Axonal ramifications of hippocampal Ca1 pyramidal cells

Abstract

Intracellular injections of Lucifer Yellow into CA1 pyramidal cells of the in vitro guinea pig hippocampal slice enabled us to examine in detail the morphology of the axons of these neurons. We also recorded the electrophysiological responses of these neurons to alvear stimulation. In our morphological examinations, we found that many axons bifurcate in the alveus, with the major branch projecting caudally toward the subiculum and the second, thinner branch projecting rostrally toward the fimbria. Either axons may bifurcate further to produce several axon branches which follow parallel paths in the alveus. These axons also have local collaterals which project into strata oriens and pyramidale. In addition, a very fine plexus of axonal processes was observed in stratum oriens located largely within the basal dendritic field of the parent cell. Our electrophysiological experiments demonstrated that neurons could be activated antidromically by stimulation of the alveus at sites both rostral ad caudal to the neuron. Weak alvear stimulation occasionally evoked small potentials which appeared similar to fast prepotentials. The local axonal ramifications may be involved in recurrent pathways mediating feedback inhibition and/or excitation. The axonal bifurcations also may provide a basis for understanding the origins of fast prepotentials elicited with antidromic stimulation.

Interlamellar CA1 network in the hippocampus

“…Two-photon imaging of intracellularly labeled CA1 pyramidal neurons shows that every CA1 pyramidal neuron that was examined sends out a longitudinally oriented axon that branches off, and is perpendicular to, the thick, well-established projection that goes toward the subiculum and fimbria. The subicular and fimbrial axonal projections agree with established data (17), but the longitudinally projecting collateral has not been described. Note that although we use the term "longitudinal" as others have, that this term was done partly for convenience: The most favorable orientation for excitatory effects was in the longitudinal plane; however, stimulation at angles oblique to the longitudinal plane was also quite effective in activating CA1 pyramidal cells.…”

“…It has been proposed that the previously described sparse associational projections to neighboring transverse lamellae in CA1 are primarily directed onto inhibitory interneurons (17,18). To investigate whether the associational axonal processes described here represent excitatory-excitatory or excitatory-inhibitory connections, we looked for immunohistochemical evidence of contact with synaptic spines of pyramidal neurons.…”

Visualization of the spread of electrical activity in rat hippocampal slices by voltage‐sensitive optical probes

“…On the other hand, MacVicar & Dudek (1981) reported that the small fast prepotentials detected at the somata of CA3 pyramidal cells are a reflexion of electrical coupling rather than of dendritic spikes. Knowles & Schwartzkroin (1981) recently suggested that the fast prepotentials are a passive reflection of action potentials in axonal collaterals of the pyramidal cells. Jeffereys (1979) found that in dentate granular cells, the action potential is generated in the soma and is only triggered by the dendritic e.p.s.p.…”

Local axon collaterals of area CA1 support spread of epileptiform discharges within CA1, but propagation is unidirectional

“…The anatomical evidence for a dense projection from CA1 to subiculum is well established (Witter and Amaral, 2004). There are examples of individual CA1 neurons with pronounced asymmetries of their axonal arbors (Finch and Babb, 1981;Knowles and Schwartzkroin, 1981;Tamamaki et al, 1987), although some cells have symmetrical axonal arbors (Finch et al, 1983). Importantly, for the development of the argument that projection collaterals contribute to the asymmetry of the intrinsic collateral system, there are several examples of CA1 pyramidal cells showing axonal extensions that reach up into stratum oriens arising from the major collaterals (Knowles and Schwartzkroin, 1981;Lorente de No, 1934).…”

“…There are examples of individual CA1 neurons with pronounced asymmetries of their axonal arbors (Finch and Babb, 1981;Knowles and Schwartzkroin, 1981;Tamamaki et al, 1987), although some cells have symmetrical axonal arbors (Finch et al, 1983). Importantly, for the development of the argument that projection collaterals contribute to the asymmetry of the intrinsic collateral system, there are several examples of CA1 pyramidal cells showing axonal extensions that reach up into stratum oriens arising from the major collaterals (Knowles and Schwartzkroin, 1981;Lorente de No, 1934). Lorente de Nó described both the local axonal arbors around individual CA1 cells (Lorente de No, 1934) (we have similar data from Golgi studies in rats, unpublished), and collaterals arising from CA1 alvear fibers that entered the ''lower parts'' of CA1.…”