Glutamate-like immunoreactivity in ascending spinofugal afferents to the rat periaqueductal grey

Connections from the rat dorsal column nuclei (DCN) to the periaqueductal gray matter (PAG)

Electrical stimulation of the dorsal columns (DCs; spinal cord stimulation; SCS) has been proposed to treat chronic neuropathic pain. SCS may activate a dual mechanism that would affect both the spinal cord and supraspinal levels. Stimulation of DCs or DC nuclei (DCN) in animals where neuropathic pain has been induced causes activation of brainstem centers including the periaqueductal gray (PAG), which is involved in the endogenous pain suppression system. Biotinylated dextran-amine (BDA) was iontophoretically injected into the DCN to analyze the ascending projection directed to the PAG. Separate injections into the gracile nucleus (GrN) and the cuneate nucleus (CunN) showed BDA-positive fibers terminating in different regions of the contralateral PAG. GrN-PAG afferents terminated in the caudal and middle portions of PAG-l, whereas CunN-PAG fibers terminated in the middle and rostral portions of PAG-l. Based on the DCN somatotopic map, the GrN sends information to the PAG from the contralateral hindlimb and the tail and the CunN from the contralateral forelimb, shoulder, neck and ear. This somatotopic organization is consistent with earlier electrophysiological and PAG stimulation studies. These fibers could form part of the DCs-brainstem-spinal cord loop, which may be involved in the inhibitory effects of SCS on neuropathic pain.

Prrxl1 is required for the generation of a subset of nociceptive glutamatergic superficial spinal dorsal horn neurons

Perception of noxious events relies on activation of complex central neuronal circuits. The spinal cord dorsal horn plays a pivotal role in the process relaying to the brain various types of somatosensory input. These functions are accomplished by distinct sensory neurons specifically organized in different laminae. They differentiate during development in a spatial-temporal order due to the expression of combinatorial sets of homeodomain transcription factors. Here we demonstrate that the differential expression of the homeodomain transcription factors Prrxl1 (DRG11), Tlx3, and Lmx1b defines various subpopulations of spinal cord dorsal horn glutamatergic early born and late born neurons. Accordingly, in the superficial dorsal horn of Prrxl1(-/-) mice, the number of glutamatergic neurons is reduced by 70%, while the number of Golgi-impregnated and noxious-induced Fos immunoreactive neurons is reduced by 85%. These results suggest a crucial role for Prrxl1 in the generation of various subpopulations of nociceptive glutamatergic superficial dorsal horn neurons.

Anatomically specific patterns of glial activation in the periaqueductal gray of the sub-population of rats showing pain and disability following chronic constriction injury of the sciatic nerve

Neuropathic pain conditions for which treatment is sought are characterized by complex behavioural disturbances, as well as "pain." Recent studies using chronic constriction injury of the sciatic nerve have shown that rats develop three distinct patterns of disability characterized by changes in social-interactions and sleep-wake cycle behaviours post-injury: (i) Persistent Disability, (ii) Transient Disability and (iii) No Disability. These patterns occur despite all rats showing identical levels of allodynia and hyperalgesia (i.e., pain). In rats, social-interactions and sleep-wake cycle behaviours are regulated in part, by neural networks, which converge on the periaqueductal grey (PAG). We sought therefore to identify neural adaptations in the PAG, 6 days following chronic constriction injury (CCI), the time at which rats in which disabilities persist are first distinguished from those without disabilities (i.e., No Disability and Transient Disability). GeneChips, RT-PCR and Western blotting revealed the select up-regulation in translation and transcription of glial fibrillary acidic protein (GFAP) and Vimentin in rats with Persistent Disability. Significant increases in GFAP immunoreactivity were localized histologically to the lateral and caudal ventrolateral columns of the PAG. This anatomically specific pattern of increased GFAP suggests activation of astrocytes by select neural pathways, which likely include afferents of both spinal and nucleus of the solitary tract (NTS) origin. The PAG columns in which astrocytes are activated play significant roles in modulating both social-interactions and the sleep-wake cycle. It is possible therefore that the persistent disabilities seen in a subgroup of CCI rats are in part a functional consequence of this specific pattern of astrocyte activation.

