Current Literature In Basic Science
Go Out and Play; Your Brain Needs the Exercise
Environmental Enrichment Restores CA1 Hippocampal LTP and Reduces Severity of Seizures in Epileptic Mice. Morelli E, Ghiglieri V, Pendolino V, Bagetta V, Pignataro A, Fejtova A, Costa C, Ammassari-Teule M, Gundelfinger ED, Picconi B, Calabresi P. Exp Neurol 2014;261:320–327.
We have analyzed the effects of environmental enrichment (EE) in a seizure-prone mouse model in which the genetic disruption of the presynaptic protein Bassoon leads to structural and functional alterations in the hippocampus and causes early spontaneous seizures mimicking human neurodevelopmental disorders. One-month EE starting at P21 reduced seizure severity, preserved long-term potentiation (LTP) and paired-pulse synaptic responses in the hippocampal CA1 neuronal population and prevented the reduction of spine density and dendrite branching of pyramidal neurons. These data demonstrate that EE exerts its therapeutic effect by normalizing multiple aspects of hippocampal function and provide experimental support for its use in the optimization of existent treatments.
Commentary The majority of mice used in laboratory research spend their adult lives in standard housing conditions predicated by the recommendations set forth in the Guide for the Care and Use of Laboratory Animals (1). Adult mice of the same sex are typically group housed in cages with a minimum of 15 in2 of floor space per mouse, which translates to cages that are 8 in (w) × 13 (l) × 6 in (h). Although the standards are currently changing, until recently, most mouse cages contained only mice and a modest amount of bedding. Almost 55 years ago, Mark Rosenzweig and his colleagues at the University of California Berkeley demonstrated, first in rats (2, 3) and later in mice (4), that altering the animal’s living condition to include toys, tunnels, and ladders was sufficient to alter brain chemistry and increase brain mass. Similar manipulations, commonly referred to as “environmental enrichment” (EE), have been reported to ameliorate cognitive deficits in several mouse models of neurological and psychiatric disease states (5). In addition to improving cognitive function, EE has been demonstrated to diminish seizure severity in rat and mouse models of epilepsy (6, 7). While there have been a number of reports investigating the putative substrates that underlie EE-induced enhanced cognition, far less is known as to how EE might modify seizures. A recent paper published by Emanuela Morelli, Veronica Ghiglieri, and their colleagues begins to address this question. To assess the impact of EE on seizures, Morelli et al. utilized a line of mice in which the presynaptic active zone protein Bassoon was deleted. Bassoon knock-out (bsn-KO) mice exhibit a progressive seizure phenotype, displaying mild epileptic seizures characterized by tremor, rigidity, and Epilepsy Currents, Vol. 15, No. 4 (July/August) 2015 pp. 223–224 © American Epilepsy Society
sporadic head–neck myoclonus at postnatal day P14. At P21, epileptic episodes in the bsn-KO mice evolve into forelimb myoclonic seizures, and by ~P50, they begin to exhibit generalized tonic–clonic seizures. To test the impact of EE on seizure progression and neuropathology, immediately following weaning (P21), one group of bsn-KO mice and WT littermate controls were transferred to the EE (10–12 mice group housed in a large cage with a running wheel, tunnels, and toys). A separate group of bsn-KO and WT mice remained under standard housing conditions. Following one month of EE (or standard housing), seizure activity was video monitored for 4 hours a day for 5 days and scored offline by hand. Although the bsn-KO mice continued to have seizures, mice exposed to EE exhibited a significant decrease in the amount of time they spent in the tonic–clonic phase during a seizure bout. Following the seizure assessment period, mice were euthanized, and in vitro hippocampal slices were prepared. At the neurophysiological level, EE appeared to affect the bsn-KO mice in a variety of ways. The bsn-KO mice exhibited a deficit in long-term potentiation (LTP; an activity dependent form of activity-dependent plasticity), which can be reversed with 4 weeks of EE. The authors also examined a form of short-term plasticity known as paired pulse facilitation (PPF) in which two synaptic stimuli are delivered in pairs, separated by a variable time interval. In region CA1 of the hippocampus, the synaptic response to the second pulse is larger than the first when the interval between the two pulses is sufficiently short. Consistent with the original description of the mice (8), Morelli et al. did not observe any deficits in PPF under normal conditions. However, after the induction of LTP, the bsn-KO mice exhibited enhanced PPF, which was reversed by EE. Although only two time points are presented for the PPF curve, these results suggest that the loss of bsn produces an alteration in metaplasticity that can be reversed by EE.
