Roland J. Boegman

Differential Effects of Scopolamine on Working and Reference Memory of Rats in the Radial Maze

Anticholinergics have often been found to impair choice accuracy in the radial maze. Some researchers have suggested that this indicates involvement of cholinergically innervated structures in cognitive mapping while others argue that these structures mediate working memory. However, most results are open to either interpretation since the baiting method did not allow a distinction between reference and working memory errors. To further test these hypotheses this study examined the effects of systemic scopolamine on radial maze performance, using a 4-out-of-8 baiting procedure. Food-deprived Wistar rats were pretrained until working memory choice accuracy stabilized to a criterion of 87% or better. Scopolamine (0.1, 0.4 and 0.8 mg/kg, IP, 30 min before a session) significantly increased the number of working memory errors (re-entries into baited arms) whereas reference memory errors (entries into never baited arms) did not change significantly. Observed deficits appeared not to be attributable to a drug-induced disruption of motivational systems. Results confirm the behavioural similarities between the memorial effects of hippocampectomy and anticholinergics, and implicate cholinergically innervated structures in working memory.

Quinolinic acid does not spare striatal neuropeptide Y-immunoreactive neurons

When infused into the striatum of the rat, the excitotoxin quinolinic acid was found to eliminate neuropeptide Y (NPY)-immunoreactive nerve cell bodies and processes within the core of the injection area in a dose-dependent manner. This finding suggests that the NPY immunoreactivity in the striatum is entirely derived from a relatively small population of striatal NPY-producing interneurons. The striatal cholinergic neurons identified by means of the di-isopropylfluorophosphate (DFP)-pharmacohistochemical procedure for acetylcholinesterase were found to be more resistant than NPY-immunoreactive cells to the action of the neurotoxin. Similar results were also obtained following striatal injections of kainic acid. The fact that the striatal NPY-immunoreactive neurons are highly sensitive to quinolinic acid is not consistent with the recent proposal that this excitotoxin can be used as an experimental model of Huntingtons disease where striatal NPY-positive neurons are selectively spared.

Protection against quinolinic acid-mediated excitotoxicity in nigrostriatal dopaminergic neurons by endogenous kynurenic acid

Endogenous excitotoxins have been implicated in the degeneration of dopaminergic neurons in the substantia nigra compacta of patients with Parkinsons disease. One such agent quinolinic acid is an endogenous excitatory amino acid receptor agonist. This study examined whether an increased level of endogenous kynurenic acid, an excitatory amino acid receptor antagonist, can protect nigrostriatal dopamine neurons against quinolinic acid-induced excitotoxic damage. Nigral infusion of quinolinic acid (60 nmoles) or N-methyl-D- aspartate (15 nmoles) produced a significant depletion in striatal tyrosine hydroxylase activity, a biochemical marker for dopaminergic neurons. Three hours following the intraventricular infusion of nicotinylalanine (5.6 nmoles), an agent that inhibits kynureninase and kynurenine hydroxylase activity, when combined with kynurenine (450 mg/kg i.p.), the precursor of kynurenic acid, and probenecid (200 mg/kg i.p.), an inhibitor of organic acid transport, the kynurenic acid in the whole brain and substantia nigra was increased 3.3-fold and 1.5-fold respectively when compared to rats that received saline, probenecid and kynurenine. This elevation in endogenous kynurenic acid prevented the quinolinic acid-induced reduction in striatal tyrosine hydroxylase. However, 9 h following the administration of nicotinylalanine with kynurenine and probenecid, a time when whole brain kynurenic acid levels had decreased 12-fold, quinolinic acid injections produced a significant depletion in striatal tyrosine hydroxylase. Intranigral infusion of quinolinic acid in rats that received saline with kynurenine and probenecid resulted in a significant depletion of ipsilateral striatal tyrosine hydroxylase. Administration of nicotinylalanine in combination with kynurenine and probenecid also blocked N-methyl-D-aspartate-induced depletion of tyrosine hydroxylase. Tyrosine hydroxylase immunohistochemical assessment of the substantia nigra confirmed quinolinic acid-induced neuronal cell loss and the ability of nicotinylalanine in combination with kynurenine and probenecid to protect neurons from quinolinic acid-induced toxicity. The present study demonstrates that increases in endogenous kynurenic acid can prevent the loss of nigrostriatal dopaminergic neurons resulting from a focal infusion of quinolinic acid or N-methyl-D-aspartate. The strategy of neuronal protection by increasing the brain kynurenic acid may be useful in retarding cell loss in Parkinsons disease and other neurodegenerative diseases where excitotoxic mechanisms have been implicated.

