An injury to one hind paw can be followed by sensitivity on both sides of the body. How does pain processing cross the brain’s midline? A study published in Communications Biology on December 9, 2023 examines the connection between the two anterior cingulate cortices. Following fibers, synaptic currents and responses to touch and heat in adult mice, the researchers find that this cortical connection can facilitate pain-related responses.

01 / Across the midline

A working connection between the two ACCs

The anterior cingulate cortex (ACC) contributes to pain, emotion and action selection. Previous work found bilateral ACC changes after injury to one side of the body, but explaining communication requires examining the connection itself. Activity on both sides alone does not show that one side drives the other.

Tracing reveals fibers crossing the corpus callosum between the ACCs, with connected neurons in both superficial and deep layers. Labeled fibers from one side appear among cells on the other, providing an anatomical basis for bilateral communication.

Original figure 1Anatomical links between the ACCs

02 / Synaptic communication

Glutamate makes the connection excitatory

Does the connection transmit signals? Stimulating fibers from one ACC evokes excitatory synaptic currents in contralateral pyramidal cells, including responses to repeated stimulation. Glutamate-receptor blockade greatly reduces these currents. Further recordings identify a major AMPA-receptor component and an additional kainate-receptor component in some cells.

Input strength also matters for calcium signaling. Weaker inputs that do not trigger spikes produce no significant increase in the recorded somatic calcium signal. Inputs that evoke action potentials, especially bursts, produce larger calcium responses. Communication across the midline can thus alter both electrical activity and intracellular signaling.

Original figure 2Excitatory currents evoked by projection fibers

03 / Greater sensitivity

Stronger communication changes withdrawal responses

In uninjured male mice, activating the connection lowers mechanical withdrawal thresholds in both hind paws and shortens hot-plate and tail-flick response latencies. Rotarod performance does not change significantly, indicating no evident accompanying change in motor performance in that assay.

Affect-related measures do not change uniformly. Time in the open arms of the elevated plus maze decreases, but entry counts do not change significantly. Open-field center exploration and conditioned place aversion also show no significant differences. Heightened stimulus sensitivity and learned avoidance of a location therefore have different outcomes here.

Original figure 5Sensory and behavioral responses in males

04 / A separate female cohort

Females also become more sensitive

Females also show lower mechanical thresholds in both hind paws and faster hot-plate responses, extending the sensory finding beyond males. In the elevated plus maze, both open-arm time and entries decrease in females, whereas the significant male change is in time spent there.

Open-field center exploration and place-aversion scores remain without significant changes in females. Increased mechanical and heat responsiveness is shared across the cohorts, while the exploration measures vary with assay and sex.

Original figure 6Sensory responses and exploration in females

05 / Region and connection

The whole region has a broader reach

The researchers also manipulate the activity of an entire unilateral ACC population. In uninjured mice, activation lowers the opposite paw’s mechanical threshold, whereas inhibition raises it. Hot-plate responses become faster with activation and slower with inhibition, demonstrating a capacity of unilateral ACC activity to modulate nociceptive responses.

The ACC also connects to several subcortical targets. When interhemispheric fibers are inhibited, however, only one paw’s mechanical threshold increases significantly in uninjured animals; the other paw and hot-plate latency show no significant change. The behavioral reach of the entire ACC and its interhemispheric connection is therefore not identical.

Original figure 7Effects of directly altering unilateral ACC activity

06 / After nerve injury

Inhibition has different time courses in the two paws

After unilateral peripheral nerve injury, both the injured-side and opposite hind paws become mechanically hypersensitive. Inhibiting the interhemispheric connection raises the injured paw’s threshold on days 3, 7, 14 and 28. A significant increase in the opposite paw occurs only on day 7. The connection contributes to bilateral sensitivity with different temporal patterns.

Conversely, activating the connection does not significantly lower the injured paw’s threshold further, but increases opposite-paw sensitivity at some time points. In the injured network, increasing and decreasing activity in the same pathway do not always yield mirror-image outcomes.

Original figure 8Interhemispheric fibers and post-injury mechanical sensitivity