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

Long term facilitation (LTF) is a serotonin-dependent augmentation of respiratory motor output (phrenic and hypoglossal) following episodic hypoxia. Since ageing influences respiratory control mechanisms and serotonergic function, we tested the hypothesis that LTF decreases with age in male rats. Young (3-4 month) and aged (13 month) male Sprague-Dawley rats were anaesthetized with urethane, vagotomized, paralysed and pump ventilated. Integrated phrenic and hypoglossal (XII) nerve activities were measured before (baseline), during and for 60 min after three 5 min episodes of isocapnic hypoxia (Pa,O2 35-45 mmHg) separated by 5 min of hyperoxia (Pa,O2 > 150 mmHg). In young rats, LTF was observed as an augmentation in peak integrated phrenic (n = 8) and XII (n = 7) amplitudes following episodic hypoxia (56 ± 14 and 73 ± 16 % (means ±s.e.m.) at 60 min post-hypoxia, respectively; both P 0.05) motor output. LTF was significantly greater in young than in aged rats in both motor outputs (P < 0.05). Decreased phrenic and XII LTF suggests that serotonergic modulation of respiratory motor output decreases in ageing male rats. We speculate that decreased serotonergic modulation may contribute to age-related breathing disorders. Although age affects the neural control of breathing, such as alterations in hypoxic or hypercapnic responses (Fukuda, 1991, 1992), little is known concerning the specific underlying neural mechanisms affected by age. In this study, we investigated the effects of age on a serotonin-dependent respiratory memory: long term facilitation (LTF) of respiratory motor output following episodic hypoxia. Episodic hypoxia or electrical stimulation of carotid chemoafferent neurons elicits a long-lasting increase of respiratory motor output (i.e. LTF) in awake and anaesthetized mammals of several species (Millhorn et al. 1980a; Cao et al. 1992; Hayashi et al. 1993; Fregosi & Mitchell, 1994; Turner & Mitchell, 1997; Powell et al. 1998). In anaesthetized rats, LTF is commonly observed as an augmentation of integrated phrenic and hypoglossal (XII) nerve burst amplitude, with a smaller increase in burst frequency (Bach & Mitchell, 1996; Powell et al. 1998; Fuller et al. 2000). Serotonin receptor activation is necessary for LTF induced by episodic stimulation of the carotid sinus nerve (Millhorn et al. 1980b;Fregosi & Mitchell, 1994) or by episodic hypoxia (Bach & Mitchell, 1996; Kinkead & Mitchell, 1999). Furthermore, hypoxia-induced LTF requires 5-HT2 receptor activation during, but not following, episodic hypoxia in anaesthetized rats (D. Fuller, A. Zabka, T. Baker & G. Mitchell, unpublished observations). Thus, serotonin receptor activation is necessary to initiate, but not to maintain, LTF. Recent evidence suggests that the relevant serotonin receptors are within the respective motor nuclei since intraspinal administration of a serotonin receptor antagonist blocks phrenic, but not XII, LTF (T. Baker & G. Mitchell, unpublished observations). The serotonergic system is influenced by age. For example, the number of serotonergic terminals, the concentrations of 5-HT and its metabolic products, 5-HT receptor density, and 5-HT reuptake protein are all affected by age, with considerable variation among different regions of the central nervous system (Van Luijtelaar et al. 1992; Ko et al. 1997). Age-related decreases in serotonergic innervation have been described in the XII nucleus of middle-aged male rats relative to young male rats (Behan & Brownfield, 1999). Although there is no specific information concerning age-associated changes in serotonergic innervation of the phrenic nucleus, 5-HT decreases with age in the dorsal and ventral horns of cervical spinal segments associated with the phrenic motor nucleus (Ko et al. 1997). Since LTF is serotonin dependent, and serotonergic innervation decreases with age in XII and possibly in the phrenic motor nucleus, we tested the hypothesis that LTF in XII and phrenic nerve activity decreases with age in male rats. The results indicate that LTF is decreased in phrenic, and completely lost in XII, motor output in middle-aged male rats. Such an effect may be of significance to our understanding of age-associated changes in both respiratory and serotonergic function.