The common neurotransmitter serotonin controls different aspects of early neuronal differentiation, although the underlying mechanisms are poorly understood. Here we report that activation of the serotonin 5-HT 7 receptor promotes synaptogenesis and enhances synaptic activity in hippocampal neurons at early postnatal stages. An analysis of G␣ 12 -deficient mice reveals a critical role of G 12 -protein for 5-HT 7 receptor-mediated effects in neurons. In organotypic preparations from the hippocampus of juvenile mice, stimulation of 5-HT 7 R/G 12 signaling potentiates formation of dendritic spines, increases neuronal excitability, and modulates synaptic plasticity. In contrast, in older neuronal preparations, morphogenetic and synaptogenic effects of 5-HT 7 /G 12 signaling are abolished. Moreover, inhibition of 5-HT 7 receptor had no effect on synaptic plasticity in hippocampus of adult animals. Expression analysis reveals that the production of 5-HT 7 and G␣ 12 -proteins in the hippocampus undergoes strong regulation with a pronounced transient increase during early postnatal stages. Thus, regulated expression of 5-HT 7 receptor and G␣ 12 -protein may represent a molecular mechanism by which serotonin specifically modulates formation of initial neuronal networks during early postnatal development.

Interjoint reflex function of the insect leg contributes to postural control at rest or to movement control during locomotor movements. In the stick insect (Carausius morosus), we investigated the role that sensory signals from the femoral chordotonal organ (fCO), the transducer of the femur-tibia (FT) joint, play in patterning motoneuronal activity in the adjacent coxa-trochanteral (CT) joint when the joint control networks are in the movement control mode of the active behavioral state. In the active behavioral state, sensory signals from the fCO induced transitions of activity between antagonistic motoneuron pools, i.e., the levator trochanteris and the depressor trochanteris motoneurons. As such, elongation of the fCO, signaling flexion of the FT joint, terminated depressor motoneuron activity and initiated activity in levator motoneurons. Relaxation of the fCO, signaling extension of the FT joint, induced the opposite transition by initiating depressor motoneuron activity and terminating levator motoneuron activity. This interjoint influence of sensory signals from the fCO was independent of the generation of the intrajoint reflex reversal in the FT joint, i.e., the "active reaction," which is released by elongation signals from the fCO. The generation of these transitions in activity of trochanteral motoneurons barely depended on position or velocity signals from the fCO. This contrasts with the situation in the resting behavioral state when interjoint reflex action markedly depends on actual fCO stimulus parameters, i.e., position and velocity signals. In the active behavioral state, movement signals from the fCO obviously trigger or release centrally generated transitions in motoneuron activity, e.g., by affecting central rhythm generating networks driving trochanteral motoneuron pools. This conclusion was tested by stimulating the fCO in "fictive rhythmic" preparations, activated by the muscarinic agonist pilocarpine in the otherwise isolated and deafferented mesothoracic ganglion. In this situation, sensory signals from the fCO did in fact reset and entrain rhythmic activity in trochanteral motoneurons. The results indicate for the first time that when the stick insect locomotor system is active, sensory signals from the proprioceptor of one leg joint, i.e., the fCO, pattern motor activity in an adjacent leg joint, i.e., the CT joint, by affecting the central rhythm generating network driving the motoneurons of the adjacent joint.

Transient A-type K ؉ channels (IA) in neurons have been implicated in the delay of the spike onset and the decrease in the firing frequency. Here we have characterized biophysically and pharmacologically an IA current in lamprey locomotor network neurons that is activated by suprathreshold depolarization and is specifically blocked by catechol at 100 M. The biophysical properties of this current are similar to the mammalian Kv3.4 channel. The role of the IA current both in single neuron firing and in locomotor pattern generation was analyzed. The IA current facilitates Na ؉ channel recovery from inactivation and thus sustains repetitive firing. The role of the IA current in motor pattern generation was examined by applying catechol during fictive locomotion induced by N-methyl-D-aspartate. Blockade of this current increased the locomotor burst frequency and decreased the firing of motoneurons. Although an alternating motor pattern could still be generated, the cycle duration was less regular, with ventral roots bursts failing on some cycles. Our results thus provide insights into the contribution of a high-voltage-activated I A current to the regulation of firing properties and motor coordination in the lamprey spinal cord.C oordinated motor patterns are generated by neural circuits, the activity of which depends on the intrinsic properties of single neurons and their synaptic interactions. Although a few neural circuits underlying rhythmic motor patterns have been identified and characterized in some detail (1-4), our understanding of their function remains incomplete. A fundamental understanding of how neural circuits generate and control behavior requires knowledge about the role of different subclasses of ion channels in the function of network neurons and thereby in the regulation of information processing, synaptic interactions, and the operation of the network as a whole. The importance of individual classes of ion channel in regulating the activity of neural networks has been studied in a few vertebrate (5-7) and invertebrate (8-10) preparations.The present study identifies a high-voltage-activated A-type K ϩ current in the lamprey spinal cord and investigates its importance in controlling the firing properties of single neurons and thereby in the generation of coordinated locomotor patterns. Transient A-type K ϩ channels can act as a control mechanism for neuronal excitability (11,12). Different A-type K ϩ channels have been cloned and characterized biophysically in single-cell expression systems (13). They can be subdivided into low-and high-voltage-activated channels, depending on their voltage activation range. Low-voltage-activated currents generally activate at potentials below the spike threshold, whereas high-voltage-activated currents are only activated by membrane potential depolarizations above the spike threshold. Lowvoltage-activated currents delay the onset of action potentials, decrease the firing frequency of neurons (11,12,14), influence dendritic integration and propagation of informati...

