Effect of Bicuculline on Thalamic Activity: A Direct Blockade of I _AHP in Reticularis Neurons

Abstract

The thalamic reticular nucleus (RTN) is the major source of inhibitory contacts in the thalamus and thus plays an important role in regulating the excitability of the thalamocortical network. Inhibition occurs through GABAergic synapses on relay cells as well as through GABAergic synapses between reticularis neurons themselves. Here we report that the role and mechanisms of this inhibition, which frequently have been studied using N-methyl derivatives of the gamma-aminobutyric acid-A (GABAA) receptor antagonist bicuculline, should be revisited. Using the whole cell patch-clamp technique in thalamic slices from young rats, we observed an enhancement by bicuculline methiodide, methobromide, and methochloride (collectively referred to as bicuculline-M; 5-60 microM) of the low-threshold calcium spike burst in RTN neurons that persisted in the presence of tetrodotoxin (1 microM) and was not reproduced in picrotoxin (100-300 microM). The effect did not involve activation of any GABA receptor subtype. Voltage-clamp recordings showed that bicuculline-M blocked the current underlying the low-threshold spike burst afterhyperpolarization (AHP), an effect that was mimicked by apamin (100 nM). Recordings from nucleated patches extracted from reticularis neurons demonstrated that this effect was not mediated by modulation of the release of an unidentified neurotransmitter but that bicuculline-M directly blocks small conductance (SK) channels. The AHP-blocking effect also was observed in other brain regions, demonstrating that although bicuculline-M is a potent GABAA receptor antagonist, it is of limited value in assessing GABAergic network interactions, which should be studied using picrotoxin or bicuculline-free base. However, bicuculline-M may provide a useful tool for developing nonpeptide antagonists of SK channels.

Excitatory Effects of Thyrotropin-Releasing Hormone in the Thalamus

“…We have shown previously that bath application of the GABA A receptor antagonist [and blocker of some K Ca2ϩ channels (Debarbieux et al, 1998)] bicuculline-methiodide can transform spindle waves into 2-3 Hz synchronized network discharges (von Krosigk et al, 1993;Bal et al, 1995a,b). To examine whether the application of TRH may disrupt these bicuculline-induced oscillations, we first evoked spindle waves with brief stimulation to the optic radiation (10 stimuli; 100 s duration; 40 -400 A amplitude; 100 Hz frequency) (Fig.…”

Cerebellar ataxia and Purkinje cell dysfunction caused by Ca 2+ -activated K + channel deficiency

“…Saggital slices (350-400 m thick) from the cerebellar vermis of 4-to 5-week-old WT and BK Ϫ/Ϫ mice were prepared and kept at 34-36°C in artificial cerebrospinal fluid with 10 M bicuculline free base [a concentration that has been shown to not measurably affect currents of Ca 2ϩ -activated K ϩ channels with small conductance (SK) or afterhyperpolarizations (AHPs) in rat brain slices (14,15) and to cause only a Ϸ20% inhibition of SK channels expressed in frog oocytes (16)] and 10 M 6,7-dinitroquinoxaline-2,3-dione (DNQX) to block synaptic transmission. Whole-cell (Axoclamp 2A, Axon Instruments, Union City, CA) and extracellular (Multiclamp 700A, Axon Instruments) recordings were obtained from PC somata under visual control.…”

Stabilization of Bursting in Respiratory Pacemaker Neurons

“…These drugs were bath applied at the following final concentrations (in M): 40 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; Tocris Cookson, Ballwin, MO), 10 M (RS)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP; Tocris Cookson), 1 M strychnine (Sigma), and 20 M bicuculline free-base (Sigma). Note that unlike bicuculline methiodide, the bicuculline free-base derivative is a specific GABA receptor antagonist and is not known to block apamin-sensitive Ca 2ϩ -activated K ϩ currents (Johnson and Seutin, 1997;Debarbieux et al, 1998).…”