Visual deprivation decreases long-term potentiation in rat visual cortical slices

The use of in vivo, ex vivo, in vitro, computational models and volunteer studies in vision research and therapy, and their contribution to the Three Rs

Much is known about mammalian vision, and considerable progress has been achieved in treating many vision disorders, especially those due to changes in the eye, by using various therapeutic methods, including stem cell and gene therapy. While cells and tissues from the main parts of the eye and the visual cortex (VC) can be maintained in culture, and many computer models exist, the current non-animal approaches are severely limiting in the study of visual perception and retinotopic imaging. Some of the early studies with cats and non-human primates (NHPs) are controversial for animal welfare reasons and are of questionable clinical relevance, particularly with respect to the treatment of amblyopia. More recently, the UK Home Office records have shown that attention is now more focused on rodents, especially the mouse. This is likely to be due to the perceived need for genetically-altered animals, rather than to knowledge of the similarities and differences of vision in cats, NHPs and rodents, and the fact that the same techniques can be used for all of the species. We discuss the advantages and limitations of animal and non-animal methods for vision research, and assess their relative contributions to basic knowledge and clinical practice, as well as outlining the opportunities they offer for implementing the principles of the Three Rs (Replacement, Reduction and Refinement).

Postnatal development and sensory experience synergistically underlie the excitatory/inhibitory features of hippocampal neural circuits: Glutamatergic and GABAergic neurotransmission

During a postnatal critical period balance of excitation/inhibition in the developing brain is highly regulated by environmental signals. Compared to the visual cortex, rare document includes effects of sensory experience on the hippocampus, which is also bombarded by sensory signals. In this study, basic and tetanized field excitatory postsynaptic potentials (fEPSPs) were recorded in CA1 area of hippocampus of light-(LR) and dark-reared (DR) rats (at 2, 4 and 6weeks of age). Also, we assessed age- and activity-dependent changes in the N-Methyl-d-aspartic acid (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors subunit compositions and, GABA producing enzymes. While the sensory deprivation increased amplitude of baseline fEPSPs, it decreased degree of potentiation of post-tetanus responses. Expression of GluA1 and GluA2 subunits of AMPA receptors was increased across age in DR rats. In contrast to LR rats, mRNA and protein expression of GluN1, GluN2A and GluN2B subunits of NMDA receptors was decreased in DR ones. Also, dark rearing diminished expression of GABA synthesis enzymes GAD65 and GAD67. These results indicate that, sensory experience adjusts synaptic plasticity and might also affect the balance of excitation/inhibition in the hippocampus.

Light deprivation improves melatonin related suppression of hippocampal plasticity

In early postnatal life, sensory inputs deeply influence development as well as function of the brain. Plasticity of synaptic transmission including its experimentally induced form, long-term potentiation (LTP), is affected by sensory deprivation in neocortex. This study is devoted to assess if dark rearing and a dark phase synthesized hormone melatonin influence LTP in the hippocampus, an area of brain involved in learning and memory. In vivo experiments were carried out on two groups of 45-days-old male Wistar rats kept in standard 12-h light/dark condition [light reared (LR) tested during the light phase] or in complete darkness [dark reared (DR)] since birth to testing. Each group, in turn, was divided to two, vehicle- and melatonin-treated, groups. Stimulating the Schaffer collaterals of CA3 area of hippocampus extracellular postsynaptic potentials (EPSPs) were recorded in the CA1 area. Having the stable baseline responses to the test pulses, the hippocampus was perfused by either vehicle or 2 microg melatonin and EPSPs were recorded for 30 min. Then, for induction of LTP, the tetanus was applied to the Schaffer collaterals and the field potentials were pooled for 120-min post-tetanus. The light deprivation resulted in a significant augmentation in the amplitude of baseline responses. Also, we observed a melatonin-induced increase in amplitude of the baseline recordings in either LR or DR animals. Tetanic stimulation elicited LTP of EPSPs in both LR and DR groups, robustly in the former where it lasted for about 90 min. Generally, melatonin inhibited the production of LTP in the two groups especially in the LR animals leading to a noticeable depression. We concluded that higher level of neuronal activity in the DR rats gives rise to a lower level of LTP. Weaker effect of melatonin on blocking the potentiation of post-tetanus EPSPs in the DR rats may be the result of a desensitization of melatonin receptors due to chronically increased levels of this hormone in the visually deprived rats.

