Excitatory Convergence of Periaqueductal Grey and Somatic Afferents in the Solitary Tract Nucleus: Role for Neurokinin 1 Receptors

Our previous studies showed that activation of somatic afferents attenuated the baroreceptor reflex via NK 1 and GABA A receptors within the nucleus of the solitary tract (NTS). The periaqueductal grey matter (PAG) can also depress baroreceptor reflex function and project to the NTS. In the present study, we have tested the possibility that the dorsolateral (dl) PAG projects to the NTS neurons that also respond to somatic afferent input. In an in situ arterially perfused, unanesthetized decerebrate rat preparation, somatic afferents (brachial plexus), cervical spinal cord and dorso-lateral PAG were stimulated electrically, while NTS neurons were recorded extracellularly. From 45 NTS neurons excited by either brachial plexus or dl-PAG stimulation, 41 received convergence excitatory inputs from both afferents. Onset latency and evoked peak discharge frequency from brachial plexus afferents were 39.4+4.7ms and 10.7+1.1Hz respectively, whereas this was 43.9+6.4ms and 7.9+1Hz following dl-PAG stimulation. As revealed using a paired pulse stimulation protocol, monosynaptic connections were found in 9 of 36 neurons tested from both spinal cord and dl-PAG. We tested NK 1 receptor sensitivity in 38 neurons that received convergent inputs from brachial plexus/PAG. 15 neurons were sensitive to selective antagonism of NK 1 receptors. CP-99,994 the NK 1 antagonist failed to alter ongoing firing activity but reduced the evoked peak discharge frequency following stimulation of both brachial plexus (from 12.3+1.8 to 7.2+1.3Hz; p<0.01) and PAG (from 7.8+1.5 to 4.5+1Hz; p<0.01). We conclude: (i) somatic brachial and PAG afferents can converge onto single NTS neurons; (ii) this convergence occurs via either direct or indirect pathways; (iii) NK 1 receptors are activated by some of these inputs.