Inhibition of spinal protein kinase C reduces nerve injury-induced tactile allodynia in neuropathic rats

Protein kinase A modulates spontaneous activity in chronically compressed dorsal root ganglion neurons in the rat

Protein kinase A (PKA) can play a critical role in the modulation of neuronal excitability. We examined the role of PKA in the modulation of abnormal spontaneous activity (SA) originating from the chronically compressed dorsal root ganglion (CCD). The L(4) and L(5) dorsal root ganglia (DRGs) were compressed by inserting a stainless steel rod into each corresponding intervertebral foramen. After 1-14 postoperative days, SA in DRG neurons with myelinated axons was recorded in vitro from teased dorsal root microfilaments. Rp-cAMPS (5-500 microM), a specific inhibitor of PKA, caused a dose-dependent decrease in the discharge rate of SA when topically applied to the DRG. The highest dose completely blocked the SA, but not the conduction of action potentials. H89 (10 microM), another PKA inhibitor, also markedly decreased SA. Sp-cAMPS (500 microM), a specific activator of PKA, increased the discharge rate of SA in all injured units tested, but did not trigger firing in silent neurons. Okadaic acid (0.1 microM), a protein phosphatase inhibitor, and forskolin (1 microM), an adenyl cyclase activator, each significantly increased the discharge rate of SA. These results strongly suggest that PKA modulates the SA in injured DRG neurons with myelinated axons.

Block by gabapentin of the facilitation of glutamate release from rat trigeminal nucleus following activation of protein kinase C or adenylyl cyclase

The effect of activation of protein kinase C (PKC) or adenylyl cyclase on release of glutamate has been investigated in a perfused slice preparation from the rat caudal trigeminal nucleus. Stimulation of PKC by phorbol 12-myristate 13-acetate (PMA) produced a concentration-dependent increase in K(+)-evoked release of [(2)H]-glutamate (maximum increase 45%, EC(50) 11.8 nM), but in the presence of gabapentin (30 microM) the facilitation of release was blocked. The adenylyl cyclase activator forskolin (FSK) also induced a concentration-dependent increase in K(+)-evoked release of [(3)H]-glutamate (maximum increase 36%, EC(50) 2.4 microM), and again this facilitatory effect was blocked by gabapentin (30 microM). We suggest that these results may be of relevance to the antihyperalgesic properties of gabapentin, in conditions where concomitant release of substance P and CGRP produces activation of PKC and adenylyl cyclase respectively.

Effects of amyloid peptides on cell viability and expression of neuropeptides in cultured rat dorsal root ganglion neurons: a role for free radicals and protein kinase C

Chronic pain caused by nerve injury and inflammation is more common in the elderly. However, mechanisms underlying this phenomenon are unclear. Higher sensitivity of sensory neurons to free radicals has been suggested as one possibility. The production of free radicals can be induced by various agents, including the highly toxic protein beta-amyloid (A beta), which is found in higher amounts in the brains of Alzheimer's Disease patients. In dorsal root ganglion (DRG) cultures exposed to A beta, we examined cellular toxicity and peptide expression, in particular calcitonin gene-related peptide (CGRP), a peptide which is abundantly expressed by nociceptive afferents and is known to be involved in pain processes. Exposure of cultured rat DRG neurons to A beta(25--35) or A beta(1--40) (10 or 20 microM for 24--96 h) increased trypan blue-stained cells in a concentration- and time-dependent manner, thus, indicating cellular toxicity. These treatments also increased the number of CGRP immunoreactive (IR) neurons while decreasing the number of neuropeptide Y- and galanin-IR neurons. The free radical scavenger, superoxide dismutase, attenuated both the toxicity and neuropeptide changes induced by A beta, thus, suggesting that oxidative stress probably contributes to these effects. Exposure of cultured DRG neurons to A beta also increased the number of protein kinase C alpha (PKC alpha)-IR neurons. The PKC inhibitors, chelerythrine chloride and Gö6976, significantly augmented A beta-induced cellular toxicity while attenuating the increases in CGRP-and PKC alpha-IR cells, supporting the notion of a protective role for PKC in A beta insults. These in vitro data suggest that A beta peptides may, in addition to causing neurotoxicity, regulate neuropeptide expression in primary afferents. This finding could be relevant to the higher incidence of neuropathic pain that occurs with ageing.