Abstract
GABAergic local circuit neurons are critical for the network activity and functional interaction of the amygdala and hippocampus. Previously, we obtained evidence for a GABAergic contribution to the hippocampal projection into the basolateral amygdala. Using fluorogold retrograde labeling, we now demonstrate that this projection indeed has a prominent GABAergic component comprising 17% of the GABAergic neurons in the ventral hippocampus. A majority of the identified GABAergic projection neurons are located in the stratum oriens of area CA1, but cells are also found in the stratum pyramidale and stratum radiatum. We could detect the expression of different markers of interneuron subpopulations, including parvalbumin and calbindin, somatostatin, neuropeptide Y, and cholecystokinin in such retrogradely labeled GABA neurons. Thus GABAergic projection neurons to the amygdala comprise a neurochemically heterogeneous group of cells from different interneuron populations, well situated to control network activity patterns in the amygdalo-hippocampal system.
Highlights
The amygdala and hippocampus are tightly connected both anatomically and functionally
No retrograde labeling was observed in the dorsal hippocampus, in the CA3 or dentate gyrus of the ventral hippocampus, or in the contralateral hemisphere, unless the injection needle had accidentally injured the fimbria/fornix bundle
We show that approximately 16% of all GABAergic neurons in the hippocampal CA1/subiculum transition zone project to the amygdala, comprising about 5% of all retrogradely labeled neurons from the amygdala in this region
Summary
The amygdala and hippocampus are tightly connected both anatomically and functionally. We have recently demonstrated a GABAergic projection from the ventral hippocampal formation to the amygdala [18]. To further characterize this projection, in the current study we analyzed the distribution in the ventral hippocampus of GABAergic neurons retrogradely labeled from the amygdala and their expression of subpopulation-specific neurochemical markers, i.e. the calcium-binding proteins parvalbumin, calbindin, and calretinin and the neuropeptides. An important role for amygdalo-hippocampal circuits in fear expression and extinction of fear memory was shown [19] Anxiety disorders such as post-traumatic stress disorder (PTSD) produce changes in neuronal activity in these circuits [20]. PV, parvalbumin; CB, calbindin; CR, calretinin; NPY, neuropeptide Y; SOM, somatostatin; CCK, cholecystokinin
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