Brain Research, 574 (1992) 329-332 © 1992 Elsevier Science Publishers B.V. All rights reserved. 0006-8993/92/$05.00

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BRES 25078

Bulbar reticular neurons relaying somatosensory information to the mesencephalic parabrachial area of the cat Haruhide Hayashi, Takashi Toda and Takayoshi Tabata Department of Physiology, School of Dentistry, Tohoku University, Seiryo-machi, Aoba-ku, Sendai 980 (Japan)

(Accepted 26 November 1991) Key words: Parabrachial area; Nucleus cuneiformis; Pulpal nerve; Bulbar reticular neuron; Spinal trigeminal nucleus; Mesencephalon; Pain

Somatosensory neurons projecting to the mesencephalic parabrachial area (MPBA), which is located ventral to the inferior colliculus and dorsal to the brachium conjunctivum, were recorded from the bulbar reticular formation of adult cats anesthetized with a-chloralose. The majority (41 of 50 neurons) were nociceptive-specificneurons responding only to noxious mechanical and/or thermal stimuli to the skin, cornea and/or oral mucosa. The size of their receptive fields was smaller than that of the intrinsic MPBA-neurons, but larger than that of the trigeminal sensory nucleus neurons. Twenty-three neurons received input from the tooth pulp nerve and 10 of 32 neurons tested responded to electrical stimulation of the vagal nerve. These results indicate that these bulbar reticular neurons receive noxious inputs and transmit them to the MPBA, which also receives input from spinal or trigeminal sensory nucleus neurons projecting directly to the MPBA. Retrograde and orthograde tracing experiments using horseradish peroxidase show that a bilateral projection arising from all spinal segments and the spinal trigeminal nucleus terminates heavily in the mesencephalic parabrachial area (MPBA), which is located ventral to the inferior colliculus, dorsal to the brachium conjunctivum and rostral to the pontine parabrachial nucleus and includes the cuneiform nucleus 5'8':5':6. Afferent and efferent connections of the MPBA with the brainstem reticular formation, hypothalamus and limbic structures, and defensive or pain-related behavior induced in animals by the MPBA stimulation suggest its probable role in the nociception or antinociception 1'2'7'~2':7. Recently, physiological studies have demonstrated that the majority of lamina I neurons in the lumbosacral spinal dorsal horn that project to the MPBA responded exclusively to noxious stimulation of their peripheral receptive fields (RFs) in the rat 1°'11 and in the cat 9. Subsequently, the sensory trigeminal nucleus neurons that project to the MPBA were reported to carry mainly nociceptive information from the oral and facial areas 3. Consistent with these studies, most of the somatosensory neurons within the MPBA were characterized as nociceptive-specific neurons 4. However, they had extremely large RFs, extending to include the whole body surface, and 69% of the MPB,~ neurons received input from the tooth pulp nerve: In contrast, the RFs of MPBA-projection neurons in the sensory trigeminal nucleus and spinal dorsal horn are small and restricted to the ipsilateral side, and few

trigeminal projection neurons have pulpal input 3'9. These large RFs and extensive inputs from the tooth pulp onto intrinsic MPBA neurons suggest that somatosensory information is transmitted not only by the direct pathway from the sensory trigeminal nucleus, but also via additional relay nuclei. Since retrograde tracing experiments show a scattered distribution of MPBA-projection neurons in the bulbar reticular formation, in addition to a heavy distribution to the trigeminal sensory nucleus 5, the aim of the present study is to fred and characterize MPBA-projecting relay neurons in the bulbar reticular formation. Nine adult cats were sedated with ketamine hydrochloride (20-25 mg/kg, i.m.). After a venous cannula was installed, they were anesthetized with pentobarbital sodium (10 mg/kg, i.v., preoperative dose) and a-chloralose (60-100 mg/kg/h, i.v., initial dose, followed by 5-10 mg/ kg/h, i.v.). The trachea was cannulated for artificial respiration. Body temperature was maintained at 37-38°C. E C G was monitored continuously. A bilateral pneumothorax was made and the animals were paralyzed with pancuronium bromide (0.4-0.5 mg/kg/h, i.v.). An adequate level of anesthesia was ensured by reference to pupillary diameter, heart rate, and nociceptive reflexes during recovery from paralysis. After the animal was fixed in a stereotaxic apparatus, the overlying cerebellum was removed and the brainstem was exposed. An array of 4 antidromic stimulating electrodes (0.2 mm diameter stainless steel; tip exposure, 0.2 mm) was positioned

Correspondence: H. Hayashi, Department of Physiology, School of Dentistry, Tohoku University, Seiryo-machi, Aoba-ku, Sendai 980, Japan.

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al. 6. Stimulation and recording sites were verified histologically by iron deposits. A total of 77 neurons were activated antidromically

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with electrical stimulation of the M P B A ( < 1 m A , 0.2-ms duration). The termination sites of the axons of 8 neu-

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rons were mapped and determined to be within the M P B A by moving the array of antidromic stimulating

Bulbar reticular neurons relaying somatosensory information to the mesencephalic parabrachial area of the cat.

Somatosensory neurons projecting to the mesencephalic parabrachial area (MPBA), which is located ventral to the inferior colliculus and dorsal to the ...
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