Local excitatory network and NMDA receptor activation generate a synchronous and bursting command from the superior colliculus

Abstract

The generation of bursting spike activity in the deeper layers of the superior colliculus (SC) is a critical determinant of decision making in the initiation of orienting behaviors, such as saccades. The bursting activity exhibits a typical threshold effect that may arise from a nonlinear signal amplification process in the deeper layers of the SC. We used whole-cell patch-clamp recordings in rat SC slices to investigate the neuronal mechanism underlying the generation of such bursting activity. We found that (1) neurons in the intermediate gray layer [stratum griseum intermediale (SGI)] produce a prolonged bursting response when released from GABA(A) receptor-mediated inhibition, (2) this GABA(A) inhibition may partially arise from inhibitory interneurons within the SGI that are driven synaptically by glutamatergic excitatory inputs to the SC, (3) the bursting is not the result of the intrinsic membrane properties of individual SC neurons but is instead produced by local circuits within the SGI, (4) the bursting is mediated by activation of NMDA receptors, and (5) the bursting can be synchronous among SGI neurons. These results suggest that activation of a local excitatory network within the deeper layers of the SC and NMDA receptor-dependent synaptic transmission after release from GABA(A) inhibition are fundamental mechanisms that may explain the nonlinear signal amplification process in the deeper layers of the SC.

Figure 1.

Simultaneous recordings from a pair of SGS and SGI neurons. Five traces are superimposed. A, Synaptic responses in SGS (top traces) and SGI (bottom traces) neurons to stimulation of LSO (arrow, 100μA) in control solution. B, Synaptic responses in SGS (top traces) and SGI (middle and bottom traces) neurons after application of 10 μm Bic. The bottom traces are slower sweep records of the middle ones. C, Drawing of biocytin-filled SGS and SGI neurons visualized after the recordings.

Figure 2.

A, Synaptic currents in an SGI neuron induced by LSO stimulation and recorded at different holding potentials (1, -70 mV; 2, -20 mV; 3, average of traces in 2; 4, +10 mV) in control solution. B, Synaptic currents in the same neuron as in A, this time recorded at a holding potential of-20 mV in the presence of 5 μm CNQX plus 50 μm d-APV. C, Synaptic currents in the same neuron in A recorded at a holding potential of -20 mV in the presence of 10 μm Bic. D, Synaptic currents in an SGI neuron induced by SGS stimulation and recorded at different holding potentials (1, -60 mV; 2, -20 mV; 3, average of traces in 2) in control solution. E, Synaptic currents in the same neuron as in D recorded at a holding potential of -20 mV in the presence of 5 μm CNQX plus 50 μm d-APV. F, Synaptic currents in another SGI neuron induced by SGS stimulation and recorded at a holding potential of -20 mV in control solution. G, Synaptic currents in the same neuron as in F recorded at a holding potential of -20 mV in the presence of 5 μm CNQX plus 50 μm d-APV.

Figure 3.

Spontaneous postsynaptic currents in an SGI neuron recorded in the presence of 5 μm CNQX plus 50 μm d-APV.A, Spontaneous postsynaptic currents recorded at different holding potentials (values given at left). B, Spontaneous postsynaptic currents in the presence of 5 μm CNQX plus 50 μm d-APV, and 10 μm Bic. C, Spontaneous postsynaptic currents recorded after washing out Bic.

Figure 4.

Input–output relationships for intrinsic and synaptic properties of an SGI neuron. A and B, Firing responses in an SGI neuron induced by depolarizing current pulses in control solution and under 10μm SR95531, respectively. C, Plots of number of action potentials against amplitude of injected currents (open circle, control; closed circle, SR95531). D and E, Synaptic responses to LSO stimulation in control solution and under 10 μm SR95531, respectively. Five traces are superimposed in D and in top panel in E. F, Plots of number of action potentials against stimulus strength (symbols as in C).

Figure 5.

Input–output relationships for intrinsic and synaptic properties of SGS (A and B) and SGI (C and D) neurons in control (1) and in the solution containing GABA antagonist (2). Individual symbols represent the plots obtained from individual neurons. Population data obtained from seven SGS neurons (Bic, n = 5; SR95531, n = 2) and seven SGI neurons (Bic, n = 3; SR95531, n = 4) are shown. Bic, 10 μm; SR95531, 10 μm.

Figure 6.

Implementation of nonlinear synaptic activity in local circuits within the SGI. A, Schematic illustration of a recording from a neuron in a small rectangular piece punched out from the SGI. Stim., Stimulating electrode; Rec., recording electrode. B, Synaptic response to stimulation within this piece in control solution. Five traces are superimposed. C, Synaptic response to stimulation within the piece of SGI in the presence of 10 μm Bic. D, Relationship between number of action potentials and stimulus strength in the presence of Bic. Filled circles and error bars represent mean and SEM, respectively.

Figure 7.

A, Synaptic response induced in an SGI neuron in an adult animal by stimulation of the LSO in control solution (1), after application of 10μm Bic (2), after application of 10μm Bic plus 50 μm d-APV (3), and after washing out d-APV (4). Stimulus strength was 200 μA. B, Synaptic response induced in an SGI neuron in a young animal by stimulation of LSO after application of 10 μm Bic (1), after application of 10 μm Bic plus 50 μm d-APV (2), and after washing out d-APV (3). Stimulation strength was 100 μA. C, Synaptic response induced in an SGI neuron by stimulation of LSO in control solution (1) and after application of 10 μm Bic (2). The intracellular solution contained 30 mm BAPTA. Stimulus strength is given at right.

Figure 8.

Simultaneous recordings from a pair of SGI neurons. A, Photomicrograph of the pair of recorded SGI neurons (injected with biocytin intracellularly). B, Recordings of current responses from one cell in voltage-clamp mode (VC) after induction of a single (1) or several (2) action potential(s) in the other cell by current injection in current-clamp mode (CC). C, Spontaneous membrane potentials in control solution (1), in the presence of 10 μm Bic and low (0.1 mm) Mg ²⁺ (2), and after washing out Bic and low Mg ²⁺ (5). C3, Faster-sweep records of segments underlined in C2. C4, Faster-sweep records of segment underlined in C3.

Figure 9.

A, Spontaneous membrane potentials recorded simultaneously from a pair of SGI neurons (cell-1 and cell-2) in the presence of 10 μm Bic and low (0.1 mm) Mg ²⁺. The intracellular solution contained 5 mm QX-314. B, Plots of membrane potentials (1) and normalized membrane potentials (2) for cell-1 against cell-2. C, Spontaneous membrane potentials recorded simultaneously from a pair of SGI neurons (cell-3 and cell-4) in the presence of 10 μm Bic and low (0.1 mm) Mg ²⁺ (1), and after application of 50 μm APV (2). The intracellular solution contained 5 mm QX-314 plus 30 mm BAPTA. D, Simultaneous recordings of synaptic responses from a pair of SGI neurons (cell-5 and cell-6) after stimulation of the SGS (arrow) in the presence of 10μm Bic and low (0.1 mm) Mg ²⁺. The intracellular solution did not contain QX-314.

Figure 10.

Schematic drawing of proposed local mechanism for the generation of burst responses after release from GABAergic inhibition. Large open and filled circles indicate excitatory and inhibitory neurons, respectively. Small circles indicate terminal boutons.