Action of furosemide on GABA- and glycine currents in rat septal cholinergic neurons in culture

Modulation of glycine receptor function: a novel approach for therapeutic intervention at inhibitory synapses?

Transmitter-gated ion channels mediate rapid synaptic transmission in the CNS and constitute important targets for many neuroactive drugs. Inhibitory glycine receptors (GlyRs) are members of the nicotinic acetylcholine receptor superfamily and inhibit neuronal firing by opening Cl(-) channels following agonist binding. In this article, we discuss recent developments in GlyR pharmacology, delineate the receptor domains that are involved in binding of agonists and allosteric modulators, and present a molecular model of the extracellular architecture of the receptor. The recent discovery of compounds that act preferentially on specific GlyR isoforms and the differential expression of these isoforms in distinct regions of the developing and adult CNS show considerable promise towards the development of drugs that act in defined glycine-mediated pathways. In particular, compounds that can potentiate GlyR function should provide leads for novel muscle relaxants in addition to sedative and analgesic agents.

Mechanisms of GABA(A) receptor blockade by millimolar concentrations of furosemide in isolated rat Purkinje cells

The action of diuretic furosemide on the GABA(A) receptor was studied in acutely isolated Purkinje cells using the whole-cell recording and fast application system. Furosemide blocked stationary component of GABA-activated currents in a concentration-dependent manner with IC(50) value > 5 mM at -70 mV. The inhibition was rapid in the onset, fully reversible and did not require drug pre-perfusion. The termination of GABA and furosemide co-application was followed by transient increase in the inward current 'tail' current, which was not observed when furosemide was continuously present in the solution. The degree of furosemide block did not depend on GABA concentration. Furosemide block increased with membrane depolarization. Five millimolar furosemide depressed GABA currents by 32.4+/-1.3% at -70 mV and by 76.7+/-5.0% at +70 mV. Analysis of the voltage dependence of the block suggests that furosemide binds at the site located within GABA(A) channel pore with a dissociation constant of 5.3+/-0.5 mM at 0 mV and electric distance of 0.27. Our results provide evidence that furosemide interacts with Purkinje cell GABA(A) receptors (most probably composed of alpha1beta2/3gamma2 subunits) through a low affinity site located in channel pore and suggest that furosemide acts as a sequential open channel blocker, which prevents the dissociation of agonist while the channel is blocked.

Shift from depolarizing to hyperpolarizing glycine action in rat auditory neurones is due to age-dependent Cl- regulation

1. The inhibitory neurotransmitter glycine can elicit depolarizing responses in immature neurones. We investigated the changes in glycine responses and their ionic mechanism in developing neurones of the rat lateral superior olive (LSO), an auditory brainstem nucleus involved in sound localization. 2. Whole-cell and gramicidin perforated-patch recordings were performed from visually identified LSO neurones in brain slices and glycine was pressure applied for 3-100 ms to the soma. Glycine-evoked currents were reversibly blocked by strychnine. They were mostly monophasic, but biphasic responses occurred in approximately 30 % of P8-11 neurones in perforated-patch recordings. 3. In whole-cell recordings from P2-11 neurones, the reversal potential of glycine-evoked currents (EGly) was determined by the transmembranous Cl- gradient and corresponded closely to the Nernst potential for Cl-, regardless of age. This indicates that Cl- is the principle ion permeating glycine receptors, but is also consistent with a low relative (10-20 %) permeability for HCO3-. The Cl- gradient also determined the polarity and amplitude of glycine-evoked membrane potential changes. 4. Leaving the native intracellular [Cl-] undisturbed with gramicidin perforated-patch recordings, we found a highly significant, age-dependent change of EGly from -46.8 +/- 1.8 mV (P1-4, n = 28) to -67.6 +/- 3.3 mV (P5-8, n = 10) to -82.2 +/- 4.1 mV (P9-11, n = 18). The majority of P1-4 neurones were depolarized by glycine ( approximately 80 %) and spikes were evoked in approximately 30 %. In contrast, P9-11 neurones were hyperpolarized. 5. In perforated-patch recordings, EGly was influenced by the voltage protocol and the glycine application interval; it could be shifted in the positive and negative direction. For a given application interval, these shifts were always larger in P1-4 than in P8-11 neurones, pointing to less effective Cl- regulation mechanisms in younger neurones. 6. Furosemide (frusemide), a blocker of cation-Cl- cotransporters, reversibly shifted EGly in the negative direction in P2-4 neurones, yet in the positive direction in P8-10 neurones, suggesting the blockade of net inward and net outward Cl- transporters, respectively. 7. Taken together, age-dependent changes in active Cl- regulation are likely to cause the developmental shift from depolarizing to hyperpolarizing glycine responses. A high intracellular [Cl-] is generated in neonatal LSO neurones which decreases during maturation.