Blockade of mGluR1 receptor results in analgesia and disruption of motor and cognitive performances: effects of A-841720, a novel non-competitive mGluR1 receptor antagonist.

BACKGROUND AND PURPOSE: To further assess the clinical potential of the blockade of metabotropic glutamate receptors (mGluR1) for the treatment of pain. EXPERIMENTAL APPROACH: We characterized the effects of A-841720, a novel, potent and non-competitive mGluR1 antagonist in models of pain and of motor and cognitive function. KEY RESULTS: At recombinant human and native rat mGluR1 receptors, A-841720 inhibited agonist-induced calcium mobilization, with IC50 values of 10.7+/-3.9 and 1.0 +/- 0.2 nM, respectively, while showing selectivity over other mGluR receptors, in addition to other neurotransmitter receptors, ion channels, and transporters. Intraperitoneal injection of A-841720 potently reduced complete Freund's adjuvant-induced inflammatory pain (ED50 = 23 micromol kg(-1)) and monoiodoacetate-induced joint pain (ED50 = 43 micromol kg(-1)). A-841720 also decreased mechanical allodynia observed in both the sciatic nerve chronic constriction injury and L5-L6 spinal nerve ligation (SNL) models of neuropathic pain (ED50 = 28 and 27 micromol kg(-1), respectively). Electrophysiological studies demonstrated that systemic administration of A-841720 in SNL animals significantly reduced evoked firing in spinal wide dynamic range neurons. Significant motor side effects were observed at analgesic doses and A-841720 also impaired cognitive function in the Y-maze and the Water Maze tests. CONCLUSIONS AND IMPLICATIONS: The analgesic effects of a selective mGluR1 receptor antagonist are associated with motor and cognitive side effects. The lack of separation between efficacy and side effects in pre-clinical models indicates that mGluR1 antagonism may not provide an adequate therapeutic window for the development of such antagonists as novel analgesic agents in humans.

TRPV1 receptors in the CNS play a key role in broad-spectrum analgesia of TRPV1 antagonists.

Vanilloid receptor type 1 (TRPV1) is a ligand-gated nonselective cation channel that is considered to be an important integrator of various pain stimuli such as endogenous lipids, capsaicin, heat, and low pH. In addition to expression in primary afferents, TRPV1 is also expressed in the CNS. To test the hypothesis that the CNS plays a differential role in the effect of TRPV1 antagonists in various types of pain, the analgesic effects of two TRPV1 antagonists with similar in vitro potency but different CNS penetration were compared in vivo. Oral administration of either A-784168 (1-[3-(trifluoromethyl)pyridin-2-yl]-N-[4-(trifluoromethylsulfonyl)phenyl]-1,2,3,6-tetrahydropyridine-4-carboxamide) (good CNS penetration) or A-795614 (N-1H-indazol-4-yl-N'-[(1R)-5-piperidin-1-yl-2,3-dihydro-1H-inden-1-yl]urea) (poor CNS penetration) blocked capsaicin-induced acute pain with the same potency. In complete Freund's adjuvant (CFA)-induced chronic inflammatory pain, oral administration of either compound blocked thermal hyperalgesia with similar potency. Furthermore, intraplantar or intrathecal administration of A-784168 blocked CFA-induced thermal hyperalgesia, suggesting that both peripheral and CNS TRPV1 receptors may play a role in inflammatory thermal hyperalgesia. The effects of the two TRPV1 antagonists were further assessed in models presumably mediated by central sensitization, including CFA- and capsaicin-induced mechanical allodynia and osteoarthritic pain. In these models, the potency of the two compounds was similar after intrathecal administration. However, when administered orally, A-784168, with good CNS penetration, was much more potent than A-795614. Together, these results demonstrate that TRPV1 receptors in the CNS play an important role in pain mediated by central sensitization. In addition, these results demonstrate that significant CNS penetration is necessary for a TRPV1 antagonist to produce broad-spectrum analgesia.

Effects of phencyclidine (PCP) and (+)MK-801 on sensorimotor gating in CD-1 mice.

