Culloty, winner of the National on Bindaree two years ago has picked up the ride on the Martin Todhunter-trained Just In Debt.
His agent Chris Broad said: "He was very much looking forward to riding the favourite, but you can't have everything go your way all the time.
"It's disappointing, but even more so for Dessie Hughes and the owners of the horse.
"They've trained the horse for the race and have probably set out all year for it."
Another Irish raider, Hedgehunter, trained by Willie Mullins, is the new market leader with the major firms at around 9-1.
But Ladbrokes predict that Jurancon II, representing the champion team of jockey Tony McCoy and trainer Martin Pipe, will start favourite. "The McCoy factor will ensure that Jurancon II will be favourite on the day," said spokesman Balthazar Fabricius.
Coral report that Amberleigh House is now the worst loser in their ante-post book, having been well-backed at all rates from 33-1 down to 12-1, and believe he could turn out to be the "housewives' choice".
The 12-year-old, who was third last year, is trained by Ginger McCain, responsible for the legendary three-time winner Red Rum.
"It would be no surprise if the Red Rum factor makes Amberleigh House one of the best-backed horses on Grand National day, and he could even end up challenging for favouritism," said Coral spokesman Simon Clare.
There was a scare for Amberleigh House's jockey Graham Lee yesterday when he was stood down for the rest of the meeting following a fall from Dionn Righ in the St James Security Handicap Chase at Sedgefield.
Sue Smith's husband Harvey reported that the Bingley stable's Grand National runners, Artic Jack and Ardent Scout, are in good form.
He was on hand to saddle the stable's two winners, Free To Run and Cardinal Mark, at Sedgefield, and said: "Both our National horses worked well this morning and seem in good order, so we can't do much more.
"We were relieved to see Ardent Scout definitely get in the race and he will be ridden by Warren Marston. Dominic (Elsworth) will be on Artic Jack."
Trainer Philip Hobbs confirmed that What's Up Boys, runner-up to Bindaree two years ago, will have a second crack at the National after an encouraging comeback from injury when fourth to Exit Swinger at Wincanton. "I was pleased with the performance of What's Up Boys, though I am not sure how strong the form might be," said the Minehead trainer.
Andrew Tulloch, clerk of the course at Aintree, watered the Mildmay Course again yesterday morning but is keeping the sprinklers away from the Grand National course, which remains officially 'Good'.
Stressing that he only turned the taps on to maintain what he considers "perfect conditions", Tulloch, having walked around yesterday, said: "The National course is on the easy side of good, and there are no plans to water at this stage it is lovely ground at the moment.
"In contrast, the Mildmay course, though still officially good, was starting to dry out, so we have had to put a few millimetres on and will monitor the situation again on Wednesday."
Tulloch revealed that the forecast rain, originally predicted to arrive in the Merseyside area on Thursday morning, has now been put back 24 hours.
He added: "They are talking of between six and eight millimetres, but, apart from that bout, we are expecting virtually no rain.
"Today has been dry and sunny and it will be a similar story tomorrow, though it might cloud over mid-afternoon, while there is only a 10% chance of the odd millimetre of rain on Thursday, and it looks like being a clear dry day for the big race on Saturday."