Depressed GABA and glutamate synaptic signaling by 5-HT1A receptors in the nucleus tractus solitarii and their role in cardiorespiratory function

Serotonin (5-HT), and its 5-HT1A receptor (5-HT1AR) subtype, is a powerful modulator of the cardiorespiratory system and its sensory reflexes. The nucleus tractus solitarii (nTS) serves as the first central station for visceral afferent integration and is critical for cardiorespiratory reflex responses. However, the physiological and synaptic role of 5-HT(1A)Rs in the nTS is relatively unknown. In the present study, we examined the distribution and modulation of 5-HT(1A)Rs on cardiorespiratory and synaptic parameters in the nTS. 5-HT(1A)Rs were widely distributed to cell bodies within the nTS but not synaptic terminals. In anesthetized rats, activation of 5-HT(1A)Rs by microinjection of the 5-HT1AR agonist 8-OH-DPAT into the caudal nTS decreased minute phrenic neural activity via a reduction in phrenic amplitude. In brain stem slices, 8-OH-DPAT decreased the amplitude of glutamatergic tractus solitarii- evoked excitatory postsynaptic currents, and reduced overall spontaneous excitatory nTS network activity. These effects persisted in the presence of GABA(A) receptor blockade and were antagonized by coapplication of 5-HT1AR blocker WAY-100135. 5-HT1AR blockade alone had no effect on tractus solitarii-evoked excitatory postsynaptic currents, but increased excitatory network activity. On the other hand, GABAergic nTS-evoked inhibitory postsynaptic currents did not change by activation of the 5-HT(1A)Rs, but spontaneous inhibitory nTS network activity decreased. Blocking 5-HT(1A)Rs tended to increase nTS-evoked inhibitory postsynaptic currents and inhibitory network activity. Taken together, 5-HT(1A)Rs in the caudal nTS decrease breathing, likely via attenuation of afferent transmission, as well as overall nTS network activity.