INCREASED LEVEL OF NEUROKININ-1 TACHYKININ RECEPTOR GENE-EXPRESSION DURING EARLY POSTNATAL-DEVELOPMENT OF RAT-BRAIN

Substance P is known to elicit diverse actions via activating multiple subtypes of tachykinin receptors, and these actions appear to be invo lved not only in synaptic transmission but also in synaptic plasticity during development of the mammalian cental nervous system. The availa bility of sensitive quantitation of individual tachykinin receptor sub types is crucial for elucidating the physiological function specifical ly mediated by activation of a particular receptor subtype. We thus at tempted to develop an assay to determine the level of messenger RNA mo lecule encoding the neurokinin-l-type tachykinin receptor and apply it for assessment of developmental changes in the neurokinin-l receptor gene expression in the rat brain to explore the role of tachykinin rec eptors during ontogeny. The assay was designed to use a competitive re verse transcription-polymerase chain reaction co-amplifying endogenous neurokinin-l receptor messenger RNA and internal standard, which enab led specific quantification of the number of neurokinin-l receptor tra nscripts, ranging from 3.1 x 10(3) to 1.3 x 10(5) molecules/mu g total RNA. The levels of neurokinin-l receptor gene expression were examine d in three different brain regions of the rat aged 0-56 days after bir th. The order of neurokinin-l receptor messenger RNA expression was hi ppocampus > cerebral cortex much greater than cerebellum at all ages e xamined except postnatal day 0, where its expression was more abundant in the cerebral cortex than in the hippocampus. From postnatal day 3 onward, the hippocampus contained 140-160% of the cortical levels. Alt hough the tachykinin receptor expression in the cerebellum was too low to be accurately assessed by conventional techniques, our assay enabl ed us to determine the amount of cerebellar neurokinin-l receptor mess enger RNA that changed in the range 7-23% of the cortical level during postnatal development. A prominent feature revealed by this assay is that the neurokinin-l receptor gene expression in the rat brain is dev elopmentally regulated. The hippocampus displayed a transient peak of neurokinin-l receptor messenger RNA at postnatal day 3 and a subsequen t gradual decrease. In the cerebral cortex, the amount of the message was highest at birth, and was followed by a moderate decrease during p ostnatal development. At 56 days after birth, the expression levels in both brain regions were down-regulated to approximately 50% of their maximal levels. The transitory pattern of gene expression was also obs erved in the cerebellum. The results of this study demonstrate that th e reverse transcription-polymerase chain reaction-based assay is usefu l to quantitate precisely the neurokinin-l tachykinin receptor message in limited tissue samples derived from discrete brain regions. Togeth er with previous findings, the increased level of neurokinin-l recepto r messenger RNA expression in immature rat brain shown by the present analysis suggests that the neurokinin-l-type tachykinin receptor may p lay a role in the synaptic plasticity associated with morphological an d functional development of the mammalian CNS. Copyright (C) 1996 IBRO .