Apomorphine and glycoprotein synthesis in rat hippocampus

Disruption of the direct perforant path input to the CA1 subregion of the dorsal hippocampus interferes with spatial working memory and novelty detection

Subregional analyses of the hippocampus suggest CA1-dependent memory processes rely heavily upon interactions between the CA1 subregion and entorhinal cortex. There is evidence that the direct perforant path (pp) projection to CA1 is selectively modulated by dopamine while having little to no effect on the Schaffer collateral (SC) projection to CA1. The current study takes advantage of this pharmacological dissociation to demonstrate that local infusion of the non-selective dopamine agonist, apomorphine (10, 15 microg), into the CA1 subregion of awake animals produces impairments in working memory at intermediate (5 min), but not short-term (10 s) delays within a delayed non-match-to-place task on a radial arm maze. Sustained impairments were also found in a novel context with similar object-space relationships. Infusion of apomorphine into CA1 is also shown here to produce deficits in spatial, but not non-spatial novelty detection within an object exploration paradigm. In contrast, apomorphine produces no behavioral deficits when infused into the CA3 subregion or overlying cortex. These behavioral studies are supported by previous electrophysiological data that demonstrate local infusion of the same doses of apomorphine significantly modifies evoked responses in the distal dendrites of CA1 following angular bundle stimulation, but produces no significant effects in the proximal dendritic layer following stimulation of the SC. These results support a modulatory role for dopamine in EC-CA1, but not CA3-CA1 circuitry, and suggest the possibility of a fundamental role for EC-CA1 synaptic transmission in terms of detection of spatial novelty, and intermediate-term, but not short-term spatial working memory or object-novelty detection.

The role of the direct perforant path input to the CA1 subregion of the dorsal hippocampus in memory retention and retrieval

Subregional analyses of the hippocampus have suggested a selective role for the CA1 subregion in intermediate/long-term spatial memory and consolidation, but not short-term acquisition or encoding processes. It remains unclear how the direct cortical projection to CA1 via the perforant path (pp) contributes to these CA1-dependent processes. It has been suggested that dopamine selectively modulates the pp projection to CA1 while having little to no effect on the Schaffer collateral (SC) projection to CA1. This series of behavioral and electrophysiological experiments takes advantage of this pharmacological dissociation to demonstrate that the direct pp inputs to CA1 are critical in CA1-dependent intermediate-term retention and retrieval function. Here we demonstrate that local infusion of the nonselective dopamine agonist, apomorphine (10, 15 microg), into the CA1 subregion of awake animals produces impairments in between-day retention and retrieval, sparing within-day encoding of a modified Hebb-Williams maze and contextual conditioning of fear. In contrast, apomorphine produces no deficits when infused into the CA3 subregion. To complement the behavioral analyses, electrophysiological data was collected. In anesthetized animals, local infusion of the same doses of apomorphine significantly modifies evoked responses in the distal dendrites of CA1 following angular bundle stimulation, but produces no significant effects in the more proximal dendritic layer following stimulation of the SC. These results support a modulatory role for dopamine in the EC-CA1, but not CA3-CA1 circuitry, and suggest the possibility of a more fundamental role for EC-CA1 synaptic transmission in terms of intermediate-term, but not short-term spatial memory.

Cholinergic and dopaminergic agents which inhibit a passive avoidance response attenuate the paradigm-specific increases in NCAM sialylation state

The influence of cholinergic and dopaminergic agents on the acquisition of a passive avoidance response in the rat is demonstrated. Trifluoperazine (0.12 mg/kg), a dopamine antagonist, inhibited task acquisition when present during training or later, during consolidation, at the 10-12 h post-training period and at no other intervening time point. Induction of amnesia was dose-dependent and was not apparent when the dose exceeded 0.12 mg/kg. This effect appears to be due to an increase in dopamine release through presynaptic receptor antagonism as similar results could be obtained by the administration of apomorphine (0.5 mg/kg), a dopamine agonist, and this effect could be antagonized by the D1 receptor selective antagonist SCH-23390. Scopolamine (0.15 mg/kg), a muscarinic antagonist, impaired acquisition of the passive avoidance response when administered during training and, separately, at the 6 h post-training period. This could not be attributed to presynaptic antagonism as oxotremorine (0.2 mg/kg), a muscarinic agonist, had no amnesic action. Administration of apomorphine or scopolamine during training and at the appropriate post-training period prevented subsequent paradigm-specific increases of neural cell adhesion molecule sialylation state in hippocampal immunoprecipitates obtained at 24 h after task acquisition and 4 h following intraventricular infusion of the labelled sialic acid precursor - N-acetyl-D-mannosamine. Oxotremorine alone did not influence neural cell adhesion molecule sialylation state. These observations provide further evidence of a regulatory role for neural cell adhesion molecule sialylation state in information storage processes.

