No abstract

The orcokinins are a family of neuropeptides recently isolated from several crustacean species. We found orcokinin-like immunoreactivity in the stomatogastric nervous systems and pericardial organs of three decapod crustacean species, Homarus americanus, Cancer borealis, and Panulirus interruptus. The neuropil of the stomatogastric ganglion was stained in adults of all three species as well as in embryonic and larval H. americanus. In H. americanus, the somata giving rise to this projection were found in the inferior ventricular nerve. Matrix-assisted laser desorption/ionization mass spectrometry mass profiling and sequencing with postsource decay led to the identification of six different orcokinin family peptides, including those previously described in other decapods and two novel shorter peptides. Application of exogenous [Ala(13)]orcokinin to the stomatogastric ganglion of H. americanus resulted in changes in the pyloric rhythm. Specifically, the number of lateral pyloric (LP) neuron spikes/burst decreased, and the phase of firing of the pyloric neurons was altered. Together, these data indicate that the orcokinins are likely to function as modulators of the crustacean stomatogastric ganglion.

We studied the effect of synaptic inputs of different amplitude and duration on neural oscillators by simulating synaptic conductance pulses in a bursting conductance-based pacemaker model and by injecting artificial synaptic conductance pulses into pyloric pacemaker neurons of the lobster stomatogastric ganglion using the dynamic clamp. In the model and the biological neuron, the change in burst period caused by inhibitory and excitatory inputs of increasing strength saturated, such that synaptic inputs above a certain strength all had the same effect on the firing pattern of the oscillatory neuron. In contrast, increasing the duration of the synaptic conductance pulses always led to changes in the burst period, indicating that neural oscillators are sensitive to changes in the duration of synaptic input but are not sensitive to changes in the strength of synaptic inputs above a certain conductance. This saturation of the response to progressively stronger synaptic inputs occurs not only in bursting neurons but also in tonically spiking neurons. We identified inward currents at hyperpolarized potentials as the cause of the saturation in the model neuron. Our findings imply that activity-dependent or modulator-induced changes in synaptic strength are not necessarily accompanied by changes in the functional impact of a synapse on the timing of postsynaptic spikes or bursts.

Purkinje neurons are central to cerebellar function and show membrane bistability when recorded in vitro or in vivo under anesthesia. The existence of bistability in vivo in awake animals is disputed. Here, by recording intracellularly from Purkinje neurons in unanesthetized larval zebrafish (Danio rerio), we unequivocally demonstrate bistability in these neurons. Tonic firing was seen in depolarized regimes and bursting at hyperpolarized membrane potentials. In addition, Purkinje neurons could switch from one state to another spontaneously or with current injection. While GABAAR or NMDAR were not required for bursting, activation of AMPARs by climbing fibers (CFs) was sufficient to trigger bursts. Further, by recording Purkinje neuron membrane potential intracellularly, and motor neuron spikes extracellularly, we show that initiation of motor neuron spiking is correlated with increased incidence of CF EPSPs and membrane depolarization. Developmentally, bistability was observed soon after Purkinje neuron specification and persists at least until late larval stages.DOI: http://dx.doi.org/10.7554/eLife.09158.001

Dopamine is a key neuromodulator of locomotory circuits, yet the role that dopamine plays during development of these circuits is less well understood. Here, we describe a suppressive effect of dopamine on swim circuits in larval zebrafish. Zebrafish larvae exhibit marked changes in swimming behavior between 3 days postfertilization (dpf) and 5dpf. We found that swim episodes were fewer and of longer durations at 3 than at 5dpf. At 3dpf, application of dopamine as well as bupropion, a dopamine reuptake blocker, abolished spontaneous fictive swim episodes. Blocking D2 receptors increased frequency of occurrence of episodes and activation of adenylyl cyclase, a downstream target inhibited by D2-receptor signaling, blocked the inhibitory effect of dopamine. Dopamine had no effect on motor neuron firing properties, input impedance, resting membrane potential, or the amplitude of spike afterhyperpolarization. Application of dopamine either to the isolated spinal cord or locally within the cord does not decrease episode frequency, whereas dopamine application to the brain silences episodes, suggesting a supraspinal locus of dopaminergic action. Treating larvae with 10 microM MPTP reduced catecholaminergic innervation in the brain and increased episode frequency. These data indicate that dopamine inhibits the initiation of fictive swimming episodes at 3dpf. We found that at 5dpf, exogenously applied dopamine inhibits swim episodes, yet the dopamine reuptake blocker or the D2-receptor antagonist have no effect on episode frequency. These results led us to propose that endogenous dopamine release transiently suppresses swim circuits in developing zebrafish.

