CT) (5 microM) evoked only a slow depolarization. When the fast depolarization evoked by 5-HT was blocked with granisetron (1 microM, 5-HT3 receptor antagonist), only a slow depolarization remained; this was abolished by the 5-HT7 receptor antagonist SB 269970 (1 microM, control: 14+/-2 mV, granisetron+SB 269970: -1+/-2 mV). The slow depolarization evoked by 5-CT was also significantly reduced by SB 269970 (control: 14+/-1 mV, SB 269970: 5+/-2 mV) suggesting a 5-HT7 receptor was activated by exogenous application of 5-CT and 5-HT. Slow excitatory postsynaptic potentials evoked by stimulating descending neural pathways (containing serotonergic fibers) were reduced by SB 269970 (control: 8+/-3 mV, SB 269970: 3+/-1 mV). However, SB 269970 had no effect on slow excitatory postsynaptic potentials evoked by stimulation of circumferential (tachykinergic) pathways (control: 7+/-1 mV, SB 269970: 6+/-1 mV). These data are consistent with the presence on enteric AH neurons of functional 5-HT7 receptors that participate in slow synaptic transmission.     

Ionic mechanisms underlying excitatory effects of serotonin on embryonic rat motoneurons in long-term culture

The actions of serotonin were investigated on motoneurons isolated from embryonic day 14 rat spinal cord and enriched by metrizamide density gradient centrifugation. Trophic support was provided by a spinal cord glial monolayer, ciliary neurotrophic factor and heat-inactivated serum. Cultures were maintained for 17–83 days and investigated using whole-cell patch-clamp recording. Serotonin evoked slow depolarizations (6.2±0.7 or 9.3±1.3 mV in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione and strychnine, ₅₀ 8.2 nM), which were reversibly blocked by 0.1 μM ketanserin. Serotonin generated synaptic potentials in motoneurons, lowered the threshold for repetitive firing and changed the slope of the current intensity–firing frequency relationship. The inward current evoked by serotonin (−147±15.2 pA) was ascribed to a complex ionic mechanism, which varied amongst neurons in the sampled population. It was due to closure of barium-sensitive potassium channels, effects on I_h and increase in a separate mixed cation current which comprised both transient voltage-sensitive and sustained components.

We conclude that serotonergic responses develop in motoneurons cultured under these conditions in the absence of serotonergic input, sensory neurons or many interneurons.     

Serotonergic modulation of neurotransmission in the rat basolateral amygdala.

Whole cell patch-clamp recordings were obtained from projection neurons and interneurons of the rat basolateral amygdala (BLA) to understand local network interactions in morphologically identified neurons and their modulation by serotonin. Projection neurons and interneurons were characterized morphologically and electrophysiologically according to their intrinsic membrane properties and synaptic characteristics. Synaptic activity in projection neurons was dominated by spontaneous inhibitory postsynaptic currents (IPSCs) that were multiphasic, reached 181 +/- 38 pA in amplitude, lasted 296 +/- 27 mS, and were blocked by the GABAA receptor antagonist, bicuculline methiodide (30 microM). In interneurons, spontaneous synaptic activity was characterized by a burst-firing discharge patterns (200 +/- 40 Hz) that correlated with the occurrence of 6-cyano-7-nitroquinoxaline-2,3-dione-sensitive, high-amplitude (260 +/- 42 pA), long-duration (139 +/- 19 mS) inward excitatory postsynaptic currents (EPSCs). The interevent interval of 831 +/- 344 mS for compound inhibitory postsynaptic potentials (IPSPs), and 916 +/- 270 mS for EPSC bursts, suggested that spontaneous IPSP/Cs in projection neurons are driven by burst of action potentials in interneurons. Hence, BLA interneurons may regulate the excitability of projection neurons and thus determine the degree of synchrony within ensembles of BLA neurons. In interneurons 5-hydroxytryptamine oxalate (5-HT) evoked a direct, dose-dependent, membrane depolarization mediated by a 45 +/- 6.9 pA inward current, which had a reversal potential of -90 mV. The effect of 5-HT was mimicked by the 5-HT2 receptor agonist, alpha-methyl-5-hydroxytryptamine (alpha-methyl-5-HT), but not by the 5-HT1A receptor agonist, (+/-) 8-hydroxydipropylaminotetralin hydrobromide (8-OH-DPAT), or the 5-HT1B agonist, CGS 12066A. In projection neurons, 5-HT evoked an indirect membrane hyperpolarization ( approximately 2 mV) that was associated with a 75 +/- 42 pA outward current and had a reversal potential of -70 mV. The response was independent of 5-HT concentration, blocked by TTX, mimicked by alpha-methyl-5-HT but not by 8-OH-DPAT. In interneurons, 5-HT reduced the amplitude of the evoked EPSC and in the presence of TTX (0.6 microM) reduced the frequency of miniature EPSCs but not their quantal content. In projection neurons, 5-HT also caused a dose-dependent reduction in the amplitude of stimulus evoked EPSCs and IPSCs. These results suggest that acute serotonin release would directly activate GABAergic interneurons of the BLA, via an activation of 5-HT2 receptors, and increase the frequency of inhibitory synaptic events in projection neurons. Chronic serotonin release, or high levels of serotonin, would reduce the excitatory drive onto interneurons and may act as a feedback mechanism to prevent excess inhibition within the nucleus.     

