Rs. Broide et al., DEVELOPMENTAL EXPRESSION OF ALPHA(7) NEURONAL NICOTINIC RECEPTOR MESSENGER-RNA IN RAT SENSORY CORTEX AND THALAMUS, Neuroscience, 67(1), 1995, pp. 83-94
The distribution of alpha(7) messenger RNA expression was characterize
d in developing rat cortex and thalamus. Northern blot analysis of neo
natal and adult cortex revealed a single messenger RNA transcript of 5
.7 kb. Using in situ hybridization with both full length and short S-3
5-labeled alpha(7) riboprobes, a distinct transient expression of mess
enger RNA within sensory cortex and thalamus, during early postnatal d
evelopment, was observed. alpha(7) transcripts were expressed in low l
evels as early as embryonic day 13 in the ventricular zone of the neoc
ortex, and as early as embryonic day 15 in the thalamic neuroepitheliu
m. A marked increase in messenger RNA levels was observed during the l
ate prenatal period in both sensory and non-sensory regions of the cor
tex and thalamus. Moderate to high levels of messenger RNA were mainta
ined into the first postnatal week, followed by a decline into adultho
od. alpha(7) messenger RNA expression was significantly higher in the
anterodorsal, lateral dorsal, ventral posterior medial and ventral pos
terior lateral thalamic nuclei of postnatal day 7 pups than in adult b
rains. Expression of messenger RNA within dorsal lateral geniculate, v
entral lateral geniculate and medial geniculate did not show a signifi
cant reduction with age. Within the developing cortex, messenger RNA e
xpression delineated the primary somatosensory, auditory and visual co
rtices in a unique laminar pattern that was consistently and significa
ntly higher than in the adult in superficial layer VI. Higher levels o
f expression were also observed in retrosplenial cortex at postnatal d
ay 7 than in the adult. Tangential sections through postnatal day 7 co
rtex revealed low levels of alpha(7) messenger RNA expression delineat
ing the primary sensory areas in layer IV, corresponding to acetylchol
inesterase-labeled thalamocortical afferents. However, these sensory a
reas exhibited higher levels of alpha(7) messenger RNA expression and
were more clearly defined in layer VI, but not by acetylcholinesterase
staining. The distribution of alpha(7) messenger RNA within the devel
oping thalamocortical system parallels the distribution of alpha-bunga
rotoxin binding sites and suggests that the receptor is localized on b
oth thalamic cells and their cortical target neurons. This transient a
nd distinct pattern of distribution of the alpha(7) neuronal nicotinic
receptor, which coincides with the major phase of thalamocortical dev
elopment, suggests that it may play a functional role in the developme
nt of cortical circuitry.