Medullary dorsal horn neurons providing axons to both the parabrachial nucleus and thalamus

Abstract

It has often been suggested that the trigemino- and spino-thalamic pathways are highly implicated in sensory-discriminative aspects of pain, whereas the trigemino- and spino-parabrachial pathways are strongly implicated in affective/emotional aspects of pain. On the other hand, the superficial laminae of the spinal dorsal horn, where many nociceptive neurons are distributed, have been reported to contain projection neurons innervating both the parabrachial nucleus (PBN) and thalamus by way of axon collaterals (Hylden et al., 1989). For the medullary dorsal horn (caudal subnucleus of spinal trigeminal nucleus: Vc), however, the existence of such neurons has not been reported. Thus, in the present study, we examined whether the Vc might contain projection neurons sending their axons to both the thalamus and PBN. Dual retrograde labeling with fluorescence dyes was attempted. In each rat, tetramethylrhodamine-dextran amine and Fluoro-gold were stereotaxically injected into the PBN and thalamic regions, respectively. The proportion of the dually labeled Vc cells in the total population of all labeled Vc cells was about 20%. More than 90% of the dually labeled neurons were distributed in lamina I (marginal zone), less than 10% of them were located in lamina II (substantia gelatinosa), and only a few (about 1%) were found in lamina III (magnocellular zone). The results indicate that some Vc neurons in the superficial laminae mediate nociceptive information directly to the PBN and thalamus by way of axon collaterals and that the vast majority of them project to the ipsilateral PBN and contralateral thalamus.

The lateral parabrachial nucleus, but not the thalamus, mediates thermosensory pathways for behavioural thermoregulation

“…However, the present study clearly demonstrates that the thalamocortical thermosensory transmission is not required for eliciting heat- or cold-avoidance behaviour, while the LPB is a brain site that plays an essential role in the central circuit for these thermoregulatory behaviours. The LPB receives numerous projections from the dorsal horn 18–20 , which include axonal collaterals from spinothalamic and trigeminothalamic projections 21, 22 . The LPB contains two separate populations of neurons responsive to either cutaneous warm- or cool-sensory input; they are activated by exposure of animals to warm or cool ambient temperature or by direct warming or cooling of the trunk skin with a water jacket 11, 12, 23 .…”

Central circuitries for body temperature regulation and fever

“…The LPB is a sensory mediating region that receives massive projections from the dorsal horn (8,25,36). These projections to the LPB include axon collaterals from thalamic projecting neurons in the spinal and trigeminal dorsal horns (57,75). An anatomical study revealed that axonal swellings from dorsal horn neurons are closely associated with postsynaptic structures of LPB neurons that project to the POA (105), supporting the view that the activation of POA-projecting LPB neurons in response to skin thermal challenges is caused by direct cutaneous thermosensory inputs from dorsal horn neurons (Fig.…”

Central neural control of thermoregulation and brown adipose tissue

“…The TRP channels in the thermoreceptor central endings may also provide a substrate for spinal cord or trigeminal nucleus temperature to influence the level of thermal effector activity. In turn, spinal and trigeminal lamina I neurons collateralize and innervate the thalamus, providing the neural substrate for cutaneous thermosensory perception and localization (Craig et al, 1994; Craig, 2002), and the pontine lateral parabrachial nucleus (LPB) (Hylden et al, 1989; Li et al, 2006) (Fig. 1), responsible for triggering involuntary (e.g., autonomic, shivering and respiratory) thermoregulatory responses.…”