- Identification of the CSF-contacting nucleus in primates
Our previous studies [4, 6] in rodents confirmed that after CB-HRP injection into the CSF of the unilateral ventricular system, it can only flow to the contra-lateral LV, 3V, Aq, 4V, CC of the spinal cord and subarachnoid space. The tracer is localised on the walls of these structures and forms a clear outline. The present study in rhesus monkey confirms that the CB-HRP can only flow within the ventricular system and cannot pass through the CSF-brain barrier and leak into the brain parenchyma. Thus, the neurons whose processes stretch into the CSF can be labelled by CB-HRP. Therefore, the CB positive neurons observed in our study can only be the CSF-contacting neurons.
The criteria for naming cranial nuclei are as follows [11-13]: (1) Similar morphology and functions: In the present study, all of the CB positive neurons are CSF-contacting neurons whose functions are specified for information connection between the brain and the CSF. (2) Similar location and occupy certain space: The CB positive neurons are located in the ventral grey of the aqueduct and 4V floor, form an independent cluster and are significantly different from the nearby structures. The three-dimensional reconstruction results show that the entire morphology of the nucleus presents “rivet-like” shape and occupies a certain spatial volume. (3) The distribution of these nuclei has an evolutionary similarity: In both, rodents as well as in primates, the CSF-contacting nucleus always initiates ventral of the xscp and at the ventral grey of the 4V floor. The core of the nucleus is always located at the ventral grey of the Aq and the nearby structures are always DR, PAG, and mlf. The CSF-contacting nucleus in the monkey brain is larger than that in the rats, however, the inner structures are similar. The morphology of the nucleus is highly homologous between the two species [9, 14]. Therefore, we have sufficient evidence suggesting that the CSF-contacting nucleus is existed objectively in primates.
- The CSF-contacting nucleus is regarded as the XIII pair of cranial nucleus
The ancient Greek scientist Gelen (129–200 AD) was the first to describe cranial nerves and summarised seven pairs of cranial nerves . Soemmerring (1755–1830, Germany) supplemented the concept and numbered the cranial nerves from I to XII, which is still in use [1, 2]. With the development of imaging techniques and microscopy, the distribution and location of the cranial nuclei in the brain parenchyma corresponding to each pair of the cranial nerves have been confirmed, and the “concept” of XII pairs of cranial nuclei is accepted. Most of these nuclei are located in the brainstem. The nerve fibres go in or out through the outer brain surface and have definite functions and clear division. For example, the midbrain oculomotor nucleus sends out the third pair of oculomotor nerves to control the extraocular muscle movement. The hypoglossal nucleus in the medulla oblongata sends out hypoglossal nerves to control the tongue movement. The trigeminal nucleus in pons receives sensory inputs from the head and the face. The common features of the XII pairs of cranial nuclei are that the neural somas are located in the brain (mainly in the brainstem), and the processes connect with the organs (somatic or visceral). They perform specific functions and are the anatomical foundations of the central nervous system modulating the activity of the organs.
The animal body is composed of not only organs, but also includes fluids such as extracellular fluid, plasma, and CSF. It is believed that the body fluids are controlled by the instructions from the brain; however, presently there is no description of any structure in the central nervous system that directly modulates the body fluids. Due to the lack of information regarding this structure, it is difficult to address why the type and content of the substance in the CSF changes, and the “lumbar anaesthesia” via CSF has higher effects such as nausea and vomiting.
The past 30 years of our studies in rodents and our present study in the monkey brain confirm that similar to the XII pairs of cranial nuclei, the CSF-contacting nucleus is located in the brainstem; however, its processes cross through the brain-CSF barrier and contact the CSF in the ventricular system. Our concept of the CSF-contacting nucleus (XIII pair of cranial nucleus) which specifically connects with the body fluids will help to bridge the gap in understanding the CNS mediated regulation of the body fluids regulation. The twelve pairs of cranial nuclei are connected to the organs and modulate sensory or motor functions. However, the XIII pair of cranial nucleus (CSF-contacting nucleus) connects and regulates the body fluids. The concept of the CSF-contacting nucleus is important and necessary for understanding the anatomical structures of the CNS as well as for understanding the coordination of the functions in life activities.
- Role of XIII pair of cranial nucleus (CSF-contacting nucleus) in body fluids regulation
In the central nervous system, the synaptic structures are the key for information transmission between the neurons . We used the well-established method of CB-HRP labelling to specifically label CSF-contacting nucleus in combination with electron microscopy [4, 5] and found that the CB-HRP positive axons are covered with a myelin sheath. The vesicles in the axon terminals are round, flattened, polygonal, and contain dense cores. It has been confirmed that the round vesicles mainly contain excitatory neurotransmitters [16, 17], flattened vesicles mainly contain inhibitory neurotransmitters [18, 19] and the dense core vesicles mainly contain protein or peptide neurotransmitters . Therefore, the neurons in the CSF-contacting nucleus participate in functional modulation via releasing excitatory, inhibitory, or peptide neurotransmitters.
Using scanning and transmission electron microscopy, we confirmed that the CB-HRP positive nerve fibres stretch into the ventricular system and contact the CSF. The nerve fibres not only have local expansions but also contain many vesicles. This unique morphological feature suggests that the CSF-contacting nucleus regulates the release of biological substances into the CSF. The nerve fibres can sense the physicochemical properties and changes in the CSF and transmit the information to the brain via different types of synapses.
According to the classic synaptic transmission theory , the information is transmitted from the pre-synaptic component to the post-synaptic component. In the brain parenchyma, we observed that the neurons in the CSF-contacting nucleus form both pre-synaptic and post-synaptic components. Besides, the CSF-contacting nucleus forms both Gray I type excitatory synapse as well as Gray II type inhibitory synapse. According to the diversity of the synapse, complicacy of the neurotransmitters, and bidirectional information transmission of the CSF-contacting nucleus, it is inferred that the nucleus participates in the complex neural circuits and is involved in the fine regulation of life activities. Due to the special structure of the CSF-contacting nucleus, we can further speculate that under certain physiological or pathological situations, the CSF-contacting nucleus nerve fibres can release biological compounds into the CSF to alter its composition. By sampling and detecting the changes in the CSF, the auxiliary diagnosis can be achieved. On the contrary, when the chemical components of the CSF change, the CSF-contacting nucleus can sense the alterations and send the information to the non-CSF-contacting structures in the brain parenchyma via different types of synapses. Hence, medical interventions via the CSF might be developed as efficient therapeutic methods. The present study provides strong morphological evidence to unveil the existence of a cranial nucleus involved in regulating the CSF.