Pharyngeal pumping

The special neuromuscular structures of the nematode pharynx described above, generate pumping during feeding. This is necessary to carry the food from the mouth to the intestine. The mechanism of ingestion has been studied in great detail in C. elegans by several researchers.

Generally, the contractions of the pharyngeal muscles dilate the lumen of the pharynx. This causes a decrease in the pharyngeal pressure and so food is sucked in. The following rapid relaxation of these muscles causes an increase of the pressure. When this is sufficient to overcome the high internal hydrostatic pressure in the body, which keeps the intestine collapsed, the pressure pushes the food towards the pharyngeal-intestinal valve.

According to different environments and physiological needs, each pharyngeal pump is composed of one or more sequential stages. The pumping rates also differ within the species and developmental stages of the nematodes.

The electrical events associated with the contractions of the pharynx can be measured.

Table 1.3.: The pharyngeal pumping rates of different nematode species in their different stages (SONG, 2003).

Stage Pumping rate

(pumps/min) T. colubriformis:

L1 73.8+-11.0

L2 13.8 +- 1.5

L3 0.00

L3 exsheathed 0.00

L4 anterior pharynx 55.8 +-43.1 Adult anterior pharynx 180.0 +-13.3

128.0 +-18.0 C. elegans (adult) 90.0 +-12.4 H. contortus (adult) 200-300 Ancylostoma caninum (adult) 120-250

In C. elegans it is believed, that the pharyngeal pumping consists of three or more stages. Avery and Horvitz (AVERY and HORV'ITZ, 1989) recorded the electrical events of pumping using extracellular techniques. They established laser ablation and found that MC and M4 had significant effects on the function of the pharynx.

Pharyngeal pumping consists of three linked steps, the sensory stimulation, the reaction of the pharyngeal nervous system and the subsequent action of the pharyngeal muscles. The sensory stimulation can come from different environmental signals such as pH, temperature, pressure, chemicals, light, position or food, is probably detected by amphidial sensory organs and sent to the pharyngeal nervous system. The pharyngeal nervous system responds to the signals by releasing neurotransmitters and neuropeptides, which then act on the receptors of nerve or muscle cells.

The rates of pharyngeal pumping in nematodes are known to be partly controlled by serotonergic and neuropeptinergic inputs (BROWNLEE et al., 1995b).

Several anti-parasitic drugs act on the same receptors as the neurotransmitters, causing significant physiological changes in the pharyngeal pumping.

Glutamate is the best-studied neurotransmitter in the pharynx of the nematodes. It is an important inhibitory transmitter, acting on GluCl receptors. Glutamate is released by M3 neurons of C.

elegans (DENT et al., 1997; LI et al., 1997) and acts on the pharyngeal muscle by opening Cl -channels. This leads to hyperpolarisation of the nerve cell membrane and finally inhibits pumping. The anthelmintic class of ML’ s is believed to bind to the same receptor (GluCl).

GABA is the major inhibitory neurotransmitter in the somatic body musculature in nematodes.

But its role in the pharyngeal muscles remains unclear. It has been found to inhibit pharyngeal pumping in A. suum, (BROWNLEE et al., 1995b; BROWNLEE et al., 1997). In contrast, GABA and its agonist piperazine show excitatory effects on pharyngeal pumping in T. colubriformis (SONG, 2003). McIntire (MCINTIRE et al., 1993b) suggest, that GABA acts on cation channels, mediated by outwards Cl- conductance. This could indicate that GABA could be an excitatory neurotransmitter in the pharyngeal muscles, acting on GABA-gated Cl-channels.

The role of ACh is unclear. Pharyngeal pumping initiated by ACh had a faster onset and shorter effect. Pharmacological and genetic data from Avery and Thomas (AVERY and THOMAS, 1997) suggest that ACh is a neurotransmitter for MC neurons, which control the initiation and the rate of pumping in C. elegans. The findings of Song (SONG, 2003) in T. colubriformis support this hypothesis. ACh is the major excitatory neurotransmitter in the somatic body musculature in nematodes, but appears to have a biphasic effect in the pharynx muscles (SONG, 2003). An initial excitatory effect is followed by the inhibition of pumping. Similar effects have also been seen in the ovijector, another muscular organ. The explanation for the biphasic effect of ACh could be that: 1. ACh acts on different types of receptors or 2. that the receptor/mechanism in the pharyngeal muscle is pharmacologically different from the receptor/mechanism in the somatic muscle (SONG, 2003). In C. elegans both, nicotinic and muscarinic receptors are believed to be involved in the action of ACh (AVERY et al., 1990). Further it is presumed, that the putative nicotinic receptor in the pharyngeal muscle is pharmacologically and genetically different from the receptor in the somatic body muscles. Song (SONG, 2003) also suggests for T. colubriformis, that the whole nicotinic pharmacology, including mechanisms and receptors is complex and

totally different to that in somatic muscles. The main effect of ACh is inhibitory, but there is an interesting initial excitatory effect, seen at low concentrations (10^-5 M to 10^-6 M) as a quickly induced excitatory peak. It is likely, that at low concentrations ACh acts on nicotinic receptors to initiate pumping. At higher concentrations ACh probably starts to act on muscarinic receptors, causing permanent excitatory action, which leads to inhibition of pumping.

The inhibitory effect is significantly reduced in LEV-resistant worms, because they are less sensitive to cholinergic drugs. ACh and LEV both cause inhibitory effects in the pharynx.

Once dissected from the body the pharynx will continue pumping. The pharynx of starved C.

elegans will pump in the absence of food under conditions, in which well-fed worms do not pump (MUNN and MUNN, 2002). Furthermore, starved worms do respond to lower concentrations of food than fed worms do (AVERY and HORVITZ, 1990).