Synaptic Mechanisms Underlying Strong Reciprocal Connectivity between the Medial Prefrontal Cortex and Basolateral Amygdala

Abstract

The medial prefrontal cortex (mPFC) plays a critical role in the control of cognition and emotion. Reciprocal circuits between the mPFC and basolateral amygdala (BLA) are particularly important for emotional control. However, the neurons and synapses that link these brain regions remain largely unknown. Here we examine long-range connections between the mouse mPFC and BLA, using whole-cell recordings, optogenetics, and two-photon microscopy. We first identify two non-overlapping populations of layer 2 pyramidal neurons that directly project to either the BLA or contralateral mPFC. We then show that pyramidal neurons projecting to the BLA receive much stronger excitatory inputs from this same brain region. We next assess the contributions of both presynaptic and postsynaptic mechanisms to this cell-type and input-specific connectivity. We use two-photon mapping to reveal differences in both the synaptic density and subcellular targeting of BLA inputs. Finally, we simulate and experimentally validate how the number, volume, and location of active spines all contribute to preferential synaptic drive. Together, our findings reveal a novel and strong reciprocal circuit that is likely to be important for how the mPFC controls cognition and emotion.

Prefrontal Cortex Drives Distinct Projection Neurons in the Basolateral Amygdala

“…In targeted recordings from labeled neurons, we measured both AMPA-R EPSCs at −70 mV and NMDA-R EPSCs at +40 mV. By comparing the EPSC amplitude ratios at pairs of cells, we were able to control for variability in viral injections and expression between slices and animals (Little and Carter, 2013; MacAskill et al, 2014; McGarry and Carter, 2016). We found that both AMPA-R and NMDA-R EPSCs are much larger at AC neurons than AS neurons (AS/AC ratio: AMPA-R = 0.29, 95% CI = 0.14–0.61, p = 0.004; NMDA-R = 0.38, 95% CI = 0.2–0.74, p = 0.008; n = 9 pairs, 7 animals) (Fig.…”

“…3A). The resulting input-specific quantal EPSCs (qEPSCs) provide a direct measure of unitary synaptic strength (amplitude) and an indirect indication of number of connections and presynaptic properties (frequency) (Goda and Stevens, 1994; Little and Carter, 2013; MacAskill et al, 2014). In the absence of stimulation, baseline miniature EPSC (mEPSC) frequency was similar at AC, AS and AH neurons in medial BLA (AS = 2.0 ± 0.3 Hz, n = 8 cells; AC = 1.7 ± 0.2 Hz, n = 15 cells; AH = 1.8 ± 0.3 Hz, n =7 cells; p = 0.2).…”

“…In the cerebral cortex, distinct projection neurons segregate into individual layers and often possess unique intrinsic and synaptic properties (Brown and Hestrin, 2009; Hattox and Nelson, 2007; Little and Carter, 2013; Morishima and Kawaguchi, 2006), but whether equivalent diversity exists for projection neurons in the BLA remains poorly understood. Recent studies indicate that different projection neurons can be distinctly regulated by local inhibitory circuits in the BLA (Vogel et al, 2016).…”

“…While the PFC engages inhibitory circuits in the amygdala (Arruda-Carvalho and Clem, 2014; Cho et al, 2013; Hubner et al, 2014; Quirk et al, 2003), it also makes direct, excitatory synapses onto projection neurons in the BLA (Arruda-Carvalho and Clem, 2014; Brinley-Reed et al, 1995; Cho et al, 2013; Hubner et al, 2014; Likhtik et al, 2005). Recently, inputs from the BLA have been shown to preferentially contact reciprocally connected neurons in the PFC (Little and Carter, 2013; McGarry and Carter, 2016). One untested possibility is that PFC inputs also selectively contact neurons in the BLA that project back to the PFC.…”

Ketamine Strengthens CRF-Activated Amygdala Inputs to Basal Dendrites in mPFC Layer V Pyramidal Cells in the Prelimbic but not Infralimbic Subregion, A Key Suppressor of Stress Responses

“…A possible stress-sensitive input to basal dendrites comes from anterograde tracing that demonstrates a major monosynaptic input projecting from the basolateral amygdala nucleus (BLA) to the basal dendrites of mPFC layer V cells (Bacon et al, 1996;Gabbott et al, 2012). Also, a direct BLA input to layer II pyramidal cells has been shown by anterograde optogenetic methods (Little and Carter, 2013). The main cortically projecting excitatory neurons of the BLA are glutaminergic pyramidal cells.…”