223
Environmental Enrichment, LTP, and Seizures
The authors also investigated the possibility that there may be a difference in the way the postsynaptic CA1 neuron responses to synaptic input. Under voltage clamp conditions, the authors measured the ratio of the synaptic currents gated by the inotropic glutamate AMPA and NMDA receptor complexes. Although the differences were modest, the authors did observe a difference in the AMPA/NMDA ratio, with the bsn-KO mice having a smaller AMPA/NMDA ratio when housed under standard conditions. Environmental enrichment appears to normalize this difference but does so by reducing the AMPA/NMDA ratio of the WT EE group. Finally, the authors examined the basic morphological properties of pyramidal neurons in the CA1 region of the hippocampus. Genetic ablation of Bassoon reduces the number of dendritic spines (spines/µm) located on both the apical and basal dendrites. Environmental enrichment appears to restore the number of spines across both dendritic fields in the bsn-KO mice and also increases WT spine numbers on the apical dendrites. Loss of Bassoon also results in a modest decrease in dendritic length of the most proximal dendrites, and this morphological change is rescued by EE. Also, it is worth noting that the neurons quantified in the WT mice exposed to EE exhibited a significant increase in dendritic length that included the more distal dendrites. While it remains unclear how EE leads to the observed changes in neurophysiology and neuroanatomy, Morelli et al. have demonstrated that EE can alter some aspects of seizure progression. These results are consistent with previous work and extend the principle to a model where the seizure onset is very early and seizures have already begun prior to the attempted intervention. In addition, Morelli et al. demonstrate changes in neurophysiology and neuronal morphology in the bsn-KO mice that appear to be altered by EE. At present, it is difficult to know how restoring LTP or altering dendritic morphology in these mice might lead to a decrease in seizure duration or severity. However, results from future studies targeted toward determining how EE leads to these changes could likely be exploited to develop interventions, and the current work provides incentive for those investigations. Could EE be used as an antiepileptic therapy? This is likely the wrong question to ask, given that there are very few epileptic patients who live in an environment that would
224
be considered comparable to the standard mouse housing conditions. The more important question appears to be: What specific component or components of EE contribute to the reduction in seizure activity? For example, is it the increased exercise? Is it the challenge of navigating an unfamiliar and novel environment? Does the EE housing condition reduce overall stress levels? Once identified, these elements could be augmented in patients with epilepsy. Conversely, if one could determine how elements of EE might translate into beneficial changes in neuroanatomy or neurophysiology, then the underlying cellular mechanisms could be exploited as therapeutic targets. by Geoffrey G. Murphy, PhD References 1. National Research Council. Guide for the Care and Use of Laboratory Animals. 8th ed. Washington, DC: The National Academies Press, 2011. 2. Krech D, Rosenzweig MR, Bennett EL. Effects of environmental complexity and training on brain chemistry. J Comp Physiol Psychol 1960;53:509–519. 3. Rosenzweig MR, Krech D, Bennett EL, Diamond MC. Effects of environmental complexity and training on brain chemistry and anatomy: A replication and extension. J Comp Physiol Psychol 1962;55:429–437. 4. La Torre JC. Effect of differential environmental enrichment on brain weight and on acetylcholinesterase and cholinesterase activities in mice. Expl Neurol 1968;22:493–503. 5. Pang TYC, Hannan AJ. Enhancement of cognitive function in models of brain disease through environmental enrichment and physical activity. Neuropharmacology 2013;64:515–528. 6. Manno I, Macchi F, Caleo M, Bozzi Y. Environmental enrichment reduces spontaneous seizures in the Q54 transgenic mouse model of temporal lobe epilepsy. Epilepsia 2011;52:e113–e117. 7. Young D, Lawlor PA, Leone P, Dragunow M, During MJ. Environmental enrichment inhibits spontaneous apoptosis, prevents seizures and is neuroprotective. Nat Med 1999;5:448–453. 8. Altrock WD, tom Dieck S, Sokolov M, Meyer AC, Sigler A, Brakebusch C, Fässler R, Richter K, Boeckers TM, Potschka H, Brandt C, Löscher W, Grimberg D, Dresbach T, Hempelmann A, Hassan H, Balschun D, Frey JU, Brandstätter JH, Garner CC, Rosenmund C, Gundelfinger ED. Functional inactivation of a fraction of excitatory synapses in mice deficient for the active zone protein Bassoon. Neuron 2003;37:787–800.
