Mu opioid receptors inhibit GABA release from parvalbumin interneuron terminals onto CA1 pyramidal cells.

Selectively activating (by optogenetics) parvalbumin-expressing (PV) interneurons induces GABA release onto CA1 pyramidal cells. Here we report that this release was attenuated by presynaptic mu opioid receptors (MORs) activation. On the other hand, conventional electric shock, presumably activating non-selectively presynaptic GABAergic terminals, also induced GABA release; however, this release showed relatively limited depression by MORs activation. The data suggest that MORs specifically inhibit GABA release from PV terminals and therefore, further support the idea that MORs contribute to homeostasis in CA1 neuro-circuit.

GABAB receptors constrain glutamate presynaptic release and postsynaptic actions in substantia gelatinosa of rat spinal cord.

The substantia gelatinosa (SG, lamina II of spinal cord gray matter) is pivotal for modulating nociceptive information from the peripheral to the central nervous system. γ-Aminobutyric acid type B receptors (GABABRs), the metabotropic GABA receptor subtype, are widely expressed in pre- and postsynaptic structures of the SG. Activation of GABABRs by exogenous agonists induces both pre- and postsynaptic inhibition. However, the actions of endogenous GABA via presynaptic GABABRs on glutamatergic synapses, and the postsynaptic GABABRs interaction with glutamate, remain elusive. In the present study, first, using in vitro whole-cell recordings and taking minimal stimulation strategies, we found that in rat spinal cord glutamatergic synapses, blockade of presynaptic GABABRs switched "silent" synapses into active ones and increased the probability of glutamate release onto SG neurons; increasing ambient GABA concentration mimicked GABABRs activation on glutamatergic terminals. Next, using holographic photostimulation to uncage glutamate on postsynaptic SG neurons, we found that postsynaptic GABABRs modified glutamate-induced postsynaptic potentials. Taken together, our data identify that endogenous GABA heterosynaptically constrains glutamate release via persistently activating presynaptic GABABRs; and postsynaptically, GABABRs modulate glutamate responses. The results give new clues for endogenous GABA in modulating the nociception circuit of the spinal dorsal horn and shed fresh light on the postsynaptic interaction of glutamate and GABA.

Presynaptic GABAB receptors differentially modulate GABA release from cholecystokinin and parvalbumin interneurons onto CA1 pyramidal neurons: A cell type-specific labeling and activating study.

Combining cell type-specific optogenetics and whole cell recordings on mouse acute hippocampal slices, we compared GABA release from cholecystokinin-expressing (CCK) and parvalbumin-expressing (PV) interneurons onto CA1 pyramidal neurons. Baclofen, a selective GABAB receptor agonist, inhibited GABAergic synaptic transmission greater from CCK terminals, compared to that from PV terminals. The N-type calcium channels on CCK and P/Q-type calcium channels on PV terminals contributed to the GABAB receptor-mediated inhibition, respectively. Our data thus provide direct evidence that GABAB receptors differentially modulate GABA release from CCK and PV interneurons, adding to an increasing list of differences between these two interneuron subtypes in modulating hippocampal pyramidal neurons.

Isoflurane decreases substantia gelatinosa neuron excitability and synaptic transmission from periphery in the rat spinal dorsal horn.

Isoflurane is an inhaled anesthetic, though its actions at the cellular level remain controversial. By using acute spinal cord slices from adult rats and the whole-cell recording technique, we found that aqueous isoflurane at the minimum alveolar concentration decreased postsynaptic neural excitability and enhanced membrane conductance, while suppressing glutamate release from presynaptic afferent onto substantia gelatinosa (lamina II) neurons in the dorsal horn. The data demonstrate that isoflurane modulates synaptic transmission from peripheral to the spinal cord via both pre- and postsynaptic effects and these actions may underlie its spinal anesthesia.

Differential sensitivity of presynaptic and postsynaptic GABAB receptors in rat ventrolateral periaqueductal gray.

The ventrolateral periaqueductal gray (PAG) in the midbrain plays a key role in the descending pain modulatory system. γ-Aminobutyric acid type B (GABAB) receptors belong to a metabotropic receptor subfamily and mediate both presynaptic and postsynaptic effects in PAG. It has been well documented that activation of GABAB receptors yields analgesia in some PAG subdivisions. In the present study, employing whole-cell patch-clamp recordings on acute rat PAG slices, we simultaneously monitored the responses of presynaptic and postsynaptic GABAB receptors. We found that the GABAB agonist, baclofen, exhibits less efficacy and potency at GABAB postsynaptic versus presynaptic receptors. This sensitivity bias may contribute to synapse homeostasis and implicate a novel pharmacotherapy treatment.

Accumulated GABA activates presynaptic GABAB receptors and inhibits both excitatory and inhibitory synaptic transmission in rat midbrain periaqueductal gray.

γ-Aminobutyric acid (GABA), the major inhibitory neurotransmitter, activates ionotropic GABAA receptors and metabotropic GABAB receptors in the central nervous system, respectively. In ventrolateral division of the midbrain periaqueductal gray (PAG), GABAB receptors play important roles in pain modulation. However, the role of endogenous GABA action in presynaptic GABAB receptors remains elusive. Using whole-cell recordings on acute PAG slices from adult rats, here, we show that ambient GABA exerts a tonic inhibition on presynaptic terminals by binding GABAB receptors. Extracellular GABA accumulated by nipecotic acid, which blocks GABA transporters, strengthened GABAB receptor-mediated presynaptic inhibition on both excitatory and inhibitory synapses. Our results indicate that PAG neurons experience GABAB receptor-mediated inhibition determined by GABA transporters. The accumulated GABA-mediated actions may indicate a therapeutic way.

[Measurement of plasma concentration and bioavailability of nolatrexed dihydrochloride in mice].

OBJECTIVE: To establish a methods based on high-performance liquid chromatogram-mass spectrum for measuring the plasma concentration of nolatrexed dihydrochloride and investigate the pharmacokinetic profile and absolute bioavailability of the drug in mice. METHODS: Nolatrexed dihydrochloride were injected intravenously at 50 mg/kg or administered orally at 200 mg/kg in mice, and blood samples were collected at various time points following drug administration. The plasma concentration of nolatrexed dihydrochloride in mice was determined using high-performance liquid chromatogram-mass spectrum. The pharmacokinetic parameters were calculated using DAS software, and the absolute bioavailability of orally and intravenously administered was assessed according to the ratio of their area under the curve (AUC). RESULTS: The method showed good linear relationship within the drug concentration range of 0.01-40 mg/L (r=0.9995, P<0.001). The recovery of nolatrexed dihydrochloride from the mouse plasma was more than 85%, and the intra- and inter-day precision expressed as the relative standard deviation was less than 15%. The half-life (T(1/2)), AUC, distribution factor and plasma clearance (CL) for intravenously administered nolatrexed dihydrochloride (50 mg/kg) were 3.020-/+0.017 h, 89.972-/+0.425 mg/L/h, 0.831-/+0.106 L/kg, and 0.556-/+0.093 L/h/kg, respectively. The T(1/2), AUC, peak time (T(max)) and peak concentration (C(max)) for orally administered drug were 5.046-/+0.191 h, 84.893-/+9.923 mg/L/h, 1.000-/+0.012 h, and 18.000-/+0.0140 mg/L, respectively. The absolute bioavailability of nolatrexed dihydrochloride in mice was 23.58%. CONCLUSION: The absolute bioavailability of nolatrexed dihydrochloride in mice determined in this study provides an experimental basis for development of the oral preparation of the drug.