Single neuron control over a complex motor program

Cycle period of a network oscillator is independent of membrane potential and spiking activity in individual central pattern generator neurons

Rhythmic motor patterns are thought to arise through the cellular properties and synaptic interactions of neurons in central pattern generator (CPG) circuits. Yet, when examining the CPG underlying the rhythmic escape response of the opisthobranch mollusc, Tritonia diomedea, we found that the cycle period of the fictive swim motor pattern recorded from the isolated nervous system was not altered by changing the resting membrane potential or the level of spiking activity of any of the 3 known CPG cell types: ventral swim interneuron-B (VSI-B), the dorsal swim interneurons (DSIs), and cerebral neuron 2 (C2). Furthermore, tonic firing in one or more DSIs or C2 evoked rhythmic bursting that did not differ from the cycle period of the motor pattern evoked by nerve stimulation, regardless of the firing frequency. In contrast, the CPG produced a large range of cycle periods as a function of temperature. The temperature sensitivity of the fictive motor pattern produced by the isolated nervous system was similar to the temperature sensitivity of the swimming behavior produced by the intact animal. Thus, although the CPG is capable of producing a wide range of cycle periods under the influence of temperature, the membrane potentials and spiking activity of the identified CPG neurons do not determine the periodicity of the motor pattern. This suggests that the timing of activity in this network oscillator may be determined by a mechanism that is independent of the membrane potentials and spike rate of its constituent neurons.

Orchestration of synaptic plasticity through AKAP signaling complexes

Significant progress has been made toward understanding the mechanisms by which organisms learn from experiences and how those experiences are translated into memories. Advances in molecular, electrophysiological and genetic technologies have permitted great strides in identifying biochemical and structural changes that occur at synapses during processes that are thought to underlie learning and memory. Cellular events that generate the second messenger cyclic AMP (cAMP) and activate protein kinase A (PKA) have been linked to synaptic plasticity and long-term memory. In this review we will focus on the role of PKA in synaptic plasticity and discuss how the compartmentalization of PKA through its association with A-Kinase Anchoring Proteins (AKAPs) affect PKA function in this process.

Imaging reveals synaptic targets of a swim-terminating neuron in the leech CNS

In the leech, the command-like neuron called cell Tr2 is known to stop swimming, but the connections from cell Tr2 to the swim central pattern generator have not been identified. We used fluorescence resonance energy transfer voltage-sensitive dyes to identify three neurons that are synaptic targets of cell Tr2. We then used electrophysiological techniques to show that these connections are monosynaptic, chemical, and excitatory. Two of the novel targets, cell 256 and cell 54, terminate swimming when stimulated. These neurons are likely to mediate swim cessation caused by cell Tr2 activity, and thus play the role of intermediate control cells in the leech CNS.

A cellular mechanism for prepulse inhibition

In prepulse inhibition (PPI), startle responses to sudden, unexpected stimuli are markedly attenuated if immediately preceded by a weak stimulus of almost any modality. This experimental paradigm exposes a potent inhibitory process, present in nervous systems from invertebrates to humans, that is widely considered to play an important role in reducing distraction during the processing of sensory input. The neural mechanisms mediating PPI are of considerable interest given evidence linking PPI deficits with some of the cognitive disorders of schizophrenia. Here, in the marine mollusk Tritonia diomedea, we describe a detailed cellular mechanism for PPI--a combination of presynaptic inhibition of startle afferent neurons together with distributed postsynaptic inhibition of several downstream interneuronal sites in the startle circuit.

Spike timing-dependent serotonergic neuromodulation of synaptic strength intrinsic to a central pattern generator circuit

Neuromodulation is often thought to have a static, gain-setting function in neural circuits. Here we report a counter example: the neuromodulatory effect of a serotonergic neuron is dependent on the interval between its spikes and those of the neuron being modulated. The serotonergic dorsal swim interneurons (DSIs) are members of the escape swim central pattern generator (CPG) in the mollusk Tritonia diomedea. DSI spike trains heterosynaptically enhanced synaptic potentials evoked by another CPG neuron, ventral swim interneuron B (VSI-B), when VSI-B action potentials occurred within 10 sec of a DSI spike train; however, if VSI-B was stimulated 20-120 sec after DSI, then the amplitude of VSI-B synaptic potentials decreased. Consistent with this, VSI-B-evoked synaptic currents exhibited a temporally biphasic and bidirectional change in amplitude after DSI stimulation. Both the DSI-evoked enhancement and decrement were occluded by serotonin and blocked by the serotonin receptor antagonist methysergide, suggesting that both phases are mediated by serotonin. In most preparations, however, bath-applied serotonin caused only a sustained enhancement of VSI-B synaptic strength. The heterosynaptic modulation interacted with short-term homosynaptic plasticity: DSI-evoked depression was offset by VSI-B homosynaptic facilitation. This caused a complicated temporal pattern of neuromodulation when DSI and VSI-B were stimulated to fire in alternating bursts to mimic the natural motor pattern: DSI strongly enhanced summated VSI-B synaptic potentials and suppressed single synaptic potentials after the cessation of the artificial motor pattern. Thus, spike timing-dependent serotonergic neuromodulatory actions can impart temporal information that may be relevant to the operation of the CPG.

Imaging reveals synaptic targets of a swim-terminating neuron in the leech CNS

In the leech, the command-like neuron called cell Tr2 is known to stop swimming, but the connections from cell Tr2 to the swim central pattern generator have not been identified. We used fluorescence resonance energy transfer voltage-sensitive dyes to identify three neurons that are synaptic targets of cell Tr2. We then used electrophysiological techniques to show that these connections are monosynaptic, chemical, and excitatory. Two of the novel targets, cell 256 and cell 54, terminate swimming when stimulated. These neurons are likely to mediate swim cessation caused by cell Tr2 activity, and thus play the role of intermediate control cells in the leech CNS.