Go Out and Play; Your Brain Needs the Exercise
Environmental Enrichment Restores CA1 Hippocampal LTP and Reduces Severity of Seizures in Epileptic Mice. Morelli E, Ghiglieri V, Pendolino V, Bagetta V, Pignataro A, Fejtova A, Costa C, Ammassari-Teule M, Gundelfinger ED, Picconi B, Calabresi P. Exp Neurol 2014;261:320–327.
We have analyzed the effects of environmental enrichment (EE) in a seizure-prone mouse model in which the genetic disruption of the presynaptic protein Bassoon leads to structural and functional alterations in the hippocampus and causes early spontaneous seizures mimicking human neurodevelopmental disorders. One-month EE starting at P21 reduced seizure severity, preserved long-term potentiation (LTP) and paired-pulse synaptic responses in the hippocampal CA1 neuronal population and prevented the reduction of spine density and dendrite branching of pyramidal neurons. These data demonstrate that EE exerts its therapeutic effect by normalizing multiple aspects of hippocampal function and provide experimental support for its use in the optimization of existent treatments.
Commentary The majority of mice used in laboratory research spend their adult lives in standard housing conditions predicated by the recommendations set forth in the Guide for the Care and Use of Laboratory Animals (1). Adult mice of the same sex are typically group housed in cages with a minimum of 15 in2 of floor space per mouse, which translates to cages that are 8 in (w) × 13 (l) × 6 in (h). Although the standards are currently changing, until recently, most mouse cages contained only mice and a modest amount of bedding. Almost 55 years ago, Mark Rosenzweig and his colleagues at the University of California Berkeley demonstrated, first in rats (2, 3) and later in mice (4), that altering the animal’s living condition to include toys, tunnels, and ladders was sufficient to alter brain chemistry and increase brain mass. Similar manipulations, commonly referred to as “environmental enrichment” (EE), have been reported to ameliorate cognitive deficits in several mouse models of neurological and psychiatric disease states (5). In addition to improving cognitive function, EE has been demonstrated to diminish seizure severity in rat and mouse models of epilepsy (6, 7). While there have been a number of reports investigating the putative substrates that underlie EE-induced enhanced cognition, far less is known as to how EE might modify seizures. A recent paper published by Emanuela Morelli, Veronica Ghiglieri, and their colleagues begins to address this question. To assess the impact of EE on seizures, Morelli et al. utilized a line of mice in which the presynaptic active zone protein Bassoon was deleted. Bassoon knock-out (bsn-KO) mice exhibit a progressive seizure phenotype, displaying mild epileptic seizures characterized by tremor, rigidity, and Epilepsy Currents, Vol. 15, No. 4 (July/August) 2015 pp. 223–224 © American Epilepsy Society
sporadic head–neck myoclonus at postnatal day P14. At P21, epileptic episodes in the bsn-KO mice evolve into forelimb myoclonic seizures, and by ~P50, they begin to exhibit generalized tonic–clonic seizures. To test the impact of EE on seizure progression and neuropathology, immediately following weaning (P21), one group of bsn-KO mice and WT littermate controls were transferred to the EE (10–12 mice group housed in a large cage with a running wheel, tunnels, and toys). A separate group of bsn-KO and WT mice remained under standard housing conditions. Following one month of EE (or standard housing), seizure activity was video monitored for 4 hours a day for 5 days and scored offline by hand. Although the bsn-KO mice continued to have seizures, mice exposed to EE exhibited a significant decrease in the amount of time they spent in the tonic–clonic phase during a seizure bout. Following the seizure assessment period, mice were euthanized, and in vitro hippocampal slices were prepared. At the neurophysiological level, EE appeared to affect the bsn-KO mice in a variety of ways. The bsn-KO mice exhibited a deficit in long-term potentiation (LTP; an activity dependent form of activity-dependent plasticity), which can be reversed with 4 weeks of EE. The authors also examined a form of short-term plasticity known as paired pulse facilitation (PPF) in which two synaptic stimuli are delivered in pairs, separated by a variable time interval. In region CA1 of the hippocampus, the synaptic response to the second pulse is larger than the first when the interval between the two pulses is sufficiently short. Consistent with the original description of the mice (8), Morelli et al. did not observe any deficits in PPF under normal conditions. However, after the induction of LTP, the bsn-KO mice exhibited enhanced PPF, which was reversed by EE. Although only two time points are presented for the PPF curve, these results suggest that the loss of bsn produces an alteration in metaplasticity that can be reversed by EE.
