Neurones projecting to the hypothalamus from the brainstem A1 catecholaminergic cell group express glutamate-R2,3 receptor immunoreactivity.

Stimulation of ascending catecholaminergic neurones of the A1 region in ventrolateral medulla by excitatory amino acids mediate neurohypophysial vasopressin secretion triggered by hypovolemic hypotension. Recent cloning of the ionotrophic excitatory amino acid receptors of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type and subsequent production of receptor recognizing antisera have made immunocytochemical detection of receptor proteins in phenotypically characterized neurones possible. Using single immunocytochemical detection of glutamate GluR1, GluR2,3, GluR4 receptor proteins we have investigated the distribution of GluR-receptor proteins in the caudal ventrolateral medulla. In the neurones of the A1 cell group, only GluR2,3-immunoreactivity was expressed whereas GluR1-immunoreactive neurones were seen in the adjacent reticular formation. Using dual immunocytochemistry in combination with retrograde Fluorogold tracing we determined the extent of co-expression of tyrosine-hydroxylase and glutamate GluR2,3 receptor protein immunoreactivity in neurones of the A1 cell group in the ventrolateral medulla that project to the area of the paraventricular nucleus of the hypothalamus. It was seen that the majority of catecholaminergic A1 neurones of the caudal VLM that project directly to the paraventricular nucleus are also immunoreactive to the Glu R2,3 receptor protein further substantiating that these neurones are directly influenced by the excitatory amino acid glutamate.