Amygdala inputs drive feedforward inhibition in the medial prefrontal cortex

Abstract

Although interactions between the amygdala and prefrontal cortex (PFC) are critical for emotional guidance of behavior, the manner in which amygdala affects PFC function is not clear. Whereas basolateral amygdala (BLA) output neurons exhibit many characteristics associated with excitatory neurotransmission, BLA stimulation typically inhibits PFC cell firing. This apparent discrepancy could be explained if local PFC inhibitory interneurons were activated by BLA inputs. Here, we used in vivo juxtacellular and intracellular recordings in anesthetized rats to investigate whether BLA inputs evoke feedforward inhibition in the PFC. Juxtacellular recordings revealed that BLA stimulation evoked action potentials in PFC interneurons and silenced most pyramidal neurons. Intracellular recordings from PFC pyramidal neurons showed depolarizing postsynaptic potentials, with multiple components evoked by BLA stimulation. These responses exhibited a relatively negative reversal potential (Erev), suggesting the contribution of a chloride component. Intracellular administration or pressure ejection of the GABA-A antagonist picrotoxin resulted in action-potential firing during the BLA-evoked response, which had a more depolarized Erev. These results suggest that BLA stimulation engages a powerful inhibitory mechanism within the PFC mediated by local circuit interneurons.

Keywords: GABA; electrophysiology; fast-spiking interneuron; in vivo intracellular recording; parvalbumin.

Fig. 1.

Pyramidal neuron responses to basolateral amygdala (BLA) stimulation. A: juxtacellularly labeled pyramidal neuron [Neurobiotin in red; parvalbumin (PV) in green]. The apical dendrite is oriented toward the apical surface. B: peristimulus time histogram (PSTH) constructed from consecutive sweeps, illustrating a typical pyramidal neuron response, a pause in spike firing (bin width = 5 ms). C: overlay of 100 traces showing excitatory response of a pyramidal neuron to BLA stimulation. D: PSTH plotting action-potential occurrences from the 100 consecutive sweeps shown in C with 1-ms bins. B and D: the stimulation artifact is represented by red, vertical lines.

Fig. 2.

Interneuron responses to BLA stimulation. A: Neurobiotin-filled interneuron colabeled with PV. Neurobiotin is shown in red and PV in green so that colocalization results in a yellow-filled cell body. B: overlay of traces showing the excitatory response of a PV-positive interneuron to BLA stimulation. C–F: raster plots (C, D) and PSTH (E, F) of the excitatory response to BLA stimulation at 2 different time scales. This neuron fired at least 1 action potential during every sweep. The red, vertical lines represent the time of stimulation.

Fig. 3.

Example of a prefrontal cortex (PFC) neuron that was recorded from and filled with Neurobiotin. A: neutral, red-stained coronal section with a Neurobiotin-filled pyramidal neuron. B: overlay of 3 sections from The Rat Brain in Stereotaxic Coordinates, illustrating sites of recorded neurons (red dots). C: Nissl-stained section, illustrating a representative stimulating-electrode placement in the BLA (arrow). The black, dashed circle represents the boundaries of the BLA. D: overlay of sections of the The Rat Brain in Stereotaxic Coordinates, illustrating placements of BLA-stimulating electrodes (red dots). [From Paxinos and Watson (1998).]

Fig. 4.

Passive membrane properties and spontaneous activity of PFC pyramidal neurons. A: trace showing spontaneous activity of a representative pyramidal neuron that displays up- and down-state transitions. B: histogram of membrane-potential values recorded in the trace shown in A, showing a bimodal distribution that can be fit to a dual Gaussian function (green lines). C: membrane-potential traces of an example neuron responding to injection of positive and negative, 100-ms current pulses. The spiking pattern seen in the most depolarized trace is typical of a regular spiking pyramidal neuron of the cerebral cortex, including spike-frequency adaptation. D: current (I)–voltage (V) plot of the traces in C. A linear function was fit to the plot, and its slope was used to estimate input resistance.

Fig. 5.

Synaptic responses evoked in PFC pyramidal neurons by BLA stimulation. A: overlay of traces showing depolarizing synaptic potentials in response to BLA stimulation. The time of stimulation and stimulus artifact are indicated with the upward-pointing arrow. When stimulation was delivered during the up state, the depolarizing postsynaptic potential (dPSP) was small compared with dPSPs evoked during the down state (downward-pointing arrows). B: overlay of several traces at a slower time scale, revealing the long-lasting hyperpolarization that follows the dPSP. C: several traces from a neuron, in which depolarization revealed a negative-reversing component in the BLA-evoked dPSP. The arrow indicates the time of stimulation, and membrane-potential values, at which responses were recorded, are indicated to the left of each trace. Depolarized traces revealed a segregation of components with different reversal potentials. The peak of the response observed at resting membrane potential (blue line) was depolarizing (down state: −78 mV). At depolarized membrane potentials, an early depolarizing component (green line) became segregated from a later hyperpolarizing component (red line). D: overlay of BLA-evoked responses in a pyramidal neuron with depolarized up states. BLA stimulation evoked a depolarizing response at negative membrane potentials (2 traces are highlighted in blue) and a hyperpolarizing response from the up state (2 representative responses are highlighted in red).

Fig. 6.

Picrotoxin (PTX) reveals a GABA component in the BLA-evoked dPSP and short-latency, BLA-evoked excitation. A: overlay of traces showing action potentials evoked by BLA stimulation in representative pyramidal neurons recorded with an electrode containing PTX (200 μM). B: raster plot (left) and PSTH (right), illustrating a response of a juxtacellularly recorded pyramidal neuron to BLA stimulation, before, during, and after local pressure ejection of PTX (vertical blue line in highlighted sweeps). The raster plot depicts consecutive sweeps (from top to bottom), with the red line indicating the time of BLA stimulation. The top PSTH shows baseline responses. Bin width is 5 ms, and the BLA stimulation time is shown with a vertical red line. The bottom PSTH shows responses after PTX pressure ejection, which unmasked a short-latency excitation in response to BLA stimulation. Insets in both PSTH illustrate the waveform of the neuron recorded. C: PTX significantly increased the magnitude of the early BLA-evoked excitatory response. The graph illustrates the firing increase by BLA stimulation (measured as the subtraction of firing/bin in the bins showing increased firing minus the prestimulation firing/bin) at baseline (left) and following PTX (right; *P < 0.05). D: PTX did not modify basal firing rates in all tested neurons.

Fig. 7.

Antidromic activation from the ventral tegmental area (VTA). A: overlay of traces showing action potentials evoked by each pulse in a 20-Hz train delivered to the VTA. Action potentials were evoked with a constant latency of 9.8 ms and can be observed to rise from the down state. B: VTA-evoked action potentials follow 200 Hz stimulation. The 1st 3 action potential are full somatodendritic spikes, and the last 2 show only the initial segment component. C: overlay of 2 traces showing a spontaneous action potential (1st from left) colliding with an antidromically evoked action potential. BLA stimulation, immediately after the spontaneously occurring action potential, failed to evoke a spike.