Tonic endovanilloid facilitation of glutamate release in brainstem descending antinociceptive pathways

Activation of transient receptor potential vanilloid-1 (TRPV1) channels in the periaqueductal gray (PAG) activates OFF antinociceptive neurons of the rostral ventromedial medulla (RVM). We examined in rats the effect of intra-ventrolateral (VL)-PAG injections of TRPV1 agonists and antagonists on the nocifensive response to heat in the plantar test, neurotransmitter (glutamate and GABA) release in the RVM, and spontaneous and tail flick-related activities of RVM neurons. The localization of TRPV1 in VL-PAG and RVM neurons was examined using various markers of glutamatergic and GABAergic neurons. Intra-VL-PAG injection of capsaicin increased the threshold of thermal pain sensitivity, whereas the selective TRPV1 antagonist 5'-iodo-resiniferatoxin (I-RTX) facilitated nociceptive responses, and blocked capsaicin analgesic effect at a dose inactive per se. Intra-VL PAG capsaicin evoked a robust release of glutamate in RVM microdialysates. I-RTX, at a dose inactive per se, blocked the effect of capsaicin, and inhibited glutamate release at a higher dose. Antinociception and hyperalgesia induced by capsaicin and I-RTX, respectively, correlated with enhanced or reduced activity of RVM OFF cells. Immunohistochemical analyses suggested that several TRPV1-immunoreactive (ir) neurons in both the VL-PAG and RVM are glutamatergic and surrounded by glutamatergic and GABAergic terminals. Our data suggest that VL-PAG neurons respond to TRPV1 stimulation by releasing glutamate into the RVM, thereby activating OFF cells and producing analgesia. The results obtained with the TRPV1 antagonist alone suggest that this pathway is tonically activated by endovanilloids.

TRPV1 Receptor Mediates Glutamatergic Synaptic Input to Dorsolateral Periaqueductal Gray (dl-PAG) Neurons

The purpose of this study was to determine the role of transient receptor potential vanilloid type 1 (TRPV1) receptor in modulating neuronal activity of the dorsolateral periaqueductal gray (dl-PAG) through excitatory and inhibitory synaptic inputs. First, whole cell voltage-clamp recording was performed to obtain the spontaneous miniature excitatory postsynaptic currents (mEPSCs) and inhibitory postsynaptic currents (mIPSCs) of the dl-PAG neurons. As 1 μM of capsaicin was applied into the perfusion chamber, the frequency of mEPSCs was increased from 3.21 ± 0.49 to 5.64 ± 0.64 Hz ( P < 0.05, n = 12) without altering the amplitude and the decay time constant of mEPSCs. In contrast, capsaicin had no distinct effect on mIPSCs. A specific TRPV1 receptor antagonist, iodo-resiniferatoxin (i-RTX, 300 nM), decreased the frequency of mEPSCs from 3.51 ± 0.29 to 2.01 ± 0.2 Hz ( P < 0.05, n = 8) but did not alter the amplitude and decay time. In addition, i-RTX applied into the chamber abolished the effect of capsaicin on mEPSC of the dl-PAG. In another experiment, spontaneous action potential of the dl-PAG neurons was recorded using whole cell current-clamp methods. Capsaicin significantly elevated the discharge rate of the dl-PAG neurons from 3.03 ± 0.38 to 5.96 ± 0.87 Hz ( n = 8). The increased firing activity was abolished in the presence of glutamate N-methy-d-aspartate (NMDA) and non-NMDA antagonists, 2-amino-5-phosphonopentanoic acid, and 6-cyano-7-nitroquinoxaline-2,3-dione. The results from this study provide the first evidence indicating that activation of TRPV1 receptors increases the neuronal activity of the dl-PAG through selective potentiation of glutamatergic synaptic inputs.

Lbx1 and Tlx3 are opposing switches in determining GABAergic versus glutamatergic transmitter phenotypes

Most neurons in vertebrates make a developmental choice between two principal neurotransmitter phenotypes (glutamatergic versus GABAergic). Here we show that the homeobox gene Lbx1 determines a GABAergic cell fate in the dorsal spinal cord at early embryonic stages. In Lbx1-/- mice, the presumptive GABAergic neurons are transformed into glutamatergic cells. Furthermore, overexpression of Lbx1 in the chick spinal cord is sufficient to induce GABAergic differentiation. Paradoxically, Lbx1 is also expressed in glutamatergic neurons. We previously reported that the homeobox genes Tlx1 and Tlx3 determine glutamatergic cell fate. Here we show that impaired glutamatergic differentiation, observed in Tlx3-/- mice, is restored in Tlx3-/-Lbx1-/- mice. These genetic studies suggest that Lbx1 expression defines a basal GABAergic differentiation state, and Tlx3 acts to antagonize Lbx1 to promote glutamatergic differentiation.

Tlx3 and Tlx1 are post-mitotic selector genes determining glutamatergic over GABAergic cell fates

Glutamatergic and GABAergic neurons mediate much of the excitatory and inhibitory neurotransmission, respectively, in the vertebrate nervous system. The process by which developing neurons select between these two cell fates is poorly understood. Here we show that the homeobox genes Tlx3 and Tlx1 determine excitatory over inhibitory cell fates in the mouse dorsal spinal cord. First, we found that Tlx3 was required for specification of, and expressed in, glutamatergic neurons. Both generic and region-specific glutamatergic markers, including VGLUT2 and the AMPA receptor Gria2, were absent in Tlx mutant dorsal horn. Second, spinal GABAergic markers were derepressed in Tlx mutants, including Pax2 that is necessary for GABAergic differentiation, Gad1/2 and Viaat that regulate GABA synthesis and transport, and the kainate receptors Grik2/3. Third, ectopic expression of Tlx3 was sufficient to suppress GABAergic differentiation and induce formation of glutamatergic neurons. Finally, excess GABA-mediated inhibition caused dysfunction of central respiratory circuits in Tlx3 mutant mice.

