Clonidine and cortical plasticity: possible evidence for noradrenergic involvement

Neuromodulatory transmitter systems in the cortex and their role in cortical plasticity

Cortical neuromodulatory transmitter systems refer to those classical neurotransmitters such as acetylcholine and monoamines, which share a number of common features. For instance, their centers are located in subcortical regions and send long projection axons to innervate the cortex. The same transmitter can either excite or inhibit cortical neurons depending on the composition of postsynaptic transmitter receptor subtypes. The overall functions of these transmitters are believed to serve as chemical bases of arousal, attention and motivation. The anatomy and physiology of neuromodulatory transmitter systems and their innervations in the cerebral cortex have been well characterized. In addition, ample evidence is available indicating that neuromodulatory transmitters also play roles in development and plasticity of the cortex. In this article, the anatomical organization and physiological function of each of the following neuromodulatory transmitters, acetylcholine, noradrenaline, serotonin, dopamine, and histamine, in the cortex will be described. The involvement of these transmitters in cortical plasticity will then be discussed. Available data suggest that neuromodulatory transmitters can modulate the excitability of cortical neurons, enhance the signal-to-noise ratio of cortical responses, and modify the threshold for activity-dependent synaptic modifications. Synaptic transmissions of these neuromodulatory transmitters are mediated via numerous subtype receptors, which are linked to multiple signal transduction mechanisms. Among the neuromodulatory transmitter receptor subtypes, cholinergic M(1), noradrenergic beta(1) and serotonergic 5-HT(2C) receptors appear to be more important than other receptor subtypes for cortical plasticity. In general, the contribution of neuromodulatory transmitter systems to cortical plasticity may be made through a facilitation of NMDA receptor-gated processes.

Involvement of nerve growth factor in visual cortex plasticity

The physiological role of nerve growth factor (NGF), the prototype member of the neurotrophin family, has been widely studied. NGF has been shown to promote survival, sprouting and differentiation of sympathetic ganglion cells and sensory neurons in the peripheral nervous system; it has also been shown to support survival and regeneration of cholinergic neurons in the central nervous system. Recent evidence indicates that NGF is also involved in the neuronal plasticity of the visual cortex. Exogenous supplies of NGF have been shown to interfere with normal processes underlying activity- and age-dependent synaptic modifications in both developing and adult visual cortex. In parallel to these physiological effects, numerous neuronal markers in the visual cortex have been found to be influenced by NGF. Several proposals have been introduced to explain the physiological role of NGF in visual cortex plasticity. Although the mechanisms underlying NGF effects in the visual cortex are still under active investigation, current evidence implies that NGF, and perhaps other neurotrophins as well, may be useful for preventing or correcting inappropriate or anomalous connections in the visual cortex, and thus for treating visual dysfunctions such as amblyopia and strabismus.

Neonatal clonidine treatment results in long-lasting changes in noradrenaline sensitivity and kindling epileptogenesis

In the present experiment we tested the hypothesis that early interference with noradrenaline transmission can have permanent consequences for brain function in adulthood. Neonatal depletion of noradrenaline by daily subcutaneous injections of clonidine results in supersensitivity to noradrenaline in adult hippocampal CA1 cells as shown in our previous microiontophoretic study. These findings were confirmed and extended here with dose-response curves. Furthermore, we tested whether this form of neonatal interference with noradrenaline also permanently affects long-lasting plasticity as revealed in kindling epileptogenesis in adulthood. The initiation of the epileptic activity after the kindling stimulation was significantly delayed in the clonidine-treated group, and all measured parameters of seizure expression tended to be retarded in comparison with saline-treated control rats. This indicates that noradrenaline supersensitivity induced by neonatal clonidine treatment retards kindling development in adulthood.

Hippocampal neuronal responsiveness to different neurotransmitters in the adult rat after neonatal interference with noradrenaline transmission

In order to test the functional significance of noradrenergic neurotransmission for brain maturation, rat pups were injected daily with clonidine from 8-21 days of postnatal life. In adulthood, single cell responses to microiontophoretically applied neurotransmitters were studied in different layers of the hippocampus. No significant differences were found in neuronal responsiveness to glutamate, GABA, or acetylcholine in the hippocampus of clonidine-treated rats as compared with age-matched controls. However, there was a significantly stronger depression of glutamate-evoked activity of CA1 pyramidal neurons by noradrenaline in the clonidine-treated rats.

Long-term suppression of the cerebral spread of a memory: effects of idazoxan and clonidine

Bitemporal injections of puromycin consistently induce amnesia of aversive maze-learning in mice when administered within 3 days of training. These bitemporal puromycin injections lose their amnestic effectiveness if the latency between training and injection is extended beyond 6 days. Consistent with other evidence, we believe that memory (in our task) "spreads" during the 6 days following training. Since previous experiments have indicated that the central noradrenergic system is involved in this process of "memory spread," we have examined the effect of stimulation or blockade of the alpha 2-receptor. To this end, we administered a single dose of the alpha 2-adrenoceptor antagonist, idazoxan, or the alpha 2-agonist, clonidine. Idazoxan (1 mg/kg, SC) had no effect on engram spread. Clonidine (25 micrograms-125 ng/kg, SC), by contrast, suppressed engram spread for at least 30 days after treatment. When mice were tested at 60 and 90 days after treatment, spontaneous recovery (i.e., engram spread) was evident in only about 50% of the clonidine treated mice. Coadministration of idazoxan with clonidine blocked the effects of clonidine on "memory spread."