Halothane induces vesicular and carrier-mediated release of [3H]serotonin from rat brain cortical slices

Isolation of Neuronal Synaptic Membranes by Sucrose Gradient Centrifugation

Sucrose gradient centrifugation is a very useful technique for isolating specific membrane types based on their size and density. This is especially useful for detecting fatty acids and lipid molecules that are targeted to specialized membranes. Without fractionation, these types of molecules could be below the levels of detection after being diluted out by the more abundant lipid molecules with a more ubiquitous distribution throughout the various cell membranes. Isolation of specific membrane types where these lipids are concentrated allows for their detection and analysis. We describe herein our synaptic membrane isolation protocol that produces excellent yield and clear resolution of five major membrane fractions from a starting neural tissue homogenate: P1 (nuclear), P2 (cytoskeletal), P3 (neurosynaptosomal), PSD (post-synaptic densities), and SV (synaptic vesicle).

Isolation of Neuronal Synaptic Membranes by Sucrose Gradient Centrifugation

Sucrose gradient centrifugation is a very useful technique for isolating specific membrane types based on their size and density. This is especially useful for detecting fatty acids and lipid molecules that are targeted to specialized membranes. Without fractionation, these types of molecules could be below the levels of detection after being diluted out by the more abundant lipid molecules with a more ubiquitous distribution throughout the various cell membranes. Isolation of specific membrane types where these lipids are concentrated allows for their detection and analysis. We describe herein our synaptic membrane isolation protocol that produces excellent yield and clear resolution of five major membrane fractions from a starting neural tissue homogenate: P1 (Nuclear), P2 (Cytoskeletal), P3 (Neurosynaptosomal), PSD (Post-synaptic Densities), and SV (Synaptic Vesicle).

The effects of volatile anesthetics on the extracellular accumulation of [(3)H]GABA in rat brain cortical slices

GABA is an inhibitory neurotransmitter that appears to be associated with the action of volatile anesthetics. These anesthetics potentiate GABA-induced postsynaptic currents by synaptic GABAA receptors, although recent evidence suggests that these agents also significantly affect extrasynaptic GABA receptors. However, the effect of volatile anesthetics on the extracellular concentration of GABA in the central nervous system has not been fully established. In the present study, rat brain cortical slices loaded with [(3)H]GABA were used to investigate the effect of halothane and sevoflurane on the extracellular accumulation of this neurotransmitter. The accumulation of [(3)H]GABA was significantly increased by sevoflurane (0.058, 0.11, 0.23, 0.46, and 0.93 mM) and halothane (0.006, 0.012, 0.024, 0.048, 0072, and 0.096 mM) with an EC50 of 0.26 mM and 35 μM, respectively. TTX (blocker of voltage-dependent Na(+) channels), EGTA (an extracellular Ca(2+) chelator) and BAPTA-AM (an intracellular Ca(2+) chelator) did not interfere with the accumulation of [(3)H]GABA induced by 0.23 mM sevoflurane and 0.048 mM halothane. SKF 89976A, a GABA transporter type 1 (GAT-1) inhibitor, reduced the sevoflurane- and halothane-induced increase in the accumulation of GABA by 57 and 63 %, respectively. Incubation of brain cortical slices at low temperature (17 °C), a condition that inhibits GAT function and reduces GABA release through reverse transport, reduced the sevoflurane- and halothane-induced increase in the accumulation of [(3)H]GABA by 82 and 75 %, respectively, relative to that at normal temperature (37 °C). Ouabain, a Na(+)/K(+) ATPase pump inhibitor, which is known to induce GABA release through reverse transport, abolished the sevoflurane and halothane effects on the accumulation of [(3)H]GABA. The effect of sevoflurane and halothane did not involve glial transporters because β-alanine, a blocker of GAT-2 and GAT-3, did not inhibit the effect of the anesthetics. In conclusion, the present study suggests that sevoflurane and halothane increase the accumulation of GABA by inducing the reverse transport of this neurotransmitter. Therefore, volatile anesthetics could interfere with neuronal excitability by increasing the action of GABA on synaptic and extrasynaptic GABA receptors.

