Galanin peptide levels in hippocampus and cortex of galanin-overexpressing transgenic mice evaluated for cognitive performance

Galanin-overexpressing transgenic mice (GAL-tg) generated on a dopamine beta-hydroxylase promoter were previously shown to express high levels of galanin mRNA in the locus coeruleus, and to perform poorly on challenging cognitive tasks. The present study employed radioimmunoassay to quantitate the level of galanin peptide overexpression in two brain regions relevant to learning and memory, the hippocampus and cerebral cortex. Approximately 4-fold higher levels of galanin were detected in the hippocampus of GAL-tg as compared to WT. Approximately 10-fold higher levels of galanin were detected in the frontal cortex of GAL-tg as compared to WT. A second cohort of GAL-tg and WT again showed high levels of galanin overexpression in GAL-tg as compared to WT in both brain regions. Correlation analyses were conducted between galanin peptide concentrations and behavioral scores on four learning and memory tasks: the Morris water maze, social transmission of food preference, standard delay fear conditioning, and trace fear conditioning. While some significant correlations were detected, neither hippocampal nor cortical galanin levels in the two cohorts of GAL-tg consistently correlated with performance across these diverse cognitive tasks. Several interpretations of these findings are discussed, including the possibility that a threshold level of galanin overexpression is sufficient to impair performance on learning and memory tasks in mice.

Lack of effect of moderate Purkinje cell loss on working memory

192 immunoglobulin G-saporin (192-sap) is an immunotoxin which targets the cholinergic basal forebrain after injection into either the ventricular system or the parenchyma of the rat brain. When injected by the i.c.v. route, 192-sap kills some cerebellar Purkinje cells in addition to its more extensive killing of the cholinergic basal forebrain. Behaviorally, i.c.v. injections of 192-sap result in impaired performance in a variety of experimental paradigms of learning and memory including a working memory task in the radial maze. The current study examined the contribution, if any, of immunotoxin-induced Purkinje cell loss to impaired performance in the radial maze. To meet this aim, we used i.c.v. injection of another immunotoxin, OX7-saporin (OX7-sap), at a dose that produced Purkinje cell loss of similar extent to that produced by i.c.v. 192-sap. We then compared these OX7-sap-injected rats with 192-sap-injected rats in a radial maze working memory task. We found a working memory impairment only in the 192-sap-injected rats. These data show that moderate Purkinje cell loss alone is insufficient to impair working memory. Furthermore, the data are consistent with the idea that the working memory deficit observed in 192-sap-injected animals is likely due to lesioning of the cholinergic basal forebrain.

Behavioral profiles of inbred strains on novel olfactory, spatial and emotional tests for reference memory in mice

Studying the behavior of genetic background strains provides important information for the design and interpretation of cognitive phenotypes in mutant mice. Our experiments examined the performance of three commonly used strains (C57BL/6J, 129S6, DBA/2J) on three behavioral tests for learning and memory that measure very different forms of memory, and for which there is a lack of data on strain differences. In the social transmission of food preference test (STFP) all three strains demonstrated intact memory for an odor-cued food that had been sampled on the breath of a cagemate 24 hours previously. While C57BL/6J and 129S6 mice showed good trace fear conditioning, DBA/2J mice showed a profound deficit on trace fear conditioning. In the Barnes maze test for spatial memory, the 129S6 strain showed poor probe trial performance, relative to C57BL/6J mice. Comparison of strains for open field exploratory activity and anxiety-like behavior suggests that poor Barnes maze performance reflects low exploratory behavior, rather than a true spatial memory deficit, in 129S6 mice. This interpretation is supported by good Morris water maze performance in 129S6 mice. These data support the use of a C57BL/6J background for studying memory deficits in mutant mice using any of these tasks, and the use of a 129S6 background in all but the Barnes maze. A DBA/2J background may be particularly useful for investigating the genetic basis of emotional memory using fear conditioning.

