Adrian J. Duszkiewicz

The synaptic plasticity and memory hypothesis asserts that activity-dependent synaptic plasticity is induced at appropriate synapses during memory formation and is both necessary and sufficient for the encoding and trace storage of the type of memory mediated by the brain area in which it is observed. Criteria for establishing the necessity and sufficiency of such plasticity in mediating trace storage have been identified and are here reviewed in relation to new work using some of the diverse techniques of contemporary neuroscience. Evidence derived using optical imaging, molecular-genetic and optogenetic techniques in conjunction with appropriate behavioural analyses continues to offer support for the idea that changing the strength of connections between neurons is one of the major mechanisms by which engrams are stored in the brain.

Adaptation to the ever-changing world is critical for survival, and our brains are particularly tuned to remember events that differ from previous experiences. Novel experiences induce dopamine release in the hippocampus, a process which promotes memory persistence. While axons from the ventral tegmental area (VTA) were generally thought to be the exclusive source of hippocampal dopamine, recent studies have demonstrated that noradrenergic neurons in the locus coeruleus (LC) corelease noradrenaline and dopamine in the hippocampus and that their dopamine release boosts memory retention as well. In this opinion article, we propose that the projections originating from the VTA and the LC belong to two distinct systems that enhance memory of novel events. Novel experiences that share some commonality with past ones ('common novelty') activate the VTA and promote semantic memory formation via systems memory consolidation. By contrast, experiences that bear only a minimal relationship to past experiences ('distinct novelty') activate the LC to trigger strong initial memory consolidation in the hippocampus, resulting in vivid and long-lasting episodic memories.

The anterior olfactory nucleus (AON), a component of the main olfactory system, is a cortical region that processes olfactory information and acts as a relay between the main olfactory bulbs and higher brain regions such as the piriform cortex. Utilizing a transgenic rat in which an enhanced green fluorescent protein reporter gene is expressed in vasopressin neurones (eGFP-vasopressin), we have discovered a population of vasopressin neurones in the AON. These vasopressin neurones co-express vasopressin V1 receptors. They also co-express GABA and calbinin-D28k indicating that they are neurochemically different from the newly described vasopressin neurons in the main olfactory bulb. We utilized the immediate early gene product, early growth response protein 1 (Egr-1), to examine the functional role of these vasopressin neurons in processing social and non-social odours in the AON. Exposure of adult rats to a conspecific juvenile or a heterospecific predator odour leads to increases in Egr-1 expression in the AON in a subregion specific manner. However, only exposure to a juvenile increases Egr-1 expression in AON vasopressin neurons. These data suggest that vasopressin neurones in the AON may be selectively involved in the coding of social odour information.