How does neurokinin 3 receptor agonism affect pathological and cognitive impairments in an Alzheimer's disease-like rat model?

Alzheimer's disease (AD) is accepted as a form of progressive dementia. Cholinergic systems are commonly affected in AD. Neurokinin 3 receptor (NK3R) is involved in learning memory-related processes. It is known that the activation of NK3R affects the release of many neurotransmitters. The aim of this project was to investigate the effects of NK3R agonist senktide administration on neurobehavioral mechanisms in the experimental AD-like rat model. 50 male Wistar albino rats were divided into Control (C), AD, Control + NK3R agonist (CS), AD + NK3R agonist (ADS), AD + NK3Ragonist + antagonist groups (ADSO). We designed AD-like model by intrahippocampal administration of Aβ1-42. After NK3R agonist + antagonist injections, open field (OF), Morris water maze (MWM) tests were applied. Cholinergic mechanism analysis from hippocampus-cortex tissues was performed by ELISA and catecholamine analysis from brain stem tissue were performed by HPLC method. The transitions from edge to center, rearing, grooming parameters were found to be reduced in final values of OF. While the group-time interaction was significant in the OF test findings, there was no significant difference between the groups. In MWM test, ADS group showed a learning level close to control group and animals in AD and ADSO groups could not learn target quadrant in MWM test. The brain stem NA and DA concentrations were not statistically significant. Hippocampal AChE-ChAT levels were supported by positive effects of senktide on learning via the cholinergic mechanisms. As a result, NK3R agonists were found to be effective in improving cognitive functions in rats with AD pathology. In the experimental AD model, positive effects of NK3R on learning memory may be mediated by cholinergic mechanisms.

Neurokinin receptor mechanisms in forebrain medial septum modulate nociception in the formalin model of inflammatory pain

The present study has explored the hypothesis that neurokinin1 receptors (NK1Rs) in medial septum (MS) modulate nociception evoked on hind paw injection of formalin. Indeed, the NK1Rs in MS are localized on cholinergic neurons which have been implicated in nociception. In anaesthetized rat, microinjection of L-733,060, an antagonist at NK1Rs, into MS antagonized the suppression of CA1 population spike (PS) evoked on peripheral injection of formalin or on intraseptal microinjection of substance P (SP), an agonist at NK1Rs. The CA1 PS reflects the synaptic excitability of pyramidal cells in the region. Furthermore, microinjection of L-733,060 into MS, but not LS, attenuated formalin-induced theta activation in both anaesthetized and awake rat, where theta reflects an oscillatory information processing by hippocampal neurons. The effects of L-733,060 on microinjection into MS were nociceptive selective as the antagonist did not block septo-hippocampal response to direct MS stimulation by the cholinergic receptor agonist, carbachol, in anaesthetized animal or on exploration in awake animal. Interestingly, microinjection of L-733,060 into both MS and LS attenuated formalin-induced nociceptive flinches. Collectively, the foregoing novel findings highlight that transmission at NK1R provide an affective valence to septo-hippocampal information processing and that peptidergic transmission in the septum modulates nociceptive behaviours.

Neuroanatomical relationship between the cholinergic and tachykininergic systems in the adult human brainstem: An immunohistochemical study

The cholinergic system plays an important role in brain homeostasis and interacts with the neuropeptidergic systems, and the functional relationships between both systems are well known. However, in the brainstem the possible physiological interactions between neurokinins and acetylcholine are unknown, although both substances have been detected in the same brainstem nuclei and have been implicated in similar functions controlled from brainstem regions such as some cranial motor nuclei. The aim of this work is to determine whether these possible physiological interactions might have a neuroanatomical basis by means of the double immunohistochemical detection of neurokinins (NK) and the enzyme choline acetyl-transferase (ChAT) in the human brainstem. No double-labelled structures were detected, although both NK and ChAT were observed in cell bodies and fibers of the same brainstem nuclei. The distribution of immunoreactive fibers is widespread, and NK and ChAT were observed in several motor cranial nerves as well as in the substantia nigra. The results obtained in the present work provide a neuroanatomical basis for possible physiological interactions between NK and ChAT that may be carried out by volume-transmission mechanisms. These interactions might participate in motor regulation or in limbic pathways as well as influence on other neurotransmitter systems such as the dopaminergic system.

Neuropeptides in learning and memory

Dementia conditions and memory deficits of different origins (vascular, metabolic and primary neurodegenerative such as Alzheimer's and Parkinson's diseases) are getting more common and greater clinical problems recently in the aging population. Since the presently available cognitive enhancers have very limited therapeutical applications, there is an emerging need to elucidate the complex pathophysiological mechanisms, identify key mediators and novel targets for future drug development. Neuropeptides are widely distributed in brain regions responsible for learning and memory processes with special emphasis on the hippocampus, amygdala and the basal forebrain. They form networks with each other, and also have complex interactions with the cholinergic, glutamatergic, dopaminergic and GABA-ergic pathways. This review summarizes the extensive experimental data in the well-established rat and mouse models, as well as the few clinical results regarding the expression and the roles of the tachykinin system, somatostatin and the closely related cortistatin, vasoactive intestinal polypeptide (VIP) and pituitary adenylate-cyclase activating polypeptide (PACAP), calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), opioid peptides and galanin. Furthermore, the main receptorial targets, mechanisms and interactions are described in order to highlight the possible therapeutical potentials. Agents not only symptomatically improving the functional impairments, but also inhibiting the progression of the neurodegenerative processes would be breakthroughs in this area. The most promising mechanisms determined at the level of exploratory investigations in animal models of cognitive disfunctions are somatostatin sst4, NPY Y2, PACAP-VIP VPAC1, tachykinin NK3 and galanin GALR2 receptor agonisms, as well as delta opioid receptor antagonism. Potent and selective non-peptide ligands with good CNS penetration are needed for further characterization of these molecular pathways to complete the preclinical studies and decide if any of the above described targets could be appropriate for clinical investigations.

Neurokinin3-R agonism in aged rats has anxiolytic-, antidepressant-, and promnestic-like effects and stimulates ACh release in frontal cortex, amygdala and hippocampus

Neurokinin-3 receptors (NK(3)-R) are localized in brain regions which have been implicated in processes governing learning and memory as well as emotionality. The effects of acute subcutaneous (s.c.) senktide (0.2 and 0.4 mg/kg), a NK(3)-R agonist, were tested in aged (23-25 month old) Wistar rats: (a) in an episodic-like memory test, using an object discrimination task (this is the first study to test for deficits in episodic-like memory in aged rats, since appropriate tests have only recently became available); (b) on parameters of anxiety in an open field test, (c) on indices of depression in the forced swimming test and (d) on the activity of cholinergic neurons of the basal forebrain, using in vivo microdialysis and HPLC. Neither the saline-, nor senktide-treated aged animals, exhibited episodic-like memory. However, the senktide-, but not the vehicle-treated group, exhibited object memory for spatial displacement, a component of episodic memory. Senktide injection also had anxiolytic- and antidepressant-like effects. Furthermore, the active doses of senktide on behavior increased ACh levels in the frontal cortex, amygdala and hippocampus, suggesting a relationship between its cholinergic and behavioral actions. The results indicate cholinergic modulation by the NK(3)-R in conjunction with a role in the processing of memory and emotional responses in the aged rat.