Modulation of a viscerosomatic reflex by electrical and chemical stimulation of hypothalamic structures in the rat

Ventromedial forebrain structures were stimulated electrically with short (10-ms) trains of pulses to test for effects on a viscerosomatic reflex. Stimulation at many hypothalamic sites led to an attenuation or even a complete inhibition of reflex activity. The most sensitive sites, however (i.e. those requiring currents of 50 microA or less to inhibit the reflex), were located in the anterior hypothalamus/preoptic area (AH/POA) and rostrally in the diagonal band of Broca (DBB). At certain sites the effects of electrical stimulation were compared with those of microinjection of an excitatory amino acid (DL-homocysteic acid) which is known to excite neuronal cell bodies and not axons. The results of this part of the study indicated that activation of cell bodies located in the ventromedial AH/POA (from the level of the optic chiasma caudally to the level of DBB rostrally) mediate, at least in part, the inhibitory effects on visceral afferent processing. These data are discussed in relation to a possible role of AH/POA in the spinal processing of nociceptive information of visceral origin.