Spike timing-dependent serotonergic neuromodulation of synaptic strength intrinsic to a central pattern generator circuit

Neuromodulation is often thought to have a static, gain-setting function in neural circuits. Here we report a counter example: the neuromodulatory effect of a serotonergic neuron is dependent on the interval between its spikes and those of the neuron being modulated. The serotonergic dorsal swim interneurons (DSIs) are members of the escape swim central pattern generator (CPG) in the mollusk Tritonia diomedea. DSI spike trains heterosynaptically enhanced synaptic potentials evoked by another CPG neuron, ventral swim interneuron B (VSI-B), when VSI-B action potentials occurred within 10 sec of a DSI spike train; however, if VSI-B was stimulated 20-120 sec after DSI, then the amplitude of VSI-B synaptic potentials decreased. Consistent with this, VSI-B-evoked synaptic currents exhibited a temporally biphasic and bidirectional change in amplitude after DSI stimulation. Both the DSI-evoked enhancement and decrement were occluded by serotonin and blocked by the serotonin receptor antagonist methysergide, suggesting that both phases are mediated by serotonin. In most preparations, however, bath-applied serotonin caused only a sustained enhancement of VSI-B synaptic strength. The heterosynaptic modulation interacted with short-term homosynaptic plasticity: DSI-evoked depression was offset by VSI-B homosynaptic facilitation. This caused a complicated temporal pattern of neuromodulation when DSI and VSI-B were stimulated to fire in alternating bursts to mimic the natural motor pattern: DSI strongly enhanced summated VSI-B synaptic potentials and suppressed single synaptic potentials after the cessation of the artificial motor pattern. Thus, spike timing-dependent serotonergic neuromodulatory actions can impart temporal information that may be relevant to the operation of the CPG.

Long-lasting activation of rhythmic neuronal activity by a novel mechanosensory system in the crustacean stomatogastric nervous system

Sensory neurons enable neural circuits to generate behaviors appropriate for the current environmental situation. Here, we characterize the actions of a population (about 60) of bilaterally symmetric bipolar neurons identified within the inner wall of the cardiac gutter, a foregut structure in the crab Cancer borealis. These neurons, called the ventral cardiac neurons (VCNs), project their axons through the crab stomatogastric nervous system to influence neural circuits associated with feeding. Brief pressure application to the cardiac gutter transiently modulated the filtering motor pattern (pyloric rhythm) generated by the pyloric circuit within the stomatogastric ganglion (STG). This modulation included an increased speed of the pyloric rhythm and a concomitant decrease in the activity of the lateral pyloric neuron. Furthermore, 2 min of rhythmic pressure application to the cardiac gutter elicited a chewing motor pattern (gastric mill rhythm) generated by the gastric mill circuit in the STG that persisted for < or =30 min. These sensory actions on the pyloric and gastric mill circuits were mimicked by either ventral cardiac nerve or dorsal posterior esophageal nerve stimulation. VCN actions on the STG circuits required the activation of projection neurons in the commissural ganglia. A subset of the VCN actions on these projection neurons appeared to be direct and cholinergic. We propose that the VCN neurons are mechanoreceptors that are activated when food stored in the foregut applies an outward force, leading to the long-lasting activation of projection neurons required to initiate chewing and modify the filtering of chewed food.

Loss of self-inhibition is a cellular mechanism for episodic rhythmic behavior

BACKGROUND

Rhythmic motor behaviors can be generated continuously (e.g., breathing) or episodically (e.g., locomotion, swallowing), when short or long bouts of rhythmic activity are interspersed with periods of quiescence. Although the mechanisms of rhythm generation are known in detail in many systems, there is very little understanding of how the episodic nature of rhythmic behavior is produced at the neuronal level.

RESULTS

Using a well-established episodic rhythm-generating neural circuit controlling molluscan feeding, we demonstrate that quiescence between bouts of activity arises from active, maintained inhibition of an otherwise rhythmically active network. We show that the source of the suppressive drive is within the circuit itself; a single central pattern generator (CPG) interneuron type that fires tonically to inhibit feeding during quiescence. Suppression of the tonic activity of this neuron by food is sufficient to change the network from an inactive to a rhythmically active state, with the cell switching function to fire phasically as part of the food-evoked rhythmogenesis. Furthermore, the absolute level of intrinsic suppressive control is modulated extrinsically by the animal's behavioral state (e.g., hunger/satiety), increasing the probability of episodes of feeding when the animal is hungry.

CONCLUSIONS

By utilizing the same intrinsic member of a CPG network in both rhythm-generation and suppression, this system has developed a simple and efficient mechanism for generating a variable level of response to suit the animal's changing behavioral demands.

Nicotinic activation of reticulospinal cells involved in the control of swimming in lampreys

In lampreys as in other vertebrates, brainstem centres play a key role in the initiation and control of locomotion. One such centre, the mesencephalic locomotor region (MLR), was identified physiologically at the mesopontine border. Descending inputs from the MLR are relayed by reticulospinal neurons in the pons and medulla, but the mechanisms by which this is carried out remain unknown. Because previous studies in higher vertebrates and lampreys described cholinergic cells within the MLR region, we investigated the putative role of cholinergic agonists in the MLR-controlled locomotion. The local application of either acetylcholine or nicotine exerted a direct dose-dependent excitation on reticulospinal neurons as well as induced active or fictive locomotion. It also accelerated ongoing fictive locomotion. Choline acetyltransferase-immunoreactive cells were found in the region identified as the MLR of lampreys and nicotinic antagonists depressed, whereas physostigmine enhanced the compound EPSP evoked in reticulospinal neurons by electrical stimulation of this region. In addition, cholinergic inputs from the MLR to reticulospinal neurons were found to be monosynaptic. When the brainstem was perfused with d-tubocurarine, the induction of swimming by MLR stimulation was depressed, but not prevented, in a semi-intact preparation. Altogether, the results support the hypothesis that cholinergic inputs from the MLR to reticulospinal cells play a substantial role in the initiation and the control of locomotion.

Cytological and molecular analysis of centromere misdivision in maize

B chromosome derivatives suffering from breaks within their centromere were examined cytologically and molecularly. We showed by high resolution FISH that misdivision of the centromere of a univalent chromosome can occur during meiosis. The breaks divide the centromere repeat sequence cluster. A telocentric chromosome formed by misdivision was found to have the addition of telomeric repeats to the broken centromere. A ring chromosome formed after misdivision occurred by fusion of the broken centromere to the telomere. Pulsed-field electrophoresis analyses were performed on the telocentric and ring chromosomes to identify fragments that hybridize to both the telomeric repeat and the B-specific centromeric repeat. We conclude that healing of broken maize centromeres can be achieved through the mechanisms of addition or fusion of telomeric repeat sequences to the broken centromere.

