Mechanoafferent neurons innervating tail of Aplysia. II. Modulation by sensitizing stimulation

The tail-withdrawal reflex of Aplysia can be sensitized by weak stimulation of a site outside the site used to test the reflex or by repeatedly stimulating the test site itself. The sensitization of tail-withdrawal responses is associated with enhanced activation of the tail motor neurons and heterosynaptic facilitation of the monosynaptic connections between the tail sensory neurons and tail motor neurons. This synaptic facilitation can occur under conditions in which neither posttetanic potentiation nor generalized changes in postsynaptic input resistance contribute to the facilitation. In addition to producing monosynaptic excitatory postsynaptic potentials (EPSPs), action potentials in tail sensory neurons often recruit longer latency polysynaptic input to the tail motor neurons during sensitization. Strong, noxious tail shock similar in intensity to that used previously for sensitization and aversive classical conditioning of other responses in Aplysia produces more heterosynaptic facilitation than does weak sensitizing stimulation. Heterosynaptic facilitation builds up progressively with multiple trials and lasts for hours. Very strong shocks to the tail can change the response characteristics of tail sensory neurons so that a prolonged, regenerative burst of spikes is elicited by a brief intracellular depolarizing pulse. This bursting response produced by sensitizing stimulation has not been described previously in Aplysia sensory neurons and can greatly amplify the synaptic input to tail motor neurons from the sensory neurons. In addition, strong shocks to the tail increase the duration and magnitude of individual sensory neuron action potentials. Sensitizing tail stimulation usually produces long-lasting depolarization of the tail motor neurons and often long-lasting hyperpolarization of the tail sensory neurons. The tail motor and sensory neurons show both increases and decreases of input resistance following sensitizing stimulation. However, the small, occasional increases in input resistance of the motor neuron are insufficient to explain the heterosynaptic facilitation produced by sensitizing stimulation. Serotonin (5-HT) application can mimic many of the effects of sensitizing tail shock, including facilitation of both tail withdrawal and the monosynaptic connections between tail sensory and motor neurons, hyperpolarizing and depolarizing responses in the tail sensory neurons, and an increase in the duration and magnitude of the sensory neuron action potential. In the nearly isolated sensory neuron soma, 5-HT usually produces a slow, decreased conductance depolarizing response, suggesting that the 5-HT-induced hyperpolarizing response see     

Serotonin acts in the synaptic region of sensory neurons in Aplysia to enhance transmitter release

An important mechanism that contributes to sensitization in Aplysia is heterosynaptic facilitation of the synaptic connections between sensory neurons (SNs) and motor neurons (MNs). Heterosynaptic facilitation, in turn, is associated with broadening of the spike in the SN. Spike broadening is readily observed in recordings from somata of SNs, and from growth cones of SNs in culture, but broadening in synaptic terminals has only been inferred. Intracellular recordings were made from somata of SNs and from somata of follower MNs. Additional recordings were made from the axons of SNs as they enter the neuropil in the pedal ganglion. Serotonin (5-HT) broadened action potentials in axons of SNs and enhanced excitatory postsynaptic potentials (EPSPs) in the MNs, even after the axons of SNs were surgically separated from their somata. These results indicate that both heterosynaptic facilitation and spike broadening in the axon are due to the local action of 5-HT and can occur independently of modulation of membrane properties in the soma.     

Forskolin mimics and blocks a serotonin-sensitive decreased K+ conductance in tail sensory neurons of Aplysia

Facilitation of synaptic connections between sensory neurons and motor neurons mediating the tail-withdrawal reflex in Aplysia is produced by sensitizing stimuli. These effects can be mimicked by perfusing the pleural and pedal ganglia of a semi-intact preparation with serotonin (5-HT). In addition to the synaptic facilitation, 5-HT produces a depolarization associated with an increase in input resistance in the sensory neurons. The 5-HT-induced changes appear to be mediated by an elevation in cAMP levels. To examine further the role of cAMP in mediating the 5-HT response we utilized the potent cyclase activator forskolin. Forskolin mimics the 5-HT response and blocks the response to subsequent 5-HT applications indicating that both 5-HT and forskolin act through a common saturable mechanism. Voltage clamp and ion substitution experiments indicate that the 5-HT response is due, at least in part, to a decrease in a resting K+ conductance.     

