Galanin fails to alter both acquisition of a two trial per day water maze task and neurochemical markers of cholinergic or serotonergic neurones in adult rats

Galanin and Cognition

Since the neuropeptide galanin’s discovery in 1983, information has accumulated that implicates it in a wide range of functions, including pain sensation, stress responses, appetite regulation, and learning and memory. This article reviews the evidence for specific functions of galanin in cognitive processes. Consistencies as well as gaps in the literature are organized around basic questions of methodology and theory. This review shows that although regularities are evident in the observed behavioral effects of galanin across several methods for measuring learning and memory, generalization from these findings is tempered with concerns about confounds and a restricted range of testing conditions. Furthermore, it is revealed that many noncognitive behavioral constructs that are relevant for assessing potential roles for galanin in cognition have not been thoroughly examined. The review concludes by laying out how future theory and experimental work can overcome these concerns and confidently define the nature of the association of galanin with particular cognitive constructs.

Effects of neurotensin in amygdaloid spatial learning mechanisms

Neurotensin (NT) acts as a neurotransmitter and/or neuromodulator and plays a role in learning and reward related processes. The central nucleus of amygdala (CeA) participates in the regulation of memory and learning mechanisms. In Morris water maze test, rats were microinjected with NT or neurotensin receptor-1 (NTS1) antagonist SR 48692 (ANT). NT significantly reduced the escape latency. Effect of NT was blocked by ANT pretreatment. Our results show that in the rat CeA NT facilitates spatial learning. We clarified that NTS1s are involved in this action.

Galanin impairs cognitive abilities in rodents: relevance to Alzheimer's disease

The neuropeptide galanin and its receptors are localized in brain pathways that mediate learning and memory. Central microinjection of galanin impairs performance of a variety of cognitive tasks in rats. Transgenic mice overexpressing galanin display deficits in some learning and memory tests. The inhibitory role of galanin in cognitive processes, taken together with the fact that overexpression of galanin occurs in Alzheimer's disease, suggests that galanin antagonists may offer a novel therapeutic approach to treat memory loss in patients suffering from Alzheimer's.

Galanin impairs cognitive abilities in rodents: relevance to Alzheimer's disease

The neuropeptide galanin and its receptors are localized in brain pathways mediating learning and memory. Central microinjection of galanin impairs performance of a variety of cognitive tasks in rats. Transgenic mice overexpressing galanin display deficits in some learning and memory tests. The inhibitory role of galanin in cognitive processes, taken together with the overexpression of galanin in Alzheimer's disease, suggests that galanin antagonists may offer a novel therapeutic approach to treat memory loss in Alzheimer's patients.

Intracerebroventricularly administered galanin does not alter operant reaction time or differentially reinforced high rate schedule operant responding in rats

Galanin (Gal) is a 29/30 amino acid neuroendocrine peptide that impairs learning and memory processes, stimulates feeding, and modulates somatosensory, sex, and stress responses. Anatomical markers for Gal are found throughout the brain, including in the caudate-putamen and substantia nigra motor regions. Many of the behavioral tests that have been used to study the involvement of Gal in complex behavioral processes are motorically demanding, but no research has specifically investigated the involvement of Gal in response initiation or the maintenance of fine motor action. Therefore, the present study examined the effects of intraventricularly administered Gal on two highly sensitive operant tasks designed to detect alteration of these response properties. Response initiation was studied using a light-dark discrimination reaction time task that required a correct response within 2.5s of a spatially and temporally uncertain stimulus onset. The ability to perform high local rates of responding was studied using an operant differential reinforcement of high rate (DRH) of responding task. Gal (10-20 microg, i.c.v.) did not alter reaction time or inter-response time distributions in either task, though did substantially reduce the total number of responses and reinforcers obtained on the DRH schedule. These results are consistent with a Gal-induced reduction of reinforcer efficacy rather than Gal-disruption of response initiation or response patterning.

Pharmacological evidence supporting a role for galanin in cognition and affect

1. Galanin is localized in brain pathways involved in both cognition and affect. 2. Galanin has inhibitory actions on a variety of memory tasks including the Morris water maze, delayed nonmatching to position, T-maze delayed alternation, starburst maze, passive avoidance, active avoidance, and spontaneous alternation. 3. Galanin may inhibit learning and memory by inhibiting neurotransmitter release and neuronal firing rate. 4. Two signal transduction mechanisms through which galanin exerts its inhibitory actions are the inhibition of phosphatidyl inositol hydrolysis and the inhibition of adenylate cyclase. 5. Galanin released during periods of burst firing from noradrenergic locus coeruleus terminals in the ventral tegmental area (VTA) may lead to symptoms of depression through inhibition of dopaminergic VTA neurons. 6. Intraventricular galanin has anxiolytic effects in a punished drinking test. Intra-amygdala galanin has anxiogenic effects in a punished drinking test.

