Analgesia precedes lower stimulus thresholds

Early after fentanyl exposure, stronger mechanical stimulation was needed to elicit withdrawal, and thermal withdrawal occurred later. Both measures then decreased below control levels. This transition from analgesia to greater sensitivity represents the opioid-associated hyperalgesia examined here.

Increased sensitivity persisted for several days before gradually recovering. The study focused on the hyperalgesic phase several hours after exposure, when behavioral responses had changed, and examined which brain processes changed alongside them.

Original figure 1The time course of analgesia and hyperalgesia

Prefrontal output cells become less excitable

In PL-mPFC layer V, fewer cells co-expressed CaMKIIα and the activity marker c-Fos. Single-cell recordings showed fewer action potentials under comparable current injection, with larger currents needed to initiate firing in hyperalgesic rats.

Activity-marker and firing changes converge on reduced excitability in this output population. Prefrontal cortex participates in pain regulation through several connections; the study focused on its output to NAc core to examine changes downstream.

Original figure 2Activity and firing of PL output cells

How does the prefrontal signal reach accumbens?

NAc core receives inputs from these prefrontal cells. PL fibers reached the region, and their recruitment elicited excitatory synaptic currents in recipient cells, placing changes at both ends within a traceable connection.

Current-blockade and recovery findings supported direct excitatory synaptic input from PL to NAc core. This glutamatergic signal enters cells that also integrate inhibitory inputs, so their firing reflects the combined state of multiple influences.

Accumbens is frequently studied in reward, but also participates in sensory and affective aspects of pain. Prefrontal–accumbens connections link cortical regulation with behavioral responses. This work focuses on the core as a specific recipient region; prefrontal signals reaching different locations may have different roles. Examining individual connections helps clarify regulation behind changes in broader regional activity.

Original figure 3Excitatory connectivity from PL to NAc core

The recipient cells’ input balance also changes

In NAc-core recipient cells from hyperalgesic rats, PL-terminal recruitment elicited smaller excitatory and larger inhibitory currents, lowering their excitation-to-inhibition ratio. The evoked input balance shifted toward inhibition relative to controls.

The cells also generated fewer action potentials under current injection and required larger currents to initiate firing. Weaker upstream excitability, altered downstream input balance and lower recipient-cell excitability form a set of functional changes in the hyperalgesic state.

Original figure 4NAc-core inputs and cellular excitability

Can recruiting the pathway improve responses?

Recruiting PL terminals in NAc core increased mechanical withdrawal thresholds and prolonged thermal withdrawal latencies in hyperalgesic rats. Stronger mechanical stimulation was needed to elicit withdrawal, and thermal responses occurred less rapidly.

Light did not produce the corresponding improvement in controls lacking the relevant light-sensitive channel. These behavioral changes support a contribution from the reduced-activity pathway to pain regulation in this state. The findings concern responses during stimulation; sustained effects remain to be examined.

Original figure 5Mechanical and thermal responses after activation

Suppressing the connection increases sensitivity in controls

Suppressing PL-to-NAc-core inputs reduced mechanical thresholds and thermal withdrawal latencies in saline-treated controls. Rats already in the hyperalgesic state did not show a further significant decrease. The same manipulation yielded different observed outcomes in the two groups.

Improved responses with activation and greater sensitivity with inhibition in controls connect pathway activity with nociceptive behavior. The study identifies a regulatory component in a repeated acute-exposure model in male rats. The relevant NAc-core cell types, links to downstream pain-modulatory networks and corresponding functions in females remain open.

Original figure 6Pathway inhibition and implications