223
Environmental Enrichment, LTP, and Seizures
The authors also investigated the possibility that there may be a difference in the way the postsynaptic CA1 neuron responses to synaptic input. Under voltage clamp conditions, the authors measured the ratio of the synaptic currents gated by the inotropic glutamate AMPA and NMDA receptor complexes. Although the differences were modest, the authors did observe a difference in the AMPA/NMDA ratio, with the bsn-KO mice having a smaller AMPA/NMDA ratio when housed under standard conditions. Environmental enrichment appears to normalize this difference but does so by reducing the AMPA/NMDA ratio of the WT EE group. Finally, the authors examined the basic morphological properties of pyramidal neurons in the CA1 region of the hippocampus. Genetic ablation of Bassoon reduces the number of dendritic spines (spines/µm) located on both the apical and basal dendrites. Environmental enrichment appears to restore the number of spines across both dendritic fields in the bsn-KO mice and also increases WT spine numbers on the apical dendrites. Loss of Bassoon also results in a modest decrease in dendritic length of the most proximal dendrites, and this morphological change is rescued by EE. Also, it is worth noting that the neurons quantified in the WT mice exposed to EE exhibited a significant increase in dendritic length that included the more distal dendrites. While it remains unclear how EE leads to the observed changes in neurophysiology and neuroanatomy, Morelli et al. have demonstrated that EE can alter some aspects of seizure progression. These results are consistent with previous work and extend the principle to a model where the seizure onset is very early and seizures have already begun prior to the attempted intervention. In addition, Morelli et al. demonstrate changes in neurophysiology and neuronal morphology in the bsn-KO mice that appear to be altered by EE. At present, it is difficult to know how restoring LTP or altering dendritic morphology in these mice might lead to a decrease in seizure duration or severity. However, results from future studies targeted toward determining how EE leads to these changes could likely be exploited to develop interventions, and the current work provides incentive for those investigations. Could EE be used as an antiepileptic therapy? This is likely the wrong question to ask, given that there are very few epileptic patients who live in an environment that would
224
be considered comparable to the standard mouse housing conditions. The more important question appears to be: What specific component or components of EE contribute to the reduction in seizure activity? For example, is it the increased exercise? Is it the challenge of navigating an unfamiliar and novel environment? Does the EE housing condition reduce overall stress levels? Once identified, these elements could be augmented in patients with epilepsy. Conversely, if one could determine how elements of EE might translate into beneficial changes in neuroanatomy or neurophysiology, then the underlying cellular mechanisms could be exploited as therapeutic targets. by Geoffrey G. Murphy, PhD References 1. National Research Council. Guide for the Care and Use of Laboratory Animals. 8th ed. Washington, DC: The National Academies Press, 2011. 2. Krech D, Rosenzweig MR, Bennett EL. Effects of environmental complexity and training on brain chemistry. J Comp Physiol Psychol 1960;53:509–519. 3. Rosenzweig MR, Krech D, Bennett EL, Diamond MC. Effects of environmental complexity and training on brain chemistry and anatomy: A replication and extension. J Comp Physiol Psychol 1962;55:429–437. 4. La Torre JC. Effect of differential environmental enrichment on brain weight and on acetylcholinesterase and cholinesterase activities in mice. Expl Neurol 1968;22:493–503. 5. Pang TYC, Hannan AJ. Enhancement of cognitive function in models of brain disease through environmental enrichment and physical activity. Neuropharmacology 2013;64:515–528. 6. Manno I, Macchi F, Caleo M, Bozzi Y. Environmental enrichment reduces spontaneous seizures in the Q54 transgenic mouse model of temporal lobe epilepsy. Epilepsia 2011;52:e113–e117. 7. Young D, Lawlor PA, Leone P, Dragunow M, During MJ. Environmental enrichment inhibits spontaneous apoptosis, prevents seizures and is neuroprotective. Nat Med 1999;5:448–453. 8. Altrock WD, tom Dieck S, Sokolov M, Meyer AC, Sigler A, Brakebusch C, Fässler R, Richter K, Boeckers TM, Potschka H, Brandt C, Löscher W, Grimberg D, Dresbach T, Hempelmann A, Hassan H, Balschun D, Frey JU, Brandstätter JH, Garner CC, Rosenmund C, Gundelfinger ED. Functional inactivation of a fraction of excitatory synapses in mice deficient for the active zone protein Bassoon. Neuron 2003;37:787–800.