Electrophysiological evidence for vasopressin V(1) receptors on neonatal motoneurons, premotor and other ventral horn neurons

Prominent arginine-vasopressin (AVP) binding and AVP V(1) type receptors are expressed early in the developing rat spinal cord. We sought to characterize their influence on neural excitability by using patch-clamp techniques to record AVP-induced responses from a population of motoneurons and interneurons in neonatal (5-18 days) rat spinal cord slices. Data were obtained from 58 thoracolumbar (T(7)-L(5)) motoneurons and 166 local interneurons. A majority (>90%) of neurons responded to bath applied AVP (10 nM to 3 microM) and (Phe(2), Orn(8))-vasotocin, a V(1) receptor agonist, but not V(2) or oxytocin receptor agonists. In voltage-clamp, postsynaptic responses in motoneurons were characterized by slowly rising, prolonged (7-10 min) and tetrodotoxin-resistant inward currents associated with a 25% reduction in a membrane potassium conductance that reversed near -100 mV. In interneurons, net AVP-induced inward currents displayed three patterns: decreasing membrane conductance with reversal near -100 mV, i.e., similar to that in motoneurons (24 cells); increasing conductance with reversal near -40 mV (21 cells); small reduction in conductance with no reversal within the current range tested (41 cells). A presynaptic component recorded in most neurons was evident as an increase in the frequency but not amplitude (in motoneurons) of inhibitory and excitatory postsynaptic currents (IPSCs and EPSCs), in large part due to AVP-induced firing in inhibitory (mainly glycinergic) and excitatory (glutamatergic) neurons synapsing on the recorded cells. An increase in frequency but not amplitude of miniature IPSCs and EPSCs also indicated an AVP enhancement of neurotransmitter release from axon terminals of inhibitory and excitatory interneurons. These observations provide support for a broad presynaptic and postsynaptic distribution of AVP V(1) type receptors and indicate that their activation can enhance the excitability of a majority of neurons in neonatal ventral spinal cord.     

Effects of 5-hydroxytryptamine on the afterhyperpolarization, spike frequency regulation, and oscillatory membrane properties in lamprey spinal cord neurons

