Locations of spinothalamic tract axons in cervical and thoracic spinal cord white matter in monkeys.

The spinothalamic tract (STT) is the primary pathway carrying nociceptive information from the spinal cord to the brain in humans. The aim of this study was to understand better the organization of STT axons within the spinal cord white matter of monkeys. The location of STT axons was determined using method of antidromic activation. Twenty-six lumbar STT cells were isolated. Nineteen were classified as wide dynamic range neurons and seven as high-threshold cells. Fifteen STT neurons were recorded in the deep dorsal horn (DDH) and 11 in superficial dorsal horn (SDH). The axons of 26 STT neurons were located at 73 low-threshold points (<30 microA) within the lateral funiculus from T(9) to C(6). STT neurons in the SDH were activated from 33 low-threshold points, neurons in the DDH from 40 low-threshold points. In lower thoracic segments, SDH neurons were antidromically activated from low-threshold points at the dorsal-ventral level of the denticulate ligament. Neurons in the DDH were activated from points located slightly ventral, within the ventral lateral funiculus. At higher segmental levels, axons from SDH neurons continued in a position dorsal to those of neurons in the DDH. However, axons from neurons in both areas of the gray matter were activated from points located in more ventral positions within the lateral funiculus. Unlike the suggestions in several previous reports, the present findings indicate that STT axons originating in the lumbar cord shift into increasingly ventral positions as they ascend the length of the spinal cord.

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.

Immunohistochemical localization of delta opioid receptors in peripheral tissues.

The distribution of delta opioid receptor (DOR) immunoreactivity (ir) was examined in various peripheral tissues of Sprague-Dawley rats and macaque monkeys, including glabrous and hairy skin, corneas, eyelids, and the lip. DOR-ir was observed in all tissues examined. In addition to the presence of DOR-immunoreactive fibers in subcutaneous nerve bundles and the papillary dermis, we report the existence of positively labeled fibers and terminals in close association with peripheral structures not traditionally assigned a primarily nociceptive function, such as hair follicles, glandular apparatus, and blood vessels. In every case, staining was restricted to small-diameter axons that appeared to terminate as free nerve endings. To further classify DOR-immunoreactive fibers, we examined the extent of colocalization between DOR and three commonly used neuronal subtype markers; tyrosine hydroxylase (TH), calcitonin gene-related peptide (CGRP), and RT-97, a monoclonal antibody which preferentially labels neurons with myelinated axons. Additional double-labeling experiments using the nonspecific neuronal marker Protein Gene Product 9.5 were performed in glabrous skin to determine the percentage of total fiber count that displayed DOR-ir. No colocalization was observed between DOR and RT-97, indicating that DOR-ir is localized to unmyelinated axons. In addition, DOR colocalized with CGRP, but did not colocalize with TH. Taken together, these data support the hypothesis that delta opioid receptors in peripheral tissues are associated with sensory fibers, but not with the terminals of postganglionic sympathetic neurons.