Differential localization of vesicular glutamate transporters and peptides in corneal afferents to trigeminal nucleus caudalis

Abstract

Trigeminal afferents convey nociceptive information from the corneal surface of the eye to trigeminal subnucleus caudalis (Vc). Trigeminal afferents, like other nociceptors, are thought to use glutamate and neuropeptides as neurotransmitters. The current studies examined whether corneal afferents contain both neuropeptides and vesicular glutamate transporters. Corneal afferents to Vc were identified using Cholera Toxin B (CTb). Corneal afferents project in two clusters to the rostral and caudal borders of Vc; regions that contain functionally distinct nociceptive neurons. Thus, corneal afferents projecting to these two regions were examined separately. Dual immunocytochemical studies combined CTb with either calcitonin gene-related peptide (CGRP), substance P (SP), vesicular glutamate transporter 1 (VGluT1) or VGluT2. Corneal afferents were more likely to contain CGRP than SP, and corneal afferents projecting to the rostral region were more likely to contain CGRP than afferents projecting caudally. Overall, corneal afferents were equally likely to contain VGluT1 or VGluT2. Together, 61% of corneal afferents contained either VGluT1 or VGluT2, suggesting that some afferents lack a vesicular glutamate transporter. Caudal corneal afferents were more likely to contain VGluT2 than VGluT1; while rostral corneal afferents were more likely to contain VGluT1 than VGluT2. Triple labeling studies combining CTb, CGRP and VGluT2 showed very few corneal afferents contain both CGRP and VGluT2; caudally (1%) and rostrally (2%). These results suggest that most corneal afferents contain a peptide or a VGluT, but rarely both. Our results are consistent with a growing literature suggesting that glutamatergic and peptidergic sensory afferents may be distinct populations.

Physiological temperatures drive glutamate release onto trigeminal superficial dorsal horn neurons

“…We classified one fundamental aspect of primary afferent neurons (TRPV1ϩ), but our data support at least four subclassifications of synaptic responses. Primary trigeminal afferents are quite diverse (Hegarty et al 2010;Kobayashi et al 2005), and, therefore, the variety of responses is not surprising. Many peripheral sensory responses, however, can show primary afferent nerve activation with decreases in temperature described in the spinal cord (Wrigley et al 2009) and Fig.…”

Capsaicin-responsive corneal afferents do not contain TRPV1 at their central terminals in trigeminal nucleus caudalis in rats

“…Painful stimulation of the cornea is transduced by these afferents and transmitted via the ophthalmic branch of the trigeminal nerve to neurons in the trigeminal nucleus caudalis (Vc). Corneal afferent terminals send their highest density projections to the caudal and rostral transition areas of Vc with cervical spinal cord and trigeminal nucleus interpolaris (Vi), respectively (Aicher et al, 2013; Belmonte et al, 2004; Hegarty et al, 2010; Marfurt and Del Toro, 1987). The peripheral ends of the corneal afferents contain members of the transient receptor potential (TRP) channel family that are thought to transduce mechanical, thermal, cold and chemical stimuli (Guo et al, 1999; Hiura and Nakagawa, 2012; Murata and Masuko, 2006; Nagata et al, 2005; Nakagawa et al, 2009; Parra et al, 2010).…”

“…Previous studies from this laboratory have successfully used cholera toxin B (CTb) to trace corneal afferents to caudal and rostral Vc (Aicher et al, 2013; Aicher et al, 2014; Hegarty et al, 2010). We have demonstrated that a substantial percentage of CTb-labeled corneal afferents contain vesicular glutamate transporter 1 (VGluT1) (33%) or VGluT2 (28%), with fewer afferents containing calcitonin gene-related peptide (CGRP) (15%) and very few afferents containing substance P (SP) (3%) (Hegarty et al, 2010).…”

“…We have demonstrated that a substantial percentage of CTb-labeled corneal afferents contain vesicular glutamate transporter 1 (VGluT1) (33%) or VGluT2 (28%), with fewer afferents containing calcitonin gene-related peptide (CGRP) (15%) and very few afferents containing substance P (SP) (3%) (Hegarty et al, 2010). We also found that the glutamatergic and peptidergic neurochemical composition differs between caudal and rostral Vc corneal afferents (Hegarty et al, 2010).…”

“…We have demonstrated that a substantial percentage of CTb-labeled corneal afferents contain vesicular glutamate transporter 1 (VGluT1) (33%) or VGluT2 (28%), with fewer afferents containing calcitonin gene-related peptide (CGRP) (15%) and very few afferents containing substance P (SP) (3%) (Hegarty et al, 2010). We also found that the glutamatergic and peptidergic neurochemical composition differs between caudal and rostral Vc corneal afferents (Hegarty et al, 2010). In later studies we found that the corneal afferents to rostral Vc preferentially contact parabrachial-projecting neurons (Aicher et al, 2013; Aicher et al, 2014) which are responsive to multi-modal corneal stimulation (Aicher et al, 2014).…”