1. Local application of 5-hydroxytryptamine (5-HT) in the area in which a dense 5-HT plexus is located in the lamprey spinal cord leads to a marked depression of the late phase of the afterhyperpolarization (AHP) following the action potential. This effect was observed in motoneurons, premotor interneurons, and giant interneurons, whereas no effect was observed in the sensory dorsal cells and edge cells. 2. The late 5-HT sensitive phase of the AHP was increased in amplitude when calcium entry was enhanced during the prolongation of action potentials caused by tetraethylammonium (TEA). Conversely, a blockade of Ca2+ entry by manganese reduced the AHP amplitude, suggesting that a calcium-dependent current, most likely carried by potassium, underlies the late phase of the AHP in these cells, as is the case in many other types of neurons. 3. The late phase of the AHP could be depressed by 5-HT although no effects were exerted on either the resting input resistance or on the shape of the action potential in 54% of the cells. The membrane conductance increase associated with the late phase of the AHP was markedly attenuated by 5-HT application. 4. In voltage-clamp experiments, Na+ currents and most K+ currents were blocked by tetrodotoxin (TTX) and TEA, respectively. Under these conditions, voltage steps elicited a slow outward current, most likely representing a Ca2+-activated K+ current, which was depressed by 5-HT application. 5. 5-HT does not appear to reduce AHP amplitude by blocking the calcium entry occurring during the action potential. No evidence was obtained for an involvement of second messengers such as adenosine-3':5'-cyclic monophosphate (cAMP), guanosine-3':5'-cyclic monophosphate (cGMP), diacyglycerol, or arachidonic acid. The effect of 5-HT on the late AHP may be due to a direct action on the calcium-dependent potassium channels or on the intracellular handling of Ca2+ ions. 6. The amplitude reduction of the AHP has a profound influence on the spike frequency regulation of any given cell; the frequency of spikes evoked by a given excitatory stimulus is therefore markedly increased by application of 5-HT. 5-HT thus increases the "gain" of the input-output relation of interneurons and motoneurons responsible for generating the locomotor rhythm. In addition, 5-HT causes a prolongation of the depolarized plateau of the N-methyl-D-aspartate (NMDA) receptor-induced membrane potential oscillations, as expected from the 5-HT-induced effects on the Ca2+-activated K+ channels that contribute to the repolarization.     

5-HT Modulation of identified segmental premotor interneurons in the lamprey spinal cord

Ipsilaterally projecting spinal excitatory interneurons (EINs) generate the hemisegmental rhythmic locomotor activity in lamprey, while the commissural interneurons ensure proper left-right alternation. 5-HT is a potent modulator of the locomotor rhythm and is endogenously released from the spinal cord during fictive locomotion. The effect of 5-HT was investigated for three segmental premotor interneuron types: EINs, commissural excitatory and commissural inhibitory interneurons. All three types of interneurons produced chemical postsynaptic potentials in motoneurons, but only those from EINs had an electrical component. The effect of 5-HT was studied on the slow afterhyperpolarization, involved in spike frequency regulation, and on the segmental synaptic transmission to motoneurons. 5-HT induced a reduction in the slow afterhyperpolarization and a depression of synaptic transmission in all three types of segmental interneurons. Thus 5-HT is a very potent modulator of membrane properties and synaptic transmission of last-order segmental premotor interneurons. Such modulation of locomotor network interneurons can partially account for the observed effects of 5-HT on the swimming pattern in lamprey.     

Pattern generation in caudal-lumbar and sacrococcygeal segments of the neonatal rat spinal cord

Glutamate is the major excitatory amino acid neurotransmitter in the CNS, including the neocortex, hippocampus, and spinal cord. Normal synaptic transmission is mainly mediated by glutamate AMPA and/or kainate receptors. Glutamate N-methyl-D-aspartate (NMDA) receptors are normally inactive and only activated when a sufficient postsynaptic depolarization is induced by the activity. Here we show that in sensory synapses of adult mouse, some synaptic responses (26.3% of a total of 38 experiments) between primary afferent fibers and dorsal horn neurons are almost completely mediated by NMDA receptors. Dorsal root stimulation did not elicit any detectable AMPA/kainate receptor-mediated responses in these synapses. Unlike young spinal cord, serotonin alone did not produce any long-lasting synaptic enhancement in adult spinal dorsal horn neurons. However, co-application of the adenylyl cyclase activator forskolin and serotonin (5-HT) produced long-lasting enhancement, including the recruitment of functional AMPA receptor-mediated responses. Calcium-sensitive, calmodulin-regulated adenylyl cyclases (AC1, AC8) are required for the enhancement. Furthermore the thresholds for generating action potential responses were decreased, and, in many cases, co-application of forskolin and 5-HT led to the generation of action potentials by previously subthreshold stimulation of primary afferent fibers in the presence of the NMDA receptor blocker 2-amino-5-phosphonovaleric acid. Our results suggest that pure NMDA synapses exist on sensory neurons in adult spinal cord and that they may contribute to functional sensory transmission. The synergistic recruitment of functional AMPA responses by 5-HT and forskolin provides a new cellular mechanism for glutamatergic synapses in mammalian spinal cord.     

