Glutamate inhibits thalamic reticular neurons

Abstract

Activation of metabotropic glutamate receptors (mGluRs) can result in long-lasting modulation of neuronal excitability. Multiple mGluR subtypes are localized within the rat thalamic reticular nucleus (TRN), and we have examined the effects of activating these different receptor subtypes on the excitability of these neurons using an in vitro slice preparation. Typical of most mGluR-sensitive preparations, the general mGluR agonist, (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD) produced a robust, long-lasting excitatory response. Surprisingly, ACPD produced a membrane hyperpolarization in some neurons. Using selective mGluR agonists, we found that activation of group II mGluRs produces the hyperpolarization, whereas the depolarization is mediated by group I mGluRs. While the polarity of the postsynaptic response (hyperpolarization vs depolarization) was dependent on the mGluR subtype activated, both actions appear to result from modification of a linear K(+) conductance. The inhibitory action of Glutamate, via group II mGluRs, provides an avenue for a disinhibitory effect that could have interesting consequences upon a well-investigated, model neuronal circuit, turning its assumed functional role upside down.

Fig. 1.

Activation of mGluRs alters excitability of a TRN neuron. These examples reflect current-clamp recording.A, Control, ACPD (125 μm,thick line under recording) produces a long-lasting membrane depolarization leading to spike discharge (spikes truncated). The depolarization is interrupted for 20–30 sec by current injection to repolarize the membrane to the pre-ACPD level (−70 mV), thereby controlling for any voltage-dependent effects while comparing pre-ACPD and post-ACPD input resistance. During the depolarization, there is an increase in baseline activity, presumably representing EPSPs originating from suprathreshold excitation by the ACPD of afferent glutamatergic inputs (e.g., from ventrobasal relay cells that are generally connected the TRN neurons in these slice preparation).A, TTX, With addition of TTX (1 μm), the ACPD-mediated depolarization persists, but the increase in baseline activity is eliminated as are, of course, the action potentials. In both Control andTTX, the increased amplitude of membrane responses to the hyperpolarizing current steps indicate an increase in neuronal input resistance. B, Control, In a different TRN neuron, the group I mGluR agonist DHPG (250 μm) produces a robust depolarization, evoking action potentials (spikes truncated) as well as a robust increase in baseline activity. B, TTX, The DHPG depolarization persists in the presence of TTX (1 μm). As inA, the larger voltage responses to hyperpolarizing current steps after the membrane potential is manually returned to the pre-DHPG level of −74 mV indicate a decrease in input resistance.C, Control, The selective group II mGluR agonist S3-C4HPG (500 μm) produces a small membrane hyperpolarization from the initial membrane potential of −65 mV (thedashed line serves as a reference to this), and the responses to current steps indicate a small decrease in neuronal input resistance in a different TRN neuron. C,TTX, The S3-C4HPG-mediated hyperpolarization persists in TTX (1 μm).

Fig. 2.

Biphasic actions of ACPD on TRN cells.A, Control, In control conditions during current-clamp recording, ACPD (125 μm) produces a depolarization associated with a large increase in spontaneous depolarizations (presumed EPSPs), and action potentials are evoked. The depolarization is interrupted by a manual return of the membrane voltage to the pre-ACPD level of −63 mV. A,TTX, With the addition of TTX (1 μm), ACPD produces an early hyperpolarization with reduced input resistance followed by a longer lasting depolarization with increased input resistance. B, Control, In a voltage-clamp recording from a different neuron, ACPD (125 μm) produces an inward current associated with an increase in small inward currents, which are presumed EPSCs.B, TTX, With the addition of TTX (1 μm), ACPD no longer produces an increase in baseline activity but does evoke an initial outward current followed by a longer-lasting inward current. See legend to Figure 3 for explanation of the downward deflections during the voltage-clamp recording.

Fig. 3.

Activation of different mGluR subtypes during voltage-clamp recording in the presence of TTX (1 μm) differentially alters K⁺ conductances.Ai, The general mGluR agonist ACPD (125 μm) produces an inward current associated with a decrease in amplitude of the downward deflections. These deflections are current responses to ramped voltage commands (−50 to −90 mV, 2 sec duration). Thus, the reduced amplitude of the deflections reflects an increase in input resistance. Aii, Expanded traces of the averaged recorded current responses (I; n = 5) versus the ramped voltage commands before (Pre-drug,thin line) and during (ACPD, thick line) the peak ACPD response. In response to ACPD, there is an inward shift of the holding current (indicative of the inward current) and decreased slope of the membrane response, indicating a decreased conductance. Aiii, Difference between the pre-drug and ACPD traces in Aii, thereby showing the ACPD-mediated current response. The extrapolated reversal potential of this current is −90 mV. Bi, From a different TRN neuron, the specific group I mGluR agonist DHPG (250 μm) produces an inward current similar to that seen in Ai with ACPD.Bii, Average of five responses as in Aiiito the ramped voltage commands before (Pre-drug) and during (DHPG) the peak agonist response. DHPG produces a decreased slope of the current response, as in Aii. Biii, The extrapolated reversal potential of the DHPG-mediated current was −94 mV. Ci, From the TRN neuron in Figure1C, the selective group II mGluR agonist S3-C4HPG (500 μm) produces a small outward current. The downward deflections that are membrane responses to voltage ramps are truncated in this illustration. Cii, Average of five responses as in Aiii to the ramped voltage commands before (Pre-drug) during (S3-C4HPG) the peak agonist response. Note the increased slope of the response to S3-C4HPG.Ciii, The extrapolated reversal potential of the increased current was −85 mV.

Fig. 4.

A, Current-clamp recording showing attenuation by mGluR antagonists of group II mGluR agonist-mediated hyperpolarization of TRN cell. A, TTX, In TTX (1 μm), the group II agonist S3-C4HPG (500 μm) produces a membrane hyperpolarization.A, MCPG + TTX, The S3-C4HPG hyperpolarization is partially attenuated by the presence of the general mGluR antagonist MCPG (500 μm). A,Wash, After a 23 min wash in TTX-containing solution, the S3-C4HPG-mediated hyperpolarization recovers. Initial Vm = −59 mV. B, Voltage-clamp recording in another TRN cell showing that the mGluR-mediated effects are suppressed by Cs⁺ in the electrode. B,Control, In control conditions, ACPD produces a robust alteration in baseline activity, presumably increasing spontaneous EPSCs via suprathreshold activation of synaptically connected relay neurons. B, TTX, In the presence of TTX (1 μm), ACPD does not alter the holding current or spontaneous baseline activity. C,TTX, During voltage-clamp recording from a different TRN neuron in TTX (1 μm), the selective group II mGluR agonist S3-C4HPG produces no obvious change in resting current levels or responses to voltage ramps.