Innervation of VIP-immunoreactive neurons by the ventroposteromedial thalamic nucleus in the barrel cortex of the rat

Abstract

We investigated the synaptic terminals of fibers originating in the ventroposteromedial thalamic nucleus (VPM) and projecting to the main input layers (IV/III) of the rat posteromedial barrel subfield. It was our aim to determine whether or not the subpopulation of vasoactive intestinal polypeptide (VIP)-immunoreactive neurons in these layers are directly innervated by the sensory thalamus. Anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHA-L) and immunohistochemistry for VIP were combined for correlated light and electron microscopic examination. Columns of cortical tissue were well defined by barrel-like patches of PHA-L-labeled fibers and boutons in layers IV and III. Within these columns VIP-immunoreactive perikarya were located mainly in supragranular layers. Marked perikarya were also seen in infragranular layers, but their immunoreactivity was often weaker. Granular layer IV, which is the main terminal field for thalamic fibers, contained fewer VIP neurons than supragranular layers. In the light microscope, however, PHA-L-labeled fibers appeared to contact the somata or proximal dendrites of 60-86% of the layer IV VIP neurons . By contrast, only 18-35% of the VIP neurons in the supragranular layers, which receive a moderately dense projection from the VPM, appeared to be contacted. PHA-L-labeled boutons were seen close to 13-25% of infragranular VIP-positive cells. Electron microscopy showed that thalamic fibers formed at most four asymmetric synapses on a single layer IV, VIP-positive neuron. Although the proportion of VIP-positive neurons with labeled synapses was lower in supragranular layers, most of them shared multiple asymmetric synapses with labeled thalamic fibers. Up to six labeled synapses were seen on individual VIP neurons in layer III. We conclude that subpopulations of VIP-immunoreactive neurons, located in layers IV, III, and II are directly innervated by the VPM. These neurons may be involved in the initial stages of cortical processing of sensory information from the large, mystacial vibrissae. Since VIP is known to be colocalized with the inhibitory transmitter GABA, it is likely that VIP neurons participate in the shaping of the receptive fields in the barrel cortex.

Innervation of interneurons immunoreactive for VIP by intrinsically bursting pyramidal cells and fast‐spiking interneurons in infragranular layers of juvenile rat neocortex

“…Recently, we have been able to describe several afferent connections, e.g. excitatory input from the ventrobasal thalamic nucleus (Staiger et al ., 1996), local excitatory input (present study) and local inhibitory input by parvalbumin‐immunoreactive boutons (Staiger et al ., 1997). Because the latter are considered to represent mainly basket (and axo‐axonic) cells, this is in accordance with the present demonstration of a rich contribution of fast‐spiking multipolar interneurons to the local innervation of layer V VIP cells.…”

Properties of bipolar VIPergic interneurons and their excitation by pyramidal neurons in the rat neocortex

“…1984;Peters & Harriman 1988;Chédotal et al . 1994a;Kawaguchi & Kubota 1996;Staiger et al . 1996;Bayraktar et al .…”

“…The existence of asymmetrical synapses onto VIP‐immunopositive bipolar cells indicates that neocortical VIPergic interneurons receive glutamatergic inputs, one of which is constituted by thalamic fibres (Hajos et al . 1988;Staiger et al . 1996).…”

Barrel cortex VIP/ChAT interneurons suppress sensory responses in vivo

“…The anatomical and physiological properties of the cortical VIP + cells [39,43,44,76], as well as their long-range inputs, are similar to those of VChIs [77][78][79][80]. However, VChIs constitute approximately 30% of the VIP + cells, and most studies of VIP + population do not distinguish between VIP + /ChAT + and VIP + /ChAT − .…”