We measured changes in the molar concentration of cytosolic Ca2' ([Ca2+1]) in individual astrocytes in culture produced by the glutamate analog quisqualate (QA) and related substances by using fura-2 digital fluorescence microscopy. In cells cultured from the cortex, hippocampus, and cerebellum, the QA analog a-amino-3-hydroxy-5-methyl-isoxaiole-4-propionate (AMPA; 10 ,uM) produced a slow increase in [Ca2J11 that was modest in amplitude (-200 nM). These effects were completely abolished by 10 ,IM

SUMMARY1. Intracellular and whole-cell patch recordings were made from sixty-seven neurones located in the nucleus tractus solitarii (NTS) in transverse slices of rat brainstem.2. Baclofen at concentrations of 2-20 ftM caused hyperpolarization from normal resting membrane potentials (Vm). This response was associated with a decrease in input resistance (Rm) tested by current pulses in discontinuous current clamp mode when membrane potential was restored to control level by current injection. In single electrode discontinuous voltage clamp mode, baclofen'at these concentrations caused a small (< 50 pA) outward current associated with increased membrane conductance measured by voltage steps from holding potentials (Vh) of -50 or -60 mV. Current-voltage relations at these Vhs and the results of varying Vh between -50 and -110 mV during responses to baclofen gave a reversal potential of -73 mV. The amplitudes of baclofen responses were related to K+ concentration tested by comparing responses in media containing 1-24 mm extracellular K', indicating that postsynaptically baclofen acts via a K+ conductance.3. These effects were still apparent in the presence of tetrodotoxin (which did not abolish all spontaneous synaptic activity) and also in medium containing a combination of C02+, the excitatory amino acid antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the GABAA antagonist bicuculline which blocked synaptic activity.4. The amplitude and frequency of spontaneous postsynaptic potentials (spPSPs) and spontaneous postsynaptic currents (spPSCs) were reduced by baclofen at concentrations (1 /LM or less) which had no effect on membrane potential or holding current in current or voltage clamp recordings respectively.5. The amplitude of evoked excitatory (evEPSPs/evEPSCs) and inhibitory (evIPSPs/evIPSCs) synaptic events elicited by electrical stimulation in the vicinity of the tractus solitarius (TS) was reduced by low concentrations of baclofen (250 nM-1 jM) which did not produce discernible postsynaptic responses.6. In order to examine the effects of baclofen on excitatory synaptic events MS 9797 P. A. BROOKS AND OTHERS without contamination with inhibitory events, stimulation of the TS was carried out in the presence of bicuculline. Conversely to investigate actions on purely inhibitory synaptic responses experiments were carried out with CNQX in the bathing solution. Inhibitory synaptic responses could still be evoked, presumably by stimulation of interneurones in the vicinity of the TS. IPSPs/IPSCs were more sensitive to baclofen than EPSPs/EPSCs.7. The effects of baclofen on membrane potential or holding current and PSP/PSCs were antagonized by 2-hydroxysaclofen (400 /M) confirming that baclofen was acting at y-aminobutyric acid (GABA)B receptors.8. The pre-and postsynaptic depressant effects of baclofen are discussed in relation to the physiological effects of baclofen application in vivo.

This study examined the electrophysiological consequences of selective activation of delta 1-, delta 2-, or mu-opioid receptors using whole-cell recordings made from visually identified lamina II neurons in thin transverse slices of young adult rat lumbar spinal cord. Excitatory postsynaptic currents (EPSCs) or potentials (EPSPs) were evoked electrically at the ipsilateral dorsal root entry zone after blocking inhibitory inputs with bicuculline and strychnine, and NMDA receptors with D-2-amino-5-phosphonopentanoic acid. Bath application of the mu receptor agonist [D-Ala2, N-MePhe4, Gly5-ol]enkephalin (DAMGO) or the delta 1 receptor agonist [D-Pen2, D-Pen5]enkephalin (DPDPE) produced a long-linear, concentration-dependent reduction in the amplitude of the evoked EPSP/EPSC. By comparison, the delta 2 receptor agonist [D-Ala2,Glu4]deltorphin (DELT) was unable to reduce the evoked EPSP/EPSC by more than 50% at 100 microM, the highest concentration tested. At concentrations that reduced evoked EPSP/EPSCs by 40-60%, neither DAMGO, DPDPE, nor DELT decreased the amplitude of the postsynaptic current produced by brief pressure ejection of (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid, suggesting a presynaptic site of action of these opioid receptor agonists. Bath application of 200 nM naltriben (NTB), a delta 2 receptor antagonist, competitively increased the EC75 of DELT by 15.3-fold, but did not antagonize either DPDPE or DAMGO. The EC75 of DELT was further increased by 169.7-fold in the presence of 1 microM NTB. However, this high concentration of NTB also increased the EC50 of DPDPE by about threefold in a noncompetitive manner and antagonized DAMGO in a noncompetitive manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Following microinjection into the nucleus tractus solitarius (NTS), the effects of glutamate on the baroreceptor reflex are poorly antagonized by kynurenic acid and DL-2-amino-5-phosphonovaleric acid, suggesting the possible involvement of metabotropic glutamate receptors in this response. The metabotropic glutamate receptor agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) depolarized neurons located medial to the tractus solitarius (TS) at the level of the area postrema in coronal sections of the rat NTS. This effect was mimicked by glutamate and was not blocked by antagonists at alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA)/kainate or NMDA receptors. 1S,3R-ACPD also produced an inward current under voltage clamp that was not accompanied by a rise in [Ca2+]i, monitored with the Ca(2+)-sensitive dye fura-2. Conversely, the muscarinic agonist carbachol produced an outward current and a rise in [Ca2+]i. 1S,3R-ACPD reduced both the excitatory and the inhibitory postsynaptic current resulting from single electrical stimuli in the region of the TS. High-frequency stimulation of the TS produced an inward current in the presence of AMPA/kainate and NMDA receptor blockers. This current had similar properties to that produced by 1S,3R-ACPD. Thus, metabotropic glutamate receptors may mediate a component of excitatory transmission in the NTS.

1. We have previously demonstrated that the metabotropic glutamate receptor (mGluR) agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate [(1S;3R)-ACPD] presynaptically inhibits evoked glutamatergic excitatory postsynaptic currents (EPSCs) in patch-clamped rat nucleus tractus solitarius (NTS) neurons recorded in thin slices. The present study investigated the ability of endogenously released glutamate to modulate EPSCs in the NTS. 2. A low-frequency tetanus of the tractus solitarius (TS) resulted in either posttetanic potentiation (PTP) (8 of 21 cells) or depression (13 of 21 cells) of monosynaptic EPSCs recorded in the presence of D(-)2-amino-5-phosphonopentanoic acid (AP5) and bicuculline. 3. The amplitude of the EPSC was not significantly affected by the bath application of the mGluR antagonist (+) alpha-methyl-4-carboxyphenylglycine (MCPG). 4. In the presence of MCPG, a low-frequency tetanus resulted in PTP of the EPSC in all neurons. PTP was significantly enhanced in those cells previously exhibiting PTP. 5. The results suggest that presynaptic mGluRs on TS projections to the NTS may be activated by endogenously released glutamate at physiologically relevant stimulus frequencies and therefore play a role in the modulation of autonomic afferent transmission.