Developmental decline in modulation of glutamatergic synapses in layer IV of the barrel cortex by group II metabotropic glutamate receptors.

Previously, we demonstrated that group II metabotropic glutamate receptors (mGluRs) reduce glutamate release from thalamocortical synapses during early postnatal development (P7-11). To further examine the role of group II mGluRs in the modulation of somatosensory circuitry, we determined whether group II mGluRs continue to modulate thalamocortical synapses until adulthood and whether these receptors also modulate intra-cortical synapses in the barrel cortex. To address these issues, we examined the effect of the group II mGluR agonists on thalamocortical excitatory postsynaptic currents (EPSCs) and intra-barrel EPSCs in slices from animals of different ages (P7-53). We found that the depression of thalamocortical EPSCs by group II mGluRs rapidly declined after the second postnatal week. In contrast, adenosine continued to depress thalamocortical EPSCs via a presynaptic mechanism in young adult mice (P30-50). Activation of group II mGluRs also reduced intra-barrel EPSCs through a postsynaptic mechanism in young mice (P7-11). Similar to the thalamocortical synapses, the group II mGluR modulation of intra-barrel excitatory synapses declined with development. In young adult animals (P30-50), group II mGluR stimulation had little effect on intra-barrel EPSCs but did hyperpolarize the neurons. Together our results demonstrate that group II mGluRs modulate barrel cortex circuitry by presynaptic and postsynaptic mechanisms depending on the source of the synapse and that this modulation declines with development.

Group II metabotropic glutamate receptors inhibit glutamate release at thalamocortical synapses in the developing somatosensory cortex.

Thalamocortical synapses provide a strong glutamatergic excitation to cortical neurons that is critical for processing sensory information. Unit recordings in vivo indicate that metabotropic glutamate receptors (mGluRs) reduce the effect of thalamocortical input on cortical circuits. However, it is not known whether this reduction is due to a reduction in glutamate release from thalamocortical terminals or from a decrease in cortical neuron excitability. To directly determine whether mGluRs act as autoreceptors on thalamocortical terminals, we examined the effect of mGluR agonists on thalamocortical synapses in slices. Thalamocortical excitatory postsynaptic currents (EPSCs) were recorded in layer IV cortical neurons in developing mouse brain slices. The activation of group II mGluRs with (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) reduced thalamocortical EPSCs in both excitatory and inhibitory neurons, while the stimulation of group I or group III mGluRs had no effect on thalamocortical EPSCs. Consistent with a reduction in glutamate release, DCG IV increased the paired pulse ratio and the coefficient of variation of the EPSCs. The reduction induced by DCG IV was reversed by the group II mGluR antagonist, LY341495, and mimicked by another selective group II agonist, (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylic acid (APDC). The mGluR2 subtype appears to mediate the reduction of thalamocortical EPSCs, since the selective mGluR3 agonist, N-acetylaspartylglutamate (NAAG), had no effect on the EPSCs. Consistent with this, we showed that mGluR2 is expressed in the barrels. Furthermore, blocking group II mGluRs with LY341495 reduced the synaptic depression induced by a short stimulus train, indicating that synaptically released glutamate activates these receptors. These results indicate that group II mGluRs modulate thalamocortical processing by inhibiting glutamate release from thalamocortical synapses. This inhibition provides a feedback mechanism for preventing excessive excitation of cortical neurons that could play a role in the plasticity and refinement of thalamocortical connections during this early developmental period.

Hemorrhage alters plasma and cardiac enkephalins and catecholamines in anesthetized dogs.

Intrinsic cardiac enkephalins participate in circulatory regulation either through the modification of vagal control or vasomotor sympathetic control. We extracted, chromatographed, and assayed plasma and myocardial enkephalins from anesthetized dogs under control conditions and during hemorrhagic hypotension (2 h at 40 mmHg). Blood samples were collected at intervals during the experiment. Blood gases were stable, pH declined to 7.1, and heart rate rose. Plasma catecholamines increased and remained high throughout hypotension. Catecholamine and enkephalin immunoreactivities (ir) were unchanged in time controls. Plasma methionine-enkephalin (ME) and peptide F increased twofold and methionine-enkephalin-arginine-phenylalanine (MEAP) and peptide B increased 10- to 30-fold during hypotension. Plasma proenkephalin (ProEnk) and other large enkephalins were unchanged during hypotension. Myocardial norepinephrine was greater in the atria and both atrial and ventricular contents were decreased after hypotension. ProEnk and peptide B accounted for > 60% of the cardiac enkephalins, and their ventricular concentrations were three to four times atrial concentrations. Myocardial MEAP concentrations were 15-25 times the ME concentrations in the same tissue extracts. Hypotension increased myocardial peptide B and ProEnk, and ME, MEAP, and peptide F were unchanged. The data demonstrate a preferential processing to or retention of MEAP rather than ME-ir enkephalins in the heart. The data also indicate that myocardial MEAP-ir enkephalins respond to changes in the circulatory environment and appear in the plasma during hemorrhagic hypotension.

Intrinsic cardiac enkephalins inhibit vagal bradycardia in the dog.

Met-enkephalin-Arg-Phe (MEAP) has been identified in acid extracts of canine heart tissue. The effects of synthetic MEAP on the vagal control of heart rate were investigated in anesthetized dogs. The arterial infusion of MEAP (3 nmol.min-1.kg-1) inhibited the bradycardia observed during electrical stimulation of the right vagus nerve by 72%. After the infusion was stopped, the responsiveness to vagal stimulation returned to normal, with a half-time between 2 and 3 min. The inhibition by MEAP was reversed by the high-affinity opiate antagonist diprenorphine (100 micrograms/kg). MEAP did not alter the negative chronotropic effect of the direct-acting muscarinic agonist methacholine. This observation suggested that MEAP exerted its effect at a site in the efferent vagal tract proximal to nodal muscarinic receptors. Increasing MEAP infusions (0.09-3.00 nmol.min-1.kg-1) produced a graded suppression of vagal bradycardia, with a half-maximal effect near 0.3 nmol.min-1.kg-1. Met-enkephalin (ME) produced responses very similar to those obtained with MEAP. The effects of ME were also blocked by prior administration of diprenorphine. Dose responses to ME were shifted to the right of those for MEAP, and half-maximal responses for ME were obtained at two to four times the dose required for MEAP. The data suggest that the intrinsic cardiac enkephalin MEAP can regulate vagal control of heart rate at physiologically achievable concentrations and may serve as a local regulator of the parasympathetic-myocardial interface.