Localization of VIP and PHI-27 messenger RNA in rat thalamic and cortical neurons

Messenger RNA (mRNA) coding for vasoactive intestinal polypeptide (VIP) and peptide histidine isoleucine (PHI-27) were localized in cortical and thalamic neurons with synthetic DNA probes complementary to the PHI-27 and VIP exon coding sequence of the rat VIP precursor gene. Hybridization signal with these probes was found widely distributed in the thalamus, neocortex, and pyriform cortex, and the distribution of hybridization signal for each probe was identical. Furthermore, the distribution of each message was correlated closely with peptide distribution demonstrated immunohistochemically. Labeling of individual neurons with in situ hybridization histochemistry was characterized by a dense accumulation of silver grains in the cytoplasm of these cells with little or no label in the nucleus. Labeled neurons in the thalamus were observed in the ventrolateral, ventromedial, ventrobasal, and lateral reticular nuclei. In the neocortex, the distribution of labeled neurons was concentrated in layers II and III with scattered cells also apparent in deeper cortical layers. Hybridization signal was limited to nonpyramidal neurons in both the neo- and pyriform cortex. The coextensive distribution of immunoreactivity and mRNA coding regions for VIP and PHI-27 establishes that these peptides are synthesized from the same precursor mRNA in the same thalamic and cortical cell groups. Although the physiological role of these peptides in thalamocortical function remains unknown, these data provide an anatomical substrate which suggests that VIP and PHI-27 may be cotransmitters in thalamic and cortical neurons.