No abstract

Visual experience during a critical period early in postnatal development can change connections within mammalian visual cortex. In a kitten at the peak of the critical period (approximately P28-42), brief monocular deprivation can lead to complete dominance by the open eye, an ocular dominance shift. This process is driven by activity from the eyes, and depends on N-methyl-D-aspartate (NMDA) receptor activation. The components of the intracellular signaling cascade underlying these changes have not all been identified. Here we show that inhibition of protein kinase A (PKA) by Rp-8-Cl-cAMPS blocks ocular dominance shifts that occur following monocular deprivation early in the critical period. Inhibition of protein kinase G by Rp-8-Br-PET-cGMPS had no effect, indicating a specificity for the PKA pathway. Enhancement of PKA activity late in the critical period with Sp-8-Cl-cAMPS did not increase plasticity. PKA is a necessary component of the pathway leading to cortical plasticity during the critical period.

Synaptic train stimulation (10 Hz · 25 s) in hippocampal slices results in a biphasic response of NAD(P)H fluorescence indicating a transient oxidation followed by a prolonged reduction. The response is accompanied by a transient tissue PO 2 decrease indicating enhanced oxygen utilization. The activation of mitochondrial metabolism and/or glycolysis may contribute to the secondary NAD(P)H peak. We investigated whether extracellular lactate uptake via monocarboxylate transporters (MCTs) contributes to the generation of the NAD(P)H response during neuronal activation. We measured the effect of lactate uptake inhibition [using the MCT inhibitor a-cyano-4-hydroxycinnamate (4-CIN)] on the NAD(P)H biphasic response, tissue PO 2 response, and field excitatory post-synaptic potential in hippocampal slices during synaptic stimulation in area CA1 (stratum radiatum). The application of 4-CIN (150-250 lmol/L) significantly decreased the reduction phase of the NAD(P)H response. When slices were supplemented with 20 mmol/L lactate in 150-250 lmol/L 4-CIN, the secondary NAD(P)H peak was restored; whereas 20 mmol/L pyruvate supplementation did not produce a recovery. Similarly, the tissue PO 2 response was decreased by MCT inhibition; 20 mmol/L lactate restored this response to control levels at all 4-CIN concentrations. These results indicate that lactate uptake via MCTs contributes significantly to energy metabolism in brain tissue and to the generation of the delayed NAD(P)H peak after synaptic stimulation.

No abstract

The cAMP-dependent protein kinase (PKA) signaling pathway plays a key role in visual cortical plasticity. Inhibitors that block activation of all PKA regulatory subunits (RI␣, RI␤, RII␣, RII␤) abolish long-term potentiation (LTP) and long-term depression (LTD) in vitro and ocular dominance plasticity (ODP) in vivo. The details of this signaling cascade, however, including the source of PKA signals and which PKA subunits are involved, are unknown. To investigate these issues we have examined LTP, LTD, and ODP in knock-out mice lacking either the two cortically expressed Ca 2ϩ -stimulated adenylyl cyclases (AC1 and AC8) or the predominant neocortical subunit of PKA (RII␤). Here we show that plasticity remains intact in AC1/AC8Ϫ/Ϫ mice, whereas ODP and LTD, but not LTP, are absent in RII␤Ϫ/Ϫ mice. We conclude that (1) plasticity in the visual cortex does not require the activity of known Ca 2ϩ -stimulated adenylyl cyclases, (2) the PKA dependence of ODP and LTD, but not LTP, is mediated by RII␤-PKA, and (3) multiple isoforms of PKA contribute to LTD.

The Cat-301 monoclonal antibody identifies aggrecan, a chondroitin sulfate proteoglycan in the cat visual cortex and dorsal lateral geniculate nucleus (dLGN). During development, aggrecan expression increases in the dLGN with a time course that matches the decline in plasticity. Moreover, examination of tissue from selectively visually deprived cats shows that expression is activity dependent, suggesting a role for aggrecan in the termination of the sensitive period. Here, we demonstrate for the first time that the onset of aggrecan expression in area 17 also correlates with the decline in experience-dependent plasticity in visual cortex and that this expression is experience dependent. Dark rearing until 15 weeks of age dramatically reduced the density of aggrecan-positive neurons in the extragranular layers, but not in layer IV. This effect was reversible as dark-reared animals that were subsequently exposed to light showed normal numbers of Cat-301-positive cells. The reduction in aggrecan following certain early deprivation regimens is the first biochemical correlate of the functional changes to the γ-aminobutyric acidergic system that have been reported following early deprivation in cats.

Kittens were given intramuscular injections of the N-methyl--aspartate (NMDA) antagonist MK-801 twice daily (morning and midday) during the peak of the period of susceptibility for ocular dominance changes. They were then exposed to light with one eye closed for 4 h after each injection. The ocular dominance of these kittens was shifted significantly less than that of kittens injected with saline and exposed to light over the same period at the same age. After recording a sample of cells for an ocular dominance histogram, the kittens were injected with the same dose of MK-801 that was used during rearing to observe its effect on the activity of single cells in the visual cortex. In the majority of cells (7/13) there was no significant change in activity. Positive evidence for a reduction in activity was seen in only a minority (3/13) of cells. In a separate series of experiments, dose-response curves were measured for cells in the visual cortex in response to iontophoresis of NMDA or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and the effect of an injection of MK-801 on these curves was measured. MK-801, at doses similar to those used in the ocular dominance experiments, had a significant effect on the dose-response curves for NMDA, but little effect on the dose-response curves for AMPA, or the visual responses of the cells. We conclude that ocular dominance shifts can be reduced significantly by a treatment that has little effect on the level of activity of cells in the visual cortex but does specifically affect the responses of the cells to NMDA as opposed to the responses to AMPA.

Brief alterations to the nature of the visual input during critical periods in the early life of cats and monkeys can result in rapid anatomical and physiological changes in the central visual pathways. The immediate early genes (IEGs) represent a possible way in which these changes could be mediated since the protein products of a number of these genes have been shown to be induced rapidly in neurons in response to a variety of transsynaptic stimuli. Immunohistochemical methods were employed to examine the tempo and pattern of expression of Fos, the protein product of the c-fos gene, induced in the visual cortex of kittens dark-reared from birth to 30 days of age by brief periods of binocular visual exposure. In visual cortical area 17, the number of Fos immunoreactive cells increased rapidly from virtually zero in control kittens that received no visual exposure, to reach high levels in animals that received between 1 and 2 hours of visual experience. Immunoreactive cells were absent in the dorsal lateral geniculate nucleus, but were numerous in the ventral lateral geniculate nucleus, and in area 17, were most numerous in the extragranular layers (2, 3 and 6) but sparse in lower layer 4 and layer 5, and virtually absent in layer 1. Substantial constitutive Fos immunoreactivity was observed in area 17 of normal 30-day-old kittens but very few immunopositive cells were evident in adult animals. However, Fos immunoreactivity was observed in the visual cortex of a dark-reared (for 30 days) adult animal following a brief period of visual exposure, a finding that suggests that Fos might serve other roles in the visual cortex in addition to those it might play uniquely during development. It is suggested that Fos, in combination with the protein products of a select number of other IEGs, may mediate a variety of rapid changes in the visual cortex including those that underlie visual system plasticity during early postnatal life.