The calcium-calmodulin-dependent kinase II (CaMKII) is required for hippocampal long-term potentiation (LTP) and spatial learning. In addition to its calcium-calmodulin (CaM)-dependent activity, CaMKII can undergo autophosphorylation, resulting in CaM-independent activity. A point mutation was introduced into the alphaCaMKII gene that blocked the autophosphorylation of threonine at position 286 (Thr286) of this kinase without affecting its CaM-dependent activity. The mutant mice had no N-methyl-D-aspartate receptor-dependent LTP in the hippocampal CA1 area and showed no spatial learning in the Morris water maze. Thus, the autophosphorylation of alphaCaMKII at Thr286 appears to be required for LTP and learning.

The authors describe how (a) the timing of hippocampal lesions and (b) the behavioral-representational demands of the task affect the requirement for the hippocampus in contextual fear conditioning. Post- but not pretraining lesions of the hippocampus greatly reduced contextual fear conditioning. In contrast, pretraining lesions of the hippocampus abolished context discrimination, a procedure in which mice are trained to discriminate between 2 similar chambers (shock context vs. no-shock context). Whereas either contextual- or cue-based strategies can be used to recognize an aversive context, discrimination between similar contexts is optimally acquired by contextual (hippocampal)-based strategies. In keeping with the lesion results, Nf1(+/-)/Nmdar1(+/-) mutant mice, which have spatial learning deficits, are impaired in context discrimination but not in contextual conditioning. Together, these data dissociate hippocampal and nonhippocampal contributions to contextual conditioning, and they provide direct evidence that the hippocampus plays an essential role in the processing of contextual stimuli.

The interaction between depression and stroke is highly complex. Post-stroke depression (PSD) is among the most frequent neuropsychiatric consequences of stroke. Depression also negatively impacts stroke outcome with increased morbidity, mortality and poorer functional recovery. Antidepressants such as the commonly prescribed selective serotonin reuptake inhibitors improve stroke outcome, an effect that may extend far beyond depression, e.g., to motor recovery. The main biological theory of PSD is the amine hypothesis. Conceivably, ischaemic lesions interrupt the projections ascending from midbrain and brainstem, leading to a decreased bioavailability of the biogenic amines – serotonin (5HT), dopamine (DA) and norepinephrine (NE). Acetylcholine would also be involved. So far, preclinical and translational research on PSD is largely lacking. The implementation and characterization of suitable animal models is clearly a major prerequisite for deeper insights into the biological basis of post-stroke mood disturbances. Equally importantly, experimental models may also pave the way for the discovery of novel therapeutic targets. If we cannot prevent stroke, we shall try to limit its long-term consequences. This review therefore presents animal models of PSD and summarizes potential underlying mechanisms including genomic signatures, neurotransmitter and neurotrophin signalling, hippocampal neurogenesis, cellular plasticity in the ischaemic lesion, secondary degenerative changes, activation of the hypothalamo-pituitary-adrenal (HPA) axis and neuroinflammation. As stroke is a disease of the elderly, great clinical benefit may especially accrue from deciphering and targeting basic mechanisms underlying PSD in aged animals.

Adult neurogenesis (i.e., proliferation and differentiation of neuronal precursors in the adult brain) is responsible for adding new neurons in the dentate gyrus of the hippocampus and in the olfactory bulb. We describe herein that adult mice mutated in the cell cycle regulatory gene Ccnd2, encoding cyclin D2, lack newly born neurons in both of these brain structures. In contrast, genetic ablation of cyclin D1 does not affect adult neurogenesis. Furthermore, we show that cyclin D2 is the only D-type cyclin (out of D1, D2, and D3) expressed in dividing cells derived from neuronal precursors present in the adult hippocampus. In contrast, all three cyclin D mRNAs are present in the cultures derived from 5-day-old hippocampi, when developmental neurogenesis in the dentate gyrus takes place. Thus, our results reveal the existence of molecular mechanisms discriminating adult versus developmental neurogeneses.

The role of adult brain neurogenesis (generating new neurons) in learning and memory appears to be quite firmly established in spite of some criticism and lack of understanding of what the new neurons serve the brain for. Also, the few experiments showing that blocking adult neurogenesis causes learning deficits used irradiation and various drugs known for their side effects and the results obtained vary greatly. We used a novel approach, cyclin D2 knockout mice (D2 KO mice), specifically lacking adult brain neurogenesis to verify its importance in learning and memory. D2 KO mice and their wild-type siblings were tested in several behavioral paradigms, including those in which the role of adult neurogenesis has been postulated. D2 KO mice showed no impairment in sensorimotor tests, with only sensory impairment in an olfaction-dependent task. However, D2 KO mice showed proper procedural learning as well as learning in context (including remote memory), cue, and trace fear conditioning, Morris water maze, novel object recognition test, and in a multifunctional behavioral system-IntelliCages. D2 KO mice also demonstrated correct reversal learning. Our results suggest that adult brain neurogenesis is not obligatory in learning, including the kinds of learning where the role of adult neurogenesis has previously been strongly suggested.New neurons are produced in the brains of adult animals, including humans, throughout their lifespan (Altman 1963;Eriksson et al. 1998). Since one of the sites of adult brain neurogenesis is hippocampal formation, a brain structure involved in learning and memory, new neurons were expected to be involved in these phenomena. Following years of research, adult brain neurogenesis, along with synaptic strengthening, synaptic elimination/ weakening, and synaptogenesis are now regarded as four major types of plasticity required for formation and retention of memories (Bruel-Jungerman et al. 2007). However, the very evidence for the role of adult hippocampal neurogenesis in learning and memory remains limited, and in most cases, indirect (Leuner et al. We showed before (Kowalczyk et al. 2004) that cyclin D2, a protein involved in cell cycle regulation, is the only cyclin D (out of D1, D2, and D3) expressed in wild-type (WT) hippocampal neuronal progenitors expanded in vitro into neurospheres. Furthermore, careful analysis of the brains of cyclin D2 knockout mice (D2 KO mice) failed to reveal BrdU-positive neurons in the dentate gyrus of the hippocampal formation. We were also unsuccessful in increasing the number of newly generated cells in the hippocampus by a variety of stimulations, including introducing the mice to a novel environment, or even by a local brain injury. Notably, while adult brain neurogenesis of D2 KO mice is missing, their developmental neurogenesis allows for the formation of the brain, with all of the major structures present, though some of them are smaller. Also, neurogenesis outside of the central nervous system seems to be unaffected in these mice (Kowalczyk et al. 20...