Highly dissimilar behaviors mediated by a multifunctional network in the marine mollusk Tritonia diomedea

Several motor networks have now been found to be multifunctional, in which one group of neurons participates in the generation of multiple behavioral motor programs. Not surprisingly, the behaviors involved are frequently closely related, often using the same or similar muscle groups. Here we describe an interneuronal network in the marine mollusk Tritonia diomedea that is involved in producing two highly dissimilar behaviors, rhythmic, muscle-based escape swimming and nonrhythmic, cilia-mediated crawling. Several observations support this conclusion. First, the dorsal swim interneurons (DSIs) of the swim central pattern generator (CPG) directly excite Pedal neuron 21 (Pd21) and Pd5, the only identified cilia-activating efferent neurons in Tritonia. Second, stimulation of a single DSI elicits beating of the foot cilia in semi-intact preparations and crawling in intact animal treadmill preparations. Third, the DSIs fire at an elevated rate for nearly 1 hr after a swim motor program, which correlates reasonably well with the period freely behaving animals were found to crawl after they swam. Fourth, silencing the tonically active DSIs after a swim motor program substantially reduces or eliminates ongoing cilia neuron firing, indicating that the DSIs are major contributors to the synaptic input driving these cells. Finally, all of the other swim CPG neurons also connect to the cilia neurons, most monosynaptically. Taken together, these observations indicate that the Tritonia swim CPG network participates in producing both escape swimming and crawling. Given the extreme differences between these behaviors---rhythmic versus tonic, muscular versus ciliary, and brief versus prolonged--these findings reveal a striking versatility for a small multifunctional network.

Highly dissimilar behaviors mediated by a multifunctional network in the marine mollusk Tritonia diomedea

Several motor networks have now been found to be multifunctional, in which one group of neurons participates in the generation of multiple behavioral motor programs. Not surprisingly, the behaviors involved are frequently closely related, often using the same or similar muscle groups. Here we describe an interneuronal network in the marine mollusk Tritonia diomedea that is involved in producing two highly dissimilar behaviors, rhythmic, muscle-based escape swimming and nonrhythmic, cilia-mediated crawling. Several observations support this conclusion. First, the dorsal swim interneurons (DSIs) of the swim central pattern generator (CPG) directly excite Pedal neuron 21 (Pd21) and Pd5, the only identified cilia-activating efferent neurons in Tritonia. Second, stimulation of a single DSI elicits beating of the foot cilia in semi-intact preparations and crawling in intact animal treadmill preparations. Third, the DSIs fire at an elevated rate for nearly 1 hr after a swim motor program, which correlates reasonably well with the period freely behaving animals were found to crawl after they swam. Fourth, silencing the tonically active DSIs after a swim motor program substantially reduces or eliminates ongoing cilia neuron firing, indicating that the DSIs are major contributors to the synaptic input driving these cells. Finally, all of the other swim CPG neurons also connect to the cilia neurons, most monosynaptically. Taken together, these observations indicate that the Tritonia swim CPG network participates in producing both escape swimming and crawling. Given the extreme differences between these behaviors---rhythmic versus tonic, muscular versus ciliary, and brief versus prolonged--these findings reveal a striking versatility for a small multifunctional network.

Motor program selection in simple model systems

In recent years, studies of molluscan and crustacean feeding circuits have greatly expanded our knowledge of how the nervous system selects specific behaviors. Increasing use of neurobehavioral studies, and examination of the roles of identified command-like or influential neurons have narrowed the gap between knowledge of circuit connectivity and understanding of the normal behavioral functions of these circuits.

Broadly tuned spinal neurons for each form of fictive scratching in spinal turtles

Behavioral choice can be mediated either by a small number of sharply tuned neurons or by large populations of broadly tuned neurons. This issue can be conveniently examined in the turtle spinal cord, which generates each of three forms of scratching-rostral, pocket, and caudal-in response to mechanical stimulation in each of three adjacent regions of the body surface. Previous research showed that many propriospinal neurons are broadly tuned to either the rostral scratch region or the pocket scratch region, but responses to caudal scratch stimulation could not be examined in that reduced preparation. In the current study, individual spinal neurons were recorded extracellularly from the gray matter of the turtle spinal cord hindlimb enlargement, while sites in the rostral, pocket, and caudal scratch regions were mechanically stimulated. Many neurons were broadly tuned to the caudal scratch region; other neurons were broadly tuned to either the pocket scratch or rostral scratch region. All three types were typically found within a single animal. These data are consistent with the hypothesis that the turtle spinal cord relies on large populations of broadly tuned neurons to select each of the three forms of scratching. In addition, neurons that were broadly tuned to each of the scratch regions were typically found in each spinal cord segment and within the same range of mediolateral and dorsoventral locations. Providing that these neurons are related to the selection and generation of the three forms of scratching, this would indicate that cells of this type are not segregated into distinct regions of the spinal cord gray matter.

Multiple types of control by identified interneurons in a sensory-activated rhythmic motor pattern

Modulatory interneurons that can drive central pattern generators (CPGs) are considered as good candidates for decision-making roles in rhythmic behaviors. Although the mechanisms by which such neurons activate their target CPGs are known in detail in many systems, their role in the sensory activation of CPG-driven behaviors is poorly understood. In the feeding system of the mollusc Lymnaea, one of the best-studied rhythmical networks, intracellular stimulation of either of two types of neuron, the cerebral ventral 1a (CV1a) and the slow oscillator (SO) cells, leads to robust CPG-driven fictive feeding patterns, suggesting that they might make an important contribution to natural food-activated behavior. In this paper we investigated this contribution using a lip-CNS preparation in which feeding was elicited with a natural chemostimulant rather than intracellular stimulation. We found that despite their CPG-driving capabilities, neither CV1a nor SO were involved in the initial activation of sucrose-evoked fictive feeding, whereas a CPG interneuron, N1M, was active first in almost all preparations. Instead, the two interneurons play important and distinct roles in determining the characteristics of the rhythmic motor output; CV1a by modulating motoneuron burst duration and SO by setting the frequency of the ongoing rhythm. This is an example of a distributed system in which (1) interneurons that drive similar motor patterns when activated artificially contribute differently to the shaping of the motor output when it is evoked by the relevant sensory input, and (2) a CPG rather than a modulatory interneuron type plays the most critical role in initiation of sensory-evoked rhythmic activity.