Galanin transgenic mice display cognitive and neurochemical deficits characteristic of Alzheimer's disease

Galanin is a neuropeptide with multiple inhibitory actions on neurotransmission and memory. In Alzheimer's disease (AD), increased galanin-containing fibers hyperinnervate cholinergic neurons within the basal forebrain in association with a decline in cognition. We generated transgenic mice (GAL-tg) that overexpress galanin under the control of the dopamine beta-hydroxylase promoter to study the neurochemical and behavioral sequelae of a mouse model of galanin overexpression in AD. Overexpression of galanin was associated with a reduction in the number of identifiable neurons producing acetylcholine in the horizontal limb of the diagonal band. Behavioral phenotyping indicated that GAL-tgs displayed normal general health and sensory and motor abilities; however, GAL-tg mice showed selective performance deficits on the Morris spatial navigational task and the social transmission of food preference olfactory memory test. These results suggest that elevated expression of galanin contributes to the neurochemical and cognitive impairments characteristic of AD.

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.

Threshold relationship between lesion extent of the cholinergic basal forebrain in the rat and working memory impairment in the radial maze

The cholinergic basal forebrain (CBF) degenerates in Alzheimer's Disease (AD), and the degree of this degeneration correlates with the degree of dementia. In the present study we have modeled this degeneration in the rat by injecting various doses of the highly selective immunotoxin 192 IgG-saporin (192-sap) into the ventricular system. The ability of 192-sap-treated rats to perform in a previously learned radial maze working memory task was then tested. We report here that 192-sap created lesions of the CBF and, to a lesser extent, cerebellar Purkinje cells in a dose-dependent fashion. Furthermore, we found that rats harboring lesions of the entire CBF greater than 75% had impaired spatial working memory in the radial maze. Correlational analysis of working memory impairment and lesion extent of the component parts of the CBF revealed that high-grade lesions of the hippocampal-projecting neurons of the CBF were not sufficient to impair working memory. Only rats with high-grade lesions of the hippocampal and cortical projecting neurons of the CBF had impaired working memory. These data are consistent with other 192-sap reports that found behavioral deficits only with high-grade CBF lesions and indicate that the relationship between CBF lesion extent and working memory impairment is a threshold relationship in which a high degree of neuronal loss can be tolerated without detectable consequences. Additionally, the data suggest that the CBF modulates spatial working memory via its connections to both the hippocampus and cortex.

The behavioral functions of the cholinergic basal forebrain: lessons from 192 IgG-saporin

Until recently our understanding of the functional neuroanatomy of the cholinergic basal forebrain (CBF) has been hindered by the lack of a lesioning technique that is truly selective. The development of the immunotoxin 192 IgG-saporin (192-sap) has greatly improved our ability to create specific lesions of the CBF. Rats with such lesions have been studied in a wide variety of behavioral paradigms of learning, memory, and attention. Complete or near-complete destruction of the CBF results in deficits in a variety of behavior paradigms including passive avoidance, spatial tasks (water and radial mazes), delayed matching to position/sample, and attentional tasks. However, interpretation of many experiments is hampered by incomplete lesions and/or concomitant damage to cerebellar Purkinje neurons. Future studies will need to address these issues. Recent development of a similar immunotoxin that is effective in primates should permit more sophisticated behavioral analysis of CBF function. Additionally, immunotoxins selective for other types of neurons, such as the noradrenergic selective anti-DBH-saporin, will permit analysis of the behavioral functions of other diffusely projecting systems and how these other systems may interact with the CBF.

Selective lesion of the cholinergic basal forebrain causes a loss of cortical neuropeptide Y and somatostatin neurons

Degeneration of the cholinergic basal forebrain (CBF) and changes in cortical neuropeptide levels have been reported in Alzheimer's disease. In the present study, we sought to determine if a selective cholinergic lesion of nucleus basalis magnocellularis (Nbm) could affect the number and distribution of neuropeptide Y (NPY) and somatostatin (SS) immunoreactive neurons in the frontoparietal and occipital cortices of rats. Brain sections were evaluated at survival times of 1, 2, 4, 8, 12, 24, 48, 78 and 100 weeks after intraventricular injection of 192-saporin, an immunotoxin directed at the low affinity neurotrophin receptor (p75NGFr), that selectively destroys the CBF. Following the immunotoxin lesion of the Nbm, the number of NPY-labeled neurons decreased 33% in the frontoparietal cortex and 60% in the occipital cortex compared to age-matched normal controls at most survival time points. A significant loss of SS-labeled neurons in both cortical regions was seen 12 weeks after 192-saporin injection with no further change up to 100-week survival time. The effect of age on neuropeptidergic populations was evaluated in normal control rats. The number of NPY and SS immunoreactive neurons in aged rats (21-26 months) decreased by 42% in the frontoparietal cortex and 27% in the occipital cortex when compared with young (3-6 months) and middle-age (9-14 months) rats. When both non-lesioned and lesioned animals with different ages were pooled for linear regression, a significant correlation was found between the number of cortical NPY- and SS-labeled neurons and cortical acetylcholinesterase (AChE) histochemical staining intensity. These findings indicate that: (1) cholinergic denervation of the Nbm is associated with an irreversible loss of neocortical NPY and SS immunoreactive neurons analogous to that observed in Alzheimer's disease and aging; (2) the degree of the loss of cortical NPY and SS immunoreactive neurons seems to be related to the extent of the reduction of cortical AChE intensity in both toxin-injected and normal aged rats. These findings may reflect a trophic dependence of NPY and SS neurons on cortical cholinergic input.

