Asymmetries in long-term and short-term plasticity at thalamic and cortical inputs to the amygdala in vivo

Abstract

Converging lines of evidence suggest that synaptic plasticity at auditory inputs to the lateral amygdala (LA) is critical for the formation and storage of auditory fear memories. Auditory information reaches the LA from both thalamic and cortical areas, raising the question of whether they make distinct contributions to fear memory storage. Here we address this by comparing the induction of long-term potentation (LTP) at the two inputs in vivo in anesthetized rats. We first show, using field potential measurements, that different patterns and frequencies of high-frequency stimulation (HFS) consistently elicit stronger LTP at cortical inputs than at thalamic inputs. Field potential responses elicited during HFS of thalamic inputs were also smaller than responses during HFS of cortical inputs, suggesting less effective postsynaptic depolarization. Pronounced differences in the short-term plasticity profiles of the two inputs were also observed: whereas cortical inputs displayed paired-pulse facilitation, thalamic inputs displayed paired-pulse depression. These differences in short- and long-term plasticity were not due to stronger inhibition at thalamic inputs: although removal of inhibition enhanced responses to HFS, it did not enhance thalamic LTP and left paired-pulse depression unaffected. These results highlight the divergent nature of short- and long-term plasticity at thalamic and cortical sensory inputs to the LA, pointing to their different roles in the fear learning system.

Figure 1. Electrode placements

A, Placements of stimulating electrodes in auditory association cortex (TE3) and auditory thalamus (MGm/PIN). Filled circles represent the electrode tips and the bars represent the distance between the tip and base of the bipolar stimulating electrode (1 mm) and thus the area of stimulation along the dorsoventral axis. B, Recording electrode placements in the LA. Because of overlap, not all individual recording sites are visible.

Figure 2. Response characteristics of thalamic and cortical inputs in vivo

A, Representative examples of neural responses recorded in the LA following stimulation of the thalamic or cortical pathway, respectively, in the same animal. Upper traces show field potential responses, whereas lower traces show the response of a single neuron recorded from the same electrode which was responsive to stimulation of both pathways (10 sweeps). Scale bar 0.5 mV (field potentials), 1.0 mV (single-unit), 5ms. Triangles indicate stimulus artifacts. Note the temporal correspondence between field potential and single-unit responses. B, Mean (± SEM) amplitude of field potentials as a function of stimulus intensity applied to the two pathways (n=16). Amplitude is defined by figure in inset.

Figure 3. Long-term potentiation at thalamic and cortical inputs

A, LTP induced by 3 series of 100 Hz trains at the thalamic (n=5) and cortical (n=5) inputs. Values represent mean (± SEM) amplitudes expressed as % of baseline values. Traces on the right show representative responses before and after LTP induction. Scale bar: 1 mV, 5 ms. B, LTP as a function of HFS frequency. Values represent LTP (mean ± SEM), measured 1 hr after induction following 25Hz (n=4 and 3), 100 Hz (n=5 and 5) and 200 Hz (n=4 and 3) stimulation (n values represent thalamic and cortical inputs, respectively). C, LTP following theta-burst stimulation (n=5 per input).

Figure 4. Responses during high-frequency stimulation of thalamic and cortical inputs

A, Initial segment of a response recorded in the LA during HFS at 100 Hz. Traces were filtered digitally as shown to remove stimulus artifacts prior to further analysis. Scale bar: 1 mV, 50 ms. B, Mean (± SEM) responses during 100 Hz HFS. Values represent the field potential relative to the pre-HFS baseline. C, Field potential (mean ± SEM) during HFS as a function of frequency (n values as in Figure 3). Values represent the mean field potential during HFS. D, Mean (± SEM) test stimulus responses prior to HFS (n=15 and 13 for thalamic and cortical inputs, respectively).

Figure 5. Divergent paired-pulse response profiles at thalamic and cortical inputs

A, Representative field potential responses during paired-pulse stimulation with a 50 ms inter-stimulus interval (ISI). Scale bar: 0.25 mV, 10 ms. B, Mean (± SEM) amplitudes of the responses to the second pulse in a pair (expressed as % of the response to the first pulse) in all animals tested (n=17). C, Representative example of a single unit responding to both thalamic and cortical paired-pulse stimulation at 50 ms ISI. Scale bar: 200 μV, 10 ms. D, Mean (± SEM) probability of eliciting a spike with the second stimulus in a pair (expressed as % of the probability associated with the first stimulus) for all neurons tested (n=9).

Figure 6. Effect of GABA_A blockade on evoked responses in the LA

A,B, Representative responses recorded in the LA in response to stimulation of thalamic and cortical inputs, respectively, at increasing intensities with either picrotoxin or saline in the recording pipette. Scale bar: 1 mV, 20 ms. C,D, Mean (± SEM) response area (see inset) as a function of stimulus intensity with either picrotoxin or saline in the recording pipette (n=12 and 16 animals for recordings with picrotoxin and saline, respectively).

Figure 7. Effect of GABA_A blockade on paired-pulse responses

A,B, examples of paired-pulse responses (50ms ISI) at thalamic and cortical inputs, respectively, recorded in the presence of saline or picrotoxin. Scale bar: 0.5 mV, 50 ms. C,D, Mean (± SEM) paired-pulse responses at the two inputs in the presence of either saline (n=16) or picrotoxin (n=12).

Figure 8. Effect of GABA_A blockade on LTP induction in the LA

Responses to 100 Hz HFS (A,B) and LTP (C,D) at thalamic and cortical inputs, respectively, with either saline (n=5 per input) or picrotoxin (n=9–11 per input) in the recording pipette. Values represent mean ± SEM.