Multiple types of control by identified interneurons in a sensory-activated rhythmic motor pattern

Modulatory interneurons that can drive central pattern generators (CPGs) are considered as good candidates for decision-making roles in rhythmic behaviors. Although the mechanisms by which such neurons activate their target CPGs are known in detail in many systems, their role in the sensory activation of CPG-driven behaviors is poorly understood. In the feeding system of the mollusc Lymnaea, one of the best-studied rhythmical networks, intracellular stimulation of either of two types of neuron, the cerebral ventral 1a (CV1a) and the slow oscillator (SO) cells, leads to robust CPG-driven fictive feeding patterns, suggesting that they might make an important contribution to natural food-activated behavior. In this paper we investigated this contribution using a lip-CNS preparation in which feeding was elicited with a natural chemostimulant rather than intracellular stimulation. We found that despite their CPG-driving capabilities, neither CV1a nor SO were involved in the initial activation of sucrose-evoked fictive feeding, whereas a CPG interneuron, N1M, was active first in almost all preparations. Instead, the two interneurons play important and distinct roles in determining the characteristics of the rhythmic motor output; CV1a by modulating motoneuron burst duration and SO by setting the frequency of the ongoing rhythm. This is an example of a distributed system in which (1) interneurons that drive similar motor patterns when activated artificially contribute differently to the shaping of the motor output when it is evoked by the relevant sensory input, and (2) a CPG rather than a modulatory interneuron type plays the most critical role in initiation of sensory-evoked rhythmic activity.

A cellular mechanism for the transformation of a sensory input into a motor command

The initiation and control of locomotion largely depend on processing of sensory inputs. The cellular bases of locomotion have been extensively studied in lampreys where reticulospinal (RS) neurons constitute the main descending system activating and controlling the spinal locomotor networks. Ca(2+) imaging and intracellular recordings were used to study the pattern of activation of RS neurons in response to cutaneous stimulation. Pressure applied to the skin evoked a linear input/output relationship in RS neurons until a threshold level, at which a depolarizing plateau was induced, the occurrence of which was associated with the onset of swimming activity in a semi-intact preparation. The occurrence of a depolarizing plateau was abolished by blocking the NMDA receptors that are located on RS cells. Moreover, the depolarizing plateaus were accompanied by a rise in [Ca(2+)](i), and an intracellular injection of the Ca(2+) chelator BAPTA into single RS cells abolished the plateaus, suggesting that the latter are Ca(2+) dependent and rely on intrinsic properties of RS cells. The plateaus were shown to result from the activation of a Ca(2+)-activated nonselective cation current that maintains the cell in a depolarized state. It is concluded that this intrinsic property of the RS neuron is then responsible for the transformation of an incoming sensory signal into a motor command that is then forwarded to the spinal locomotor networks.

A cellular mechanism for the transformation of a sensory input into a motor command

The initiation and control of locomotion largely depend on processing of sensory inputs. The cellular bases of locomotion have been extensively studied in lampreys where reticulospinal (RS) neurons constitute the main descending system activating and controlling the spinal locomotor networks. Ca(2+) imaging and intracellular recordings were used to study the pattern of activation of RS neurons in response to cutaneous stimulation. Pressure applied to the skin evoked a linear input/output relationship in RS neurons until a threshold level, at which a depolarizing plateau was induced, the occurrence of which was associated with the onset of swimming activity in a semi-intact preparation. The occurrence of a depolarizing plateau was abolished by blocking the NMDA receptors that are located on RS cells. Moreover, the depolarizing plateaus were accompanied by a rise in [Ca(2+)](i), and an intracellular injection of the Ca(2+) chelator BAPTA into single RS cells abolished the plateaus, suggesting that the latter are Ca(2+) dependent and rely on intrinsic properties of RS cells. The plateaus were shown to result from the activation of a Ca(2+)-activated nonselective cation current that maintains the cell in a depolarized state. It is concluded that this intrinsic property of the RS neuron is then responsible for the transformation of an incoming sensory signal into a motor command that is then forwarded to the spinal locomotor networks.

Immunocytochemical localization of pedal peptide in the central nervous system of the gastropod mollusc Tritonia diomedea

Tritonia pedal ganglion peptides (TPeps) are a trio of pentadecapeptides isolated from the brain of the nudibranch Tritonia diomedea. TPeps have been shown both to increase the beating rate of ciliated cells of Tritonia and to accelerate heart contractions in the mollusc Clione limacina. Here we examine the immunocytochemical distribution of TPeps in the Tritonia central nervous system. We found the brain and buccal ganglia to be rich sources of TPep immunoreactivity. Specific cells in both structures, some of them previously identified, were immunoreactive. Moreover, immunoreactive fibers were seen connecting ganglia and exiting almost all the major nerves. In the brain, we found that the paired, ciliated statocysts apparently receive TPep innervation. In addition, we observed unstained cell bodies in each buccal ganglion with extensive TPep immunoreactive projections surrounding their somata and primary neurites. Similar projections were not observed in the brain. We also compared the TPep immunoreactivity with that of SCP(b) in the buccal ganglia. We observed many neurons and processes that were immunoreactive to both peptides. One neuron that contains both TPep- and SCP(b)-like peptides (B12) has an identified role in the Tritonia feeding network. Together, these findings suggest that TPeps may play an active role in the central nervous system of Tritonia as neurotransmitters modulating orientation, swimming, and feeding.

Revisiting the concept of identifiable neurons

Although eutely in nematodes was known, giant neurons in several taxa and unique motor neurons to leg muscles in decapod crustaceans, the idea that many animals have many identifiable neurons with relatively consistent dynamical properties and connections was only slowly established in the late 1960s and early 1970s. This has to be one of the important quiet revolutions in neurobiology. It stimulated a vast acquisition of specific information and led to some euphoria in the degree and pace of understanding activity of nervous systems and consequent behavior in terms of neuronal connections and properties. Some implications, problems and opportunities for new discovery are developed. The distribution of identifiable neurons among taxa and parts of the nervous system is not yet satisfactorily known. Their evolution may have been a case of several independent inventions. The degree of consistency has been quantified only in a few examples and the plasticity is little known. Identified neurons imply identifiable circuits but whether this extends to discrete systems, functionally definable, seems likely to have several answers in different animals or sites. Very limited attempts have been made to extend the concept to cases of two or ten or a hundred fully equivalent neurons, on all kinds of criteria. These attempts suggest a much smaller redundancy and vaster number of types of neurons than hitherto believed. Theory as well as empirical information has not yet interpreted the range of systems from those with small sets of relatively reliable neurons to those with large numbers of parallel, partially redundant units. The now classical notion of local circuits has to be extended to take account of and find roles for the plethora of integrative variables, of evidence for neural processing independent of spikes and classical synapses, of spatial configurations of terminal arbors and dendritic geometry, of modulators and transmitters, degrees of rhythmicity (regularity varying several orders of magnitude), and of synchrony. Adequate language and models need to go beyond 'circuits' in any engineering sense. Identifiable neurons can contribute to a broad spectrum of issues in neurobiology.

