title: Encoding of corneal input in two distinct regions of the spinal trigeminal nucleus in the rat: cutaneous receptive field properties, responses to thermal and chemical stimulation, modulation by diffuse noxious inhibitory controls, and projections to the parabrachial area.
abstract: To determine whether corneal input is processed similarly at rostral and caudal levels of the spinal trigeminal nucleus, the response properties of second-order neurons at the transition between trigeminal subnucleus interpolaris and subnucleus caudalis (Vi/Vc) and at the transition between subnucleus caudalis and the cervical spinal cord (Vc/C1) were compared. Extracellular single units were recorded in 68 Sprague-Dawley rats under chloralose or urethan/chloralose anesthesia. Neurons that responded to electrical stimulation of the cornea at the Vi/Vc transition region (n = 61) and at laminae I/II of the Vc/C1 transition region (n = 33) were classified regarding 1) corneal mechanical threshold; 2) cutaneous mechanoreceptive field, if present; 3) electrical input characteristics (A and/or C fiber); 4) response to thermal stimulation; 5) response to the small-fiber excitant, mustard oil (MO), applied to the cornea; 6) diffuse noxious inhibitory controls (DNIC); and 7) projection status to the contralateral parabrachial area (PBA). On the basis of cutaneous receptive field properties, neurons were classified as low-threshold mechanoreceptive (LTM), wide dynamic range (WDR), nociceptive specific (NS), or deep nociceptive (D). All neurons recorded at the Vc/C1 transition region were either WDR (n = 19) or NS (n = 14). In contrast, 54% of the Vi/Vc neurons had no cutaneous receptive field. Of those Vi/Vc neurons that had a cutaneous receptive field, 57% were LTM, 25% were WDR, and 18% were D. All Vc/ C1 neurons responded to noxious thermal and MO stimulation. Only 22 of 47 and 13 of 19 Vi/Vc corneal units responded to thermal or MO stimulation, respectively. At the Vc/C1 transition region, 12 of 17 neurons demonstrated DNIC, whereas at the Vi/Vc transition region, DNIC was present in only 4 of 26 neurons. Of 15 Vc/C1 corneal units, 12 could be antidromically activated from the contralateral PBA (average latency 6.29 ms, range 1.8-26 ms). None of 22 Vi/Vc corneal units tested could be antidromically activated from the PBA. These findings suggest that neurons in laminae I/II at the Vc/C1 transition and at the Vi/Vc transition process corneal input differently. Neurons in laminae I/II at the Vc/C1 transition process corneal afferent input consistent with that from other orofacial regions. Corneal-responsive neurons at the Vi/Vc transition region may be important in motor reflexes or in recruitment of descending antinociceptive controls.
authors.affiliation: Department of Neuroscience, Brown University/Rhode Island Hospital, Providence 02903, USA.
authors.forename: I D
authors.initials: ID
authors.lastname: Meng
authors.forename: J W
authors.initials: JW
authors.lastname: Hu
authors.forename: A P
authors.initials: AP
authors.lastname: Benetti
authors.forename: D A
authors.initials: DA
authors.lastname: Bereiter
mesh_terms: D000777:Anesthetics / Q000494:pharmacology; D000818:Animals; D003315:Cornea / Q000187:drug effects / Q000294:innervation / Q000502:physiology*; D004558:Electric Stimulation; D005110:Extracellular Space / Q000502:physiology; D006358:Hot Temperature; D008465:Mechanoreceptors / Q000187:drug effects / Q000502:physiology*; D009412:Nerve Fibers / Q000502:physiology; D009475:Neurons, Afferent / Q000502:physiology; D009619:Nociceptors / Q000187:drug effects / Q000502:physiology*; D011149:Pons / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012867:Skin / Q000294:innervation*; D013268:Stimulation, Chemical; D014279:Trigeminal Nucleus, Spinal / Q000033:anatomy & histology / Q000187:drug effects / Q000502:physiology*
chemical_list: D000777:Anesthetics