Inhibitory responses in Aplysia pleural sensory neurons act to block excitability, transmitter release, and PKC Apl II activation

Expression of the 5-HT(1Apl(a)) receptor in Aplysia pleural sensory neurons inhibited 5-HT-mediated translocation of the novel PKC Apl II in sensory neurons and prevented PKC-dependent synaptic facilitation at sensory to motoneuron synapses (Nagakura et al. 2010). We now demonstrate that the ability of inhibitory receptors to block PKC activation is a general feature of inhibitory receptors and is found after expression of the 5-HT(1Apl(b)) receptor and with activation of endogenous dopamine and FMRFamide receptors in sensory neurons. Pleural sensory neurons are heterogeneous for their inhibitory response to endogenous transmitters, with dopamine being the most prevalent, followed by FMRFamide, and only a small number of neurons with inhibitory responses to 5-HT. The inhibitory response is dominant, reduces membrane excitability and synaptic efficacy, and can reverse 5-HT facilitation at both naive and depressed synapses. Indeed, dopamine can reverse PKC translocation during the continued application of 5-HT. Reversal of translocation can also be seen after translocation mediated by an analog of diacylglycerol, suggesting inhibition is not through blockade of diacylglycerol production. The effects of inhibition on PKC translocation can be rescued by phosphatidic acid, consistent with the inhibitory response involving a reduction or block of production of this lipid. However, phosphatidic acid could not recover PKC-dependent synaptic facilitation due to an additional inhibitory effect on the non-L-type calcium flux linked to synaptic transmission. In summary, we find a novel mechanism downstream of inhibitory receptors linked to inhibition of PKC activation in Aplysia sensory neurons.     

Layer-specific serotonergic facilitation of IPSC in layer 2/3 pyramidal neurons of the visual cortex

Serotonin (5-hydroxytryptamine, 5-HT) inhibits the induction of long-term synaptic plasticity in layer 2/3 of the visual cortex at the end of its critical period in rats. However, the cellular and molecular mechanisms remain unclear. Since inhibitory influence is crucial in the induction of synaptic plasticity, the effect of 5-HT on inhibitory transmission was investigated in layer 2/3 pyramidal neurons of the primary visual cortex. The amplitude of inhibitory postsynaptic current (IPSC), but not excitatory postsynaptic current, evoked by stimulation of the underlying layer 4, was increased by ～20% with a bath application of 5-HT. The amplitude of miniature IPSC was also increased by the application of 5-HT, while the paired-pulse ratio was not changed. The facilitating effect of 5-HT on IPSC was mediated by the activation of 5-HT(2) receptors. An increase in intracellular Ca(2+) via release from inositol 1,4,5-trisphosphate (IP(3))-sensitive stores, which was confirmed by confocal Ca(2+) imaging, and activation of Ca(2+)/calmodulin-dependent kinase II (CaMKII) were involved in the facilitation of IPSC by 5-HT. However, 5-HT failed to facilitate IPSC evoked by the stimulation of layer 1. These results suggest that activation of 5-HT(2) receptors releases intracellular Ca(2+) via IP(3)-sensitive stores, which facilitates GABA(A)ergic transmission via the activation of CaMKII in layer 2/3 pyramidal neurons of the visual cortex in a layer-specific manner. Thus facilitation of inhibitory transmission by 5-HT might be involved in regulating the information flow and the induction of long-term synaptic plasticity, in a pathway-specific manner.     

