Effects of nucleus basalis lesions on cerebral cortical concentrations of corticotropin-releasing hormone (CRH)-like immunoreactivity in the rat

Long-range inputome of cortical neurons containing corticotropin-releasing hormone

Dissection of the neural circuits of the cerebral cortex is essential for studying mechanisms underlying brain function. Herein, combining a retrograde rabies tracing system with fluorescent micro-optical sectional tomography, we investigated long-range input neurons of corticotropin-releasing hormone containing neurons in the six main cortical areas, including the prefrontal, somatosensory, motor, auditory, and visual cortices. The whole brain distribution of input neurons showed similar patterns to input neurons distributed mainly in the adjacent cortical areas, thalamus, and basal forebrain. Reconstruction of continuous three-dimensional datasets showed the anterior and middle thalamus projected mainly to the rostral cortex whereas the posterior and lateral projected to the caudal cortex. In the basal forebrain, immunohistochemical staining showed these cortical areas received afferent information from cholinergic neurons in the substantia innominata and lateral globus pallidus, whereas cholinergic neurons in the diagonal band nucleus projected strongly to the prefrontal and visual cortex. Additionally, dense neurons in the zona incerta and ventral hippocampus were found to project to the prefrontal cortex. These results showed general patterns of cortical input circuits and unique connection patterns of each individual area, allowing for valuable comparisons among the organisation of different cortical areas and new insight into cortical functions.

The effects of corticotropin-releasing factor on the cortical EEG are reduced following adolescent nicotine exposure

Although smoking is highly prevalent among adolescents, relatively little is known about the lasting neurobehavioral consequences associated with adolescent nicotine exposure. Prior studies from our laboratory suggest that adolescent nicotine exposure induces an anxiogenic profile in adult rats. Corticotropin-releasing factor (CRF) systems are important modulators of anxiety and response to stress. Since acute nicotine administration has been shown to stimulate central CRF activity, the purpose of this study was to examine the effects of adolescent nicotine exposure on CRFs modulation of the cortical and hippocampal EEG in adult rats. Male Sprague-Dawley rats were exposed to nicotine (5 mg/kg/day) between postnatal days 35-40 using transdermal nicotine patches. Six weeks after nicotine exposure ended, the effects of intracerebroventricular administration of CRF (0.01-1.0 microg/5 microl) on EEG activity in the cortex and hippocampus were assessed in nicotine-exposed rats and age-matched control rats. The overall effects CRF were consistent with previous reports. CRF decreased low to moderate frequency EEG activity (1-32 Hz) and increased high frequency EEG activity (32-50 Hz). However, in nicotine-exposed rats the effects of CRF on the frontal and parietal cortical EEG were blunted by 30-50% compared to control rats. A similar pattern of decreased response to CRF was not observed in the hippocampus. These blunted effects of CRF on the cortical EEG suggest that long-term changes in systems responsive to CRF result from adolescent nicotine exposure. Given the role of CRF systems in behavioral responses to stress and anxiety, these data suggest that adolescent nicotine exposure may produce long-term decreases in neurophysiological responses to stress.

Interaction between the cholinergic system and CRH in the modulation of spatial discrimination learning in mice

Both cholinergic and CRH systems have been linked to cognitive processes such as learning and memory, and neuroanatomical as well as neurochemical evidence suggests important interactions between these two systems. Moreover, recent reports of pro-mnestic effects of CRH open the possibility that CRH could have beneficial effects in animals with cholinergic dysfunction. In a first experiment, spatial discrimination of C57BL/6 mice treated with various doses of scopolamine (0.5--2.0 mg/kg IP) was tested in a two-choice water maze task. Scopolamine, but not methylscopolamine, impaired accuracy and decreased responsivity. In contrast, similar doses of the nicotinic antagonist mecamylamine had no effect on choice accuracy but altered responsivity, as indicated by increased errors of omission and a reduction in swim speed during early experimental stages. ICV CRH (0.5--1.0 microg) also failed to significantly affect accuracy, but a strong tendency was observed to impair percentage correct responses. Measures of responsivity, such as errors of omission, choice latency and distance traveled, and of thigmotaxis were not significantly affected by CRH. However, initial swim speed was reduced by the peptide. Combined treatment with scopolamine (0.5 mg/kg IP) and CRH (0.5 microg ICV) had only mild, and primarily independent, effects, but overall suggested that concomitant blockade of muscarinic receptors and activation of the CRH system would rather act synergistically to disrupt spatial discrimination learning. Synergistic effects were also observed when animals receiving a combination of mecamylamine (2.0 mg/kg IP) and CRH (0.5 microg ICV) were tested, both in terms of responsivity and thigmotaxis, and there was limited evidence that part of these effects were potentiating. Thus, the cholinergic and CRH systems interact in the modulation of learning, but CRH, contrary to prediction, worsens the impairment caused by cholinergic blockade.

