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.