Afferents to a midbrain periaqueductal grey region involved in the 'defense reaction' in the cat as revealed by horseradish peroxidase. II. The diencephalon.

Horseradish peroxidase injections were made at the same sites, within the midcollicular portion of the midbrain periaqueductal grey region (PAG), at which electrical stimulation and microinjections of excitatory amino acids elicited defensive behaviour. The diencephalic afferents to these sites were then studied. Seventy eight percent of the labelled diencephalic neurons were found within the tuberal portion of the hypothalamus, specifically in 3 regions: (a) the ventromedial hypothalamic nucleus and adjacent perifornical area; (b) the region of the tuber cinereum, ventral and medial to the ventromedial hypothalamic nucleus; (c) the region of paraventricular and parvocellular hypothalamic nuclei. A control injection of HRP at a site, in the tegmentum immediately adjacent to the PAG, at which electrical stimulation elicited defensive behaviour, but excitatory amino acid injections did not, suggested that the extensive labelling of neurones within the tuberal region of the hypothalamus was specific to the HRP injections being made at PAG sites at which excitatory amino acid injections elicited defensive behaviour.