On the action of noradrenaline microinjected into the paraventricular nucleus of rat hypothalamus

Abstract

The microinjection of noradrenaline (NA) into the hypothalamic paraventricular nucleus (PVN) of the rat results in feeding. This response was shown; contrary to previous reports; to be mediated through both a-1 and a-2 NA receptors. Selective blockade of these two receptor sub-types, in conjunction with re-uptake blockade was used to examine the individual contributions of each receptor type to the whole response. It is suggested that the previously reported a-2 receptor specificity of the response to microinjected NA is a result of the location of these receptors. The post-synaptic a-1 receptor being located close to the pre-synaptic re-uptake mechanism, whilst the post-synaptic a-2 receptor is located outside the synapse and thus away from the re-uptake mechanism. The re-uptake mechanism acts to create a concentration difference of microinjected NA between the two receptor sub-types, resulting in a higher concentration and thus a preferential action at a-2 receptors. The involvement of the paraventricular NA system in stress induced eating was examined using a tail pinch procedure. Microinjection of NA antagonists into PVN prior to the onset of the pinch had no effect on the duration or latency of the eating response, thus there was no evidence for the involvement of this system in tail pinch elicited feeding. Further to the suggestion that the NA a-2 receptor is extra-synaptic whilst a-1 is intrasynaptic, the actions of NA were examined at a second site. NA microinjected into the ventral striatum elicited a vigorous locomotor response, although the origins of this showed a clear priming effect. However, this response was unaffected by prior microinjection of NA a-antagonists, preventing an analysis of receptor involvement comparable with that performed in PVN.