GPR

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The Gastropyloric Receptor (GPR) neurons are muscle receptors in the stomatogastric ganglion of decapod crustaceans (Katz et al., 1989). These neurons have been most extensively studied in the crab (Cancer borealis), but they have been found in all decapods that have been examined (Katz, Thesis, 1989).

Function

The GPR cells are muscle stretch receptors. In the crab, they are activated by contraction of the medial tooth muscle of the gastric mill (Katz et al, 1989).

Anatomy

Diagram of GPR innervation of the foregut. From Katz and Harris-Warrick, 1989

Peripheral innervation pattern

In crabs, there are two distinct types of GPR cells. GPR1 innervates the gm8a muscle. GPR2 innervates the gm9 and cpv3a muscles. In lobsters (Homarus), there are 4 GPR cells, which all innervate the same set of muscles. In crayfish, there are two GPR cells, which also co-innervate the same set of muscles.

Central projection pattern

Each GPR cell has an axon that projects to the stomatogastric ganglion (STG), where it branches. The axon continues through the stomatogastric nerve and branches at the superior oesophageal nerve (son), sending a branch to each commissural ganglion (CoG).

Neurotransmitters

Serotonin

The GPR cells are serotonergic in crabs, hermit crabs, lobsters (Homarus), and shrimp. But in the spiny lobster (Panulirus) they are not serotonergic. Although it has not been conclusively demonstrated, it is likely that serotonin (5-HT) is a borrowed transmitter in the GPR cells. In other words, serotonin is not synthesized in these neurons, but is taken up by transporters. The evidence for this is that serotonin immunoreactivity is often very weak unless the preparation is bathed in serotonin.

Acetylcholine

The GPR cells elicit rapid EPSPs that are blocked by a variety of nicotinic acetylcholine antagonists (Katz and Harris-Warrick, 1989).

Allatostatin

The GPR cells were found to be allostatin-immunoreactive in a number of species (Skiebe, 1999).

Physiology

The GPR cells make monosynaptic connections onto neurons in the stomatogastric ganglion (STG) and in the commissural ganglia (CoGs). In the STG, the GPR cells excite DG with monosynaptic cholinergic EPSPs and induce plateau potential properties with 5-HT.

In the crab the GPR cells display two modes of activity in response to muscle movement: spiking and bursting (Katz et al, 1989). In the latter mode the neuron exhibits endogenous bursting properties that might play a role in coding sensory information (Birmingham et al, 1999). Spiking and bursting appear to originate from distinct spike initiations zones. The evidence for this is the observation of different spike sizes in extracellular recordings measured close to the innervated muscles.

A number of neuromodulatory substances including allatostatin and 5-HT can modify the response of GPR2 to muscle stretch (Birmingham et al, 2003).

References

  1. Beenhakker MP, DeLong ND, Saideman SR, Nadim F, Nusbaum MP. Proprioceptor regulation of motor circuit activity by presynaptic inhibition of a modulatory projection neuron. J Neurosci. 2005 Sep 21;25(38):8794-806. PMID: 16177049
  2. Billimoria CP, DiCaprio RA, Birmingham JT, Abbott LF, Marder E. Neuromodulation of spike-timing precision in sensory neurons. J Neurosci. J Neurosci. 2006 May 31;26(22):5910-9. PMID: 16738233
  3. Birmingham JT, Billimoria CP, DeKlotz TR, Stewart RA, Marder E. Differential and history-dependent modulation of a stretch receptor in the stomatogastric system of the crab, Cancer borealis. J Neurophysiol. 2003 Aug 27; 90(6):3608–3616. PMID: 12944539
  4. Birmingham JT, Szuts ZB, Abbott LF, Marder E. Encoding of muscle movement on two time scales by a sensory neuron that switches between spiking and bursting modes. J Neurophysiol. 1999 Nov;82(5):2786-97. PMID: 10561445
  5. Blitz DM, Beenhakker MP, Nusbaum MP. Different sensory systems share projection neurons but elicit distinct motor patterns. J Neurosci. 2004 Dec 15;24(50):11381-90. PMID: 15601944
  6. Katz PS, Eigg MH, Harris-Warrick RM. Serotonergic/cholinergic muscle receptor cells in the crab stomatogastric nervous system. I. Identification and characterization of the gastropyloric receptor cells. J Neurophysiol. 1989 Aug;62(2):558-70. PMID: 2769347
  7. Katz PS, Harris-Warrick RM. Serotonergic/cholinergic muscle receptor cells in the crab stomatogastric nervous system. II. Rapid nicotinic and prolonged modulatory effects on neurons in the stomatogastric ganglion. J Neurophysiol. 1989 Aug;62(2):571-81. PMID: 2769348
  8. Katz PS, Harris-Warrick RM. Neuromodulation of the crab pyloric central pattern generator by serotonergic/cholinergic proprioceptive afferents. J Neurosci. 1990 May;10(5):1495-512. PMID: 2332793
  9. Katz PS, Harris-Warrick RM. Recruitment of crab gastric mill neurons into the pyloric motor pattern by mechanosensory afferent stimulation. J Neurophysiol. 1991 Jun;65(6):1442-51. PMID: 1875253
  10. Kiehn O, Harris-Warrick RM. Serotonergic stretch receptors induce plateau properties in a crustacean motor neuron by a dual-conductance mechanism. J Neurophysiol. 1992 Aug;68(2):485-95. PMID: 1527571
  11. Kilman V, Fenelon VS, Richards KS, Thirumalai V, Meyrand P, Marder E. Sequential developmental acquisition of cotransmitters in identified sensory neurons of the stomatogastric nervous system of the lobsters, Homarus americanus and Homarus gammarus. J Comp Neurol. 1999 Jun 7;408(3):318-34. PMID: 10340509
  12. Skiebe P. Allatostatin-like immunoreactivity in the stomatogastric nervous system and the pericardial organs of the crab Cancer pagurus, the lobster Homarus americanus, and the crayfish Cherax destructor and Procambarus clarkii. J Comp Neurol. 1999 Jan 5;403(1):85-105. PMID: 10075445
  13. Tierney AJ, Godleski MS, Rattananont P. Serotonin-like immunoreactivity in the stomatogastric nervous systems of crayfishes from four genera. Cell Tissue Res. 1999 Mar;295(3):537-51. PMID: 10022973
  14. Zhang B, Harris-Warrick RM. Multiple receptors mediate the modulatory effects of serotonergic neurons in a small neural network. J Exp Biol. 1994 May;190:55-77. PMID: 7964396
  15. Zhang B, Harris-Warrick RM. Calcium-dependent plateau potentials in a crab stomatogastric ganglion motor neuron. I. Calcium current and its modulation by serotonin. J Neurophysiol. 1995 Nov;74(5):1929-37. PMID: 8592186