Incorporation of [3H]fucose in rat hippocampal structures after conditioning by perforant path stimulation and after LTP-producing tetanization

The contribution of glycoprotein synthesis to functional synaptic changes and to the formation of memory traces was investigated by autoradiographic determination of the incorporation of [3H]fucose into the hippocampal structures of rats. In the first experiment, the fucose incorporation was measured after induction of post-tetanic long-term potentiation (LTP) in granular cell synapses by repeated tetanization (200 cps) of the perforant path, and after stimulation of this hippocampal input by the same number of impulses with very low frequency (0.2 cps) not producing LTP. In the second experiment, the incorporation of fucose was determined after an active avoidance training using the stimulation of the perforant path by impulse trains of 15 cps as conditioning stimuli, and after a session of corresponding unpaired stimulations of the perforant path. Unstimulated animals were used in both experiments to measure the basal glycosylation. LTP-producing tetanization resulted only in a slight increase of incorporation into the ipsilateral hippocampal structures without significant differences to similar changes after the corresponding control stimulation with single impulses. After a session of unpaired stimulation of the perforant path with impulse trains of 15 cps only slight and inconsistent changes of incorporation occurred in the hippocampus too. However, after conditioning by the corresponding perforant path stimulation as conditioned stimulus, considerable increases of incorporation were observed in all structures of the ipsilateral hippocampus, when compared to the unpaired control stimulation. An enhanced labeling occurred also in some structures of the contralateral hippocampus mainly receiving commissural inputs. The results suggest again, that the activation of one single hippocampal afferent, even if producing LTP, would not be sufficient to induce an increased glycosylation of neuronal proteins. The increase of glycoprotein formation seems to require the convergence of several inputs, which can be assumed to occur during learning. Therefore, LTP of a single synaptic population seems not to represent the complete long-lasting memory trace, but only one of its components, or a preceding transient storage mechanism.

Dopamine-induced changes in protein phosphorylation and polyphosphoinositide metabolism in rat hippocampus

Effects of dopamine (DA) on endogenous phosphorylation of hippocampal proteins and polyphosphoinositides were studied in subcellular fractions from a crude mitocondrial/synaptosomal preparation. DA induced a concentration-dependent decrease in the in vitro phosphorylation of the protein B-50 (-22.1% at 10(-5) M DA), whereas no changes were found in phosphoproteins in other subcellular fractions. Treatment of hippocampal slices with 5 X 10(-4) M DA resulted in a 45.8% increase in post hoc phosphorylation of B-50 in SPM and it affected post hoc phosphorylation of several proteins in a cytosolic fraction. In vitro phosphorylation of SPM with DA (5 X 10(-4) M) increased endogenous TPI phosphorylation (+51.6%), whereas treatment of slices with DA (5 X 10(-4) M) resulted in a 39.4% decrease in post hoc TPI phosphorylation. This decrease could be blocked by haloperidol. Significant changes induced by DA (5 X 10(-4) M) were also found in 32P-incorporation into PA (in vitro: -32.4% and post hoc: +39.3%), but were not found in DPI labeling. The data provide evidence for DA-induced changes in phosphorylation of proteins and polyphosphoinositides in rat hippocampal SPM.

