The hippocampal opioid system : role in spatial learning

Abstract

The main aim of the present thesis was to study the role of the endogenous hippocampal opioid system, and in particular, the role of dynorphin B and nociceptin, in spatial learning.

The effects of bilateral infusion (using chronic cannulae) of dynorphin B into the dorsal hippocampus (CA3-region), an area containing a high number of dynorphin-containing neurons, were examined in the Morris swim-maze, a spatial task dependent on hippocampal functioning. In this test, rats are required to locate a hidden platform in a pool filled with water using extra-maze cues. The results show that dynorphin B impaired spatial learning at all doses tested (1, 3.3 and 10 nmol/rat) and reached its maximum effect already at the 1 nmol/rat dose. Retention performance was not affected indicating that dynorphin B mainly affects acquisition but not memory processes. The impairing effect was blocked by the selective [kappa]-antagonist, nor-binaltorphimine, indicative of [kappa]-receptor involvement. No effect on locomotion could be seen, as examined in a separate experiment. Since the septohippocampal pathway has been shown to play an important role in learning and memory, the effect of dynorphin B on basal acetylcholine relase in the dorsal hippocampus was examined using in vivo microdialysis. The effect of dynorphin B on spatial learning appears not to involve basal ACh release, since dynorphin B perfused via a microdialysis probe did not alter basal hippocampal acetylcholine release. The mechanism by which dynorphin B affects spatial learning (acquisition) probably involves either glutamate- or GABA-transmission or both.

The metabolism of dynorphins was examined in tissue homogenates of various brain areas (hippocampus, striatum and substatia nigra) using HPLC and mass spectrometry. It was found that dynorphin B was metabolized into the shorter fragments, Leu-enkephalin and to a smaller extent, Leu-enkephalin-Arg6. The ratio of these formed fragments differed between the tissues tested, which may be of physiological relevance.

Nociceptin, infused into the CA3 region of the dorsal hippocampus at a dose of 10 nmol/rat severly impaired spatial learning and decreased exploratory behaviour without affecting swim speed. Nociceptin also impaired retention performance, indicating an effect on both acquisition and memory processes. Using lower doses of nociceptin, it was shown that the peptide had a biphasic dose-effect curve on learning, impairing at a high dose (3.3 nmol/rat) and facilitatory at lower doses (1 and 0.33 nmol/rat). Both the impairing and the facilitatory effect could be blocked by a selective antagonist, showing the involvement of the ORL1-receptor. Nociceptin had no effect on swim speed or on locomotion at these doses. The effects of nociceptin on spatial learning are apparently not mediated by the cholinergic system, since no effect of nociceptin perfusion on basal acetylcholine release was observed. Rather, it probably involves either glutamate- or GABA-transmission or both. In this regard, dynorphin B and nociceptin appear similar in action. However, low doses of nociceptin had the unique property of facilitating spatial learning.

The degradation of nociceptin was studied after bilateral infusion of nociceptin (10 nmol/rat) into the rat hippocampus in vivo. Mass spectrometric analysis showed that two fragments were formed by stepwise fragmentation, nociceptin (1-13) and nociceptin (1-9). The nociceptin (1-13) fragment was infused into the dorsal hippocampus and shown to have no effect on spatial learning, suggesting a loss of affinity for the ORL1-receptor, in agreement with receptor binding data.

Microinjection of endomorphin-2, a [my]-receptor ligand, into the dorsal hippocampus impaired spatial learning at a high dose (10 nmol/rat) but had no effect at lower doses (3.3 and 1 nmol/rat). No effect on swim speed was observed.

On the other hand, Leu-enkephalin, an endogenous [delta]-receptor ligand, had no effect on spatial learning or swim speed at the doses tested (10 and 3.3 nmol/rat). Taken together, the results suggest an important role for hippocampal opioid systems, and dynorphin and nociceptin in particular, in spatial learning. These observations have potential relevance for Alzheimer's disease and age-related disorders of memory.