Effects of acute ethanol administration on methionine-enkephalin expression and release in regions of the rat brain

The dopaminergic mesolimbic system plays a key role in mediating the reinforcing properties of ethanol and other drugs of abuse. Ethanol reinforcement and high alcohol drinking behaviour have been suggested to involve the ethanol-induced activation of endogenous opioid systems. Ethanol may alter opioidergic transmission at different levels, including opioid peptide biosynthesis and release, as well as binding to opioid receptors. The aim of this work was to investigate the effects of different ethanol doses on methionine-enkephalin (Met-enk) release from the rat nucleus accumbens (NAcc). Ethanol effects were also studied on Met-enk content in the NAcc, prefrontal cortex (PFC) and caudate-putamen (CP). Met-enk release was studied by microdialysis in Wistar anesthetized rats and peptide concentrations were quantitated by radioimmunoassay. Ethanol was administered by intraperitoneal injection after a 2-h basal release period. Ethanol doses of 0.5, 1 and 2.5 g/kg induced a 2.7-, 4.9- and 3.4-fold increase in Met-enk release from the NAcc. However, ethanol responses followed different kinetics, with earliest effects observed with the highest ethanol dose. In comparison, a 2.5-fold increase in peptide release was produced by 100 mM KCl. Ethanol, at a dose of 2.5 g/kg, induced a significant 66.7% decrease in Met-enk content in the NAcc, as well as a 76.4% reduction in peptide levels in the CP. Lower ethanol doses did not alter Met-enk content in these regions. On the other hand, an ethanol dose of 0.5 g/kg produced a non-significant decrease in Met-enk levels in the PFC. Our results suggest that ethanol-induced changes in enkephalin expression and release in regions of the mesocorticolimbic and nigrostriatal pathways could be involved in ethanol central effects. Released enkephalins by ethanol may modulate the dopaminergic activity of mesolimbic neurons and play a critical role in ethanol reinforcement mechanisms.

MDMA (3,4-methylenedioxymethamphetamine)-mediated distortion of somatosensory signal transmission and neurotransmitter efflux in the ventral posterior medial thalamus

MDMA (3,4-methylenedioxymethamphetamine, Ecstasy) is reported to enhance tactile sensory perception, an effect that is believed to contribute to its popularity as a recreational drug. To date, no literature exists that addresses the neurophysiological mechanisms underlying the effects of MDMA on somatosensation. However, MDMA interactions with the serotonin transporter protein (SERT) are well known. The rat trigeminal somatosensory system has been studied extensively and receives serotonergic afferents from the dorsal raphe nucleus. Given that these fibers express SERT, they should be vulnerable to MDMA-induced effects. We found that short-term low-dose MDMA administration (3 mg/kg i.p.) led to a significant increase in 5-hydroxytryptamine (5-HT) efflux in the ventral posterior medial (VPM) thalamus, the main relay along the lemniscal portion of the rodent trigeminal somatosensory pathway. We further evaluated the potential for MDMA to modulate whisker-evoked discharge (WED) of individual neurons in this region. After surgically implanting stainless steel 8-wire multichannel electrode bundles, we recorded spike train activity from single cells of halothane-anesthetized rats while mechanically activating the whisker pathway. We found that short-term low-dose MDMA (3 mg/kg i.p.) increased the spontaneous firing rate but reduced the magnitude and duration of WED in individual VPM thalamic neurons. It is noteworthy that the time course of drug action on neuronal firing patterns was generally consistent with increased 5-HT efflux as shown from our microdialysis studies. Based on these results, we propose the working hypothesis that MDMA may "distort" rather than enhance tactile experiences in humans, in part, by disrupting normal spike firing patterns through somatosensory thalamic relay circuits.