Galanin and spatial learning in the rat. Evidence for a differential role for galanin in subregions of the hippocampal formation

Anatomical, neurochemical and behavioural evidence support a role for galanin in hippocampally mediated functions such as spatial learning and memory. To obtain more precise information on this role, galanin (3 nmol/rat) was infused via bilateral chronic cannulae into different areas of the hippocampal formation which are characterized by different galanin receptor subtypes and also by different galanin innervation patterns. The effects of infused galanin on spatial learning were examined in the Morris swim maze. Infusions of galanin into both the dorsal and ventral dentate gyrus, which mainly contain GAL-R2 receptor mRNA and a high degree of galanin-noradrenaline coexistence, significantly retarded spatial acquisition without affecting swim speed or performance in the visible platform test. This spatial learning deficit was fully blocked by pretreatment with the non-selective galanin antagonist M35. Analysis of retention performance suggested that the major effect of intrahippocampal galanin is mediated via a specific disruption of acquisition mechanisms of importance for performance in the probe trial. Galanin infused into the ventral CA1 (a mainly GAL-R1 receptor mRNA expressing region) or into anterior, ventral CA3 regions did not produce any deficits in spatial learning compared to control animals. These results suggest that galanin mediates its action on spatial learning mainly through the GAL-R2 receptor subtype in areas where most of the galanin is present in noradrenergic terminals. A possible role for the GAL-R1 receptor subtype in cognition in the dorsal and ventral hippocampus remains to be defined. The results suggest a differential functional role for galanin and galanin receptor subtypes within subregions of the hippocampal formation.

Galanin receptor subtypes

The neuropeptide galanin, which is widely expressed in brain and peripheral tissues, exerts a broad range of physiological effects. Pharmacological studies using peptide analogues have led to speculation about multiple galanin receptor subtypes. Since 1994, a total of three G-protein-coupled receptor (GPCR) subtypes for galanin have been cloned (GAL1, gal2 and gal3). Potent, selective antagonists are yet to be found for any of the cloned receptors. Major challenges in this field include linking the receptor clones with each of the known physiological actions of galanin and evaluating the evidence for additional galanin receptor subtypes.

Effect of neuropeptides on cognitive function

Recent evidence indicates that, in addition to the involvement of cholinergic and other neurotransmitter systems, various neuropeptides that occur in cortical and subcortical brain regions have a role in cognitive behavior. This evidence results largely from behavioral studies in rodents and other animals, following peptide administration and only in a very few cases from similar studies in human subjects. Several neuropeptides studied appear to enhance or produce changes conducive to improvement in cognitive performance and these include vasopressin, corticotrophin-releasing hormone (CRH), somatostatin, substance P, neuropeptide Y, and thyrotrophin-releasing hormone (TRH), while one peptide, galanin, has been reported to inhibit cognitive processes. Of those neuropeptides that improve performance, only TRH has been shown recently to attenuate the memory impairment of human subjects and Alzheimer patients treated with an anticholinergic drug, and this review describes a series of complimentary studies in adult and aged rodents that contribute to our understanding of the possible mechanisms involved in the role of TRH in cognition.

Central cholinergic systems and cognition

The organization and possible functions of basal forebrain and pontine cholinergic systems are reviewed. Whereas the basal forebrain cholinergic neuronal projections likely subserve a common electrophysiological function, e.g. to boost signal-to-noise ratios in cortical target areas, this function has different effects on psychological processes dependent upon the neural network operations within these various cortical domains. Evidence is presented that (a) the nucleus basalis-neocortical cholinergic system contributes greatly to visual attentional function, but not to mnemonic processes per se; (b) the septohippocampal projection is involved in the modulation of short-term spatial (working) memory processes, perhaps by prolonging the neural representation of external stimuli within the hippocampus; and (c) the diagonal band-cingulate cortex cholinergic projection impacts on the ability to utilize response rules through conditional discrimination. We also suggest that nucleus basalis-amygdala cholinergic projections have a role in the retention of affective conditioning while brainstem cholinergic projections to the thalamus and midbrain dopamine neurons affect basic arousal processes (e.g. sleep-wake cycle) and behavioral activation, respectively. The possibilities and limitations of therapeutic interventions with procholinergic drugs in patients with Alzheimer's disease and other neurodegenerative disorders in which basal forebrain cholinergic neurons degenerate are also discussed.

