Pentobarbital enhances gamma-aminobutyric acid-mediated excitation without altering synaptic plasticity in rat hippocampus.

BACKGROUND: Synaptic plasticity is thought to provide a molecular mechanism for learning and memory. N-methyl-d-aspartate receptor-mediated plasticity requires that N-methyl-d-aspartate receptor activation coincides with postsynaptic depolarizing potentials (DPSP(A)'s). Pentobarbital, in high concentrations, enhances DPSP(A)'s, but high concentrations suppress synaptic plasticity, probably by impairing glutamatergic transmission. Here we tested the hypothesis that low concentrations of pentobarbital can enhance DPSP(A)'s and modify the induction of synaptic plasticity. METHODS: Studies were performed in vitro on rat hippocampal slices. With glutamate transmission blocked, intracellular recording from CA1 neurons was used to investigate the influence of 5 microM pentobarbital on DPSP(A)'s and neuron excitability evoked by high frequency (100 Hz) stimulation. With glutamate transmission intact, extracellular recording was used to examine the effect of 5 microM pentobarbital on the induction of long-term depression and long-term potentiation of synaptic transmission by conditioning stimuli applied to the Schaffer collateral pathway. RESULTS: High frequency stimulation generated typical DPSP(A)'s that were mediated by gamma-aminobutyric acid(A) receptors and dependent upon HCO3-. Pentobarbital (5 microM) increased the amplitude, but not the width, at half-maximal amplitude of DPSPA's (P < 0.01). Pentobarbital increased the probability of action potential generation during the DPSP(A)'s. Pentobarbital did not alter the induction of long-term depression or long-term potentiation. CONCLUSIONS: Despite increasing the amplitude of DPSP(A)'s, 5 microM pentobarbital did not alter the induction of synaptic plasticity by a range of conventional conditioning stimuli. These results do not support the hypothesis that excitatory effects of pentobarbital may alter synaptic plasticity.

Levetiracetam reduces anesthetic-induced hyperalgesia in rats.

BACKGROUND: As part of an increase in excitability, small doses of pentobarbital, propofol, and midazolam induce an increased sensitivity to pain. Specific therapy to prevent or reduce this excitability may offer advantages over current clinical management with analgesics and sedatives. The pharmacological profile of the novel antiepileptic drug, levetiracetam, suggests that it may reduce the intensity of the excitatory stages of anesthesia. METHODS: We examined the influence of levetiracetam on the reduction of the nociceptive reflex threshold in rats by sedative doses of pentobarbital, propofol, and midazolam. Measurements of nociceptive reflex threshold to pressure and heat were made and then repeated after intraperitoneal injection of saline or one of three doses of levetiracetam (100, 200, 500 mg/kg). Pentobarbital (30 mg/kg), propofol (30 mg/kg), or midazolam (1.9 mg/kg) were then administered. The reflex threshold was measured every 10 min, starting at 5 min after the sedative injection, until 65 min had elapsed. RESULTS: Levetiracetam did not alter nociceptive reflex threshold in nonsedated animals (P = 0.11) or influence the degree or duration of sedation. The three anesthetic/sedative drugs reduced the nociceptive reflex threshold by 20%-30% of control values. Levetiracetam reduced the hyperreflexia associated with pentobarbital and midazolam (P < 0.05), but not propofol. CONCLUSIONS: These findings support further investigation into the role of levetiracetam in the prevention of anesthetic-induced excitability.

Spinal carbonic anhydrase contributes to nociceptive reflex enhancement by midazolam, pentobarbital, and propofol.

BACKGROUND: Systemic administration of acetazolamide blocks nociceptive hyperreflexia induced by pentobarbital. The authors assessed the effect of intrathecal carbonic anhydrase inhibitors (CAIs) on nociceptive reflex enhancement by pentobarbital, propofol, and midazolam. METHODS: Twenty-seven rats with chronic indwelling subarachnoid catheters were studied. Nociceptive paw reflex latency (PWL) for paw withdrawal from radiant heat was measured in forelimbs and hind limbs. Measurements were obtained under control conditions, 15 min after lumbar intrathecal injection of 10 microl artificial cerebrospinal fluid containing the CAIs acetazolamide or ethoxyzolamide, and during the 55 min after intraperitoneal injection of three sedative drugs: 30 mg/kg pentobarbital, 50 mg/kg propofol, or 1.9 mg/kg midazolam. RESULTS: Control values of PWL averaged 10.9 +/- 1.5 s in the forelimbs and 11.1 +/- 1.6 s in the hind limbs (P = 0.18). Intrathecal injection of 50 microm ethoxyzolamide reduced PWL by 8% and 4% in the forelimbs and hind limbs, respectively (P = 0.01); all other CAI injections had no effect on PWL. Following anesthetic injection, PWL in the forelimbs was reduced by approximately 35-40% of control values; in the hind limbs, CAI treatment decreased the PWL reduction to 8-16% for pentobarbital (P < 0.001), 30-32% for propofol (P < 0.02), and 9-16% for midazolam (P < 0.001). The hind limb reduction of hyperreflexia by CAI was less for propofol than for midazolam or pentobarbital (P < 0.002). CONCLUSION: Spinal carbonic anhydrase contributes to nociceptive hyperreflexia induced by pentobarbital and midazolam and to a lesser extent with propofol. These findings are consistent with a role for carbonic anhydrase in nociceptive signal enhancement by these drugs.