Absence of long-term depression in the visual cortex of glutamic Acid decarboxylase-65 knock-out mice

LTP and LTD vary with layer in rodent visual cortex

Mechanisms of plasticity in the visual cortex have been studied with long-term potentiation (LTP), long-term depression (LTD) and ocular dominance plasticity (ODP). It is now possible to compare results from these three forms of plasticity using knockout mice, and also by pharmacological manipulations. A review of the literature shows that if both LTP and LTD are completely abolished, then ODP will also be abolished. In other situations, there is little correlation. We hypothesize that this lack of correlation occurs because the mechanisms for LTP and LTD vary with layer in the visual cortex, and results show that they do.

Contribution of GABAergic inhibition to synaptic responses and LTD early in postnatal development in the rat superior colliculus

We studied the development of optic tract evoked field potentials (FP) in the rodent superior colliculus (SC) and the effect of GABA antagonists upon their development and upon induction of long-term depression (LTD). Brain slices were cut from Lister Hooded rats. The optic tract was stimulated while recording from the superficial grey layer. GABAergic inhibition was assessed by adding 100 microm picrotoxin and 3 microm CGP55845 antagonists to block GABA A,B,C receptors. LTD was induced with a 50 Hz, 20 s tetanus. At age P2, the FP consisted only of a presynaptic spike. The GABA antagonists had no effect. By P4, the FP consisted of a presynaptic spike, a longer latency population spike, and a field excitatory postsynaptic potential (fEPSP). The fEPSP was slightly prolonged by the GABA antagonists at this age. By P7-P14, a prominent FP with trailing fEPSP was recorded. The GABA antagonists usually had a large effect, with the fEPSP increasing in both amplitude and duration. A mature FP was usually recorded in P15-P23 slices where the GABA antagonist effect remained substantial. LTD could be induced in 17 of 30 control slices from rats aged P4-P26. The average fEPSP amplitude after tetanus was 77.9% of control. Pre-treatment with GABA antagonists produced a short-term potentiation (average 114.0%), rather than LTD, in 14 of 19 cases. This STP was followed by a more prolonged potentiation in 12 of the 14 cases. We conclude that GABAergic inhibitory circuits mature before eye opening and that GABA contributes to induction of LTD in the developing SC.

Prepulse inhibition deficits in GAD65 knockout mice and the effect of antipsychotic treatment

Recent postmortem studies in humans suggest that defects in GABAergic neurotransmission might contribute to the neuropathology associated with schizophrenia. Disturbances in GABAergic systems may also contribute to the sensorimotor gating deficits classically observed in schizophrenic patients, including deficits in prepulse inhibition (PPI). To explore the relationship, the current study examined the integrity of PPI and startle habituation in knockout (KO) mice that lack the GABA synthesizing enzyme glutamic acid decarboxylase 65 (GAD 65). GAD65 KO mice displayed normal baseline and habituated startle responses, which did not differ from GAD65 wild-type (WT) or heterozygous (HET) mice. However, GAD65 KO mice showed robust deficits in PPI which were reversed by the atypical antipsychotic agent clozapine. These results lend support to the view that abnormalities in GABAergic systems might contribute to the basic pathophysiological mechanisms in schizophrenia.

Layer variations of long-term depression in rat visual cortex

In vitro long-term depression (LTD) is thought to be a model for the loss of cortical responsiveness to an eye deprived of vision during the critical period. Using whole cell recording, the present study investigates the mechanisms of LTD in vitro across layers in developing rat visual cortex. LTD was induced in layers II/III, V, and VI but not layer IV with 10-min 1-Hz stimulation paired with postsynaptic depolarization. LTD in layers II/III and V could be blocked by the N-methyl-D-aspartate (NMDA) receptor antagonist D-aminophosphonovaleric acid (D-AP5) but not by 100 microM (2S)-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (LY341495), a metabotropic glutamate receptor inhibitor. In contrast, LTD in layer VI was blocked by 100 microM LY341495 and (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) but not D-AP5 and partially blocked by application of guanosine 5'-O-(2-thiodiphosphate) thilothium salt (GDP-beta-S) in patch pipette, suggesting an involvement of postsynaptic group I metabotropic glutamate receptors (mGluRs). These results indicate that LTD in developing rat visual cortex varies with layer: LTD was absent in layer IV, suggesting a unique plasticity mechanism at geniculocortical synapses; LTD in layers II/III and V depends on NMDA receptors but not mGluRs, and LTD in layer VI requires mGluRs but not NMDA receptors.

