Lateral Protocerebrum

Structure of the lateral protocerebrum

The lateral protocerebrum (LPC) is a non-glomerular neuropil and surrounds glomerular neuropile such as mushroom bodies. In silkmoth, immunohistochemistry for soluble guanylyl cyclase and labeling of projection neurons revealed that information for the major component of the pheromone, bombykol, is relayed in a specific triangular region now called the delta area in the inferior lateral protocerebrum (DILPC). In contrast, information for the minor component of the pheromone, bombykal, is relayed outside this conspicuous area, but also in the infero-lateral PC. General odorant information is relayed in the posterior lateral PC (Seki et al., 2005). Similar functional areas also exist in Drosophila (Jefferis et al., 2007). In the cockroach, thermo- and hygrosensory projection neurons innervate areas different from those where plant and pheromone information is processed (Nishino et al., 2003).

Function of lateral protocerebrum
The functions of the LPC are poorly understood compared to other more conspicuous neuropils. Both anatomical and electrophysiological data indicate that pheromone, plant odor, thermo- and hygrosensory information could be integrated in the LPC. Arborization patterns of the LPC neurons are hardwired and develop even in the absenceof olfactory input (Heimbeck et al., 2001; Tanaka et al., 2004). Thus, in contrast to the mushroom bodies, which play an important role in learning and memory, it is thought that the LPC is involved in experience-independent instinctive olfactory behaviors. In Drosophila, ablation of mushroom bodies using hydroxyurea treatment did not affect odor-induced courtship behavior of males. This result suggests that neuronal pathway from the antennal lobe to the LPC is important in instinctive behavior (Kido and Ito, 2002). Some neurons with axon collaterals in the LPC are crucial for the initiation of mating behavior (Broughton et al., 2004). Since blockade of neurotransmission in Drosophila mushroom bodies impairs attractive but not repulsive olfactory conditioning, the LPC is likely to be involved in aversive conditioning (Wang et al., 2003).

Neurons in the lateral protocerebrum
Input neurons: Antennal lobe projection neurons have dendrites in glomeruli of the antennal lobe and project to the protocerebrum, namely the mushroom bodies and the LPC. These neurons are the main input neurons of the LPC. One pathway projects first to the mushroom bodies and from there sends collaterals to the LPC. A second pathway first innervates the LPC and from there sends collaterals to the mushroom bodies. A third group of AL PNs only projects to the LPC. AL Pns that exclusively have MB projections have so far not been identified. Output neurons: The LPC output neurons receive synaptic inputs in the LPC and project to other protocerebrum regions. One output neuron type with projections in the calyx and the lobes of the MB has been characterised morphologically and electrophysiologically (Nishino and Mizunami, 1998; Strausfeld and Li, 1999; Perez-Orive et al., 2002). Other types of output neurons type projecting to protocerebral target regions have so far only been investigated morphologically (Tanaka et al., 2004).


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