Muscarinic Modulation of Synaptic Transmission and Short-Term Plasticity in the Dorsal and Ventral Hippocampus.

Muscarinic neurotransmission is fundamentally involved in supporting several brain functions by modulating flow of information in brain neural circuits including the hippocampus which displays a remarkable functional segregation along its longitudinal axis. However, how muscarinic neuromodulation contributes to the functional segregation along the hippocampus remains unclear. In this study we show that the nonselective muscarinic receptor agonist carbachol similarly suppresses basal synaptic transmission in the dorsal and ventral CA1 hippocampal field, in a concentration-depended manner. Furthermore, using a ten-pulse stimulation train of varying frequency we found that carbachol changes the frequency filtering properties more in ventral than dorsal hippocampus by facilitating synaptic inputs at a wide range of input frequencies in the ventral compared with dorsal hippocampus. Using the M2 receptor antagonist gallamine and the M4 receptor antagonist tropicamide, we found that M2 receptors are involved in controlling basal synaptic transmission and short-term synaptic plasticity (STSP) in the ventral but not the dorsal hippocampus, while M4 receptors participate in modulating basal synaptic transmission and STSP in both segments of the hippocampus. These results were corroborated by the higher protein expression levels of M2 receptors in the ventral compared with dorsal hippocampus. We conclude that muscarinic transmission modulates excitatory synaptic transmission and short-term synaptic plasticity along the entire rat hippocampus by acting through M4 receptors and recruiting M2 receptors only in the ventral hippocampus. Furthermore, M4 receptors appear to exert a permissive role on the actions of M2 receptors on STSP in the ventral hippocampus. This dorsoventral differentiation of muscarinic modulation is expected to have important implications in information processing along the endogenous hippocampal circuitry.

Septotemporal variation of information processing in the hippocampus of Fmr1 KO rat.

Introduction Fragile X messenger ribonucleoprotein (FMRP) is a protein involved in many neuronal processes in the nervous system including the modulation of synaptic transmission. Loss of FMRP produces the fragile X syndrome (FXS), a neurodevelopmental disorder affecting synaptic and neuronal function and producing cognitive impairments. However, the effects of FXS on short-term processing of synaptic inputs and neuronal outputs in the hippocampus have not yet been sufficiently clarified. Furthermore, it is not known whether dorsal and ventral hippocampus are affected similarly or not in FXS. Method We used a Fmr1 knock-out (KO) rat model of FXS and recordings of evoked field potentials from the CA1 field of transverse slices from both the dorsal and the ventral hippocampus of adult rats. Results Following application of a frequency stimulation protocol consisting of a ten-pulse train and recordings of fEPSP, we found that the dorsal but not ventral KO hippocampus shows altered short-term synaptic plasticity. Furthermore, applying the frequency stimulation protocol and recordings of population spikes, both segments of the KO hippocampus display altered short-term neuronal dynamics. Conclusions These data suggest that short-term processing of synaptic inputs is affected in the dorsal, not ventral FXS hippocampus, while short-term processing of neuronal output is affected in both segments of the FXS hippocampus in a similar way. These FXS-associated changes may have significant impact on the functions of the dorsal and ventral hippocampus in individuals with FXS.

α7 nicotinic acetylcholine receptors induce long-term synaptic enhancement in the dorsal but not ventral hippocampus.

Agents that positively modulate the activity of α7nAChRs are used as cognitive enhancers and for the treatment of hippocampus-dependent functional decline. However, it is not known whether the expression and the effects of α7nAChRs apply to the entire longitudinal axis of the hippocampus equally. Given that cholinergic system-involving hippocampal functions are not equally distributed along the hippocampus, we comparatively examined the expression and the effects of α7nAChRs on excitatory synaptic transmission between the dorsal and the ventral hippocampal slices from adult rats. We found that α7nAChRs are equally expressed in the CA1 field of the two segments of the hippocampus. However, activation of α7nAChRs by their highly selective agonist PNU 282987 induced a gradually developing increase in field excitatory postsynaptic potential only in the dorsal hippocampus. This long-term potentiation was not reversed upon application of nonselective nicotinic receptor antagonist mecamylamine, but the induction of potentiation was prevented by prior blockade of α7nAChRs by their antagonist MG 624. In contrast to the long-term synaptic plasticity, we found that α7nAChRs did not modulate short-term synaptic plasticity in either the dorsal or the ventral hippocampus. These results may have implications for the role that α7nAChRs play in specifically modulating functions that depend on the normal function of the dorsal hippocampus. We propose that hippocampal functions that rely on a direct α7 nAChR-mediated persistent enhancement of glutamatergic synaptic transmission are preferably supported by dorsal but not ventral hippocampal synapses.

