The morphology of growing thalamocortical axons in developing rat

Abstract

Morphological and electrophysiological properties of calbindin D-28k-, GABA- and dopamine-β-hydroxylase-immunopositive neurons were investigated in the caudal nucleus of tractus solitarius of rats, using a patch-clamp whole-cell recording combined with intracellular staining and immunocytochemistry. Calbindin D-28K- and GABA-positive neurons had a small cell body (10.9±0.3 μm in diameter) and were distributed throughout the caudal nucleus of tractus solitarius. Double fluorescence immunocytochemistry revealed that calbindin- and GABA-positive neurons formed distinct subpopulations. Calbindin- and GABA-positive neurons double stained for biocytin showed extensive axon collaterals within the nucleus of tractus solitarius and some calbindin-positive, but not GABA-positive neurons, had also projection axons leaving the nucleus of tractus solitarius. Dopamine-β-hydroxylase-immunopositive neurons had a small (10.8±0.3 μm) or large (17.2±0.4 μm) cell body. Neurons with a small cell body were observed in the dorsomedial nucleus at the level of the area postrema, and in the area postrema, while neurons with a large cell body were observed in the medial nucleus throughout the caudal nucleus of tractus solitarius. Double fluorescence immunocytochemistry revealed that almost all small dopamine-β-hydroxylase-positive were also immunoreactive for calbindin, while large dopamine-β-hydroxylase-positive neurons were not. Double staining for dopamine-β-hydroxylase and biocytin showed that neurons with a small cell body had moderate axon collaterals. On the contrary, neurons with a large cell body had a few, if any, axon collaterals and a projection axon which could leave the nucleus of tractus solitarius.Following stimulation of the tractus solitarius, all neurons with a small cell body exhibited a polysynaptic excitatory response (type I neurons), while dopamine-β-hydroxylase-immunopositive neurons with a large cell body exhibited a monosynaptic excitatory response (type II neurons) or an excitatory followed by an inhibitory response (type III neurons). Spontaneous and evoked excitatory postsynaptic currents of (type I neurons) calbindin- or GABA-positive neurons were reversibly blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. Spontaneous and evoked inhibitory postsynaptic currents of type III neurons were reversibly blocked by bicuculline. Type II neurons showed no spontaneous excitatory nor inhibitory postsynaptic currents.It was concluded that the three kinds of chemically-defined neurons formed distinct neuronal subpopulations in the caudal nucleus of tractus solitarius in terms of synaptic responses and morphological characteristics such as cell size and axonal trajectory.