1. Male CD-1 mice were tested for prepulse inhibition (PPI) following administration of PCP and the PCP site ligand, (+)MK-801, as well as the dopamine (DA) agonist (-)-apomorphine and DA releaser d-amphetamine. Similar to reports in rats, PCP (0.36-36.0 mumol/kg), (+)MK-801 (0.03-3.0 mumol/kg), (-)-apomorphine (3.3 and 10.0 mumol/kg) and d-amphetamine (3.0 and 8.0 mumol/kg) significantly reduced PPI when administered prior to testing. 2. Because PCP also binds to sigma receptors, the authors tested the sigma ligand (+)-3-PPP at (118 mumol/kg) which marginally increased the PPI. 3. Haloperidol (1.1 mumol/kg) pretreatment attenuated the reduction in PPI following (-)-apomorphine (10.0 mumol/kg), however no effects of haloperidol or clozapine pretreatment on (+)MK-801 disruption of PPI were observed. 4. Because of the pharmacological similarities between mouse data and previously published rat data, it is concluded that the mouse is a viable alternative to the rat for testing PPI.

Influence of separate and combined septal and amygdala lesions on memory, acoustic startle, anxiety, and locomotor activity in rats.

The septohippocampal system and the amygdala have been implicated in cognitive and emotional processes. A series of experiments was conducted to examine the effects of separate and combined lesions of these areas on a variety of behaviors, including: startle responses to acoustic stimuli; sensory gating, using prepulse inhibition of acoustic startle; anxiety, using the elevated plus-maze; locomotor activity in an open field; and memory, using both a spatial discrimination version of the Morris water maze and the inhibitory (passive) avoidance test. Both septal and fimbria-fornix lesions markedly impaired the acquisition of spatial information in the water maze, had anxiolytic-like effects in the elevated plus-maze, increased reactivity to footshock, and had marginal effects on prepulse inhibition and baseline startle. Septal and fimbria-fornix lesions also increased locomotor activity in the later stages of a session of open field exploration, but only septal lesions produced "freezing" during the early portion of this session and during inhibitory avoidance training. Amygdala lesions markedly impaired prepulse inhibition of acoustic startle. Amygdala lesions also attenuated the effects of septal lesions on freezing in the open field and on footshock reactivity, but did not affect the anxiolytic-like effects or hyperactivity associated with septal lesions. Amygdala lesions by themselves had no significant effect on water maze performance, but significantly potentiated the effects of septal lesions. These results suggest that there are dissociations between the effects of septal and fimbria-fornix lesions and that the interactions between the amygdala and septum in cognitive and emotional processes are task dependent.

The influence of Bay K 8644 treatment on (+/-)-epibatidine-induced analgesia.

The purpose of this investigation was to determine if analogous to (-)-nicotine's analgesic effect, the analgesic effect of the recently characterized potent nicotinic acetylcholine receptor (nAChR) agonist (+/-)-epibatidine was altered in response to treatment with the calcium channel agonist (+/-)-Bay K 8644. In addition, the effects of the enantiomers, (+)-Bay K 8644, reported to be a calcium channel antagonist, and (-)-Bay K 8644, reported to be a calcium channel agonist were examined. (+/-)-Bay K 8644 (2.8 mumol/kg; i.p.) produced a large analgesic response in mice in the hot-plate paradigm that rapidly dissipated by 30 min after treatment. This analgesic effect of (+/-)-Bay K 8644 was not prevented by pre-treatment with the nicotinic antagonist mecamylamine (5 mumol/kg; i.p.). Treatment with non-analgesic doses of the calcium channel agonists (+/-)- and (-)-Bay K 8644 (1.4 mumol/kg; i.p.) significantly potentiated the analgesic effect of (+/-)-epibatidine (0.05 mumol/kg; i.p.). Potentiation of (+/-)-epibatidine's analgesic effect occurred when the agonists were administered prior to (+/-)-epibatidine or after (+/-)-epibatidine as long as analgesia testing was conducted 15 to 30 min after Bay K 8644 treatment. Pre-treatment with the calcium channel antagonist (+)-Bay K 8644 was found to attenuate (+/-)-epibatidine-induced analgesia. When given after (+/-)-epibatidine, (+)-Bay K 8644 had no effect on (+/-)-epibatidine's analgesic effect. These data provide additional in vivo evidence that altering calcium dynamics can modulate neuronal nAChR function.