Serotonin modulates the properties of ascending commissural interneurons in the neonatal mouse spinal cord

The interneuron populations that constitute the central pattern generator (CPG) for locomotion in the mammalian spinal cord are not well understood. We studied the properties of a set of commissural interneurons whose axons cross and ascend in the contralateral cord (aCINs) in the neonatal mouse. During N-methyl-D-aspartate (NMDA) and 5-HT-induced fictive locomotion, a majority of lumbar (L2) aCINs examined were rhythmically active; most of them fired in phase with the ipsilateral motoneuron pool, but some fired in phase with contralateral motoneurons. 5-HT plays a critical role in enabling the locomotor CPG to function. We found that 5-HT increased the excitability of aCINs by depolarizing the membrane potential, reducing the postspike afterhyperpolarization amplitude, broadening the action potential, and decreasing the action potential threshold. Serotonin had no significant effect on the input resistance and sag amplitude of aCINs. These results support the hypothesis that aCINs play important roles in coordinating left-right movements during fictive locomotion and thus may be component neurons in the locomotor CPG in neonatal mice.     

Differences between steady-state and transient post-synaptic potentials elicited by stimulation of the sural nerve

Summary In cat medial gastrocnemius motoneurons, single stimuli to the cutaneous sural nerve evoke a post-synaptic potential with a mixture of depolarization and hyperpolarization, depolarization being dominant in type F cells and hyperpolarization in type S cells. This pattern is consistent with previous reports showing that activation of the sural nerve can sometimes reverse the normal order of motor unit recruitment by inhibiting S motor units while simultaneously exciting F motor units. However, during repetitive stimulation for 1–2 s, we found that the hyperpolarizing component of the sural input to medial gastrocnemius motoneurons was not persistent, but instead gave way to depolarization after the first 30 ms. The net steady-state response after 0.5–1.0 s of stimulation was depolarization in all cells, regardless of motor unit type. This suggests that tonic sural input may be incapable of producing prolonged recruitment reversals.     

A role for 5-hydroxytryptamine in descending inhibition of spinal sexual reflexes

Summary Neurons in the region of the rostral nucleus paragigantocellularis (nPGi) mediate the inhibition of spinal sexual reflexes. Anatomical and pharmacological evidence is presented supporting a role for 5-hydroxytryptamine (5-HT) in this inhibition. Neurons in the rostral nPGi project to the ventral horn in the vicinity of the pudendal motoneurons. A significant number (78% ipsilateral) of these neurons contain 5-HT. Anterograde tracing with Phaseolus leucoagglutinin (PHA-L) confirmed the nPGi projection to pudendal motoneuron and interneuronal areas of the lumbar cord. 5-HT immunoreactive fibers and presumptive terminals surround the pudendal motoneurons. Urethral stimulation, in the anesthetized male rat, elicited penile erections, ejaculation and rhythmic contractions of the perineal muscles, we have used the term coitus reflex to describe this response. Intrathecal injection of 5-HT (4–50 µg) abolished the coitus reflex. Methysergide (1–10 mg/kg i.v.) prevented the 5-HT induced blockade of the coitus reflex. These data support the hypothesis that 5-HT is involved in the descending inhibition of spinal sexual reflexes.     

Excitatory postsynaptic currents in response to different synaptic inputs of frog spinal motoneurons

