Heterosynaptic long-term potentiation of inhibitory interneurons in the lateral amygdala

Abstract

Long-term potentiation (LTP) of synaptic transmission in the lateral amygdala (LA) is believed to underlie the formation and retention of fear memories. To explore the role of inhibitory transmission in amygdala plasticity, we recorded from LA inhibitory interneurons in vitro before and after tetanization of the thalamo-LA pathway, one of the major inputs to LA involved in fear learning. Tetanization resulted in LTP of the EPSPs elicited in both the tetanized thalamic pathway and the untetanized cortical pathway to LA. This LTP was NMDA-dependent and associated with a decrease in paired-pulse facilitation in both pathways. In LA excitatory cells, LTP of interneurons resulted in an increase in the amplitude of GABAergic IPSPs in both input pathways. Finally, isolated GABAergic IPSPs between inhibitory and excitatory neurons could be potentiated as well. Plasticity of inhibitory transmission within the LA may therefore contribute significantly to LA-mediated functions, such as fear conditioning.

Figure 1.

Whole-cell recordings from LA neurons. A, Schematic of stimulating and recording electrode placement. CE, Central nucleus; B, basal nucleus; ec, external capsule; ic, internal capsule. B, Response of an excitatory (left), and inhibitory (right) cell to a positive current injection of 0.15 nA.

Figure 2.

Heterosynaptic LTP of inhibitory neurons. A, Mean ± SE percentage of EPSP slope relative to baseline in response to thalamic (left) or cortical (right) afferent stimulation. A tetanus was given to the thalamic pathway at time 0. Traces (averages of 5 responses) from individual experiments before and after tetanus are shown in the insets. Calibration: 5 mV, 40 msec. B, PPF in inhibitory neurons in the thalamic (top) and cortical (bottom) input pathways before (Pre), 30 min, and 60 min after LTP induction (with a tetanus given to the thalamic pathway). PPF was examined with a 50 msec interpulse interval and is measured as the slope of the second EPSP as a percentage of the slope of the first EPSP. C, Mean ± SE percentage of EPSP slope relative to baseline in response to thalamic (filled squares) or cortical (open circles) afferent stimulation in the presence of 50 μm APV with a tetanus given to the thalamic pathway at time 0. D, Thalamic and cortical inputs to LA do not show cross-facilitation. Paired-pulse ratios were calculated by dividing the second EPSP response by the first response obtained from the corresponding primed-pulse sequence. Calibration: 4 mV, 50 msec.

Figure 3.

Heterosynaptic LTP of IPSPs in excitatory neurons. A, Tetanization of thalamic afferents induces LTP of thalamo-LA EPSPs and LTP of thalamo-LA and cortico-LA IPSPs. Mean ± SE percentage of EPSP slope (left) or IPSP maximum amplitude (right) relative to baseline in response to thalamic afferent (filled squares) or cortical afferent (open circles) stimulation. A tetanus was given to the thalamic pathway at time 0. Traces are from individual experiments before (1) and 60 min (2) after tetanus. Calibration: 5 mV, 50 msec. B, Tetanization of cortical afferents induces LTP of cortico-LA EPSPs and LTP of IPSPs in both pathways. Mean ± SE percentage of EPSP slope (left) or maximum IPSP amplitude (right) relative to baseline in response to cortical afferent (open circles) or thalamic afferent (filled squares) stimulation, with a tetanus given at time 0. Traces from individual experiments before (1) and 60 min after (2) tetanus are shown on the right. Calibration: 5 mV, 50 msec.

Figure 4.

LTP of GABAergic synapses. A, Diagram of the circuitry within the LA. Potentiation of IPSPs in excitatory neurons (exc) can reflect changes at inputs to inhibitory cells (inh, 1) and/or changes of GABAergic synapses between inhibitory and excitatory neurons (2). B, Loading excitatory cells with the calcium chelator BAPTA (10 mm) reduced IPSP LTP. Mean ± SE percentage of IPSP maximum amplitude in excitatory cells in response to thalamic (filled squares) or cortical (open circles) input stimulation. A tetanus was given to the thalamic pathway at time 0. Traces from individual experiments showing thalamo-LA (left) and cortico-LA (right) responses before and 60 min after tetanus are shown above. Calibration: 5 mV, 50 msec. C, GABAergic IPSPs between inhibitory and excitatory neurons were isolated with CNQX (10 μm) and APV (50 μm). Mean ± SE percentage of IPSP maximum amplitude in response to stimulation from an electrode placed within the dorsal tip of the LA. A tetanus was given at time 0. Traces from an individual experiment before and 60 min after tetanus are shown in the inset. Calibration: 3 mV, 50 msec. D, PPD of isolated GABAergic IPSPs before (Pre), 30, and 60 min after LTP induction. PPD was examined with a 50 msec interpulse interval and is measured as the amplitude of the second IPSP as a percentage of the amplitude of the first IPSP.