Central noradrenergic lesioning using anti-DBH-saporin: anatomical findings

The ability to create lesions of discrete neuronal populations is an important strategy for clarifying the function of these populations. The power of this approach is critically dependent upon the selectivity of the experimental lesioning technique. Anti-neuronal immunotoxins offer an efficient way to produce highly specific neural lesions. Two previous immunotoxins have been shown to be effective in both the CNS and PNS. They are OX7-saporin, which is targeted at Thy1, and 192-saporin, which is targeted at the low affinity neurotrophin receptor, p75NTR. In the present study, we sought to determine if an immunotoxin targeted at the neurotransmitter synthesizing enzyme, dopamine beta-hydroxylase (DBH), could selectively destroy central noradrenergic neurons after intraventricular administration. This immunotoxin, which consists of a monoclonal antibody to DBH coupled by a disulfide bond to saporin (a ribosome inactivating protein), has been shown to be selectively toxic to peripheral noradrenergic sympathetic neurons in rats after systemic injection. In the present study, immunohistochemical and Cresyl violet staining showed that the noradrenergic neurons of the locus coeruleus are destroyed bilaterally after intraventricular (i.c.v.) injection of 5, 10, and 20 micrograms of anti-DBH-saporin (alpha-DBH-sap) into rats. Complete bilateral lesioning of the A5 and A7 cell groups occurred at the two higher doses. Lesions of the A1/C1 and A2/C2/C3 cell groups were incomplete at all three doses. Dopaminergic neurons of the substantia nigra and ventral tegmental area and serotonergic neurons of the raphé, all monoaminergic neurons that do not express DBH, survived all alpha-DBH-sap doses. The cholinergic neurons of the basal forebrain, which are selectively killed by i.c.v. injection of 192-saporin, and cerebellar Purkinje cells which are killed by OX7-saporin, were not killed by alpha-DBH-sap. These results show that alpha-DBH-sap efficiently and selectively destroys CNS noradrenergic neurons after i.c.v. injection. The preferential destruction of locus coeruleus, A5, and A7 over A1/C1 and A2/C2/C3 may be due to more efficient access of the immunotoxin to these neurons and their terminals after i.c.v. injection.

Loss of nucleus basalis magnocellularis, but not septal, cholinergic neurons correlates with passive avoidance impairment in rats treated with 192-saporin

Intraventricular injection of 192-saporin, an immunotoxin directed at the low affinity neurotrophin receptor (p75NGFr), selectively destroys cholinergic neurons in the basal forebrain (CBF). In the present study, we sought to determine if there was a correlation between degree of CBF neuron destruction and degree of passive avoidance behavioral impairment. 192-saporin caused a decrease in the number of p75NGFr + neurons in both nucleus basalis magnocellularis (Nbm) and medial septal nucleus/diagonal band of Broca (MS/DBB). All rats had >95% loss of the p75NGFr + cholinergic neurons in the MS/DBB, but there was variation in the extent of the Nbm cell loss. A significant correlation was found between the severity of impairment of passive avoidance learning and the magnitude of the loss in the number of p75NGFr + neurons in the Nbm. Step-through latency also correlated significantly with the magnitude of loss of AChE histochemical staining intensity in dorsolateral neocortex ipsilateral to the injection of 192-saporin. These data show that >95% loss of cholinergic neurons in MS/DBB is not sufficient to impair passive avoidance learning. However, in the presence of severe loss of cholinergic neurons from the MS/DBB, the resulting deficit in passive avoidance behavior is proportional to the degree of cholinergic neuron loss from the Nbm. These results are interpreted as support for the hypothesis that the cholinergic projection from Nbm to neocortex plays a role in passive avoidance behavior.