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Potassium channels play an important role in controlling neuronal firing and synaptic interactions. Na+-activated K+ ( KNa) channels have been shown to exist in neurons in different regions of the CNS, but their physiological function has been difficult to assess. In this study, we have examined if neurons in the spinal cord possess KNa currents. We used whole cell recordings from isolated spinal cord neurons in lamprey. These neurons display two different KNa currents. The first was transient and activated by the Na+ influx during the action potentials, and it was abolished when Na+ channels were blocked by tetrodotoxin. The second KNa current was sustained and persisted in tetrodotoxin. Both KNa currents were abolished when Na+ was substituted with choline or N-methyl-d-glucamine, indicating that they are indeed dependent on Na+ influx into neurons. When Na+ was substituted with Li+, the amplitude of the inward current was unchanged, whereas the transient KNa current was reduced but not abolished. This suggests that the transient KNa current is partially activated by Li+. These two KNa currents have different roles in controlling the action potential waveform. The transient KNa appears to act as a negative feedback mechanism sensing the Na+ influx underlying the action potential and may thus be critical for setting the amplitude and duration of the action potential. The sustained KNa current has a slow kinetic of activation and may underlie the slow Ca2+-independent afterhyperpolarization mediated by repetitive firing in lamprey spinal cord neurons.

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The modulation of neuronal excitability by group I metabotropic glutamate receptors (mGluRs) was studied in isolated lamprey spinal cord. At resting potential, application of the group I mGluR agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) slightly depolarized the cells. However, at depolarized membrane potentials, this agonist induced repetitive firing. When Na+ channels were blocked by TTX, DHPG induced a slight depolarization at rest that increased in amplitude as the neurons were held at more depolarized membrane potentials. In voltage-clamp conditions, DHPG application induced an inward current associated with a decrease in membrane conductance when cells were held at -40 mV. At resting membrane potential, no significant change in the current was induced by DHPG, although a decrease in membrane conductance was seen. The conductance blocked by DHPG corresponded to a leak current, since DHPG had no effect on the voltage-gated current elicited by a voltage step from -60 to -40 mV, when leak currents were subtracted. The leak current blocked by DHPG is mediated by fluxes of both K+ and Na+. The subtype of group I mGluR mediating the block of the leak current was characterized using specific antagonists for mGluR1 and mGluR5. The inhibition of the leak current was blocked by the mGluR1 antagonist LY 367385 but not by the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP). The DHPG-induced blockage of the leak current required phospholipase C (PLC)-activation and release of Ca2+ from internal stores as the effect of DHPG was suppressed by the PLC-blocker U-73122 and after depletion of intracellular Ca2+ pools by thapsigargin. Our results thus show that mGluR1 activation depolarizes spinal neurons by inhibiting a leak current. This will boost membrane depolarization and result in an increase in the excitability of spinal cord neurons, which could contribute to the modulation of the activity of the spinal locomotor network.

Metabotropic glutamate receptors (mGluRs) act as modulators in the CNS of vertebrates, but their role in motor pattern generation in particular is primarily unknown. The intracellular signaling mechanisms of the group I mGluRs (mGluR1 and mGluR5), and their endogenous role in regulating locomotor pattern generation have been investigated in the spinal cord of the lamprey. Application of the group I mGluR agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) produced oscillations of the intracellular Ca2+ concentration ([Ca2+]i) in neurons. The oscillations were blocked by the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) but not by the mGluR1 antagonist 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester. These [Ca2+]i oscillations were abolished by a phospholipase C blocker and after depletion of internal Ca2+ stores by thapsigargin but did not involve protein kinase C activation. Furthermore, they were dependent on Ca2+ influx, because no [Ca2+]i oscillations were produced by DHPG in a Ca2+-free solution or after blockade of L-type Ca2+ channels. The mGluR5 is activated by an endogenous release of glutamate during locomotion, and a receptor blockade by MPEP caused an increase in the burst frequency. Thus, our results show that mGluR5 induces [Ca2+]i oscillations and regulates the activity of locomotor networks through endogenous activation.