GABAergic parvalbumin-immunoreactive large calyciform presynaptic complexes in the reticular nucleus of the rat thalamus

In the reticular thalamic nucleus of the rat, nearly all neurons are parvalbumin-immunoreactive. We found that in addition, though superficially similar to large parvalbumin-immunoreactive neurons, also numerous peculiar parvalbumin-immunoreactive complexes are present in the reticular thalamic nucleus which are not identical with parvalbumin-immunoreactive perikarya, as shown by nuclear variation curves. Light and electron microscopic immunocytochemical studies revealed that these parvalbumin-immunoreactive complexes are brought about by parvalbumin-immunoreactive calyciform terminals which establish synapses with large, parvalbumin-immunonegative dendritic profiles. Transection of thalamo-reticular connections did not cause any alteration of calyciform terminals in the reticular thalamic nucleus. Nuclear counterstaining revealed that parvalbumin-immunoreactive calyciform terminals originated from local parvalbumin-immunoreactive interneuronal perikarya, which, depending of the length of the “neck” protruding from the perikaryon, establish somato-dendritic, axo-dendritic or dendro-dendritic synapses. Light and electron microscopic immunocytochemical investigations prove that the parvalbumin-immunoreactive calyciform complexes contain also GABA, that are likely to be inhibitory. In accordance with literature data, our results suggest that parvalbumin-immunoreactive GABAergic calyciform terminals in the reticular thalamic nucleus may be instrumental in intrinsic cell-to-cell communications and, as such, may be involved in synchronisation of thalamo-cortical oscillations, in the production of sleep spindles and in attentional processes.