Further evidence for a possible excitatory serotonergic synapse on raphe neurons of pons and lower midbrain

Abstract Iontophoretic techniques were employed to investigate the response of neurons in the nucleus raphe pontis and median raphe nucleus to 5HT. Neurons were identified by response to stimulation of nucleus paragigantocellularis lateralis (NPL) as driven (D), inhibited (I) or non-responsive (NR) cells. Of 35 D cells tested, 33 were excited by 5HT and 2 inhibited. Of 10 I cells tested, 7 were inhibited by 5HT and 3 excited. Of 56 NR cells tested, 5HT excited 30, inhibited 23 and produced no response for 3. Combining this and previous work, 100 D cells have been tested of which 97 were excited by 5HT. This 97% excitatory response of D cells to 5HT provides a strong suggestion that the NPL-to-raphe projection represents an excitatory serotonergic pathway in the central nervous system.