Effect of nicotine, lobeline, and mecamylamine on sensory gating in the rat.

In normal subjects, if an acoustic startle stimulus is immediately preceded by a small brief change in background noise intensity, the magnitude of the subsequent startle response is decreased. This prepulse inhibition (PPI) of an acoustic startle response has been shown to be associated with sensorimotor gating. PPI is disrupted in schizophrenic patients and has been linked to attentional disorders characteristic of this disease. We tested the effects of (-)-nicotine, (0.19, 0.62, and 1.9 mumol/kg IP) (equivalent to 0.03, 0.1, and 0.3 mg/kg base) and the nicotinic cholinergic receptor (nAChR) channel blocker, mecamylamine (5.0 and 50 mumol/kg IP) (equivalent to 1.0 and 10.0 mg/kg) on PPI of the acoustic startle response in the rat. Nicotine increased the PPI at the lowest prepulse signal levels but not at the stronger levels. Mecamylamine was without effect at 5.0 mumol/kg, but the 50 mumol/kg dose decreased the inhibition at both weak and strong prepulse (PP) levels. Mecamylamine (5.0 mumol/kg) pretreatment did not block the (-)-nicotine-induced increase in PPI. Lobeline (0.19, 0.62, 1.9, and 6.2 mumol/kg IP) (equivalent to 0.071, 0.23, 0.71, and 2.3 mg/kg) was without effect. These results are consistent with a mecamylamine-insensitive effect of nicotine to improve gating in normal rats. The nAChR subtype involved in producing nicotine's increase of PPI needs further investigation.

The effects of aging on hippocampal and cortical projections of the forebrain cholinergic system.

It has been proposed that disruption of cholinergic input to the hippocampus and cortex contributes to the learning and memory deficits associated with aging. The data reviewed here, however, suggest that the oft-stated generalization that normal aging is characterized by disruption of cholinergic input to the hippocampus and cortex is not entirely correct. Instead it appears that age-related changes are not consistently found on measures such as the activity of ChAT or the content of ACh in these regions, basal levels of ACh release in cortex, and the number of cholinergic neurons in the basal forebrain (source of cholinergic input to the hippocampus and cortex). These observations suggest that unlike Alzheimer's disease, normal aging does not reliably produce a degeneration of the cholinergic innervation of the hippocampus and cortex. The responsivity of the cholinergic system, however, is altered during normal aging. ACh synthesis and stimulation-induced release of ACh are diminished in aged animals. Further, the electrophysiological response of postsynaptic neurons to ACh is reduced during aging. Although some regional differences in these age-related changes may be present, the generalization that the functioning of the cholinergic system is impaired during aging is probably accurate. Thus, investigation of these changes in the dynamic properties of cholinergic input to the hippocampus and cortex during aging may provide clarification of the relationship between cholinergic dysfunction and age-related decline in learning and memory and may also provide a more reasonable rationale for treatment approaches.

Bombesin-like immunoreactivity in the central nervous system of capsaicin-treated rats: a radioimmunoassay and immunohistochemical study.

The neuroanatomical distribution of bombesin-like immunoreactivity (BLI) in the rat central nervous system was investigated using radioimmunoassay and immunohistochemistry. Whereas cross-reactivity of the bombesin antiserum with substance P was problematic in the immunohistochemical experiments, no significant cross-reactivity with substance P was apparent in the radioimmunoassay. Results from the radioimmunoassay studies reveal particularly high concentrations of BLI in the hypothalamus, thalamus, medulla and spinal cord. Adult rats treated neonatally with capsaicin displayed significant depletions of somatostatin-like and substance P-like immunoreactivity and a small, statistically significant, reduction of BLI in the cervical spinal cord. Capsaicin treatment significantly reduced substance P-like immunoreactivity, but not somatostatin-like immunoreactivity, in the medulla and resulted in a small BLI depletion of borderline statistical significance in this brain region. Neonatally administered capsaicin treatment had no effect on the thalamic concentration of any of these three neuropeptides and neurotensin-like immunoreactivity was unchanged in all brain regions studied. These results suggest that the source of some of the BLI found in the spinal cord may be capsaicin-sensitive dorsal root ganglion cells.