Effects of scopolamine and unilateral lesions of the basal forebrain on T-maze spatial discrimination and alternation in rats

Cholinergic systems are thought to play a role in memory. It has been suggested that cholinergic neurons, possibly the cortically projecting cells of the nucleus basalis magnocellularis, are differentially involved in working and reference memory. To evaluate this hypothesis the effects on memory of scopolamine (0, 0.3, 0.6 mg/kg) or unilateral kainic acid (4.7 nmoles in 1 microliter) lesions of the basal forebrain of rats were tested. Working memory, the recall of recent events of transient importance that is vulnerable to interference, was tested using a T-maze alternation task; reference memory, information stored over the long term that is relatively resistant to interference, was evaluated using a spatial discrimination task in the T-maze. The differential sensitivity of the two tasks to interference effects was confirmed by the finding that the insertion of a 30-sec delay between trials significantly reduced performance in the alternation but not the spatial discrimination task. Furthermore, scopolamine or the lesions significantly impaired alternation but not spatial discrimination performance. Biochemical assays of the kainate-injected brains confirmed that the cortical cholinergic marker, choline acetyltransferase, was significantly reduced. These results support the hypothesis that working and reference memory may be differentially controlled by cholinergic systems.

Modulation of striatal quinolinate neurotoxicity by elevation of endogenous brain kynurenic acid

Nicotinylalanine, an inhibitor of kynurenine metabolism, has been shown to elevate brain levels of endogenous kynurenic acid, an excitatory amino acid receptor antagonist. This study examined the potential of nicotinylalanine to influence excitotoxic damage to striatal NADPH diaphorase (NADPH‐d) and γ‐aminobutyric acid (GABA)ergic neurones that are selectively lost in Huntingtons disease. A unilateral injection of the N‐methyl‐D‐aspartate (NMDA) receptor agonist, quinolinic acid, into the rat striatum produced an 88% depletion of NADPH‐d neurones. Intrastriatal infusion of quinolinic acid also produced a dose‐dependent reduction in striatal GABA content. Nicotinylalanine (2.3, 3.2, 4.6, 6.4 nmol 5 μl−1, i.c.v.) administered with L‐kynurenine (450 mg kg−1), a precursor of kynurenic acid, and probenecid (200 mg kg−1), an inhibitor of organic acid transport, 3 h before the injection of quinolinic acid (15 nmol) produced a dose‐related attenuation of the quinolinic acid‐induced loss of NADPH‐d neurones. Nicotinylalanine (5.6 nmol 5 μl−1) in combination with L‐kynurenine and probenecid also attenuated quinolinic acid‐induced reductions in striatal GABA content. Nicotinylalanine (4.6 nmol, i.c.v.), L‐kynurenine alone or L‐kynurenine administered with probenecid did not attenuate quinolinic acid‐induced depletion of striatal NADPH‐d neurones. However, combined administration of kynurenine and probenecid did prevent quinolinic acid‐induced reductions in ipsilateral striatal GABA content. Injection of nicotinylalanine, at doses (4.6 nmol and 5.6 nmol i.c.v.) which attenuated quinolinic acid‐induced striatal neurotoxicity, when combined with L‐kynurenine and probenecid produced increases in both whole brain and striatal kynurenic acid levels. Administration of L‐kynurenine and probenecid without nicotinylalanine also elevated kynurenic acid, but to a lesser extent. The results of this study demonstrate that nicotinylalanine has the potential to attenuate quinolinic acid‐induced striatal neurotoxicity. It is suggested that nicotinylalanine exerts its effect by increasing levels of endogenous kynurenic acid in the brain. The results of this study suggest that agents which influence levels of endogenous excitatory amino acid antagonists such as kynurenic acid may be useful in preventing excitotoxic damage to neurones in the CNS.

Differential sensitivity of neuropeptide Y, somatostatin and NADPH-diaphorase containing neurons in rat cortex and striatum to quinolinic acid

The tryptophan metabolite quinolinic acid (QUIN) was injected unilaterally into rat cerebral cortex or striatum in order to determine whether the neurotoxin would destroy neuropeptide Y (NPY)- and somatostatin (SS)-immunoreactive, and NADPH-diaphorase (NADPH-D)-containing neurons. Following intrastriatal injections of QUIN, NPY and SS immunoreactivity and NADPH-D-activity was absent in the injection core area. In contrast, cortical NPY- and SS-immunoreactive cells and NADPH-D-containing neurons were resistant to QUINs neurotoxicity. These results suggest that in contrast to striatal neurons, cortical SS- and NPY-containing neurons do not express N-methyl-D-aspartate receptors.

Neurosteroids and reward: allopregnanolone produces a conditioned place aversion in rats.

The neurosteroid 3alpha-hydroxy-5alpha-pregnan-20-one (allopregnanolone) has been reported to have rewarding properties in mice tested for place conditioning. Another study found that allopregnanolone reduced dopamine (DA) output in the nucleus accumbens (NAc) of rats. As many rewarding stimuli increase accumbens DA, these results may appear contradictory. Thus, the present study examined the rewarding properties of allopregnanolone in rats tested for place conditioning using an unbiased conditioning procedure. In control studies, a place preference was observed following conditioning with intraperitoneal (2.0 mg/kg) or intracerebroventricular (i.c.v.) (100 microg/0.5 microl) amphetamine. Conditioning with i.c.v. allopregnanolone produced a significant aversion at a dose of 5.0 microg (in 5.0 microl) and a near aversion at 25.0 microg (in 8.3 microl); doses of 0 microg (i.e., vehicle alone, in 10 microl) or 30.0 microg (in 10 microl) produced little effect on place preference. During conditioning, locomotor activity was stimulated by amphetamine using either route of administration, but allopregnanolone had no significant main effect on locomotor activity. Thus, there was a dissociation between the effects of drugs on locomotor activity vs. place conditioning. Results show that i.c.v. amphetamine produces a place preference, whereas allopregnanolone produces either no effect or an aversion, depending on the dose.

