Secondary hair cells and afferent neurones of the squid statocyst receive both inhibitory and excitatory efferent inputs

Microtransplantation of cellular membranes from squid stellate ganglion reveals ionotropic GABA receptors

The squid has been the most studied cephalopod, and it has served as a very useful model for investigating the events associated with nerve impulse generation and synaptic transmission. While the physiology of squid giant axons has been extensively studied, very little is known about the distribution and function of the neurotransmitters and receptors that mediate inhibitory transmission at the synapses. In this study we investigated whether γ-aminobutyric acid (GABA) activates neurotransmitter receptors in stellate ganglia membranes. To overcome the low abundance of GABA-like mRNAs in invertebrates and the low expression of GABA in cephalopods, we used a two-electrode voltage clamp technique to determine if Xenopus laevis oocytes injected with cell membranes from squid stellate ganglia responded to GABA. Using this method, membrane patches containing proteins and ion channels from the squid's stellate ganglion were incorporated into the surface of oocytes. We demonstrated that GABA activates membrane receptors in cellular membranes isolated from squid stellate ganglia. Using the same approach, we were able to record native glutamate-evoked currents. The squid's GABA receptors showed an EC(50) of 98 μmol l(-1) to GABA and were inhibited by zinc (IC(50) = 356 μmol l(-1)). Interestingly, GABA receptors from the squid were only partially blocked by bicuculline. These results indicate that the microtransplantation of native cell membranes is useful to identify and characterize scarce membrane proteins. Moreover, our data also support the role of GABA as an ionotropic neurotransmitter in cephalopods, acting through chloride-permeable membrane receptors.

Pre- and postsynaptic excitation and inhibition at octopus optic lobe photoreceptor terminals; implications for the function of the 'presynaptic bags'

Synaptic transmission was examined in the plexiform zone of Octopus vulgaris optic lobes using field-potential recording from optic lobe slices. Stimulation of the optic nerve produced pre- and postsynaptic field potentials. Transmission was abolished in calcium-free seawater, L- glutamate or the AMPA/Kainate receptor blocker CNQX (EC(50), 40 microm), leaving an intact presynaptic field potential. ACh markedly reduced or blocked and d-tubocurarine augmented both pre- and postsynaptic field potentials, while alpha-bungarotoxin and atropine were without effect. Paired-pulse stimulation showed short-term depression of pre- and postsynaptic components with a half-time of recovery of approximately 500 ms. The depression was partially relieved in the presence of d-tubocurarine (half-time of recovery, 350 ms). No long-term changes in synaptic strength were induced by repetitive stimulation. A polyclonal antibody raised against a squid glutamate receptor produced positive staining in the third radial layer of the plexiform zone. No positive staining was observed in the other layers. Taking into account previous morphological data and our results, we propose that the excitatory terminations of the photoreceptors are in the innermost layer of the plexiform zone where the transmitter is likely to be glutamate and postsynaptic receptors are AMPA/kainate-like. Thus, the function of the terminal bags is to provide a location for a presynaptic cholinergic inhibitory shunt. The results imply that this arrangement provides a temporal filter for visual processing and enhances the perception of moving vs. stationary objects.

A model biological neural network: the cephalopod vestibular system

Artificial neural networks (ANNs) have become increasingly sophisticated and are widely used for the extraction of patterns or meaning from complicated or imprecise datasets. At the same time, our knowledge of the biological systems that inspired these ANNs has also progressed and a range of model systems are emerging where there is detailed information not only on the architecture and components of the system but also on their ontogeny, plasticity and the adaptive characteristics of their interconnections. We describe here a biological neural network contained in the cephalopod statocysts; the statocysts are analogous to the vertebrae vestibular system and provide the animal with sensory information on its orientation and movements in space. The statocyst network comprises only a small number of cells, made up of just three classes of neurons but, in combination with the large efferent innervation from the brain, forms an 'active' sense organs that uses feedback and feed-forward mechanisms to alter and dynamically modulate the activity within cells and how the various components are interconnected. The neurons are fully accessible to physiological investigation and the system provides an excellent model for describing the mechanisms underlying the operation of a sophisticated neural network.

