The cells of origin of the spinohypothalamic tract in cats

Various cutaneous and visceral stimuli alter the discharge rates of neurons in the hypothalamus. Changes in the activity of hypothalamic neurons are thought to play important roles in eliciting neuroendocrine, autonomic, and affective responses to somatosensory and visceral stimuli. Information from peripheral structures has been considered generally to reach the hypothalamus via multisynaptic ascending pathways. Recently, a direct projection from the spinal cord to the hypothalamus was demonstrated in rats. The goal of this study was to determine whether a similar projection exists in cats. Either wheat germ agglutinin conjugated to horseradish peroxidase, a mixture of this tracer and the B subunit of cholera toxin conjugated to horseradish peroxidase, or fast blue was injected into the hypothalamus of cats. Injections were centered in the hypothalamus in seven cats and did not spread to the thalamus, zona incerta or midbrain. After these injections, retrogradely labeled neurons were observed bilaterally in each of the 17 spinal segments that were examined. A total of approximately 400–500 labeled neurons was observed in alternate sections through these segments in the most effective cases. Roughly 70% of the labeled neurons were located contralaterally. Labeled neurons were found predominantly in the deep dorsal horn, the intermediate zone/ventral horn and in the area surrounding the central canal. A few were also noted in the superficial dorsal horn. The first and second sacral segments contained the largest numbers of retrogradely labeled neurons in the spinal cord. The number of spinohypothalamic tract neurons observed in this study in cats was roughly an order of magnitude smaller than that previously reported for rats. This finding suggested either that the spinohypothalamic tract is relatively small in cats or that our tracing techniques did not label many spinohypothalamic tract neurons in cats. To test the sensitivity of one of our tracing techniques, control injections of wheat germ agglutinin conjugated to horseradish peroxidase that filled the ventrobasal thalamus were made in two cats. In both cases, thousands of spinal cord neurons were labeled. In summary, our results indicate that a spinohypothalamic tract exists in cats. However, our findings also suggest that the total number of spinohypothalamic tract neurons in cats may be an order of magnitude smaller than it is in rats.