David L. Tauck

Characterization of GABA- and glycine-induced currents of solitary rodent retinal ganglion cells in culture

Ganglion cells were fluorescently labeled, dissociated from 7- to 11-day-old rodent retinas, and placed in tissue culture. Whole-cell recordings with patch electrodes were obtained from solitary cells lacking processes, which permitted a high-quality space clamp. Both GABA (1-200 microM) and glycine (10-300 microM) produced large increases in membrane conductance in virtually every ganglion cell tested, including ganglion cells from different size classes in both rats and mice. Taurine evoked responses similar to those of glycine, but considerably greater concentrations of taurine (150-300 microM) were necessary to observe any effect. Since 20 microM GABA produced approximately the same response as 100 microM glycine, the effects of these two concentrations were compared under various conditions. When recording with chloride distributed equally across the membrane, the reversal potential of the agonist-induced currents was approximately 0 mV. When the internal chloride was reduced by substitution with aspartate, the reversal potential shifted in a negative direction by about 42 mV, indicating that the current was carried mainly by chloride ions. Strychnine (1-5 microM) completely and reversibly blocked the actions of glycine (100 microM) but not those of GABA (20 microM); however, higher concentrations of strychnine (20 microM) nearly totally inhibited the current elicited by GABA (20 microM). The responses to glycine (100 microM) were not affected by bicuculline methiodide (20 microM) or picrotoxinin (20 microM). In contrast, bicuculline methiodide (10 microM) and picrotoxinin (10 microM) reversibly blocked the current evoked by GABA (20 microM); d-tubocurarine (100 microM) only slightly decreased the response to GABA (20 microM). The antagonists were effective over a wide range of holding potentials (-90 mV to +30 mV). The responses to a steady application of both GABA and glycine decayed in a few seconds when recorded under conditions of both symmetric and asymmetric chloride across the membrane. During this decay the current and conductance decreased simultaneously, reflecting receptor desensitization rather than a change in the driving force for chloride caused by agonist-induced ionic fluxes. The time-course of desensitization was usually described by a single exponential with time constants for GABA (20 microM) and glycine (100 microM) of 4.0 +/- 1.6 s and 4.4 +/- 1.9 s (mean +/- S.D.), respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuromodulation in invertebrate sensory systems: from biophysics to behavior

SUMMARY Neuromodulation may enhance the ability of sensory circuits to respond appropriately to widely variable environmental stimuli. The functional significance of neuromodulation will emerge from understanding the effects of modulators not just on single cells and synapses, but also on networks and the behavior of intact animals. With their relatively simple circuitry and large identifiable cells, invertebrate nervous systems offer insights into the complex roles of neuromodulators in modifying networks to meet the changing needs of the animal. Here we describe the role of neuromodulation in several invertebrate sensory systems that have been studied at a variety of levels, from the biophysical up to the behavioral.

Glycine synergistically potentiates the enhancement of LTP induced by a sulfhydryl reducing agent

Two selective modulators of N-methyl-D-aspartate (NMDA) receptor function, dithiothreitol (DTT) and glycine, each dramatically enhanced long-term potentiation (LTP) in area CA1 of the hippocampus slice. Glycine synergistically potentiated the effect of DTT. Kynurenate, but not strychnine, antagonized the modulatory effect of glycine on LTP, while 2-amino-5-phosphonovalerate blocked LTP in all cases. Neither oxidation with 5-5-dithio-bis-2-nitrobenzoic acid nor exposure to the oxidized form of DTT had any effect on LTP. These data suggest that in vivo the reducing potential of local environments may interact with endogenous glycine to regulate NMDA receptor function.

Ethanol as a general anesthetic: actions in spinal cord.

Ethanol, usually studied in relation to intoxication, is also capable of producing general anesthesia. The most common standard of anesthetic potency is the concentration which produces immobility in response to a noxious stimulus. This concentration will be referred to as the anesthetic concentration. Immobilization is a spinal effect. Ethanol effects were studied in spinal cord from 2-7-day-old rats at concentrations which included the anesthetic concentration in both adult rats (97 mM) and 6-7-day-old rats (235 mM). At neonatal but not adult anesthetic concentrations, ethanol depressed monosynaptic reflex amplitude (mediated by glutamate AMPA receptors + compound action potential). At both neonatal and adult anesthetic concentrations ethanol reversibly depressed the population excitatory postsynaptic potential (pEPSP) (glutamate AMPA and NMDA receptors), the slow ventral root potential (NMDA + metabotropic receptors), and the dorsal root potential (GABA(A) receptors, via glutamate-excited interneurons). Effects were greater on NMDA receptor-mediated components than on AMPA-receptor-mediated components of the pEPSP and greater on NMDA than on metabotropic receptor-mediated components of the slow ventral root potential. The profile of ethanol effects on spinal cord resembles that of inhalation general anesthetics. The results show that both AMPA and NMDA receptor-mediated transmission are sensitive to ethanol and that enhancement of GABAergic neurotransmission is overridden by depression of excitation to the interneurons. They provide no obvious explanation for ethanols lower general anesthetic potency in the neonate.

Redox modulation of NMDA receptor-mediated synaptic activity in the hippocampus.

The sulfhydryl reducing agent dithiothreitol (DTT) and the oxidizing agent 5,5-dithio-bis-2-nitrobenzoic acid (DTNB) reversibly modulate the component of synaptic potentials mediated by N-methyl-D-aspartate (NMDA) receptors in slices of hippocampal area CA1. DTT (1 mM) reversibly potentiates NMDA receptor-mediated synaptic potentials while DTNB (1 mM) has the opposite effect. However, treatment of the slices with the irreversible sulfhydryl alkylating agent N-ethylmaleimide (300 microM) prevents DTNB from reversing the potentiation induced by DTT. These results further implicate the redox modulatory site as a regulator of the NMDA receptor-channel complex in vivo.

Lymnaea stagnalis and the development of neuroelectronic technologies

The recent development of techniques for stimulating and recording from individual neurons grown on semiconductor chips has ushered in a new era in the field of neuroelectronics. Using this approach to construct complex neural circuits on silicon from individual neurons will require improvements at the neuron/semiconductor interface and advances in controlling synaptogenesis. Although devices incorporating vertebrate neurons may be an ultimate goal, initial investigations using neurons from the pond snail Lymnaea stagnalis have distinct advantages. Simple two‐cell networks connected by electrical synapses have already been reconstructed on semiconductor chips. Furthermore, considerable progress has been made in controlling the processes that underlie chemical synapse formation in Lymnaea. Studies of Lymnaea neural networks on silicon chips will lead to a deeper understanding of the long‐term dynamics of simple neural circuits and may provide the basis for reliable interfaces for new neuroprosthetic devices.