Differential action of 7-nitro indazole on rat brain nitric oxide synthase

We examined the dose-response characteristics of brain nitric oxide synthase (NOS) inhibition following intraperitoneal administration of 7-nitro indazole (7-NI). 7-NI inhibited striatal, hippocampal, cortical, cerebellar and nigral NOS activity in a dose-dependent manner. NOS activity in the striatum and hippocampus could not be inhibited more than 60% while cerebellar and nigral activity was depleted by at least 85%, indicating that 7-NI has differential effects in different brain regions. ED50 values obtained from the 7-NI dose-response curves of the striatum and hippocampus were significantly higher than the ED50 values obtained from the cortex, cerebellum and substantia nigra, further confirming the differential actions of 7-NI. In addition, inhibition of NOS activity 4.5 h following a maximal dose of 7-NI demonstrated differential recovery. At this time point, the cerebellum and hippocampus were more inhibited than the striatum, cortex and substantia nigra. Therefore, the extent of recovery from this inhibition was independent of the level of maximal NOS inhibition in the different brain regions. We suggest determining the extent and duration of NOS inhibition resulting from 7-NI administration prior to using it to study the role of neuronal nitric oxide (NO) in various systems.

Quinolinate-induced cortical cholinergic damage: modulation by tryptophan metabolites

Certain products of tryptophan metabolism interact with excitatory amino acid receptors to produce or protect against excitotoxicity. In this study, the action of several tryptophan metabolites, yielded by the kynurenine pathway, on cortical cholinergic toxicity was evaluated following focal injection into the rat nucleus basalis magnocellularis (nbM). Metabolites were injected singly or in combination with a fixed dose of quinolinic acid (QUIN). Cholinergic toxicity, or protection against it, was evaluated by measurements of choline acetyltransferase (ChAT) activity or potassium-evoked release of [3H]acetylcholine [( 3H]ACh) from slices of the frontoparietal cortex, from the injected and uninjected sides. Focal injections of QUIN and 3-hydroxyanthranilic, but not kynurenic, picolinic, quinaldic or anthranilic acid, produced a dose-related decrease in ChAT activity, with QUIN being more potent. Kynurenic, picolinic, quinaldic and anthranilic acid, co-injected into the nbM with QUIN (120 nmol), produced dose-related antagonism of the neurotoxicity associated with QUIN alone. Picolinic acid also prevented the reduction in cortical [3H]ACh release induced by injections of QUIN. Kynurenic and picolinic acid produced a complete blockade of QUINs effect on cortical ChAT activity, while quinaldic and anthranilic acid produced a partial blockade. The order of effectiveness against QUIN was kynurenic greater than picolinic greater than quinalidic or anthranilic acid. Evaluation of thin sections following Cresyl violet staining indicated that injections of QUIN produced neuronal loss and glial proliferation, while co-injections of picolinic or quinaldic acid with QUIN protected neurons. These findings show that several tryptophan metabolites have the potential to either produce or antagonize cholinergic toxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional and neurochemical cortical cholinergic impairment following neurotoxic lesions of the nucleus basalis magnocellularis in the rat

The effect of kainic and quinolinic acid on cortical cholinergic function was examined following injections of these agents into the nucleus basalis magnocellularis (nbm) or into the frontoparietal cortex. The release of cortical 3H-acetylcholine (3H-ACh), high affinity choline uptake (HACU) and acetylcholinesterase was measured 7 days following injections of saline (control), kainic acid (4.7 nmoles) and quinolinic acid (60, 150 and 300 nmoles) into the nbm. These cortical cholinergic parameters were also examined after injections of saline (control), kainic acid (9.4 nmoles) and quinolinic acid (300 nmoles) into the fronto-parietal cortex. The release of 3H-ACh, HACU and AChE was significantly reduced in animals injected with kainic or quinolinic acid into the nbm. Histological examination of stained sections showed a loss of cell bodies in the region of the nbm and the globus pallidus. The size of the lesion produced by quinolinic acid was proportional to the dose injected into the nbm. In animals injected with kainic acid or quinolinic acid into the cerebral cortex, the release of 3H-ACh, HACU and AChE was not significantly reduced when compared with control animals, although histological examination of stained cortical sections showed a marked loss of cortical neurons. The results show that quinolinic acid, an endogenous neuroexcitant, produces a deficit of cholinergic function similar to that described in the cortical tissue of patients with senile dementia of Alzheimers type. The toxic effects of quinolinic acid on cortical cholinergic function are due to its action on cholinergic cell bodies in the nbm.(ABSTRACT TRUNCATED AT 250 WORDS)