Adrenergic neuromodulation of transmitter release in rat neocortex

Abstract

Presynaptic Ca{u00B2}{u207A} stores modulate transmitter release via presynaptic metabotropic glutamate receptors (group I). I hypothesized that presynaptic {u03B1}1-adrenoreceptors ({u03B1}1-ARs) have the potential to cause Ca{u00B2}{u207A} mobilization from stores, as they cause PIP{u2082} hydrolysis and IP{u2083} production. I found in individual layer II/III pyramidal neurones a mEPSC rate of 42 {u00B1} 1 Hz. Adding noradrenaline (NA; 10 {u03BC}M; after 5') increased this rate by 50 {u00B1} 11% with no effect on amplitude. After blocking ({u03B2}-and {u03B1}{u2082}-ARs with propranolol and yohimbine, NA still increased mEPSC frequency by 62 {u00B1} 11% suggesting {u03B1}{u2081}-ARs activation. The {u03B1}{u2081}-AR agonist cirazoline increased the frequency by the same amount. Displacement experiments using both agonists and antagonists confirmed that NA bound to the {u03B1}{u2081}-AR. These findings were further supported by immunohistochemical labelling, which showed localisation of {u03B1}{u2081}B-ARs to these nerve terminals. To verify the signalling cascade activated by {u03B1}{u2081}-ARs, both the phospholipase C inhibitor edelfosine and membrane permeable IP{u2083}R blocker 2-APB caused a drop in mEPSC frequency, which subsequent NA application did not relieve. Emptying Ca{u00B2}{u207A} stores with cyclopiazonic acid reduced mEPSC frequency by the same amount, as did chelating intracellular Ca{u00B2}{u207A} with BAPTA-AM. When two receptors signalling via Gq were activated subsequently, the mEPSC frequency did not increase. This may indicate that these presynaptic receptors may not necessarily be signalling independently of each other. In 101 pairs of connected pyramidal neurones, an average EPSC amplitude of -31.4 {u00B1} 3.6 pA with a CV of 0.6 {u00B1} 0.1 was recorded. 60% of these EPSCs displayed long-term depression and the remainder showed a correlation between CV{u207B}{u00B2} and mean EPSC. As a consequence, the quantal current of the population was estimated to be 6.0 {u00B1} 0.6 pA, giving rise to a quantal content of 4.6 {u00B1} 0.6. This correlation indicates that at these synapses, the size of the EPSCs is largely governed by the number of release sites but not by release probability. Also, these EPSCs showed little paired-pulse plasticity. In NA, the EPSC depressed by 66 {u00B1} 23%. On average, both paired-pulse plasticity and release-independence remained unchanged. The depression was caused by {u03B1}{u2081}-AR activation as evidenced by cirazoline; it also was not caused by Ca{u00B2}{u207A} release from presynaptic stores indicating that the depression was caused by an early step in the signalling cascade. The depression was not correlated with presynaptic factors. After a burst of action potentials, recovery from depression was much faster, with recovery amplitudes being larger than the first EPSC in the stimulus sequence. This indicates that NA dramatically alters the synaptic dynamics. When store release was blocked, the EPSCs depressed by 33 {u00B1} 11% suggesting that store release increases each EPSC amplitude. This aspect is consistent with the expectations raised by the mEPSC data above. A potential mechanism is presented and discussed. I found that NA is a powerful tool to differentiate spontaneous from evoked transmitter release, as it's effect on both of these modes is opposite suggesting that the molecular elements for evoked and spontaneous transmitter release are distinct.