1. Ninety-six neurons in the lumbar enlargement of urethan-anesthetized rats were antidromically activated from the contralateral hypothalamus. The antidromic stimulating electrode was moved systematically within the hypothalamus until antidromic activation could be produced with currents of less-than-or-equal-to 50-mu-A (18.6 +/- 10.8-mu-A; mean +/- SD). The points at which antidromic activation thresholds were lowest were found in several regions of the hypothalamus but were concentrated in the optic tract and the supraoptic decussation. 2. The recording locations of 79 spinohypothalamic tract (SHT) neurons were marked and recovered. Twenty-nine were located in the superficial dorsal horn (SDH), 42 in the deep dorsal horn (DDH), 4 in the intermediate zone, and 2 in the gray matter surrounding the central canal. Two additional marks were located in the dorsal lateral funiculus (DLF). 3. The responses of 46 SHT neurons were examined during innocuous and noxious mechanical stimulation of their receptive fields. Forty-eight percent of recorded SHT neurons responded to both innocuous and noxious stimuli (wide dynamic range, WDR) and 39% responded only to noxious stimuli (high threshold, HT). Therefore 87% of SHT neurons responded preferentially or exclusively to noxious mechanical stimulation. Nine percent of SHT neurons responded exclusively to innocuous manipulation of joints and muscles. Four percent of SHT neurons responded only to innocuous tactile stimuli (low threshold, LT). WDR, HT, and LT neurons were recorded widely throughout the dorsal horn; no relationship was found between the locations of recording sites in the dorsal horn and the response types of the neurons. SHT neurons that responded to stimulation of muscle, tendon, or joint were recorded deep in the gray matter. 4. The effects of heating the receptive fields were determined for 25 SHT neurons. Fourteen (56%) responded to thermal stimuli. Six (43%) of the responsive neurons responded at low frequencies to innocuous warming (38-41-degrees-C) but more vigorously to noxious (greater-than-or-equal-to 45-degrees-C) heating. The other eight responded only to noxious heat. Eighteen percent (3/17) of tested SHT neurons were activated by noxious cooling of their receptive fields. 5. Cutaneous receptive fields of most recorded SHT neurons were small, typically involving areas as small as two or three toes on the ipsilateral hindlimb; the largest receptive fields covered the entire paw. These findings indicate that relatively precise information about the location of innocuous and noxious stimuli is conveyed directly to the hypothalamus by SHT neurons. 6. In eight cases, we attempted to activate SHT neurons from a transverse grid of closely spaced stimulation points bilaterally across the entire upper cervical spinal cord. In all cases, a single lowest threshold point was found in the contralateral white matter, suggesting that SHT neurons each issue a single projecting axon that ascends in the contralateral white matter. 7. The locations of 29 additional SHT axons were determined. A stimulating electrode was moved systematically through the contralateral white matter in the upper cervical cord until antidromic activation could be achieved with currents of less-than-or-equal-to 30-mu-A (13.7 +/- 7.4-mu-A). The lowest threshold points were located in the DLF for 57% of the examined SHT neurons, in the ventral lateral funiculus (VLF) for 38% of SHT neurons, and in the ventral funiculus for 5% of neurons. No association was found between the location of the recording site in the lumbar dorsal horn and the location of the low-threshold point in the cervical lateral funiculus; the DLF and VLF contained approximately equal numbers of axons of neurons recorded in either SDH or DDH. 8. Antidromic stimulation was attempted bilaterally throughout much of the diencephalon. Eight of 13 (62%) examined SHT neurons had axons that crossed from the side contralateral to the cell body to the ipsilateral side. The present and previous findings indicate that SHT axons cross the midline within the supraoptic decussation of the hypothalamus. 9. In 11 cases, after an SHT axon was located within the hypothalamus, a stimulating electrode was systematically moved across the mediolateral extent of the rostral diencephalon/caudal telencephalon while delivering 500-mu-A pulses. Nine of the tested SHT neurons could not be antidromically activated at this level. In two cases, the results suggested that the SHT axons projected rostrally through the hypothalamus to, but not anterior to, the level of the preoptic area. These findings suggest that the axons of the majority of neurons antidromically activated in the hypothalamus terminated within the hypothalamus and did not ascend through it to the telencephalon. 10. Although systematic attempts were not made to determine whether SHT neurons also projected to the thalamus, six SHT neurons were antidromically activated from low threshold points in the ventrobasal thalamus. These findings indicate that at least some SHT neurons project to (or through) the thalamus. 11. The conduction velocity (CV, mean +/- SD) of SHT axons between the lumbar cord and the hypothalamus was 21.7 +/- 6.4 m/s. Axons of SHT neurons conducted at 29.5 +/- 8.2 m/s between lumbar and upper cervical levels, and conduction slowed to 12.3 +/- 4.8 m/s from upper cervical levels to the hypothalamus. The reduced CVs of SHT axons within the brain suggest that SHT axons may issue collateral projections within the brain stem. No significant differences were found between CVs for neurons recorded in the SDH or DDH or between those classified as WDR or HT. 12. In the hypothalamus, the maximum effective spread of currents of 30-mu-A was found to be approximately 400-mu-m (13.3-mu-m/mu-A). In the cervical cord, current pulses of 30-mu-A spread effectively not more than 600-mu-m. The lowest thresholds for antidromic activation in the hypothalamus were < 50-mu-A and were surrounded in the hypothalamus by points at which the antidromic thresholds were higher. In addition, these lowest threshold points were typically > 1 mm from the thalamus or midbrain. Taken together, these results virtually eliminate the possibility that cells identified as SHT neurons projected to the thalamus or midbrain and not to the hypothalamus. 13. The SHT provides a direct route by which nociceptive information originating in the spinal cord may reach areas in the hypothalamus that produce autonomic, neuroendocrine, and emotional/motivational responses to noxious stimuli.