Effects of ventral hippocampal galanin on spatial learning and on in vivo acetylcholine release in the rat

The neuropeptide galanin coexists in the medial septum and diagonal band of Broca with a population of acetylcholine neurons which project mainly to the ventral hippocampus. The present studies investigated the role of ventral hippocampal galanin in spatial learning in the male rat using a spatial learning task. In addition, the effects of galanin on cholinergic function were monitored by in vivo microdialysis and high-performance liquid chromatography. Bilateral microinjections of galanin (3 nmol/ rat) via chronic cannulae placed into the ventral hippocampus (i.v.h.) produced a slight but significant impairment of acquisition of the spatial task, while the 1 nmol dose of galanin facilitated acquisition. The 6 nmol dose of galanin failed to affect performance. A trend for an impairment of long-term memory retention (examined seven days after the last training session) was observed after 3 nmol of galanin, while the 1 nmol dose facilitated retention performance. Scopolamine (0.1 mg/kg, s.c.) caused a marked impairment of acquisition. Galanin (3 nmol/rat) given i.v.h. failed to modify the acquisition impairment caused by scopolamine (0.1 mg/kg, s.c.). These results suggest that galanin given i.v.h. produces a biphasic dose-dependent effects on spatial learning. In freely moving rats, galanin (3 nmol/10 microliters) given into the lateral ventricle (i.c.v.) did not affect basal acetylcholine release. In contrast, perfusion (100 min) with galanin (0.1 or 0.3 nmol/1.25 microliters/min) through the ventral hippocampal probe resulted in a reduction of basal acetycholine release which was dose-dependent and reversible. Galanin given i.c.v. (3 nmol/10 microliters) or through the probe (0.3 nmol/1.25 microliters/min) attenuated the increase in acetylcholine release evoked by the muscarinic antagonist scopolamine (0.1 mg/kg, s.c.; 0.001 nmol/1.25 microliters/min through the probe). The galanin plus scopolamine combinations produced a 50% lower increase in the extracellular acetylcholine concentrations than scopolamine alone. This suggests that the mechanism(s) behind scopolamine- and galanin-induced stimulation of acetylcholine differ. These results indicate that ventral hippocampal galanin plays a role in cognition and that it has a powerful and modulatory effect on cholinergic transmission. However, the effects of exogenous galanin on spatial learning cannot be directly related to changes in in vivo cholinergic transmission in the ventral hippocampus. These discrepancies may relate to effects on subtypes of galanin receptors with different functional coupling. In addition, other hippocampal neurotransmitter systems (e.g. noradrenergic neurons) important for cognitive functions may also be modulated by ventral hippocampal galanin.

Improvement by dynorphin A (1-13) of galanin-induced impairment of memory accompanied by blockade of reductions in acetylcholine release in rats

1. Human galanin (0.32 nmol per rat, i.c.v.), an endogenous neuropeptide, administered 30 min before acquisition or retention trials, significantly impaired the acquisition of learning and recall of memory in a step-through type passive avoidance performance. 2. The role of dynorphin A (1-13) in learning and memory is controversial. Dynorphin A (1-13) (0.5 nmol per rat, i.c.v.) administered 5 min before galanin injection, completely antagonized these impairments. 3. Galanin significantly decreased acetylcholine release in the hippocampus 40 to 120 min after injection as determined by in vivo brain microdialysis. This peptide also decreased acetylcholine release, albeit to a lesser extent, from the frontal cortex. 4. Dynorphin A (1-13) (0.5 nmol per rat, i.c.v.) 5 min before galanin injection, completely blocked the decrease in extracellular acetylcholine concentration induced by galanin. 5. These antagonistic effects of dynorphin A (1-13) were abolished by treatment with norbinaltorphimine (5.44 nmol per rat, i.c.v.), a selective kappa-opioid receptor antagonist, 5 min before dynorphin A (1-13). 6. Dynorphin A (1-13) (0.5 nmol) itself had no effect on learning and memory and on the acetylcholine concentration in the hippocampus or the frontal cortex in normal rats. 7. These results suggest that the neuropeptide dynorphin A (1-13) ameliorates the galanin-induced impairment of learning and memory accompanied by abolition of reductions in acetylcholine release via kappa-opioid receptors.

Galanin receptor binding sites in the temporal and occipital cortex are minimally affected in Alzheimer's disease

Galanin receptor binding sites were examined in the inferior temporal and medial occipital gyri of patients with Alzheimer's disease (AD) and matched control subjects using quantitative autoradiography. In the inferior temporal gyrus, galanin binding was reduced selectively in layers V-VI of the AD cases compared to controls, the magnitude of the reduction (45%) being similar to that of choline acetyltransferase activity (40%) in this region. In the medial occipital gyrus, galanin binding in the AD cases was not different from controls in any cortical layer despite a reduction in choline acetyltransferase activity. Galanin binding did not correlate with the densities of neuritic plaques in either temporal or occipital gyri. Thus, despite a significant cortical cholinergic deficit in AD, there is an anatomically selective reduction of cortical galanin receptor binding sites suggesting that the majority of galanin receptors are not located on cholinergic terminals in the human cerebral cortex.