去除感觉神经元胞体促进离体培养的海兔感觉神经突触形成

目的 研究去除离体培养的海兔感觉神经元（SN）与运动神经元（MN,L7）互相接触形成的突触终末数量的变化。方法 体外培养无脊椎海生动物海兔（Aplysia）SN和MN,L7,两者互相接触形成突触联系fSN／L7）,培养到第4d时,将部分感觉神经细胞的体部去除（SN／L7,-CB）,其中一部分应用5-HT处理,另一部分应用5-HT＋茴香霉素（Anisomysin）处理,将仍保留细胞体部（SN／L7,＋CB）的离体培养突触作为对照组,24h后。利用免疫组织化学方法观察突触终末数目的变化。结果与对照组相比,应用5-HT后24h,5-HT处理组突触终末的数目显著增加（P〈0．01）;而5-HT＋茴香霉素组的突触终末数目却没有明显改变。结论 在长时程易化条件下,去除感觉神经细胞体可能使突触终末的数量增加,而这种感觉神经突触可塑性的变化可以被蛋白质合成抑制剂茴香霉素所抵消。     

Serotonergic control of cerebellar mossy fiber activity by modulation of signal transfer by rat pontine nuclei neurons

Serotonergic modulation of precerebellar nuclei may be crucial for the function of the entire cerebellar system. To study the effects of serotonin (5-HT) on neurons located within the pontine nuclei (PN), the main source of cerebellar mossy fibers, we performed standard intracellular recordings from PN neurons in a slice preparation of the rat pontine brain stem. Application of 5 microM 5-HT significantly altered several intrinsic membrane properties of PN neurons. First, it depolarized the somatic membrane potential by 6.5 +/- 3.5 mV and increased the apparent input resistance from 49.5 +/- 14.6 to 62.7 +/- 21.1 MOmega. Second, 5-HT altered the I-V relationship of PN neurons: it decreased the inward rectification in hyperpolarizing direction, but increased it when depolarizing currents were applied. Third, it decreased the rheobase from 0.32 +/- 0.14 to 0.24 +/- 0.14 nA without affecting the firing threshold. Finally, the amplitude of medium-duration after hyperpolarizations was reduced from -14.9 +/- 2.0 to -12.3 +/- 2.4 mV. Together, these 5-HT effects on the intrinsic membrane properties result in an increase in excitability and instantaneous firing rate. In addition, application of 5 microM 5-HT also modulated postsynaptic potentials (PSPs) evoked by electric stimulations within the cerebral peduncle. The amplitude, maximal slope, and integral of these PSPs were reduced to 46.2 +/- 23.4%, 45.7 +/- 23.7%, and 61.4 +/- 28.4% of the control value, respectively. In contrast, we found no change in the decay and voltage dependence of PSPs. To test modulatory effects on short-term synaptic facilitation, we applied pairs of electrical stimuli at intervals between 10 and 1,000 ms. 5-HT selectively enhanced the paired-pulse facilitation for interstimulus-intervals >20 ms. The alteration of paired-pulse facilitation points to a presynaptic site of action for 5-HT effects on synaptic transmission. Pharmacological experiments suggested that pre- and postsynaptic effects of 5-HT were mediated by two different kinds of 5-HT receptors: changes in intrinsic membrane properties were blocked by the 5-HT(2) receptor antagonist cinanserin while the reduction of PSPs was prevented by the 5-HT(1) receptor antagonist cyanopindolol. In conclusion, 5-HT increases the excitability of PN neurons but decreases the synaptic transmission on them. The selective enhancement of synaptic facilitation may, however, allow high-frequency inputs to effectively drive PN neurons, thus the PN may act as a high-pass filter during periods of 5-HT release.     

Neurotransmitters producing and modulating opaline gland contraction in Aplysia californica

