Analysis of gamma-aminobutyric acidergic synaptic contacts in the thalamic reticular nucleus of the monkey

Gamma-aminobutyric acidergic (GABAergic) neurons in the thalamic reticular nucleus (TRN) spontaneously generate a synchronous bursting rhythm during slow-wave sleep in most mammals. A previous study at the electron microscopic level in cat anterior TRN has suggested that synchronous bursting activity could result from the large number of presumably GABAergic dendrodendritic synaptic contacts. However, little is known about the synaptology of the monkey thalamic reticular nucleus and whether it contains dendrodendritic contacts. To address this issue, we examined tissue obtained from Macaca fascicularis that was prepared for electron microscopy using postembedding techniques to demonstrate GABA immunoreactivity. Examination of the anterior (motor) and posterior (somatosensory) portions of the TRN disclosed the following: The majority of synaptic contacts (87.5% of 958) were formed by axon terminals showing no GABA immunoreactivity and making asymmetric synaptic contacts on dendrites or cell bodies. A further 6.4% of synaptic contacts was composed of GABA-immunoreactive presynaptic terminals making symmetric contacts with the dendrites of TRN neurons. The majority resembled the pleomorphic vesicle containing F-terminals seen in the dorsal thalamus and known to originate from axons of TRN. A subset or possible second class did not resemble any previously described class of GABA-immunoreactive terminals in the TRN. Both classes of these terminals making symmetric contacts may originate wholly or partially within the nucleus. There was one dendrodendritic synaptic contact and only a small number (3.2%) of axodendritic contacts with synaptic vesicles visible both pre- and postsynaptically. We conclude that dendrodendritic contacts are probably not responsible for the synchronized bursting neuronal activity seen in the slow-wave sleep of monkeys, and that, if TRN neurons are coupled synaptically, the most likely mechanism is through the synapses formed by recurrent axon collaterals of TRN neurons onto TRN dendrites.