Long term potentiation varies with layer in rat visual cortex

Long term potentiation (LTP) in various layers of rat visual cortex was studied in 90 cells with visually identified, whole-cell recordings. LTP was induced in layer II/III, layer V or layer VI with theta burst stimulation (TBS), but was not observed in layer IV. In the presence of a NMDA antagonist, D-AP5, in the bath solution, potentiation was blocked in layer II/III, some depression was seen in layer V, and potentiation still remained in layer VI. After addition of a specific mGluR1 antagonist, LY367385, to the bath solution, LTP was reduced in layer II/III and layer V, and was blocked in layer VI. After a specific mGluR5 antagonist, MPEP was applied in the bath solution, LTP was enhanced in layer VI, and blocked in layer V. We conclude that: (1) LTP in layer VI is different from other layers, depending on mGluR1, but not NMDA receptors. (2) In layer II/III, LTP is NMDA-dependent and is not blocked by group I mGluR antagonists. (3) LTP in layer V is both NMDA receptor and mGluR5 receptor-dependent. (4) LTP was not induced in layer IV with TBS.

Controlling the critical period

Neuronal circuits are shaped by experience during 'critical periods' of early postnatal life. The ability to control the timing, duration, and closure of these heightened levels of brain plasticity has recently become experimentally possible. Two seemingly opposed views of critical period mechanism have emerged: (1) plasticity may be functionally accessed throughout life by appropriately modified stimulation protocols, or (2) plasticity is rigidly limited to early postnatal life by structural modifications. This overview synthesizes both perspectives across a variety of brain regions and species. A deeper understanding of critical periods will form the basis for novel international efforts to "nurture the brain".

Demonstration of functional coupling between gamma -aminobutyric acid (GABA) synthesis and vesicular GABA transport into synaptic vesicles

l-Glutamic acid decarboxylase (GAD) exists as both membrane-associated and soluble forms in the mammalian brain. Here, we propose that there is a functional and structural coupling between the synthesis of gamma-aminobutyric acid (GABA) by membrane-associated GAD and its packaging into synaptic vesicles (SVs) by vesicular GABA transporter (VGAT). This notion is supported by the following observations. First, newly synthesized [(3)H]GABA from [(3)H]l-glutamate by membrane-associated GAD is taken up preferentially over preexisting GABA by using immunoaffinity-purified GABAergic SVs. Second, the activity of SV-associated GAD and VGAT seems to be coupled because inhibition of GAD also decreases VGAT activity. Third, VGAT and SV-associated Ca(2+)calmodulin-dependent kinase II have been found to form a protein complex with GAD. A model is also proposed to link the neuronal stimulation to enhanced synthesis and packaging of GABA into SVs.

Developmental plasticity of mouse visual acuity

Monocular deprivation in mice between postnatal days 19 and 32 has been reported to significantly shift ocular dominance within the binocular region of primary visual cortex; however, it is not known whether visual deprivation in mice during this physiologically defined critical period also results in amblyopia, as it does in other mammals. We addressed this uncertainty by psychophysically assessing in adulthood (postnatal day 70 or older) the grating acuity of normal and monocularly deprived mice, using the Visual Water Task. The visual acuity of mice tested with their nondeprived eyes was equivalent to that of normal mice ( approximately 0.5 cycles/degree); however, acuity measured with eyes monocularly deprived of vision transiently between postnatal days 19 and 32 was reduced by over 30% ( approximately 0.31 cycles/degree). Identical binocular deprivation produced a significant, but smaller, decrease in acuity ( approximately 0.38 cycles/degree). The effects of monocular and binocular deprivation were long lasting and occurred only if visual deprivation occurred between postnatal days 19 and 32. These data indicate that the deleterious effects of early visual deprivation on visual acuity in mice are similar to those reported in other mammals, and together with electrophysiological evidence of ocular dominance plasticity, suggest that the mechanisms of mouse visual plasticity are fundamentally the same as that in other mammals. Therefore, the mouse is probably a good model for investigating the basic cellular and molecular mechanisms underlying visual developmental plasticity and amblyopia.

Dark rearing alters the development of GABAergic transmission in visual cortex

We studied the role of sensory experience in the maturation of GABAergic circuits in the rat visual cortex. Between the time at which the eyes first open and the end of the critical period for experience-dependent plasticity, the total GABAergic input converging into layer II/III pyramidal cells increases threefold. We propose that this increase reflects changes in the number of quanta released by presynaptic axons. Here, we show that the developmental increase in GABAergic input is prevented in animals deprived of light since birth but not in animals deprived of light after a period of normal experience. Thus, sensory experience appears to play a permissive role in the maturation of intracortical GABAergic circuits.