1. Opaline release in Aplysia provides a simple model system for examining the biochemical and electrophysiological mechanisms underlying glandular secretion and its modulation. The opaline gland is a large multivesicular structure, which is innervated by at least three large identified motor neurons located within the right pleural ganglion (28). In this paper we have investigated the roles of dopamine (DA), acetylcholine (ACh), and serotonin (5-HT) in this simple neural system. 2. DA infusion produces a gland contraction that is similar to the response produced by neural activity in the previously identified opaline motor neurons. 3. The gland response to DA infusion is not affected by blocking synaptic transmission in the gland, suggesting that DA acts directly on muscle cells surrounding the gland and not through interposed interneurons. 4. In addition to its effect of producing contractions of the gland, DA enhances the size of subsequent neurally evoked gland contractions and increases the size of excitatory junctional potentials (EJPs) recorded from the opaline gland. Thus DA may have an additional modulatory role in opaline release. 5. DA antagonists such as fluphenazine and haloperidol inhibit the gland response to DA and also block the gland contraction produced by neural activity in the identified motor neurons. In addition, the DA antagonists reversibly block the EJPs recorded from the gland cells. Compounds known to block the effects of ACh or 5-HT have no effect on the dopamine-induced gland contractions, the contractions produced by firing the motor neurons, or the EJPs evoked by motor neuron stimulation. These results suggest that DA may be the neurotransmitter used by the identified opaline motor neurons. 6. ACh produces a decrease in pressure recorded from the lumen of the opaline gland that can be blocked by hexamethonium. 7. While 5-HT does not directly produce a contraction, treatment of the gland with the transmitter increases the size of subsequent gland responses produced either by DA infusion or activity in the opaline motor neurons. This enhancement has a relatively slow onset and long duration and persists for more than 15 min after the serotonin is washed out. In 60% of the experiments 5-HT also increased the size of the EJPs recorded from the opaline gland. The results suggest a modulatory role for serotonin in opaline secretion similar to the one it plays in other neural systems in Aplysia.     

Involvement of protein kinase C in serotonin-induced spike broadening and synaptic facilitation in sensorimotor connections of Aplysia.

1. Plasticity at the connections between sensory neurons and their follower cells in Aplysia has been used extensively as a model system to examine mechanisms of simple forms of learning. Earlier studies have concluded that serotonin (5-HT) is a key modulatory transmitter and that it exerts its short-term actions via cAMP-dependent activation of protein kinase A. Subsequently, it has become clear that other kinase systems such as protein kinase C (PKC) also may be involved in the actions of 5-HT. 2. Application of phorbol esters, which activate PKC, produced a slowly developing spike broadening but had little effect on excitability (a process known to be primarily cAMP dependent). Moreover, the effects of phorbol esters and 5-HT on spike duration were not additive, suggesting that they may share some common mechanisms. 3. The protein kinase inhibitor staurosporine suppressed both 5-HT-induced slowly developing spike broadening and, under certain conditions, facilitation of transmitter release. Staurosporine did not inhibit 5-HT-induced enhancement of excitability. The effectiveness of staurosporine on spike broadening was dependent on the time at which spike broadening was examined after application of 5-HT. Staurosporine appeared to have little effect on spike broadening 3 min after application of 5-HT, whereas it inhibited significantly 5-HT-induced spike broadening at later times. The staurosporine-insensitive component of 5-HT-induced spike broadening may be mediated by cAMP. 4. The results suggest that the activation of PKC plays a key role in components of both 5-HT-induced spike broadening and facilitation of synaptic transmission.(ABSTRACT TRUNCATED AT 250 WORDS)     

5-HT_2受体-磷脂酶C的激活易化大鼠基底外侧杏仁核突触可塑性的研究

为了探讨5-HT2受体激动剂盐酸2,5-二甲氧基-4-碘苯基丙烷(DOI)对杏仁核突触可塑性的调节作用,本研究在杏仁核脑片上记录基底外侧杏仁核(BLA)场电位,应用单串的θ频率波刺激(TBS)诱导突触可塑性,观察DOI对TBS诱导的突触可塑性的影响,及5-HT2受体拮抗剂、磷脂酶C抑制剂能否抑制DOI的作用。结果显示:单串的TBS刺激外囊,在BLA仅诱导约为10min的短时程增强。灌流液中加入100μmol/L DOI 20min,对基础的场电位没有作用。但在DOI存在的情况下,单串的TBS即可诱导长时程增强,强直刺激30min后,增强的场电位斜率仍维持在基础值的(162.5±9.7)%(n=9,P<0.01)。DOI对TBS诱导的突触可塑性的易化作用可被5-HT2A/2C受体拮抗剂ketanserin和PLC抑制剂U73122所抑制。以上结果提示5-HT2A/2C受体的激活可通过磷脂酶C通路易化杏仁核的突触可塑性。     

