Pentobarbital Depressant Effects Are Independent of GABA Receptors in Auditory Thalamic Neurons

Abstract

Pentobarbital, a general anesthetic, has received extensive study for its ability to potentiate inhibition at GABAA subtype of receptors for GABA. Using whole cell current-clamp techniques and bath applications, we determined the effects of pentobarbital and GABA receptor antagonists on the membrane properties and tonic or burst firing of medial geniculate neurons in thalamic slices. Pentobarbital (0.01–200 μM) induced depressant effects in 50 of 66 neurons (76%). Pentobarbital hyperpolarized neurons by a mean of 3 mV and decreased the number of action potentials in tonic firing, evoked by current pulse injection from near the resting potential. Pentobarbital also decreased burst firing or low threshold Ca2+-spikes, evoked by current pulse injection into neurons at potentials hyperpolarized from rest. The blockade of tonic and burst firing, as well as low threshold Ca2+-spikes, was surmountable by increasing the amplitude of input current. The GABAA receptor antagonists, bicuculline (100 μM) and picrotoxinin (50–100 μM), did not block the depressant effects of pentobarbital (10 μM). The GABAB receptor antagonist, saclofen (200 μM), and GABAC receptor antagonist, (1,2,3,6-tetrahydropyridine-4-yl)methylphosphinate (10–50 μM), did not significantly alter the depressant effects. Pentobarbital produced excitatory effects (0.1–50 μM) on 11 neurons (17%) but had no effects on 5 neurons (7%). The excitation consisted of approximately 3 mV depolarization, increased tonic and burst firing and the rate of rise and amplitude of low threshold Ca2+ spikes. These effects were associated with a increase in input resistance. In contrast, the depressant effects of pentobarbital correlated to a decreased input resistance measured with hyperpolarizing current pulse injection (IC50 = 7.8 μM). Pentobarbital reduced Na+-dependent rectification on depolarization and lowered the slope resistance over a wide voltage range. Tetrodotoxin eliminated both Na+-dependent rectification and the pentobarbital-induced decrease in membrane resistance at depolarized voltages in two-thirds of the neurons. The pentobarbital-induced decrease in membrane resistance at voltages hyperpolarized from rest was not evident during co-application with Cs+, known to block the hyperpolarization-activated rectifiers. In summary, the pentobarbital acted at low concentrations to depress thalamocortical neurons. The depression resulted from decreased rectification on depolarization, which no longer boosted potentials over threshold, and an increased conductance that shunted spike generation. The depressant effects of pentobarbital did not involve known types of GABA receptor interactions.

General anesthesia changes gap-evoked auditory responses in guinea pigs

“…The second mechanism is GABA independent, namely the direct effect of PB on neuronal membrane potential. It has been reported that PB can cause hyperpolarization and therefore decrease the rate of action potential; this effect is not blocked by GABA antagonists [36]. Since the gap response threshold is determined by the recovery speed of neurons from the forward masking/inhibition effect of the pre-gap marker, the PB effect on gap response threshold must be considered as a combination of its depressant effect on neuronal membrane properties and inhibition due to forward masking.…”

Responses of inferior colliculus neurons to sounds presented at different rates in anesthetized albino mouse

“…Some effects of barbital may be connected with blocking (or enhancing the inactivation) of potential-dependent Na + conductance. The last effect was quite prominent in the auditory thalamus (Wan and Puil, 2002). Slowly inactivated Na + channels have been demonstrated in several animal models (Barlett and Wang, 2004;Toib et al, 1998).…”

“…In some cases, the depressing effects of pentobarbital did not involve known types of GABA receptor interactions (Wan and Puil, 2002). A prominent effect of some general anesthetics can be connected with an increase of potassium conductance (Ries and Puil, 1999).…”

“…The examination of the pentobarbital depressant effects upon the internal properties of the mammalian thalamocortical units had been provided (Wan and Puil, 2002;Wan et al, 2003). These studies clearly demonstrated multiple sites of barbiturate action, including a decrease in input resistance and the inducing of outward ion current.…”

Long-term synaptic changes in two input pathways into the lateral nucleus of the amygdala underlie fear extinction

“…One possibility is that the massive inhibition due to anesthesia diminished the reactivity of the sensory network and consequently blocked the auditory CS transmission from reaching the amygdala. In fact, general anesthetics, including sodium pentobarbital, are known to inhibit auditory transmission by altering neuronal activities in the inferior colliculus (Kuwada et al 1989), the auditory thalamus (Wan and Puil 2002), and the auditory cortex (Gaese and Ostwald 2001). To circumvent this problem, we employed the brain-stimulation CS, utilizing the known direct connection between the MGm and the LA.…”