Effects of subchronic and chronic melatonin treatment on somatostatin binding and its effects on adenylyl cyclase activity in the rat frontoparietal cortex

Involvement of the cAMP-dependent pathway in the reduction of epileptiform bursting caused by somatostatin in the mouse hippocampus

The cyclic AMP pathway is major signal transduction system involved in hippocampal neurotransmission. Recently, the peptide somatostatin-14 (SRIF) has emerged as a key signal that, by activating its receptors, inhibits epileptiform bursting in the mouse hippocampus. Little is known on transduction mechanisms, which may mediate SRIF function in native cell/tissues. Using a well-established model of epileptiform activity induced by Mg(2+)-free medium with 4-aminopyridine [0 Mg(2+)/4-aminopyridine (4-AP)] in mouse hippocampal slices, we demonstrated that protein kinase A (PKA)-related signaling is upregulated by hippocampal bursting and that treatment with SRIF normalizes this upregulation. We also demonstrated that the SRIF-induced inhibition of PKA impairs phosphorylation of the NMDA receptor subunit NR1. Extracellular recordings of the 0 Mg(2+)/4-AP-induced hippocampal discharge from the CA3 region demonstrated that treating slices with compounds, which interfere with PKA activity, prevent SRIF inhibition of epileptiform bursting. Our results suggest that SRIF modulation of hippocampal activity may involve PKA-related signaling.

An update on somatostatin receptor signaling in native systems and new insights on their pathophysiology

The peptide somatostatin (SRIF) has important physiological effects, mostly inhibitory, which have formed the basis for the clinical use of SRIF compounds. SRIF binding to its 5 guanine nucleotide-binding proteins-coupled receptors leads to the modulation of multiple transduction pathways. However, our current understanding of signaling exerted by receptors endogenously expressed in different cells/tissues reflects a rather complicated picture. On the other hand, the complexity of SRIF receptor signaling in pathologies, including pituitary and nervous system diseases, may be studied not only as alternative intervention points for the modulation of SRIF function but also to exploit new chemical space for drug-like molecules.

Perfusion of melatonin into the prefrontal cortex disrupts the circadian rhythm of acetylcholine but not of locomotor activity

Extracellular concentrations of acetylcholine (ACh) in the prefrontal cortex displayed a circadian rhythm, with a maximum increase in the dark phase of the light:dark cycle. The increase in ACh related well to the circadian rhythm of the same rats in which a maximal increase of locomotor activity in the dark phase also was observed. Local perfusion of melatonin (200-500 microm), in a dose-dependent manner, disrupted the ACh rhythm in the prefrontal cortex but did not affect the circadian rhythm of locomotor activity. It is suggested that the change in the cholinergic transmission during a circadian period in the prefrontal cortex may be under the control of the biological clock through the action of melatonin; however, the prefrontal cortical ACh cycle seems not to be related to the regulation of locomotor activity.