Physiological studies of spinohypothalamic tract neurons in the lumbar enlargement of monkeys.

Recent anatomic results indicate that a large direct projection from the spinal cord to the hypothalamus exists in monkeys. The aim of this study was to determine whether the existence of this projection could be confirmed unambiguously using electrophysiological methods and, if so, to determine the response characteristics of primate spinohypothalamic tract (SHT) neurons. Fifteen neurons in the lumbar enlargement of macaque monkeys were antidromically activated using low-amplitude current pulses in the contralateral hypothalamus. The points at which antidromic activation thresholds were lowest were found in the supraoptic decussation (n = 13) or in the medial hypothalamus (n = 2). Recording points were located in the superficial dorsal horn (n = 1), deep dorsal horn (n = 10), and intermediate zone (n = 4). Each of the 12 examined neurons had cutaneous receptive fields on the ipsilateral hindlimb. All neurons responded exclusively or preferentially to noxious stimuli, suggesting that the transmission of nociceptive information is an important role of primate SHT axons. Twelve SHT neurons were also antidromically activated from the thalamus. In all cases, the antidromic latency from the thalamus was shorter than that from the hypothalamus, suggesting that the axons pass through the thalamus then enter the hypothalamus. These results confirm the existence of a SHT in primates and suggest that this projection may contribute to the production of autonomic, neuroendocrine, and emotional responses to noxious stimuli in primates, possibly including humans.