Second Hippocampal Zinc-rich Synaptic System

Abstract

AMONG ‘peculiarities of uncal hippocampus’ revealed by silver-sulphide impregnation I have elsewhere¹ reported the presence of zinc-rich synaptic bags, of a smaller order of magnitude than in the main hippocampus, extending from CA4 into the inner molecular stratum of the fascia dentata and within the ‘tail’ of the end-bulb of the mossy fibre system in the guinea pig. There is one further hippocampal region, remote from the uncus and from the mossy fibre system, that exhibits similar smaller-order zinc-rich synaptic bags. It extends as a narrow zone, readily seen in well-impregnated transverse sections under medium magnifications (Fig. 1), from the apex of the subiculum along the junction of alveus and stratum oriens over the whole length and breadth of CA1. Higher magnifications show that, whereas in subiculum proper lighter brown synaptic entities outline alveo-perpendicular apical dendrites, in the alveo-oriens zone darker-impregnating entities outline dendritic processes that are multipolar and weave a loose alveo-parallel network. The somata of origin of these skeining dendrites can be ascertained by counterstaining, rather indefinitely in most normal preparations, but confidently in certain pathological circumstances (to be described elsewhere). They lie within the network, leaving little doubt that they are the somata of the ‘horizontal neurones’ displayed in this zone by Golgi methods. Golgi preparations reveal no conspicuous axonal swellings or dendritic spines in this neighbourhood and show most of the horizontal dendrites to lie distal to the terminal bushwork of the basal dendrites of the CA1 pyramidal cells. Hence, the zinc-rich entities here would seem to be most probably axo-dendritic synapses with ‘axon terminals’ of the intermediate type defined electronmicroscopically by Blackstad², the axons concerned being presumably CA1 pyramidal ones before they are myelinated, or collaterals thereof, since the synaptic entities are shrunken here after ‘selective ablation’ of CA1 pyramidal cell somata by hypoglycæmic seizures, much as in the end-bulb³. Further electron-microscope, as well as enzyme, investigations are obviously needed in this realm.