Antagonism by glibenclamide of the effect of morphine in hippocampal preparations

Modulation of memory with septal injections of morphine and glucose: effects on extracellular glucose levels in the hippocampus

The concentration of glucose in the extracellular fluid (ECF) of the hippocampus decreases substantially during memory testing on a hippocampus-dependent memory task. Administration of exogenous glucose, which enhances task performance, prevents this decrease, suggesting a relationship between hippocampal glucose availability and memory performance. In the present experiment, spontaneous alternation performance and task-related changes in hippocampal ECF glucose were assessed in rats after intraseptal administration of morphine, which impairs memory on a spontaneous alternation task, and after co-administration of intraseptal glucose, which attenuates that impairment. Consistent with previous findings, spontaneous alternation testing resulted in a decrease in hippocampal ECF glucose levels in control rats. However, rats that received intraseptal morphine prior to testing showed memory impairments and an absence of the task-related decrease in hippocampal ECF glucose levels. Intraseptal co-administration of glucose with morphine attenuated the memory impairment, and ECF glucose levels in the hippocampus decreased in a manner comparable to that seen in control rats. These data suggest that fluctuations in hippocampal ECF glucose levels may be a marker of mnemonic processing and support the view that decreases in extracellular glucose during memory testing reflect increased glucose demand during memory processing.

ATP-sensitive potassium channel blockade enhances spontaneous alternation performance in the rat: a potential mechanism for glucose-mediated memory enhancement

Peripheral and central injections of D-glucose enhance learning and memory in rats, and block memory impairments produced by morphine. The mechanism(s) for these effects is (are) as yet unknown. One mechanism by which glucose might act on memory and other brain functions is by regulating the ATP-sensitive potassium channel. This channel may couple glucose metabolism and neuronal excitability, with channel blockade increasing the likelihood of stimulus-evoked neurotransmitter release. The present experiments explored the effects of intra-septal injections of glucose and the ATP-sensitive potassium channel blocker glibenclamide on spontaneous alternation behavior in the rat. Intra-septal injections of glucose (20 nmol) or glibenclamide (10 nmol), 30 min prior to plus-maze spontaneous alternation performance, significantly enhanced alternation scores compared to rats receiving vehicle injections. Glibenclamide enhanced spontaneous alternation performance in an inverted-U dose-response manner. Individually sub-effective doses of glucose (5 nmol) and glibenclamide (5 nmol) significantly enhanced plus-maze alternation scores when co-injected into the septal area. Glibenclamide (10 nmol), when co-administered with morphine (4 nmol) 30 min prior to Y-maze spontaneous alternation performance, attenuated the performance-impairing effects of morphine alone. The present findings show that intra-septal injections of the direct ATP-sensitive potassium channel blocker glibenclamide, both alone and in conjunction with a sub-effective dose of glucose, enhance spontaneous alternation performance and attenuate the performance-impairing effects of morphine. The similarity of the results obtained with glibenclamide and glucose, together with their similar actions on ATP-sensitive potassium channel function, suggests that glucose may modulate memory-dependent behavior in the rat by regulating the ATP-sensitive potassium channel.

Chronic morphine administration enhances the expression of Kv1.5 and Kv1.6 voltage-gated K+ channels in rat spinal cord

Prolonged opiate administration leads to the development of tolerance and dependence. These phenomena are accompanied by selective regulation of distant cellular proteins and mRNAs, including ionic channels. Acute opiate administration differentially affects voltage-dependent K+ currents. Whereas, opiate activation of K+ channels is well established opioid-induced inhibition of K+ conductance has also been studied. In this study, we focused on the effect of chronic morphine exposure on voltage-dependent Shaker-related Kv1.5 and Kv1.6 K+ channel gene expression and on Kv1.5 protein levels in the rat spinal cord. Several experimental approaches including in-situ hybridization, RNAse protection, reverse transcriptase-polymerase chain reaction (RT-PCR), Western blotting and immunohistochemistry were employed. We found that motor neurons are highly enriched in Kv1.5 and Kv1.6 mRNA and in Kv1.5 channel protein. Moreover, we found significant increases in the amount of mRNA encoding for these two K+ channels and in Kv1.5 channel protein in the spinal cord of morphine-treated rats, compared with controls. For example, quantitative in-situ hybridization, revealed a 2.1 +/- 0.15- and 2.3 +/- 0.5-fold increase in Kv1.5 and Kv1.6 channel mRNA levels, respectively. Similar results were obtained by semiquantitative RT-PCR analyses. Kv1.5 protein level was increased by 1.9-fold in the spinal cord or morphine-treated rats. Our results suggest that Kv1.5 and Kv1.6 Shaker K+ channels play an important role in regulating motor activity that increases in mRNA and protein levels of the spinal cord K+ channels after chronic morphine exposure could be viewed as a cellular adaptation which compensates for a persistent opioid-induced inhibition of K+ channel activity. These alterations may account, in part, for the cellular events leading to opiate tolerance and dependence.