Long term facilitation of respiratory motor output decreases with age in male rats

Long term facilitation (LTP) is a serotonin-dependent augmentation of respi ratory motor output (phrenic and hypoglossal) following episodic hypoxia. S ince ageing influences respiratory control mechanisms and serotonergic func tion, we tested the hypothesis that LTF decreases with age in male rats. 2. Young (3-4 month) and aged (13 month) male Sprague-Dawley rats were anaest hetized with urethane, vagotomized, paralysed and pump ventilated. Integrat ed phrenic and hypoglossal (XII) nerve activities were measured before (bas eline), during and for 60 min after three 5 min episodes of isocapnic hypox ia (P-a,(O2) 35-45 mmHg) separated by 5 min of hyperoxia (P-a,P-O2 > 150 mm Hg). 3. In young rats, LTF was observed as an augmentation in peak integrat ed phrenic (n = 8) and XII (n = 7) amplitudes following episodic hypoxia (5 6 +/- 14 and 73 +/- 16% (means +/- S.E.M) at 60 min post-hypoxia, respectiv ely; both P < 0.05). In aged rats, LTF was significantly increased compared to baseline in phrenic (25 <plus/minus> 8% at 60 min, P < 0.05), but not i n XII (4 <plus/minus> 7 %, P > 0.05) motor output. LTF was significantly gr eater in young than in aged rats in both motor outputs (P < 0.05). 4. Decre ased phrenic and XII LTF suggests that serotonergic modulation of respirato ry motor output decreases in ageing male rats. We speculate that decreased serotonergic modulation may contribute to age-related breathing disorders.