Ultrastructure of the isthmic nucleus and identification of the synaptic contacts received by the neurons of the crossed isthmotectal projection in Rana esculenta.
The isthmic nucleus receives its major input from the ipsilateral optic tectum and projects to both tecta. It has been regarded as a relay station that is not involved in further processing of visual information. Recent studies on the connectivity and neurochemistry of this nucleus yielded substantial new data suggesting a more important role as being a simple relay. Since data on the ultrastructure of this nucleus are scarce the aim of this study was to obtain further information about the synaptic organisation of the nucleus. The frog Rana esculenta was used for our studies. The projection neurons of the isthmic nucleus were retrogradely labelled with HRP via the crossed isthmotectal tract. The tracer has been visualised with the diaminobenzidine reaction and the synaptic contacts have been studied with the electron microscope. Analysis of the synapses and profile types showed that both symmetrical and asymmetrical synapses are present in the nucleus. The dominant synapse type is axodendritic. Presynaptic profiles contain primarily round clear vesicles, but flattened vesicles also occur. Besides the axodendritic synapses, axosomatic contacts were also found in low number. Two kinds of dense core vesicles (large round or large elongated) were seen coexisting in the above described terminals. Close membrane appositions between the cortical cells suggest the probability of electrotonic coupling. Analysis of the relative frequency of synaptic profiles in different locations of the nucleus showed that the medullar neuropile is inhomogeneous. Our data support the view that the organisation of the isthmic nucleus is more diverse than that has been supposed until recently raising new functional considerations regarding isthmic information processing.
Pollák
E
E
Department of General Zoology and Neurobiology, Janus Pannonius University, Pées, Hungary.
Tóth
P
P
Gábriel
R
R
Lázár
G
G
D000818:Animals; D001370:Axonal Transport; D001921:Brain / Q000502:physiology / Q000648:ultrastructure*; D005260:Female; D006735:Horseradish Peroxidase; D008297:Male; D008854:Microscopy, Electron; D009474:Neurons / Q000502:physiology / Q000648:ultrastructure*; D011893:Rana esculenta; D013477:Superior Colliculi / Q000502:physiology / Q000648:ultrastructure*; D013569:Synapses / Q000502:physiology / Q000648:ultrastructure*
D006735:Horseradish Peroxidase