Inhibitory contribution to suprathreshold corticostriatal responses: an experimental and modeling study

Abstract

Neostriatal neurons may undergo events of spontaneous synchronization as those observed in recurrent networks of excitatory neurons, even when cortical afferents are transected. It is necessary to explain these events because the neostriatum is a recurrent network of inhibitory neurons. Synchronization of neuronal activity may be caused by plateau-like depolarizations. Plateau-like orthodromic depolarizations that resemble up-states in medium spiny neostriatal neurons (MSNs) may be induced by a single corticostriatal suprathreshold stimulus. Slow synaptic depolarizations may last hundreds of milliseconds, decay slower than the monosynaptic glutamatergic synaptic potentials that induce them, and sustain repetitive firing. Because inhibitory inputs impinging onto MSNs have a reversal potential above the resting membrane potential but below the threshold for firing, they conform a type of "shunting inhibition". This work asks if shunting GABAergic inputs onto MSNs arrive asynchronously enough as to help in sustaining the plateau-like corticostriatal response after a single cortical stimulus. This may help to begin explaining autonomous processing in the striatal micro-circuitry in the presence of a tonic excitatory drive and independently of spatio-temporally organized inputs. It is shown here that besides synaptic currents from AMPA/KA- and NMDA-receptors, as well as L-type intrinsic Ca(2+)- currents, inhibitory synapses help in maintaining the slow depolarization, although they accomplish the role of depressing firing at the beginning of the response. We then used a NEURON model of spiny cells to show that inhibitory synapses arriving asynchronously on the dendrites can help to simulate a plateau potential similar to that observed experimentally.

Fig. 1

Corticostriatal suprathreshold response. a Schematic of experimental arrangement showing the positions of recording and stimulating electrodes and the cellular elements involved. b Photograph of the area of interest. After biocytin instillation in the cortex (Ctx) (see “Methods”), several fibers can be traced anterogradely into the neostriatum (NSt). Squares (dashed lines) show common recording sites, and are seen at larger magnification at the bottom (a and b). Note abundant labeled fibers. c A single stimulus of increasing strength delivered at the cortex evokes subthreshold postsynaptic potentials. An action potential is reached at threshold intensity (arrow). d Suprathreshold activity (black traces) is superimposed to subthreshold traces from (c) (gray traces). A plateau potential with repetitive firing is elicited at the topmost trace (rat PD > 45 d, sharp electrode recording). e This cell and most cells in the next figures were identified as medium spiny projection neurons. f A similar plateau potential can be induced in spiny neurons from younger animals (rat PD = 20 d, whole-cell recording). g Voltage-clamp recordings of the same responses only show exponentially decaying currents with action currents firing at the beginning of the response. h Current–voltage relationship (I–V plot) obtained in current-clamp mode. The typical firing of spiny neostriatal neurons exhibits inward rectification and delayed tonic firing. i Filled circles: an I–V plot obtained in voltage-clamp mode (empty circles represent the I–V plot in H, after axis permutation). Both I–V plots coincide showing minimal interference from series resistance or bridge balance. Horizontal calibrations h, i: 50 ms

Fig. 2

Contribution of NMDA receptors to the corticostriatal response. (a, b) Families of corticostriatal responses obtained after a single cortical stimulus of increasing strength, before (a) and after the NMDA-receptors are blocked with 50 μM APV (b). c Superimposition of the larger responses in (a) and (b). d Intensity–response relationships in a sample of neurons. Data were fitted with where A(i) = amplitude of response as a function of stimulus strength (normalized to threshold); k = steepness factor (how easy corticostriatal afferents are recruited by stimulation strength); i = stimulus strength (normalized to threshold intensity); ih = stimulus strength necessary to attain half maximal amplitude; A _max = maximal response amplitude. e Superimposition as in (c), but with four slightly suprathreshold shocks at 20 Hz as corticostriatal stimulus

Fig. 3

Synaptic and intrinsic depolarizing components of the corticostriatal response. a Superimposition of a suprathreshold corticostriatal response firing a single action potential in control (black trace) and after adding 50 μM APV to the bath saline (gray trace). b A digital subtraction of the traces in (a). Note that after a single shock the depolarization contributed by NMDA current may last hundreds of milliseconds. c Superimposition of a suprathreshold corticostriatal response firing a burst of action potentials in control (black trace) and after adding 400 nM calciseptine, a blocker of Ca_V1 class (L) Ca²⁺-channels to the bath saline (gray trace). d A digital subtraction of the traces in (c). Note that after a single shock the depolarization contributed by Ca_V1 class (L) Ca²⁺-channels may last hundreds of milliseconds. e Nonscaled superimposition of the traces in (b) and (d) to compare their duration and time courses with that of the corticostriatal subthreshold and threshold responses (mostly AMPA/KA receptor currents). Note that when AMPA/KA synaptic potentials are already decaying, the NMDA and L current components are turning on, explaining the prolonged duration of the corticostriatal response after a single shock. f Voltage traces after successive blockage of, first, the L current, and then, the NMDA current

Fig. 4

Role of synaptic inhibition during the corticostriatal response. a Superimposition of a suprathreshold corticostriatal response firing a train of action potentials in control (black trace) and after adding 10 μM bicuculline a GABA_A receptor antagonist to the bath saline (gray trace). Note that GABAergic current was restraining firing at the beginning of the response, but later, it was helping to maintain the level of depolarization. b A digital subtraction of the traces in (a) shows that the bicuculline-sensitive component of the corticostriatal response is biphasic: the first 50–70 ms is lower than the response in the presence of bicuculline and all the time left is above it (hundreds of milliseconds); in this case two spikes occur when the bicuculline-sensitive component is depolarizing. Note that the first component of the subtraction is not hyperpolarizing (<−80 mV in a) but it is just more negative than threshold for firing. Superimpositions in Fig. 5e, f take this fact into account. c The time course of the bicuculline-sensitive component is compared with that of the APV-sensitive (NMDA-) and (calciseptine-sensitive) Ca_V1 (L) Ca²⁺-currents components. Note that the GABAergic component decays faster in part explaining the termination of the plateau-potential. d 1–3 A plateau potential evoked at different holding potentials (black trace). Note that at depolarized potentials it becomes hyperpolarizing (there is a local minimum, dot and dashed gray line). Addition of 10 μM bicuculline blocks local minima (gray traces). e Relative plateau amplitude (local minima/maxima) plotted against its absolute membrane potential yields a reversal potential ≈−63 mV

Fig. 5

A NEURON-model of a medium spiny neuron to illustrate the contribution of inhibition during the corticostriatal response. a A biocytin-filled medium spiny neostriatal neuron reconstructed with a camera lucida. Inset: details of spiny dendritic shaft. b A compartmental model (see “Methods”) of a spiny neuron, with the spines necessary to add glutamatergic synaptic inputs (rest of spines surface is compensated with dendritic surface to preserve electrotonic length and passive parameters) (see Supplementary material). c Characteristic delay to first spike, tonic firing and inward rectification for depo- and hyper-polarizing intracellular current steps of equal strength. d Similar characteristics in the model neuron. e Suprathreshold corticostriatal response showing a train of action potentials and a plateau-like depolarization lasting hundreds of milliseconds (black trace). The inhibitory component of a corticostriatal response obtained by subtraction is superimposed (gray trace). It is the same trace as in Fig. 4b, c. f A similar simulated corticostriatal response in the model neuron (black trace) with a similarly subtracted inhibitory component (gray trace). Inhibitory components are not scaled for amplitude but are scaled to match the duration of the corticostriatal responses