Multifunctional neuron CC6 in Aplysia exerts actions opposite to those of multifunctional neuron CC5

The controls of somatic and autonomic functions often appear to be organized into antagonistic systems. This issue was explored in the bilaterally paired C cluster neuron, CC6, which was found to have properties that suggested that it might function antagonistically to the previously identified multiaction neuron, CC5. Similar to CC5, CC6 is an interganglionic neuron that sends its sole axon to the ipsilateral and contralateral pedal and pleural ganglia. Synaptic inputs to CC6 were opposite to those of CC5. For example, CC6 receives inhibitory inputs from mechanical touch to the lips and tentacles and is excited by firing of C-PR, a neuron involved in the control of a head extension response. Also during rhythmic buccal mass movements CC6 receives synaptic inputs that are out of phase with those received by CC5. CC6 is inhibited during a fictive locomotor program, whereas CC5 is excited, but unlike CC5, the inputs to CC6 are not rhythmic. CC6 has extensive mono- and polysynaptic outputs to many identified and unidentified neurons located in various central ganglia. Firing of CC6 evoked ipsilateral contraction of the transverse muscles of the neck, whereas CC5 contracts longitudinal neck muscles. CC6 monosynaptically inhibits the pedal artery shortener neuron, whereas CC5 monosynaptically excites the pedal artery shortener neuron. Specific motor neurons in the pedal ganglion receive synaptic inputs of opposite sign from CC5 and CC6. Although the inputs and most of the effects of CC6 were opposite to those of CC5, both cells were found to produce polysynaptic excitation of the abdominal ganglion neuron RBhe, a cell whose activity excites the heart. CC5 and CC6 appear to be multifunctional neurons that form an antagonist pair.

Paradoxical actions of the serotonin precursor 5-hydroxytryptophan on the activity of identified serotonergic neurons in a simple motor circuit

Neurotransmitter synthesis is regulated by a variety of factors, yet the effect of altering transmitter content on the operation of neuronal circuits has been relatively unexplored. We used electrophysiological, electrochemical, and immunohistochemical techniques to investigate the effects of augmenting the serotonin (5-HT) content of identified serotonergic neurons embedded in a simple motor circuit. The dorsal swim interneurons (DSIs) are serotonergic neurons intrinsic to the central pattern generator (CPG) for swimming in the mollusc Tritonia diomedea. As expected, treatment with the serotonin precursor 5-hydroxytryptophan (5-HTP) increased the intensity of serotonin immunolabeling and enhanced the potency of synaptic and modulatory actions elicited by the DSIs. It also greatly enhanced the ability of the DSIs to evoke rhythmic CPG activity. After 5-HTP treatment, microvoltammetric measurements indicated an increase in a putative 5-HT electrochemical signal during swim CPG activation. Paradoxically, the spiking activity of the serotonergic neurons decreased to a single burst at the onset of the rhythmic motor program, whereas the overall duration of the episode remained about the same. 5-HTP treatment gradually reduced the rhythmicity of the CPG output. Thus, more serotonin did not result in a more robust swim motor program, suggesting that serotonin synthesis must be kept within certain limits for the circuit to function correctly and indicating that altering neurotransmitter synthesis can have serious consequences for the output of neural networks.

Paradoxical actions of the serotonin precursor 5-hydroxytryptophan on the activity of identified serotonergic neurons in a simple motor circuit

Neurotransmitter synthesis is regulated by a variety of factors, yet the effect of altering transmitter content on the operation of neuronal circuits has been relatively unexplored. We used electrophysiological, electrochemical, and immunohistochemical techniques to investigate the effects of augmenting the serotonin (5-HT) content of identified serotonergic neurons embedded in a simple motor circuit. The dorsal swim interneurons (DSIs) are serotonergic neurons intrinsic to the central pattern generator (CPG) for swimming in the mollusc Tritonia diomedea. As expected, treatment with the serotonin precursor 5-hydroxytryptophan (5-HTP) increased the intensity of serotonin immunolabeling and enhanced the potency of synaptic and modulatory actions elicited by the DSIs. It also greatly enhanced the ability of the DSIs to evoke rhythmic CPG activity. After 5-HTP treatment, microvoltammetric measurements indicated an increase in a putative 5-HT electrochemical signal during swim CPG activation. Paradoxically, the spiking activity of the serotonergic neurons decreased to a single burst at the onset of the rhythmic motor program, whereas the overall duration of the episode remained about the same. 5-HTP treatment gradually reduced the rhythmicity of the CPG output. Thus, more serotonin did not result in a more robust swim motor program, suggesting that serotonin synthesis must be kept within certain limits for the circuit to function correctly and indicating that altering neurotransmitter synthesis can have serious consequences for the output of neural networks.

Control of cricket stridulation by a command neuron: efficacy depends on the behavioral state

Crickets use different song patterns for acoustic communication. The stridulatory pattern-generating networks are housed within the thoracic ganglia but are controlled by the brain. This descending control of stridulation was identified by intracellular recordings and stainings of brain neurons. Its impact on the generation of calling song was analyzed both in resting and stridulating crickets and during cercal wind stimulation, which impaired the stridulatory movements and caused transient silencing reactions. A descending interneuron in the brain serves as a command neuron for calling-song stridulation. The neuron has a dorsal soma position, anterior dendritic processes, and an axon that descends in the contralateral connective. The neuron is present in each side of the CNS. It is not activated in resting crickets. Intracellular depolarization of the interneuron so that its spike frequency is increased to 60-80 spikes/s reliably elicits calling-song stridulation. The spike frequency is modulated slightly in the chirp cycle with the maximum activity in phase with each chirp. There is a high positive correlation between the chirp repetition rate and the interneuron's spike frequency. Only a very weak correlation, however, exists between the syllable repetition rate and the interneuron activity. The effectiveness of the command neuron depends on the activity state of the cricket. In resting crickets, experimentally evoked short bursts of action potentials elicit only incomplete calling-song chirps. In crickets that previously had stridulated during the experiment, short elicitation of interneuron activity can trigger sustained calling songs during which the interneuron exhibits a spike frequency of approximately 30 spikes/s. During sustained calling songs, the command neuron activity is necessary to maintain the stridulatory behavior. Inhibition of the interneuron stops stridulation. A transient increase in the spike frequency of the interneuron speeds up the chirp rate and thereby resets the timing of the chirp pattern generator. The interneuron also is excited by cercal wind stimulation. Cercal wind stimulation can impair the pattern of chirp and syllable generation, but these changes are not reflected in the discharge pattern of the command neuron. During wind-evoked silencing reactions, the activity of the calling-song command neuron remains unchanged, but under these conditions, its activity is no longer sufficient to maintain stridulation. Therefore stridulation can be suppressed by cercal inputs from the terminal ganglia without directly inhibiting the descending command activity.

