Tritiated phorbol-12,13-dibutyrate [3H]PDBu), a phorbol ester, was utilized to autoradiographically localize protein kinase C (PKC) in the cat visual cortex. Thin, slide-mounted sections of adult cat brain were used to characterize binding of [3H]PDBu. This was found to be saturable, reversible, and more readily displaced by phorbol ester than by synthetic diacylglycerols. Binding sites displayed a tissue concentration of 20 pmol/mg protein, and a dissociation constant of 8.0 nM. [3H]PDBu was slow to associate with its receptor, requiring 9.5 h to reach equilibrium. Autoradiography revealed that PKC is heterogeneously distributed in the cat brain, and displays a laminar-specific pattern in the visual cortex. This laminar distribution undergoes marked changes during the first two months of postnatal life. In the visual cortex of neonatal kittens, [3H]PDBu binding is confined to layers I and V. Layer III acquires high levels of binding by postnatal day 15, layer II by 28 days, and layer VI becomes labelled by 40 days of age. Adult animals exhibit high levels of binding in all laminae except layer IV. Age-dependent changes in PKC's laminar distribution do not seem to be correlated with specific anatomical, neurochemical, or behavioural events during development. PKC appears to be associated with cell bodies or processes intrinsic to the visual cortex, and is probably not located on the terminals of cortical afferents.

We review efforts to further understand the development and nature of sensory processing mechanisms in the cat visual cortex. In vitro autoradiographic and homogenate assay techniques have been employed to determine the laminar distribution and characteristics of various neurotransmitter and neuromodulator receptor populations during postnatal development. Each receptor population shows a distinct laminar-specific pattern of binding, which, in most cases, is age-dependent. Changes in receptor number and affinity are also observed during postnatal development. These findings indicate that major alterations in the basic chemical circuitry of cat visual cortex are a normal feature of postnatal maturation and may play a role in plasticity mechanisms.

We have examined the characteristics of various receptors in cat visual cortex during postnatal development. These included beta-adrenergic, GABA, benzodiazepine and acetylcholine receptors. For each population of receptor the number (Bmax) and affinity (Kd) were examined as a function of postnatal age (3 days-adult). For all receptors examined, the Bmax increased during development from low early values to a peak within the critical period. The Kd also changed during development for most receptors. The simultaneous alterations in Bmax and Kd necessitate defining a term which takes both of these receptor properties into consideration. This term, called receptor sensitivity (RS), provides a more comprehensive measure of receptor function than either Bmax or Kd alone. Using this measure, we find that receptor sensitivity is low near birth for the 4 receptor populations studied, rises to a peak within the first two months of life, and then declines to near-neonatal levels for 3 of the 4 receptor populations.

In vitro receptor binding techniques were used to study the characteristics, distribution, and ontogenesis of muscimol binding sites in cat visual cortex. [3H] Muscimol, a GABA agonist, labelled a single population of binding sites with a KD of 18 nM in adult cats. Specific binding was saturable, reversible and was blocked by the addition of GABA or (+) bicuculline. Autoradiograms revealed that the highest density of [3H] muscimol binding occurred in cortical layer IV. Similar patterns of [3H] muscimol binding were observed at all ages examined, although the binding densities differed. The peak [3H] muscimol binding density, corrected for amount of protein, occurred at 3 months postnatally. In 3 day old and adult cats binding density was 41% and 69%, respectively, of the peak value.

1. The number, affinity, and laminar distributions of various receptors in cat visual cortex were examined during postnatal development using homogenate and in vitro autoradiographic techniques.

Acetylcholine appears to act as a modulator of neuronal activity in cat visual cortex and, like noradrenaline, may be involved with cortical plasticity mechanisms during the critical period. To explore possible ACh involvement in these mechanisms we have examined acetylcholine binding sites in cat visual cortex during development using [3H]QNB, a muscarinic antagonist. At 3 days postnatal [3H]QNB preferentially labelled binding sites in layer IV. During development the pattern of binding reversed, so that by 95 days postnatal layer IV was the least densely labelled. The number of binding sites increased during development peaking at 1 month postnatal. The Kd of [3H]QNB binding sites increased to 95 days postnatal, with a peak value of 0.76 nM. The results show that during development, and especially within the critical period, changes in [3H]QNB binding site distribution, number and affinity occur.

In vitro receptor binding techniques were used to study the characteristics and distribution of [3H]muscimol binding sites in cat visual cortex. [3H]muscimol, a specific GABA agonist, labeled a single population of binding sites with aKd of 18 nM. Specific binding was saturable, reversible, and was blocked by the addition of GABA or (+)-bicuculline. Autoradiograms revealed that the highest density of [3H]muscimol binding sites occurred in cortical layer IV. Little variation between the various visual cortical areas was noted in contrast to marked regional heterogeneity within subcortical structures.