Physiological changes of premotor nonspiking interneurons in the central compensation of eyestalk posture following unilateral sensory ablation in crayfish

We investigated how the physiological characteristics and synaptic activities of nonspiking giant interneurons (NGIs), which integrate sensory inputs in the brain and send synaptic outputs to oculomotor neurons innervating eyestalk muscles, changed after unilateral ablation of the statocyst in order to clarify neuronal mechanisms underlying the central compensation process in crayfish. The input resistance and membrane time constant in recovered animals that restored the original symmetrical eyestalk posture 2 weeks after operation were significantly greater than those immediately after operation on the operated side whereas in non-recovered animals only the membrane time constant showed a significant increase. On the intact side, both recovered and non-recovered animals showed no difference. The frequency of synaptic activity showed a complex pattern of change on both sides depending on the polarity of the synaptic potential. The synaptic activity returned to the bilaterally symmetrical level in recovered animals while bilateral asymmetry remained in non-recovered ones. These results suggest that the central compensation of eyestalk posture following unilateral impairment of the statocyst is subserved by not only changes in the physiological characteristics of the NGI membrane but also the activity of neuronal circuits presynaptic to NGIs.

Synaptic interactions between crista hair cells in the statocyst of the squid Alloteuthis subulata

Intracellular injections of the fluorescent dye Lucifer yellow into the various cell types within the anterior transverse crista segment of the statocyst of squid revealed that the primary sensory hair cells and both large and small first-order afferent neurons have relatively simple morphologies, each cell having a single, unbranched axon that passes directly into the small crista nerve that innervates the anterior transverse crista. However, the small first-order neurons have short dendritic processes occurring in the region of the sensory hair cells. The secondary sensory hair cells have no centripetal axons, but some have long processes extending from their bases along the segment. Simultaneous intracellular recordings from pairs of the different cell types in the anterior transverse crista segment demonstrated that electrical coupling is widespread; secondary sensory hair cells are coupled electrically along a hair cell row, as are groups of primary sensory hair cells. Secondary sensory hair cell also are coupled to neighboring small first-order afferent neurons. However, this coupling is rectifying in that it only occurs from secondary sensory hair cells to first-order afferent neurons. Direct electrical stimulation of the small crista nerve to excite the efferent axons revealed efferent connections to both the primary sensory hair cells and the small first-order afferent neurons. These efferent responses were of three types: excitatory or inhibitory postsynaptic potentials and excitatory postsynaptic potentials followed by inhibitory postsynaptic potentials. The functional significance of the cell interactions within the crista epithelium of the statocyst of squid is discussed and comparisons drawn with the balance organs of other animals.

Structure and function of the Nautilus statocyst

The two equilibrium receptor organs (statocysts) of Nautilus are avoid sacks, half-filled with numerous small, free-moving statoconia and half with endolymph. The inner surface of each statocyst is lined with 130,000-150,000 primary sensory hair cells. The hair cells are of two morphological types. Type A hair cells carry 10-15 kinocilia arranged in a single ciliary row; they are present in the ventral half of the statocyst. Type B hair cells carry 8-10 irregularly arranged kinocilia; they are present in the dorsal half of the statocyst. Both type of hair cells are morphologically polarized. To test whether these features allow the Nautilus statocyst to sense angular accelerations, behavioural experiments were performed to measure statocyst-dependent funnel movements during sinusoidal oscillations of restrained Nautilus around a vertical body axis. Such dynamic rotatory stimulation caused horizontal phase-locked movements of the funnel. The funnel movements were either in the same direction (compensatory funnel response), or in the opposite direction (funnel follow response) to that of the applied rotation. Compensatory funnel movements were also seen during optokinetic stimulation (with a black and white stripe pattern) and during stimulations in which optokinetic and statocyst stimulations were combined. These morphological and behavioural findings show that the statocysts of Nautilus, in addition to their function as gravity receptor organs, are able to detect rotatory movements (angular accelerations) without the specialized receptor systems (crista/cupula systems) that are found in the statocysts of coleoid cephalopods. The findings further indicate that both statocyst and visual inputs control compensatory funnel movements.

The effect of L-glutamate on the afferent resting activity in the cephalopod statocyst

The effects of bath application of L-glutamate and of excitatory amino acid agonists and antagonists on the resting activity of afferent crista fibers were studied in isolated preparations of the statocyst of the cuttlefish, Sepia officinalis. L-Glutamate (threshold 10(-5) M) and its agonists quisqualate and kainate (thresholds 10(-6) M) increased the resting activity in a dose-dependent manner. Glutamine (threshold 10(-5) M) was also excitatory, while D-glutamate had no effect. Also, no obvious excitatory effects were seen for NMDA and L-aspartate, nor was any antagonistic effect seen for the selective NMDA-receptor antagonist D-2-amino-5-phosphonovaleric acid (D-AP-5). The spider toxin Argiotoxin636 (threshold 10(-11) M), 2-amino-4-phosphonobutyric acid (AP-4), glutamic acid diethyl ester (GDEE), gamma-D-glutamylaminomethyl-sulfonic acid (GAMS), and kynurenic acid decreased the resting activity and effectively blocked or reversed the effect of L-glutamate and its non-NMDA agonists. Preliminary experiments with statocysts from the squid Sepioteuthis lessoniana and the octopod Octopus bimaculoides gave comparable results. All data show that in cephalopod statocysts L-glutamate, via non-NMDA receptors, has an excitatory effect on the activity of afferent fibers, an effect consistent with its possible function as a hair cell transmitter.