Dishabituation of the Tritonia escape swim

When repeatedly elicited, the oscillatory escape swim of the marine mollusc Tritonia diomedea undergoes habituation of the number of cycles per swim. Although similar in most respects to habituation observed in vertebrates and other invertebrates, one key feature, dishabituation, has been surprisingly difficult to demonstrate. Here we evaluate the hypothesis that this is due to interference from short-term sensitization, which is manifested as a reduction in swim onset latency, that occurs simultaneously during habituation training. Robust dishabituation was obtained using a multisession habituation protocol designed to allow this sensitization to dissipate before the dishabituatory stimulus was applied. These results extend the similarity of habituation in Tritonia to that described in other species, strengthening the usefulness of this preparation as a model system for studies of the cellular basis of habituation.

Anatomical and functional differentiation of glutamatergic synaptic innervation in the neocortex

Pyramidal neurons are the principal neurons of the neocortex and their excitatory impact on other pyramidal neurons and interneurons is central to neocortical dynamics. A fundamental principal that has emerged which governs pyramidal neuron excitation of other neurons in the local circuitry of neocortical columns is differential anatomical and physiological properties of the synaptic innervation via the same axon depending on the type of neuron targeted. In this study we derive anatomical principles for divergent innervation of pyramidal neurons of the same type within the local microcircuit. We also review data providing circumstantial and direct evidence for differential synaptic transmission via the same axon from neocortical pyramidal neurons and derive some principles for differential synaptic innervation of pyramidal neurons of the same type, of pyramidal neurons and interneurons and of different types of interneurons. We conclude that differential anatomical and physiological differentiation is a fundamental property of glutamatergic axons of pyramidal neurons in the neocortex.

Central pattern generator for escape swimming in the notaspid sea slug Pleurobranchaea californica

Escape swimming in the notaspid opisthobranch Pleurobranchaea is an episode of alternating dorsal and ventral body flexions that overrides all other behaviors. We have explored the structure of the central pattern generator (CPG) in the cerebropleural ganglion as part of a study of neural network interactions underlying decision making in normal behavior. The CPG comprises at least eight bilaterally paired interneurons, each of which contributes and is phase-locked to the swim rhythm. Dorsal flexion is mediated by hemiganglion ensembles of four serotonin-immunoreactive neurons, the As1, As2, As3, and As4, and an electrically coupled pair, the A1 and A10 cells. When stimulated, A10 commands fictive swimming in the isolated CNS and actual swimming behavior in whole animals. As1-4 provide prolonged, neuromodulatory excitation enhancing dorsal flexion bursts and swim cycle number. Ventral flexion is mediated by the A3 cell and a ventral swim interneuron, IVS, the soma of which is yet unlocated. Initiation of a swim episode begins with persistent firing in A10, followed by recruitment of As1-4 and A1 into dorsal flexion. Recurrent excitation within the As1-4 ensemble and with A1/A10 may reinforce coactivity. Synchrony among swim interneuron partners and bilateral coordination is promoted by electrical coupling among the A1/A10 and As4 pairs, and among unilateral As2-4, and reciprocal chemical excitation between contralateral As1-4 groups. The switch from dorsal to ventral flexion coincides with delayed recruitment of A3, which is coupled electrically to A1, and with recurrent inhibition from A3/IVS to A1/A10. The alternating phase relation may be reinforced by reciprocal inhibition between As1-4 and IVS. Pleurobranchaea's swim resembles that of the nudibranch Tritonia; we find that the CPGs are similar in many details, suggesting that the behavior and network are primitive characters derived from a common pleurobranchid ancestor.

C-PR neuron of Aplysia has differential effects on "Feeding" cerebral interneurons, including myomodulin-positive CBI-12

Head lifting and other aspects of the appetitive central motive state that precedes consummatory feeding movements in Aplysia is promoted by excitation of the C-PR neuron. Food stimuli activate C-PR as well as a small population of cerebral-buccal interneurons (CBIs). We wished to determine if firing of C-PR produced differential effects on the various CBIs or perhaps affected all the CBIs uniformly as might be expected for a neuron involved in producing a broad undifferentiated arousal state. We found that when C-PR was fired, it produced a wide variety of effects on various CBIs. Firing of C-PR evoked excitatory input to a newly identified CBI (CBI-12) the soma of which is located in the M cluster near the previously identified CBI-2. CBI-12 shares certain properties with CBI-2, including a similar morphology and a capacity to drive rhythmic activity of the buccal-ganglion. Unlike CBI-2, CBI-12 exhibits myomodulin immunoreactivity. Furthermore when C-PR is fired, CBI-12 receives a polysynaptic voltage-dependent slow excitation, whereas, CBI-2 receives relatively little input. C-PR also polysynaptically excites other CBIs including CBI-1 and CBI-8/9 but produces inhibition in CBI-3. In addition, firing of C-PR inhibits plateau potentials in CBI-5/6. The data suggest that activity of C-PR may promote the activity of one subset of cerebral-buccal interneurons, perhaps those involved in ingestive behaviors that occur during the head-up posture. C-PR also inhibits some cerebral-buccal interneurons that may be involved in behaviors in which C-PR activity is not required or may even interfere with other feeding behaviors such as rejection or grazing, that occur with the head down.

