Peripheral nerve injury reduces the excitation-inhibition balance of basolateral amygdala inputs to prelimbic pyramidal neurons projecting to the periaqueductal gray

Abstract

Cellular and synaptic mechanisms underlying how chronic pain induces maladaptive alterations to local circuits in the medial prefrontal cortex (mPFC), while emerging, remain unresolved. Consistent evidence shows that chronic pain attenuates activity in the prelimbic (PL) cortex, a mPFC subregion. This reduced activity is thought to be driven by increased inhibitory tone within PL circuits. Enhanced input from the basolateral amygdala (BLA) to inhibitory neurons in PL cortex is one well-received mechanism for this circuit change. In mice, we used retrograde labeling, brain slice recordings, and optogenetics to selectively stimulate and record ascending BLA inputs onto PL neurons that send projections to the periaqueductal gray (PAG), which is a midbrain structure that plays a significant role in endogenous analgesia. Activating BLA projections evoked both excitatory and inhibitory currents in cortico-PAG (CP) neurons, as we have shown previously. We measured changes to the ratio of BLA-evoked excitatory to inhibitory currents in the spared nerve injury (SNI) model of neuropathic pain. Our analysis reveals a reduced excitation-inhibition (E/I) ratio of BLA inputs to PL-CP neurons 7 days after SNI. The E/I ratio of BLA inputs to CP neurons in neighboring infralimbic (IL) cortex was unchanged in SNI animals. Collectively, this study reveals that the overall E/I balance of BLA inputs to PL neurons projecting to the PAG is reduced in a robust neuropathic pain model. Overall, our findings provide new mechanistic insight into how nerve injury produces dysfunction in PL circuits connected to structures involved in pain modulation.