IN-SITU HYBRIDIZATION ANALYSIS OF AMPA RECEPTOR SUBUNIT GENE-EXPRESSION IN THE DEVELOPING RAT SPINAL-CORD

In early postnatal life the acquisition of mature morphological and mo lecular features of motor neurons is influenced by synaptic activity w ithin the spinal cord. Glutamatergic synaptic neurotransmission is bel ieved to play a central role in this process. We hypothesize that the repertoire of glutamate receptors expressed by neurons in the young sp inal cord differ from those expressed in adults and such receptors sup port activity-dependent developmental plasticity. To explore this idea , we used in situ hybridization histochemistry to determine the distri bution, temporal expression, and potential subunit composition of lpha -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors in the d eveloping rat spinal cord and compared these findings with those in ad ult rats.We find qualitative and quantitative changes in lpha-amino-3- hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit gene expre ssion over the first month of postnatal life. lpha-amino-3-hydroxy-5-m ethyl-4-isoxazolepropionic acid receptor subunit genes GluR1, 2 and 4 are expressed at greater levels throughout the spinal cord of the neon ate versus the adult animals. The developmental down-regulation is mos t pronounced for GluR1 transcripts, less for GluR2 and GluR4 transcrip ts, and minimal for GluR3 transcripts. Analysis of flip and hop splice variants of each subunit show that receptors expressed by adult motor neurons are potentially composed of the subunits GluR1 flop, GluR2 fl ip, GluR3 flip and flop, and GluR4 flip. In neonatal motor neuron all subunits are potentially expressed (except GluR2 flop) with quantitati vely the dominent subunits being the flip splice variants of GluR1, 2 and 4. Receptors in the substantia gelatinosa undergo equally dramatic , developmentally independent changes. Changes in the lpha-amino-3-hyd roxy-5-methyl-4-isoxazolepropionic acid receptor subunit composition a re likely to have an important effect on the electrophysiological prop erties of motor neurons and may form part of the molecular identity of neurons capable of undergoing activity-dependent developmental plasti city.