Serotonin gating of cortical and thalamic glutamate inputs onto principal neurons of the basolateral amygdala

Abstract

The basolateral amygdala (BLA) is a key site for crossmodal association of sensory stimuli and an important relay in the neural circuitry of emotion. Indeed, the BLA receives substantial glutamatergic inputs from multiple brain regions including the prefrontal cortex and thalamic nuclei. Modulation of glutamatergic transmission in the BLA regulates stress- and anxiety-related behaviors. Serotonin (5-HT) also plays an important role in regulating stress-related behavior through activation of both pre- and postsynaptic 5-HT receptors. Multiple 5-HT receptors are expressed in the BLA, where 5-HT has been reported to modulate glutamatergic transmission. However, the 5-HT receptor subtype mediating this effect is not yet clear. The aim of this study was to use patch-clamp recordings from BLA neurons in an ex vivo slice preparation to examine 1) the effect of 5-HT on extrinsic sensory inputs, and 2) to determine if any pathway specificity exists in 5-HT regulation of glutamatergic transmission. Two independent input pathways into the BLA were stimulated: the external capsule to mimic cortical input, and the internal capsule to mimic thalamic input. Bath application of 5-HT reversibly reduced the amplitude of evoked excitatory postsynaptic currents (eEPSCs) induced by stimulation of both pathways. The decrease was associated with an increase in the paired-pulse ratio and coefficient of variation of eEPSC amplitude, suggesting 5-HT acts presynaptically. Moreover, the effect of 5-HT in both pathways was mimicked by the selective 5-HT_1B receptor agonist CP93129, but not by the 5-HT_1A receptor agonist 8-OH DPAT. Similarly the effect of exogenous 5-HT was blocked by the 5-HT_1B receptor antagonist GR55562, but not affected by the 5-HT_1A receptor antagonist WAY 100635 or the 5-HT₂ receptor antagonists pirenperone and MDL 100907. Together these data suggest 5-HT gates cortical and thalamic glutamatergic inputs into the BLA by activating presynaptic 5-HT_1B receptors.

Keywords: Basolateral amygdala; Glutamatergic transmission; Serotonin receptor.

Figure 1. Effects of 5-HT on evoked EPSCs in cortical and thalamic glutamate inputs onto BLA pyramidal neurons

5-HT (10 µM) reversibly decreased the amplitude of eEPSCs in either cortical (A, B) or thalamic pathway (C,D). The decrease of eEPSCs amplitude during 5-HT application was associated with an increase of paired pulse ratio in either cortical (E, F) or thalamic pathway (G, H). **, p<0.01 vs baseline.

Figure 2. 5-HT_1B receptor agonist mimicked the effect of 5-HT on eEPSCs in BLA

A, B) 5-HT_1B receptor agonist CP93129 (10 µM) reduced the amplitude of eEPSCs in cortical pathway, which effect was accompanied by an increase of paired pulse ratio (C, D). E,F) Mixed 5-HT_1B/D agonist sumatriptan (10 µM) reduced the amplitude of eEPSCs in cortical pathway. The decrease of eEPSCs amplitude was associated with increases of PPR (G,H). I,J) In thalamic pathway CP 93129 (10 µM) decreased the amplitude of eEPSCs, which associated with an increase of paired pulse ratio. *,**, p<0.05 and 0.01 respectively vs baseline.

Figure 3. 5-HT_1B antagonist blocked the effect of 5-HT on eEPSCs

Selective 5-HT_1B antagonist GR55562 (20 µM) significantly attenuated the effect of 5-HT (10 µM) on eEPSCs in cortical pathway(A,B).

Figure 4. Effect of 5-HT_1A activation on cortical eEPSCs

A,B) 5-HT_1A agonist 8-OH DPAT slightly increased the amplitude of eEPSCs in cortical pathway. C,D) Blockade of 5-HT_1A receptor with WAY 100635 (1 µM) has no effect on 5-HT (10 µM) inhibition of cortical eEPSCs.

Figure 5. 5-HT₂ receptor antagonists have no effect on 5-HT inhibition of EPSCs in cortical pathway

In the presence of 5-HT_2A antagonist MDL 100907 (10 µM) (A) or non-selective 5-HT₂ antagonist pirenperone (10 µM)(B), 5-HT (10 µM) significantly decreased the amplitude of cortical eEPSCs, which is not different from 5-HT effect in control ACSF (C).