Neuromodulation intrinsic to the central pattern generator for escape swimming in Tritonia

Extrinsic neuromodulatory inputs to central pattern generators (CPGs) can alter the properties and synaptic interactions of neurons in those circuits and thereby modify the output of the CPG. Recent work in a number of systems has now demonstrated that neurons intrinsic to CPG can also evoke neuromodulatory actions on other members of the CPG. Such "intrinsic neuromodulation" plays a role in controlling the CPG underlying the escape swim response of the nudibrach mollusc, Tritonia diomedea. The dorsal swim interneurons (DSIs) are a bilaterally represented set of three serotonergic neurons that participate in the generation of the rhythmic swim motor program. Serotonin released from these CPG neurons functions both as a fast neurotransmitter and as a slower neuromodulator. In its modulatory role, serotonin enhances the release of neurotransmitter from another CPG neuron, C2, and also increases C2 excitability by decreasing spike frequency adaptation. These neuromodulatory actions intrinsic to the CPG may be important for the initial self-configuration of the system into a function CPG and for experience-dependent changes in the output such as behavioral sensitization and habituation.

Prepulse inhibition of the Tritonia escape swim

Presenting a weak stimulus just before a strong, startle stimulus reduces the amplitude of the ensuing startle response in humans and other vertebrates. This phenomenon, termed "prepulse inhibition" (PPI), appears to function to reduce distraction while processing sensory input. To date, no detailed neural mechanism has been described for PPI. Here we demonstrate PPI in the marine mollusk Tritonia diomedea, which has a nervous system highly suitable for cellular analyses. We found that a 100 msec vibrotactile prepulse prevented the animal's escape swim response to a closely following 1 sec tail shock. This inhibition was highly transient, with a significant effect lasting just 2.5 sec. These findings indicate that the Tritonia escape swim response undergoes a form of PPI phenomenologically similar to that observed in vertebrates. Further tests showed that the vibrotactile stimulus had no inhibitory effect if applied after tail shock, while the animal was preparing to swim, but it acted to terminate swims once they were actively under way. As a first step toward a cellular analysis of PPI, we recorded from neurons of the swim circuit in a semi-intact preparation and found that the vibrotactile stimulus used in the behavioral experiments also prevented the tail shock-elicited swim motor program. These results represent the first explicit demonstration of PPI in an invertebrate and establish Tritonia as a model system for analyzing its physiological basis.

Prepulse inhibition of the Tritonia escape swim

Presenting a weak stimulus just before a strong, startle stimulus reduces the amplitude of the ensuing startle response in humans and other vertebrates. This phenomenon, termed "prepulse inhibition" (PPI), appears to function to reduce distraction while processing sensory input. To date, no detailed neural mechanism has been described for PPI. Here we demonstrate PPI in the marine mollusk Tritonia diomedea, which has a nervous system highly suitable for cellular analyses. We found that a 100 msec vibrotactile prepulse prevented the animal's escape swim response to a closely following 1 sec tail shock. This inhibition was highly transient, with a significant effect lasting just 2.5 sec. These findings indicate that the Tritonia escape swim response undergoes a form of PPI phenomenologically similar to that observed in vertebrates. Further tests showed that the vibrotactile stimulus had no inhibitory effect if applied after tail shock, while the animal was preparing to swim, but it acted to terminate swims once they were actively under way. As a first step toward a cellular analysis of PPI, we recorded from neurons of the swim circuit in a semi-intact preparation and found that the vibrotactile stimulus used in the behavioral experiments also prevented the tail shock-elicited swim motor program. These results represent the first explicit demonstration of PPI in an invertebrate and establish Tritonia as a model system for analyzing its physiological basis.

Information processing with frequency-dependent synaptic connections

The efficacy of synaptic transmission between two neurons changes as a function of the history of previous activations of the synaptic connection. This history dependence can be characterized by examining the dependence of transmission on the frequency of stimulation. In this framework synaptic plasticity can also be examined in terms of changes in the frequency dependence of transmission and not merely in terms of synaptic strength which constitutes only a linear scaling mechanism. Recent work shows that the frequency dependence of transmission determines the content of information transmitted between neurons and that synaptic modifications can change the content of information transmitted. Multipatch-clamp recordings revealed that the frequency dependence of transmission is potentially unique for each synaptic connection made by a single axon and that the class of pre-postsynaptic neuron determines the class of frequency dependence (activity independent), while the unique activity relationship between any two neurons could determine the precise values of the parameters within a specific class (activity dependent). The content of information transmitted between neurons is also formalized to provide synaptic transfer functions which can be used to determine the role of the synaptic connection within a network of neurons. It is proposed that deriving synaptic transfer functions is crucial in order to understand the link between synaptic transmission and information processing within networks of neurons and to understand the link between synaptic plasticity and learning and memory.

Sensitization of the Tritonia escape swim

When repeatedly elicited, the oscillatory escape swim of the marine mollusc Tritonia diomedea undergoes habituation of the number of cycles per swim. Previous work has shown that this habituation is accompanied by sensitization of another feature of the behavior: latency to swim onset. Here we focused on the behavioral features of sensitization itself. Test swims elicited 5 min after a strong sensitizing head stimulus differed in several ways from control swims: sensitized animals had shorter latencies for gill and rhinophore withdrawal, a shorter latency for swim onset, a lower threshold for swim initiation, and an increased number of cycles per swim. Sensitized animals did not, however, swim any faster (no change in cycle period). A separate experiment found that swim onset latency also sensitized when Tritonia came into contact with one of their natural predators, the seastar Pycnopodia helianthoides, demonstrating the ecological relevance of this form of nonassociative learning. These results define the set of behavioral changes to be explained by cellular studies of sensitization in Tritonia.

Role of sensory-evoked NMDA plateau potentials in the initiation of locomotion

Reticulospinal (RS) neurons constitute the main descending motor system of lampreys. This study reports on natural conditions whereby N-methyl-D-aspartate (NMDA)-mediated plateau potentials were elicited and associated with the onset of locomotion. Reticulospinal neurons responded in a linear fashion to mild skin stimulation. With stronger stimuli, large depolarizing plateaus with spiking activity were elicited and were accompanied by swimming movements. Calcium imaging revealed sustained intracellular calcium rise upon sensory stimulation. Blocking NMDA receptors on RS neurons prevented the plateau potentials as well as the associated rise in intracellular calcium. Thus, the activation of NMDA receptors mediates a switch from sensory-reception mode to a motor command mode in RS neurons.

