The crustacean stomatogastric ganglion (STG) is modulated by numerous neuropeptides that are released locally in the neuropil or that reach the STG as neurohormones. Using 1,5-diaminonaphthalene (DAN) as a reductive screening matrix for matrix-assisted laser desorption/ ionization (MALDI) mass spectrometric profiling of disulfide bond-containing C-type allatostatin peptides followed by electrospray ionization quadrupole time-of-flight (ESI-Q-TOF) tandem mass spectrometric (MS/MS) analysis, we identified and sequenced a novel C-type allatostatin peptide (CbAST-C1), pQIRYHQCYFNPISCF-COOH, present in the pericardial organs of the crab, Cancer borealis. Another C-type allatostatin (CbAST-C2), SYWKQCAFNAVSCFamide, was discovered using the expressed sequence tag (EST) database search strategy in both C. borealis and the lobster, Homarus americanus, and further confirmed with de novo sequencing using ESI-Q-TOF tandem MS. Electrophysiological experiments demonstrated that both CbAST-C1 and CbAST-C2 inhibited the frequency of the pyloric rhythm of the STG, in a state-dependent manner. At 10 −6 M, both peptides were only modestly effective when initial frequencies of the pyloric rhythm were >0.8 Hz, but almost completely suppressed the pyloric rhythm when applied to preparations with starting frequencies < 0.7Hz. Surprisingly, these state-dependent actions are similar to those of the structurally unrelated allatostatin A and allatostatin B families of peptides.

To determine the sensory information encoded and the relative contributions of these two transmission modes, M-cell responses to acoustic stimuli in air were recorded intracellularly. Excitatory postsynaptic potentials (EPSPs) evoked by both short 100-to 900-Hz "pips" and longer-lasting amplitude-and frequency-modulated sounds were dominated by fast, repetitive EPSPs superimposed on an underlying slow depolarization. Fast EPSPs 1) have kinetics comparable to presynaptic action potentials, 2) are maximal on the distal lateral dendrite, and 3) are insensitive to GluR antagonists. They presumably are coupling potentials, and power spectral analysis indicated they constitute a high-pass signal that accurately tracks sound frequency and amplitude. The spatial profile of the slow EPSP suggests both proximal and distal dendritic sources, a result supported by predictions of a multicompartmental model and the effects of AMPAR antagonists, which preferentially reduced the proximal component. Thus a second class of afferents generates a portion of the slow EPSP that, with sound stimuli, demonstrate that the dominant mode of transmission at LMCE synapses is electrical. The slow EPSP is a dynamic, low-pass representation of stimulus strength. Accordingly, amplitude and phase information, which are segregated in other systems, are faithfully represented in the M-cell.

In this study, we address the impact of temperature acclimation on neuronal properties in the Mauthner (M-) system, a brain stem network that initiates the startle-escape behavior in goldfish. The M-cell can be studied at cellular and behavioral levels, since it is uniquely identifiable physiologically within the intact vertebrate brain, and a single action potential in this neuron determines not only whether a startle response will occur but also the direction of the escape. Using animals acclimated to 15 degrees C as a control, 25 degrees C-acclimated fish showed a significant increase in escape probability and a decrease in the ability to discriminate escape directionality. Intracellular recordings demonstrated that M-cells in this population possessed decreased input resistance and reduced strength and duration of inhibitory inputs. In contrast, fish acclimated to 5 degrees C were behaviorally similar to 15 degrees C fish and had increased input resistance, increased strength of inhibitory transmission, and reduced excitatory transmission. We show here that alterations in the balance between excitatory and inhibitory synaptic transmission in the M-cell circuit underlie differences in behavioral responsiveness in acclimated populations. Specifically, during warm acclimation, synaptic inputs are weighted on the side of excitation and fish demonstrate hyperexcitability and reduced left-right discrimination during rapid escapes. In contrast, cold acclimation results in transmission weighted on the side of inhibition and these fish are less excitable and show improved directional discrimination.

Prepulse inhibition (PPI) is typically associated with an attenuation of auditory startle behavior in mammals and is presumably mediated within the brainstem startle circuit. However, the inhibitory mechanisms underlying PPI are not yet clear. We addressed this question with complementary behavioral and in vivo electrophysiological experiments in the startle escape circuit of goldfish, the Mauthner cell (M-cell) system. In the behavioral experiments we observed a 77.5% attenuation (PPI) of startle escape probability following auditory prepulse-pulse stimulation. The PPI effect was observed for prepulse-pulse interstimulus intervals (ISIs) ranging from 20 to 600 ms and its magnitude depended linearly on prepulse intensity over a range of 14 dB. Electrophysiological recordings of synaptic responses to a sound pulse in the M-cell, which is the sensorimotor neuron initiating startle escapes, showed a 21% reduction in amplitude of the dendritic postsynaptic potential (PSP) and a 23% reduction of the somatic PSP following a prepulse. In addition, a prepulse evoked a long-lasting (500 ms) decrease in M-cell excitability indicated by 1) an increased threshold current, 2) an inhibitory shunt of the action potential (AP), and 3) by a linearized M-cell membrane, which effectively impedes M-cell AP generation. Comparing the magnitude and kinetics of inhibitory shunts evoked by a prepulse in the M-cell dendrite and soma revealed a disproportionately larger and longer-lasting inhibition in the dendrite. These results suggest that the observed PPI-type attenuation of startle behavior can be correlated to distinct postsynaptic mechanisms mediated primarily at the M-cell lateral dendrite.