Rapid eye movement sleep deprivation modifies expression of long-term potentiation in visual cortex of immature rats

During rapid eye movement (REM) sleep, activity of non-retinal origin is propagated into central visual-system pathways in a manner similar, in pattern and intensity, to central visual-system activity that is exogenously generated in waking. It has been hypothesized that REM sleep, which is more abundantly represented early in life than later, functions to provide adjunct 'afferent' input for shaping synaptic connectivity during brain maturation. Here we present data that support this proposal. Recent studies have described a developmentally regulated form of in vitro long-term potentiation (LTP) in the visual cortex that is experience- and age-dependent. In immature rats, suppression of retinal activation of the visual system by removal of visual experience (dark rearing) extends the age when the developmentally regulated form of LTP can be produced. This study tests whether suppression of REM-state activation of the visual system also lengthens the developmental period in which this specific form of LTP can be elicited. Young rats were deprived of REM sleep by the multiple-small-platforms-over-water method during the typically latest week for induction of such LTP in slices of visual cortex. After this week, we could still induce LTP in slices from nearly all the REM-sleep-deprived rats (8/9) but not from age-matched rats that had not lost REM sleep (0/5). The control rats had been housed on large platforms that allow the animals to obtain REM sleep. Only body weights and the concentration of thyrotrophin-releasing hormone in the hypothalamus distinguished home-caged, normal-sleeping controls from both groups of platform animals. On all measures, stress levels were not dissimilar in the two platforms groups. After 7 days of behavioral suppression of REM sleep in immature rats, and consequent reduction of the intense, extra-retinal activity endogenously generated during this sleep state, we found that the period was extended in which developmentally regulated synaptic plasticity (LTP) could be elicited in slices of visual neocortex. These studies support the role of REM sleep and its associated neuronal activity in brain maturation.

The role of N-methyl-D-aspartate receptors in synaptic plasticity of rat visual cortex in vitro: effect of sensory experience

We examined the role of N-methyl-D-aspartate (NMDA) receptors in synaptic plasticity of visual cortex of light (LR) and dark (DR) reared adult rats in vitro. Layer IV stimulation resulted in field potentials in layer II/III, consisting of two excitatory postsynaptic potentials (EPSP) called EPSP1 and EPSP2. Tetanic stimulation induced long-term potentiation (LTP) in EPSP2 of both LR and DR visual cortices. NMDA receptor antagonist D, L-2-amino-5-phosphono-valeric acid (AP5) completely blocked the LTP of EPSP2 in DR visual cortex while it reduced slightly the extent of LTP of EPSP2 in LR ones. Another NMDA receptor antagonist ketamine blocked potentiation of EPSP1 as well as EPSP2 in both groups. Our findings demonstrate that dependency of LTP on NMDA receptors and/or sensitivity of these receptors to the antagonists are different in LR and DR animals.

Differential effect of dark rearing on long-term potentiation induced by layer IV and white matter stimulation in rat visual cortex

In the earlier work, we showed that primed-burst stimulation (PBs) is an effective protocol to induce long-term potentiation (LTP) in layer II/III of adult rat visual cortex in vitro. In the present study, we investigated effects of dark rearing on potentiation of layer II/III responses to stimulation of layer IV or the underlying white matter in the visual cortex in vitro. Long-term potentiation was induced by PBs applied to white matter or layer IV of the cortex in light and dark reared rats. Regardless of the stimulation site, layer II/III field potentials consisted of two components. In general, the latency of responses in dark reared rats was shorter than that in light reared ones. Whereas PBs of layer IV produced LTP of two components in both the groups, that of white matter induced an appreciable potentiation of the second component in both groups and the first component only in dark reared rats. These results indicate that PBs of either white matter or layer IV can gain access to the modifiable synapses that are related to the second component of layer II/III responses in light and dark reared visual cortex, but accessibility of the modifiable synapses that are related to first component depends on the tetanization site. The dark rearing enhances accessibility of the modifiable synapses that are related to the first component following PBs of the white matter. It is suggested that the immaturity of inhibitory circuits and/or better function of excitatory ones in the visual cortex of dark reared rats may contribute to the enhanced accessibility of the first component.