Serotonergic modulation in aplysia. II. Cellular and behavioral consequences of increased serotonergic tone

Serotonin (5-HT) plays an important role in sensitization of defensive reflexes in Aplysia and is also involved in several aspects of arousal, such as the control of locomotion and of cardiovascular tone. In the preceding paper, we showed that tail-nerve shock, a noxious stimulus that readily induces sensitization, increases the firing rate of a large number of serotonergic neurons throughout the CNS. However, the functional consequences of such an increase in serotonergic tone are still poorly understood. In this study, we examined this question by using the 5-HT precursor 5-hydroxytryptophan (5-HTP) to specifically increase 5-HT release in the CNS. Increased tonic 5-HT release after 5-HTP treatment was manifested by facilitation of sensorimotor (SN-MN) synapses, increased firing rate of serotonergic neurons in the pedal and abdominal ganglia, and enhanced 5-HT release evoked by tail-nerve shock. When 5-HTP was administered to freely moving animals, it produced a strong arousal response characterized by increased locomotion and heart rate, which was reminiscent of the defensive arousal reaction triggered by noxious stimulation such as tail-shock. In contrast, 5-HTP actually inhibited the tail-induced siphon-withdrawal reflex. It is possible that 5-HT-induced facilitation of SN-MN synapses was counteracted by inhibition of polysynaptic reflex pathways between SNs and MNs, resulting in transient behavioral inhibition of the reflex, which could favor escape locomotion and/or respiration shortly after an aversive stimulus. We conclude that a major function associated with the activation of the Aplysia serotonergic system evoked by noxious stimuli is the triggering of a defensive arousal response. It is known that tail-shock-induced serotonergic activation contributes to memory encoding at least in part by facilitating SN-MN synapses. However, this effect in isolation might not be sufficient for the behavioral expression of sensitization.     

Evolution of Learning in Three Aplysiid Species: Differences in Heterosynaptic Plasticity Contrast with Conservation in Serotonergic Pathways

We investigated the neurobiological basis of variation in sensitization between three aplysiid species: Aplysia californica , Phyllaplysia taylori and Dolabrifera dolabrifera . We tested two different forms of sensitization induced by a noxious tail shock: local sensitization, expressed near the site of shock, and general sensitization, tested at remote sites. Aplysia showed both local and general sensitization, whereas Phyllaplysia demonstrated only local sensitization, and Dolabrifera lacked both forms of learning. We then investigated a neurobiological correlate of sensitization, heterosynaptic modulation of sensory neuron excitability by tail-nerve stimulation. We found (1) an increase in sensory neuron (SN) excitability after both ipsilateral and contralateral nerve stimulation in Aplysia , (2) a smaller and shorter-lasting increase in Phyllaplysia , and (3) no effect in Dolabrifera . Because sensitization in Aplysia is strongly correlated with serotonergic (5-HT) neuromodulation, we hypothesized that the observed interspecific variation in sensitization and SN neuromodulation might be correlated with variation in the anatomy and/or functional response of the serotonergic system. However, using immunohistochemistry, we found that all three species showed a similar pattern of 5-HT innervation. Furthermore, they also showed comparable 5-HT release evoked by tail-nerve shock, as measured with chronoamperometry. These observations indicate that interspecific variation in learning is correlated with differences in SN heterosynaptic plasticity within a backgound of evolutionary conservation in the 5-HT neuromodulatory pathway. We thus hypothesize that evolutionary changes in learning phenotype do not involve modifications of the 5-HT pathway per se , but rather, changes in the response of SNs to the activation of this or other neuromodulatory pathways upon noxious stimulation.     

