Submitted on May 19, 2008
Revised on August 29, 2008
Accepted on September 2, 2008
HYPOGLOSSAL PREMOTOR NEURONS OF THE INTERMEDIATE MEDULLARY RETICULAR REGION EXPRESS CHOLINERGIC MARKERS
Denys V. Volgin1, Irma Rukhadze1, and Leszek Kubin1*
1 University of Pennsylvania
* To whom correspondence should be addressed. E-mail: lkubin{at}vet.upenn.edu.
The inspiratory drive to hypoglossal (XII) motoneurons originates in the caudal medullary intermediate reticular (IRt) region. This drive is mainly glutamatergic, but little is known about the neurochemical features of IRt XII premotor neurons. Prompted by the evidence that XII motoneuronal activity is controlled by both muscarinic (M) and nicotinic cholinergic inputs and that the IRt region contains cells that express choline acetyl transferase (ChAT), a marker of cholinergic neurons, we investigated whether some IRt XII premotor neurons are cholinergic. In 7 rats, we applied single-cell reverse transcription-polymerase chain reaction to acutely dissociated IRt neurons retrogradely labeled from the XII nucleus. We found that over half (21/37) of such neurons expressed mRNA for ChAT and one third (13/37) also had M2 receptor mRNA. In contrast, among the not retrogradely labeled IRt neurons, only 4 out of 29 expressed ChAT mRNA (p<0.0008), and only 3 out of 29 expressed M2 receptor mRNA (p<0.04). The distributions of other cholinergic receptor mRNAs (M1, M3, M4, M5 and nicotinic 
4 subunit) did not differ between IRt XII premotor and not retrogradely labeled IRt neurons. In an additional 3 rats with retrograde tracers injected into the XII nucleus and ChAT immunohistochemistry, 5-11% of IRt XII premotor neurons located at, and caudal to, the area postrema were ChAT-positive, and 27-48% of ChAT-positive caudal IRt neurons were retrogradely labeled from the XII nucleus. Thus, the pre- and postsynaptic cholinergic effects previously described in XII motoneurons may originate, at least in part, in medullary IRt neurons.