Rescue of sharp wave-ripples and prevention of network hyperexcitability in the ventral but not the dorsal hippocampus of a rat model of fragile X syndrome.

Fragile X syndrome (FXS) is a genetic neurodevelopmental disorder characterized by intellectual disability and is related to autism. FXS is caused by mutations of the fragile X messenger ribonucleoprotein 1 gene (Fmr1) and is associated with alterations in neuronal network excitability in several brain areas including hippocampus. The loss of fragile X protein affects brain oscillations, however, the effects of FXS on hippocampal sharp wave-ripples (SWRs), an endogenous hippocampal pattern contributing to memory consolidation have not been sufficiently clarified. In addition, it is still not known whether dorsal and ventral hippocampus are similarly affected by FXS. We used a Fmr1 knock-out (KO) rat model of FXS and electrophysiological recordings from the CA1 area of adult rat hippocampal slices to assess spontaneous and evoked neural activity. We find that SWRs and associated multiunit activity are affected in the dorsal but not the ventral KO hippocampus, while complex spike bursts remain normal in both segments of the KO hippocampus. Local network excitability increases in the dorsal KO hippocampus. Furthermore, specifically in the ventral hippocampus of KO rats we found an increased effectiveness of inhibition in suppressing excitation and an upregulation of α1GABAA receptor subtype. These changes in the ventral KO hippocampus are accompanied by a striking reduction in its susceptibility to induced epileptiform activity. We propose that the neuronal network specifically in the ventral segment of the hippocampus is reorganized in adult Fmr1-KO rats by means of balanced changes between excitability and inhibition to ensure normal generation of SWRs and preventing at the same time derailment of the neural activity toward hyperexcitability.

Increased Inhibition May Contribute to Maintaining Normal Network Function in the Ventral Hippocampus of a Fmr1-Targeted Transgenic Rat Model of Fragile X Syndrome.

A common neurobiological mechanism in several neurodevelopmental disorders, including fragile X syndrome (FXS), is alterations in the balance between excitation and inhibition in the brain. It is thought that in the hippocampus, as in other brain regions, FXS is associated with increased excitability and reduced inhibition. However, it is still not known whether these changes apply to both the dorsal and ventral hippocampus, which appear to be differently involved in neurodegenerative disorders. Using a Fmr1 knock-out (KO) rat model of FXS, we found increased neuronal excitability in both the dorsal and ventral KO hippocampus and increased excitatory synaptic transmission in the dorsal hippocampus. Interestingly, synaptic inhibition is significantly increased in the ventral but not the dorsal KO hippocampus. Furthermore, the ventral KO hippocampus displays increased expression of the α1GABAA receptor subtype and a remarkably reduced rate of epileptiform discharges induced by magnesium-free medium. In contrast, the dorsal KO hippocampus displays an increased rate of epileptiform discharges and similar expression of α1GABAA receptors compared with the dorsal WT hippocampus. Blockade of α5GABAA receptors by L-655,708 did not affect epileptiform discharges in any genotype or hippocampal segment, and the expression of α5GABAA receptors did not differ between WT and KO hippocampus. These results suggest that the increased excitability of the dorsal KO hippocampus contributes to its heightened tendency to epileptiform discharges, while the increased phasic inhibition in the Fmr1-KO ventral hippocampus may represent a homeostatic mechanism that compensates for the increased excitability reducing its vulnerability to epileptic activity.

Early life stress influences basal ganglia dopamine receptors and novel object recognition of adolescent and adult rats.