Excitatory postsynaptic currents (EPSCs) evoked by the primary afferents (dorsal root; DR) and the descending lateral column (LC) fibers were studied in frog spinal motoneurons under voltage clamp with two separate electrodes. The average rise time and half-width of the EPSCs were shorter for LC-EPSCs than for DR-EPSCs, though the values of the parameters for LC- and DR-EPSCs were distributed within a similar range. The relation between the amplitudes of the EPSP and EPSC was almost linear. The amount of current required to generate a 1 mV increment in the EPSP was 5.0 +/- 2.3 nA for the DR-EPSC and 3.8 +/- 1.2 nA for the LC-EPSC. The decay time was shortened by hyperpolarization and prolonged by depolarization in DR- and LC-EPSCs and spontaneous EPSCs. The reversal potential ranged from -30 to -5 mV and was almost identical for DR- and LC-EPSCs and spontaneous EPSCs in individual motoneurons. The current-voltage relation was linear from -100 to +50 mV for these EPSCs. Spontaneous EPSCs became more prominent and frequent during a large hyperpolarization or a large depolarization. These results suggest that the ionic mechanisms underlying EPSC are similar for the functionally different excitatory synapses located on motoneurons.     

Endogenous monoamine receptor activation is essential for enabling persistent sodium currents and repetitive firing in rat spinal motoneurons

The spinal cord and spinal motoneurons are densely innervated by terminals of serotonin (5-HT) and norepinephrine (NE) neurons arising mostly from the brain stem, but also from intrinsic spinal neurons. Even after long-term spinal transection (chronic spinal), significant amounts (10%) of 5-HT and NE (monoamines) remain caudal to the injury. To determine the role of such endogenous monoamines, we blocked their action with monoamine receptor antagonists and measured changes in the sodium currents and firing in motoneurons. We focused on persistent sodium currents (Na PIC) and sodium spike properties because they are critical for enabling repetitive firing in motoneurons and are facilitated by monoamines. Intracellular recordings were made from motoneurons in the sacrocaudal spinal cord of normal and chronic spinal rats (2 mo postsacral transection) with the whole sacrocaudal cord acutely removed and maintained in vitro (cords from normal rats termed acute spinal). Acute and chronic spinal rats had TTX-sensitive Na PICs that were respectively 0.62 +/- 0.76 and 1.60 +/- 1.04 nA, with mean onset voltages of -63.0 +/- 5.6 and -64.1 +/- 5.4 mV, measured with slow voltage ramps. Application of 5-HT2A, 5-HT2C, and alpha1-NE receptor antagonists (ketanserin, RS 102221, and WB 4101, respectively) significantly reduced the Na PICs, and a combined application of these three monoamine antagonists completely eliminated the Na PIC, in both acute and chronic spinal rats. Likewise, reduction of presynaptic transmitter release (including 5-HT and NE) with long-term application of cadmium also eliminated the Na PIC. Associated with the elimination of the Na PIC in monoamine antagonists, the motoneurons lost their ability to fire during slow current ramps. At this point, the spike evoked by antidromic stimulation was not affected, suggesting that activation of the transient sodium current was not impaired. However, the spike evoked after a slow ramp depolarization was slightly reduced in height and rate-of-rise, suggesting decreased sodium channel availability as a result of increased channel inactivation. These results suggest that endogenous monoamine receptor activation is critical for enabling the Na PIC and decreasing sodium channel inactivation, ultimately enabling steady repetitive firing in both normal and chronic spinal rats.     

刺激三叉上核引起三叉神经中脑核神经元的突触反应(英文)

目的:旨在研究三叉上核(Su5)对三叉神经中脑核(Me5)神经元的活动是否发挥着重要的调节作用,从而参与对颌运动的调节。方法:本研究通过全细胞电流钳技术,刺激生后30～43d大鼠脑片上三叉上核并记录Me5神经元反应。结果:Me5神经元静息膜电位为(-53.5±0.5)mV;所有Me5神经元在超极化和去极化时分别显示为内向、外向整流;同时去极化引起神经元放电。刺激三叉上核引起4种类型的Me5神经元的反应,即逆向动作电位、GABAA、AMPA/kainate和NMDA等受体介导的反应,这些反应各占32%、36%、20%和12%。钳制电位在-60mV左右时,诱发的GABA能突触后电位为(1.08±0.45)mV,膜电位水平时;刺激引起的AMPA/Kainate受体介导的电流大小为(0.98±0.51)mV;钳制电位在-45mV左右时,NMDA受体介导的谷氨酸电流为(2.40±0.75)mV。结论:三叉上核神经元可通过突触由GABA和谷氨酸信号系统调节Me5神经元活动。