Heterogeneity of neuronal firing type and morphology in retrosplenial cortex of male F344 rats

The rodent granular retrosplenial cortex (gRSC) has reciprocal connections to the hippocampus to support fear memories. Although activity-dependent plasticity occurs within the RSC during memory formation, the intrinsic and morphological properties of RSC neurons are poorly understood. The present study used whole-cell recordings to examine intrinsic neuronal firing and morphology of neurons in layer 2/3 (L2/3) and layer 5 (L5) of the gRSC in adult male rats. Five different classifications were observed: regular-spiking (RS), regular-spiking afterdepolarization (RS_ADP), late-spiking (LS), burst-spiking (BS), and fast-spiking (FS) neurons. RS_ADP neurons were the most commonly observed neuronal class, identified by their robust spike frequency adaptation and pronounced afterdepolarization (ADP) following an action potential (AP). They also had the most extensive dendritic branching compared with other cell types. LS neurons were predominantly found in L2/3 and exhibited a long delay before onset of their initial AP. They also had reduced dendritic branching compared with other cell types. BS neurons were limited to L5 and generated an initial burst of two or more APs. FS neurons demonstrated sustained firing and little frequency adaptation and were the only nonpyramidal firing type. Relative to adults, RS neurons from juvenile rats (PND 14-30) lacked an ADP and were less excitable. Bath application of group 1 mGluR blockers attenuated the ADP in adult neurons. In other fear-related brain structures, the ADP has been shown to enhance excitability and synaptic plasticity. Thus, understanding cellular mechanisms of the gRSC will provide insight regarding its precise role in memory-related processes across the lifespan.NEW & NOTEWORTHY This is the first study to demonstrate that granular retrosplenial cortical (gRSC) neurons exhibit five distinctive firing types: regular spiking (RS), regular spiking with an afterdepolarization (RS_ADP), late spiking (LS), burst spiking (BS), and fast spiking (FS). RS_ADP neurons were the most frequently observed cell type in adult gRSC neurons. Interestingly, RS neurons without an ADP were most common in gRSC neurons of juvenile rats (PND 14-30). Thus, the ADP property, which was previously shown to enhance neuronal excitability, emerges during development.

Predominance of late-spiking neurons in layer VI of rat perirhinal cortex

Recent work demonstrated the importance of perirhinal cortex (PR) in a variety of behavioral tasks and disease processes. Studies from our laboratory revealed that some layers of PR contain neurons with unusual properties. Here we report a detailed examination of the cellular neurobiology of layer VI of PR, using whole-cell recordings and biocytin cell fills in horizontal rat brain slices. The most striking finding is that an overwhelming majority ( approximately 86%) of neurons are late-spiking (LS) cells, which can delay the onset of their spike trains by several seconds or more relative to the onset of a depolarizing current step. LS neurons previously have been shown to exist only in very small numbers in a limited number of other cortical regions. Anatomical reconstructions have revealed that the LS neurons vary greatly in morphology, including both pyramidal and nonpyramidal cells. Another surprising physiological finding is the fact that single-spiking (SS) neurons are the second most common cell type ( approximately 7%). SS neurons issue only a single action potential even in response to extreme depolarization. They have been seen previously in the amygdala, but never in cortex. A third remarkable finding is that there are almost no regular spiking (RS) neurons, unlike all other cortical regions that have been studied. This unique abundance of LS neurons in layer VI, along with the presence of SS neurons and the absence of RS neurons, demonstrates that layer VI of PR is unlike any other cortical region that has been studied to date.