HTR1A and Respiratory insufficiency due to muscle weakness: Previous animal studies of serotonin agonists have found that stimulating 5HT1A, 5HT7, and 5HT4a receptors—all present on respiratory neurons in the rhythm-generating pre-Bӧtzinger complex—attenuated opioid-induced respiratory depression (Manzke et al., 2003; Meyer et al., 2006).