Primed-bursts induced long-term potentiation in rat visual cortex: effects of dark-rearing

Theta burst stimulation (TBS) and primed bursts (PBs) stimulation are among the effective tetanic stimulations for induction of long-term potentiation (LTP) in the hippocampus. Recent studies have indicated that TBS is effective in LTP induction of layer III synapses of neocortex, only if applied to layer IV. However, the possibility of neocortical LTP induction using PBs has not been investigated yet. Sensory deprivation greatly influences the development of neocortex. According to the effect of sensory deprivation on synaptic plasticity of developing neocortex, we studied the induction of LTP by PBs in visual cortical slices of control and dark-reared rats. The results showed that application of PBs to layer IV could effectively induce LTP of layer II/III field potentials. These potentials are consisted of two components: pEPSP1, (population excitatory postsynaptic potential 1) and pEPSP2. In control slices PBs led to selective potentiation of pEPSP2. Visual deprivation increased the incidence of LTP of pEPSP1 and decreased the amount of LTP of pEPSP2. These findings showed that PBs could be used as an effective tetanic stimulation to study the synaptic plasticity in neocortex. The effects of visual deprivation on PBs-induced LTP are consistent with its role in the development of excitatory system in neocortex.

Visual deprivation increases capability of layer II/III for epileptiform activity in the rat visual cortical slices

Effects of visual deprivation on the induction of epileptiform activity were studied in layer II/III of 29-39-day-old rat primary visual cortex. Field potentials were evoked by stimulation of layer IV in slices from control (CON) and dark-reared (DR) rats. Picrotoxin (PTX)-induced epileptiform activity was characterized by spontaneous and evoked epileptic field potentials (EFPs). The results showed that DR slices demonstrate greater susceptibility for induction of spontaneous EFP. PTX-induced changes in the characteristics of evoked field potentials also showed higher tendency of DR animals to generate epileptiform activity. In both groups, field potentials consisted of pEPSP(1) (population excitatory postsynaptic potential 1, i.e., first negativity) and pEPSP(2) (second negativity), respectively. There was no significant difference between the characteristics of field potentials in CON and DR slices. PTX significantly increased amplitude and duration of pEPSP(2), but it had no significant effect on pEPSP(1). Effects of PTX on pEPSP(2) were significantly higher in DR slices. It is concluded that visual deprivation results in a heightened potential in layer II/III of the rat visual cortex to generate PTX-induced epileptiform activity.

Development of a transient increase in recurrent inhibition and paired-pulse facilitation in hippocampal CA1 region

Paired-pulse recurrent inhibition (RI) of population spike (PS) and facilitation (PPF) of field excitatory postsynaptic potential (EPSP) were studied in the CA1 region of hippocampal slices taken from Wistar rats aged from 9 days to 16 months. The comparison of three different paired-pulse protocols revealed the antidromic-orthodromic (A-O) stimulation as the most reliable in quantifying the strength of fast (peaking at 10 ms) and slow (peaking at 200 ms) components of recurrent inhibition. Fast RI, present but weak at 9 days, progressively increased to reach its maximal strength at 30 days, declining in adult (2 m) and middle-aged (16 m) animals. Slow RI was replaced by facilitation at 9 days while it was absent at 15 days. It reached adult values at 30 days. A reduction of the test response at interpulse interval (IPI) of 2-4 ms was strong in developing and adult animals, but was significantly decreased in 16 m. At maximal stimulation PPF was expressed as an enhancement of the slow rather than the fast phase of the EPSP and was particularly strong with a prominent N-methyl-D-aspartate dependent component. A very characteristic selectivity for a prominent PPF at stimulation frequency of 5 Hz appeared first at the 18th day and increased gradually to reach a maximum at the 30th day, after which it declined to very low values in middle-aged animals. A similar developmental pattern was observed in slices taken from rats reared in complete darkness, suggesting a strong innate origin. The ability of hippocampal circuits for plastic gating of information appears to be transiently enhanced at the completion of the first postnatal month as it can be exercised at a wider part of the frequency spectrum, with maximal inhibition and potentiation especially at the frequency of theta rhythm.

Co-regulation of long-term potentiation and experience-dependent synaptic plasticity in visual cortex by age and experience

Long-term potentiation (LTP) is a lasting enhancement of excitatory synaptic transmission that follows specific patterns of electrical stimulation. Although the mechanism of LTP has been intensively studied, particularly in the hippocampus, its significance for normal brain function remains unproven. It has been proposed that LTP-like mechanisms may contribute to naturally occurring, experience-dependent synaptic modifications in the visual cortex. The formation of normal binocular connections within the visual cortex requires simultaneous input from both eyes during a postnatal critical period that can be delayed by rearing animals in complete darkness. To explore the role of LTP in this experience-dependent maturation process, we induced LTP in visual cortical slices taken at different ages from light-reared and dark-reared rats. Susceptibility to LTP coincides with the critical period and, like the critical period, can be prolonged by rearing animals in darkness. These findings support the hypothesis that LTP reflects a normal mechanism of experience-dependent synaptic modification in the developing mammalian brain.