Hippocampus
-
In urethane-anesthetized rats, cortical regions which provide distal dendritic excitation of the dentate gyrus and CA1 of the dorsal hippocampus were studied using current source density analysis. Electrical stimulation of the lateral perforant path (LPP) in the lateral angular bundle, lateral entorhinal cortex (LEC), and amygdala-entorhinal transition (TR) resulted in a current sink in the outer molecular layer of the dentate gyrus accompanied by proximal sources; this sink-source pattern is distinctly different from the source-sink-source pattern evoked by medial perforant path stimulation. The progressive decrease of the sink latency following stimulation of the TR, LEC, and LPP (11.6, 7.8, and 3.6 ms, respectively, at the dorsal blade of the dentate gyrus) suggests a possible sequence of orthodromic activation of these structures. ⋯ We conclude that the PRh does not provide a significant excitatory input to the DG or CA1. We have found distinct dendritic excitation of the dentate gyrus by the lateral versus medial perforant paths, and by fiber (LPP and MPP) versus cortical (LEC and TR) stimulation. We also emphasize that processing in the entorhinal cortex is important in the temporal shaping of the signals afferent to the hippocampus.
-
Intracellular recordings were used to study the electrophysiological properties of rat subicular neurons in a brain slice preparation in vitro. Cells were classified as bursting neurons (n = 102) based on the firing pattern induced by depolarizing current pulses. The bursting response recorded at resting membrane potential (-66.1 +/- 6.2 mV, mean +/- SD n = 94) was made up of a cluster of fast action potentials riding on a slow depolarization and was followed by an afterhyperpolarization. ⋯ Tetraethylammonium (10 mM)-sensitive, outward rectification became apparent in the presence of TTX. These results suggest that neurons in the rat subiculum can display voltage-dependent bursts of action potentials as well as membrane rectification in the depolarizing and hyperpolarizing directions. These results also indicate that activation of a voltage-gated Na+ conductance may be instrumental in the initiation of bursting activity.
-
A precise description of the timing and route traveled by axons traversing the telencephalic midline through the ventral hippocampal commissure (HC) is essential for understanding the role it plays in the formation of the corpus callosum (CC). A normal baseline of HC development was described in B6D2F2 hybrid mice and then compared with two inbred strains of mice displaying callosal agenesis, BALB/cWah1 (50% CC defect) and 129/J (70% CC defect), their F2 hybrid (C129F2-33% CC defect), and a recombinant inbred strain (RI-1-100% CC defect) derived from pairs of C129F2 mice. Embryos weighing from 0.25 g to 0.70 g (E14.5-E17) were collected and fixed by perfusion. ⋯ Initial HC crossing occurred at about 0.470 g (E16.25) in BALB mice and about 0.520 g (E16.5) in 129 mice. In the RI-1 embryos, first HC crossing was estimated at about 0.750 g (E17.5), although several older embryos showed no crossing. These results show the importance of the HC for successful CC formation and suggest that absent CC arises as a consequence of a developmental defect which affects the formation of the hippocampal commissure prior to arrival of CC axons at midplane.