Thyrotropin releasing hormone (TRH) and its analog, RGH-2202, accelerate maturation of cerebellar neurons in vitro

We have studied the "trophic" action of thyrotropin releasing hormone (TRH) in cultured cerebellar granule cells, a pure and homogenous population of glutamatergic neurons. As an index of neuronal maturation, we have measured the uptake of d-[³H]aspartate (a non-metabolizable analog of glutamate) at different days of maturation in vitro (DIV). In control cultures, d-[³H]Aspartate increased linealy during maturation reaching plateau values between 7 and 9 DIV; daily addition of TRH tartrate (TRH-t) or RGH-2202 (a TRH analog) accelerated in a concentration-dependent manner the maturation profile of d-[³H]aspartate uptake. This effect was more pronounced for RGH-2202; in cultures treated daily with RGH-2202, d-[³H]aspartate uptake was fully expressed after 3 DIV. Neither TRH-t nor RGH-2202 significantly increased d-[³H]aspartate uptake in mature cells, excluding a direct action on the glutamate transport system. Both compounds specifically potentiated the increase in [³H]inositol monophosphate formation (but not the stimulation of ⁴⁵Ca²⁺ influx) induced by N-methyl-d-aspartate (NMDA) receptor agonists, without affecting the stimulation of inositol phospholipid hydrolysis by quisqualate or carbamylcholine. We suggest that, in cultured cerebellar granule cells, TRH and RGH-2202 enhance the trophic action of endogenous glutamate by amplifying some of the intracellular events that follow the influx of extracellular Ca²⁺ through NMDA-gated ion channels.