Changes in activities of fucokinase and fucosyltransferase in rat hippocampus after acquisition of a brightness discrimination reaction

Activities of enzymes involved in utilization of the glycoprotein precursor L-fucose (fucokinase and fucosyltransferase) were studied in rat hippocampal tissue after acquisition of a brightness discrimination reaction. Fucokinase activity was increased immediately after training, while fucosyltransferase revealed decreased values. However, 7 hr after training fucokinase activity showed normal values, while fucosyltransferase activity rose in trained animals over active and passive controls. The results are discussed in the light of a regulatory role that fucokinase and fucosyltransferase may play in fucose utilization under altered functional conditions.

Mechanisms of dopamine induced changes in hippocampal glycoprotein metabolism

In rat hippocampal slices incubated in the presence of dopamine, a relatively strong correlation was observed between changes in the incorporation of 3H-fucose into total proteins and the formation of GDP-3H-fucose. However, in hippocampal homogenate the incorporation of 14C-fucose from GDP-14C-fucose was not stimulated by dopamine. In contrast, the incorporation of 3H-fucose was stimulated by dopamine to a similar extent observed in hippocampal slices. Furthermore, in hippocampal slices dopamine did not increase the activity of fucosyltransferase. These results, together with our previous findings, suggest that the increased incorporation of fucose induced by dopamine in the hippocampal slices may be due to a receptor-mediated cAMP-dependent regulation, which controls the rate of fucosylation of acceptor-glycoproteins either at the level of fucose phosphorylation or of formation of GDP-fucose rather than the activity of fucosyltransferase.

Apomorphine and glycoprotein synthesis during consolidation

The dopamine agonist, apomorphine, was injected intrahippocampally immediately after acquisition of a brightness discrimination task, which was motivated by footshock in rats. This led to an increase in the incorporation of L-fucose into total proteins which were measured in the hippocampus 7-9 hours later. In behavioral experiments, the same application improved the retention of a learned task. A possible linkage between increased glycoprotein synthesis and improvement of the retention of a new learned behavior due to the action of apomorphine is discussed.

Dopamine induced changes in L-fucose incorporation into proteins of rat hippocampus and corpus striatum during postnatal development

In 60 day old (adult) male Wistar rats dopamine caused a dose-dependent increase of L-fucose incorporation into total proteins of both hippocampus and corpus striatum slices up to +47.8 +/- 6.0% (n=6) and +53.2 +/- 8.5% (n=20), respectively, when compared to corresponding controls. Under these conditions the dopamine concentration leading to a maximum stimulation of fucose incorporation was 5 X 10(-4) M in hippocampus and 1 X 10(-3) M in corpus striatum. In the latter tissue the range of dopamine concentrations causing a significant elevation in incorporation rates was larger than in hippocampal tissue. In the corpus striatum of 9 day old rats dopamine was ineffective, but by 30 days the transmitter stimulated fucose incorporation rate reached the maximum observed for any age studied. This developmental pattern seems to be related to the ontogenesis of dopamine receptor sites or dopamine sensitive adenylate cyclase formation in this brain structure. In the hippocampus the postnatal development of dopamine induced augmentation of glycoprotein synthesis showed a longer latency, but the maximum effect was also seen in 30 day old animals. These results support our assumption that at the end of the postnatal differentiation period the glycoprotein synthesis in brain tissue may be controlled (at least to some extent) by the state of dopaminergic receptors and/or of dopamine sensitive adenylate cyclase.

Increase in in vivo (3H) spiperone binding in the rat hippocampal formation and striatum after repeated treatment with haloperidol

An increase in in vivo (3H) spiperone binding was observed in rat hippocampal formation and striatum after repeated treatment with haloperidol. This suggests that in hippocampus as well as in striatum prolonged blockade of dopaminergic transmission by a neuroleptic agent results in the development of a supersensitivity of the dopamine receptors.

The influence of dopamine on the incorporation of different sugars into total proteins of hippocampal slices

Dopamine increases the incorporation of L-fucose and of D-mannose to a similar significant degree, whereas the incorporation of D-galactose as well as of N-acetyl-D-glucosamin into the total proteins of hippocampal slices was only slightly enhanced. The incorporation of N-acetyl-neuraminic acid was not influenced by dopamine. The results suggest that the effect of dopamine on glycoprotein formation seems mainly to depend on the kind of nucleotides necessary for activation of sugars and not on the sugar's final position in the glycan chain.