Limited contributions of serotonin to long-term hyperexcitability of Aplysia sensory neurons

Serotonin (5-HT) has provided a useful tool to study plasticity of nociceptive sensory neurons in Aplysia. Because noxious stimulation causes release of 5-HT and long-term hyperexcitability (LTH) of sensory neuron somata and because 5-HT treatment can induce long-term synaptic facilitation of sensory neuron synapses, a plausible hypothesis is that 5-HT also induces LTH of the sensory neuron soma. Prolonged or repeated exposure of excised ganglia to 5-HT produced immediate hyperexcitability of sensory neurons that showed little desensitization, but the hyperexcitability decayed within minutes of washing out the 5-HT. Prolonged or repeated treatment of either excised ganglia or dissociated sensory neurons with various concentrations of 5-HT failed to induce significant LTH even when long-term synaptic facilitation was induced in the same preparations. Use of a high-divalent cation solution to reduce interneuron activity during 5-HT treatment failed to enable the induction of LTH in excised ganglia. Pairing 5-HT application with nerve shock failed to enhance LTH produced by nerve shock or to reveal covert LTH produced by 5-HT. The induction of LTH by nerve stimulation was enhanced rather than inhibited by treatment with methiothepin, a 5-HT antagonist reported to block various 5-HT receptors and 5-HT-induced adenylyl cyclase activation. This suggests that endogenous 5-HT may have inhibitory effects on the induction of LTH by noxious stimulation. Methiothepin blocked immediate hyperexcitability produced by exogenous 5-HT and also inhibited the expression of LTH induced by nerve stimulation when applied during testing 1 day afterward. At higher concentrations, methiothepin reduced basal excitability of sensory neurons by mechanisms that may be independent of its antagonism of 5-HT receptors. Several observations suggest that early release of 5-HT and consequent cAMP synthesis in sensory neurons is not important for the induction of LTH by noxious stimulation, whereas later release of 5-HT from persistently activated modulatory neurons, with consequent elevation of cAMP synthesis, may contribute to the maintenance of LTH.     

Serotonin receptor antagonists discriminate between PKA- and PKC-mediated plasticity in aplysia sensory neurons

Highly selective serotonin (5-hydroxytryptamine, 5-HT) receptor antagonists developed for mammals are ineffective in Aplysia due to the evolutionary divergence of neurotransmitter receptors and because the higher ionic strength of physiological saline for marine invertebrates reduces antagonist affinity. It has therefore been difficult to identify antagonists that specifically block individual signaling cascades initiated by 5-HT. We studied two broad-spectrum 5-HT receptor antagonists that have been characterized biochemically in Aplysia CNS: methiothepin and spiperone. Methiothepin is highly effective in inhibiting adenylyl cyclase (AC)-coupled 5-HT receptors in Aplysia. Spiperone, which blocks phospholipase C (PLC)-coupled 5-HT receptors in mammals, does not block AC-coupled 5-HT receptors in Aplysia. In electrophysiological studies, we explored whether methiothepin and spiperone can be used in parallel to distinguish between the AC-cAMP and PLC-protein kinase C (PKC) modulatory cascades that are initiated by 5-HT. 5-HT-induced broadening of the sensory neuron action potential in the presence of tetraethylammonium/nifedipine, which is mediated by modulation of the S-K+ currents, was used an assay for the AC-cAMP cascade. Spike broadening initiated by 5 microM 5-HT was unaffected by 100 microM spiperone, whereas it was effectively blocked by 100 microM methiothepin. Facilitation of highly depressed sensory neuron-to-motor neuron synapses by 5-HT was used as an assay for the PLC-PKC cascade. Spiperone completely blocked facilitation of highly depressed synapses by 5 microM 5-HT. In contrast, methiothepin produced a modest, nonsignificant, reduction in the facilitation of depressed synapses. Interestingly, these experiments revealed that the PLC-PKC cascade undergoes desensitization during exposure to 5-HT.     

Activation of serotonin receptors modulates synaptic transmission in rat cerebral cortex

