Urethane inhibits the GABAergic neurotransmission in the nucleus of the solitary tract of rat brain stem slices

Abstract

Because urethane is a widely used anesthetic in animal experimentation, in the present study, we evaluated its effects on neurons of the nucleus of the solitary tract (NTS) in brain stem slices from young rats (25-30 days old). Using the whole cell configuration of the patch-clamp technique, spontaneous postsynaptic currents (sPSCs) and evoked excitatory postsynaptic currents (eEPSCs) were recorded. Urethane (20 mM) decreased by approximately 60% the frequency of GABAergic sPSCs (1.0 +/- 0.2 vs. 0.4 +/- 0.1 Hz) but did not change the frequency, amplitude, or half-width of glutamatergic events or TTX-resistant inhibitory sPSCs [miniature inhibitory postsynaptic currents (IPSCs)]. Miniature IPSCs were measured in the presence of urethane plus 1 mM diazepam (1 mM), and no changes were seen in their amplitude. This suggests that the GABA concentration in the NTS synapses is set at saturating level. We also evaluated the effect of urethane on eEPSCs, and no significant change was observed in the amplitude of N-methyl-d-aspartate [NMDA; 44.2 +/- 11.5 vs. 37.6 +/- 10.6 pA (holding potential = 40 mV)] and non-NMDA currents [204.4 +/- 35.5 vs. 196.6 +/- 31.2 pA (holding potential = -70 mV)]. Current-clamp experiments showed that urethane did not alter the action potential characteristics and passive membrane properties. These data suggest that urethane has an inhibitory effect on GABAergic neurons in the NTS but does not change the spontaneous or evoked excitatory responses.

Cardiovascular function of a glutamatergic projection from the hypothalamic paraventricular nucleus to the nucleus tractus solitarius in the rat

“…For example, chemical stimulation of the PVN may have resulted in the release of vasopressin into the circulation and exaggerated the anesthetic-induced vasopressin-dependent vascular tone. Moreover, patch-clamp experiments on brain slice preparations have shown that urethane decreased the frequency of GABAergic, but not glutamatergic, spontaneous postsynaptic currents (Accorsi-Mendonca et al, 2007). In our study, it is possible that the tone of GABAergic inputs to the PVN (Li et al, 2006) is decreased by the anesthetic and the PVN neurons are rendered more excitable.…”

Synaptic transmission of baro- and chemoreceptors afferents in the NTS second order neurons

“…In studies performed in both anesthetized and awake rats, we documented that different anesthetics affect the neurotransmission at the NTS level (Machado and Bonagamba, 1992). Experiments using brainstem slices also showed that NTS neurons and the modulation of synaptic transmission are strongly affected by urethane, one of the most common anesthetic used in cardiovascular experiments (Accorsi-Mendonça et al, 2007).…”

“…In studies performed in both anesthetized and awake rats, we documented that different anesthetics affect the neurotransmission at the NTS level (Machado and Bonagamba, 1992). Experiments using brainstem slices also showed that NTS neurons and the modulation of synaptic transmission are strongly affected by urethane, one of the most common anesthetic used in cardiovascular experiments (Accorsi-Mendonça et al, 2007).In a series of experiments performed in awake rats to study the glutamatergic neurotransmission of the baroreflex in the NTS, we verified that bilateral microinjections of non-selective (kynurenic acid) or selective (NMDA, AP-5) glutamate ionotropic receptors antagonists into NTS were effective in blocking the bradycardia in response to baroreflex activation or to microinjections of L-glutamate or a selective agonist (NMDA) into the NTS, but not the sympathoinhibitory component of the responses (Colombari et al Almado and Machado, 2005). Therefore, these functional studies evaluating the neurotransmission of baroreceptors afferents in the NTS of unanesthetized rats indicate how complex the processing of the baroreceptors afferents at the NTS level under physiological conditions and in vitro studies is without anesthetics, such electrophysiological studies using brainstem slices are useful to provide a better understanding about the synaptic…”

Interaction between glutamatergic and nitrergic mechanisms mediating cardiovascular responses to l-glutamate injection in the diagonal band of Broca in anesthetized rats

“…Under this condition, hypotensive mechanisms would be unveiled. Because urethane has been reported to inhibit GABAergic neurotransmission in the NTS (Accorsi-Mendonca et al, 2007), another hypothesis for these opposing blood pressure responses would be a possible resetting of the cardiovascular tone due to similar effects of urethane on GABAergic neurotransmission in the dbB. However, microinjection of GABA A receptor antagonist into the dbB has been reported to cause pressor responses in urethaneanesthetized rats Nasimi and Hatam, 2005), suggesting that GABAergic neurotransmission is not depressed in the dbB under urethane anesthesia.…”