SPINOTHALAMIC AND SPINOHYPOTHALAMIC TRACT NEURONS IN THE CERVICAL ENLARGEMENT OF RATS .1. LOCATIONS OF ANTIDROMICALLY IDENTIFIED AXONS IN THE THALAMUS AND HYPOTHALAMUS

1. Seventy-seven neurons in the cervical enlargement of rats anesthetized with urethan were initially antidromically activated using currents less than or equal to 30 mu A from the contralateral posterior thalamus. A goal of these experiments was to determine the course of physiologically characterized spinal axons within the diencephalon. Therefore, in 38 cases, additional antidromic mapping was done throughout the mediolateral extent of the diencephalon at multiple anterior-posterior planes. 2. Electrolytic lesions marking the recording sites were recovered for 71 neurons. Thirty-one were located in the superficial dorsal horn (SDH); 39 were in nucleus proprius or the lateral reticulated area of the deep dorsal horn (DDH), and one was in the ventral horn. 3. Eight of 38 (21%) neurons that were tested for more anterior projections could only be antidromically activated with currents less than or equal to 30 mu A from sites in the contralateral posterior thalamus. Such neurons are referred to as spinothalamic tract (STT) neurons. Lesions marking the lowest threshold points for antidromic activation were located in or near the posterior thalamic group (Po). At more anterior levels, considerably higher currents were required for antidromic activation or it was not possible to activate the neurons with currents up to 500 mu A. Four of these neurons were physiologically characterized and each responded preferentially to noxious mechanical stimuli (wide dynamic range, WDR). Each of the three neurons that were tested responded to noxious heat stimuli. These findings confirm anatomic studies that have shown that a number of STT axons terminate in Po and suggest that such axons that originate in the cervical enlargement carry nociceptive input from the upper extremity. 4. In 15 cases, electrode penetrations were made systematically throughout much of the contralateral ventrobasal complex (VbC). In 17 cases, penetrations were made throughout the intralaminar nuclei contralaterally, including the central lateral nucleus (CL). Surprisingly, only one of the examined axons was antidromically activated with low currents from CL and one from VbC, although both of these nuclei are known to receive sizeable inputs from the STT. 5. Many of the axons (27 of the 38 tested, 71%) that were initially antidromically activated from the contralateral posterior thalamus could also be antidromically activated with low currents (less than or equal to 30 mu A) and at increased latencies from sites located anteriorly in the contralateral hypothalamus. Such neurons are referred to as spinothalamic tract/spinohypothalamic tract (STT/SHT) neurons. Lesions marking the lowest threshold points for antidromic activation in the hypothalamus were concentrated in or near the supraoptic decussation, optic tract, and caudal part of the optic chiasm. Responses of 20 of these neurons to innocuous and noxious mechanical stimuli were determined. Eleven were classified as WDR neurons. The other nine were classified as high threshold (HT) neurons. Fourteen of 17 (82%) responded to noxious heat. These results confirm our earlier studies and demonstrate that many nociceptive neurons in the cervical enlargement project to both the thalamus and hypothalamus. distances from the lowest threshold points were generated to determine the effective spread of current within the hypothalamus. The maximum effective spread of currents of 30 mu A was similar to 400 mu m (13 mu m/mu A). The mean threshold for antidromic activation at the lowest threshold points was similar to 15 mu A, and thus it appears that the effective current spread away from the lowest threshold points was probably <200 mu m.