Single unit electrical activity has been recorded from thirty-four viscero-somatic neurones in the dorsal horn of the lower thoracic spinal cord (T9-T11) of chloralose-anaesthetized rats. All neurones were driven by natural and/or electrical stimulation within their somatic receptive fields and gave excitatory responses to electrical stimulation of the ipsilateral splanchnic nerve. Descending influences on these neurones were tested by electrical and chemical (microinjections of DL-homocysteic acid) stimulation of sites in the rostral hypothalamus. 2. The electrical activity of most viscero-somatic neurones (64%) was inhibited by electrical stimulation at sites throughout the anterior hypothalamus-preoptic region. In any one cell, responses to stimulation of visceral and somatic afferent fibres were inhibited to the same extent and any on-going activity was also depressed. Only one cell was driven by the conditioning stimulus and the electrical activity of the remaining cells (n = 7) was unaffected. 3. At certain hypothalamic sites the effects of electrical conditioning stimulation on the responses of viscero-somatic neurones were compared with those of local microinjection of DL-homocysteic acid. Electrical stimulation at all sites tested (n = 7) led to an inhibition of on-going and evoked neuronal activity. At two hypothalamic sites, both located in the ventral part of the preoptic area, microinjection of DL-homocysteic acid resulted in a complete abolition of the responses to the test stimuli and in a cessation of any on-going activity. Microinjection of DL-homocysteic acid at the remaining five sites had no detectable influences on dorsal horn activity. 4. The results of this study include the first description of input properties of viscero-somatic neurones in the lower thoracic spinal cord of the rat. In addition, these results demonstrate that transmission of visceral and somatic information through these neurones can be modulated by pathways that originate in the anterior hypothalamus-preoptic region of the ventromedial forebrain.
