Differential expression of rat and human type I metabotropic glutamate receptor splice variant messenger RNAs

The type I metabotropic glutamate receptor (mGlu1) messenger RNA and protein are known to be widely expressed in rat brain, but knowledge of the regional expression of splice variants other than mGlu1a is limited. Probes were designed for in situ hybridization that specifically recognize each of the carboxy-terminal splice variants mGlu1a, -1b, -1c and -1d. The novel rat mGlu1d sequence was obtained by polymerase chain reaction and the predicted protein is highly homologous to the human sequence but contains both conservative and radical substitutions and is slightly longer (912 vs 908 amino acids). Each rat mGlu1 splice variant messenger RNA was found in a unique expression pattern. The messenger RNA encoding mGlu1a was abundant in cerebellar Purkinje cells and in mitral and tufted cells of the olfactory bulb. Strong expression was also detected in hippocampal interneurons, and neurons of the thalamus and substantia nigra, while moderate expression was found in colliculi and cerebellar granule cells. The mGlu1b messenger RNA was strongly expressed in Purkinje cells, hippocampal pyramidal neurons, dentate gyrus granule cells and lateral septum, and moderately expressed in striatal, superficial cortical and cerebellar granule neurons. The mGlu1d messenger RNA was expressed in all regions where mGlu1a and -1b were detected; abundant in Purkinje cells, mitral and tufted cells, and hippocampal principal neurons and interneurons, strong in thalamus and substantia nigra, and moderate in lateral septum, cortex, striatum and colliculi. Human mGlu1 splice variant expression in the cerebellum matched that found for the rat. No specific signal was found with a probe capable of hybridizing to the rat mGlu1c splice junction, although another probe designed against a more 3' sequence of mGlu1c gave strong signals in the cerebellum and hippocampus, and moderate signals in thalamus and colliculi. It is concluded that mGlu1d messenger RNA is widely expressed, that mGlu1a and -1b messenger RNAs are expressed in almost complementary patterns and that formation of the mGlu1c splice junction is a rare event.