Neuroprotective effects of 2,4-dimethoxybenzylidene anabaseine (DMXB) and tetrahydroaminoacridine (THA) in neocortices of nucleus basalis lesioned rats

The nicotinic alpha7 agonist dimethoxybenzilidene anabaseine (DMXB) and cholinesterase inhibitor tetrahydroaminoacridine (THA) were investigated in a trans-synaptic model for neocortical atrophy and degeneration following nucleus basalis lesions. Bilateral lesions reduced parietal neuronal density in layers II-V 8 months later. DMXB administered i.p. daily to rats for 3 months attenuated this loss in layers II-V at a 1 mg/kg i.p. dose. A lower, 0.2 mg/kg i.p. dose, was neuroprotective in layer IV only. THA (1 mg/kg i.p.) also protected against neocortical Nissl-staining deficits.

Effects of chronic nicotine and pilocarpine administration on neocortical neuronal density and [3H]GABA uptake in nucleus basalis lesioned rats

We investigated the possible long-term neuroprotective roles of (-)nicotine and muscarinic agonist, pilocarpine, in the neocortices of rats receiving bilateral nucleus basalis lesions. Ibotenic acid-lesioned animals eventually displayed a 15-20% reduction in the density of neocortical Nissl staining neurons in layers II, III and VI, as well as a 27% loss in high-affinity GABA uptake 8 months post-lesioning. Deficits were not observed at earlier intervals. (-)Nicotine (0.2 mg/kg, i.p.) or (-)nicotine plus pilocarpine (1 mg/kg, i.p.) attenuated these losses when administered once daily to rats from 5-8 months post-lesioning. Pilocarpine alone had no protective effect on neuronal density or GABA uptake. These results suggest that nicotine receptor activation may counteract neocortical neuronal loss/atrophy following loss of ascending basal forebrain neurons.

Effect of various neurotransmitters and neuropeptides on the release of corticotropin-releasing hormone from the rat cortex in vitro

Corticotropin-releasing hormone (CRH), in addition to its neuroendocrine role, may act as a central neurotransmitter. Cerebral cortical CRH may have an important role in behavioral and neurodegenerative disorders. To gain an understanding of factors that may influence cortical CRH, we investigated the effect of several neurotransmitters and neuropeptides on the release of immunoreactive CRH (iCRH) from various cerebral cortical regions [frontal (FC), parietal (PC), temporal (TC), and occipital (OC)] in vitro. The hypothalamic release of iCRH was also evaluated under the same experimental conditions. Basal release of iCRH was approximately 2-fold, and KCl-stimulated iCRH release was approximately 4-fold higher in the hypothalamus than in any of the cortical regions. Cortical iCRH release was stimulated by 10 nM somatostatin (SRIF) in PC and 1 nM neuropeptide Y (NPY) in TC. Cortical iCRH release was inhibited by 1 and 10 nM acetylcholine (ACh), 0.1 microM glutamate, and 10 nM NPY. These effects were confined to the FC and/or PC. Hypothalamic iCRH release was stimulated by 1 and 10 nM ACh, 10 microM GABA, and 1 and 10 nM serotonin but was inhibited by 10 nM SRIF and 1 microM GABA. Growth hormone-releasing hormone did not affect cortical or hypothalamic iCRH release. These results demonstrate that CRH release from the cerebral cortex and the hypothalamus are under different regulatory mechanism(s). Furthermore, they indicate that the release of CRH in various cortical regions may be regulated differentially by the same neurotransmitter.

Somatostatin in Alzheimer's disease and depression

Somatostatin (somatotropin release-inhibiting factor, SRIF) was originally discovered (1) during the purification of growth hormone-releasing factor from rat hypothalamus and was subsequently isolated and characterized (2) in 1972 from ovine hypothalamus. Since its initial characterization, SRIF has been shown to fulfill criteria for a neurotransmitter and to directly modulate neuronal activity as well as acting as an inhibitory factor regulating endocrine and exocrine secretion. Alterations in cerebrospinal fluid (CSF) concentrations of SRIF have been reported in several diseases exhibiting prominent cognitive dysfunction, including Alzheimer's disease (AD), major depression, Huntington's chorea, multiple sclerosis, schizophrenia and Parkinson's disease, while evidence for regional brain tissue concentration deficits in SRIF are more specific for AD. This mini-review will focus on the studies reporting alterations in CSF and postmortem tissue concentrations of SRIF in AD and depression.