The cerebral cortex receives an extensive serotonergic (5-hydroxytryptamine, 5-HT) input. Immunohistochemical studies suggest that inhibitory neurons are the main target of 5-HT innervation. In vivo extracellular recordings have shown that 5-HT generally inhibited cortical pyramidal neurons, whereas in vitro studies have shown an excitatory action. To determine the cellular mechanisms underlying the diverse actions of 5-HT in the cortex, we examined its effects on cortical inhibitory interneurons and pyramidal neurons. We found that 5-HT, through activation of 5-HT(2A) receptors, induced a massive enhancement of spontaneous inhibitory postsynaptic currents (sIPSCs) in pyramidal neurons, lasting for approximately 6 min. In interneurons, this 5-HT-induced enhancement of sIPSCs was much weaker. Activation of 5-HT(2A) receptors also increased spontaneous excitatory postsynaptic currents (sEPSCs) in pyramidal neurons. This response desensitized less and at a slower rate. In contrast, 5-HT slightly decreased evoked IPSCs (eIPSCs) and eEPSCs. In addition, 5-HT via 5-HT(3) receptors evoked a large and rapidly desensitizing inward current in a subset of interneurons and induced a transient enhancement of sIPSCs. Our results suggest that 5-HT has widespread effects on both interneurons and pyramidal neurons and that a short pulse of 5-HT is likely to induce inhibition whereas the prolonged presence of 5-HT may result in excitation.     

Swimming in Aplysia brasiliana: behavioral and cellular effects of serotonin

1. Aplysia brasiliana is a marine mollusk that swims by repeated metachronal flapping movements of its bilateral fleshy parapodia. Animals with bilateral cerebropedal connective (CPC) lesions do not swim when suspended above the substrate, although tonic CPC stimulation can elicit normal parapodial flapping. Although the parapodial opener-phase (POP) cells, a previously identified group of neurons, fire synchronous bursts of efferent spikes in-phase with parapodial opening movements in both intact animals and dissected preparations, they are not likely to be primary parapodial motoneurons. These cells receive one or more large, apparently monosynaptic excitatory postsynaptic potentials (EPSPs) during CPC stimulation that are effective in producing the swimming motor program (SMP). 2. In suspended CPC-lesioned animals, injections of serotonin (5-HT) that produce an average hemolymph concentration of 10(-5) M induced full-amplitude parapodial flapping. Selected episodes of flapping were similar in frequency to normal suspended swimming. 3. In suspended CPC-lesioned animals, 5-HT injections elicited an apparently normal swimming motor program that was associated with synchronous bursts of large-amplitude efferent spikes in the parapodial nerves. In many semi-intact preparations, exposing the circumoesophageal ganglia to 5-HT elicited a similar rhythmic motor program, but usually at a lower frequency than during normal swimming or during tonic CPC stimulation. 4. In isolated-ganglion preparations, bath application of 5-HT produced immediate depolarization and tonic firing of individual POP neurons, followed by smooth and regular bursting in the apparent absence of synaptic input. In such preparations, the motor program elicited by bath-applied 5-HT differed from the one elicited by tonic CPC stimulation in that the 5-HT-elicited rhythmic bursting usually was not synchronous in different POP neurons. Tonic CPC stimulation during bath applications of 5-HT produced immediate synchronization of bursts among the POP neurons. 5. Hyperpolarization (or depolarization) of a POP neuron during bath application of 5-HT increased (or decreased) the burst period, but membrane polarization did not change the burst period elicited during tonic CPC stimulation.     

Conditioned changes of synaptic transmission in the motor cortex of the cat

Intracellular recordings were made from 117 neurons in the motor cortex of anesthetized cats. The pyramidal tract (PT) and VL nucleus of thalamus were stimulated in order to activate the neurons from two directions. 1. PT cells were conditioned by antidromic trains (10--50 cps for 4--15 s) and by paired PT and VL stimuli with different intervals and sequences. The VL-EPSPs were examined before and after conditioning, to find differences in efficacy in giving rise to spikes. The conditioning procedures resulted in a remarkable facilitation of VL-EPSPs, manifesting itself as a significant rise of efficacy in generating spikes, a shortening of peak latency and in some cases, an enhancement of background firing. 2. In non-PT neurons the same conditioning procedures elicited heterosynaptic facilitation and a rise in firing activity. 3. Intracellularly injected square wave pulses also resulted in facilitation of VL-EPSPs. 4. Pairings of PT and VL stimuli were more effective than trains in evoking conditioned changes. 5. Plastic modifications were observed in the 13.7% of the neurons subjected to conditioning procedures. 6. The authors assume that synchronous activity of the pre- and postsynaptic neurons is a highly important condition for plastic changes in the efficacy of synaptic transmission.     

