LACK OF INTRINSIC GABAERGIC NEURONS IN THE RODENT NUCLEUS REUNIENS

Little is documented about the inhibitory neurons of the midline thalamic nucleus reuniens (Re) in rodents. Because Re provides a major anatomical bridge between the medial prefrontal cortex and hippocampus, defining its inhibitory organization is critical for circuit models and interpreting rodent behavior, particularly in cognitive domains. This study aimed to determine whether the rodent Re contains intrinsic GABAergic neurons and to characterize their spatial distribution. Considering this, we used two complementary
approaches: immunostaining for GAD67, which reflects the cytoplasmic capacity for GABA synthesis, and direct GABA immunostaining. Both stainings targeted Re, with the reticular thalamic nucleus (Rt) used as a positive control due to its predominantly GABAergic neuronal population. In mouse brain slices, GAD67 immunoreactivity in Re did not exhibit the distinct somatic labeling pattern observed in Rt. Instead, staining in Re was comparable to negative-control sections, indicating an absence of GAD67-positive neuronal somata. Notably, clusters of GAD-immunoreactive puncta were present in the dorsal Re neuropil without associated somatic labeling, a pattern consistent with inhibitory axon terminals originating from extrinsic afferents rather than from local interneurons. Sporadically, cytoplasmic GABA immunoreactivity was detected within the core of Re. The GABA staining displayed a distinct cellular pattern without colocalization with GAD67, suggesting that these cells may produce or accumulate GABA through a metabolic pathway independent of the GAD67 enzyme. Together, the absence of clear GAD67-positive neuronal somata supports the hypothesis that the nucleus reuniens lacks intrinsic GABAergic neurons. Ongoing experiments in rats will determine whether this phenomenon is conserved across species.