We studied the effects of dopamine on the stomatogastric ganglion (STG) of the lobster, Homarus americanus. The two pyloric dilator (PD) neurons are active in the pyloric rhythm, have somata in the STG, and send axons many centimeters to innervate muscles of the stomach. Dopamine application to the stomatogastric nervous system when the PD neurons were rhythmically active evoked additional action potentials during the PD neuron interburst intervals. These action potentials were peripherally generated at a region between the STG and the first bilateral branch, approximately 1 cm away from the STG, and traveled antidromically to the neuropil and orthodromically to the pyloric dilator muscles. Focal applications of dopamine to the nerves showed that spikes could be initiated in almost the entire peripheral axon of the PD neurons. Dopamine also evoked spikes in isolated peripheral axons. The concentration threshold for peripheral spike initiation was at or below 10-9 m dopamine. Thus, the peripheral axon can play an important role in shaping the output signaling to the muscles by the motor neuron.

We studied the developmental acquisition of three of the cotransmitters found in the gastropyloric receptor (GPR) neurons of the stomatogastric nervous systems of the lobsters Homarus americanus and Homarus gammarus. By using wholemount immunocytochemistry and confocal microscopy, we examined the distribution of serotonin‐like, allatostatin‐like, and FLRFNH2‐like immunoreactivities within the stomatogastric nervous system of embryonic, larval, juvenile, and adult animals. The GPR neurons are peripheral sensory neurons that send proprioceptive information to the stomatogastric and commissural ganglia. In H. americanus, GPR neurons of the adult contain serotonin‐like, allatostatin‐like, and Phe‐Leu‐Arg‐Phe‐amide (FLRFNH2)‐like immunoreactivities. In the stomatogastric ganglion (STG) of the adult H. americanus and H. gammarus, all of the serotonin‐like and allatostatin‐like immunoreactivity colocalizes in neuropil processes that are derived exclusively from ramifications of the GPR neurons. In both species, FLRFNH2‐like immunoreactivity was detected in the STG neuropil by 50% of embryonic development (E50). Allatostatin‐like immunoreactivity was visible first in the STG at approximately E70–E80. In contrast, serotonin staining was not clearly visible until larval stage I (LI) in H. gammarus and until LII or LIII in H. americanus. These data indicate that there is a sequential acquisition of the cotransmitters of the GPR neurons. J. Comp. Neurol. 408:318–334, 1999. © 1999 Wiley‐Liss, Inc.

The neuropeptide, red pigment concentrating hormone (RPCH), strengthened the inhibitory synapse from the lateral pyloric (LP) neuron to the pyloric dilator (PD) neurons in the pyloric network of the stomatogastric ganglion (STG) of the lobster, Homarus americanus. RPCH produced several-fold increases in the amplitude of both action potential-mediated and non-impulse-mediated transmission that persisted for as long as the peptide remained present. Because the LP to PD synapse is the only feedback to the pacemaker kernel of the pyloric network, which consists of the electrically coupled two PD neurons and the anterior burster (AB) neuron, it might have been expected that strengthening the LP to PD synapse would increase the period of the pyloric rhythm. However, the period of the pyloric rhythm increased only transiently in RPCH, and a transient increase in cycle period was observed even when the LP neuron was hyperpolarized. Phase response curves were measured using the dynamic clamp to create artificial inhibitory inputs of variable strength and duration to the PD neurons. Synaptic conductance values seen in normal saline were ineffective at changing the pyloric period throughout the pyloric cycle. Conductances similar to those seen in 10(-6) M RPCH also did not evoke phase resets at phases when the LP neuron is typically active. Thus the dramatic effects of RPCH on synaptic strength have little role in modulation of the period of the pyloric rhythm under normal operating conditions but may help to stabilize the rhythm when the cycle period is too slow or too fast.