Effects on breathing of putative neurotransmitters in the rostral hypothalamus of the rat

The putative neurotransmitters norepinephrine (NE) and thyrotropin releasing hormone (TRH) are normally present in the rostral hypothalamic region (RHT) of the rat, and our aim was to evaluate possible effects of these agents on ventilatory regulations associated with this region. Using haloperidol-tranquilized Sprague-Dawley rats, microinfusions of both NE and TRH into the RHT resulted in an increase in rate, but not depth, of breathing. Control infusions and control infusion sites, mainly in the posterior hypothalamus, yielded no significant effect on breathing rate. Since NE and TRH can inhibit the discharge of some cells in the RHT, it was possible that the observed effects on breathing were due to depression of an inhibitory neural pathway. This idea was further tested by performing microinfusions using lidocaine. Evidence suggests that lidocaine can inhibit neural discharge in the central nervous system and that inhibitory pathways may be preferentially affected. Lidocaine produced effects on breathing comparable to NE and TRH, thereby supporting the proposition that inhibition of neural pathways in the RHT can stimulate breathing.