Removal of spike frequency adaptation via neuromodulation intrinsic to the Tritonia escape swim central pattern generator

For the mollusc Tritonia diomedea to generate its escape swim motor pattern, interneuron C2, a crucial member of the central pattern generator (CPG) for this rhythmic behavior, must fire repetitive bursts of action potentials. Yet, before swimming, repeated depolarizing current pulses injected into C2 at periods similar those in the swim motor program are incapable of mimicking the firing rate attained by C2 on each cycle of a swim motor program. This resting level of C2 inexcitability is attributable to its own inherent spike frequency adaptation (SFA). Clearly, this property must be altered for the swim behavior to occur. The pathway for initiation of the swimming behavior involves activation of the serotonergic dorsal swim interneurons (DSIs), which are also intrinsic members of the swim CPG. Physiologically appropriate DSI stimulation transiently decreases C2 SFA, allowing C2 to fire at higher rates even when repeatedly depolarized at short intervals. The increased C2 excitability caused by DSI stimulation is mimicked and occluded by serotonin application. Furthermore, the change in excitability is not caused by the depolarization associated with DSI stimulation or serotonin application but is correlated with a decrease in C2 spike afterhyperpolarization. This suggests that the DSIs use serotonin to evoke a neuromodulatory action on a conductance in C2 that regulates its firing rate. This modulatory action of one CPG neuron on another is likely to play a role in configuring the swim circuit into its rhythmic pattern-generating mode and maintaining it in that state.

Removal of spike frequency adaptation via neuromodulation intrinsic to the Tritonia escape swim central pattern generator

For the mollusc Tritonia diomedea to generate its escape swim motor pattern, interneuron C2, a crucial member of the central pattern generator (CPG) for this rhythmic behavior, must fire repetitive bursts of action potentials. Yet, before swimming, repeated depolarizing current pulses injected into C2 at periods similar those in the swim motor program are incapable of mimicking the firing rate attained by C2 on each cycle of a swim motor program. This resting level of C2 inexcitability is attributable to its own inherent spike frequency adaptation (SFA). Clearly, this property must be altered for the swim behavior to occur. The pathway for initiation of the swimming behavior involves activation of the serotonergic dorsal swim interneurons (DSIs), which are also intrinsic members of the swim CPG. Physiologically appropriate DSI stimulation transiently decreases C2 SFA, allowing C2 to fire at higher rates even when repeatedly depolarized at short intervals. The increased C2 excitability caused by DSI stimulation is mimicked and occluded by serotonin application. Furthermore, the change in excitability is not caused by the depolarization associated with DSI stimulation or serotonin application but is correlated with a decrease in C2 spike afterhyperpolarization. This suggests that the DSIs use serotonin to evoke a neuromodulatory action on a conductance in C2 that regulates its firing rate. This modulatory action of one CPG neuron on another is likely to play a role in configuring the swim circuit into its rhythmic pattern-generating mode and maintaining it in that state.

A pair of identified interneurons in Aplysia that are involved in multiple behaviors are necessary and sufficient for the arterial-shortening component of a local withdrawal reflex

A bilateral pair of cerebral interneurons, called CC5, contribute to the generation of a number of different behaviors involving head movements. Each cell sends its axon to the ipsilateral and contralateral pedal and pleural ganglia. A weak tactile stimulus to the head excites the ipsilateral CC5; a strong stimulus excites both the ipsilateral and contralateral cells. Firing of CC5 produces powerful shortening of the ipsilateral pedal artery (PA) by means of monosynaptic excitation of the pedal artery shortener (PAS) neuron, the single motor neuron for the artery. A weak touch to a tentacle excites the ipsilateral PAS and evokes a local withdrawal response accompanied by shortening of the ipsilateral PA. In vivo recording of the pedal artery nerve (PAn) showed that PAS was activated bilaterally during defensive head withdrawal elicited by a strong stimulus and was activated unilaterally by a weak stimulus. The responses were eliminated by cutting the ipsilateral cerebral-pleural connective (C-PLC). Electrical stimulation of the cerebral-pleural connective provided evidence that all of the excitatory input to PAS via this connective is provided by CC5. A variety of experimental results indicates that during a local withdrawal reflex of the tentacle, CC5 is necessary and sufficient for the unilateral PA-shortening component of the response and therefore functions as a command neuron for a component of the behavior. The data suggest that during defensive head withdrawal, the two CC5 neurons may act conjointly as a two-neuron command system that is necessary and sufficient for the bilateral arterial-shortening component of the behavior.

A pair of identified interneurons in Aplysia that are involved in multiple behaviors are necessary and sufficient for the arterial-shortening component of a local withdrawal reflex

A bilateral pair of cerebral interneurons, called CC5, contribute to the generation of a number of different behaviors involving head movements. Each cell sends its axon to the ipsilateral and contralateral pedal and pleural ganglia. A weak tactile stimulus to the head excites the ipsilateral CC5; a strong stimulus excites both the ipsilateral and contralateral cells. Firing of CC5 produces powerful shortening of the ipsilateral pedal artery (PA) by means of monosynaptic excitation of the pedal artery shortener (PAS) neuron, the single motor neuron for the artery. A weak touch to a tentacle excites the ipsilateral PAS and evokes a local withdrawal response accompanied by shortening of the ipsilateral PA. In vivo recording of the pedal artery nerve (PAn) showed that PAS was activated bilaterally during defensive head withdrawal elicited by a strong stimulus and was activated unilaterally by a weak stimulus. The responses were eliminated by cutting the ipsilateral cerebral-pleural connective (C-PLC). Electrical stimulation of the cerebral-pleural connective provided evidence that all of the excitatory input to PAS via this connective is provided by CC5. A variety of experimental results indicates that during a local withdrawal reflex of the tentacle, CC5 is necessary and sufficient for the unilateral PA-shortening component of the response and therefore functions as a command neuron for a component of the behavior. The data suggest that during defensive head withdrawal, the two CC5 neurons may act conjointly as a two-neuron command system that is necessary and sufficient for the bilateral arterial-shortening component of the behavior.

Intrinsic and extrinsic neuromodulation of motor circuits

Neuromodulation of motor circuits by extrinsic inputs provides enormous flexibility in the production of behavior. Recent work has shown that neurons intrinsic to central pattern-generating circuits can evoke neuromodulatory effects in addition to their neurotransmitting actions. Modulatory neurons often elicit a multitude of different effects attributable to actions at different receptors and/or through the release of co-transmitters. Differences in neuromodulation between species can account for differences in behavior. Modulation of neuromodulation may provide an additional level of flexibility to motor circuits.