Effects of light hatching on synapse number and size in the intermediate and medial part of the hyperstriatum ventrale of the domestic chick

Post-synaptic GABAB receptors—possible controllers of coincidence detection?

Post-synaptic GABA(B) responses (slow, late hyperpolarisations which can be eliminated by perfusion with phaclofen) can be recorded in vitro from many, but not all, neurones in the intermediate medial hyperstriatum ventrale (IMHV). The IMHV is an area of the chick forebrain which is remarkable for its plasticity, and for its essential role in two specific types of early learning-imprinting, and a form of one-trial passive-avoidance learning. Post-synaptic GABA(B) responses are strongly statistically associated with other properties (such as high membrane resistance) which are, themselves, dependent on a bird's past history. There is also evidence that their incidence changes with prior training in vivo and with age. GABA(B) hyperpolarisations are always offset to a varying extent by excitatory NMDA components. These two components follow a very similar time-course, so that the duration and (to a lesser extent), the magnitude of a response is controlled by the balance between the two systems. The evidence suggests that this balance fluctuates, and that its fluctuations determine the extent to which any neurone can function as a coincidence detector.

Estimation of numerical density and mean synaptic height in chick hippocampus 24 and 48 hours after passive avoidance training

The effects of passive avoidance learning on synaptic morphology and number in the dorsolateral hippocampus of chick were investigated at 24 and 48 h after training. Chicks of both sexes were used. The numerical density of synapses and mean synaptic height were determined using design-based quantitative electron microscopic techniques. Our results suggest that after training there is a significant increase in synaptic density in the dorsolateral hippocampus of chicks at both 24 and 48 h, and also that the mean synaptic height was significantly different between trained and control groups. The increase in synaptic density was due to shaft (type II) synapses. It is known that during synaptogenesis, shaft synapses are formed first and are then converted to spine synapses. The only hemispheric asymmetry was found in the 24 h water-trained (W-trained) males where the numerical density of spine synapses was significantly higher in the left hippocampus. No significant differences due to gender in either numerical synaptic density or synapse height were observed at either 24 and 48 h. Comparison of the 24 h with 48 h groups showed an increase in shaft synaptic density over time in the W-trained groups, and an increased density of both shaft and spine synapses with time in methylanthranilate-trained (MeA-trained) chicks. These results demonstrate that the dorsolateral hippocampus of the chick shows synaptic changes at both 24 and 48 h after training and implicates this region in the long-term memory process.

Effects of nitric oxide release in an area of the chick forebrain which is essential for early learning

Extracellular recording techniques were used to study the effects of the nitric oxide releasing agents diethylamine-NO (DEA-NO) and S-nitroso-N-acetyl-penicillamine (SNAP) on synaptic transmission in the intermediate and medial part of the hyperstriatum ventrale (IMHV), a part of the domestic chick forebrain that is essential for some forms of early learning. The field response evoked by local electrical stimulation was recorded in the IMHV in an in vitro slice preparation. DEA-NO (100-200 mgr) significantly depressed the field response in a concentration dependent and reversible manner. However, the depression produced by perfusion with 400 mgr DEA-NO, was not reversed following washout of the drug. With 400 mgr DEA-NO, NO reaches a maximum concentration of 10 mgr at 2 min of perfusion, and then declines slowly. SNAP (400 mgr) produced an effect similar to 400 mgr DEA-NO. Neither the immediate nor the longer-term depressive effect of NO is mediated by activation of guanylyl cyclase because in the presence of both low and high doses of ODQ, a potent and selective inhibitor of NO-stimulated guanylyl cyclase, NO produced the same depression of the field response. There is evidence however that the IMHV possesses c-GMP responsive elements since direct perfusion of 8-Br-cGMP (1 mM) produced a long-term but not an immediate depression. The long-term depression produced by 400 mgr DEA-NO was eliminated in the presence of either a selective adenosine A(1) receptor antagonist or an ADP-ribosyltransferase inhibitor. It was also possible to prevent the long-term effect in the presence of tetraethyl ammonium a K(+)-channel blocker. These results suggest that the NO may be acting presynaptically in a synergistic fashion with the adenosine A(1) receptor to depress transmitter release.