Analysis of 5-HT-induced short-term facilitation at Aplysia sensorimotor synapse during bursts: increased synaptic gain that does not require ERK activation

The 5-HT-induced synaptic plasticity of Aplysia sensorimotor synapses has typically been probed by firing a single presynaptic spike. In this study, 5-HT-induced synaptic plasticity was probed with brief bursts of spikes (10 Hz, 1 s), which are more behaviorally relevant stimuli. Because such bursts provide a greater challenge to the release machinery than single spikes, their use may reveal additional aspects of synaptic modulation, and, in particular, the role of extracellular signal-regulated protein kinase (ERK), which has recently been implicated in several examples of short- and long-term synaptic plasticity. Excitatory postsynaptic currents (EPSCs) were characterized by their amplitudes. In addition, two kinetic measurements, time to peak and decay time constant, were determined for the initial and last EPSCs of each burst. Application of 5-HT produced a uniform increase in gain by facilitating each EPSC elicited during a burst of spikes without affecting the kinetics of the initial or last EPSC. These data suggest that short-term facilitation during a burst is mediated largely by processes such as those that affect the size of the releasable pool or rate of vesicle mobilization rather than by an increase in the duration of the presynaptic action potential. An ERK cascade inhibitor (U0126) had no effect on the 5-HT-mediated facilitation of either the initial EPSC or EPSCs elicited late in the burst.     

5-HT inhibits synaptic transmission in rat subthalamic nucleus neurons in vitro

The subthalamic nucleus (STN) plays a pivotal role in normal and abnormal motor function. We used patch pipettes to study effects of 5-HT on synaptic currents evoked in STN neurons by focal electrical stimulation of rat brain slices. 5-HT (10 microM) reduced glutamate-mediated excitatory postsynaptic currents (EPSCs) by 35+/-4%. However, a much higher concentration of 5-HT (100 microM) was required to inhibit GABA-mediated inhibitory postsynaptic currents (IPSCs) to a comparable extent. Concentration-response curves showed that the 5-HT inhibitory concentration 50% (IC50) for inhibition of IPSCs (20.2 microM) was more than fivefold greater than the IC50 for inhibition of EPSCs (3.4 microM). The 5-HT-induced reductions in EPSCs and IPSCs were accompanied by increases in paired-pulse ratios, indicating that 5-HT acts presynaptically to inhibit synaptic transmission. The 5-HT1B receptor antagonist NAS-181 significantly antagonized 5-HT-induced inhibitions of EPSCs and IPSCs. These studies show that 5-HT inhibits synaptic transmission in the STN by activating presynaptic 5-HT1B receptors.     

The Effects of Antibodies Against Proteins of the s100 Group on Neuron Plasticity in Sensitized and Non-Sensitized Snails

The effects of antibodies to a total fraction of s100 proteins and protein s100b on the activity of defensive behavior command neurons LPl1 and RPl1 were studied in common snails, using non-sensitized animals and animals which had acquired nociceptive sensitization. In non-sensitized snails, application of antibodies against s100 or s100b (0.1 mg/ml) induced membrane depolarization, increased membrane permeability, and suppressed slow excitatory postsynaptic potentials in the responses of neurons to sensory stimulation. Acquisition of sensitization in snails in the presence of antibodies to s100 or s100b (0.1 mg/ml) led to significantly less marked facilitation of synaptic transmission and smaller increases in neuron membrane excitability than in cells of control sensitized animals. The difference in synaptic facilitation in the neurons of control sensitized snails and neurons in sensitized snails given antibody was comparable with the magnitude of synaptic depression due to antibody in non-sensitized animals. At a dose of 0.01 mg/ml, antibody had no effect on these measures of neuron activity. It is suggested that s100 proteins, particularly s100b, are involved in the mechanisms regulating excitability, the membrane potential, and synaptic transmission in command neurons in untrained snails, as well as in the mechanism of plasticity of the electrogenic membranes of nerve cells during the acquisition of nociceptive sensitization.