Glutamate release in midbrain periaqueductal gray by activation of skeletal muscle receptors and arterial baroreceptors

We have previously reported that both skeletal muscle receptor and arterial baroreceptor afferent inputs activate neurons in the dorsolateral (DL) and lateral regions of the midbrain periaqueductal gray (PAG). In this study, we determined whether the excitatory amino acid glutamate (Glu) is released to mediate the increased activity in these regions. Static contraction of the triceps surae muscle for 4 min was evoked by electrical stimulation of the L7 and S1 ventral roots in cats. Activation of arterial baroreceptor was induced by intravenous injection of phenylephrine. The endogenous release of Glu from the PAG was recovered with the use of a microdialysis probe. Glu concentration was measured by the HPLC method. Muscle contraction increased mean arterial pressure (MAP) from 98 +/- 10 to 149 +/- 12 mmHg (P < 0.05) and increased Glu release in the DL and lateral regions of the middle PAG from 0.39 +/- 0.10 to 0.73 +/- 0.12 microM (87%, P < 0.05) in intact cats. After sinoaortic denervation and vagotomy were performed, contraction increased MAP from 95 +/- 12 to 158 +/- 15 mmHg, and Glu from 0.34 +/- 0.08 to 0.54 +/- 0.10 microM (59%, P < 0.05). The increases in arterial pressure and Glu were abolished by muscle paralysis. Phenylephrine increased MAP from 100 +/- 13 to 162 +/- 22 mmHg and increased Glu from 0.36 +/- 0.10 to 0.59 +/- 0.18 microM (64%, P < 0.05) in intact animals. Denervation abolished this Glu increase. Summation of the changes in Glu evoked by muscle receptor and arterial baroreceptor afferent inputs was greater than the increase in Glu produced when both reflexes were activated simultaneously in intact state (123% vs. 87%). These data demonstrate that activation of skeletal muscle receptors evokes release of Glu in the DL and lateral regions of the middle PAG, and convergence of afferent inputs from muscle receptors and arterial baroreceptors in these regions inhibits the release of Glu. These results suggest that the PAG is a neural integrating site for the interaction between the exercise pressor reflex and the arterial baroreceptor reflex.

GABA(A) receptor regulation of kyphotic nursing and female sexual behavior in the caudal ventrolateral periaqueductal gray of postpartum rats

Bilateral lesions of the ventrolateral caudal periaqueductal gray inhibit lordosis and kyphosis, the postures of female sexual receptivity and maternal nursing that are characterized respectively by dorsoflexion and ventroflexion of the spinal column. These lesions also inhibit the solicitation behaviors that accompany lordosis, but they do not impair retrieval or licking of pups. We tested the hypothesis that reproductive behaviors affected by these lesions are tonically inhibited by activity of the GABA(A) receptor via site-specific manipulations of receptor activity. Rats were bilaterally implanted during pregnancy with guide cannulae aimed at the caudal periaqueductal gray and ovariectomized on day 1 postpartum. Microinfusions (0.25 microl/side) of saline or drug took place on days 5 and 7 postpartum into the dorsolateral column and on days 9 and 11 into the ventrolateral column. Five minutes post-infusion dams were reunited with their pups and their maternal behavior was observed for 30 min. Feminine sexual behaviors were evaluated post-weaning after another set of microinfusions in the ventrolateral caudal periaqueductal gray. Potential facilitation of kyphosis and lordosis was tested with the GABA(A) antagonist bicuculline (15 ng/side) during sub-threshold conditions, i.e., non-suckling pups or sub-threshold ovarian hormone dosages; potential inhibition of these postures was tested with the GABA(A) agonist muscimol (125 ng/side) during optimal conditions, i.e., suckling pups or supra-threshold ovarian hormone treatments. Dorsolateral drug manipulations were ineffective. In the ventrolateral periaqueductal gray bicuculline significantly increased and muscimol significantly decreased kyphosis, lordosis, and sexual solicitations compared with saline. Retrieval and licking of pups were not altered by GABA(A) manipulations. These findings suggest that the reproductive postures of female rats, lordosis and kyphosis, as well as sexual solicitations, are tonically inhibited by the neurotransmitter GABA within the ventrolateral caudal periaqueductal gray in the midbrain. In contrast, retrieval and licking of pups appear to be under separate neurochemical or neuroanatomical control, or both. Further, this tonic inhibition is likely relieved by excitatory somatosensory inputs to this site, from mounting and suckling respectively.