Somato-dendritic synapses in the nucleus reticularis thalami of the rat

Upregulated expression of oncomodulin, the beta isoform of parvalbumin, in perikarya and axons in the diencephalon of parvalbumin knockout mice

The calcium-binding proteins parvalbumin, calbindin D-28k, calretinin and calcineurin are present in subsets of GABAergic gigantic calyciform presynaptic terminals of the reticular thalamic nucleus (RTN). Previously it was hypothesized that GABA and calcium-binding proteins including parvalbumin are not only colocalized in the same neuron subpopulation, but that GABA synthesis and parvalbumin expression could be also genetically regulated by a common mechanism. Moreover, parvalbumin expression levels could influence GABA synthesis. For this, we analyzed GABA immunoreactivity in RTN gigantic calyciform presynaptic terminals of parvalbumin-deficient (PV-/-) mice. With respect to GABA immunoreactivity we found no differences compared to wild-type animals. However, using a polyclonal parvalbumin antibody raised against full-length rat muscle parvalbumin on brain sections of PV-/- mice, we observed paradoxical parvalbumin immunoreactivity in partly varicose axons in the diencephalon, mainly in the lamina medullaris externa surrounding the thalamus. A detailed immunohistochemical, biochemical and molecular biological analysis revealed this immunoreactivity to be the result of an upregulation of oncomodulin (OM), the mammalian beta isoform of parvalbumin in PV-/- mice. In addition, OM was present in a sparse subpopulation of neurons in the thalamus and in the dentate gyrus. OM expression has not been observed before in neurons of the mammalian brain; its expression was restricted to outer hair cells in the organ of Corti. Our results indicate that the absence of parvalbumin has no major effect on the GABA-synthesizing system in RTN presynaptic terminals excluding a direct effect of parvalbumin on this regulation. However, a likely homeostatic mechanism is induced resulting in the upregulation of OM in selected axons and neuronal perikarya. Our results warrant further detailed investigations on the putative role of OM in the brain.

Calcium-binding proteins in GABAergic calyciform synapses of the reticular nucleus

Large calyciform synapses in the rat reticular thalamic nucleus are characterized by the presence of gamma-aminobutyric acid. Presynaptic terminals are also loaded with calcium-binding proteins such as parvalbumin, calbindin, calretinin and calcineurin. The number of calyciform terminals containing gamma-aminobutyric acid and parvalbumin is 2005 in young adult rats; calbindin is present in 1,500, calretinin in 850 and calcineurin in 560 calyciform terminals. Developmental studies revealed that gamma-aminobutyric acid and calcium-binding proteins are virtually absent from calyciform terminals at birth but their occurrence increased considerably during postnatal life, suggesting increasing regulation of presynaptic calcium signaling during postnatal life. It is concluded that synaptic activity of large calyciform gamma-aminobutyric acid-containing synapses of the reticular thalamic nucleus is mediated, regulated or accompanied by calcium ions.

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.