Somatostatin-like immunoreactivity characterizes neurons of the nucleus reticularis thalami in the cat and monkey

Abstract

This report presents immunohistochemical evidence for the occurrence of a somatostatin-related peptide in neurons of the thalamic reticular nucleus, a nucleus known to modulate thalamic activity by γ-aminobutyric acid (GABA)-mediated inhibition. In eight cats and kittens and one rhesus monkey, sections through the nucleus reticularis thalami and adjoining regions of the thalamus, subthalamus and mesencephalic tegmentum were incubated in anti-somatostatin antisera and processed by the Sternberger peroxidase-antiperoxidase or Coons direct immunofluorescence methods. In each brain, intense somatostatin-like immunoreactivity was found in neurons of the nucleus reticularis thalami. The immunoreactivity was localized to the perinuclear region of the neurons in both species. Somatostatin-like immunoreactivity appeared in only a sharply restricted subset of other zones thought to have an embryologic origin in the ventral thalamus (including, in the cat, two nuclei that project to the amygdala: the nucleus subparafascicularis and the peripeduncular nucleus of the mesodiencephalic tegmentum). An incidental finding was that the pars lateralis of the cat's substantia nigra contains a system of fine fibers expressing somatostatin-like immunoreactivity. In the nucleus reticularis of the cat, immunoreactive neurons were observed regularly in experiments with antisera generated against somatostatin-14 and were also found in preliminary experiments with an antiserum against the 'non-somatostatin; (1 to 14) sequence of somatostatin-28. These observations, together with the perinuclear location of the immunoreactivity, suggest that a somatostatin precursor molecule may be present or at least that the neurons contain both the larger and smaller somatostatin-related peptides. The neurons of the nucleus reticularis also showed moderate acetylcholinesterase activity but were negative in tests of immunoreactivity to antisera raised against avian pancreatic polypeptide and Met-enkephalin. We conclude that the presence of somatostatin-like immunoreactivity may be a defining characteristic of the nucleus reticularis thalami and suggest that the functional activity of the neurons of the nucleus reticularis may involve not only a GABA mechanism but also the action of somatostatin or a related neuropeptide.