Environmental stimuli in early life are recognized to affect brain development and behavior. Mother-pup interaction constitutes a determinant stimulus during this critical period. It is known that the dopaminergic system undergoes significant reorganization during adolescence and that dopamine receptors are involved in recognition memory. Based on the above, we examined the effects of brief and prolonged maternal separation during the neonatal period (15 or 180 min daily) on basal ganglia dopamine receptors and on the behavior in the novel object recognition task of adolescent and adult male rats. Using the NOR task, we observed that the discrimination index (DI) was decreased in rats with brief maternal separations independent of age. Using receptor autoradiography, we observed that brief maternal separation induced decreases in D1, D2 and D4 receptor binding levels in adult basal ganglia nuclei, while prolonged maternal separation induced increases in D1 receptor binding levels in caudate - putamen (CPu) of adolescent rats. With immunoblotting experiments, we found decreases in D1 and increases in D2 total protein levels in CPu of adult rats with prolonged maternal separations. Α positive correlation was observed between DI and D1 binding levels in CPu, internal globus pallidus and substantia nigra, and D2 binding levels in nucleus accumbens core in adult rats, using the Pearson correlation coefficient. Our results indicate that the long-lasting effects of neonatal mother-offspring separation on dopamine receptors depend on the duration of maternal separation and age and that this early life experience impairs recognition memory in adolescent and adult rats. Furthermore, the present results suggest that modulation of striatal dopamine receptors might underlie the reduced recognition memory of adult rats with brief neonatal maternal separations.

Age-dependent modulation of short-term neuronal dynamics in the dorsal and ventral rat hippocampus.

Brain aging is associated with alterations in the behavioral capacity to process information, due to mechanisms that are still largely unclear. Short-term neuronal activity dynamics are basic properties of local brain networks profoundly involved in neural information processing. In this study, we investigated the properties of short-term changes in excitatory synaptic transmission and neuronal excitation in the CA1 field of dorsal and ventral hippocampal slices from young adult and old rats. We found that short-term synaptic plasticity (i.e. short-term dynamics of input to CA1 circuit) does not significantly differ between young and old dorsal or ventral hippocampus. However, short-term dynamics of hippocampal output differ markedly between young and old rats. Notably, age-dependent alterations in short-term neuronal dynamics were detected mainly in the dorsal hippocampus. Thus, the dorsal hippocampus of young rats can detect and facilitate transmission of 1-30 Hz input and depress transmission of higher-frequency input. In contrast, the old dorsal hippocampus appears unable to transmit information in a frequency-dependent discriminatory manner. Furthermore, the amplification of steady-state output at frequencies < 40 Hz is considerably lower in the old than the young dorsal hippocampus. The old ventral hippocampus did not show major alterations in short-term processing of neural information, though under conditions of intense afferent activation, neuronal output of the ventral hippocampus is depressed at steady-state more in old than in young rats. These results suggest that aging is accompanied by alterations in neural information processing mainly in the dorsal hippocampus, which displays a narrower dynamic range of frequency-dependent transient changes in neuronal activity in old compared with young adult rats. These alterations in short-term dynamics may relate to deficits in processing ongoing activity seen in old individuals.

Neonatal maternal separation affects metabotropic glutamate receptor 5 expression and anxiety-related behavior of adult rats.

Exposure to early life stress leads to long-term neurochemical and behavioral alterations. Stress-induced psychiatric disorders, such as depression, have recently been linked to dysregulation of glutamate signaling, mainly via its postsynaptic receptors. The role of metabotropic glutamate receptor 5 (mGluR5) in stress-induced psychopathology has been the target of several studies in humans. In rodents, blockade of mGluR5 produces antidepressant-like actions, whereas mice lacking mGluR5 exhibit altered anxiety-like behaviors and learning. In this study, we used well-known rodent models of early life stress based on mother-infant separation during the first 3 weeks of life in order to examine the effects of neonatal maternal separation on mGluR5 expression and on anxiety-related behavior in adulthood. We observed that brief (15 min) neonatal maternal separation, but not prolonged (3 h), induced increases in mGluR5 mRNA and protein expression levels in medial prefrontal cortex and mGluR5 protein levels in dorsal, but not ventral, hippocampus of adult rat brain. Behavioral testing using the open-field and the elevated-plus maze tasks showed that brief maternal separations resulted in increased exploratory and decreased anxiety-related behavior, whereas prolonged maternal separations resulted in increased anxiety-related behavior in adulthood. The data indicate that the long-lasting effects of neonatal mother-offspring separation on anxiety-like behavior and mGluR5 expression depend on the duration of maternal separation and suggest that the increased mGluR5 receptors in medial prefrontal cortex and hippocampus of adult rats exposed to brief neonatal maternal separations may underlie their heightened ability to cope with stress.