Efferent control of posterior canal afferent receptor discharge in the frog labyrinth

EPSP and spike discharges were intracellularly recorded from 90 afferent fibres of the posterior nerve in the isolated frog labyrinth and the effects of electrical activation of the efferent system were tested. Posterior canal efferent synapses were activated, via an axon reflex, by electrical shocks to the anterior-horizontal nerves. The afferent resting discharge of all fibres tested was affected by efferent stimulation: 39 units were inhibited (43%) and 51 (57%) were facilitated. The efferent system was activated with several stimulation frequencies in the 10-200 Hz range applied for different times (250 ms-10 s). By changing the stimulus parameters, inhibition did not reverse to facilitation or vice versa. Facilitation appeared within the train above a threshold frequency of about 10 Hz. The peak response was readily reached within the first second and then, with long lasting stimulation, a marked adaptation ensued. The increase in firing rate was independent of previous resting activity. The relationship between frequency and facilitatory response is described by a logarithmic function over the 30-200 Hz range tested. At the end of short trains a consistent post-stimulation after-discharge appeared, whose intensity is positively related to stimulation frequency and time. Inhibition was achieved at stimulation rates above 10 Hz, and a post-stimulation rebound discharge was evident, which was linearly dependent on previous stimulation rate. The latency values of both inhibitory and facilitatory effects were measured by taking into account either the EPSP release rate or the spike discharge modifications at all the frequencies tested. Latency proved to decrease exponentially with increasing stimulation time from a minimal value of 3 ms to a maximum of 200 ms, with minor differences between inhibition or facilitation. These long latency values, the presence of a threshold frequency and the stimulus- and frequency dependence indicate that the efferent synapses must be activated repetitively to produce detectable effects on the afferent discharge; this is in line with the discharge pattern of the efferent system fibres physiologically measured in some systems. The present results show that the dual central control of the crista ampullaris of frog posterior canal is potentially capable of setting the receptor population at a variable level of sensitivity to mechanical stimuli, with profound modifications in the canal transfer function and the spike encoding mechanism.

Convergent inputs to Octopus oculomotor neurones demonstrated in a brain slice preparation

A brain slice preparation was developed to permit recordings to be made from neurones in the anterior lateral pedal lobe of the octopus CNS. These are the first intracellular recordings obtained from identified neurones in the cephalopod CNS. The neurones had membrane resting potentials of around -40 mV and action potentials up to 20 mV in amplitude. Spontaneous synaptic potentials could be seen in some cells. All of the oculomotor neurones, identified by stimulation of the anterior oculomotor nerve to evoke an antidromic potential, received an excitatory input from the visual system, shown by electrical stimulation of the optic tract, and some also received an excitatory input from the statocyst, as shown by electrical stimulation of the macula nerve.

Sensory evoked potentials in unanesthetized unrestrained cuttlefish: a new preparation for brain physiology in cephalopods

Up to five microelectrodes inserted through short hypodermic needles in the cranial cartilage of Sepia officinalis recorded potentials while the cuttlefish moved freely in a small enclosure. Compound field potentials and unit spikes were seen during ongoing, spontaneous activity and after sensory stimulation. Ongoing activity resembles that reported for octopus, with maximum power usually below 20 Hz. Amplitude varies greatly but has not been seen to shut off or turn on abruptly and globally as in octopus. Evoked potentials, focally large after flashes of light consist of several waves; the first is largest, positive and peaks at ca. 35 ms (called P35), followed by ca. P75, P95, N110 and smaller waves or oscillations lasting more than 0.5 s. The Upper Following Frequency (highest flashing rate the potentials can follow 1:1), without averaging, is greater than 15 flashes/s (20-22 degrees C); at 20/s the 1:1 following lasts for 1 or 2 s. The Lower Fusion Frequency of averaged responses is less than 30/s. Gentle tapping of the tank wall evokes local, brief, fast potentials. No responses have been found to loud air-borne clicks and tone bursts with principal energy at 300 Hz or to electric fields in the bath at 50-100 microV/cm. In a few loci relatively large slow Omitted Stimulus Potentials have been seen following the end of a train of flashes at more than 5/s; these are by definition event related potentials and a special, central form of OFF response.